This system has proved quite ineffectual in preventing the ignition of stored petroleum by lightning, and in confining the fire when once it has broken out. Relying upon the fact that petroleum will not ignite unless it is vaporized, E. A. L. Roberts, of Titusville, Pennsylvania, has devised a lightning-proof tank, as shown in Fig. 1021: a, oil-space; b, diaphragm ; c, balance-pipe ; d, filling and emptying oil-pipe ; e, inlet and overflow water pipe ; f, vent-pipe ; g, water layer above the oil ; water layer beneath the oil. The tank is first filled with water by the pipe d, entering the tank immedi ately under the diaphragm ; the admission of water is continued until it has passed up the balance-pipe c, and filled the space g, driving out the air by the vent f. Petroleum is then forced through d, dis placing the water, which passes up c into g, the sur plus escaping by the outlet e. Sufficient water is left to form a layer of about 6 in. on the bottom, and at least as great a depth remains above the diaphragm.
When the vent f is closed, no air can mingle with the petroleum, and no evaporation can take place. In order to draw oil out, water is forced in by e.
A plan projected by Donny embraces the storage in both bulk and barrels, so as to be free from danger of ignition by any ordinary occurrence, such as lightning, and to confine the fire and the burning material in case of a conflagration. The cisterns are constructed of concrete, with vaulted roofs, preferably below the surface of the ground ; if above, the walls must be kept moist, to prevent leakage. For storage in bulk, a number of tanks are formed of concrete, communicating by siphon-pipes, into and out of which the oil is passed by pumps. The barrels are stored in long concrete vaults, closed by a double ayetem of airtight doors, made of light sheet iron, and ao arranged that if one is blown out, the second will fall into its place. Supposing the doors to act, in case of an explosion, the first doer will be blown away, when the second resumes its place, shutting off air, and smothering the fire ; should the doors not act, the burning oil will flow along a passage specially provided for it into an immense cistern, and meantime the air-supply can be cut off by banking up earth in the doorways.
With regard to the material for the construction of petroleum receptacles, Dr. Stevenson Macadam states that lead will spoil lamp-oil in a week, or less if bright ; iron does net detract from the illuminating qualities, but deepens the colour, and causes a rusty deposit ; zinc, solder, and galvanized iron are all deleterious. Metals which do not seriously damage the oil, but which still cause its deterioration by contact prolonged for months, are tin, copper, and tinned copper, common solder containing lead being excluded from use in their manufacture. Stoneware, slate, and enamelled iron are recommended as superior to all metals.
A very curious circumstance, which may be turned to some practical account, is that the addition of a little powdered soapwort (Saponaria offieinalis), digested in water, causes petroleum to form a solid mucilage, and that the subsequent application of a little phenol (carbolic acid) causes it to resume perfect limpidity.
Separation of the Constituents, and their Uses.—Crude petroleum is usually a dark greenish brown liquid, of somewhat offensive odour, having a density varying from 40° to 48° B. (0.820-0.782 sp. gr.), and composed of not less than 30 distinct hydrocarbons capable of separation by heat. To prepare the oil for commerce, it is freed from both the heaviest and the lightest members. The operations are directed to the separation of the following matters :— (1) The light oils, which are highly volatile and inflammable ; (2) the heavy oils, which do not illuminate well, but are good lubricators ; (3) tarry matters ; (4) colouring matters ; (5) malodorous matters. This involves 3 or 4 distinct processes :—(1) Fractional distillation ; (2)
agitation with sulphuric acid ; (3) agitation with hydrates of soda and ammonia ; (4) washing with water ; (5) occasionally a second distillation after the acid and alkali treatment.
The distillation is effected in an iron still, provided with a condenser-coil. (Several forms of still used in fractionizing coal-tar will be found described on pp. 641-4, and much information bearing upon the subject is scattered throughout the article on Coal-tar Products, pp. 641-684 ; see also Paraffin.) The matters first issuing from the still are very volatile gases, which escape con densation at ordinary temperatures, but which, by cooling and compressing, may be converted into the volatile liquids rhigolene and cymogene. As the distillation proceeds, the issuing matters take a liquid form at ordinary temperatures, and increase in density, from 95° B. (0.629 sp. gr.) downwards. These oils may be separated according to their densities as they come over ; but it is more usual first to collect in one receiver all the oils that pass over between 95° and 65° B. (0.629-0.723 sp. gr.), constituting (a) " crude naphtha," and to effect the breaking-up of this crude naphtha by a subsequent operation. When the distillate shows a density of 65°-59° B. (0.723-0.748 sp. gr.), it is run into the (b) "kerosene " tank, until the density reaches about 38° B. (0.838 sp. gr.), or the colour deepens to yellow. The next portion is then collected as (c) paraffin-oil, until nothing but pitch or coke remains in the still, the density of the last products being about 25° B.(0.906 sp. gr.). The distillate (a) "crude naphtha," by redistillation, is broken up into " gasolene," or " light naphtha," ordinary " naphtha," and " benzine." The " kerosene" or lamp-oil, forms the bulk of the product. This is agitated with about 2 per cent. by volume of sulphuric acid, to remove. the disagreeable odour and a portion of the colour. Thus partially cleansed, it is washed with water, then with alkali (hydrate of soda or ammonia) to correct the remaining traces of acid, then with water to remove the taint of alkali. Sometimes it is redistilled at a higher temperature than before to remove the small percentage of naphtha or benzine still present. Finally it is exposed in open tanks, under glass, to the sun, for 24 hours or so, to complete the bleaching and sweetening. Tho extra price at which kerosene is sold tempts many distillers to neglect the separation as just detailed, and to mix as much benzine and naphtha as possible with the kerosene. The tendency of this is to reduce the flashing-point (see p. 1479) in a remarkable degree, and to render the oil totally unsafe for illuminating purposes ; an oil flashing at 113°F. was reduced to 103° F. by the addition of 1 per cent. of naphtha, and to 83° F. by the addition of 5 per cent., while with 20 per cent., the mixture actually burned at 50° F. The annexed table shows at a glance the densities, proportions, uses, and relative market values of the several products of the fractional distillation of crude petroleum :— The substance called " vaseline " is the residue from the distillation of petroleum, purified by an elaborate process. It is a pale-yellow, translucent, slightly fluorescent, semi-solid, of sp. gr. 0.840 at 54° (129° F.), insoluble in water, slightly soluble in alcohol, freely in ether, and miscible in all proportions with fixed and volatile oils. Being unchangeable, it has been proposed as a basis for ointments, perfumes, and other compounds where fat is usually employed.