Abstract: A method is disclosed for the purification of an organic heat transfer fluid without thermal degradation which comprises passing contaminated transfer fluid to a purification zone wherein the fluid is heated to produce a vapor-liquid phase in which the liquid fraction thereof is caused to flow in a narrow flowpath within the purification zone to insure maximum purification temperature and minimum purification zone vessel wall temperature. The heating may be effected by an electric resistance heating means such as bayonet-type electric heaters.

Abstract: An electrically heated still or purifier for the distillation purification of heat transfer fluids such as diphenyl, diphenyl ether and mixtures thereof which are contaminated with high boiling thermal decomposition products. The design incorporates an internal draft tube to induce natural circulation of the boiling liquid and thereby enhance heat transfer coefficients. The purifier is especially adapted for operation with Dowtherm A at temperatures of 700.degree.-750.degree. F. for removal of thermal decomposition products and corresponding superatmospheric pressures for Dowtherm A containing low boiling contaminants or other low boiling organic fluids without danger of overheating the heat transfer fluid.