The effect of crystallization conditions, crystal morphology and size on pressure filtration of l-glutamic acid and an aromatic amine

Abstract

The influence of crystal morphology and chord length distribution on porosity, cake resistance and compressibility has been investigated for l-glutamic acid and an aromatic amine derivative. At higher initial supersaturation ratios, and hence higher crystal growth rates, both investigated systems produced spherulitic particles which could be compared with respect to filtration behaviour, along with the more conventional polyhedral and needle-like morphologies of l-glutamic acid. The cake resistance of l-glutamic acid measured at 2 bar pressure difference was 2.0 × 10 10 m/kg for the obtained polycrystalline spherulites, which is more than 40-fold higher than the cake resistance of monocrystalline needles. The cake resistance of polyhedral particles was 3 times lower than for the needle-like crystals although the needle-like crystals exhibit 30% higher cake porosity and the mean chord length is approximately the same for both crystal morphologies. For aromatic amine spherulites, the cake resistance at 2 bar was found to be lower for small compact spherulites (2.4 × 10 9 m/kg) of a narrow chord length distribution than for more compressible particles with open morphology (6.0 × 10 9 m/kg). It was found that spherulites in general give higher cake resistance values than polyhedra and needles. Comparably high cake resistance values for the spherulites are accounted for by lower mean chord length values as compared to polyhedral and needle-like particles and the cake compression in the course of filtration, which is assumed to occur predominantly directly above the filter cloth. The low mean chord length of spherulites measured at the end of the crystallization process is partly caused by bulk and surface nucleation followed by growth of plate-like particles as well as fragmentation of the fragile spherulites. Hence, the supersaturation should be kept at moderate levels in industrial crystallization processes to avoid spherulites in these systems, because of the high filter cake resistances associated with their formation.