The Russian process of continuous distillation differs from the American method only in using a series of a dozen or more stills, each of which is heated to a definite steady temperature. The crude oil passing from one still to another encounters these successively higher temperatures, which cor respond to the boiling points of the different petro leum products. Each still constantly gives off a distillate of uniform character, while the series of stills gives the same range of distillates as are obtained by the gradual application of in creased heat in the intermittent system. The pos sibility of supplying the crude oil to the stills as fast as the distillates pass off results in important economies of time, less waste of fuel, and a mini mum of injury to the plant by avoiding the cool Ping and reheating of the still. This process, how ever, is not well adapted to American conditions because of differences in the nature of the crude oils, and in the products most desired. The Ameri can refiner, in general, aims to produce as much kerosene or lubricating oils as possible, whereas in Russia the enormous demand for the residuum, or astatki for fuel, makes it nearly as valuable as any other product. There is, therefore, little in ducement to increase the yield of kerosene and reduce the quantity of residuum by employing the cracking process, which can be done only in inter mittent distillation.
The first distillates obtained from the crude oil by either process usually have to be redistilled or purified before they can be used. Any sulphur which is present must be removed either in the first process or subsequently. One method makes use of copper oxide in the first condenser, or in a specially constructed still, the sulphur by chemical union being removed in the form of a copper sulphide, from which the copper can be reclaimed and used over and over. Another method makes the separa tion by treating the distillates successively with sulphuric acid, caustic soda, and litharge in agita tor tanks built for the purpose, the removal in this case being in the form of a sulphide of lead. This treatment for sulphur is one of the most important and yet most troublesome processes of all, since the presence of a very small percentage of sulphur im parts a highly disagreeable odor to any distillate. No product can be sold until the last trace of sul phur has been removed.
The naphtha distillate, where obtained in impor tant quantities, may be roughly separated into dif ferent grades, or cuts, known as gasoline, commer cial naphtha, and benzine. When the division is made by the stillman, as they come from the condenser, washing with acid, water, caustic soda, and water again, in the metal agitators, to purify and deodorize is the only further treat ment necessary before they are ready for ship ment. More often, however, all the naphtha dis
tillate goes into a single cut as it comes from the condenser, is subjected as a whole to the deodoriz ing and purifying treatment and is then redis tilled and divided into the three fractions men tioned above. This redistillation of the naphtha is done in a special still heated by steam, and with the outlet, through which the vapors reach the condenser, rising for some distance before it actu ally enters the condenser coil. This arrangement is introduced to prevent any liquid from being car ried over into the condenser with the gas. The condenser for the naphtha still also differs from the others in having two coils of pipe, the first of which has a "back trap," or pipe leading back to the still, so that any heavier oils present, con densing quickly, will be returned to the still. The main body of the naphtha distillate is condensed in the second coil of pipe, and is cut into standard grades by the usual separating-box method, but, in order to secure the very lightest of the products, it is necessary to use a third coil surrounded by a freezing mixture of salt and ice. The different cuts obtained from this distillation are immedi ately ready for use as soon as tested to prove their quality.
The distillate of illuminating oil, or kerosene, as we know it, if used just as it comes from the orig inal still, has all the disadvantages which Kier's "carbon oil" presented, charring the wicks, giv ing off an unpleasant odor, and rapidly turning to a dark color after standing, all owing to the pres ence of various impurities. The illuminating "cut," therefore, is given the same sort of purifi cation treatment as is applied to the naphtha. Testing and grading for sale then complete the last stages in the production of kerosene.
The manufacture of lubricating oils, and par affin or wax complete the principal processes of refining. Some lubricating oils are produced by the processes known as sunning or reducing, de pending on the evaporation of the lighter products either by exposing the crude in open tanks or by gently heating it with steam. This method of treat ment is said to have originated from the observa tion that certain oils spilled on the streams of the oil regions were thickened by evaporation, and be came fit for lubricating purposes without further treatment. Experiments with different oils showed the possibility of making natural lubricators in this way from special grades of crude petroleum. So-called "sunned oils" and "reduced oils" are still to be found on the market, but by far the greater proportion of machine oils are products of distillation.