PGP Lectures 3 & 4 PDF

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 har...

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.