Kinetics of hexamine crystallization in industrial scale

The values of kinetic parameters for hexamine crystallization from aqueous solution, necessary for control of industrial crystallizers were calculated in this study. Hexamine is an intermediate chemical mainly used in pharmaceutical and resin industries. In Meissner process, the major industrial method of hexamine production, formaldehyde and ammonia in gaseous states are introduced into the reaction vessel, and crystalline hexamine is continuously produced. The reaction between formaldehyde and ammonia is very fast and thus the whole process is controlled by the crystallization stage. However, the kinetic information reported in the literature for industrial crystallization of hexamine is very limited. Mechanistic modeling of hexamine crystallization was performed using mass and population balance equations. These equations were solved using the backward difference method. Kinetic parameters in the growth and nucleation rate equations were determined by fitting the mechanistic model to the industrial data obtained in the present study from an industrial MSMPR crystallizer. To determine the population densities and supersaturations, both the solid and liquor phases were sampled during the transient step of particle size evolution. To optimize the parameters, the differences between the model-predicted data and the plant data were minimized using the nonlinear unconstrained method in an optimization subroutine. The growth and nucleation rate equations for hexamine crystallization in industrial scale were calculated as G=0.7S1.47 and B°=9138MT0.74S0.551, respectively. The growth and nucleation rate equations obtained in this study can be used to simulate the crystallization stage of hexamine plants.