Hybrid separation processes combining vacuum distillation with fractional crystallization, partial melting, and granulation

Abstract

The physicochemical fundamentals of the combined processes of distillative crystallization (also called distillation crystallization or distillative freezing) and distillative sweating (also called distillation sweating or distillation melting) are considered. Results of the experimental study of the kinetics of separation of binary and ternary organic mixtures of various forms (liquids, powders, continuous layers, and granules) are presented. The high efficiency of the combined processes that make it possible to rapidly decrease the impurity content by one to two orders of magnitude after a single stage is shown. It is found that these processes can be used for effective separation of the mixtures of a eutectic composition when conventional crystallization is not applicable. Dependences of the important kinetic characteristic—an effective diffusion coefficient—on pressure are determined for a number of mixtures. An empirical formula for estimating its values is proposed. It is found that the kinetic features of the combined processes conducted under nonequilibrium conditions can cause both a decrease and an increase in the efficiency of separation of the mixtures of different types in comparison with calculated values for the equilibrium process. A method is developed that makes it possible to combine distillative crystallization with the granulation of a melt in a semicontinuous or continuous process on a cooled movable belt. The method ensures the additional purification of solidifying granules from volatile impurities. Results of calculating the main characteristics of the combined process for purifying naphthalene granules in the batch and continuous modes are given.