Patent ID: 7947845

Claim:
A process for purifying removal of a crystal of acrylic acid, methacrylic acid, N-vinylpyrrolidone or p-xylene from a suspension thereof in mother liquor, comprising: feeding a suspension including a crystal to a wash column which has a metal wall which encloses a process chamber; releasing mother liquor from the process chamber while retaining the crystal to form a crystal bed in the process chamber from the suspension conducted into the process chamber; conveying the crystal bed within the process chamber such that at least one force other than gravity acts in the process chamber in a conveying direction of the crystal bed and conveys the crystal bed within the process chamber; conducting a pure melt comprising a molten crystal removed in a purifying manner within the process chamber in countercurrent to the crystal bed such that a wash front which divides the crystal bed into a mother liquor zone and into a pure melt zone forms in the crystal bed; and providing a specific heat flow through the metal wall of the wash column into the process chamber of the wash column, wherein the metal wall of the wash column which encloses the process chamber is enclosed by a shell comprising a thermal insulation material, the shell comprising the thermal insulation material has a water vapor barrier whose environment facing away from the process chamber is permeable to the ambient air and whose surface facing away from the process chamber has a temperature which is above the dew point of the ambient air, and the specific heat flow flowing into the process chamber of the wash column through its metal wall is >0.1 W/m 2 and <10 W/m 2 .