THE past few years have brought a widespread realisation of the economies that may be effected by reducing the evaporation losses that occur while volatile petroleum products are being stored or transported. These losses. are objectionable for two reasons: first, because evaporation results in the loss of a definite volume of the product that might otherwise be marketed; and second, because evaporation decreases the quality of the remaining product due to the fact that the valuable light fractions are the ones that evaporate first. Analytical and experimental investigations by various individuals and companies, have done much to bring about a more thorough understanding of the conditions giving rise to loss by evaporation, and methods for measuring the losses accurately have been devised. The information obtained from these investigations has formed a basis for the design of various types of equipment for reducing the losses or eliminating them entirely.
The general subject of evaporation losses may well be introduced by a brief discussion of the mechanism of evaporation. The molecules of any liquid are always in motion at a velocity depending on the temperature and the character of that particular liquid. At the surface, some of the molecules break away and exist as a gas. This transition from liquid to gas at the surface is called evaporation. When any volatile liquid such as gasoline is placed in a closed container with a space above the surface, the molecules leaving the surface hit the walls and top of the container and rebound, some of them returning eventually to the liquid. As evaporation proceeds, a condition is finally reached for which the number of molecules returning to the liquid is exactly equal to the number leaving it. The space above the liquid, usually called the vapour space, is then said to be saturated. The impact of the molecules striking against the walls of the container causes a pressure known as vapour pressure. This pressure depends only on the temperature of the liquid surface, and is entirely independent of the size of the vapour space. If a vacuum exists in the space above the liquid surface, saturation will be accomplished very rapidly. If, however, the space contains air, the rate of saturation will be much slower, but the final number of molecules for each unit of vapour space at any given temperature will be the same in either case. When both air and vapour are present, each exerts the same pressure that it would if the other were not present, and the total pressure in the vapour space is the sum of the two partial pressures. Before going further, it may be well to distinguish between evaporation and boiling, as these terms are * Research Department, Chicago Bridge and Iron Works, Chicago, Illinois, U.S.A. sometimes confused.