CASING-HEAD GASOLINE The extraction of gasoline from the vapors from oil wells has developed into an important business. It is so intimately related to the oil industry that a general treatise on the petroleum industry would not be complete without some mention of the casing-head gasoline business.
Definition of Casing-head Gasoline.—Natural gas is classed as "dry" gas or "wet" gas. Dry gas is defined as gas that fails to show any of the light hydrocarbon vapors that emanate from oil. However, recent experiments have shown that gases once con sidered dry do carry small quantities of light vapors.
"Wet" gas is defined as gas that carries light hydrocarbon vapors that emanate from oil. These vapors when obtained by the condensation and refrigeration or the absorption systems give light products of the benzine series.
Casing-head gasoline is gasoline obtained by catching the "wet" gas from the flow lines leading from the casing head of an oil well, and condensing or absorbing the vapors. These vapors emanate from the oil and are obtained from gas traps at the wells and from traps placed on the storage tanks for crude oil.
Three important methods of obtaining the gasoline from the vapors are in use: (1) The compression and refrigeration system; (2) the absorption; (3) combinations of 1 and 2.
Compression and Refrigeration Process.—The compression and refrigeration process consists primarily in compressing the " wet " gas under high pressure to a small volume, and then condensing the volatile vapors by using low temperatures.
This process calls for a compressor to compress the gas, and cooling or condensing coils to condense the vapor after it leaves the compressor. Collecting tanks for the condensate are also necessary. The general procedure is described on page 225.
Absorption Process.—The absorption process consists in compressing the gas under low pressure to a small volume, and then passing it through an absorbing medium, generally mineral seal oil. The mineral seal oil, which has absorbed the light petroleum vapors, is then collected in a still, and distilled. The vapors given off are then condensed and collected in proper storage tanks.
The two processes are employed as follows : The compression and refrigeration processes are used for rich gases running from 1 to 12 gal. per 1000 cubic feet and the absorption system is used for gases having from ;10 to 1 gal. per 1000 cubic feet. The latter system is best applied where there are large volumes of low grade "wet" gas, such as are found in the gas lines running from gas fields to the large cities. Indeed, some of the early gases once classed as "dry" gases are now successfully treated by the absorption system.
The big gas wells of high volume generally carry small pro portions of casing-head gasoline. Wells of 10,000,000 to 80,000, 000 cu. ft. have very small quantities of gasoline, but with such large volumes even a small amount of gasoline, say ;10 gal. per 1000 cubic feet, will be profitable.
The older an oil well the richer the gasoline contents. Some old wells at Glenn .Pool, Oklahoma, run as high as 12 gal. per 1000 cubic feet.
Vacuum pumps on wells increase the richness of the gasoline contents.
Absorption Methods.—There are two classes of absorption methods: one for large volumes of rich gases at low pressures-40 to 50 lbs., and one for lean gases at high pressures-100 to 300 lbs. In the first type the absorber has a diameter of 12 ft. and a height of 48 ft. (See Fig. 96A.) In the second type the absorber has a diameter of 20 to 30 in. and heights of 30 to 50 ft. (See Fig. 96B.) The system of baffle boards in the absorbers is in each case much the same as shown in Fig. 97, which gives the details of an absorber.
The first type can treat 2,500,000 cu. ft. per day.
The second type can treat from 5,000,000 to 6,000,000 cu. ft. per day for each absorber. The absorbers are usually placed in batteries.
The extraction of casing-head gasoline by the absorption method demands: (a) Gathering lines.
(b) A flow tower in which the gas and absorbing medium can meet and mix.
(c) Receptacles for the absorbate.
(d) Stills for distillation of the mineral seal oil.
(e) Condenser coils or refrigerator system.
(f) Tanks for the condensed gasoline product.
There are, of course, auxiliary pump systems, and tanks for the mineral seal oil.
The mineral seal oil, however, is used over and over again.
There is some loss, say 5 per cent, occasioned each time by leak age and some vaporization, but the loss is negligible.
The mixing towers or absorbers are generally vertical cylin drical towers 30 to 50 ft. high, although horizontal absorbers are in use. The liquid mineral seal oil flows from the top of the tower downward over baffle plates. The gas is allowed to enter at the bottom of the tower and in passing upward meets the oil. The light vapors in the gas are absorbed. At high pressures 7 to 10 gal. of mineral seal oil per 1000 cu. ft. of gas are necessary.
Separation of the gasoline vapors from the absorbate is obtained by distilling the mineral seal oil in a simple still. Condensation is obtained through air-cooled pipes with water passing over them.
Some casing-head plants use both systems. The "lean" products from the compression are treated by the absorption system, and may deliver good yields of gasoline.