In the earlier refineries the stills, the capacity of which varied from 25 to 80 barrels, usually consisted of a vertical cylinder, constructed of castor wrought-iron, with a boiler-plate bottom and a cast-iron dome, on which the " goose-neck " was bolted.
The operation was, however, completely revolutionized in the United States by the introduction of the " cracking process," and by the division of the distillation into two parts, one consisting in the removal of the more volatile constituents of the oil, and the other in the distillation (which is usually conducted in separate stills) of the residues from the first distillation, for the production of lubricating oils and paraffin.
Various arrangements have been proposed and patented for the continuous distillation of petroleum, in which crude oil is supplied to a range of stills as fast as the distillates pass off.
American stills of the former type are constructed of wrought-iron or steel, and are about 30 ft.
The stills were formerly completely bricked in, so that the vapours should be kept fully heated until they escaped to the condenser, but since the introduction of the " cracking process," the upper part has usually been left exposed to the air.