Influence of different scales of mixing in reaction crystallization

Experiments on semibatch reaction crystallization of benzoic acid are repor ted. The conditions in an agitated tank are simulated by a loop reactor by which feed point mixing conditions can be controlled separately from the ma croscale circulation rate. Hydrochloric acid is fed into a circulating solu tion of sodium benzoate and the influence of macromixing, mesomixing and mi cromixing on the product crystal mean size is evaluated. The product mean s ize increases with increasing circulation rate in the loop, with increasing feed point mixing intensity, with decreasing feed rate and with decreasing feed pipe diameter. Increased mixing intensity on any level leads to large r product crystals, but especially the rate of mesomixing is of importance. The influence of the feed pipe diameter is opposite to predictions by avai lable theories and cannot be explained by backmixing into the feeding pipe. All results can be correlated quite well against a dimensionless mixing ef ficiency defined as the ratio of the reactant feeding time to the mixing ti me. The mixing time is the sum of the time constants for mesomixing and mic romixing. A new mesomixing time constant is defined as being proportional t o the ratio of the feed pipe diameter and the velocity of the bulk flow pas sing the feed pipe. (C) 2001 Published by Elsevier Science Ltd.