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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 decided by the predominance of some or other group of hydrocarbons in crude petroleum and by the presence of compounds containing sulphur, nitrogen or oxygen.

1. 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 diesel- 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.

2. Naphthenic Hydrocarbons [Cycloalkanes]

Their general formula is CnH2n. They were discovered by V.V. Markovnikov, a prominent Russian chemist, when studying petroleum of Caucasian deposits.

In their chemical properties; naphthenic hydrocarbons are similar to paraffins, 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.

3. Benzene Hydrocarbons [Arenes]

Arenes of the benzene series have the general formula 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.

4. Unsaturated Hydrocarbons [Olefins]

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

5. 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, diesel- fuel and light oil distillates of petroleum and are corrosion- aggressive; they are removed from petroleum fractions by leaching. Naphthenic acids and their salts are widely used in industry as components of greases, for impregnation 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 to 2.5%], sulphur [0.8-7%], and nitrogen [up to 1%]. Low molecular tar compounds can partially be distilled off 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 products as bitumen, coke, insulating and impregnating materials.

All tar-asphaltene products are usually classed into neutral resins soluble in light gasoline; asphaltenes [ the products of polymerization of neutral resins and oxyacids] 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.

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.

6. 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 corrosion 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 temperatures.

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 hydrogen 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 temperatures and pressures are applied, however, organic sulphur compounds are destroyed together with high - molecular hydrocarbons 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 distillation might be desalted and dehydrated. The content of sulphurous compounds in petroleum products can be lowered by various methods of refining, mainly by hydrogen refining.

7. Nitrogen Compounds

The content of nitrogen compounds 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 nitrous 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 vapours].

8. 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-As. 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 composition of petroleum?

What are the main constituents of petroleum ?

Which series of hydrocarbon are present in petroleum ?

Which series of hydrocarbon are formed during processing of petroleum ?

What can you say about 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 are the difference and the similarity in structure and properties between paraffinic and naphthenic hydrocarbons ?

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

What are the applications of benzene hydrocarbons ?

What can you say about the properties of olefins and diolefins?

What are the applications of olefins and diolefins?

What are the applications of naphthenic acid?

What is the elemental composision of tar-asphaltene compounds?

How can you class tar-asphaltene products?

Which types of sulphurous compounds are present in petroleum products?

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

How are nitrogen compounds distributed?