Materials, Methods & Technologies, Volume 7, 2013

Abstract: This paper investigates how to improve the crystallization (isolation) stage in producing an optical brightener used for bleaching textiles, paper, and plastics, focusing on the stilbene derivative Belophor OB. Using a laboratory reactor setup that models industrial conditions, the authors examine how the choice and composition of oxidizing (salting) agents, along with process parameters such as temperature, hydrodynamics (Reynolds number), and post addition holding time, affect crystallization kinetics, crystal size distribution, product concentration, and impurity removal. Twelve oxidizing agent variants were tested, including individual acids (sulfuric, hydrochloric, acetic) and their mixtures. The best results were achieved with a hydrochloric plus acetic acid mixture at a 1:3 ratio, which produced the highest concentration of the target substance (about 44.5 g/dm³) and larger, more uniform crystals. Temperature optimization showed that around 60°C maximized target concentration (about 45 g/dm³) and improved filtration performance while minimizing chloride impurities. Hydrodynamic studies indicated that increasing mixing intensity improved uniformity up to Re ≈ 1600, while higher values reduced crystal size and broadened the fraction range. Additional experiments on aging time demonstrated crystal growth over roughly 25 to 35 minutes, enabling crystal sizes around 40 to 45 μm and reducing filtration losses. Based on these findings, the authors propose practical industrial recommendations, including oxidizing agent composition, temperature, stirring regimes, and holding time, which reduce chloride content to roughly 0.8 to 1.0% and lower product losses to about 0.5%.