SCHEME FOR COMPRESSION AND REFRIGERATION PROCESS The flow sheet shown in Fig. 100 gives the compression and refrigeration system.
The gas from the field mains enters the intake receiver at (1). The gas is taken into the low-stage compressors at (2) and is compressed to pressures of 40 to 80 lb. per square inch. The gas passes through the trap (3) where any lubricating oil or other low-grade condensate is caught.
The product then passes into the low-pressure cooling coils (4). In these coils part of the vapors having the lowest range of specific gravity are condensed and drawn off into the tanks at (5). The uncondensed vapors pass off as the arrows indicate and are carried to the high pressure cylinders (compressors) (10). The condensate in (5) is carried off into the "make tank" (7) where any water in it is drawn off and blending is also carried on. From there the product goes to the storage tanks (8) and from there to the loading racks (9).
The condensed gas run to (10) is compressed at high pressures of from 200 to 300 lb. per square inch and from there goes to the high-pressure cooling coils (12) and into the accumulator tanks (13). The condensed product is taken off through (14) and into the "make tank" (7).
The uncondensed gas coming from the tanks (13) is carried to the double cooling coils, and into the accumulation tank (16). Part of the product is condensed here and is run into tank (7). The residual gas is carried through the coils (15) and through the tank (16) where more of the condensed product is obtained and carried to tank (7).
The final residue of gas and vapor, the most difficult con densable portions, are carried through the expansion engine (19). From (19) the condensed portions accumulate in the tank (20) and are drawn off through the automatic trap (21) and from there to the "make tank" (7). This is the last stage of separation. The residual gas passes from (20) and is allowed to expand rapidly through the coils (15) so that it cools the vapors coming off from (13). The final gas passes off from (15) to the gas line (22). This completes the scheme.
It must be realized that the schemes presented merely give the general outline. No two plants are exactly alike and changes are made as improvements take place, but the flow sheets outlined give a fair idea cf both systems.
Blending.—The product obtained from the compression and refrigeration system, or from the absorption system, ranges from 85 to 100°Be. It is very "wild" or volatile. It.evaporates much more rapidly than gasoline and, accordingly, the raw product must be very carefully sealed. Also this product requires careful handling and is subject to many restrictions in trasportation. To overcome some of these difficulties the product is blended with naphtha of lower specific gravity. The naphtha of 50 to 54° is blended with this higher condensate to form a product of about 68°Be. This is accomplished by filling a tank half full with cas ing-head and then pumping in naphtha which settles toward the bottom, and mixes with the condensate.
The blended product is then "weathered" or exposed to the air until the product has a vapor tension of 10 lb. per square inch. This product has a specific gravity of 60 to 65°Be. and can be transported without difficulty.
This blended gasoline is fully as good for internal-combination engines as the straight gasoline obtained from refining crude oil.
Reduction of Heating Values.—It is interesting to note that treatment of gas from gas lines reduces the heating value of the resultant gas. A natural gas consisting of methane, ethane, propane, butane and pentane will have a much higher heating content than pure methane. It is not unusual to have such a mixture of gases and vapors give 1700 B.t.u. per 1000 cubic feet as against 950 B.t.u. for methane gas. After treating the "wet" gas the B.t.u. of the resultant lean gas may register 1000 or 1100 B.t.u. cu. ft.
Amount of Gasoline Obtained Lower in Summer.—There is a difference in the amount of casing-head gasoline obtainable in summer and winter from plants. In the hot summer days the yield is as much as 30 per cent lower than in the winter months. The amount of refined product depends greatly on the differences in temperature. When the outside air is 30° the vapors in the compressors will have a temperature of say 180°F. This means that in the summer the compressor has a temperature of 180° and the cooling atmosphere temperature is 80 to 90°. The chill ing effect is, therefore, much less in summer than winter and, as a result, the amount of casing-head gasoline obtained is less.