Teilkondensation und nichtadiabate Rektifikation binärer Gemische in Füllkörperrohren

Abstract

AbstractPartial condensation and non‐adiabatic rectification of binary mixtures in packed columns. Vertical tube assemblies are preferred for partial condensation of vapour mixtures. The mixture of vapours is fed through the tubes in countercurrent flow to the falling condensate, while coolant flows through the jacket. Only with very large ratios of length L to diameter d of the tubes, i.e. L/d 200, does the separatory effect of partial condensation, owing to rectification mass transfer between the phases, exceed the enrichment effect of partial condensation. Use of ring‐shaped packings drastically reduces the characteristic diameter, so that the additional separatory effect occurs at relatively short tube lengths. In the present paper, experimental results concerning the separation efficiency of tubes packed with Raschig rings for non‐adiabatic rectification and partial condensation of the “negative” binary mixture n‐heptane/benzene are reported and evaluated with the aid of the extended two‐film theory. It is found that the resistance to mass transfer averaged over the length of the tube lies entirely on the liquid side on non‐adiabatic rectification. Correlation of the Sherwood number of the condensate taken over the length of the tube and the condensate loading clearly shows that the usual calculation of partial condensation with average total mass transfer coefficients does not give physically meaningful results in the case of liquid‐side resistance to transport. Calculating procedures which account for the variation of the partial mass transfer coefficients along the tube should lead to improved evaluation or prediction of separation efficiency.