why is diesel economically viable than gasoline (petroleum)
What is alkylation?
Gasoline (or petrol) is a petroleum-derived liquid mixture consisting primarily of hydrocarbons, used as fuel in internal combustion engines.
Gasoline is separated from crude oil via distillation. The bulk of a typical gasoline consists of hydrocarbons between 5 to 12 carbon atoms per molecule. The various refinery streams produce gasoline of different characteristics.
Important streams of gasoline are:
• Reformate, produced in a catalytic reformer with a high octane and high aromatics content, and very low olefins (alkenes).
• Catalytically Cracked Gasoline or Catalytically Cracked Naphtha, produced from a catalytic cracker, with a moderate octane, high olefins (alkene) content, and moderate aromatics level.
• Product from a hydrocracker, contains medium to low octane and moderate aromatic levels.
• Natural Gasoline, directly from crude oil contains low octane, low aromatics (depending on the crude oil), some naphthenes (cycloalkanes) and zero olefins (alkenes).
• Alkylate, produced in an alkylation unit, with a high octane and which is pure paraffin (alkane), mainly branched chains.
• Isomerate, which is made by isomerising natural gasoline to increase its octane rating and is very low in aromatics and benzene content.
Octane Rating of Gasoline
Octane number is a figure of merit representing the resistance of gasoline to premature detonation when exposed to heat and pressure in the combustion chamber of an internal combustion engine. Such detonation is wasteful of the energy in the fuel and potentially damaging to the engine.
Premature detonation is indicated by knocking or ringing noises that occur as the engine operates. If an engine running on a particular gasoline makes such noises, they can be lessened or eliminated by using a gasoline with a higher octane number.
Additives to gasoline - To increase octane number
The discovery that lead additives reduced the knocking property of gasoline in internal combustion engine led to the widespread adoption of the practice in the 1920s and therefore more powerful higher compression engines. The most popular additive was tetra-ethyl lead.
However, with the recognition of the environmental damage caused by lead, and the incompatibility of lead with catalytic converters found on virtually all automobiles since 1975, this practice began to wane in the 1980s. Most countries are phasing out leaded fuel. Different additives have replaced the lead compounds. The most popular additives include aromatic hydrocarbons, ethers and alcohol (usually ethanol or methanol).
Additives to gasoline - To increase combustion capacity
Oxygenate blending increases oxygen to the fuel in oxygen-bearing compounds such as MTBE, ethanol and ETBE, and so reduces the amount of carbon monoxide and unburned fuel in the exhaust gas, thus reducing smog. MTBE use is being phased out in some countries due to issues with contamination of ground water.
Ethanol and to a lesser extent the ethanol derived ETBE are a common replacements. Especially ethanol derived from bio-matter such as corn, sugar cane or grain is frequent, this will often be referred to as bio-ethanol. An ethanol-gasoline mix of 10% ethanol mixed with gasoline is called gasohol.
Diesel is produced from petroleum, and is sometimes called petro-diesel when there is a need to distinguish it from diesel obtained from other sources. As a hydrocarbon mixture, it is obtained in the fractional distillation of crude oil between 250 ºC and 350 ºC at atmospheric pressure.
Diesel is considered to be a fuel oil and is about 18% denser than gasoline. The density of diesel is about 850 grams per liter (g/l) whereas gasoline has a density of about 720 g/l, or about 18% less. Diesel is generally simpler to refine than gasoline and often costs less.
Diesel fuel often contains higher quantities of sulfur than gasoline. High levels of sulfur in diesel are harmful for the environment. It prevents the use of catalytic diesel particulate filters to control diesel particulate emissions, as well as more advanced technologies such as nitrogen oxide (NOx) absorbers, to reduce emission.
However, lowering sulfur reduces the lubricity of the fuel, meaning that additives must be put into the fuel to help lubricate engines. Biodiesel is an effective lubricant. Diesel contains approximately 18% more energy per unit of volume than gasoline, which, along with the greater efficiency of diesel engines, contributes to fuel economy.
Energy content of fuels
Gasoline contains about 45 mega joules per kilogram (MJ/kg). A high octane fuel such as liquid petroleum gas (LPG) has lower energy content than lower octane gasoline, resulting in an overall lower power output at the regular compression ratio of an engine that runs on gasoline. However, with an engine tuned to the use of LPG, this lower power output can be overcome. This is because higher - Octane fuels allow for higher compression ratio.
Fuel type MJ/L MJ/kg
Diesel 40.9 63.47
Gasoline 29.0 45
Gasohol (10% ethanol + 90 % gasoline) 28.06 43.54
LPG 22.16 34.39
Ethanol 19.59 30.40
Methanol 14.57 22.61
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