Another very convenient arrangement for the reception and distribution of the distillates is shown in Fig. 88. It is much used in this country and is very similar in principle to the one previously described. The distillate enters at a and passes through the siphon c. The gases are forced to pass out at b while the distillate runs into the cast-iron box d. The latter is provided with the glass i i, through which the progress of the distillate can be observed. From the box the distillate runs through the distributing pipes, distribution being effected by means of the three-way cocks e e and the ordinary cocks ƒƒ. Through the pipes ggg the distillate runs into the collecting tanks.

In Figs. 86 and 87 is shown an open method of distribution tanks. With sufficient condensation and complete separation of the gases this method is, in many cases, to be preferred to closed distribution. The separation of the fractions is more simple, and, when several stills are in operation, the control of the progress and the quality of the distillates is more readily accomplished. As seen from the illustration, the arrangement is very simple. The distillates leave the condensing pipes a a a on the front side of the cooling box in the small building, pass through the siphon U and run according to their quality-— which is regulated by the application of smaller or larger sheetiron pipes-into the gutters R,, R2, R3, R. These gutters of cast-iron are as long as the front side of the condensing apparatus and run parallel with it. On one end the gutters are provided with a small pipe and a cock H, by means of which the distribution of the distillates is effected. The condensing pipes a a a, at a short distance from the point where they leave the condensing apparatus, are provided with the vertical pipes 1 g, etc., and with the cocks h, ha, h. The gas shut off by the syphon ascends in these pipes and-when the cocks are open-through the horizontal pipe G into the gas holder, to be used for illuminating and heating purposes.

When in the commencement the evolution of illuminating gas is still slight, or when towards the end of the operation it

becomes so large that the gas-holder cannot hold it, the cock L is opened and the gas escapes into the open air.

Tanks for the Distillates.

Through the distributing pipes the distillate runs into the collecting tanks, the size and number of which depend on the mode of working, so that no definite directions can be given in this respect. Regarding their arrangement the following may be laid down as a general rule: The total capacity of the tanks should, chiefly for the sake of safety, be at least equal to that of the still, because as they have to be at a sufficient distance from the still they form the first and safest place of storage for the oil, and in case a still should leak, its contents may be conducted into the collecting tanks through a provisional conduit.

The construction of the collecting tanks varies. They are, as a rule, of a cylindrical shape, closed in most cases and provided with man-holes. They are generally made of iron, though wood is sometimes used. They are now almost universally placed underground in order to give the distillate a free fall from the distributor; with the latter they are connected by a system of pipes. To prevent their contents from freezing they are provided with a steam coil, and at the lowest point are furnished with a contrivance for the discharge of the separated water. A contrivance may here be mentioned, which has been found useful in practice when it becomes necessary to pump one or the other of the fluids from a vessel containing two fluids of different specific gravities. For this purpose the suction pipe, which reaches nearly to the bottom of the vessel, carries a movable double elbow with a horizontal pipe. By lifting or sinking the elbow the fluid may be sucked off at every level.

Process of Distillation.

Distillation is carried on either periodically or continuously. Continuous distillation will be discussed later on.

Periodical

distillation is used everywhere, even in the largest refineries, when the supply of crude oil is not steady and sufficiently large. The still is filled in the manner previously described by means of pumps, or from tanks placed at a higher level. To allow for the expansion of the oil, the still, according to its shape, is filled to full. The crude oil is brought into the still either cold or preparatorily heated by the residuum, after which the actual distilling process commences.

The fire being kindled it is at first kept low, because if any water is present, which is recognized by a suspicious bumping, it is capable of hurling out the contents of the still, if there is not sufficient empty space. With a gentle fire the fluid is agitated without causing injury, and the first traces of the lightest products of distillation and of water appear in the condensing apparatus. The quantity of water constantly increases until the water in the still boils. This is designated "boiling of the still." The quantity of water then decreases suddenly, and a quite uniform stream of distillate runs from the "outlets." The fire is now somewhat increased and kept low only when oils containing benzine are distilled until the change to illuminating oil is made. During the distillation of illuminating oil a strong fire is kept up. When distillation has progressed so far that specifically heavier and darker oils appear, the fire is slackened, and the distillate runs off, even in the largest refineries, in a stream not thicker than the finger. At this stage of distillation a decomposition of the oil, the so-called "cracking," takes place. If circumstances permit to keep up distillation in this manner with a slackened fire, a considerable portion of the heavy oils may be converted into light oils and mixed with the illuminating oil.

According to the yield of the oils these general rules have to be modified. With oils poor in illuminating oil, the residuum of which is not very valuable, distillation must be forced more and the decomposition of the heavy oils can be continued longer than with oils rich in illuminating oil, or with oils the residuum of which is valuable for the fabrication of lubricating

The products of distillation may be divided into two principal groups: a. Light oils, essences, or, as commonly called, benzine. b. Illuminating oil or petroleum. Distilling for heavy oils, after illuminating oil, is but seldom done in the crude oil still. The heavy oils remain in the residuum, which is discharged and then worked by itself.

The benzine obtained in distilling crude oil can but seldom be used as such, it being generally subjected to redistillation and purification in apparatuses to be described further on. Sometimes, however, a separation is effected while distilling the crude oil, and the product thus obtained is either used as it is, or subjected to chemical purification. The lightest products, such as petroleum-ether, etc., are generally caught, gasolene and ligroin being also obtained. This mode of working is, however, not advisable, because the products thus obtained are of less value; the carrying along of heavy oils with these light products being unavoidable, so that they can never be obtained free from fat and water-clear. In every well-arranged refinery working crude oils containing benzine, only one light product, viz., crude benzine, is obtained in distilling crude oil. The point at which the distiller "cuts off" his benzine and directs the product into the illuminating oil tank varies according to the kind of oil he manufactures, the quality of the oil being largely determined by local conditions, by the laws regarding the burning point, and by commercial usage. Where no importance is attached to the specific gravity and burning point, the distiller will endeavor to introduce into the illuminating oil as much as possible of the heavier constituents of the essences, and increase the gravity by the addition of heavy oils. In other localities he will separate as much benzine as is necessary to obtain a good product. In almost all civilized countries the test of 21° Abel' is the permitted limit. As a rule, a specific gravity of 0.740 to 0.760, which corresponds to a boiling point of 3020 F., may be designated as the point at which the dis

1 See Chapter on "Tests."

tiller should cut off his bezine and direct the product into the illuminating oil tank.

The properties, as well as the specific gravity, of the first product of distillation, viz., the crude benzine (also called crude naphtha) vary very much. It is a very mobile liquid of a slightly yellowish color, due to the presence of heavy oil, and of a penetrating odor caused by a very volatile body containing sulphur; its specific gravity varies between 0.700 and 0.730.

The second principal product of distillation consists of the illuminating oil distillate. When only an average quality is to be produced from a certain crude oil no further fraction is made, distillation being continued as long as color, specific gravity and inflammability will permit. The average specific gravity illuminating oil must show to be marketable varies in different localities. Thus the commercial varieties produced in the United States show from 44° to 45° Bé. (0.805 to 0.810 specific gravity) while the Russian illuminating oils show 41° to 42° Bé. (0.820 to 0.824 specific gravity). Galician and Roumanian illuminating oils generally show 43° to 44° Bé. (0.810 to 0.815 specific gravity.)

While no definite directions can be given, the product obtained between the specific gravities 0.750 to 0.860 and 0.870 is considered illuminating oil distillate. If besides the ordinary commercial varieties, special qualities, such as head-light oil, are to be produced, the lighter products of distillation are kept out until the Beaumé scale indicates 56°. In this process a large percentage of oil between 62° and 56° is obtained, which may be returned to the benzine tank, but is generally reserved for the manufacture of a lower grade of oil.

The properties of illuminating oil distillate vary very much. It generally represents a very mobile fluid of a yellow to brownish, and sometimes red-brown color, and shows strong fluorescence, mostly playing into blue, but sometimes, with too rapid distillation, into green. Its odor also varies very much; oils which contain much oxygen have the characteristic odor of creosote, while other oils, for instance Russian, have an