The early refineries were mainly small plants with a few vertical iron stills resembling giant cheese boxes, and having a capacity of twenty-five to seventy-five barrels each. As the industry ex panded, however, and made constantly increasing demands on the capacity of the refineries, larger and larger stills were introduced. A horizontal cylinder still was found to offer various advan tages over the old cheese-box style, and the cylin der form, with a capacity of about 600 barrels, is the type now generally used in this country.
Each still may have its own condenser, or sev eral stills may be connected with a common con denser, although the former arrangement is pref erable. In either case the condenser is the same, consisting of coils of three- or four-inch pipe sev eral hundred feet long, and ordinarily kept cool by thousands of gallons of water pumped over them daily. The hot vapors entering the con denser from the still come in contact with the cold pipe and return to liquid form, in the same way as steam on a winter day will collect on the cold glass of a window and trickle down the pane in tiny streams of water. The refined product of a dozen condensers may be turned into a single receiving tank until the limit of its capacity is reached, and then other similar tanks are pressed into service.
Between the condenser and the receiving tanks, the distilled oil has to pass through the stillhouse and undergo the keen scrutiny of the stillman, on whose skill the success of the entire process de pends. The condensed distillates make their en trance to the stillhouse through a V-shaped tube, such as are commonly inserted in drain pipes to prevent the passage of sewer gas, and which serves much the same purpose here. A vertical pipe on the condenser side of the V allows the uncondensable gases from the still, that is, those vapors which will condense only at very low temperature, either to escape into the air or to be led away to be burned under the still from which they came. The con densed distillate, now in the liquid form again, passes through the V tube and enters the stillhouse in what is called the separating box, a triangular, cast-iron affair. A glass door on one side of the box enables the stillman to watch both the color of the oil and the size of the stream as it enters the box.
In this way, from the knowledge of long experience, he knows how to regulate his fires under the stills, and from occasional samples of the distillate he can determine when a different grade of oil has begun to vaporize in the still and is coming through the condenser. Shutting one valve and opening an other close at hand turns the stream into a differ ent receiving tank. So the process goes as long as separation is possible, or until some special re quirements make it desirable to stop the distilla tion at a certain point.
The actual process of distillation consists in carefully separating the different hydrocarbon compounds which make up the crude petroleum. These "fractions," as the different compounds are called, are determined more or less arbitrarily by their weight as compared with an equal bulk of water, and by the ease with which they give off inflammable vapors.
Distillation may be done by what is known as the intermittent process, in which the major part of the operation is carried on in one still heated to successively higher temperatures by gradually in creasing the fires beneath it. This method is most commonly used in the United States. Distillation may also be done by the continuous process, in which the crude oil is pumped through a series of stills, each succeeding one being heated to a con stant temperature higher than that of the one pre ceding. .
In the intermittent process, the crude oil in the still is subjected to a gradually increasing tempera ture, so that the different fractions pass off to the condenser in the order of their volatility. The lighter and more volatile compounds, that is, those boiling at low temperatures, are vaporized first, the heavy, less volatile compounds not appearing until the highest temperatures are reached. Dif ferent petroleums vary so widely in character, and the number of possible products is so large that each kind requires special treatment to secure the particular products for which it is best adapted. The distilling business, therefore, becomes decid edly intricate when examined in detail, and a high degree of skill must be exercised in manipulating the process so that it will yield the largest quantity and best quality of the valuable oils.