STORAGE OF VOLATILE LIQUIDS HUGH HARVEY Shell Oil Company, New York City
losses during the storage of volatile liquids can be substantial. In addition, when inflammable materials are being handled, the fire hazard cannot be ignored. These facts are perhaps nowhere better understood than in the petroleum industry which is called upon to store millions of gallons of highly volatile liquids every year. Ordinary gasolme, for example, has a vapor pressure of 6 . 5 ~s.. i. a. a t 70°F. Let us assume that we have this gasoline in a covered storage tank, vented to the atmosphere. According to Dalton's Law, then, there are in the vapor space above the liquid four molecules of vaporized gasolme for every five molecules of air. This condition exists, of course, only under static conditions. Normally, air currents will cause some of the vapor to escape, and approximately 40 per cent of the escaping gases will be gasolme. Losses of this type are referred to as windage losses; they account for approximately one-half of the standing storage loss in tanks not equipped with vapor tight roofs. Storage tanks are also subject to the diurnal fluctuation in temperature. If the tanks are vented to the atmosphere, air is inhaled a t night and exhaled during the day, thereby leading to breathing losses. In this process, condensation of gasolme takes place on the under side of the roof plates as a result of the convection currents set up during the inhaling process. Since the roof plates readily absorb heat, the condensed vapors evaporate rapidly and are vented during the exhaling period. Through a combmation of breathing and windage as much as 1500 gallons of grtsoline out of 275,000 can be lost in 20 days. This loss, of cgurse, is cumulative and over a period of years can be quite substantial. Another factor contributing to vapor loss in vented tanks occurs during emptying and filling. During withdrawal, air is drawn in through the roof vents and is soon saturated with hydrocarbon vapors. In the filling process the air-vapor mixture is of course forced out through the vents leading to what are known as filling losses. Finally, we have losses arising from the boiling of volatile liquids; for example, motor gasoline will boil a t around 130°F., while some of the lighter fractions like isobutane going into its manufacture boil a t about 10°F. Taken altogether, these vapor properties of p6troleum products present a very complex problem to the engineers. Theseemingly obvious answer to storage is of course totally enclosed containers, but these immediately become pressure vessels, the design and construction of which are highly specialized techniques. Furthemore, the cost of pressure vessels is no small matter, though there is little doubt that the accumulated VAPOR
savings resulting from their use, not to mention the added degree of safety they bring, make them a good investment, especially for large capacity tanks and highly volatile liquids. There are many types of pressure vessels designed for storing volatile liquids and gases, the most familiar being the gas holders in gas plants and the cylinders used for transporting compressed gases like hydrogen and oxygen. The gas holders are the variable volume type of pressure vessel and are rarely used a t pressures above 10 p. s. i. Cylinders have, of course, a fixed volume and can withstand pressures as high as 2000 p. s. i. or more. These extremely high pressures, however, do not enter into the handling of petroleum products. For some of the extremely volatile fractions, like butane and propane, cylmdrical tanks are sometimes used, but when large quantities are involved, pressures rarely exceed 250 p. s. i. For this purpose, a cylindrical shell with hemispherical heads can be used, and tanks holding as much as 12,000barrels have been built in this manner. By far the best closed container for handling volatile liquids is obtained by using spherical construction. Vessels of this type are of two classes, the sphere and spheroid. For liquids requiring pressures of from 25 to 100 lbs. per sq..in. or higher, the hortonsphere is used. These are true spheres and have the smallest surface area for a given volume. Stresses are equally distributed when the internal pressure is uniform. Hortonspheres may be 65 feet in diameter and have a capacity of 25,000 bbls. or 144,000 cubic feet. The advantages of large-scale storage are twofold. Large
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X h ~ l h h o r o . Great Lakes Pipe Line Company Faidax. Oklahoma. Did You Ev,er Wonder Why Gaeolin. Tanks
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JOURNAL OF CHEMICAL EDUCATION
198
containers are less subject to transient temperature variation than small containers and, therefore, pressure conditions inside vary over a narrower range. From an economic point of view, a 10,000-bbl. hortonsphere requires only one set of pipe fittings, whiie this corresponding capacity stored in cylindrical tanks might require many times this number of fittings. In some cases spherical vessels are used to store natural gas. Vessels of this type are two concentric spheres with a layer of insulating material three feet thick between them. Under these conditions 50,000,000 cubic feet of natural gas a t normal temperature and pressure can be stored in 97,000 cubic feet. The gas is first compressed to about 600 p. s. i. and then cooled to -130°F. by ethylene and ammonia. At this stage, it is suddenly allowed to expand to about 3 p. s. i. in well-insulated vessels, and the temperature drops to -250°F. When stored in the insulated spheres, pressure remains low over long periods.
The Hortonspheroid is of two types, the plain and the noded. These vessels are designed to conform to the shape assumed by a rubber bag containing gas and liquid under pressure; this is an oblate spheroid. The plain spheroid is shown in Figure 1 and the noded spheroid in the Frontispiece. Plain spheroids usually handle only pressures up to 30 p. s. i. while the noded type rarely exceed 15 to 20 p. s. i. The advantage of the noded spheroid is the great capacity that can be gained by such construction. Some units handle 100,000 bbls. or more of motor fuel. Top and bottom are connected by internal structural members which serve to offset gas pressure and support the top. Plain spheroids employ no inside framing. Both types rest on the ground inside a steel cradle, and design is standardized so that soil-loading is never excessive. These vessels are particularly useful in cold climates when containers with floating roofs may experience difficultybecause of ice and snow.
ORIGIN of COALS, OIL, ASPHALT, 8 NATURAL GAS PLANT MATERIAL I
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Dr. E I N ~ h l . Reprinted horn the Carnrgk Technical (Octobu. 19161.
