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Energy II. Explain the process of fractional distillation of crude oil with a well labelled diagram & different fractions obtained with their uses. [ S18, W17, S17, S16, S15, S14, S13]. Working of Internal Combustion Engine. KNOCKING.
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Explain the process of fractional distillation of crude oil with a well labelled diagram & different fractions obtained with their uses. • [S18, W17, S17, • S16, S15, S14, S13]
KNOCKING • Explosive combustion of air-petrol vapour mixture produce shock waves which hit the cylinder wall and piston of IC engine, creating loud metallic sound is known as knocking in petrol engine. • Knocking in petrol engines is due to early combustion of fuel. Pre ignition (Knocking) in petrol engine depends on fuel structure and compression ratio. • The knocking tendency of hydrocarbon in gasoline is: • straight chain paraffin > branched chain paraffins > olefins (alkenes) > cyclo-paraffins > Aromatics
State how knocking in petrol is related with its chemical structure.[S18, W17, S16, W13] Why good spark ignition fuels are poorer compression ignition fuels & vice-versa?[S13] The knocking tendency decreases with compactness of the molecules, double bonds and cyclic structure. • With normal paraffins (alkane chain) the knocking tendency increases with increases in the length of molecule of hydrocarbon chain. E.g. Octane No. of n-butane, n- pentane, n-hexane & n- heptane are 90, 60, 29 & so on. But this is opposite in diesel engine. • Branched chain paraffins have higher anti-knocking properties than the normal paraffins further as the no. of branches in structure increase, the knocking tendency decreases. E.g. 2 methyl hexane has octane no. 55 while 2, 2 dimethyl pentane has octane no. 80. • Alkenes (Olefins) have higher anti-knocking properties than corresponding alkanes (paraffins). Further as the double bond approaches to center of chain, the knocking tendency gets decreases. But this is opposite in diesel engine. • Aromatics hydrocarbons like benzene, toluene have high octane no. Alcohols are more resistance to detonation. But this is opposite in diesel engine. • Cracked and reformed gasoline have higher anti-knocking tendency than straight run gasoline. • Alcohols have less knocking property in petrol engines but this is opposite in diesel engine.
Octane Number • It shows the extent or degree of anti-knocking property in a gasoline. • Octane no. is a rating of gasoline as its anti-knocking property. • The rating or scale is based on the anti- knocking properties of standard sample i.e. iso-octane and n-heptane. • The octane No. was first introduced by Edger in 1927. • Thus octane No. of gasoline is defined as the % of iso-octane in a mixture of iso-octane and n-heptane.
Antiknocking agents in petrol or Improving Octane number. [W16, S14] A) Doping: The knocking can be reduced or antiknocking properties of gasoline are increased by adding anti knocking agents like TEL, TML, etc. and this process is called doping. • Antiknocking agents: Knocking of petrol engine may be reducedby addition of organolead compound into it. The substance added to control knocking is called antiknocking agent. The common commercial antiknocking agents used are: • TEL: Tetraethyl lead, (C2H5)4Pb • TML: Tetramethyl lead, (CH3)4Pb • A mixture of TEL & TML • An extensively used antiknocking agent is tetraethyl lead. • TEL is added in the form ethyl fluid and petrol is called leaded petrol. • Ethyl fluid contains 63% TEL, 26% dibromoethane and 9% dichloroethane. • TEL is believed to react with the peroxy compounds and decompose them. Thus knocking is prevented. • Dibromoethane and dichloroethane are added to remove lead as volatile lead halide (air pollutant) which escapes through exhaust. • Addition of 1-2 cm3 of ethyl fluid to a liter of petrol increases the octane number by 8-10 units. • Since TEL is extremely poisonous, gasoline containing TEL is always colored for identification and safety.
B) Blending: To increase the octane rating of fuel benzol and alcohol is added. This process is called Blending. However, relatively large amount of blending agent should be added as the effect solely proportional to the amount of blending agent added and its octane number. • Benzole: Benzole is chemical obtained during the destructive distillation of coal and coaltar. • Composition of Benzole is: Benzene(75%), Toulene(15%), Xylene(10%). • Since the octane no. of gasoline increases with the increase in aromatics constituents • The main disadvantage to use high percentage of benzole in aviation gasoline is benzene which has freezing point of about 5C • Therefore only addition upto 20% can be employed. • Methyl Tertiary Butyl Ether(MTBE): MTBE is the most adaptable octane enhancing oxygenate • MTBE is produced by a simple reaction of isobutylene with methanol. • This compound provides oxygen (of ether group) for combustion of petrol in I.C. engine, thereby reducing considerably the formation of peroxy compounds (which cause knocking). • Power alcohol: Power alcohol is gasoline blends containing ethanol • Blends containing up to 25% of alcohol with petrol are used. • The main objective of the power alcohol was to reduce oil imports and provide an alternative to non-renewable energy source, gasoline.
C) Unleaded Petrol: Unleaded petrol is one which does not contain any lead compound (like TEL). To improve octane number concentration of high octane components like isooctane, isopentane, ethylbenzene, isopropylbenzene, etc. is increased by process of reforming. In addition to it, blends like MTBE is added to improve octane value. Advantages • The harmful effect of discharge of poisonous leaded compounds through exhaust is avoided. • This eliminates the pollution level of lead in atmosphere. • Unleaded petrol permits the attachment of catalytic converters to exhaust pipe. The catalyst (Rhodium) converts the toxic gases like CO & NO to non toxic gases CO2 & N2 respectively. Consequently pollution level is reduced to great extent.
KNOCKING • In diesel engine, the diesel is not combustible by a spark. • It requires high temperature, of 550-700oC is produced by compression of air. Then the diesel is injected along with air. • At high temperature, liquid diesel evaporates and gets ignited and temp & pressure further increases. The hydrocarbon molecules undergo atomization & then combustion. The atomization step is slow & requires time. • If there is time lag(Difference in time between fuel injection and ignition) in the combustion of fuel in diesel engine, then it produces large extent of vibrations along with hammering sound called as Diesel knock. • The rate of burning of diesel oil in diesel engine is very slow & it produces very loud sound and vibrations in diesel engine, called as knocking in diesel engines. Thus the cause of knocking in diesel engine is the delay in combustion of fuel.
KNOCKING • The knocking tendency of hydrocarbon in diesel is • Aromatic > cycloparaffins > olefins (alkenes) > branched chain paraffin’s > straight chain paraffins. https://youtu.be/DZt5xU44IfQ
CatalyticCracking • Cracking [S18, W15, S13]: The process in which heavy oils (higher hydrocarbons) having higher boiling points are converted into lower hydrocarbons having lower boiling points using high temperature, is called as cracking. • On fractional distillation of crude petroleum oil, only 20-30% petrol (straight run petrol) & 30-40% intermediate oils (such as diesel oil, kerosene) & 25-50% of residual fuel is obtained. https://youtu.be/Xsqlv4rWnEg
Objective of Cracking [W18, S17, W15] • Petrol obtained by this process has good anti-knocking properties and higher octane number because the cracked petrol is rich in branched, unsaturated, aromatic hydrocarbon as compared to straight run petrol i.e. it is of superior quality. • The yield of the lower boiling point fraction can be increased by converting higher boiling point fraction to lower ones Significance of Cracking: • Petrol obtained by this method produce very less knocking in petrol engine. Because the cracked petrol contain branched chain molecules, aromatic molecules & unsaturated molecules. Hence octane No. of such petrol is more than straight run petrol. • By this method, higher hydrocarbons (heavy oils) having higher boiling points are converted into lower hydrocarbons having lower boiling points. • Saturated hydrocarbons are converted to unsaturated hydrocarbons.
