四级冲刺练习阅读164_大学英语阅读_大学英语-查字典英语网

四级冲刺练习阅读164

　　Crude oil wells flow at varying rates, from ten to thousands of barrels per hour. Petroleum products are always measured in 42-gallon barrels.

　　Petroleum products vary greatly in physical appearance: thin, thick, transparent or opaque, but their chemical composition is made up of only two elements; carbon and hydrogen, which form compounds called hydrocarbons. Other chemical elements found in union with the hydrocarbons are few and are classified as impurities. Trace elements are also found, but these are of such minute quantities that they are disregarded. The combination of carbon and hydrogen forms many thousands of compounds which are possible because of the various positions and joinings of these two atoms in the hydrocarbon molecule.

　　The various petroleum products are refined from the crude oil by heating and condensing the vapors. These products are the so-called light oils, such as gasoline, kerosene, and distillate oil. The residue remaining after the light oils are distilled is known as heavy or residual fuel oil and is used mostly for burning under boilers. Additional complicated refining processes rearrange the chemical structure of the hydrocarbons to produce other products, some of which are used to upgrade and increase the octane rating of various types of gasolines.