Text 26

Catalytic Reforming

Because higher- octane gasoline permit the building of engines that extract more power from gasoline, there has been a constant push toward higher octanes since differences is octane quality were first recognized. A major factor in this development has been the large - scale use of catalytic reforming to raise octane ratings of gasoline stocks. The first commercial unit, Hydroformer, went on stream just before World War II, and the process proved to be a major source of aromatics and aviation gasoline for military uses. However, catalytic reforming did not "catch on" until about 1950, when Haensel and others at the Universal Oil Products Co. demonstrated that platinum catalysts could be used commercially despite their high cost. By 1955, catalytic reforming processes had almost completely supplanted thermal reforming. Catalytic processes not only give higher quality products, they give a higher yields as well.

Reactions

In catalytic reforming, the principal object is to convert other hydrocarbon to aromatics. The reason may be seen by comparing the octane numbers of some corresponding hydrocarbons [Table 1]. Thus, high conversions to aromatics result in high octane products. There is a loss in volume, because aromatics are denser than other hydrocarbons; however, the loss is small in comparison with the loss [to gas and tar] suffered in thermal reforming. Other reactions of some importance in catalytic reforming are cracking and isomerization.

Table 1

Research rating Motor rating

n-Heptane

2- Methylhexane

Heptene-2

Methylcyclohexane

2,3-Dimethylpentane

2,2,3-Trimethylbutane[triptane]

Toluen 0

42

73

75

91

113

120 0

45

57

71

89

101

104

Production of Aromatics. Because aromatics contain less hydrogen than do other hydrocarbons, dehydrogenation is the primary reaction. Of the nonaromatics, cyclohexane derivatives are dehydrogenated most readily:

C6H11CH3  C6H5CH3 + 3H2

Methylcyclohexan Toluene

Cyclopentane derivatives react similarly, but they require a preliminary isomerization to cyclohexan derivatives:

C5H9CH2CH3  C6H11CH3  C6H5CH3 + 3H2

Ethylcyclopentane Methylcyclohexane Toluene

Conversion of paraffins to aromatics involves a cyclization step. For normal heptane, the reaction may be written:

n-C7H16  C6H11CH3 + H2

C6H11CH3  C6H5CH3 + 3H2

To undergo these reactions, a paraffin must have at least six carbon atoms in a chain or be isomerizable to such a compound.

Aliphatic olefins can also be converted to aromatics directly, but this fact is of little practical significance because such olefins are readily hydrogenated to paraffins under the conditions used in catalytic reforming. Thus , aliphatic olefins behave as paraffins, with the exception that they deactivate the catalyst more rapidly. Similarly, cyclic olefins behave as naphtenes.

Hydrocracking. Under the conditions employed in catalytic reforming, cracking completes with dehydrogenation reactions. Because high hydrogen pressure are used, any olefins that form are saturated immediately, and the reaction usually called "hydrocracking". Whether hydrocracking occurs in one step or in two is of little consequence. In either case, a typical over-all reaction is:

n-C8H18 + H2  C3H8 + n-C5H12

Octane Propane Pentane

Because lower-boiling paraffins have higher octane numbers, hydrocracking improves octane ratings; however, the improvement is less than if the paraffins were converted to aromatics. Also, there is considerable loss of gasoline to butanes and lighter materials, and the vapour pressure of the debutanized product is raised. Increasing the vapour pressure reduces the amount of butane that can be blended into the product to make a finished gasoline; thus, the effective yield of gasoline is reduced still further.

Hydrocracking of naphthenes also occurs to some extent. Cyclopentane derivatives are more susceptible than cyclohexane derivatives, especially over catalysts with little isomerization activity. The first step in the hydrocracking of naphthenes is probably scission of the ring:

C5H9CH2CH3 + H2  n-C7H16

The paraffins are formed may react further to produce aromatics, or it may be hydrocracking.

Isomerization. With some catalysts, paraffins are isomerized under reforming conditions. Usually, isomerization of paraffins does not have a large effect on octane quality because the production of highly branched paraffins are small. If the paraffins in a given charge were chiefly normal, their isomerization would have a large effect on octane. In most instances, however, paraffins in the charge are mixtures of isomers; therefore the isomerizing activity of a catalyst is important chiefly for the isomerization of cyclopentane derivatives.

Catalysts

Although aromatics can be produced from either hydrocarbons without catalysts, severe conditions are required, and yields are low. To obtain acceptable yields, dehydrogenation catalysts must be employed. Those of commercial interest include platinum on alumina, platinum on sillica-alumina, chromia on alumina, molybdena on alumina, and cobalt molybdate on alumina. The ideal catalyst would convert all other hydrocarbons selectiely to aromatics rapidly, with only a small catalyst inventory. Such a catalyst would not promote hydrocracking, and it would have to operate under conditions thermodynamically favourable to production of aromatics. To the extent that a catalyst deviates from this conditions it is a poorer catalyst. Derivations may be either in the selectivity of the catalyst toward the production of aromatics or in the activity of the catalyst for the several reactions that actually occur. Selectivity is determined by the relative rates of the competing reactions- dehydrogenation to aromatics and hydrocracking, and isomerization in so far as it affects the other two. Activity is determined by the magnitude of the rate constants. Platinum catalysts appear to be the most selective and the most active, as well as the most expensive.

Dehydrogenation of Naphthenes. Selectivities of catalysts depend to some extent on the make-up of the feed stock. Alkylcyclohexanes are readily converted to aromatics by all dehydrogenation catalysts, provided that the reaction conditions are favourable thermodynamically. For the conversion of alkylcyclopentanes, on the other hand, there are large differences. Because alkylcyclopentanes require an isomerization step, their conversion to aromatics depends upon the isomerization activity of the catalyst. Published data on platinum, molybdena, and chromia catalysts show that platinum has the highest isomerization activity, chromia the lowest. Even with platinum catalysts, the isomerization reaction is the rate-controlling step. Thus the conversion of alkylcyclopentanes to aromatics is lower than the conversion of alkylcyclopentanes to aromatics is lower than the conversion of cyclohexane derivatives; consequently there is more opportunity for hydrocracking, and yields of aromatics are poorer.

Dehydrocyclization of Paraffins. Data on platinum, molibdena, and chromia catalysts have also been published for the conversion of paraffins to aromatics. When operating in the pressure range normally use in catalytic reforming, platinum is the most effective catalyst, chromia the least. The poor results obtained with chromia catalyst are surprising, inasmuch as high conversions of n-heptane to toluene are obtained at low pressures. Apparently, the chromia catalyst has the unusual property of adsorbing hydrogen so strongly at higher pressure that paraffins can not readily reach its surface.

Reaction Mechanism

Extensive studies have been made to elucidate the mechanism of reforming over platinum catalysts. Such catalysts are duel-functional; they contain platinum as a dehydrogenating agent and an acidic material, such as chlorine, fluorine, or alumina-promoted silica, as an isomerization agent. In commercial catalysts, enough platinum is used to ensure that the dehydrogenation activity is large in comparison with the isomerization activity.

Although only traces of olefins can exist under reforming conditions, they apparently are intermediates in the reactions. Both naphthenes and paraffins are dehydrogenated to olefins [in trace amounts] on dehydrogenation sites in the catalyst. Cyclohexenes continue to dehydrogenate rapidly to aromatics. Alkylcyclopentanes transfer to acid sites, where they are isomerized to cyclohexanes; the cyclohexanes then pass back to dehydrogenation sites, where they are converted to aromatics. Alkyl olefins also transfer to acid sites where they may either isomerize to other alkyl structures or cyclize to naphthenes. The isomerized olefins pass back to dehydrogenation sites, where alkyl olefins are hydrogenated to paraffins and cyclohexenes are dehydrogenated to aromatics.

In view of the low isomerization activity of chromia catalysts, the excellent results obtained with them at low pressure suggest that n-heptane is easier to aromatize than are its isomers. This idea is also suggested by data on the conversion of n-heptane over a platinum catalyst; the ratio of aromatics production to hydrocracking was higher at low conversions [where n-heptane predominates in the reactants] than at higher conversion [where isoheptanes predominate]. It has also been shown that paraffins with more than seven carbon atoms are converted more readily to aromatics than are heptanes. All these observations fit the hypothesis that naphthene intermediates are not formed from paraffins over platinum catalysts by linking of two end [primary] carbon atoms. It has been suggested that platinum catalysts form derivatives of cyclopentane by the linkage of second and sixth carbon atoms; the alkylcyclopentanes so formed isomerize to alkylcyclohexanes, which are dehydrogenated to aromatics. This mechanism could not apply for cgromia catalysts, which have little isomerization activity. When n-heptane is processed over a chromia catalyst, the second and seventh carbon atoms appear to link up to form methylcyclohexane directly.

Exercises

Answer the following question

1. What is the purpose of catalytic reforming ?

2. Why had catalytic reforming supplanted thermal reforming ?

3. Do you know why conversion other hydrocarbons to aromatics is principle of catalytic reforming ?

4. What are the main reactions in catalytic reforming ?

5. Would isomerization have a large effect on octane number?

6. By what is the activity of catalysts determined?

7. What is the characteristic of platinum catalysts?

8. Why is the conversion of alkylcyclopentanes to aromatics lower than that of cyclohexane derivatives?

9. What does the selectivity of catalysts depend on?

10. Which catalyst has the highest isomerization activity?

11. Why is the activity of the dehydrocyclization of paraffins low at pressure range normally used in catalytic reforming?

12. What are dual-functional catalysts?

13. Which reaction is performed in dehydrogenation sites?

14. Which reaction is performed in acid sites?

15. Why are the paraffins with more than seven carbon atoms converted to aromatics easier than heptanes?

16. Can you show the mechanism of conversion of paraffins to aromatics?

Petroleum products

The products obtained from petroleum can be classed into four groups: I-fuels; II- lubricating oils, paraffins, etc.; III-miscellaneous petroleum products; and IV-chemical and petrochemical products.

Group I includes liquefied hydrocarbon gases, fuels for carburettor engines [gasolines], fuels for jet (kerosene) and turbojet engines , Diesel fuels, boiler fuels; Group II-various lubricating oils, paraffins, ceresins and petrolatum; Group III- plastic greases, bitumenns, coke, etc; and group IV- hydrocarbons of various classes which serve as starting materials for organic or petrochemical synthesis.

I. Liquefied hydrocarbon gases and fuels

Liquefied hydrocarbon gases consist mainly of propane and butane and sometimes may contain small quantities of propylene and butylene. They have found the widest application as domestic fuel which may be commercial propane (at least 93 % of propane), commercial butane (at least 93 % of butane) or their mixture (in winter time, with a greater proportion of propane).

Liquefied gases or their constituents of higher purity are used as starting materials for the manufacture of various chemical products and olefines [by pyrolysis].

a) Fuels for carburettor Engines

This group of fuels includes aviation and motor gasolines and tractor kerosene. An important characteristic of these fuels is the pressure of sasturated vapours, kPa, which should be 29.3 to 47.9 for aviation gasolines, 66.5 to 93.4 for motor gasolines (not more than 66.5 for summer grades).

The fractional composition of fuels is also of large importance. For instance, the 10% boiling point of gasoline (the p[oint at which 10% of the fuel boils off) can characterize the starting properties and reliability of an engine starting under various conditions, in particular, at a low temperature of the ambient air. The 50% boiling point of gasoline characterizes the speed of engine heating during starting, the smoothness of switching from one operating mode to another, and the stability of engine operation. The 90% and 97.5% boiling points of aviation gasoline and the temperature of the end of boiling of motor gasoline determine the homogeneity of the fuel mixture, i.e. the completeness of fuel combustion in the engine. This is extremly important, since with incomplete combustion of the fuel, liquid substances can penetrate into the crankcase and dilute the lubricating oil and thus cause a quick wear of the engine. Besides, incomplete combustion causes a stronger pollution of the air.

Antiknock rating is another important characteristic of fuels which determines their proper combustion ic carburettor engines. With detonation (knock-type) combustion, the rate of flame front propagation increases very quickly and causes explosion, or knock, in an engine; as a result, the engine may quickly be put out of operation. The antiknock rating of fuels is evaluated in terms of the octane number (ON).

Aviation Gasolines (state Standard GOST 1012-72). They are used as fuel for carburettor-engine planes and helicopters. In the USSR, aviation gasoline is avalable in the following grades: B-70, B-100/130, and B-91/115. The grading includes a letter B and a number indicating the octane number or two numbers: The numerator indicating the octane number and the denominator, the rating. Aviation gasolines are prepared by mixing (compounding) of a base gasoline (obtained by catalytic cracking or catalytic reforming), high-octane components (isooctane, alkyl gasoline, isopentane, benzene hydrocarbons, etc.), tetraethyl lead (TEL) and other additives raising the octane number, and of inhibitors i.e. substances preventing fuel oxidation (with aviation gasoline, oxy- diphenylamine is used for the purpose). These components are taken in proportions required to make gasoline of the desired grade and quality.

The boiling-off points of gasolines should not exceed the following temperatures: 90% boiling-off point 1450C; 50% 1050C; and 10% from 750 to 880C. The content of TEL (g/kg of gasoline) should be not more than 2.7 for grade B-100/130 and from 2.5 to 3.3 for other grades, except for B-70 which contains no TEL.

Motor Gasolines. These grades of gasoline are employed in automobile carbuurettor engines. One of the most important indices of their quality is the anti-knock rating which is expressed in terms of the octane number.

Otane number is numerically equal to the content of isooctane (% by volume) in a mixture with n- heptane, which is equivelent in its detonation intensity in a single cylinder engine to the fuel being tasted under standard conditions. The octane numbers of isooctane is taken conditionally to be 100 and that of n-haptane, zero. The octane numbers are determined by using mixture of these two hydrocarbons. The current control of fuels is done by using what is called secondary reference fuels having various values of the octane number.

Octane numbers can be determined by various methods. The motor method uses apparatuses of the type IT9-2M and UIT-65 to measure the octane number of motor and aviation gasolines. Motor gasolines can also be tested by what is called the research method in apparatuses of the type IT9-6 and UIT-65 (State Standard GOST 8226-66). The temperature method(State Standard GOST 3337-52) with the use of IT9-5 apparatus is employed to determine the antiknock of high- octane aviation gasolines (ON 100 or higher). The pressurization method (State Standard GOST 3368-68) with the use of IT9-1 apparatus is used to determine the rating of aviation gasolines in rich mixtures.

The octane number of gasoline increases on addition of benzene hydrocarbons and isomeric paraffin hydrocarbons and also on a decrease of the point of full boiling-off. If these measures fail to give gasoline with a desired octane number, an antiknock agent is added. Various metalorganic and organic substances can be used as antiknock agents.The most popular antiknock is tetraethyl lead Pb(C2H5)4 in the form of ethyl liquid.

All hydrocarbons can be written in the following order of increasing effect of TEL on octane number: paraffin-naphenes-benzenes-olefins. With an increase in the content of TEL in gasoline, its effectiveness diminishes.The sensitivity of gasolines to TEL decreases sharply with an increasing concentration of sulphur which reacts with lead and petrifies the effect of TEL. For that reason, the starting materials of certain processes and some grades of gasoline are purified from sulphur compuonds before adding TEL.

The grades of motor gasoline produced in the USSR are as follows: A-66, A-72, A-76, AI-93 and AI-98 (the digits are octane numbers). All these grade can be ethylated, except for A-72. The content of TEL in them should not exceed 0.6 g/kg in A-66, 0.41 g/kg in A-76 and 0.82 g/kg in AI-93 and AI-98. The octane number of grade AI-93 and AI_98 is measured by the research method and that of the other grades, by the motor method. For easier operation of engines, motor gasolines are manufactured as summer and winter kinds, the latter, as has been given earlier, having a higher pressure of saturaed vapours. Besides, they have (except for grade AI-980 different temperature of boiling-off of intermediate fractions and of the end of boiling. The fractional composition of motor gasolines is given below (numbers in numerators and denominators are boiling-off points respectively for summer and winter kinds of gasoline, 0C).

A-66 A-72, A-76

AI-93 AI-98

Beginning of boiling, at least

Boiling-off points,lower limit:

10%

50%

90%

End of boiling 35/-

79/65

125/125

195/160

205/185 35/-

70/55

115/100

180/160

195/185 35/-

70/-

115/-

180/-

195/-

New, more efficient makes of automobile engines have a high compression ratio and can be run only on high-octane gasoline. Motor gasolines are prepared by mixing (compounding) various components: high-octane gasolines of catalytic cracking and catalytic reforming, alkylates and isomerizates of light fractions of preliminary distillation. For preparation of gasoline with lower octane numbers (especially of grade A-66), use is also made of gasolines of thermal cracking and coking, gasolines obtained by straight-run distillation of petroleum, which have a higher temperature of boiling off, and dearomatized products (refined petroleum) obtained in the manufacture of benzene hydrocarbons by catalytic reforming of gasoline fractions.

Since tars, if present in gasoline, can disturb the operation of engines, their content in gasolines is limited at 7mg/100ml in grade A-66 and 5 mg/100ml in the other grades. The chemical stability of gasolines is checked by determining the induction period which should constitute at the manufacturer at least 450 min for grade A-66, 600 min for A-72, and 900 min for the other grades.

b) Fuels for diesel engines.

In Diesel engines, air is compressed and its temperature rises and Diesel fuel injected into the engine is ignited by the hot air. The capability of diesel fuels for self- ignition is measured in term of cetane number.

Cetane number is the index of ignitability of diesel fuel, which is equal numerically (in per cent) to the content of cetane (n-hexadecane C16H34) in a mixture with -methylnaphthalene C11H10, which possesses the same ignitability in a single-cylinder engine under standard testing conditions as the fuel being examined. The cetane number of cetane proper is taken equal to 100 and that of -methylnaphthalene, zero. The cetane number depends on fuel composition: the highest cetane number is shown by paraffin hydrocarbons, a lower, by naphthenes and the lowest, by bezene hydrocarbons which for that reason are undesirable in Diezel fuels. The cetane number can be raised by mixing Diezel fuel with certain components containing paraffin hydrocarbons of normal structure or by giving special additives.

Diesel engines are divided into three classes: high-speed engines (above 1000 rpm) for agricultural machines, Diesel locomotives, cross-country vehicles, etc.; medium-speed engines (500-1000 rpm) for large locomotives and as auxiliary motors on ships; and low-speed engines (less than 500 rpm) employed as main marine engines and Diesel-generators.

Depending on the content of sulphur in the original petroleum, diezel fuel fractions may be low-sulphurous (up to 0.2 % S) and sulphurous (0.7 to 1.8 % S). The content of sulphur can be reduced by hydrogen refining. Low-sulphur fuels are advantageous in being less corrosive and less liable to carbonization; besides, they form exhaust gases low in sulphurous and sulphuric anhydrides. The viscosity of diezel fuels also standardized to ensure proper atomization and reliable operation of the fuel-supply system. Heavy fractions in the fuel can cause in complete combustion and smokes in exhaust gases and carbonization in the engine.

Medium-speed diezel engines can be run heavy distillate fuels and low-speed ones, on fuels obtained by dilution of fuel oils by distillates, including diesel fractions, to obtain the desired viscosity from 36 to 67 mm2/s at 500C]. The setting point of the mixture may be from -5 to 50C.

c) Boiler Oils (fuel oils)

Fuel oils are used in many branches of national economy, in particular, at thermal power plants.

According to the State Standard GOST 10585-75, fuel oil is graded as folows: marine grades F-5 and F-12 (light fuel), furnace fuel oil grade 40 (medium) and furnace fuel oil grade 100 (heavy fuel). The characteristics of fuel oils may vary appreciably in different grades. For instance, the relative viscosity of fuel oils should be respectively: not more than 5 and 12 mm2/s at 500C for marine grades and 8 and 16 mm2/s at 800C for furnace grades 40 and 100. Fuel oils of a higher viscosity have a higher flash point, which is specified at 80 and 90 for marine grades F-5 and F-12 (in a closed cruible) and at 900 and 1100C (in an open crucible) for furnace grades 40 and 100. The setting point of fuels is limited at -50 to 250C (or up to 42 for fuel oils obtained from high-paraffin petroleum). According to the sulphur content, fuel oils of each grade are divided into low-sulphurous (up to 0.5% S), medium-sulphurous (0.51 to 1.0 per cent), and high- sulphurous (1.01 to 3.5 %).

The quality of fuel oils is decided by the following characteristics: viscosity, which determines the ease of transportation of the fuel and the propable degree of heating for effective atomization; setting point, which determines the conditions of storage and application of the fuel at various temperatures of the air; sulphur content, which determines the degree of corrosion of the engine and the exhaust of sulphurous compounds to the atmosphere. One of the decisive charateristics of fuel oils is the heat of combustion (calorific value) which depends on their composition. The low calorific value of low-sulphurous and medium-sulphurous fuel oils (recanculated to dry fuel) must be not less than 41454 kJ/kg for furnace grades, 40470 kJ/kg for furnace grade 40, 40530 kJ/kg for furnace grade 100.

Fuel oil grades are chosen according to the conditions of operation of engines. Thicker (and cheaper) grades are commonly used at stationary plants where the fuel can be heated up and filtered. Magine grades of fuel oil (employed in marine power plants) differ from furnace grades in having a lower content of ash, water, sulphur and tars.

Fuel oils are prepared by mixing residual products of preliminary distillation (residual fuel oil, semigoudron and goudron) with residual products of thermal and some catalytic processes (cracking residue, gasoil, reflux, polymers) and residual products of oil manufacture.

d) fuels for Jet and Gas-turbine Engines

Fuels for aviation jet engines are divided into two main groups: for subsonic and supersonic speeds. The latter must have an elevated density and a sufficienly high calorific value to ensure the required power of the engine and the desired flight range. At higher speeds of flight, fuel is heated much more.

Jet-engine fuels (aviation kerosene] are kerosene fractions of preliminary distillation of petroleum having the temperature of the begining of boiling from 1500 to 1950C and the boiling-off point from 2500 to 3150C. Fuels for jet engines must easily be vaporizable, have a hight calorific value (the lowest calorific value being not less than 42950-44 160 kJ/kg], high thermal stability, a low temperature of the begining of crystallization (not higher than -600C) and cause no corrosion of engine elements. Jet-engine fuels of the highest thermal stability are obtained by catalytic refining in hydrogen under pressure.

Gas-turbine fuels for terrestrial machines differ from aviation kerosene by a wider fractional composition and higher content or sulphur (up to 3 %). Their relative viscosity at 500C must be not more than 2.

Exercises

Answer the following question

1. How many group can petroleum products be classed into? What are they?

2. Which group do liquefied hydrocarbon gases belong to?

3. What are the main compositions of liquefied hydrocarbon gases?

4. How many important characteristic have fuels for caburettor engines got? What are they?

5. What happen if there is incomplet combustion?

6. What does the antiknok rating determine?

7. What is the octane number?

8. Which kind of hydrocarbons have the higher octane number?

9. What do they do to increase the octane number of gasoline?

10. What are the limit content of TEL in the grades of motor gasoline in USSR?

11. The winter kind of motor gasoline has a hihger pressure of saturated vapors than the summer, hasn't it?

12. What is the chemical stability of gasoline checked?

13. Which fuels is the term cetane number used for?

14. What is cetane number?

15. What does cetane number depend on?

16. What is order of increasing the cetane number of hydrocarbon?

17. Which method are used to raise cetane number?

18. How are fuel oils classified?

19. What is the quality of fuel oils decided by?

20. What is decisive characteristic of fuel oils?

21. Fuel oil grades are chosen according to the conditions of operation of engines, aren't they?

22. Where are thicker grades commonly uesed?

23. How do they prepare fuel oils?

24. How do they classify fuels for aviation jet engines?

25. What are the characteristics of fuel for let engines?

II. Lubricants, Products of Oil-paraffin Processing and Other Petroleum Products

In addition to high-quality fuels, lubricating materials are also essential for normal operation of various engines and mechanisms. All lubricants can be divided into four types: gaseous, solid, liquid, and semisolid (thickened), or greases.

Some gases can react with metals to form a lubricating film which lowers friction and wear. Solid lubriants, such as graphite or molybdenum disulphide, are employed at very high temperatures and under heavy loads where ordinary lubricants, including oils and greases, are ineffective. In this section, we shall discuss only lubricating oils and greases.

a) Lubricating and other Oil

Petroleum processing industry manufactures minaral oils of many kinds: motor oils (aviation, diesel and automobile grades), industrial oils, turbine oils, electroinsulating oils, compressor oils, ectc.

Viscosity is the most important characteristic of all kinds of oil. On the one hand, it should be sufficiently low to ensure lubrication and easy start of engines at low temperatures and, on the other, sufficiently high to lubricate properly even the hottest parts of an engine. This requirement is met by oils having a high viscosity index. Other important chaaracteristics of oils are their oxidation stability at the elevated temperature, a low setting point (especially for winter grades), godd anticorrosive properties, and others.

All grades of lubricating oils for modern machenisms and engines, especially for diesel engines and the like, contain additives which improve their performance.

Diesel and automobile oils are made by mixing purified residual and distillate oils.

Aviation Oils. These are employed for lubricating of aviation piston engines. They are prepared from goudron residue after distillation of specially selected petroleum grades by deep refining with selective solvents and sometimes by mixing with distillate oils.

Aviation engines operate under heavy loads and at high temperatures, so that the oils for them will have a high chemical stability and great lubricating power.

Industrial Oils. These are intended for lubrication of machines and mechanism of industrial equipment which operate at relatively low temperatures and of pairs of machines and engines not subjected to the effect of steam, hot air or gases. There is no strict scientific classification of industrial oils. They are commonly classified by their viscosity and by the conditions and fiels of application. Depending on viscosity, industial oils are divided into light (3.5-10 mm2/s at 500C), medium (10-58 mm2/s at 500C), and heavy (11-96 mm2/s at 500C). Depending on the conditions of application, they are classed into oils for light and moderate speeds and loads and heavy-duty oils, and by the fields of application, into oils for gear transmissions, slip guides, spindles, instrument oils, break-in oils, and special oils. These grades of oil are prepared using base oils of selective purification produced from eastern grades of petroleum.

Industrial oils should be pressure-and corrosion-resisting, retain their fluidity at the working temperatures, and be stable against foaming and oxidation.

Turbine oils. These are used for lubrication of bearings and auxiliary machanisms of turbomachines (steam and gas turbines, turbocompressors, hydraulic turbines, marine turbines, etc.); they are also used as pressure fluids. Turbine oils without additives are produced by contact acid refining, those with additives are manufactured by selective refining from low-sulphuruos and sulphuruos grades of petroleum. Additives improve their antioxidizing, deemulsifying, anticorrosive and antifoaming properties. Some grades oils contain antiwear additives. Turbine oils must have a high chemical stability and separate easily from water which enters occasionally the lubrication system.

Insulating oils. Oils are liquid dielectrics and therefore can be used for insulation of current-conducting elements of electric equipment (transformers, capacitors, cables, etc.). Insulating oils also serve for removing heat and favour quick are extinction between electric contacts. This group of oils includes transformer, capacitor and cable oils.

Transformer oils have found the widest application in the group. They are intended for long operation at 70-80 0C in the atmosphere of air and for that reason should possess a very high chemical stability and should not form low- molecular acids on oxidation. Besides, transformer oils should naturally have dielectric characteristics. Most grades of transformer oils have a viscosity of not more than 9 mm2/s at 50 0C, exceptions being grade ATM-65, arctic transformer oil, with a viscosity of not more than 3.5 mm2/s at 50 0C amd grade T-1500 (for equipment of transmission lines for 1500kV) whose viscosity is limited at 8 mm2/s at 50 0C. Transformer oils cannot be replaced by other kinds of oil.

Compressor oils. These oils serve for lubrication of cyliders, valves and piston rods of compressor operating at temperatures of 200-2500C and pressures of 20-25 Mpa. The main requirement to compressor oils is that they should have an appropriate oxidation stability. Compressor oil grade 12 M, of a kinematic viscosity of 11-14 mm2/s at 100C, is intended for single-stage horizontal and vertical compressors for a pressures 0.7-0.8 MPa and for two-stage compressor for an average pressure up to 5 Mpa. Compressor oil grade 19T, of a kinematic viscosity of 17-21 mm2/s at 1000C, is employed in multi-stage high-pressure compressors (for 20-30 MPa). The oxidation stability of these oils is ensured by deep refining.

Oils for steam engines. They are divided into two main groups: for saturated-steam and for superheated-steam machines. These oils are distinguished by a low evaporability and a high viscosity (the kinematic viscosity at 1000C is 9-13 mm2/s for cylinder oil grade 2 and 44-64 mm2/s for grade 52 Vapor oil). Cylinder oils of the first group are prepared from distillates and those of the second group, from residues by deasphalting with propane or by distillation of goudron in high vacuum.

Synthetic oils. These oils are essentially organic or elementoorganic compounds [containing silicon, iron, etc.] and are intended for heavy-duty applications.

b. Pareffine, ceresins and petroleum

Pareffines. These are soft [liquid] or solid petroleum products of crystalline structure obtained from distillates of paraffinic and high- paraffinic grades of petroleum.

Solid petroleum paraffins are crystalline products of white to bright- brown colour, depending on the amount of oil. The oil content in paraffines may vary within 0.8-0.5 per cent for high- purified grades and up to 2.2-2.3 and even 5 per cent for other grades.

Special grades of paraffine are manufactured in the USSR for application in food industry. They are obtained by deep refining of raw paraffins and employed mainly for impregnation of packing materials either contacting loose fry foodstuffs [grade P-2 with the oil content up to 0.9 per cent by mass] or non-contacting [grade P-3, oil content up to 2.3 per cent by mass]. Paraffin grade P-1 [oil cotent up to 0.5 per cent by mass] is used for the same purposes as grade P-2, and also in candy industry.

The fusion point of paraffin grades P-1, P-2, P-3 is respectively 54, 52 and 500C.

Ceresins. Ceresin is mixture of solid hydrocarbons obtained in processing and refining of ozokerite, unpurified petroleum ceresin or their mixtures. Ceresins are used for making greases, wax alloys, insulating material, etc.

The most important characteristic of cerecins is the dropping point, 0C. According to the state standard GOST 2488-73, the dropping point is the basis of grading of ceresins [the grade of ceresin manufactured in the USSR are disignated respectively 80, 75, 67 and 57]. The volume resistivity at 1000C is specified only for grade 80 ceresin: it should be not less than 1*1012 ohm cm.

Synthetic high-fusion ceresin has the highest dropping point. It is a mixture of solid hydrocarbons of the methane series, mostly of normal structure, which are obtained by synthesis of carbon monoxide and hydrogen [Fisher-tropsch process ]. According to the state GOST 7658-74, the dropping point of this of ceresin must be not less than 1000C and the volume resistivity at 1000C, not less than 1014 ohm cm.

Medical [liquefied] Petrolatum. This product is obtained by fusion of ceresin, paraffin, purified petrolatum or their mixtures with petroleum oil. Its dropping point is 37-500C.

Capacitor Petrolatum. It is employed for filling in and imprenating of capacitors. Its kinamatic viscosity at 600C must be at least 28 mm2/s. An important specified characteristic of this product is the volume resistivity, which must be at least 1*1012 ohm cm at 1000C.

Exercises

Answer the following question

1. How many type can lubricants be divided? What are they?

2. What are effects of lubricants?

3. Viscoity is the most important characteristic of all kinds of lubricant, isn't it?

4. How does the viscosity of lubricating oils must be like?

5. What are characteristics of lubricating oils?

6. What are purposes of aviation oils?

7. What are characteristics of aviation oils?

8. How can industrial oils be classified?

9. How can additive type for industrial oils be prepared?

10. What are properties of industrial oils?

11. What are turbine used for?

12. What are insulating oils include?

13. Which grade have the widest application?

14. What are applications of compressor oils?

15. How can they prepare cylinder oils for steam engines?

16. What are applications of solid petroleum paraffins?

17. What is ceresin?

18. What is ceresin used for?

19. The most important characteristic of ceresins is the dropping point, isn't it?

20. How can they prepare medical petrolatum?

21. What is capacitor petrolatum?

III. Miscellaneous Petroleum Products

Greases. Grease is a thick salvelike product consisting of oil and a thickener. Various soaps [calcium- sodium, aluminium, lithium, barium, etc.] are commonly used as thickeners. Greases thickened by hydrocarbon components [ceresin, paraffin or petrolatum] are mainly employed for protective coastings. They are physically and chemically stable, but their operating range is limited to temperatures of 50-600C. Special greases are also produced, in which various compounds are used instead of oil as a liquid base.

Greases are employed in cases where mineral oils cannot ensure proper lubrication of machines and mechanism, and also for tightening gaps. Greases are often used as slushing compounds; they protect mechanisms against corrosion during storage and then can serve as lubricants in operation.

Petroleum Bitumens. Bitumens are usually obtained by oxidation of goudrons from heavy tarry grades of petroleum, and also by mixing with asphalt, extracts of oil manufacture and asphaltile. The main characteristics of bitumens are; [needle] penetration, ductility, and softening temperature which characterizes the thermal stability of bitumen. The penetration and ductility at low temperature determine, in combination, the capacity of bitumen to retain its alasticity.

Petroleum bitumens are mainly used in road construction. Dirt and gravel roads are sometimes imprenated by liquid bitumen obtained by dilution of bitumen with a less viscous petroleum product, such as fuel oil. Some special grades of bitumen are made for application in civil engineering and for manufacture of paints and varnishes, electroinsulating materials, etc.

Petroleum acids and their sails. Petroleum acids, mainly naphtenic, are present in some grades of petroleum. They are separated during alkali refining of fuel and oil distillates as sodium salts [soaps] and employed for manufacture of naphtenate soap, acidol and acidol- naphtenate soap. Naphtenate soap [contain 43 per cent of petroleum acids] is a mixture of sodium soaps of petroleum acids, minaral oil and water. Acidol [contains 42- 50 per cent of petroleum acids] consists of petroleum acids with an admixture of minaral oil. Acidol - naphtalenate soap [67-70 per cent of petroleum acids] is a mixture os free naphtenic acids and their dodium soaps. All these products are employed as subtitutes of fats in manufacture of industrial soaps, since they possess high emulsifying and foaming properties. They are also used in the textile industry for dyeing, for wood imprenation, as drying agents in paints and for some other purposes. A general requirement to these products is that the content of mineral oil be not above a specified limit.

Solvents. The paint- and- varnish industry uses most widely gasoline [fraction 45-1700C], while spirit [fraction 165-2000C], and solvent naphta [mixture of xylenes] as solvents. In the food industry, the commonest solvents are extraction gasoline [fraction 70-950C] and petroleum ether [fractions 40-700C and 70-1000C]. In other industries, these and some other solvents [including benzene] can be employed. All solvents are specified for the content of benzene and unsaturated hydrocarbons and sulphur compounds.

Solvents are usually obtained from accompanying petroleum gases and low- sulphurous petroleum and in gas fractionation, preliminary distillation of petroleum and in catalytic reforming [from refining products]. The disired fraction in sometimes separated in deep- distillation plants. In many casess, the fractions obtained are specially puriffied [mostly to minimize the content of benzene hydrocarbons and sulhpur compounds].

Domestic [illumination] kerosene. Domestic kerosene is obtained by straigt- run distillation of petroleum. It should have a specified composition to ensure normal burning [mostly paraffine hydrocarbons], burn without forming fly ash [the height of non- smoky flame should be not less than 20 mm], and have an approriate brightness of flame.

Coke. This is a product of petroleum coking, used for making electrodes, abrasives and some other materials and as a solid fuel. Electrode coke has the highest industrial significane for electrolytic manufacture of aluminum and making of artificial graphites which are used as antifriction materials in mechanical engineering.

Commercial carbon [carbon black]. This is an amorphous substance usually in the form of a powder with black spherical particles 30-40 m in diameter. Commercial carbon may be named channel black or furnace black depending on the method of manufacture. It is employed as a filler in the rubber and paint and varnish industry and as a dyer in the manufacture of printing ink, ebonite, electrodes, etc. The principal standardized characteristics of carbon black are as follows; adsorption, dispersion, colouring power, absence of foreign inclusions, uniform distribution [in rubber mixtures], and fractional composition.

At petroleum-processing plants, carbon black can be made from gases, green oil [obtained in pyrolysis of kerosene -solar oil fractions], coke residue [from coking plants], gas oil of catalytic cracking, and extracts of oil processing [more often after thermal treatment in cracking plants], coal tar pitch, and aromatized extracts from gas oil of secondary processes.

Softners. Residual products of straight run distillation of petroleum ['softener' fuel oil], shale oil, and some products of oil processing can be used as softeners. They are employed in the rubber industry and as softeners of ruber mixtures in rubber regeneration.

Exercises

Answer the following question

1. What is the composition of grease?

2. What are characteristics of grease?

3. What are grease employed?

4. How are bitumens prepared?

5. What are the main characterictic of bituments?

6. What is the main application of petroleum bituments?

7. What are other applications of petroleum bituments?

8. What is the main application of product of petroleum acids?

9. What are other applications of product of petroleum acids?

10. Which products of petroleum are used as solvents ?

11. Which industries are solvents used in?

12. How are solvents obtained?

13. How is domestic kerosene obtained?

14. Which properties that flame of domestic kerosene should have?

15. What is coke used for?

16. How many is the dimension of particles of commercial carbon?

17. What is commercal carbon used for?

18. What are the principal standardized characteristic of commercial carbon?

19. What are softeners used for?

IV. Products of petrochemical and basic organic synthesis.

The industry of basic organic and petrochemical synthesis is a link between petroleum processing and chemical- recovery coke industries and all other branches of organic synthesis. It provides the latter with the required starting materials - organic products and, besides, supplies national economy with many valuable final products. Very many products of basic organic and petrochemical synthesis are intermediate, rather than end products. These include, in particular, many organic compounds of chlorine in which chlorine atoms can be substituted by other or groups of atoms.

Starting substanes for polymer materials. Their manufacture plays an important part in basic organic synthesis and petrochemical synthesis. It provides starting materials for the manufacture of plastics, synthetic rubber, synthetic lacquers, glues, film materials, fibres, etc. Polymer materials are now made in hundreds of kinds having various properties and diverse applications. The most important among them are polyethylene, polystyrene, polyvinyl choride, polypropylene, and synthetic rubbers. Many of them are used as starting materials for manufacture of commercial goods. For instance, various rubber articles, including tyres, are made from synthetic rubbers; many articles made from polyethylene and polypropylene can successfully replace non - ferrous metals, etc.

Plastifiers and other auxiliary subbstances for polymer materials. Along with the basic materials for manufacture of synthetic polymers, plastifiers and various auxiliary materials are also of large importance: they either facilitate the process of synthesis or impvove the properties of final products. For instance, plastifiers [softeners] are added [in an amount of up to 30-40 per cent] to certain polymers [especially to synthetic rubbers and polyvinyl chloride] to improve the plastic and elastic characteristics of these materials.

Among various types of plastifiers, one of the most important groups includes high - boiling esters [dibutyl phthalate, dioctyl phthalate, tricresyl phthalate] and some esters of higher alcohols and dicarboxylic acids and of higher carboxylic acids and diatomic alcohols. Softeners obtained at petroleum processing plants are used in the manufacture of synthetic rubbers. Other auxiliary substances used in polymer technology [and in other processes] include initiators, catalysts, inhibitors, regulators, etc.

Synthetic surfactants and detergents. Surfactants and detergents are used very widely in domestic life as powders and liquids for washing and cleaning. These substances are distinguished by a combination of hydrophobic and hydrophilic groups in the molecule. During washing, this facilitates wetting of the fabric and passage of dirt to the washing water. All surfactants and detergents are divided into ionogetic and non- ionogenic, depending on the presence or absence of groups capable of dissociating in aqueous solutions. Ionogenic substances, in turn, may be either anion- or cation- active, with their surface- active properties being determined respectively by anions or cations.

Most of anion- active surfactants are sodium salts of sulfonic acids and acid esters of sulphuric acid, in particular, [1] alkylarylsulfonates with a C10- C15 alkyl group; [2] alkylsulfonates with 12-18 carbon atoms; and alkylsulphates with an alkyl group roughly of the same length:

p-RC6H4SO2ONa R SO2ONa ROSO2ONa

In recent time, non- ionogenic substances have fuond a wide use. They are synthesized from ethylene oxide and various organic compounds- carboxylic acids, alcolhols, amines, etc. having active hydrogen atoms. Their hydrophilic propeties are due to a [CH2CH 2O]n chain obtained by successive attachment of molecules of ethylene oxide: RO-[CH2CH 2O]n-H. In order to improve their washing ability and lower the consumption, detergent substances might be mixed with various additives and these compositions are called washing means [in contrast to washing substances, or detergents, proper]. Such compositions [for instance, washing powders] contain sodium phosphate, pyrophosphate and hexamethaphosphate, sodium silicate, sulphate, carbonate, etc.

Sythetic fuels, lubricants and additives. This group includes synthetic motor and let fuels, lubricating oils, addditives, antifreezing agents, braking and pressure fluids.

Solvent and extractive agents. Synthetic solvents and extractive agents may belong to various groups of organic compound: chlorine derivatives, ancohols, cellosolves, ethers, ketones, esters, etc.

Miscellaneous products. These include insecticides, medicaments and explosives.

Exercises

Answer the following question

1. What are products of petrochemical and basic organic synthesis?

2. Most of products of petrochemical and basic organic synthesis are intermediate, aren't they?

3. Why does manufacture of polymer materials play an important part in basic organic and petroleum synthesis?

4. What are the most important polymer materials?

5. What are polymer materials used for?

6. What are effects of plasitifiers and other auxiliary substances?

7. What are auxiliary substances used for?

8. What is structure of molecules of surfactants and detergents?

9. What is difference between ionogenic and non- ionogenic substances?

10. How are hydrophilic properties of non- ionogenisc substances obtained?

11. What should we do to improve washing ability and to low the consumption?

Composition of petroleum

Petroleum is a natural mixture of various hydrocarbons and their derivatives containing sulphur, nitrogen, oxygen, metals, etc.

The main constituents of petroleum - hydrocarbons - may differ in the number of carbon and hydrogen atoms in the molecule and in the molecular structure. Petroleum hydrocarbons may relate to the following groups or series: paraffins [saturated, or stable hydrocarbons, alkanes], naphthenes [cycloankanes], and benzene hydrocarbons [arenes]. In most grades of petroleum, paraffins and naphthenes prevail. During processing of petroleum, unsaturated hydrocarbons [olefins and diolefins] may also form. The specific properties of petroleum products are dicided by the predominace of some or other group of hydrocarbons in crude petroleum and by the presence of compounds containing sulphur, nitrogen or oxygen.

Paraffin hydrocarbons [alkanes]. Their general formula is CnH2n+2, where n is the number of carbon atoms. Each next hydrocarbon can be obtained from the previous one by substituting a methyl group CH3 for the extreme hydrogen atom in the chain:

CH4 C2H6 C3H8 C4H10

methane ethane propane butane

The paraffin hydrocarbons are the most stable of the lot because all valence bonds are fully satisfied as indicated by the single linkage. Most reactions involve the replacement of by hydrogen atoms with other atoms, the carbon linkage remains stable.

Under common conditions, the hydrocarbons from CH4 to C4H10 are gaseuos, those from C5H12 to C15H32 are liquids [they enter the composition of gasoline, kerosene and diesei- fuel fractions], and those from C16H34 are solid [paraffins].

Beginning from the fourth term in the series [butane C4H10], hydrocarbons may exist in two or more forms differing in the structure. For instance, butane may exist in two forms: n- butane and isobutane. Compounds which have the same chemical formula but a different atomic structure are called isomers.

The number of isomers increases for each next hydrocarbon in the series. Hydrocarbons of the formula C13H28 may have 802 isomers, those of the formula C14H30, 1858, and so on. Thus, the composition of petroleum is quite complicated. Isomers possess different physical and chemical properties. For instance, heptane of normal structure [n- C7H16] has an octane number of zero, whereas isooctane [iso- C7H16] has an octane number of 100.

Naphthenic Hydrocarbons [Cycloalkanes]. Their general formula is CnH2n. They were discovered by V.V. Markovnikov, a prominent Russian chemist, when studying petroleums of Caucasian deposits.

In their chemical properties; naphthenic hydrocarbons are similar to paraffines, but differ from the latter in having a cyclic structure.

Cyclopentane and cyclohexane derivatives are especially important for the quality of petroleum and petroleum products.

Benzene Hydrocarbons [Arenes]. Arenes of the benzene series have the general formula is CnH2n-6. The cyclic structure of arenes differs from that of naphtenes by the presence of double bonds on the aromatic ring. If one or more atoms of hydrogen in the ring are replaced by a methyl [-CH3] or an ethyl [-C2H5] group, other arenes [toluene, xylenes and ethylbezene] are formed. Arenes are a valuable raw material for chemical technology and the manufacture of antinock gasoline.

Unsaturated Hydrocarbons [Olefines]. Hydrocarbons of the ethylene series have general formula is CnH2n-2, are characterized by a double bond in the molecule [ethylene C2H4, propylene C3H6, butylenes C.4H8, amylenes C5H10, etc.] and may be of either normal or isomeric structure.

They are not present in crude petroleum, but constitute an appreciable part of the products obtained in the thermal and some catalytic processes of petroleum processing. These hydrocarbons have a high reactivity and are used for the manufacture of some important products, such as polyethylene, polypropylene, ethylene and propylene oxides and their derivatives.

Along with olefines, some less saturated hydrocarbons, with two double bonds in the structure, such as diolefines, can form in petroleum processing. These are extremely unstable and for that reason should not be present in final petroleum products. Some of them [ butadien C4H6 and isoprene C.5H8] are obtained intentionally from petroleum and used for the manufacture of synthetic rubber and like products.

Oxygen- containing compounds. These include naphthenic acids, phenols and tar- asphaltene compounds.

Naphthenic acids are compounds containing a carboxyl group-COOH. Their density is from 0.96 to 1.05 g/cm3 and general formula, CnH2n-2O2. Naphthenic acids are strongly smelling oily liquids. They may be present in kerosene, diezel- fuel and light oil distillates of petroleum and are corrosion- aggressive; they are removed from petroleum fractions by leaching. Naphthenic and their salts are widely used in industry as components of greases, for imprenation of fabrics and footwear, etc.

Phenols are contained only in some grades of petroleum and are liberated together with naphthenic acids during leaching of distillates.

Tar- asphaltene compounds may be present in petroleum in considerable quantities [from traces to 25% and even more]. They are complex high- molecular compounds containing carbon[82-87.4%], hydrogen [10.3-12.5%], oxygen [up yo 2.5%], sulphur [0.8-7%], and nitrogen [up to 1%]. Low molecular tar compounds can partially be distilled of together with petroleum distillates, while high molecular ones remain in fuel-oil fractions and especially in oil residue [goudron]. The presence of tar in these products makes them dark and promotes carbonization in cylinders of internal combustion engines. Tar- asphaltene products are harmful is white petroleum products and oils, but are desirable constituents in such produsts as bitumen, coke, isulating and imprenating materials.

All tar-asphaltene products are usually classed into neutral resins soluble in light gasoline; asphaltenes [ the products of polymerization of neutral resins and oxyacits] which are insoluble in light gasoline, but soluble in benzene, chloroform and carbon bisulphide; asphaltogenous acids and their anhydrides of acid nature. which are insoluble in light gasoline, but soluble in alcohol. As has been shown experimentally by N. I. Chernozhukov and S. E. Krein, petroleum hydrocarbons are oxidized simultaneously in two directions:

All the three types of tar-asphaltene compounds are high-molecular compounds of unsaturated nature containing oxygen and sulphur. At normal temperature they are very thick and viscous liquids or are solid and have a density above 1.0 g/cm3. The content of tar-asphaltene compounds is greater in petroleum grades of higher density and in those high in sulphur.

Sulphur compounds. In Vietnam grades of petroleum, the content of sulphur is small. Sulphur is present in petroleum and petroleum products mostly in combined state, i.e. in the form of organic sulphur compounds. Sulphur compounds of the following types may be found in petroleum products: mercaptans RSH [where R is a hydrocarbon radical]; sulphides RS, disulphides RS-SR, thiophene C4H4S and its derivatives, and sometimes hydrogen sulphide and elemental sulphur. Hydrogen sulphide and mercaptans which have acid properties, and elemental sulphur form a group of active sulphur compounds which can cause strong corrossion of equipment and pipelines.

Another group includes sulphides and disulphides which are neutral at low temperatures, but are thermally unstable; at 130-1600C they decompose [with breaking of C-S bonds] and form hydrocarbons, mercaptans and hydrogen sulphide. A third group includes thiophane and thiophene and their derivatives, such as benzothiophene.

Like benzene hydrocarbons they have a low reactivity and are relatively stable at elevated temteratures.

High- molecular sulphur compounds are unstable and can be oxidized under relatively soft conditions; the products of oxidation increase the content of tar in petroleum products. In the atmosphere of hydrogen, they are reduced to corresponding hydrocarbons and hydrogen sulphide; this is the basis of the processes of hidrogen refining [hydrofining] of petroleum and petroleum products.

In straight distillation of petroleum [without destruction] the content of sulphur increases from lighter fraction to heavier ones, with the residue having the highest concentration of sulphur. When higher temperature and pressure are applied, however, organic sulphur compounds are destroyed together with high - molecular hydrocarbon to form hydrogen sulphide and mercaptans which are corrosive and toxic. Corrosion is enhanced in the presence of water vapours and hydrochloric acid which forms by decomposition of calcium and magnesium chlorides contained in undesalted petroleum.

In order to diminish corrosion and improve labour conditions, petroleum before dictillation might be desalted and dehydrated. The content of sulphurous compounds in petroleum products can be lowerd by various methods of refining, mainly by hydrogen refining.

Nitrogen Compounds. The content of nitrogen is usually greater in heavier grades of petroleum. Nitrogen compounds are divided into basic, which contain nuclei of pyridine and quinoline, and neutral, which contain pyrrol and indol homologues.

In petroleum processing, nitrogen compounds are distributed between fractions much like sulphur compounds, i. e. their concentration increases from lighter fractions to heavier ones, and the largest amount [65-75%] is concentrated in the residue.

Among nitrogen compounds, porphyrins occupy a special place. They may be present in petroleum either in free state [four pyrrol rings] or as complexes containing organic nitrogen compounds and organic derivatives of vanadium and nickel. Notwithstanding the high thermal stability of nitrogen compounds in the technological processes, they decompose partially, which is detected by the formation of ammonia. Certain refining processes [for instance, hydrogen refining] can remove an appreciable portion of sulphurous compounds [as hydrogen sulphide] and a part of nitrogen compounds [as ammonia] and oxygen compounds [as water vapour].

Mineral Substances. Mineral substances are found in petroleum only in very small concentrations [provided that crude petroleum has been refined properly from mechanical impurities at the oil well]. As has been established by combustion of many samples of petroleum, the elements found in the ash form [in the decreasing order] the following row: S-O-N-V-P-K-Ni-I-Si-Ca-Fe-Mg-Na-Al-Mn-Pb-As-Cu-Ti-V-Sn. The total amount of ash in various grades of petroleum may vary from a few thousandths of a per cent to 0.8 per cent.

Exercises

Answer the following question

What is the elemental composision of petroleum?

What is the main constituents of petroleum ?

Which series of hydrocarbon are present in petroleum ?

Which series of hydrocarbon are formed during processing of petroleum ?

What are the chemical properties of paraffin hydrocarbons ?

What are the physical properties of paraffin hydrocarbons ?

Which compounds are called isomers?

What can you say about the chemical and physical properties of isomers?

What is the difference and similarity in structure and properties between paraffinic and naphthenic hydrocarbons ?

What is the difference and similarity in structure and properties between naphthenic and benzene hydrocarbons ?

What are applications of benzene hydrocarbons ?

What can you say about properties of olefins and diolefins?

What are applications of olefins and diolefins?

What are applications of naphthenic acid?

What is the elemental composision oftat-asphaltene compounds?

How can you class tar-asphaltene products?

Which types of sulphurous compounds are present in petroleum products?

How can the content of of sulphurous compounds in petroleum products be lowered?

How are nitrogen compounds distributed?

Basic Physico-chemical properties

of petroleum and petroleum products

Density. The density of petroleum and petroleum products can be expressed in either absolute or relative values. The relative density is the ratio of the density of a petroleum product at temperature t2 to the density of distilled water at temperature t1. The density of petroleum products is normally measured at 200 C and that of water, at 40 C. Since the latter is taken as unity, the numerical values of the relative and absolute density coincide.

To find the absolute density  [kg/m3 or g/cm3] the mass of a product is divided by its volume, i. e. =m/V.

The density of petroleum and petroleum products depends on the content and the composition of light low-boiling [which have a low density] and heavy high-boiling constituents [fractions]. Indeed, among the components having roughly the same boiling point, paraffin hydrocarbons have the lowest density and benzene hydrocarbons have the highest value, with that of naphthalenes being in the middle. This is why density is one of the principal characteristics of petroleum and petroleum products.

The density of petroleum and petroleum products decreases with the increasing temperature and their volume recpectively increases. The temperature relationship for density can be expressed by Mendeleev's formula:

dt4 = d204 - a[t-20]

where dt4 is the relative density of a product at temperature t; d204 is the relative density of a product at 200C

Molecular Mass. This is one of the basic physico-chemical characteristics of petroleum and petroleum products. The molecular mass of paraffin hydrocarbons can be found approximately by using the formula:

M = 60+0.3t+0.001t2

where t is the average temperature of boiling of a petroleum fraction, 0C; it is calculated as the arithmetic mean of the temperatures at which equal volumes of the liquid, say, 10% fraction, are distilled off.

The relationship between the molecular mass and relative dendity of petroleum fractions is determinded by the following empirical formula:

M = 44.29d1515/1.03- d1515.

Using this formula, it is also possible to fine [with a certain approximation] the molecular mass of all classes of hyfrocarbons.

Boiling Point. Fractional Composition. The boiling point of a liquid os the temperature at which the pressure of vapours is equal to the external pressure; on reaching this point, vaporization, which up to that moment occured from the surface only, begins in bulk of the liquid [at the bottom and walls of the vessel being heated], where vapour bubbles are formed; this is what is called the boiling proper. If vapours are not removed off the liquid surface during heating, an equilibrium is established between the liquid and vapour phase. Vapours in equilibrium with the liquid are called saturated. At a higher temperature of heating of a liquid, vaporization occurs more intensively, more vapours are formed above the liquid, and the pressure of saturated vapours is higher.

The boiling point of a liquid depends on the external pressure. For instance, water at a pressure of 0.1 MPa boils at 1000C. At a higher pressure, say 0.4 MPa, boiling begins only at 1440C. Thus , the boiling point is higher at a higher external pressure and at a lower external pressure os in vacuum, water boils at a lower temperature. The same effect of pressure is found in other liquids. This phenomenon is utilized in vacuum distillation of fuel oil.

Petroleum and petroleum products can be separated into individual hydrocarbons only with certain difficulties. Usually separation is carried out by distillation which gives simpler mixtures are called fractions. They boil not at a constant temperature, but in a temperature range betwwen the point of the begining of boiling and that of its end. Depending on the boiling points and contents of various hydrocarbons, a product may have different boiling ranges, i. e. may have a different fractional composition.

All petroleum products obtained from crude petroleum by distillation are essentially fractions that can boil off within particular temperature ranges. For instance, gasiline fractions boil off within 35-2050C, kerosene fraction within 150-315 0C, diezel-fuel fractions within 350-4200C, light oil distillates within 350-420 0C, heavy oil distillates within 420-490 0C, and oil residues at temperatures above 490 0C.

Thermal Properties of Petroleum and Petroleum products. These properties are of high pratical importance for calculating the heat balance of all processes associated with heating or cooling.

Specific heat is the quality of heat needed to heat up 1 kg of substance by 10C. The approximate value of specific heat, kJ/kg.K, are as follows: petroleum 2.1, petroleum vapours 2.1, water 4.19.

With the specific heat of a petroleum product being known , it is possible to calculate the quantity of heat for heating. For this, the specific heat is multiplied by the mass of the product [kg] and by the difference between the final and initial temperature [0C]. The specific heat of petroleum products increases with increasing temperature and is higher for products of lower density.

Specific latent heat of evaporation is the quantity of heat spent to vaporize 1 kg of a liquid at its boiling point [this characteristic is called latent, since the heat is spent for evaporation and the temperature of the product remains constant during heating]. The average values of the latent heat of evaporation at the atmospheric pressure, kJ/kg, are as follows: water 2257, gasoline 293.3-314.3, kerosene 230-251, diesel fuels 209-213, oils 167-209. Thus the latent heat of evaporation decreases with increasing density and malecular mass of petroleum products, and also with increasing temperature and pressure.

The heat of condensation is the quantity of heat liberated by vapours during their condensation and is numerically equal to the latent heat of evaporation.

The latent heat of fusion is the quantity of heat absorbed during fusion of 1 kg of a solid at the melting point.

The heat of combustion [calorific value] of fuel is the quantity of heat liberated by the fuel on full combustion. A distinction is made between the high and low heat of combustion: the former [Qh} takes into account the heat of condensation of the water present in the fuel and formed during combustion [it is taken conditionally that the combustion prodducts contain liquid water rather than water vapours]. The low head of combustion, Ql, implies that the water of the fuel and the water formed by combustion gases [i.e. it is lower than the high heat of combustion by the quantity of heat spent for evaporation of moisture of the fuel and of the water formed through combustion of hydrogen in the fuel].

Viscosity [internal friction]. Viscosity is the ability of a liquid [or gas] to resist the motion of a layer relative to other layers. As regards petroleum products, a distinction is made between dynamic, kinematic and relative viscosity.

Dynamic viscosity  is measured in pascal-second [Pa s]. The dynamic viscosity of selected liquids is as follows:

Pa s

Ether[at 180C]

Gasoline[at 200C]

Kerosene[at 200C]

Alcohol[at 180C]

Water[at 200C]

Glycerine[at 180C]

Spindle oil[at 200C]

Cylinder oil[at 200C]

Caster oil[at 180C]

0.000026

0.0045

0.0017

0.00166

0.001006

1.10

0.042

0.35

1.20

An inverse value of dynamic viscosity is called fluidity.

In process calculations and for testing the quality of many petroleum products, use is made of kinematic viscossity , which is the ratio of the dynamic viscosity  to the relative density of a liquid, d, at the same temperature, i.e.

 = /d

Kinematic viscosity  is measured in square metre [square millimetre] per second[m2/s, mm2/s].

In practical calculations, especially for quality control of petroleum products, use is often made of relative viscosity which is the time of efflux of 200 ml of a petroleum product at the testing temperature related to the time of refflux of the same volume of distilled water at 200C [the time of refflux of 200 ml of water at 200C is what is called the water number of a viscosimeter].

Viscosity- temperature relationships. Viscosity becomes lower with encreasing temperature and vice versa. The patern of variation of viscosity with temperature is an important characteristic of petroleum products, especially of lubricating oils. These variations can be determined by various methodes, for instance, by the ratio of the viscosity at 50 0C to that at 1000C, which is now specified for many lubricating oils, or by the viscosity index; the latter is found from monograms for the known values of viscosity at 50 0C and 1000C. With a higher ratio of viscosities, the temperature curve of viscosity is steeper and on the contrary with a lower ratio, the curve is less steep and the quality of the oil is better.

The Setting and Fusion points. When being cooled, petroleum and petroleum products gradually loss mobility and can set [solidify] notwithstanding the fact that they contain some substances that might be liquid at the temperature considered.

The setting [sodification] point of a petroleum product is the temperature at which the product loses mobility under strictly specified testing conditions. The loss of mobility and freezing of petroleum and petroleum products depend mainly on the content of hydrocarbons which are solid [at the nomal temperature]. The higher the content of such hydrocarbons [in dissolved or crystalline state], the more quikly the product loses its mobility during cooling, i.e. the products has a relatively high setting point. Tarry products and asphaltenes can retard somewhat the crystallization of solid hydrocarbons, that is why the setting point of detarred products is always higher than that of the distillates from which they have been obtained.

During cooling to their setting point. white petroleum products pass through a number of intermediate stages- the stage of turbidity [blushing] and that of the beginning of crystallization. The highest temperature at which crystals [say, of benzene, etc.] can be detected in the cooled fuel by nakes eye is called the temperature of the beginning of crystallization, or the chilling temperature [point]. The temperature at which crystals of hydrocarbons [mainly of paraffins] start to precipitate and make the product turbid is called the blushing temperature [point]. Along with the temperature of chilling of liquid petroleum products, the temperature of fusion of some products which are solid at nomal temperature [paraffin and ceresin] is also practical importance.

The fusion point is the temperature at which a solid produst becomes liquid under strictly specified testing conditions.

With these constants being known it is possible to select properly the method of petroleum processing and take the required measures to ensure pipeline transportation, especially in winter time, and also to choose the methods of storage and transportation of solid products having a high chilling point.

Flash and Ignition Points. Self- ignition temperature.Explosibility. The fire hazard of petroleum products is judged upon by their flash, ignition and self- ignition temperatures [point]. At lower values of these characteristics, a product is more fire- hazardous.

The flash point is the temperature at which a mixture of air and vapours of a product being heated under standard conditions ignites on contact with an ignition source, but the product proper is not ignited and the flame is damped. For light petroleum products [with the flash point not above 500C] the flash point is measured in a closed apparatus and that of heavier products [with the flash point above 70 0C] can be determined in an open vessel. The product to be tested is poured into the apparatus and a thermometer is put inside. With light products, the apparatus is covered by a lid with a window which can closed by a gate. During the test, the window is opened periodically and a burner is brought close to it. In an open appararus, the burner is moved close to the liquid surface. Tests in an open apparatus give a higher value of a flash point, since the vapour formed are partially dissipated to the surroundings.

In further heating, a petroleum product can ignite at a certain temperature. This temperature is called the ignition point.

There is a certain relationship between the fracrtional coposition of a product and its flash and ignition points: lighter hydrocarbons in its composition lower these points. For instance, gasoline has the flash point below - 500C, whereas the flash point of fuel oil is above 1100C.

According to international recommendations, easily igniting liquids include those flash point is below 610C [in a closed vessel] or 660C [in an open vessel]. These liquids, which can be ignited by a short action or even a small ignition source [say, a spark] and without preliminary heating.

The temperature of self- ignition of a petroleum product is lower at a higher content of heavy hydrocarbons. This is the temperature at which a product ignites spontaneuosly on contact with the air, i.e. in the absence of flame or spark. Some products, such as fuel oils, goudron, soot and coke, self- ignite quite easily at temperature slightly above 300 0C. Self- ignition usually occurs in untight pipelines and apparatus in which petroleum products are at temperature above their ignition point. It is therefore essential to check the equipment for tightness to prevent self- ignition and fires.

Explosibility. In petroleum processing plants, mixtures of vapours of some products with air may be explosive. Such mixtures may form in open air, in closed premises and inside processing equipment. A mixture of vapours of a product with air become explosive when the concentration of the vapours in mixture exceeds a definite limit. At lower concentrations, the mixture is not explosion hazardous, since the greatest portion of the heat evolved in the ignition zone is spent to heat up the air. A mixture can not explode, too, if it contains little air and therefore there is not enough oxygen to sustain combustion.

The lowest concentration of vapours of a petroleum product [or other substance] in the air at which explosion is probable is called the lower explosive limit and the highest concentration of vapours at which explosion is still possible is respectively the upper explosive limit. The concentration range between the two limit in which explosion can take place on contact with open fire [or spark] is called the explosibility range.

The upper and lower explosive limits and the explosibility are different for various vapours and gases. The explosibility ranges for some vapours and gases obtained at petroleum processing plants [percent] are as follows: gasoline 0.8 to 5.1; kerosene 1.4 to 7.4; propane 2.1 to 9.5; methane 5 to 15; ammonia 15 to 28; ethylene 3 to 32; hydrogen sulphide 4.3 to 46; carbon monoxide 12.5 to 74; hydrogen 4 to 74; and acetylene 2.3 to 81.

The highest permissible concentration of vapours of a product in working premises depends on the composition of that product. The selected products it is as follows [mg/m3]: 100 for gasoline fuels, 300 for gasoline solvents, 5 for benzene ans methanol, 50 for toluene and xylene, 10 for pure hydrogene sulphide, 3 for mixture of hydrogen sulphide with C1- C5 hydrocarbons, and 5 for phenol.

Exercises

Answer the following question

1. How can they express the density of petroleum ans petroleum products?

2. what is the relative density of petroleum product ?

3. How can you calculate the absolute density?

4. Is there the relationship between the density of petroleum products and the poiling point of them? What is it?

5. Is there the relationship between the density of petroleum products and their temperature and their volume?

6. What is the boiling point of a liquid?

7. What is the relationship between the bpoiling point and the external pressure ?

8. What is the fractions?

9. What is the specific heat?

10. How can you calculate the quantity of heat for heating?

11. What is the specific latnet heat of e Is there the relationship between the density of petroleum products and the evaporation?

12. How can you understand the term"latent"?

13. How can you distinguish the high and low heat of combustion?

14. What is the viscosity of liquid?

15. How many type of viscosity of liquid do you know?

16. What is the relationship between the viscosity index and the quality of the oil?

17. What is the setting point?

18. Why is the setting point of detared products higher than that of the distillates from which they have been obtained?

19. What is the fusion point?

20. What is the flash point?

21. How can you measure the flash point?

22. What is the ignition point?

23. What is the temperature of self-ignition?

24. When does the explosipility happen?

Distillation

The property that differentiates most petroleum products from each other is "volatility", or tendency to vaporize. More volatile products are called "lighter", less volatile products, "heavier". The volatility of a product is determined, of course, by the boiling points of its components. Inusmuch as distillation separates liquid by boiling points, distillation is the principal separation process.

Theory of Distillation

The basic principle of distillation is simple. When a solution is boiled, the lighter components vaporize preferentially and the solution is separated into a lighter overhead product and a heavier residue. For most petroleum applications, this simple operation does not suffice, and multistage units must be emloyed. Such units consist of dylindrical columns, or "towers", through whish vapor and liquid streams pass countercurrently. Depending upon circumstances, feed may be charged at any point in the column. Products are withdrawn from the top and bottom and sometimes from intermediate points as well. Liquid withdrawn from the bottom is usually reboiled to supply vapors to the column; vapors from the top are condensed and a portion is returned as '"reflux". It seem paradoxical to build complex and expensive equipment to separate out an overhaed product and then to return part of it to the separarion zone. Indeed, many of technologists of the time considered refluxing foolish when it was first introduced. We may conclude from this that the function of reflux is somswhat obscure. Why it is used in multistage unit can best be illustrated by analogy with singlestage operations.

Staging. Consider a singlestage distillation system in which a solution is heated until half of it vaporizes, the vapor being separated from the liquid and condensed. Suppose that a two-component solution is processed in this system to concentrate the lighter component in the overhead fraction. Suppose further that the desired concentration is not attained. A more concentrated product could be obtained dy charging the overhead to a second unit, and this procedure could be repeated until the desired concentration was obtained. Similarly, the heavier component could be concentrated in the bottoms cut by reprocessing succesive residues. In either case, the yield of the desired product would be low. and large amounts of intermediate materials would be made. Yields could be improved by returning each intermediate material with the next charge to the preceding stage. By this means, all the original charge wouldd be recovered ultimately in one or the other of the desired products. Such an operation is diagramed in Fig.1a; each stage in thid diagram includes equipment to vaporize a portion of the charge and to condense the vapors. Although the indicated operation is possible, equipment would be complex and expensive, and labor and energy requirements would be high. The equipment could be simplified somewhat by converting each batch stage to continuous operation as shown in Fig.1b, but the equipment would still be complex and the operation expensive. The next step is to eliminate vaporization and condensation equipment from the intermediate steps by permitting the vapor from each stage to pass directly into the stage above, where it mixes with the liquid from the next higher stage; the contained heat in the vapor substitutes for indirect heating of the liquid. Now all that remains is to house all the intermediate steps in a single column, and we have the modern distillation unit shown in Fig. 1c.

Column Sections. The part of the column above the feed inlet is called the "rectifying section", and the part below it is called the "stripping section". The two sections have different purposes. One serves to increase the purity of a product; the other increases its recovery. In Fig. 1a, for example, stages 2, 4, and 6, which correspond to the rectifying section, increase the purity of the light product taken overhead. The liquid leaving stage 1 contains a considerable amount of the light component, and steps 3, 5, and 7, which correspond to the stripping section, strip the light component out and thereby improve its recovery in the overhead. For the heavy product, the functions of the two sections are reversed; the rectifying section improves recovery, the stripping section, purity. In some applications only one or the other of these two sections is required, depending upon the particular purity and recovery requirements of the operation.

Extractive ans Azeotropic Distillation

Because distillation separates by virtue of differences in volatility, disstillation can not normally be used to separate close- boiling materials. However, when the materials to be separated are chemically dissimilar, modified distillation procedures can be used. Examples are the separation of butenes from butanes and of toluene from isooctanes. In such cases; an extraneous liquid can be added which has an affinity for one of the components in the charge; as a result the relative volatilities of the original components change, and separation becomes possible. If the added material is less volatile than the original components, it is added at the top of the column and withdrawn from the bottom, and the operation is called extractive distillation. If the added material is more volatile than the original components, it is added at the top of the column or with the feed and is withdrawn in the overhead product; the operation is then called azeotropic distillation.

Solvents and Entrainers. In extractive distillation, the extraneous liquid is called a solvent; in azeotropic distillation, it is called entrainer. In either case, its effectiveness is determined by its concentration in the liquid phase. Consequently, the boiling point of an entrainer is limited; it must be about as volatile as the lighter feed components so that it will pass overhead, but it must not be so volatile that it will disappear from the downflowing liquid stream much above the bottom of the tower. An entrainer must be separable, of course, from the overhead product- by distillation or by some other technique. Similarly, a solvent in extractive distillation must be separable from the bottoms product. How the entrainer or the solvent is separated from the overhead or bottoms product is an important consideration, because large volumes must be used. To be effective in changing the relative volatilities of the original components, an entrainer or a solvent must constitute at least 40% of the liquid phase [60], and its concentration is usually much higher.

Effects of Reflux. In extractive distillation, reflux has two opposing effects. By increasing the counterflow of liquid and vapor, increasing the reflux promotes the separation. However, increasing the reflux lowers the concentration of the solvent in the liquid streams; this lessens its effect in spreading the volatilities of the original feed components and thus retards their separation. Because of these conflicting effects, there is apt to be sharp optimum in the reflux rate for an extractive distillation operation.

Feed Preparation. Only narrow-boiling materials are charged to extractive or azeotropic distillation. The reason may be seen most readily from an example. Consider extractive distillationfor the separation of toluene from a mixture with isooctane, which nomally boil very closely to toluene. Lower- boiling materials [like hexane and benzene] and higher - boiling materials [like isononanes] are first separaated by ordinary distillation.The sharpness of removing the light ends affects only the amount of material charged to extractive distillation. On the other hand, the purity of the toluene product will depend upon the sharpness of prefactionating the heavy ends out of the feed.

How poor removal of heavy ends affects product purity may be seen by considering the nomal volatilities of the feed components and how they are affected by the presence of a solvent. Toluene and isooctane boil together, and isononanes are about half as volatile. In the concentration isually employed, a solvent approximately doubles the volatilities of the paraffins relative to toluene. In the presence of the solvent, then, the isononanes have about the same volatility as toluene, and their separation is very difficult, and sometimes impossible.

Even when heavy materials can be taken overhead in extractive distillation, they may be very undesirable in the feed. When phenol is used as the solvent, for example, volatility relationships are such that heavy paraffins in the overhead tend to carry some phenol with them. Phenol is expensive, and only small losses can be tolerated.

Exercises

Answer the following question

1. What is the principle of distillation?

2. What are the distilling towers?

3. Where can they withdraw product of distillation?

4. What is the reflux?

5. What do they do to increase the pure of products?

6. What are the rectifying section and stripping section?

7. What are the purposes of them?

8. When must they add the extraneous liquid in distillation?

9. What is the extractive distillation?

10. What is the azeotropic distillation?

11. What are a solvent andanentrainer?

12. What is the boiling point of entrainer like?

13. What is the important characteristic of solvent and entrainer?

14. Which materials are used in extractive or azeotropic distillation?

Catalytic Reforming

Because higher- octane gasoline permit the building of engines that extract more power from gasoline, there has been a constant push toward higher octanes since differences is octane quality were first recognized. A major factor in this development has been the large - scale use of catalytic reforming to raise octane ratings of gasoline stocks. The first commercial unit, Hydroformer, went on stream just before World War II, and the process proved to be a major source of aromatics and aviation gasoline for minitary uses. However, catalytic reforming did not "catch on" until about 1950, when Haensel and others at the Universal Oil Products Co. demonstrated that platinum catalysts could be used commercially despite their high cost. By 1955, catalytic reforming processes had almost completely supplanted thermal reforming. Catalytic processes not only give higher quality products, they give a higher yields as well.

Reactions

In catalytic reforming, the principal object is to convert other hydrocarbon to aromatics. The reason may be seen by comparing the octane numbers of some corresponding hydrocarbons [Table 1]. Thus, high conversions to aromatics result in high octane products. There is a loss in volume, because aromatics are denser than other hydrocarbons; however, the loss is small in comparison with the loss [to gas and tar] suffered in thermal reforming. Other reactions of some importance in catalytic reforming are cracking and isomerization.

Table 1

Research rating Motor rating

n-Heptane

2- Methylhexane

Heptene-2

Methylcyclohexane

2,3-Dimethylpentane

2,2,3-Trimethylbutane[triptane]

Toluen 0

42

73

75

91

113

120 0

45

57

71

89

101

104

Production of Aromatics. Because aromatics contain less hydrogen than do other hydrocarbons, dehydrogenation is the primary reaction. Of the nonaromatics, cyclohexane derivatives are dehydrogenated most readily:

C6H11CH3  C6H5CH3 + 3H2

Methylcyclohexan Toluene

Cyclopentane derivatives react similarly, but they require a preliminary isomerization to cyclohexan derivatives:

C5H9CH2CH3  C6H11CH3  C6H5CH3 + 3H2

Ethylcyclopentane Methylcyclohexane Toluene

Coversion of paraffins to aromatics involves a cyclization step. For normal heptane, the reaction may be written:

n-C7H16  C6H11CH3 + H2

C6H11CH3  C6H5CH3 + 3H2

To undergo these reactions, a paraffin must have at least six carbon atoms in a chain or be isomerizable to such a compound.

Aliphatic olefins can also be converted to aromatics directly, but this fact is of little practical significance because such olefins are readily hydrogenated to paraffins under the conditions used in catalytic reforming. Thus , aliphatic olefins behave as paraffins, with the exception that they deactivate the catalyst more rapidly. Similarly, cyclic olefins behave as naphtenes.

Hydrocracking. Under the conditions employed in catalytic reforming, cracking completes with dehydrogenation reactions. Because high hydrogen pressure are used, any olefins that form are saturated immediately, and the reaction usually called "hydrocracking". Whether hydrocracking occurs in one step or in two is of little consequence. In either case, a typical over-all reaction is:

n-C8H18 + H2  C3H8 + n-C5H12

Octane Propane Pentane

Because lower-boiling paraffins have higher octane numbers, hydrocracking improves ontane ratings; however, the improvement is less than if the paraffins were converted to aromatics. Also, there is considerable loss of gasoline to butanes and lighter materials, and the vapor pressure of the debutanized product is raised. Increasing the vapor pressure reduces the amount of butane that can be blended into the product to make a finished gasoline; thus, the effective yield of gasoline is reduced still further.

Hydrocracking of naphthenes also occurs to some extent. Cyclopentane derivatives are more susceptible than cyclohexane derivatives, especially over catalysts with little isomerization activity. The first step in the hydrocracking of naphthenes is probably scission of the ring:

C5H9CH2CH3 + H2  n-C7H16

The paraffins are formed may react further to produce aromatics, or it may be hydrocracking.

Isomerization. With some catalysts, paraffins are isomerized under refarming conditions. Usually, isomerization of paraffins does not have a large effect on octane quality because the production of highly branched paraffins are small. If the paraffins in a given charge were chiefly normal, their isomerization would have a large effect on octane. In most instances, however, paraffins in the charge are mixtures of isomers; therefore the isomerizing activity of a catalyst is important chiefly for the isomerization of cyclopentane derivatives.

Catalysts

Although aromatics can be produced from either hydrocarbons without catalysts, severe condotions are required, and yields are low. To obtain acceptable yields, dehydrogenation catalysts must be employed. Those of commercial interest include platinum on alumina, platinum on sillica-alumina, chromia on alumina, molybdena on alumina, and cobalt molybdate on alumina.The ideal catalyst would convert all other hydrocarbons selectiely to aromatics rapidly, with only a small catalyst inventory. Such a catalyst would not promote hydrocracking, and it would have to operate under conditions thermodynamically favorable to production of aromatics. To the extent that a catalyst deviates from this conditions it is a poorer catalyst. Derivations may be either in the selectivity of the catalyst toward the production of aromatics or in the activity of the catalyst for the several reactions that actually occur. Selectivity is determined by the relative rates of the competing reactions- dehydrogenation to aromatics and hydrocracking, and isomerization in so far as it affects the other two. Activity is determined by the magnitude of the rate constants. Platinum catalysts appear to be the most selective and the most active, as well as the most expensive.

Dehydrogenation of Naphthenes. Selectivities of catalysts depend to some extent on the make-up of the feed stock. Alkylcyclohexanes are readily converted to aromatics by all dehydrogenation catalysts, provided that the reaction conditions are favorable thermodynamically. For the conversion of alkylcyclopentanes, on the other hand, there are large differences. Because alkylcyclopentanes require an isomerization step, their conversion to aromatics depends upon the isomerization activity of the catalyst. Published data on platinum, molybdena, and chromia catalysts show that platinum has the highest isomerization activity, chromia the lowest. Even with platinum catalysts, the isomerization reaction is the rate-controlling step. Thus the conversion of alkylcyclopentanes to aromatics is lower than the conversion of alkylcyclopentanes to aromatics is lower than the conversion of cyclohexane derivatives; consequently there is more opportunity for hydrocracking, and yields of aromatics are poorer.

Dehydrocyclization of Paraffins. Data on platinum, molibdena, and chromia catalysts have also been published for the conversion of paraffins to aromatics. When operating in the pressure range normally use in catalytic reforming, platinum is the most effective catalyst, chromia the least. The poor results obtained with chromia catalyst are surprising, inusmuch as high conversions of n-heptane to toluene are obtained at low pressures. Apparently, the chromia catalyst has the unusual property of adsorbing hydrogen so strongly at higher pressure that paraffins can not readily reach its surface.

Reaction Mechanism

Extensive studies have been made to elucidate the mechanism of reforming over platinum catalysts. Such catalysts are duel-functional; they cotain platinum as a dehydrogenating agent and an acidic material, such as chlorine, fluorine, or alumina-promoted silica, as an isomerization agent. In commercial catalysts, enough platinum is used to ensure that the dehydrogenation activity is large in comparision with the isomerization activity.

Although only traces of olefins can exist under reforming conditions, they apparently are intermediates in the reactions. Both naphthenes and paraffins are dehydrogenated to olefins [in trace amounts] on dehydrogenation sites in the catalyst. Cyclohexenes continue to dehydrogenate rapidly to aromatics. Alkylcyclopentanes transfer to acid sites, where they are isomerized to cyclohexanes; the cyclohexanes then pass back to dehydrogenation sites, where they are converted to aromatics. Alkyl olefins also transfer to acid sites where they may either isomerize to other alkyl structures or cyclize to naphthenes. The isomerized olefins pass back to dehydrogenation sites, where alkyl olefins are hydrogenated to paraffins and cyclohexenes are dehydrogenated to aromatics.

In view of the low isomerization activity of chromia catalysts, the exellent results obtained with them at low pressure suggest that n-heptane is easier to aromatize than are its isomers. This idea is also suggeted by data on the conversion of n-heptane over a platinum catalyst; the ratio of aromatics production to hydrocracking was higher at low conversions [where n-heptane predominates in the reactants] than at higher conversion [where isoheptanes predominats]. It has also been shown that paraffins with more than seven carbon atoms are converted more raedily to aromatics than are heptanes. All these observations fit the hypothesis that naphthene intermediates are not formed from paraffins over platinum catalysts by linking of two end [primary] carbon atoms. It has been suggected that platinum catalysts form derivatives of cyclopentane by the linkage of second and sixth carbon atoms; the alkylcyclopentanes so formed isomerize to alkylcyclohexanes, which are dehydrogenated to aromatics. This mechanism could not apply for cgromia catalysts, which have little isomerization activity. When n-heptane is processed over a chromia catalyst, the second and seventh carbon atoms appear to link up to form methylcyclohexane directly.

Exercises

Answer the following question

1. What is the purpose of catalytic reforming ?

2. Why had catalytic reforming supplanted thermal reforming ?

3. Do you know why conversion other hydrocarbons to aromatics is principle of catalytic reforming ?

4. What are the main reactions in catalytic reforming ?

5. Would isomerization have a large effect on octane?

6. What is the activity of catalysts determined by?

7. What is the characteristic of platinum catalysts?

8. Why is the conversion of alkylcuclopentanes to aromatics lower than that of cyclohexane derivatives?

9. What does selectivity of catalysts depend on?

10. Which catalyst has the highest isomerization activity?

11. Why is activity of dehydrocyclization of paraffins low at pressure range normally used in catalytic reforming?

12. What are dual-functional catalysts?

13. Which reaction is performed in dehydrogenation sites?

14. Which reaction is performed in acid sites?

15. Why are paraffins with more than seven carbon atoms converted to aromatics than heptanes?

16. Can you show the mechanism of conversion of paraffins to aromatics?

Thermal processes

At high temperatures, the bonds between atoms in molecules of hydrocarbons are weakened and can break to form new compounds. In any homologous series, lighter [low-boiling] hydrocarbons split less easily than high-boiling ones. Along with splitting into lighter hydrocarbons, other transformations can take place, in particular, packing of molecules in which heavier fractions from preliminary petroleum processing are decomposed at elevated temperatures are call thermal processes. In petroleum processing industry, the most common processes of this type are thermal cracking, coking, and pyrolysis.

Thermal cracking, usually carried out at pressures up to 5 MPa and temperatures of 420-550 0C, is a process in which the starting material is changed qualitatively with the formation of new compounds having different physicochemical properties. Depending on the composition of the starting material and the process conditions, the yield of gasoline cracking is 7-30 % of the mass of the starting material; the process also gives some other products: gaseous, liquid and solid [coke].

Coking of residue is done at temperatures of 445-5600C [still coking] or 485-5400C. Depending on the quality of the starting material and the type and conditions of the process, it may yield 15-18 % of commercial coke, 49-77.5 % of liquid products [including 7-17 % of gasoline fractions] and 5-12 % of gases [up to C4].

Pylolysis of distillates and light hydrocarbons [from ethane to butane] is usually effected at 650-8500C. The main object of pyrolysis is to produce ethylene and propylene; earlier, it was aimed at producing aromatic [benzene] hydrocarbons.

In 1930-1950's, pyrolysis played an important part as a method for increasing the manufacture of gasolines for carburettor engines. At a later time, the quality of gasolines produced in thermal cracking plants could no more satisfy the rising requirements of consumers. Upon development of catalytic processes, thermal cracking still retains its role mainly for the manufacture of low-viscous fuel oils from residue products of preliminary petroleum processing , and also of gas oils intermediate products for making carbon black. The processes of coking are being developed further, mainly to satisfy the demands for coke, especially electrode coke. Liquid products of coking are utilized for increasing production of white petroleum products. Pyrolysis is being developed rapidly in association with increasing demands for olefin materials for the chemical and petrochemical industries.

Thermal Cracking

In 1890, V.G. Shukhov, a famous Russian scientist, designed the first cracking plant for producing light petroleum products from fuel oil. Later, as the need for automobile gasoline increased, a system with reaction chambers was developed, in which the starting material, preheated to the reaction temperature in the furnace coil, was retained and subjected to cracking up to the formation of coke. The time of filling of the reactor with coke determined the length of the whole working cycle of the plant. At a later time, the reaction chamber was replaced by the reaction volume formed in radiant pipes of a furnace. To prevent the clogging of the apparatus with coke, the reaction products were chilled at the exit from the furnace by the cold starting material [quench] which stopped the cracking process [in particular, Winker-Koch plants operated by this principle]. In later years, further improvements have been made in thermal cracking in foreign countries and in the USSA where the process was implemented in 1927-28.

As has been given earlier, the principal reaction of thermal cracking is the decomposition [or cracking ] reaction. Among various hydrocarbons, paraffins can be cracked most easily. Then follow naphthenic hydrocarbons. Benzene hydrocarbons are most stable against cracking. In any homologous series, hydrocarbons of a higher molecular mass are cracked more readily. Thus heavier fraction of petroleum products are less stable and can be cracked more easily than lighter ones. Brief data on the chemistry and mechanisms of cracking of the principal classes of hydrocarbons will be given below.

Paraffin hydrocarbons. Cracking of commercial paraffins which consist mainly of C24H50, C25H52 and C26H54 hydrocarbons forms paraffin hydrocarbons and olefins composed of 12, 13, or 14 carbon atoms, i.e. roughly one-half of the carbon atoms in the original paraffin. This is an indication of that the breakdown of C-C bonds in cracking of paraffins of high molecular mass occurs in the middle of a molecule. The new paraffin hydrocarbons formed by cracking can in turn break down into simpler molecules say a molecule of a paraffin hydrocarbon and that of an olefin, for instance:

4250C

C12H26  C6H14 + C6H12

dodecan hexane hexene

[paraffinic] [paraffinic] [olefinic]

At higher temperatures of cracking of paraffinic hydrocarbons, reactions in which the breakdown of molecules occurs at the end portion of the chain begin to prevail over those in which molecules break in the middle. The larger fragment of a broken molecule is an olefin and the smaller one is the paraffinic hydrocarbon [gaseous] or hydrogen. Isoparaffinic hydrocarbons are thermally less stable than those of the normal structure. The rate of the reaction at a given temperature increases almost linearly with the molecular mass. This is true of all groups of hydrocarbons.

Olefinic Hydrocarbons. These are the principal ones among all unsaturated hydrocarbons produced by cracking. They prevail as gaseous compounds [from ethylene C2H4 to butylene C4H8] and liquid ones [from amylenes C5H10 to pentadecenes C15H30]. Cyclic olefins and diolefins form in relatively small quantities. In contrast to paraffinic hydrocarbons, olefins undergo appreciably more diverse primary reactions during cracking, the most important among them being polymerization reactions [i.e. combination of a few molecules into a single molecule] and depolymerization reactions, especially at an early stage of the process . Polymerization is the main reaction at moderately high and high pressures; it can occur not only between like molecules, but also between unlike molecules of olefins, for instance:

C2H4 + C3H6  C5H10

At later stages of the process, olefins are dehydrogenated partially and form diolefins, which typically have two double bonds, and hydrogen or split into diolefins and paraffinic hydrocarbons:

CH3- CH2- CH= CH2  CH2= CH- CH= CH2 + H2

butylene divinyl

[olefin] [diolefin]

Secondary reaction between olefins and diolefins may give cycloolefins which are present in cracking products in very small quantities. Olefins can transform into cyclic hydrocarbons [naththenes];

n-hexene -1  cyclohexane

Naphthenic hydrocarbons. The main reaction in cracking of these hydrocarbons are dealkylation [splitting of paraffinec side chains ] and dehydrogenation of hexacyclic naphthenic naphthenic hydrocarbons into benzene hydrocarbons; the two reaction can occur simultaneously.

Dehydrogenation of hexacyclic naphthenes in thermal cracking with the formation of benzene hydrocarbons is of minor importance. Owing to the dealkylation reaction taking place in thermal cracking, naphthenic and benzene hydrocarbons loss most of their long side chains. Paraffinic side chains in turn break to form gaseous and low-boiling paraffinic hydrocarbons and olefins. In high-temperature processes, naphthenic rings can break; the result is that hydrocarbons lose their cyclic structure and that polycyclic structure are partially decycled [if they had several rings]. In that case, paraffinic, olefinic and naphthenic hydrocarbons form.

Benzene Hydrocarbons.These are obtained by dehydrogenation of the cycloolefins or naphthenes which were formed at earlier stages of the process. Benzen hydrocarbons are quite stable at high temperatures, especially benzene, toluene and xylenes. The main reaction in cracking of benzene hydrocarbons with alkyl chains are dealkylation and condensation. Condensation may occur between the molecules of benzene hydrocarbons [or some other unsaturated hydrocarbons]. This gives polycyclic benzene hydrocarbons which cancondense further to asphaltenes and coke.

Sulphur compounds. They are decomposed in cracking and form hydrogen sulpide. Cyclic sulphur-organic compounds, such as thiophene and thiophane, have the greatest stability against decomposition. Hydrogen sulphide and elemental sulphur [as the product of oxidation of hydrogen sulphide] which form in cracking of sulphurous petroleum gredes can cause strong corrosion of process equipment.

Innert tars and asphaltenes. These may contain various heterocyclic compounds [usually including oxygen, sulphur, nitrogen and some metals]. In cracking they form gases, liquid products and large amount of coke. The yield of coke in cracking of asphaltenes may reach 60% and that in cracking of tars 7-20% [depending on the molecular mass of tars].

Since the starting materials for industrial thermal cracking are usually mixtures of many hydrocarbons of complicated structure, many reaction can occur simultaneously and the mechanism of thermal cracking can not be explained in detail. It is assumed however, that most reaction of thermal cracking can be described by the theory of formation of free radicals.

Exercises

Answer the following question

1. What are called thermal processes ?

2. What are the most common processes of thermal processes ?

3. What are products of thermal cracking ?

4. What are products of coking?

5. What are products of pyrolysis?

6. Who designed the first cracking plant?

7. Which type of hydrocarbon can be cracked most easily?

8. Which C-C bonds are broken down in cracking high paraffins at lower temperature ?

9. Which C-C bonds are broken down in cracking high paraffins at higher temperature ?

10. What are the products of cracking high molecular mass paraffins at higher temperature ?

11. What relationship is there between the rate and temperature of the reaction?

12. What are the primary reaction of olefins in thermal cracking condition?

13. What are the second reaction of olefins in thermal cracking condition?

14. Which reactions happen with naphthenic hydrocarbons in thermal cracking condition?

15. Why are gaseous and low-boiling parafinic hydrocarbons and olefins formed in thermal cracking of naphthenic hydrocarbons ?

16. What are the main reactions of benzene hydrocarbons ?

17. Which compounds can be obtained in cracking of benzene hydrocarbons ?

18. Which sulphurous compounds are formed in cracking ?

19. What is the main product in cracking tars and asphaltenes?

20. What is the main mechanism of thermal cracking ?

Catalytic processes

A typical feature of catalytis processes is the ues of catalysts, i.e. substances wgich can accelerate [or decelerate] the reactions and cause the formation of new hydrocarbons ans other substances not present in the starting material. Catalytic processes occur under softer conditions [at lower temperatures and pressures] than thermal ones, but may involve the reactions which are impossible in purely thermal processes.

A catalyst usually consists of an active substance [which determines the course of desirable reactions] applied onto a carrier substance [mostly alumina] having a largely extended asurface. In some cases, some other substances [promotors] are added to improve characteristics of catalytic process. The particles [granules] of catalytic process an enormous porosity and therefore a very large internal surface area. The activity of a catalyst is due mainly to the surface of pores rather than to their wxternal surface. The name of a catalyst depends on the process where it is to be used, for instance, reforming catalysts, cracking catalysts, ect.

The technico-economical characteristics of a catalytic process are determined by the quality of the starting material and the process conditions, as well as by the properties of the catalyst used. The capability of a catalyst to accelerate the rate of desirable reactions and retain the rate of unwanted ones at aconstant low level is called selectivity. Activity is another important characteristic of catalysts; it is estimated in term of the yield of the end product relative to the use of the starting material. In particular, the catalyst activity in catalytic cracking is determined as the yield of gasoline [end process].

Catalysts can participate in process reactions in a stationary [fixed-bed] or moving [circulating] state. In both cases, they gradually lose their activity and selectivity owing to ageing. This may be accelerated under more rigid process conditions. Along with normal ageing of catalyst may also take place. This occurs often when the process is run under abnormal conditions,say at an excessively high temperature. Many catalysts can be affected by certain substances containing sulphur, nitrogen and heavy metals [V, Ni, and other] and by water in the starting material.

Catalysts can be regenerated to rectore their activity and partially, the selectivity, which is usually done by removal [burning-off] of the coke deposits setted on catalyst particles during operation. By another method, the properties of catalysts [especially of fixed-bed type] are restored by gradually raising the temperature in the reactor. With circulating catalysts, a fresh catalyst is added in portions to compensate for the loss of the catalyst in the system.

Catalyst processes make it possible to remove unwanted impurities, for instance, sulphurous compounds, and to convert certain hydrocarbons into the products which cannot be obtained by preliminary distillation of petroleum or in thermal processes.

Brief Description of Catalyst processes

Catalytic cracking is the process of conversion of high-boiling petroleum fraction into high-octane base components of aviation and automobile gasolinesand middle distillates.

Industrial processes of catalytic cracking are based on contacting the starting material with an active catalyst under approriate conditions to convert a considerable portion of the material into gasoline and other light on the particles of the catalyst and thus reduce sharply the activity, in particular, the cracking ability. The activity of the catalyst is restored by burning off the carbon prcipitates [usually called coke] in air.

There exist many types and systems of catalyst cracking plants, those with circulating flow of the catalyst, especially in a fluidized bed, being most popular.

Catalytic reforming is employ widely to obtain high-octane gasoline fraction. Reforming of gasoline or gasoline fractions in combination with various methods of separation of benzene hydrocarbons, for instance, with solvent extraction, make it possible to produce benzene hydrocarbons [benzene, toluene, xylenes and higher aromatics] for the petrochemical and chemical industries.

Catalytic reforming processes are based on contacting the starting material with an active catalyst usually containing platinum. The yield of reformate may vary within 63 to 85 % of the mass of the starting material. The catalyst is regenerated periodically to restore its activity. A feature of importance is that the catalytic reforming occurs in a medium of hydrogen-containing gas at high temperatures and pressures. The hydrogen formed in various reactions of reforming is removed from the system as an excess of hydrogen-containing gas. The tigh content of hydrogen is the gas mixture [up to 80% by volume] makes it possible to utilize it in hydrigenation processes, in particular, for hydrofining of diesel fuels.

Hydrogenation processes occur in the medium of hydrogen at eleated temperatures and pressures. They can be used for preparing high-quality products from sulphurous and high-sulphurous petroleum grades, the yield and quality of these product being varied depending on the degree ofdestruction and the prevailing reactions. Among the processes of this kind, hydrofining of various fractions and products is most important.

Hydrofining of petroleum distillates ans products is one of the most popular catalytic processes, especially for streating sulphurous and high-sulphrous petroleum grades. The process is carried out in a hydrogen medium at a pressure of 3-5 MPa. The main object of hidrofining of petroleum distillate and products is to reduce the content of sulphur and other harmful compounds in them. These substances are destructed in the process, and the destruction products [hydrogen sulphide and ammonia] are removed fromthe system with gases.

Hydrofining processes are based on contacting petroleum distillates and products with a fixed-bed or circulating catalyst , usually alumina-cobalt-molybdena or alumina-nikel-molybdena. The process takes place in the medium of hydrogen at elevated temperatures and pressures so as to convert 95-99% of the starting material into the refined product or distillate [hydrogenate]. Minpr quantities of gasoline, hydrogen sulphide and ammonia also form in the process.

Alkylation is a process by which isoparaffinic hydrocarbons are combined with olefins to form higher-boiling isoparaffinic hydrocarbons which can use as high-octane components in aviation and automobile gasolines. Other kind of alkylation are also in use, in particular, alkylation of benzene y by olefins [for instance, alkylation of benzene by propylene to make iospropylbenzene].

Up to quite recently, catalyst alkylation of isobutane was carried uot by butylenes in the presence of sulphuric or hydrofluoric acid as a catalyst. In modern plants, alkylation of isobutane is done by using the materials containing ethylene, propylene and even amylenes, as well as butylenes.

Alkylation processes may differ in the starting material, catalysts, productivity, and especially in the design of catalyst plants. With the use of sulphuric acid as a catalyst, the alkylation process is characterized by a low temperature of the reaction and the necessity to maintain a high concentration of isobutane and olefins in the reaction zone. The total yield of alkylate from olefinic starting materials is 1.5-1.8 units perunit volume of the starting material, depending on the quality of the material and the process conditions. The significance and scope of alkylation increase with the rising production of high- octane automobile gasolines having a low content of TEL.

Isomerization is the process of conversion of relatively loe-octane paraffinic hydrocarbons [mostly C5¬¬-C6 and their mixtures]into corresponding isoparaffinic hydrocarbons having a high octane number. In industrial isomerization plants using various catalysts, including alumo-platinum ones, the yield of isomerizates attains 97%. The process of isomerization takes place in hydrogen atmosphere. As in other processes, the catalyst is regenerated peridically.

Izomerizates are used together with alkylates for preparationof high-quality gasolines, by compounding them with high-aromatic gasolines of catalytic cracking and reforming.

Novel catalyst processes, in paricula disproportionation. are being paid much attention now. The process is based on converting two molecules of a hydrocarbon into two unlike molecules, one having by on carbon atom more and the other, by one atom less than the original molecules, for instance:

2C3H6  C2H4 + C4H8

The process is carried out at 66-2600C and a pressure of 1.4-4.1 MPa, with the starting material being supplied at a high rate [10 to 100 h -1]. Disproportional takes place with a high selectivity: the total yield of ethylene and butylene attains 97% of the propylene converted and the degree of conversion of the latter, up to 45%. Disproportional can be employed for making benzene from toluene [2C7H8  C6H6 + C8H8 ] to replace the less efficient process of toluene alkylation.

In industial practice, a number of processes are often combined in a single plant [for instance, hydrogen cracking and catalytic reforming]. This make it possible to process low-octane starting materials into high- octane gasoline with a hihg concentration of benzene hydrocarbons [obtained by reforming] and isoparaffinic ones [obtained by hydrogen cracking]. In this combined technique, the process of hydrogen cracking occurs without hydrogen supply from the outside.

1. What is the typical feature of catalytic processes?

2. What can you say about the conditions of catalytic processes?

3. What is the composition os a catalyst of catalytiv processes?

4. What is the main characteristic of a catalyst?

5. What is the selectivity of a catalyst?

6. How can catalysts participate in process reaction?

7. When does the quick ageing happen?

8. What can affect the activity and selectivity of catalysts?

9. How can you regenenate catalysts?

10. Do you know what the main catalytic characteristics are?

11. Can you show the basis of catalytic cracking?

12. What is the purpose of catalytic reforming?

13. Which codition do catalytic reforming happen in?

14. What is the purpose of hydrogenation processes?

15. Which process is most important in hydrogenation processes?

16. What is basis of hydrofining process?

17. Can you define the alkylation reaction?

18. Which alkylation processes are used in petroleum processing?

19. What was catalytic alkylation of isobutane carried out by?

20. What do alkylation processes depend on?

21. What is akylation processes characterixed by with the use of sulphuric acid as a catalyst?

22. What is purpose of isomerization?

23. What is disproportionation?

24. How can you say about the characteristic of gasoline obtained by catalytic processes?

Catalytic cracking

There are two main types of catalyst cracking: one is carried out in the presence of a catalyst -porous solid particles of a definite composition and structure; the other is also carried out with a catalyst, but in a hydrogen atmosphere at a high pressure [up to 30 MPa] and a slightly reduced temperature [hydrocracking].

As compared to thermal cracking, catalytic cracking gives lower yields of methane, ethane and olefins, but higher yields of C3 and C4 hydrocarbons and of gasolines high in benzene and isoparaffinic hydrocarbons. This is the principal advantage of catalytic cracking over themal cracking. Aluminosilicates are used most often as cracking catalysts now. In recent time, zeolite-containing [crystlline aluminosilicate] catalysts with rare-earth additives have come into wide use.

The main object of catalytic cracking is to produce high-octane components for automobile or, less frequently, for aviation gasolines. The process gives the hihgest yield of white products with any kind of . The by-products obtained in catalytic cracking plants include gases, catalytic gas oils [light grades boiling off up to 3500C and heavier ones, which begin to boil above 3500C] and coke which precipitates on the catalyst and is burned off in regeneration.

The operation of catalytic cracking plants can be characterized by what is called cracking ratio, i.e. the relative quantity of the starting material converted into gasoline, gas and coke. Thus, the depth of conversion is 100 ninus the yield of gas oil [in per cent]. In singke cracking, the cracking ratio does not esceed 55%, whereas in deeper kinds of cracking [recycle cracking] it may reach 80% by mass. In some cases use is made of the cracking efficiency, which is the ratio of the total yield of debutanized gasoline and C4 fraction to the cracking ratio. The cracking efficiency is usually 0.75 to 0.80.

a. Principal reactions of catalytic cracking

In the cracking process, the contact of crude petroleum with a catalyst results in the formation of gas, gasoline, coke and some liquid products with the boiling temperature above the boiling -off temperature of gasoline. These products from by the following principal reactions.

Cracking of hydrocarbons with the formation of lighter molecules: for instance, an n-butyl radical splits from a molecule of n-butylbenzene to form benzene and butylene. The molevules of cetane C16H34 give on splitting C8H18, C8H16 and some other hydrocarbons. The rate of hydrocarbon splitting increases substantially with increasing temperature, which make it possible to control the process, i.e. to increase or diminish the yields of certain products by changing the temperature.

Dehydrogenation. In this reaction, only hydrogen molecules split from hydrocarbon molecules. A typical example is the catalytic reaction of dehydrocylization of methylcyclohexane C7H14 [naphthenic hydrocarbon], which give up three hydrogen molecules and converts into toluene. Part of the hydrogen liberated in dehydrogenation is attached in catalyst cracking to olefinic hydrocarbons, thus reducing the content of unsaturated hydrocarbons in catalytic cracking gasolines.

Isomerization is characterized by that the atoms in a molecule change their positions, but their number remains the same. Isomerization of normal paraffinic hydrocarbons gives hydrocarbons of a branched structure, for instance, isopentane form from n-pentane.

Hydrogenation. In this reaction, the molecules of the starting material attach hydrogen and thus form new compounds more saturated in hydrogen. for instance, octylene [an olefinic hydrocarbon] is converted into octane by the reaction:

C8H16 + H2  C8H18

The hydrogenation reaction is quite common and can take place not with olefins, but with other classes of hydrocarbons as well. For instance, cyclohexane can be obtained by hydrogenation of benzene.

Polymerization. In this reaction two or more molecules combine into a single large molecule. For example, two molecules of ethylene are polymerization into a higher boiling hydrocarbon, butylene. Using polymerization, gaseous olefinic hydrocarbons [ethylene, propylene, butylenes] can be converted into liquid or even solid hydrocarbons of a higher molecular mass.

In catalytic cracking, the rate of breakdown of paraffinic hydrocarbons is higher at a higher molecular mass. At the ordinary temperatures of catalytic cracking,i.e. 450-5200C, catalysts have almost no effecton light paraffinic hydrocarbons: propane and butane, white high-boiling paraffins undergo deep changes. For instance, the cracking rate of cetane, whose boiling temperature is 2870C is roughly 13 times that heptane which boils at 980C. The oleffins formed on breakdown of normal paraffinic hydrocarbons are isomerized, partially saturated by hydrogen and convert into paraffinic hydrocarbons of a branched structure and a lower molecular mass. Olefins can be subjected to catalytic cracking much more easily than paraffinic hydrocarbons. The reactions of splitting, isomerization, polymerization and hydrogen attachment are vey typical of them. Some other reactions are also possible, by which olefins are converted into benzene hydrocarbons and high boiling compounds.

Catalytic cracking of naphthenic hydrocarbons occurs at higher rates than that of paraffinic ones and gives more light liquid products and less gas. Besides, naphthenic hydrocarbons give many benzene hydrocarbons on splitting of hydrogen atoms. Distillates high in naphthenic hydrocarbons are a valuable starting material for catalytic cracking. They give more gasoline and of higher quality than do distillates of a similar fractional composition obtained from paraffinic grades petroleum.

The nuclei of bnzene hydrocarbons are thermally stable ans split insignificantly even at 450-500oC. On the contrary, the molecules of benzene hydrocarbons with side paraffinic chains are cracked easily: their bonds break mainly in sites of attachment of a side chain to the benzene nucleus. Benzene hydrocarbons with no side chains in the molecule and paraffinic hydrocarbons of nomal structure turn tobe most stable againt catalytic cracking. Hydrocarbons of other homologous series [with the same number of carbon atoms in the molecule], such as olefinic, naphthenic, aromatic with long side chains, are less stable and can be cracked morw easily.

b. Starting materials and products of the process.

Starting materials. The starting materials for catalytic cracking are various distillate fractions obtained by atmospheric or vacuum distillation of crude petroleum. In catalytic cracking plats for obtaining the starting material are used, in particular, distillates with the boiling-off range of 220-3600C and relative density of 0.83-0.87. The plants for making the compounds of automobile gasoline use heavier disstilates with the boiling-off range of 300-5500C and relative density of 0.87-0.93. In some cases, starting materials of an intermediate composition can be used, such as mixtures of various distillates obtained in preliminary processing of petroleum [atmospheric or vacuum distillation] and in secondary processes of preparation of fuels and oils; these mixtures can be used only for making automobile gasolines. In recent time, attempts have been made to process low-ash fuel oils and deasphatizates by catalytic cracking.

The starting material must contain no fractions boiling below 1900C, since they remain practically unchanged upon catalytic cracking and lower the octane numbr of the final gasoline.

The processing of starting materials containing harful impurities involves certain difficulties, in particular, stronger corrosion of equipment and heavier coking of the catalyst, which may result in a lower yield of gasoline and lower productivity of the plant. Metal compounds can be present in distillates owing to carry-over of goudron droplets into the top portion of the column. Some compounds are volatile at high temperatures. For that reason, the operation of a vacuum column should be careefully checked and sometimes itis advisable to lower the boiling-off temperature of a vacuum distillate to be used for catalytic cracking.

The coking ability of the starting mashould usually be not less than 0.25%. The materials with the coking ability of up to 0.7% can be processed of the regenerator has an extra capacity for coke burn-off. Moist material should not be used for processing, since moisture can disturb the process conditions, in particular, raise the pressure in the reactor, disturb the normal circulation of the catalyst, increase the quality of the end products. In some cases, this mayform emergency situations. The composition of the starting material can also influence the yield and quality of the products of catalytic cracking.

Products of catalytic cracking. Catalytic cracking plants produce up to 20% [by mass] of gases [containing hydrogen and light hydrocarbons up to 60% of high- octane components of automobile gasolines, and up to 2.5-8% of coke, the balance [except for losses] being light and heavy gas oils. Some plants make unstable gasolines which are further delivered to gas separation. Besides, catalytic cracking for production of the base aviation component may give ligroin and polymers as by-products, and also motor gasoline- an intermediate product which is subjected to catalytic reforming at the second stage.

Wet gas. Its composition is characterized by a high concentration of isomeric hydrocarbons, in particular of isobutane, which increases the value of the gas as of an intermediate product for further processing. The wet gas obtained by catalytic cracking of light and heavy distillates has roughly the following composition [in % by mass]

These data disregard steam, hydrogen suphide and inert gases which may be present in various minor amounts in gases of catalytic cracking.

Wet gas and unstable gasoline from catalytic cracking plants are fed into an absortion- gas frctionation plant for separarion of light gases. Apart from stable gasoline, the products obtained in such a plant include propane-propylene, butane-butylene and pentane-amilene fraction. Propane-propylene and butane-butylene fractions are further polymerized and alkylated to prerare gasoline components or are used in petrochemical processes [propane and butane can also be used as domestic fuel].

Unstable gasoline. It is stabilized to obtain a stable component for preparing high-octane automobile and aviation gasolines.

Light catalytic gas oil. As compared to the products of similar fractional composition obtained by preliminary distillation of petroleum, light catalytic gas oil [a distillate with the beginning of boiling at 175-2000C and the and of boiling at 320-3500C] has a lower cetane number [up to 25], higher content of sulphur [roughly the same as in crude petroleum] and benzene hydrocarbons [up to 55 %], and a certain concentration of unsaturatd hydrocarbons. The setting temperature of these gas oils is however substantially lower than that of the starting material for catalytic cracking. Under more rigid conditions of the process, and without increase in recurculation light gas oil is produced in smaller amoints and with a lower cetane number, but with a higher concentration of benzene hydrocarbons.

Light catalytic gas oil is utilized as the starting material for manufacture of commercial carbon [carbon black], as a component in commercial grades of fuel oil, and for some other purposes. In rare cases, it can be used as a component of diesel fuel, provided that other components of the fuel produced by preliminary distillation have a higher cetane nimber and a reduced content of sulphur [compared to the standard value]. In some cases, lihgt catalytic gas oil is extracted; the refined layer with a reduced content of benzene ys and a higher cetane nimber used as a component of diesel fuels and the extracted layer, which is high inbenzene hydrocarbons, is a valuable by- product for preparing carbon black.

Heavy catalytic gas oil is the liquid residue of catalytic cracking. Its quality depend mainly on the process conditions and the boiling -off temperature of the light gas oil produced. Heavy gas oil often contains many mechanical impurities [rests of the catalyst]. Its sulphur content is usually higher than that of the starting material used for cracking. Heavy catalytic gas oil is usedf for making fuel oils and carbon black.

Catalytic cracking

1. How many main types of catalytic cracking are there? What are they?

2. Is there difference between products of thermal cracking and catalytic cracking?

3. What is the principal advantage of catalytic cracking over themal cracking?

4. Which compound are used as cracking catalysts?

5. What is main object of catalytic cracking?

6. Do you know what by-products are?

7. What can the operation of catalytic cracking plants be characterized?

8. What is cracking efficiency?

9. How many principal reactions happen in catalytic cracking?

10. Which molecules are formed in cracking of hydrocarbons?

11. How are the rate of hydrocarbon splitting depend on temperature ?

12. Which compounds are formed in dehydrocyclization of methylcyclohexane C7H14?

13. Why is the content of unsaturated hydrocarbons in catalytic cracking gasolines reduced?

14. What is isomerization characterized by?

15. What is polymerization?

16. What is the rate of breakdown of paraffins depend on?

17. How can the molecular mass of hydrocarbons effect on rate of cracking?

18. Can olefins be subjected to catalytic cracking more easily than paraffinic hydrocarbons, can't they?

19. Which reaction happen with olefin in catalytic cracking?

20. How can you say about content of naphthenic hydrocarbons in starting material for catalytic cracking?

21. What can you say about reaction ability of benzene hydrocarbons?

22. Which hydrocarbons are more stable in catalytic cracking?

23. Which materials are used as starting materials for catalytic cracking?

24. Which starting material are used in catalytic cracking plants for obtaining the components of base aviation gaspline?

25. The same question for making the components of automobile gasoline?

26. Why aren't fraction boiling below 1900C used in catalytic cracking ?

27. Why is it advisable to lower the boiling -off temperature of a vacuum distillate to be used for catalytic cracking?

28. Why shouldn't moist material be used for catalytic cracking ?

29. What are products of catalytic cracking?

30. What are by-products of catalytic cracking?

31. What is the composition of wet gas?

32. How can they separate light gasses from wet gas?

33. What can you say about composition of light gas oil?

34. What is light catalytic gas oil used?

35. What do quality of heavy catalytic gas oil depend on?