DIGESTER, a closed boiler in which a tem perature above the boiling point can be obtained, the hot water and steam disintegrating or di gesting the substance treated. The invention of the digester is credited to Denis Papin, a French scientist. He noted that anything boiled in an open vessel was not subjected to a heat above 212° F., as the water went off into steam at that temperature and was lost. Accordingly he constructed a boiler with a closed top to retain the steam and succeeded in securing a tempera ture of 400° F., which served to readily digest bones. This digester came into use for the man ufacture of soups and gelatines from bones that had been previously wasted or thrown to the dogs. Within recent years the digester has found a place in a number of industries. The lard or grease tank of a slaughter-house is a digester. In some American cities digesters have been employed to receive garbage and se cure a product of some value from the refuse. Tannin is extracted from nutgalls by a digester, in which the material is saturated with ether.
The digester has found its most extensive use, however, in paper-maleing. The wood, which is abraded by grinders from the log, is introduced in a form resembling fine sawdust to the digester, and comes out pulped, ready to be rolled into paper. Sulphite digester is the com mon name for these mechanisms owing to the fact that a bisulphite solution is employed in the process of digesting. This serves as a solvent,
but being acid would attack the iron or steel of which the digester is constructed. To pro tect the iron shell various linings are employed. The Won Antemp digester employs a lining made of a continuous sheet of lead placed against the inner sides of the shell, and held in place by two courses of acid-proof brick. The Mitscherlich digester is lined with acid-proof brick laid in Portland cement. The Graham digester is made of sheets of boiler plate, with the lead lining soldered on before bending. The Partington digester is made spherical, so that the lead lining tends to maintain its place from its form; the lead is also burned to the iron. Pusey & Jones build a digester of welded steel, with double shells and no lining. The Schenck digester is also unlined, being made of deoxidized bronze, and in both of these last named the erosion is claimed to be very slight.
Most sulphite digesters are built in the form of an upright cylinder, with one or more man holes at the top, through which the ground wood is introduced. Steam is admitted by coils and a high temperature maintained for about eight hours. When properly cooked, the liquid is run off and the digested material withdrawn by manholes in the bottom. Digesters are used in the hydrolysis of starch to glucose by means of sulphuric or hydrochloric acid.