Volatility of gasoline

Like propane, butanes are obtained from natural gas liquids and from refinery gas streams. The C4 acyclic paraffin consists of two isomers n-butane and isobutane (2-methylpropane). The physical as well as the chemical properties of the two isomers are quite different due to structural differences, for example, the vapor pressure (Reid method) for n-butane is 52 Ib/in., while it is 71 Ib/in. for isobutane. This makes the former a more favorable gasoline additive to adjust its vapor pressure. However, this use is declining in the United States due to new regulations that reduce the volatility of gasolines to 9 psi, primarily by removing butane. ... 

Butane(s) -12 to -1 11-30 Increases volatility of gasoline, advantageous in cold climates... 

Environmental studies suggest that light gasoline components are detectable in the atmospheres of large metropolitan areas. New environmental laws limit the volatility of gasoline, so refiners must use other processing streams to meet volatility requirements. However, the fuel must provide performance to consumers, for example, by minimizing... 

D. Zudkevitch, A. K. S. Murthy, J Gmehling. Thermodynamic aspects of reformulation of automotive fuels, Part 1. The effects of oxygenates on the vapor pressures and volatilities of gasolines. J. Hydrocarbon Processing, 93-100, June 1995. 

Section 8.2.1 lists octane numbers for heavy naphtha and some normal and iso-paraffins. N-butane has a high octane number (92), so refiners blend as much of it as possible into gasoline. But n-butane also evaporates easily, which means that tighter restrictions on the RVP (volatility) of gasoline limit its C4 content (see Section 8.2.2). In many locales, this creates an excess of n-butane. By converting it into isobutane, which is consumed in alkylation units, refiners can reduce or eliminate this excess of n-butane. 

RVP is used to characterize the volatility of gasolines and crude oils. The RVP of a mixtiure is determined experimentally according to... 

Small concentrations of volatile components in a liquid mixture may accumulate in the vapor space of a container over time and appreciably reduce the flash point relative to the reported closed-cup value. This may be the result of degassing, chemical reaction or other mechanism. An example is bitumen [162]. Similarly, if a tank truck is not cleaned between deliveries of gasoline and a high flash point liquid such as kerosene or diesel oil, the mixture might generate a flammable atmosphere both in the tmck tank and the receiving tank. Contamination at the thousand ppm level may create hazards (5-1.4.3 and 5-2.5.4). Solids containing upward of about 0.2 wt% flammable solvent need to be evaluated for flammable vapor formation in containers (6-1.3.2). 

Evaporative emissions from vehicle fuel systems have been found to be a complex mixture of aliphatic, olefinic, and aromatic hydrocarbons [20,24,33]. However, the fuel vapor has been shown to consist primarily of five light paraffins with normal boiling points below 50 °C propane, isobutane, n-butane, isopentane, and n-pentane [33]. These five hydrocarbons represent the more volatile components of gasoline, and they constitute from 70 to 80 per cent mass of the total fuel vapor [24,33]. 

There are many components of gasoline that readily dissolve in water and are transported as solutes in the groundwater. Most gasoline products are volatile and can release gas into the soil void in gaseous form, particularly in the unsaturated zone. Besides these three forms, gasoline components can be adsorbed by the soil matrix and exist in the soil as adsorbates. 

Most gasoline constituents are volatile organics. Volatilization depends on the potential volatility of the compounds and on the soil and environmental conditions, which modify the vapor pressure of the chemicals. Factors affecting volatility are water content, clay content, surface area, temperature, surface wind speed, evaporation rate, and precipitation. 

The fate of gasoline in the subsurface is dependent on its interaction with soil and groundwater, volatilization, chemical reaction, biodegradability, and its movement, which in turn depends on the properties of both gasoline and the underground structure. 

Similar to gasoline, the properties of DNAPLs such as immiscibility with water, volatility, and solubility of some of its components cause the presence of multiphase (pure product, solute, gas, and adsorbate) products and movement that is typical of the phenomena associated with DNAPL release. The theory associated with the interaction of gasoline with soil is applicable to DNAPLs. However,... 

We take as an example the fate of benzene ((/Ur,) as it migrates with groundwater flowing through an aquifer. Benzene is a common contaminant because it makes up much of the volatile fraction of gasoline and other petroleum products. It is a suspected carcinogen with an MCL (maximum contamination level) set by the US Environmental Protection Agency of 5 qg kg-1. 

They make up the most volatile portion of gasoline and are sometimes added to propane to be marketed as bottled gas. Most n-butane, however, is converted to butadiene, which is used to make synthetic rubber and latex paints. 

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