« PreviousContinue »
the fact that nearly all condensates obtained under high pressure (200 to 300 pounds per square inch) boil at temperatures near 0° C. Of course, the boiling point is modified by the presence of other constituents. Pentane, the next highest homologue to butane, boils at 36.4° C. (97.5° F.).
GASOLINE-PLANT METHODS AND EQUIPMENT.
USE OF COMPRESSORS.
Single and two stage compressors are generally used in gasolineplant operations. Single-stage compressors are generally used where pressures of 110 pounds per square inch are not exceeded, although they are used at some plants where pressures up to 150 pounds per square inch are employed. At higher pressures, higher temperatures are produced. One of the makers of compressors states that with a 100-pound compressor volumetric efficiencies of 65 per cent are obtained, and with 50-pound compressors efficiencies of 80 to 85 per A 150-pound compressor gives only about 50 per cent efficiency.
In the initial stage the gas is subjected to pressures varying from 20 to 50 pounds per square inch. Probably 35 pounds per square inch is an average value. The temperature of the gas may rise to 450° F. (232o C.) in the low-compressor cylinder, but no liquefaction of gas or condensation of vapor will occur. As stated elsewhere, gases can not be liquefied by the application of pressure alone at temperatures above their critical temperature, and at higher temperatures they would have a greater capacity than ever for the vapors present, so that no condensation of the vapors would occur.
COOLING OF MIXTURE.
During the next stage of operation, the mixture passes through a water-cooled 2-inch iron pipe, probably 100 feet long for every 100 cubic feet of gas passed per minute. The cooling is effected at the same pressure as maintained in the low-stage compressor.
Usually little liquid is obtained at this stage of the operation, but that obtained is sometimes collected in the main receiving tank of the plant or in separators provided for the purpose. At a few plants, however, the condensate collected is considerable, being as much as 10 per cent or more of the total, but this yield is exceptional. The quantity of condensate obtained must, of course, principally depend upon the extent of the saturation of the original gas mixture with gasoline vapors. The quantity of gasoline vapors in the gas mixture
• One firm that installs gasoline plants allows 1 square foot of radiating surface for each cubic foot of natural gas passing through the coils, an allowance stated to be largely in excess of the needs. The cooling. coils are frequently equipped with a by-pass, so that only part of the coil need be used in cold weather.
depends upon many conditions, such as the quantity of gasoline peculiar to the oil, temperature and pressure conditions in the well, intimateness of contact between the oil and gas in the well, composition of the gas, and the treatment the gas receives in the plant.
After the mixture of gas and vapor has left the first cooler it is conducted to the high-pressure cylinder of the compressor and subjected to a pressure varying, perhaps, from 250 to 350 pounds per square inch. The temperature is raised probably to 250° C. (482° F.) in the compression cylinder. Under these conditions no liquefaction or condensation occurs for reasons already mentioned in connection with the discussion of the low-stage compressor. The mixture, still under pressure, is next forced through 2-inch pipe coils, on which water of ordinary temperature falls. In some plants the coils are immersed in tanks of water. An average temperature of probably 32° C. (90° F.) in summer and 4° C. (°39 F.) in winter is maintained. The temperature is further reduced by expanding the gas under the pressure mentioned through a valve into a pipe that envelops the final section of pipe through which the compressed gas travels on its way to the accumulator tank. Expansion against a reduced pressure of 30 pounds per square inch occurs at some plants. A temperature of about 4° C. (39° F.) is normal for a plant using this plan.
F. P. Peterson, general manager in the state of Oklahoma for the Riverside Western Oil Co., informed the authors that according to his experience the results obtained from direct expansion of the gases usually fall far short of expectation and that efforts in the direction of more efficient cooling by expansion are being made through what is termed a system of closed expansion. This system involves the use of a cylinder with a piston and a means of resistance against which the gas is made to do work in expanding. Water cooling is avoided as in several other different types of installation.
In the standard type of device for cooling and compressing, as above described, methane and ethane are not liquefied, but butane and, in some plants, propane are. A heavy condensation of gasoline vapors usually takes place, the final mixture containing principally butane, pentane, and hexane. Propane and also heptane may be present. There will also be found some of the gases methane and ethane dissolved in the liquid produced.
CHANGES IN GAS ON COMPRESSION AND COOLING.
The gas, in passing through the compression and cooling coils, undergoes several changes; one has to do with the condensation of vapor, another with the liquefaction of gas, and a third with the
solution of gases and vapors in the liquid produced. The vapor condensation depends simply upon the reduced capacity of the different gases to carry the vapors when subjected to increased pressure and lowered temperature. The liquefaction of natural gas depends upon the temperatures and pressures at which the gases butane and propane are liquefied at their partial pressures. The solubility depends upon the solubility of the gases in the liquids produced at the particular temperature and pressures encountered. The three changes mentioned are so intimately concerned with each other that one factor can not be disturbed without affecting the others. For instance, such a pressure above and temperature below those at present ordinarily used could be maintained as to increase to some extent condensation of the most desired constituents, pentane, hexane, and heptane, but with increasing pressure and lowered temperature more of the undesirable gaseous constituents would become liquefied. These, when exposed to the atmosphere, immediately volatilize, carrying with them some of the gasoline constituents. At increasing pressures more of the gases methane and ethane dissolve. With release of pressure they would escape with violent agitation and further loss of the condensate.
Present methods of treating the raw gas have resulted from actual plant experiments on a large scale. Little record of these experiments has ever been published. The standard equipment herein mentioned is the result to date. Improvements in the process are bound to take place, but they must follow established laws of condensation, liquefaction, solubility, etc. The authors know of commercial ventures that were failures because of the management's lack of knowledge of the fundamental laws of physics and chemistry and of the nature of the material they were dealing with.
For the purpose of picturing just the physical changes that take place in the process a diagram such as that constituting figure 8 is instructive.
Suppose one starts with a natural gas at atmospheric temperature and pressure, and suppose it contains a certain weight of a vapor of a liquid hydrocarbon, M, which exerts a part of the total pressure of the mixture. This vapor must be dry, saturated, or superheated. Assuming it to be superheated the condition of 1 pound of it may be represented by the point a, figure 8. In the diagram the ordinates represent pressures and the abscissas, volumes. The point a fixes the volume Va and the pressure Pa of a pound of the vapor at the temperature t. Now the pressure is increased until the point b is reached, representing the boiling point of the liquid at a certain temperature and pressure; condensation begins and continues at constant pressure until complete at bb. The line abb,c is the isothermal line at the temperature t.
As the vapor of the liquid hydrocarbon constitutes only a part of the gas mixture, as was first assumed, in order for condensation to begin it is necessary that the gas mixture be compressed to pressures above atmospheric, depending on the partial pressure of the vapor, in order to have a pressure (ordinate of b) on the vapor. In practice, however, the problem is complicated. The amount and character of the vapors are only imperfectly known. The most suitable pressures for any given plant operation can be obtained only by actual trial
FIGURE 8.-Diagram illustrating physical changes that take place in the production of gasoline from natural gas. DETAILS OF EQUIPMENT OF GASOLINE PLANTS.
In figures 9 and 10 are shown the plan and elevation of a plant for making gasoline from natural gas by the compression method.
The gas from the wells enters the plant by means of a gas line. After passing through a drip tank a, for the removal of oil that might be carried with the gas, it partly circles the compressor building and enters the low-stage compressor; after compression it is conducted to the low-stage cooling coils, and thence to the high-stage compressor e. From this compressor it passes to the cooling coils c, and is from them expanded into the cooling coils d.