The invention relates to a method and an apparatus for the mass separation of a liquid mixture through fractional crystallization, wherein the crystal layer is crystallized on the indirectly cooled wall of the crystallization zone. The mass transfer occurs in thin boundary layers which are produced by rising gas bubbles which glide as closely as possible along the heat-dissipating surfaces. The method is well suited for products which form only a soft crystal layer with a rough surface. It is carried out in a vertical pipe heat exchanger into the pipes of which (6) there extend into the lower part thereof gas-inlet pipes (7) which are mounted in the middle and are secured with centering pins (8). The maximum inside diameter of the pipes is dependent upon the optimum thickness of the crystal layer and of the concentration of the crystallizable component in the feedstock.
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