CRACKING PETROLEUM. The process in petroleum distillation known as (cracking'" is the subjecting of certain distillation products to a degree of heat so far above their boiling points that they are decomposed into component parts whose boiling points are lower. It is accomplished in an ordinary fire-still by con densing vapors arising from the boiling petro leum upon the cool dome of the still, whence they drop into the superheated oil below, where they are broken up into lighter oils and instantly vaporized, passing over into the condenser. As ordinarily carried on the process aims to in crease the yield of illuminating oil (kerosene) by thus cracking the heavier oils which distil over immediately after the kerosene vapors have ceased, that is, above the temperature of 625° F. When this temperature is reached the fire under the still is subdued, and the distilla tion continued slowly up to 700°, during which period the heavier oils dripping back are cracked into gasoline and kerosene. The additional amount of kerosene obtained from the oil is from 18 to 28 per cent.
The cracking process has more recently been invoked to secure a larger amount of gasoline from crude oil, and from kerosene, to meet the enormous demand for motor fuel. Several processes were invented, or at least patented, almost simultaneously. The Burton process in extensive operation in the United States keeps the entire contents of the still and also of the condenser under a pressure of 60 to 75 pounds to the square inch, through the manipulation of a valve, which is opened occasionally to avoid the liquefaction of the gases in the tubes. The syrupy residue of the distillation is distilled at atmospheric pressure, and its distillate returned to the cracking still. Some crude oils by this
process are made to yield as much as 60 per cent in crude gasoline distillate. This is im mediately available for use in marine motors, but requires deodorizing for use in automobiles.
The Hall process (American), patented in 1913, found a warm reception in England and on the Continent, and many large plants are us ing it. The raw material in this process is common gas oil, and the gasoline yield reaches 70 per cent of ((motor spirit') as it is called, hav ing. from 18 to 31 per cent more power than ordinary standard gasoline.
In the Rittman process the petroleum vapor is passed into a tube heated to 850° F., under very heavy pressures, ranging up to 500 pounds to the square inch. This vapor is then con densed under pressure, and the resulting liquid distilled for gasoline. The yield is from 60 to 70 per cent of the original bulk of the oil, in small quantities.
Other processes first subject the oil to great heat under heavy pressure, and then distil it; or take the vapors of boiling oil through iron tubes containing a red-hot sponge of some metal; or vary the operation either in the heat and pres sure in the still, or in a decomposer, or by some other manipulation, the essential features being heat and pressure. All of these processes are successful in some degree in securing an in creased yield of the sought-for gasoline. Con sult Bacon, R. F., and Hamor, W. A., The American Petroleum Industry' (New York 1916).