Boiling Point of Gasoline
Gasoline as used by vehicles is a mixture of roughly 230 different chemicals. Gasoline formulations vary depending upon the location, time of the year, environmental regulations, and availability. If the PEAC user looks up the boiling point of gasoline on the PEAC tool, the temperature 102o F (or 39o C) is displayed. But the information is misleading. Gasoline boils over a range of temperatures, with the most volatile components starting to boil away at roughly 102oF. The less-volatile components will boil at higher temperatures. Gasoline boils over range of temperatures, between 39 and 200o C (102oF and 392o F) typically, the temperature range varies depending upon the formulation. The final boiling point of the last residual of gasoline might be typically 225oC (437oF). This is in contrast to a pure chemical such as heptanes (one of the components of gasoline) which boils at a single temperature (209o F; 98oC).
Petroleum refining begins with the distillation of crude oil into fractions of different boiling ranges, usually called “light naphtha”, “heavy naphtha”, “kerosene”, “light gas oil”, “heavy gas oil”, and “reduced crude”. The naphtha fractions obtained by distillation are also called “virgin naphtha” or “straightrun gasoline”. The hydrocarbon products obtained by distillation depend greatly upon the type of crude oil being distilled. Kerosene and light gas oil fractions (also called middle distillates) are used in the production of kerosene, jet fuel, diesel fuel, and furnace oils. The heavy gas oil may be used for heavy diesel fuel, industrial fuel oil, and bunker fuel. All of these are mixtures of various hydrocarbon compounds with a range of boiling points. If the PEAC user looks up the boiling point for fuel oil, jet fuel, naphtha, or other petroleum distillate, a single temperature is displayed representing a temperature at the lower end of the boiling point range.
The lower boiling point hydrocarbon distillates are more valuable because they are major components of gasoline. A major petroleum refining step is hydrocracking, where higher boiling hydrocarbons are broken down or cracked forming lower boiling point hydrocarbons. The higher boiling point hydrocarbons are subjected to hydrogen and heat in the presence of a catalyst which results in the formation of lower molecular weight, lower boiling point hydrocarbons. The catalyst, which becomes fouled with carbon, is regenerated.
A typical breakdown of modern gasoline (excluding additives and oxygenated compounds) might be 15% n-paraffins (examples: pentane, hexane, heptane, octane, decane, etc.); 30% iso-paraffins (examples: 1-methylpropane, 2-methylbutane, 2,2,3-trimethylbutane, etc.); 12% cycloparaffins (example: cyclohexane, cyclopentane, etc.); 35% aromatics (examples: benzene, toluene, ethyl benzene, 1,3,5-trimethylbenzene; m-xylene, etc.); and 8% olefins (examples: 2-pentene, 2-methylbutene, cyclopentene, etc.). The octane number of the gasoline is a function of the components.
The U.S. Environmental Protection Agency has specified that gasoline contain a minimum of 2% oxygen by weight to reduce automotive emissions and improve air quality in polluted areas. This can be done by adding alcohols, notably ethanol, to gasoline to supply the oxygen component. Until recently, refiners have added methyl tert-butyl ether (MTBE) to gasoline to supply the oxygen component; a gasoline composition of 12% MTBE would meet the 2% oxygen by weight requirement. But MTBE proved to be a dangerous pollutant itself, contaminating groundwater from leaking gasoline tanks at fuel stations.
Modern refiners add detergents (usually an amide compound and alkylammonium dialkyl phosphate to prevent the formation of contaminants in the carburetor or fuel injectors. Light lubricants may be added to help lubricate cylinders and top piston rings. Deicing and anticorrosion additives are also in modern gasolines. Organic dyes are also added to identify brands and grades of gasoline.
In summary, gasoline is a mixture of many different chemicals. Many of the components of modern gasoline are also individually listed in the PEAC tool. The mixture boils over a temperature range rather than at a single temperature.
