SEMIBATCH REACTION CRYSTALLIZATION OF BENZOIC-ACID

An experimental study of a semibatch reaction crystallization is presented. Dilute hydrochloric acid is fed to a stirred solution of sodium benzoate to crystallize benzoic acid. The weight mean size of the product crystals increases with increasing stirring rate, reaches a maximum, and then decreases again. Larger crystals may be produced if the reactant feed point is positioned close to the outlet stream of the impeller. At equal power input the influence of stirrer type is negligible. Decreasing reactant concentrations or feed rate increases the crystal size significantly. Experimental results are explained qualitatively focusing on nucleation and growth conditions and on feed point mixing. The feed point micromixing brings reactants together to generate supersaturation and allow for nucleation. Continued mixing, however, may partially dilute supersaturation before nucleation takes place or may restrict nuclei growth, thus promoting more efficient Ostwald ripening in the bulk. This may result in high bulk supersaturations which in turn hampers the dilution effects.