PETE 203: Properties of oil Prepared by: Mr. Brosk Frya Ali Koya University, Faculty of Engineering, Petroleum Engineering Department 2013 2014 Lecture no. (2): Crude oil chemistry and composition
5. Crude oil Chemistry The crude oil mixture is composed of the following groups: 1. Hydrocarbon compounds 2. Non hydrocarbon compounds 3. Organo-metallic compounds and inorganic salts The chemical properties (physical also) of crude oil depends on its unique mixture of molecule. 5.1 Hydrocarbon compounds:- Hydrocarbons may be gaseous, liquid, or solid at normal temperature and pressure, depending on the number and arrangement of the carbon atoms in their molecules. Those with up to 4 carbon atoms are gaseous; those with 20 or more are solid; those in between are liquid.
The principal constituents of most crude oils are hydrocarbon compounds. All hydrocarbon classes are present in the crude mixture, except alkenes and alkynes. Alkanes (paraffins) are saturated hydrocarbon. They have only single bonds such as CH 4, C 2 H 6, C 3 H 8, with general formula C n H 2n+2. The naming according to IUPAC ended with ane which means single. Alkenes (Olefins) are unsaturated hydrocarbon. They have double bonds such as Ethene (Ethylene) C 2 H 4, propene C 3 H 6, butene C 4 H 8 with general formula C n H 2n. The naming according to IUPAC ended with ene which means double. Alkynes are unsaturated hydrocarbon. They have triple bonds such as Ethyne (acetylene) C 2 H 2, propyne C 3 H 6, with general formula C n H 2n - 2. The naming according to IUPAC ended with yne which means triple.
The following is a brief description of the different hydrocarbon classes found in all crude oils. 5.1.1 Alkanes (paraffins) Methane, Ethane, propane and butane are gaseous hydrocarbons at ambient temperatures and atmospheric pressure. They are usually found associated with crude oils in a dissolved state. Normal alkanes (n-alkanes, n-paraffins) are straight-chain hydrocarbons having no branches. Branched alkanes are saturated hydrocarbons with an alkyl substituent or a side branch from the main chain. A branched alkane with the same number of carbons and hydrogen as an n-alkane is called an isomer.
For example, butane (C 4 H 10 ) has two isomers, n-butane and 2-methyl propane (isobutane). As the molecular weight of the hydrocarbon increases, the number of isomers also increases.
Pentane (C5H12) has three isomers; hexane (C6H14) has five. The following shows the isomers of hexane: Crude oils contain many short, medium, and long-chain normal and branched paraffins.
5.1.2 Cycloparaffins (Naphthenes) Saturated cyclic hydrocarbons, normally known as naphthenes, are also part of the hydrocarbon constituents of crude oils. Their ratio, however, depends on the crude type. The cycloalkanes contain only single bonds, and have the general formula C n H 2n. Cyclomethane and cycloethane obviously cannot exist, but cyclopropane, cyclobutane, cyclopentane etc can exist.
The lower members of naphthenes in crude oil are cyclopentane, cyclohexane, and their mono-substituted compounds. Cyclohexanes, substituted cyclopentanes, and substituted cyclohexanes are important precursors for aromatic hydrocarbons. The examples shown here are for three naphthenes of special importance.
If a naphtha fraction contains these compounds, the first two can be converted to benzene C 6 H 6, and the last compound can dehydrogenate to toluene C 7 H 8 during processing. Heavier petroleum fractions such as kerosine and gas oil may contain two or more cyclohexane rings bonded through two neighboring carbons. The content of cycloparaffins in petroleum varies up to 60% of the total hydrocarbons.
5.1.3 Aromatic Compounds Aromatic hydrocarbons are unsaturated hydrocarbons which have one or more benzene ring, to which hydrogen atoms are attached according to the formula C n H n. Benzene ring is hexagonal ring arrangement found in benzene and other aromatic compounds, consisting of six carbon atoms with alternating single and double bonds between them, and with each carbon atom bonded to a hydrogen atom, or to other atoms or groups of atoms in derivatives of benzene.
Toluene (C 7 H 8 ) and xylene (C 8 H 10 ) are also mononuclear aromatic compounds found in variable amounts in crude oils. Benzene, toluene (C 7 H 8 ), and xylenes (C 8 H 10 ) (BTX) are important petrochemical intermediates as well as valuable gasoline components. Separating BTX aromatics from crude oil distillates is not feasible because they are present in low concentrations. Enriching a naphtha fraction with these aromatics is possible through a catalytic reforming process.
The table shows the percentage by weight of hydrocarbons in the crude oils 5.2 Non-hydrocarbon Compounds Table 2.1 Composition by weight of hydrocarbons in the crude oil Hydrocarbons Average Paraffins 30 % Napthenes 49 % Aromatics 15 % Asphaltic 6 % Various types of non-hydrocarbon compounds occur in crude oils and refinery streams. The most important are the organic sulfur, nitrogen, and oxygen compounds. Traces of metallic compounds are also found in all crudes. The presence of these impurities is harmful and may cause problems to certain catalytic processes. Fuels having high sulfur and nitrogen levels cause pollution problems in addition to the corrosive nature of their oxidization products.
5.2.1 Sulfur Components Sulfur in crude oils is mainly present in the form of organo-sulfur compounds. Hydrogen sulfide is the only important inorganic sulfur compound found in crude oil. Its presence, however, is harmful because of its corrosive nature and it can kill an operator in 10 seconds (at concentration 1000 ppm) Fortunately sulphides have a highly obnoxious (horrible) smell which gives some warning of their danger.
If one of the hydrogen atoms is replaced by a hydrocarbon group, the compound is called a mercaptan or thiol. Such compounds are formed during the distillation of crude oils. Mercaptans are acidic sulfur compounds. If both of the two hydrogen atoms are replaced by hydrocarbon groups, the compound is called a sulfide or thioether. Organosulfur compounds may generally be classified as acidic and non-acidic. Acidic sulfur compounds are the thiols (mercaptans). Thiophene, sulfides, and disulfides are examples of non-acidic sulfur compounds found in crude fractions.
Examples of some sulfur compounds from the two types are:
Sour crudes contain a high percentage of hydrogen sulfide. Because many organic sulfur compounds are not thermally stable, hydrogen sulfide is often produced during crude processing. High-sulfur crudes are less desirable because treating the different refinery streams for acidic hydrogen sulfide increases production costs. The presence of sulfur compounds in finished petroleum products often produces harmful effects. For example, in gasoline, sulfur compounds promote corrosion of engine parts. In addition, mercaptans in hydrocarbon solution cause the corrosion of copper and brass in the presence of air.
In diesel fuels, sulfur compounds increase wear and can contribute to the formation of engine deposits. Gasoline with a sulfur content between 0.2 and 0.5% has been used without obvious harmful effect. Most sulfur compounds are removed from petroleum streams through hydro-treatment processes (Hydrodesulphurization) In this process hydrogen sulfide is produced and the corresponding hydrocarbon released. Hydrogen sulfide is then absorbed in a suitable absorbent and recovered as sulfur.
5.2.2 Nitrogen Components Organic nitrogen compounds occur in crude oils either in a simple heterocyclic form as in pyridine (C 5 H 5 N) and pyrrole (C 4 H 5 N), or in a complex structure as in porphyrin. Porphyrins are a group of organic compounds which occur in nature. One of the best known is heme, the pigment in red blood cells.
Nitrogen compounds in crude oils are complex and distillation may give rise to nitrogen compounds. The nitrogen content in most crudes is very low and does not exceed 0.1 %.In some heavy crudes, however, the nitrogen content may reach up to 0.9 % wt. Nitrogen compounds are more thermally stable than sulfur compounds and accordingly are concentrated in heavier petroleum fractions and residues. Nitrogen has to be removed from crude oil because Nitrogen impurities in hydrocarbon fuels have a severe environmental impact resulting from the contribution nitrogen oxides (produced during combustion) to acid rain; Nitrogen impurities are effective catalyst poisons that slow down the processing of crude oil.
Nitrogen compounds are removed by hydrodenitrogenation to ammonia. For example, pyridine is denitrogenated to ammonia and pentane: Nitrogen compounds in crudes may generally be classified into basic and non-basic categories. Basic nitrogen compounds are mainly those having a pyridine ring, and the non-basic compounds have a pyrrole structure. Both pyridine and pyrrole are stable compounds due to their aromatic nature.
The following are examples of organic nitrogen compounds.
5.2.3 Oxygen Components Oxygen compounds in the crude oils are more complex than the sulfur types. However; their presence in petroleum streams is not poisonous to processing catalysts. Many of oxygen compounds found in crude oils are weakly acidic. They are carboxylic acids -C(=O)OH or -COOH, cresylic acid, phenol, and naphthenic acid. A cresol molecule has a methyl group substituted onto phenol molecule. Cresylic acid is any of several acids derived from petroleum and coal tar that boil above 204 C, contain varying amounts of xylene and cresol. Naphthenic acids are mainly cyclopentane and cyclohexane derivatives having a carboxylic (-COOH).
Naphthenic acids in the naphtha fraction have a special commercial importance and can be extracted by using dilute caustic solutions. Non acidic oxygen compounds such as esters, ketones, and amide are less valuable than acidic compounds. The total acid content of most crudes is generally low, but may reach as much as 3%, as in some California crudes The following shows some of the oxygen compounds commonly found in crude oils.
Acidic Oxygen Compounds Non-Acidic Oxygen Compounds
5. 3 Metallic Components Many metals occur in the crude oils. Some of the more abundant are sodium (Na), calcium (Ca), magnesium (Mg), aluminum (Al), iron (Fe), vanadium (V) and nickel (Ni). They are present either as inorganic salts, such as sodium and magnesium chloride, or in the form of organometallic compounds, such as those of nickel and vanadium. Calcium and magnesium can form salts or soaps with carboxylic acids. These compounds acts as emulsifiers and their presence is undesirable. Although metals in crudes are found in trace amounts, their presence is harmful and should be removed.
When crude oil is processed sodium and magnesium chlorides produce hydrochloric acids, which is very corrosive. Desalting crude oils is a necessary step to reduce these salts. Vanadium and nickel are poisons to many catalysts and should be reduced to very low levels. Solvent extraction processes used to reduce the concentration of heavy metals in petroleum residues.