On the mechanisms of formation of spherical agglomerates

Spherical agglomerates of benzoic acid have been successfully prepared by semi-batch, agitated vessel, drowning-out crystallization in water–ethanol–toluene mixtures. Benzoic acid is dissolved in ethanol, toluene is added and this mixture is fed at constant rate to the agitated crystallizer containing water. The influence of the amount of bridging liquid and the feeding rate on the product particle size distribution, morphology, and mechanical compression characteristics have been investigated. Compression characteristics for single agglomerates are compared with data on bed compression. With increasing amount of bridging liquid the particle size and strength increases and morphology improves. Particle size decreases and the fracture force increases with increasing feeding rate but the morphology remains unchanged. Using toluene as opposed to chloroform as the bridging liquid leads to improved product properties. Experiments have also been performed to reveal the mechanisms of the formation of the agglomerates. The results show that along the course of the process the properties of the particles change gradually but substantially. Particle size and number increases along with increasing feed. The spherical shape does not appear immediately but develops gradually, and is shown to be very much the result of the agitation of the slurry.