As early as 1823 the brothers Dubinin erected a refinery in the village of Mosdok, and in 1846 applied to Prince Woronzoff for a subsidy for extending the use of petroleum-distillates in the Caucasus.
The distillates obtained are usually purified by treatment, successively, with sulphuric acid and solution of caustic soda, followed by washing with water.
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.
If a complex mixture be operated upon, and a separation effected by collecting the distillates in several portions, the operation is termed "fractional distillation."
For the purpose of collecting the distillates in fractions, many forms of receivers have been devised.
Wood-spirit, petroleum and coal-tar distillates are condensed in plant of the latter type.