PGP Lectures 3 & 4: Hydrocarbons

For example, butane (C4H10) 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 o...

For example, butane (C4H10) 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. 6.1.1 Alkanes (Paraffins) Uses: Paraffins are widely used in various applications, including fuels (i.e., gasoline and kerosene), lubricants, and as raw materials in chemical synthesis. Paraffin wax, derived from higher alkanes, is used in candles, cosmetics, and packaging. Sources: Paraffins can be derived from natural sources (like crude oil and natural gas) or can be produced through chemical processes.  6.1.2 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 = CnH2n. Five-membered (cyclopentane) and six membered (cyclohexane) rings are especially abundant in nature and have received special attention. Cyclo-methane and cyclo-ethane 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 C6H6, and the last compound can dehydrogenate to toluene C7H8 during processing. Heavier petroleum fractions such as kerosene 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. How do Cycloparaffins differ from Paraffins? Paraffins Cycloparaffins closed-chain (cyclic) hydrocarbons with a open-chain (acyclic) general formula of CnH2n​. The carbon hydrocarbons with a atoms forma ring structure, which means Sturcutur general formula that the number of hydrogen atoms is of CnH2n+2​. They have reduced because some carbon atoms are straight or branched chains bonded to each other instead. Generally, cycloparaffins can exhibit different reactivity compared to Reactivity paraffins due to strain in their ring structure, leading to different reaction pathways. Cycloparaffins tend to have different physical properties compared to their linear counterparts, such as boiling points and melting points. Properties The presence of a ring can lead to different van der Waals interactions and thus affect the physical characteristics. 6.1.3 Olefins The olefins, are unsaturated hydrocarbons. The olefins are probably not present in crude petroleum, but they are found in cracked products. 6.1.3 Olefins Ethylene is the most important olefin for producing petrochemicals, and therefore, many sources have been sought for its production. Why?  6.1.3 Olefins Because it serves as a fundamental building block in the synthesis of a wide range of chemicals and materials. Its extensive use in the production of plastics, solvents, and other commercial chemicals, largely due to its high reactivity and versatility, makes it a key player in the petrochemical industry. Ethylene is also referred to as the "king of petrochemicals," 6.1.3 Olefins Olefins are characterized by their higher reactivities compared to paraffinic hydrocarbons.  6.1.4 Aromatic Compounds Aromatic hydrocarbons are unsaturated hydrocarbons which have one or more benzene ring (have one or more planar six-carbon rings called benzene rings C6H6), to which hydrogen atoms are attached according to the formula CnHn. 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.  6.1.4 Aromatic Compounds They are important precursors for many commercial chemicals and polymers such as phenol, trinitrotoluene (TNT), nylons, and plastics. Aromatic compounds are characterized by having a stable ring structure. Accordingly, they do not easily add to reagents such as halogens and acids as do alkenes. 6.1.4 Aromatic Compounds Aromatic hydrocarbons are susceptible, however, to substitution reactions in presence of a catalyst. Aromatic hydrocarbons are generally nonpolar. They are not soluble in water, but they dissolve in organic solvents such as hexane, diethyl ether, and carbon tetrachloride.  Toluene (C7H8) and xylene (C8H10) are also mononuclear aromatic compounds found in variable amounts in crude oils. Benzene, toluene (C7H8), and xylenes (C8H10) (BTX) are important petrochemical intermediates as well as valuable gasoline components. BTX compounds are important in the petrochemical industry due to their utility as building blocks for a wide range of chemical products as well as their applications in solvents and fuels. They are commonly obtained from crude oil through refining processes such as catalytic reforming  Separating BTX aromatics from crude oil distillates is not feasible because they are present in low concentrations. The low concentration makes it difficult to isolate them effectively without significant energy and resource expenditure, making the process economically unviable. Actually the light aromatics (BTX) do not exceed even 5% of crudes of general nature. Enriching a naphtha fraction with these aromatics is possible through a catalytic reforming process. Aromatic hydrocarbons in crude oi  The table shows the percentage by weight of hydrocarbons in the crude oils Table 2.1 Composition by weight of hydrocarbons in the crude oil Hydrocarbons Average Paraffins 30 % Napthenes 49 % Aromatics 15 % Asphaltic 6%  6.2 Non-hydrocarbon Compounds 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.  6.2.1 Sulfur Components Sulfur in crude oils is mainly present in the form of organo- sulfur compounds. Hydrogen sulfide (H2S) 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. Aliphatic & Aromatic Hydrocarbons. 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.  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. The more Sulphur in the crude, the more reduction in the market value of it.  6.2.2 Nitrogen Components Organic nitrogen compounds occur in crude oils either in a simple heterocyclic form as in pyridine (C5H5N) and pyrrole (C4H5N), or in a complex structure as in porphyrin. Prophyrins are a group of organic compounds which occur in nature. One of the best known is heme, the pigment in red blood cells. In vertebrates, an essential member of the porphyrin group is heme, which is a component of hemoproteins, whose functions include carrying oxygen…  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.  6.2.3 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  6. 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. Next Lecture Classification of Crude Oil 35 Assignment Why are aromatic compounds characterized by having a stable ring structure? Due to October 03, 2024, at 11:59 PM Note: 1- No more than 30 words. 2- Be careful about copying each other. 36  References Speight, J.G., 2001. Handbook of petroleum analysis. Coker, A.K., 2018. Petroleum Refining Design and Applications Handbook, Volume 1. John Wiley & Sons. 37 THANKS 38