Gasoline- The Petrol That We Used Everyday

          Petroleum (from Latin: 'petra' (rock) + Latin: oleum (oil)) or crude oil is a naturally occurring flammable liquid consisting of a complex mixture of hydrocarbons of various molecular weights and other liquid organic compounds, that are found in geologic formations beneath the Earth's surface. A fossil fuel is formed when large quantities of dead organisms, usually zooplankton and algae, are buried underneath sedimentary rock and undergo intense heat and pressure.


Gasoline

          Gasoline, or petrol is a transparent, petroleum-derived liquid that is used primarily as a fuel in internal combustion engines. It consists mostly of organic compounds obtained by the fractional distillation of petroleum, enhanced with a variety of additives. Some gasoline also contain ethanol as an alternative fuel. Under normal ambient conditions, its material state is liquid, unlike liquefied petroleum gas or natural gas.

Volatility

          Gasoline is more volatile than diesel oil, Jet-A, or kerosene, not only because of the base constituents, but also because of additives. Volatility is often controlled by blending with butane, which boils at −0.5 °C. The volatility of petrol is determined by the Reid vapor pressure (RVP) test. The desired volatility depends on the ambient temperature. In hot weather, petrol components of higher molecular weight and thus lower volatility are used. In cold weather, too little volatility results in cars failing to start.

Energy Value

          Energy is obtained from the combustion of gasoline by the conversion of a hydrocarbon to carbon dioxide and water. The combustion of octane follows this reaction:

2 C₈H₁₈ + 25 O₂ ---> 16 CO₂ + 18 H₂O

Gasoline contains about 35 MJ/L (9.7 kW·h/L, 132 MJ/US gal, 36.6 kWh/US gal) or 13 kWh/kg. The lower energy content (per liter) of LPG in comparison to gasoline is due mainly to its lower density. Energy content per kilogram is higher than for gasoline.

Density

          The specific gravity (or relative density) of gasoline ranges from 0.71–0.77 kg/l (719.7 kg/m³ ; 0.026 lb/in³; 6.073 lb/US gal; 7.29 lb/imp gal), higher densities having a greater volume of aromatics. Gasoline floats on water; water cannot generally be used to extinguish a gasoline fire, unless used in a fine mist.

 

Chemical Analysis and Production

          Gasoline is produced in oil refineries. Material that is separated from crude oil via distillation, called virgin or straight-run gasoline, does not meet the required specifications for modern engines (in particular octane rating; see below), but will comprise part of the blend.

Some of the main components of gasoline: isooctane, butane, 3-ethyltoluene, and the octane enhancer MTBE.

          The bulk of a typical gasoline consists of hydrocarbons with between four and 12 carbon atoms per molecule (commonly referred to as C4-C12).The various refinery streams blended to make gasoline have different characteristics. Some important streams are:

• straight-run gasoline is distilled directly from crude oil. Once the leading source of fuel, its low octane rating required lead additives. It is low in aromatics (depending on the grade of crude oil), containing some naphthenes (cycloalkanes) and no olefins. About 0-20% of gasoline is derived from this material, in part because the supply of this fraction is insufficient and its RON is too low.
• reformate, produced in a catalytic reformer with a high octane rating and high aromatic content, and very low olefins (alkenes). Most of the benzene, toluene, and xylene (the so-called BTX) are more valuable as chemical feed stocks and are thus removed to some extent.
•  cat cracked gasoline or cat cracked naphtha, produced from a catalytic cracker, with a moderate octane rating, high olefins (alkene) content, and moderate aromatics level.
• hydrocrackate (heavy, mid, and light) produced from a hydrocracker, with medium to low octane rating and moderate aromatic levels. 
• alkylate is produced in an alkylation unit, involving the addition of isobutane to alkenes giving branched chains but low aromatics. 
•  isomerate is obtained by isomerizing low octane straight run gasoline to iso-paraffin (like isooctane).

          Overall, a typical gasoline is predominantly a mixture of paraffin (alkanes), naphthenes (cycloalkanes), and olefins (alkenes). The actual ratio depends on the oil refinery that makes the gasoline, as not all refineries have the same set of processing units; crude oil feed used by the refinery; the grade of gasoline, in particular, the octane rating.

          Currently, many countries set limits on gasoline aromatics in general, benzene in particular, and olefin (alkene) content. Such regulations led to increasing preference for high octane pure paraffin (alkane) components, such as alkylate, and is forcing refineries to add processing units to reduce benzene content.

Environmental Considerations

          Combustion of 1 US gallon (3.8 L) of gasoline produces 8,788 grams (19.37 lb) of carbon dioxide (2.3 kg/l), a greenhouse gas. The main concern with gasoline on the environment, aside from the complications of its extraction and refining, is the potential effect on the climate. Un-burnt gasoline and evaporation from the tank, when in the atmosphere, react in sunlight to produce photochemical smog. Addition of ethanol increases the volatility of gasoline, potentially worsening the problem.

          The chief risks of such leaks come not from vehicles, but from gasoline delivery truck accidents and leaks from storage tanks. Because of this risk, most (underground) storage tanks now have extensive measures in place to detect and prevent any such leaks, such as monitoring systems.

 

Toxicity

          The material safety data sheet for unleaded gasoline shows at least 15 hazardous chemicals occurring in various amounts, including benzene (up to 5% by volume), toluene (up to 35% by volume), naphthalene (up to 1% by volume), trimethylbenzene (up to 7% by volume), methyl tert-butyl ether (MTBE) (up to 18% by volume, in some states) and about ten others. Hydrocarbons in gasoline generally exhibit low acute toxicities, with LD50 of 700 – 2700 mg/kg for simple aromatic compounds. Benzene and many antiknocking additives are carcinogenic.

Flammability

          Uncontrolled burning of gasoline produces large quantities of soot. Like other alkanes, gasoline burns in a limited range of its vapor phase and, coupled with its volatility, this makes leaks highly dangerous when sources of ignition are present. Gasoline has a lower explosion limit of 1.4% by volume and an upper explosion limit of 7.6%. If the concentration is below 1.4% the air-gasoline mixture is too lean and will not ignite. If the concentration is above 7.6% the mixture is too rich and also will not ignite. However, gasoline vapor rapidly mixes and spreads with air, making unconstrained gasoline quickly flammable. Many accidents involve gasoline being used in an attempt to light bonfires; the gasoline readily vaporizes after being poured and mixes with the surrounding air.