Crystallization as a concentration and purification method was studied. The concentration study focused on the study of freeze concentration. The aqueous solutions in the experiments were black liquor from the pulp manufacturing industry and various sugar solutions, such as sucrose, pentose, and betaine solutions. The main subjects of study were the kinetics of ice crystallization, the use of ripening techniques to increase the average crystal size, and the separation of ice produced from the concentrate. In the kinetic study the influence of mass transfer, heat transfer, and surface kinetics on the crystal growth was investigated. Mass transfer seemed to be the dominating factor, whereas heat transfer limits the crystal growth in, for instance, dilute sugar solutions. The purification study was based on boundary layer theory. The experiments were carried out using an aqueous potassium sulphate solution in continuous MSMPR-crystallizers of 2, 10, and 50 liter, and using naphthalene - 2-methylnapthalene and stearic acid containing two its dimers as impurities in a zone melting vessel. Boundary layer theory explains the concentration at the interface between the crystal and bulk liquor in melt crystallization. In the case of potassium sulphate it was concluded that a factor other than boundary layer conditions dominates the purity of the growing crystals. A high-pressure sweating method for the final purification of the crystalline product was developed. To study the nucleation rate in a batch crystallizer, a concentration response method was derived. In that method, the desupersaturation was registered indirectly by measuring the electrical conductivity. The method was studied using an aqueous potassium chloride solution.
|Number of pages||111|
|Journal||Acta Polytechnica Scandinavica, Chemical Technology and Metallurgy Series|
|Publication status||Published - 1996|
|MoE publication type||A1 Journal article-refereed|