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.
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.
The stills employed in Russia and Galicia are usually smaller than those already described.
In the American petroleum refineries it is found that sufficient cracking can be produced by slow distillation in stills of which the upper part is sufficiently cool to allow of the condensation of the vapours of the less volatile hydrocarbons, the condensed liquid thus falling back into the heated body of oil.
In the earlier stages of the development of the manufacture of mineral lubricating oils, the residues were distilled in cast-iron stills, and the lubricating properties of the products thus obtained were injured by overheating.
The modern practice is to employ horizontal cylindrical wrought-iron or steel stills, and to introduce steam into the oil.
Glass stills heated by a sand bath are sometimes employed in the final distillation of sulphuric acid; platinum, and an alloy of platinum and iridium with a lining of gold rolled on (a discovery due to Heraeus), are used for the same purpose.
Cast iron stills are provided with a hemispherical head or dome, generally attached to the body of the still by bolts, and of sufficient size to allow for any frothing.
In its more complete form a still has in addition the following fittings: - The dome is provided with openings to admit (I) the axis of the stirring gear (in some stills the stirring gear rotates on a horizontal axis which traverses the side and not the head of the still), (2) the inlet and outlet tubes of a closed steam coil, (3) a tube reaching to nearly the bottom of the still to carry live steam, (4) a tube to carry a thermometer, (5) one or more manholes for charging purposes, (6) sight-holes through which the operation can be watched, and (7) a safety valve.
This appears particularly in their attitude toward revenue officers sent to discover and close illicit stills for the distilling from Indian corn of so-called " moon-shine " whisky (consisting largely of pure alcohol).
On the large scale it is obtained by distilling Chile saltpetre with concentrated sulphuric acid in horizontal cast iron stills, the vapours being condensed in a series of stoneware Woulfe's bottles.
Owing to the impossibility of employing any metal in contact with the acid, the " chlorine stills," where the above reaction is carried out, must be made of acid-proof stones or " chemical " stoneware.
Moreover it is of a most disagreeable kind, as the waste "still-liquor," containing very much free hydrochloric acid and even some free chlorine, forms a most deleterious impurity when finding its way into drains or watercourses, apart from the intolerable nuisance caused by the escapes of chlorine from the stills and otherwise, which cannot be at all times avoided.
Both these reactions are carried out in tall cylindrical columns or " stills," consisting of a number of superposed cylinders, having perforated horizontal partitions, and provided with a steam-heating arrangement in the enlarged bottom portion.
The periodic process depends on the interaction between manganese dioxide (pyrolusite), sulphuric acid, and a bromide, and the operation is carried out in sandstone stills heated to 60° C., the product being condensed as in the continuous process.
This furnace is also very well adapted for impure acids, unsuitable for platinum or platinum-gold stills on account of the crusts forming at the bottom of the retorts; and it is more and more coming into use both in Great Britain and on the Continent.
American stills of the former type are constructed of wrought-iron or steel, and are about 30 ft.