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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 thermal 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 highest yield of white products with any kind of petroleum. 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 single cracking, the cracking ratio does not exceed 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.

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 molecules of cetane C16H34 give on splitting C8H18, C8H16 and some other hydrocarbons. The rate of hydrocarbon splitting increases substantially with increasing temperature, which makes 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 only 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 polymerized 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 effect on 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 very 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 gases. 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 of petroleum.

The nuclei of benzene hydrocarbons are thermally stable and split insignificantly even at 450-500 oC. 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 normal structure turn to be most stable against 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 more easily.

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 plants for obtaining the components of base aviation gasoline, lighter types of 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 components of automobile gasoline use heavier distillates 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 number of the final gasoline.

The processing of starting materials containing harmful 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 vacuum 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 carefully checked and sometimes it is advisable to lower the boiling-off temperature of a vacuum distillate to be used for catalytic cracking.

The coking ability of the starting material should 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 flow rate of vapours in the rectification column, and impair the quality of the end products. In some cases, this may form 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 C4], 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 refining 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.

These data disregard steam, hydrogen sulphide 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 absorption- gas fractionation plant for separation 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 prepare 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 end of boiling at 320-3500C] has a lower cetane number [up to 25], higher contents of sulphur [roughly the same as in crude petroleum] and benzene hydrocarbons [up to 55 %], and a certain concentration of unsaturated 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 recirculation light gas oil is produced in smaller amounts 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 number and a reduced content of sulphur [compared to the standard value]. In some cases, light catalytic gas oil is extracted; the refined layer with a reduced content of benzene hydrocarbons and a higher cetane number is used as a component of diesel fuels and the extracted layer, which is high in benzene 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 used for making fuel oils and carbon black.

Exercises

Answer the following question

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

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

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

4. Which compounds are used as cracking catalysts?

5. What is the main object of catalytic cracking?

6. What are the by-products of catalytic cracking?

7. By 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 does the rate of hydrocarbon splitting depend on temperature ?

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

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

14. By what is the isomerization characterized?

15. What is polymerization?

16. On what is the rate of breakdown of paraffins depend?

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

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

19. Which reaction happen with olefin in catalytic cracking?

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

21. What can you say about the 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 materials are used in catalytic cracking plants for obtaining the components of base aviation gasoline?

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

26. Why aren't fractions 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 the products of catalytic cracking?

30. What are the by-products of catalytic cracking?

31. What is the composition of wet gas?

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

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

34. What is the light catalytic gas oil used for?

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

36. What can you say about the composition of heavy gas oil?

37. What can heavy catalytic gas oil be used for?