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Rh lease. The main line runs to the compressor station or plant. There are a few plants extracting gasoline from a volume as small as 5,000 cub. ft. of gas per day, while some of the large plants are extracting gasoline from a volume of two and three million cub. ft. per day. The amount of gas necessary to make a profitable proposition is not only dependent upon the volume of gas but also on the quality of the gas. To further assist in the production, a vacuum pump or compressor is installed in the same building with the booster com- pressor. The object of the vacuum pump is to pump the gas from the wells and create a vacuum which materially increases the flow of the gas. Vacuum has been used in oil wells to increase the produc- tion even when gas could not be used for making gasoline.

" There are two processes for extracting gasoline. The one most commonly used is that of compression. The other is the absorption process, which is not only used with casing-head gas but also with natural gas, commonly called " lean gas," which carries as low as one-tenth of a gallon or less of gasoline per one thousand cubic feet. This process is used with gas at high as well as low pressure.

" In the compression process, the equipment consists of one or more two-stage compressors, coils, accumulating tanks, electric generator and other accessories. The casing-head gas is compressed to a pressure of from 50 to 300 pounds and then passed through a system of coils on which cold water is constantly dripping. This cools the gas, condensing the gasoline from it, the liquid being sepa- rated into respective accumulating tanks and the residue gas passing off to be used for power or heating purposes. After the gasoline is collected in the accumulating tanks, it passes into blending tanks, where it is blended with naphtha and other blending mediums to lower the gravity so as to permit of shipping without severe loss through evaporation and to make the shipping of it a safe matter.

" The absorption process is a method of passing the gas through oil and separating the gasoline vapour from it by absorption of gasoline into the oil. There are two general designs in absorption plants, one of which uses the horizontal and the other the vertical absorbers. With either system the oil is used over and over again."

Work of experimentation has been done on a process for extracting gasoline from natural gas by a method whereby natural gas is passed through absorbers containing charcoal, the gasoline absorbed by the charcoal and then distilled out.

Marketing of Products. -Expansion of the use of petroleum prod- ucts has resulted in many improvements in marketing methods and great additions to marketing equipment and facilities. In the United States a large number of tank cars is used in transporting products from refineries to consuming centres and to ports for export shipment. The tank car is still usedto some extent to carry crude oil from the field to refineries, but chiefly this is when a new field is opened up and before pipe-line connexions have been made. Tank cars are widely used in Europe for petroleum products.

There were 49,901 petroleum tank cars in the United States and Canada on Jan. I 1914, and in 1921 the number had increased to 137,493. Rigid requirements are enforced in tank-car construc- tion in the United States. Cars must be of steam boiler quality, and exceptional strength is prescribed for the frame. The insulated tank car to handle highly volatile casing-head gasoline has been successfully introduced and is also being adopted for transporting straight-run gasoline because it reduces loss by evaporation. The growth of motor-vehicle gasoline demand, particularly extensive and rapid in the United States, has created new distributing methods and devices. In that country and Canada gasoline filling installations for motor cars have been established in the cities and on every road of any importance. Thousands of filling stations have been built. Gasoline is delivered from tank wagons or steel barrels to the tanks built underground at the filling stations. The oil is lifted from these tanks by pumps (which measure the amount given by each stroke of the pump) and delivered through a hose into the automobile's tank. Lubricating oil at some stations is delivered in much the same way. An innovation is the visible pump at filling stations.

Fuel-oil bunkering stations have been established along the ocean routes to meet the increase in the oil-burning naval and merchant fleets of all countries. These stations include large storage tanks and berth and loading facilities in some cases, while in many instances they are simply storage reservoirs and oil is loaded on steamers from barges supplied from these stations. It is estimated that the number of fuel-oil stations for steamships located on trading routes approaches 300.

AUTHORITIES: Manual for the Oil and Gas Industry (1918); David T. White, John D. Northrop, E. Russell Lloyd, Mineral Resources of the United States; World Atlas of Commercial Geology (1921) ; Statements on Mexico by E. de Golyer; Frederick G. Clapp, Review of Present Knowledge Regarding the Petroleum Resources of South America; Ralph Arnold, Conservation of the Oil and Gas Re- sources of the Americas; A. Beeby Thompson, Oil Field Development and Petroleum Mining and Oil Fields of Russia; Victor Ross, Evo- lution of the Oil Industry; R. F. Bacon and W. A. Hamor, The Ameri- can Petroleum Industry; G. B. Richardson, U.S. Geological Survey Reports; V. F. Marsters, Geology of the Peruvian Fields; R. H. Johnson and L. G. Huntley, Oil and Gas Production; C. P. Bowie, Oil Storage Tanks and Preventing Oil and Gas Fires; J. H. Wiggins, Evaporation Loss of Petroleum in the Mid-Continent Field; H. Barringer, Oil Storage, Transport and Distribution; H. F. Mason,

U.S. Bureau of Mines Reports; J. M. Wadsworth, Removal of the Lighter Hydro-Carbons. . . by Continuous Distillation; Roy Cross, Handbook of Petroleum, Asphalt and Natural Gas; I. C. White, The Rapid Exhaustion of Ohio's Natural Gas Resources; V. C. Alderson, Oil Shale Industry; Albert Lidgett, Petroleum. (L. M. F.)

PETROLOGY (see 21.323). During 1910-21 there was a steady advance in all departments of petrology, and new develop- ments occurred which were not only interesting in themselves but gave promise of being important in the future. Up to a com- paratively recent time petrology was in the main a descriptive science. The discovery of the application of the microscope to the study of very thin sections of rocks opened up a new field of investigation and showed how defective had been the means employed by the older geologists. It became possible to identify even the smallest mineral grains and to ascertain their relations to one another and the manner in which they were built together from the rock mass. For many years the description of the microscopic characters of minerals and rocks was held to be, if not the only, at least the most important, part of petrographical literature. The processes by which rocks are altered by atmos- pheric agencies and by underground water were revealed in detail. A great body of literature was accumulated, and a great diversity of rock types soon came to be recognized, some of them occurring in abundance in many parts of the world and others rare and exceptional either in their mineral composition or in their minute structures. Textbooks of petrography were in general a de- scription of the recognized rock types, their composition, struc- ture and the stages of their decay, with notes on their geographi- cal distribution and their geological age. Chemical analyses were used principally as a means of identifying the classes to which individual rocks belonged and as a guide to the minerals of which the rocks consisted.

In time it came to be recognized that this aspect of the subject, if not unscientific, was at least incomplete. More attention of recent years has been directed to other problems connected with rocks such especially as the conditions of their origin, their chemical classification and the physical laws which determine what minerals shall be formed, in what order they will crystallize and through what stages they will pass when subjected to cooling, pressure and metamorphism. In fact, the effect of the laws of physics and chemistry on the mineral composition and structure of rocks had become by 1921 a branch of petrology which was rapidly developing. It would be a mistake to regard this as wholly a new development, for many of the older geologists such as Hall Gregory Watt, Daubree, Fouque and Levy, Sorby, Morozewich had carefully studied these problems and had made some notable discoveries. Essentially, however, it was necessary to attack these questions by experimental methods. The old charcoal furnace was very unsatisfactory and difficult to regulate; the gas furnace in all respects was much superior; but the advent of the electric furnace has placed in the hands of the experimentalist a weapon of enormously greater power and far more manageable. There is no difficulty now in attaining temperatures such as occur in the deeper parts of the earth's crust and in the interior of volcanoes and in maintaining these temperatures quite steady for several days or weeks if necessary. The electric pyrometer has replaced the gas thermometer and the older Seger and Wedgwood cones, and has reached such precision that an error of one or two degrees is all that need be expected in measuring temperatures up to 1600 Centigrade. By means of the electric arc, temperatures can be obtained which are beyond those which exist in the upper parts of the earth's crust, but furnaces of this type have been little used in these researches. Very high pressures can be easily obtained provided the temperature is low and there is no necessity to study the action of compressed gases. It is less easy, however, to perform experiments by which the action of steam and other gases on molten rock magmas at temperatures about 1000 and under pressures over too atmospheres can be exactly determined. More than one investigator has now been able to attain this, and a very correct reproduction of the conditions under which igneous rocks crystallize is consequently possible in the laboratory.