Introduction to Petroleum and Coal Chemistry (CHM225) PDF

# INTRODUCTION TO PETROLEUM AND COAL CHEMISTRY (CHM225) ## Introduction Petroleum is a dark colored viscous oil found deep in earth's crust. Chemically, it is a mixture of various types of hydrocarbons along with some oxygen, sulfur, and nitrogen containing compounds. The average composition of crude petroleum is given below. | Element | Percentage | |---|---| | C | 80-85% | | S | 0.1-0.5% | | H | 10-14% | | O and N | Negligible | Petroleum along with oil and coal is classified as fossil fuel. Fossil fuels are formed when sea plants and animals die and the remains become buried under several thousands feet of silt, sand, or mud. Fossil fuels take millions of years to form, and hence petroleum is considered as a non-renewable energy source, i.e. sources that cannot be renewed and will eventually run out. Fossils are drawn on finite resources that will eventually dwindle, becoming too expensive or too environmentally damaging to retrieve. Petroleum in its natural form when first collected is usually named crude oil and it can be clear, green, or black and may be either thin like gasoline or thick like tar. There are several major oil producing regions around the globe. "The Kuwait and Saudi Arabia" crude oil fields are the largest, although the Middle East oil from other countries in the region such as Iran and Iraq, also make up a significant part of world production figures. The North Sea oil fields are still fairly full and are arguably the second most influential oil field in economic terms. Texas, the World's major oil region, is now almost completely dry. In 1859, Edwin Drake Sauk, the first known oil well in Pennsylvania since then, oil and petroleum production figures grew exponentially. Originally, the primary use of petroleum was as a lighting fuel once it had been distilled and turned into kerosene. When Edison opened the world's first electricity-generating plant in 1882, the demand for kerosene began to drop. Most people tend to believe that petroleum is mostly used to power internal combustion engines in the form of gasoline or petrol. It is used for a vast array of applications, although our automobiles and other forms of transport do consume the highest quantity of petroleum. In its thickest form, the almost black petroleum, is named bitumen. This is used for paving roads, for the back top is an excellent water repellent and is used for roofing. The world has limited supply of petroleum and current estimates tell us that within the next few decades mankind will have completely depleted this valuable natural resource. Although measures have been taken to ensure that there are cheap renewable alternative sources. It is still obvious that mankind face a serious problem when petroleum supplies finally run out. ## Origin of Petroleum Several theories have been put forward to explain the formation of petroleum. However, only 3 have been widely accepted and they are given below: ### 1. Carbide Theory: According to this theory, hydrocarbons present in petroleum are formed by the action of water on inorganic carbides organic carbides in turn are formed by the reaction of metal and carbon under high temperature and pressure inside the earth. $Ca + 2C \xrightarrow{High\ temp\ and\ high\ pressure} Ca C_2;\ Calcium\ carbide$ $Ca C_2 + 2H_2O \rightarrow Ca(OH)2 + C_2H_2$ $acetylene$ $4Al+3C\rightarrow Al_4C_3;\ Aluminum\ carbide$ $Al_4C_3 + 12H_2O\rightarrow 4Al(OH)3 + 3CH_4$ $Methane$ These lower hydrocarbons then undergo hydrogenation and polymerization to give various types of hydrocarbons (paraffin, aromatic and cycloparaffins) (a) $C_2H_2 + H_2 \rightarrow C_2H_4$ $Acetylene \rightarrow ethylene$ $C_2H_4 + H_2 \rightarrow C_2 H_6$ $ethylene \rightarrow ethane$ (b) $3C_2 H_2 \xrightarrow{polymerization} C_6H_6$ $acetylene \rightarrow benzene$ (c) $3C_2 H_4 \xrightarrow{polymerization} C_6H_{12}$ $ethylene \rightarrow cyclohexane$ (d) $2C_2 H4 \rightarrow CH_3CH = CH CH_3$ $Butene-2$ $CH_3CH = CH CH_3 + H_2 \rightarrow CH_3CH_2 CH_2CH_3$ $butane$ However, this theory failed to explain the following facts: 1. Presence of nitrogen and sulfur compounds 2. Presence of chlorophyll and haemlin 3. Presence of optically active compounds ### 2. Engler's Theory: According to Engler, petroleum is of animal origin. Engler (1900) suggested that petroleum is formed by the decay and decomposition of marine animals under high pressure and temperature. The sulfur dioxide gas given out by the volcanoes beside the seaside kills the fish and other sea animals which go on piling beside the volcano. After hundreds of years, these animals start decomposing under the influence of high pressure and temperature to form petroleum. Engler’s theory is supported by the following facts. 1. Experimental destructive decomposition of fish oil and other animal fats under high pressure and temperature give a product similar to natural petroleum. 2. Presence of brine or sea water together with petroleum. 3. Presence of nitrogen and sulfur compounds 4. Presence of optically active compounds 5. Presence of fossils (the remains of animals and plants which dies long ago) in the petroleum area. However, this theory fails to account for the presence of chlorophyll (green colouring matter of plants) in petroleum. Moreover, it does not explain the presence of coal deposits in the vicinity of the oil fields. ### 3. Modern theory: According to modern views, petroleum is believed to be formed by the decay and decomposition of marine animals as well as that of vegetable organism of the prehistoric forests, i.e. it is animal as well as plant origin. It is thought that due to some upheavals or earthquakes, these prehistoric forests and sea animals got buried under the crust of earth. Due to prolonged action of high temperature and pressure in the interior of earth for ages, the biological matter decomposed into petroleum. The modern theory explains most of the facts; 1. It explains the presence of brine and coal in the vicinity of petroleum. Presence of brine explained on the basis of animal origin while the presence of coal is explained by plant origin 2. It explains the presence of N and S compounds and also of chlorophyll and haemlin. It also explains the presence of optically active compounds. ## Occurrence Petroleum is found well deep below the earth. The depth may be 5000 feet or more varying from place to place. The oil is found deep below the impervious rock floating over salt water or brine. It is often associated with natural gas (mainly methane) which exerts pressure on the oil surface and drives it out with great velocity through natural openings. America is the largest oil producing country of the world followed by Russia, Venezuela, Mexico, Rumania, etc. ## Desalting of Crude Oil in Refinery Crude oil introduced to refinery processing contains many undesirable impurities such as sand, inorganic salts, drilling mud, polymer, corrosion by-product etc. The salt content in the crude oil varies depending on source of the crude oil. When a mixture from many crude oil sources is processed in refinery, the salt content can vary greatly. The salts that are most frequently present in crude oil are calcium, sodium and magnesium chloride. The purpose of desalting is to remove these undesirable impurities, especially salts and water, from the crude oil prior to distillation. If these compounds are not removed from the oil, several problems can arise in the refining process. The salt can be removed by a process unit called Desalter. The most concerns of the impurities in crude oil are: 1. The inorganic salts can be decomposed in the crude oil pre-heat exchangers and heaters. As a result, hydrogen chloride gas is formed which condenses to liquid HCl at overhead system of distillation column, that may cause serious corrosion of equipment. 2. To avoid corrosion due to salts in the crude oil, corrosion control can be used. But the by-product from the corrosion control of oil field equipment consists of particulate iron sulfide and oxide. Precipitation of these materials can cause plugging of heat exchanger trains, tower trays, heater tubes etc. In addition, these materials can cause corrosion to any surface they precipitate on. 3. The sand and silt can cause significant damage due to abrasion or erosion to pumps, pipe lines, etc. 4. The calcium naphthanate compound in the crude unit residue stream, if not removed can result in the production of lower grade coke and deactivation of catalyst of FCC (Fluid catalytic cracking) unit. ## THE DESALTING PROCESS Crude oil passes through the cold preheat train and is then pumped to the Desalters by crude change pumps. The recycled water from the desalters is injected in the crude oil containing sediments and produces salt water. This fluid enters in the static mixer which is a crude/water dispenser, maximizing the interfacial surface area for optimal contact between both liquids. The wash water shall be injected as near as possible the emulsifying device, to avoid a first separation with crude oil. Wash water can come from various sources including relatively high salt sea water, stripping water etc. the static mixers are installed upstream the emulsifying devices to improve the contact between the salt in the crude oil and the wash water injected in the line. The oil/water mixture is homogeneously emulsified in the emulsifying device. ## DESALTING PROCESS The emulsifying device (as a value) is used to emulsify the dilution water injected upstream in the oil. The emulsification is important for contact between the salty production water contained in the oil and the wash water. Then the emulsion enters the desalters where it separates into two phases by electrostatic coalescence. The electrostatic coalescence is induced by the polarization effects resulting from an external electric source. Polarization of water droplets pull them out from oil-water emulsion phase. Salt being dissolved in these water droplets, is also separated along the way. The produced water is discharged to the water treatment system (effluent water). It can also be used as wash water from mud washing process during operation. A desalting unit can be designed with single stages or two stages. In the refineries, the two stages desalting system is normally applied, that consist of 2 electrostatic coalescers (Desalter). ## Benefits of crude oil desalting 1. Increase crude throughput 2. Less plugging scaling, coking of heat exchanger and furnace tuber. 3. Less corrosion in exchanger, fractionators, pipelines etc. 4. Better corrosion control in CDU (Crude oil Distillation Unit) overhead. 5. Less erosion by solids in control/valves, exchanger, furnace, pumps. 6. Saving of oil from slops from waste oil. By removing the suspended solids, they are not carried into the burner and eventually flue gas, where they would cause problems with environment; complain such as flue gas opacity norms. ## MINING OF PETROLEUM The natural gas present over the oil exerts pressure on the crude oil and drives it out with great velocity through the natural openings. In case of artificial drilling, mines are bored till the oil-bearing region is reached. Sometimes the oil rushes out through these holes (bores) because of the pressure of the natural gas but as the pressure of gas subsides air pressure is applied through pumps to force the oil out of the well. The crude oil thus obtained is conveyed by pipelines to refineries which are usually located at a log distance from the oil field because of the following two reasons; 1. Safety Purpose: Oil fields are very susceptible to fire and hence distillation which involves the use of high temperature must be carried out at a distant place. 2. Cost Purpose: Refineries are located at points well connected with consuming or exporting centres to avoid the unnecessary cost of transportation of the refined products. ## Refining of Petroleum Petroleum comes from the ground as a viscous, dark coloured liquid containing a lot of impurities (e.g sand, brine, etc). It has unpleasant smell because of the presence of sulphur compounds. Technically it is called crude oil. Crude oil is a mixture of hydrocarbons (aliphatic, alicyclic and aromatic) along with small amount of nitrogen, sulphur and oxygen compounds. Crude oil as such is of little importance. However, it can be separated into a number of useful fractions by fractional distillation. This process of dividing petroleum into different useful fractions with different boiling ranges is called refining of petroleum. The fractional distillation of petroleum is carried out continuously in a specially designed tall fractionating tower or column made of steel. It is provided with a large number of horizontal stainless steel trays. Each tray is provided with a small chimney covered with a loose cap. The crude oil heated to about 400°C in an iron-steel is introduced near the bottom of the tower. The tower is hot towards the lower end and comparatively cooler at the upper end. As the vapours of the oil rise up the fractionating column, they become cooler and get condensed at the trays. The highest boiling fraction condenses first at the bottom and the lowest boiling fraction at the top. Outlets are provided at the side of the column at suitable heights to withdraw a number of fractions. The names, approximate composition, boiling range and uses of the various fractions are tabulated in the table below. The uncondensed gases escape at the head of the column. These are liquefied to give the modern domestic fuel under the name of liquefied petroleum gases (LPG). LPG consisting mainly of methane, ethane, propane, and butane is supplied in cylinders under pressure. The residue left in the resort is a black and tarry mass termed as asphalt or petroleum coke. ## COMPOSITION AND USES OF MAIN PETROLEUM FRACTION | Name of the Fractions | Temperature Range of Condensation | Approximate Composition | Uses | |---|---|---|---| | Uncondensed gas | Up to 30°C | $C_1-C_5$ | Domestic fuel, synthesis of organic chemicals, production of carbon black | | Gasoline petrol on fractionation gives: (i) Petroleum ether (ii) Gasoline or petrol | 30-200-C 30-80°C 80-200°C | $C_5-C_{10}$ $C_5-C_6$ $C_5-C_{10}$ | As a solvent for fats oils, vanishes and rubber. Fuel for the internal combustion engines of automobiles and aeroplane, solvent and dry cleaning. | | Kerosene | 200-300°C | $C_{10}-C_{16}$ | Illuminant, fuel for stores, for making gas oil | | Heavy oil on fractionation gives: (i) Gas oil (ii) Fuel oil (iii) Diesel oil | 300-350 | $C_{16} - C_{18}$ | Fuel diesel engines, for conversion to gasoline by cracking | | Residual oil, on fractionation gives: (i) Lubricating oil (ii) Paraffin (iii) Petroleum Jelly (vase Line) | 350-400°C $C_{18} - C_{40}$ $C_{18} - C_{20}$ $C_{20} - C_{30}$ $C_{30} - C_{40}$ | Lubrication candles, boot polishes, wax paper, taprolin, cloth and electrical Insulation. In medicines, cosmetics, toilets and lubricants | | Residue which may either be: (i) Pitch (Asphalt) (ii) Or petroleum coke | | | Used in water proofing or roots, road making stabilizer for wood and metal. Used as fuel. Used as fuel | ## PURIFICATION OF GASOLINE AND KEROSENE Gasoline (either from crude oil/or synthetic) and kerosene contain some undesirable unsaturated olefins and sulphur compounds. The unsaturated hydrocarbons are oxidized and polymerized, and hence cause gum and sludge on storing. On the other hand sulfur compounds lead to corrosion of internal combustion engine and also affect tetra-ethyl lead which is added to gasoline to obtain a good quality petro. So these undesirable contents (olefins and sulfur compounds) must be removed from gasoline and kerosene. The following processes are used for this purpose. 1. Treatment with Concentrated Sulphuric Acid: The gasoline or kerosene oil fraction is agitated with sulphuric acid when aromatic compounds like thiophene etc are converted into their sulphonic acids. The contents are allowed to settle and the upper layer is withdrawn and treated with alkali to remove excess of acid. It is finally washed with water. 2. Treatment with Liquid Sulphur dioxide: During the treatment with H2SO4, some aromatic compounds may be destroyed. However, if the purification is done by sulphur dioxide (SO2) no loss of aromatic hydrocarbon occurs. The fraction to be purified, especially kerosene, is shaken with liquid sulfur dioxide and allowed to settle. The two layers: lower layer (extract) containing liquid SO2 with aromatic compounds and the upper (raffinate) containing kerosene and the rest of SO2 are separated. SO2 may be removed from the two layers easily by heating. 3. Sweetening process (Doctors Treatment): Mercaptans are removed by adding sodium plumbite in the presence of alkali. Free sulfur, if not present in oil in sufficient quantity, is also added during the treatment. Mercaptans are converted into disulphides as below. $RSH + Na2PbO2 + S \rightarrow R.S.S.R + PbS + 2NaOH$ $Thiol \rightarrow sodium\ plumbite$ 4. Treatment with Adsorbents: Olefins and colouring matter of gasoline are removed by passing it over adsorbents like kieselguhr, fuller's earth, clay, etc which adsorbs preferentially only the colour and olefins. The olefins and coloured compounds are removed. The purified gasoline is blended with additives to make is suitable for use in internal combustion engine. Some quantities (0.001% of the gasoline) of inhibitors or anti-oxidants are added into the purified gasoline in order to retard oxidation of olafins (i.e olefin peroxides) which causes the formation of gums on storage. Common inhibitors are butlyamino phenols, benzylamino phenols and phenylene diamines. ## Purification of Diesel and lubricating Oil Diesel oil containing sulphur compound is treated by hydro-desulphurization in which the oil vapour is hydrogenated under pressure at 400°C in the presence of cobalt-molybdenum catalyst supported on alumina. Hydro-desulphurization removes sulphur compounds in the diesel as hydrogen sulphide and simultaneously decreases the proportion of unsaturated hydrocarbons. Hydrogen sulphide is removed from the condensed product by extraction with diethanolamine and the solvent can be removed for recycling by fractional distillation. Lubricating oils are obtained by vacuum distillation of heavy residues of paraffin base crude oil. The lubricating oils are dewaxed and extracted with phenol or furfural to remove aromatics followed by liquid propane to remove aliphatics. ## PROCESSES FOR INCREASING THE YIELD OF GASOLINE (PETROL) Years ago, kerosene was considered to be the most important fraction of petroleum, but with the growth of automobile industry, the consumption of petrol increased tremendously. At present the demand for petrol is much greater than other fractions of petroleum. Therefore, it has become necessary to develop some ways of increasing the production of petrol. The following types of approaches were used. 1. Cracking 2. By the polymerization and alkylation of lower olefins (C2 - C4) obtained during cracking 3. By synthetic methods (synthetic petrol). ## Cracking Cracking may be defined as the process of the decomposition of the higher molecular weight hydrocarbons having high boiling point to the lower molecular weight hydrocarbon of low boiling point. It is the name given to the breaking up of large hydrocarbon. Hydrocarbons with shorter carbon chains are more useful. So sometimes we cut longer chains into shorter ones using the process of cracking. A long alkane is heated, vapourised and passed over a ceramic catalyst to produce a shorter alkane and an alkene. $C_8H_{18}\xrightarrow{cracking}C_4H_{10} + C_4H_8$ $Octane \rightarrow Butane + Butene$ $C_{10}H_{22}\xrightarrow{cracking}C_5H_{12} + C_5H_{10}$ $decane \rightarrow n-pentane + n-pentene$ $(b.p. \ 174°C) \rightarrow (b.p.\ 36.1°C)$ $C_{10}H_{22}\xrightarrow{cracking}C_7H_{16} + C_3H_6$ $Decane \rightarrow Heptane + Propene$ $C_{15}H_{32} \rightarrow 2C_2H_4 + C_3H_6 + C_8H_{18}$ $pentadecane \rightarrow Ethene + Propene + Octane$ For instance, the ethene and propene are important materials for making plastics or producing other organic chemicals. The octane is one of the molecules found in petroleum (gasoline). The source of the large hydrocarbon molecules is often the naphtha fraction or the gas oil fraction from the fractional distillation of crude oil (petroleum). These fractions are obtained from crude oil as liquids, but are re-vaporised before cracking. Nearly 50% of today's gasoline is obtained by cracking. It is important to note that the petrol obtained by cracking is of superior quality than that of straight run gasoline because the process of cracking also increase the octane number of petrol and this reduces its anti knocking properties. The process of cracking involves the following chemicals changes. 1. Higher hydrocarbons are converted to lower hydrocarbons by C-C cleavage. The boiling point of all the products is lower than the initial alkane. 2. Straight chain alkanes are converted to branched chain hydrocarbons (isomerization). 3. Saturated hydrocarbons are converted to unsaturated hydrocarbons 4. Ring closure (cyclization) of aliphatic alkanes may occur. Cracking process are of two types; 1. Thermal cracking 2. Catalytic cracking ## Thermal Cracking: When cracking is carried out simply by the application of heat and pressure, it is known as thermal cracking. It may be carried out in liquid phase or in the vapour phase. 1. Liquid Phase Thermal Cracking: In this process, the heavy oil or residual oil is cracked at a temperature of 475-530°C under high pressure, usually 7-10 atmospheric pressure to keep the reaction product in liquid state. The cracked products are separated in a fractionating column. Three important fraction (products) are obtained in the following percentages. - Cracked gasoline: 30-35% - Cracking gases: 10-45% - Cracking residue (cracked fuel oil) 50-55% The cracked gasoline has an octane number of about 70. The cracking gases (alkanes and alkenes, viz ethene, ethylene, propane, propylene, butane etc) after separation from each other constitute the raw materials for petrochemicals. The cracking residue is used as a fuel (fuel oil). 2. Vapour Phase Thermal Cracking: In this type of thermal cracking petroleum fraction of low boiling range like kerosene oil or other oil with similar boiling range (but not heavy or residual oil) is heated at a temperature of 670 – 720°C under atmospheric pressure. In this type, although the yield of gasoline is good (approximately 70%), the octane number is again about 70. ## Catalytic Cracking: When cracking is done in the presence of a catalyst, usually a mixture of silicate, it is known as catalytic cracking. Catalytic cracking is superior in many respects to thermal cracking. 1. It does not require high pressure 2. The yield of gasoline is high because of the formation of more branched isoparaffins (like isobutene) and aromatics (like benzene). 3. The presence of isoparaffins and aromatic further improves the antiknock quality of gasoline. Cracked gasoline obtained by this method is found to possess an octane number of 70-80. 4. About 70% of the raw materials by weight is converted into gasoline, the yield of gaseous products (alkanes and alkenes) is about 12-15% while only 4 - 6% of the raw material is converted into coke. 5. Feedstocks with a high sulphur content are permissible since some desulphurization with the formation of hydrogen sulphide takes place 6. No external fuel is necessary since the heat required for cracking is derived from the coal embedded in the catalyst. Products from catalytic cracking units are also more stable due to a lower olefin content in the liquid products. This reflects a higher hydrogen transfer activities, which needs to more saturated products from delayed coking units, for example. Of the various catalytic cracking processes, fluid catalytic cracking is the most modern and widely used. In the fluid flow cat-cracker, there is a large cylindrical container know as regenerator with a conical top and bottom. A second container, longer and slimmer, is the reactor. The process is a continuous one with a sustained input of heavy oil fractions and continuous output of petrol while the catalyst itself circulates continuously between the reactor and the regenerator. The most satisfactory catalysts are activated synthetic or natural aluminium silicates, but synthetic ones are preferred since they are more active and possess greater mechanical strength and thermal stability than the natural ones. Incorporating zeolites (crystalline alumina-silica) with silica/alumina catalysts improves selectivity towards aromatic. An important structure feature of zeolites is the presence of holes in the crystal lattice, which are formed by the silica-alumina tetrahedral. The feedstock which is generally a fraction obtained by vacuum distillation of primary distillation residue or sometimes a heavy gas oil is pumped through pre-heaters and then along a feed pipe to a vertical pipe (the generator stand pipe) where it meets the catalyst heated to about 600°C. The hot catalyst causes the oil to vapourize instantly. The mixture of oil and catalyst power passes into the reactor, in which bulk of the catalyst settles down as a fluid bed. Cracking starts immediately on contact with the catalyst and the reaction continues throughout the bed which is held at a temperature of about 480°. The cracked vapours together with a small amount of catalysts pass though internal cyclone separators, in which bulk of the catalyst is removed and returned to the reactor. From the reactor, the vapours and the remaining catalyst pass to a fractionating column in which gasoline and gases leave the top of the column while the light and heavy gas fractions are taken off. From two different outlets, and the residual heavy oil with some catalyst passes to a settler from which the cracked fuel oil is removed in quantitative amounts and the remaining sherry of oil catalyst is recycled to the reactor. In the course of cracking in the reactor, the catalyst, although not taking part in the chemical reactions, gradually gets covered with carbon which deactivated it. This spent catalyst falls to the bottom of the reactor, where it passes through a pipe and meets a stream of which blows it to the regenerator, in which the carbon is burnt off at high temperature of about 600°C. From the regenerator the very hot catalyst is led to meet the oil feed. ## The main advantages of catalytic cracking over thermal cracking 1. Good products quality 2. Less coke formation 3. Temperature and pressure uniformity in operation ## Hydrocracking: Hydrocracking is essentially catalytic cracking in the presence of hydrogen. It is one of the most versatile petroleum refining schemes adopted to process low value stocks. Generally, the feedstocks are not suitable for catalytic cracking because of their high metal sulphur, nitrogen and asphaltene contents. The process can also use feeds with high aromatic content. Products from hydrocracking processes lack olefinic hydrocarbons. The product slate ranges from light hydrocarbon gases to gasolines to residues. Depending on the operation variables, the process could be adapted for maximizing gasoline, jet fuel or diesel production. Basically. Catalytic hydrocracking involves three primary chemical processes. 1. Cracking of high-boiling, high molecular weight hydrocarbons found in petroleum crude oil into lower-boiling, lower molecular weight hydrocarbons. 2. Hydrogenating unsaturated hydrocarbons (whether present in the original feedstock or formed during the cracking of the high-boing, high molecular weight feedstock hydrocarbons) to obtain saturated hydrocarbons usually referred to as paraffins or alkanes. 3. Hydrogenating any sulphur, nitrogen or oxygen compounds in the original feedstock into gaseous hydrogen sulphides, ammonia and water. The following four reactions are provided as examples of the complex reactions involved in the primary processes. * Reaction 1: Addition of hydrogen to aromatics converts them into hydrogenated rings. These are the readily cracked using acid catalyst. * Reaction 2: Acid catalyst cracking opens paraffins rings, breaks larger paraffins into smaller pieces and creates double bonds. * Reaction 3: Addition of hydrogen to olefinic double bond to obtain paraffins * Reaction 4: Isomerization of branched and straight - chain paraffins Hydrocracking catalysts consist of active metals on solid, acidic supports and have a dual function specifically a cracking function and a hydrogenation function. The cracking function is provided by the acid catalyst support and the hydrogenation function is provided by the metals. The solid acidic support consists of amorphous oxides such as silica-alumina, crystalline zeolite or a mixture of amorphous oxides and crystalline zeolite. Cracking and isomerization reactions (reactions 2 & 4 above) take place on acidic support. Metals provide the hydrogenation reactions (reations 1 and 3 above). The metals that provide the hydrogenation functions can be noble-metals e.g. palladium and platinum or the base metals (i.e non-noble metals) e.g. molybdenum, tungsten, cobalt or nickel. Catalysts life cycle has a major impact on the economics of hydrocracking. Cycles can be as short as 1 year or as long as 5 years. Two years are typical. The main products from hydrocracking are jet fuel, diesel, high octane petrol fractions and LPG with very low sulphur and other contaminant content. As a hydrogen-addition process, hydrocracking provides high yields of valuable distillates without producing low-grade by-products, e.g. heavy oils, gas or coke. Hydrocracking requires high hydrogen and high pressure. ## Polymerization; Combination of two or more molecules of the same molecule to give a higher molecule without eliminating of anything is known as polymerization. Lower olefins (containing 2 to 4 carbon atoms) obtained during cracking gaseous fraction can be polymerized in the presence of catalyst (H2SO4 or H3PO4) to give higher olefins which can be hydrogenated as gasoline hydrocarbons. The best known example is that of isobutene. Isobutene is passed into 60-70% sulphuric acid at 20-25% and the resulting solution is heated to about 100°C to give di-isobutene as the major product along with small amount of tri-isobutene and higher polymers. $CH_2 CH_3$ $CH_3$ $CH_3-C = CH - C - CH_3 \rightarrow 2CH_3-C= CH_2$ $CH_2$ $$ $2, 4, 4-trimethyl-2-pentene\rightarrow isobutene$ $CH_3$ $CH_2$ $ CH_3-CH-CH_2-C - CH_3 \xrightarrow{Ni\ H_2} CH_2=C-CH_2-C-CH_3 $ $CH_2$ $CH_2 $ $$ $2, 4, 4-trimethyl pentane (iso-octane) \rightarrow 2, 4, 4- trimethyl-1-pentene $ The neutralized hydrocarbon layer is fractionally distilled to separate the dimer from higher polymers. The dimer fraction is catalytically hydrogenated under pressure over nickel as a catalyst at 160° to give iso-octane. However, the above process was soon replaced by co-polymerising process in which a mixture of isobutene and n-butane in 67-92% sulphur acid is heated at 75-100° followed by hydrogenation to give a quantitative yield of the product with high octane number of 97-98. Thus polymerization of olefins gives high octane number petrol. ## Alkylation: Replacement of a hydrogen atom of the hydrocarbon by an alkyl group is known as alkylation. The most important example is the reaction of isobutene and isobutane in the presence of anhydrous liquid hydrogen fluoride at room temperature to give iso-octane. $CH_3$ $CH_2$ $CH_2$ $CH_3-CH - CH_3 + CH_2 = C – CH_3\xrightarrow{anhydrous\ HF} CH_3-CH-CH_2-C-CH_3$ $Isobutane \rightarrow isobutene \rightarrow iso-octane$ $ CH_3$ Thus alkylation also gives petrol of high octane number. ## Synthetic Petrol: The fear that the world's supplies of natural petroleum may not last long has compelled the chemists to think of a synthetic product resembling natural gasoline and the product was named synthetic petrol. The chief methods for preparing synthetic petrol are given below. ### 1. Bergius Process: This method uses coal as the raw material which is a mixture of high molecule organic compounds deficient in hydrogen. In practice the finely powdered coal is made into a paste with heavy oil or coal tar and then a catalysts composed of organic compound of tin is added. The coal past along with the catalyst is prepared and then pumped to the converter where it is heated to 400 – 450°C under 200-250atm in the presence of hydrogen. $catalyst$ $Coal dust + H_2 \xrightarrow{450°C,\ 200atm} mixture of Hydrocarbons$ $ (suspended\ in\ Heavy\ oil) \xrightarrow{(i)H_2\ (ii)crackimg} crude\ oil$ Hydrogenation takes place to form higher saturated compounds which undergo cracking and hydrogenation processes to yield mixture of alkanes. Thus the vapours leaving the converter upon condensation give crude oil or synthetic petroleum. Crude oil is fractionally distilled to give (i) petrol (ii) middle oil and (iii) heavy oil. The middle oil is again hydrogenated in the vapour phase in presence of solid catalyst to give more gasoline. Actually, the processing of middle oil gives four times the gasoline obtained by the primary hydrogenation to the heavy oil is again used for making a paste with fresh coal dust. ### 2. Fischer-Tropsch process: The raw material used in this process is hard coke. This is converted into a mixture of carbon monoxide and hydrogen by passing steam over red hot coke (water gas reaction). $C + H_2O\xrightarrow{12000} + CO + H_2$ $water\ gas$ From a part of the water gas, carbon monoxide is removed by converting it into carbon dioxide which is obtained by passing the water gas with extra steam over a promoted (reduced) iron oxide catalyst at 450°. $CO + H_2O \xrightarrow{Fe} CO_2 + H_2$ The residual carbon dioxide is again removed by absorption is ammonical cuprous formats solution. Hydrogen so obtained is mixed with the rest of the water gas in a ratio of synthesis gas i.e hydrogen and carbon monoxide in a ratio of 2:1. The synthesis gas is passed at atmospheric pressure over a cobalt-thorium oxide catalyst on kieselgular at 180-200°. The reaction products are predominantly straight chain paraffins and olefins, as show below. $(2n + 1) H_2 + nCO \rightarrow C_nH_{2n+2} + nH_2O$ $n\ paraffins$ $2nH_2 + n CO \rightarrow C_nH_{2n} + nH_2O$ $synthesis\ gas \rightarrow olefins$ Although the fischer-Tropsch process requires a cheap raw material (coke and steam). It cannot compete economically with petroleum as a source of gasoline. Hence in Germany the process is mainly employed as a source of hydrocarbons for chemical production. ## KNOCKING AND OCTANE NUMBER OF GASOLINE The important properties of a fuel for use in automobiles include the boiling range, startability, vapour pressure and resistance to knocking. The latter property is particularly important. In an internal combustion engine, the mixture of fuel (gasoline vapour) and air is highly compressed before it is ignited in order to have maximum efficiency. Ignition of a mixture of gasoline vapour and air by a spark causes a compression wave to pass through the un-burnt gas mixture. Under certain conditions, this wave can compress the mixture to above its self-ignition temperature, resulting in explosive burning and the characteristic metallic sound called knocking and causing fuel wastage and increased engine wear. Under high compression, it was observed that certain types of straight-run gasoline caused knocking while under the same conditions, cracked gasoline should less tendency to knock. Thus the cracking process also increases the resistance of gasoline to knock in addition to its main function of increasing the yield of gasoline for which purpose it was originally devised. The phenomena of knocking is not yet fully understood. However it has been found

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