• crystallization;
  • kinetic;
  • crystal growth rate;
  • thermodynamic;
  • chemical potential


A growth kinetic model has been developed from a rigorous thermodynamic perspective to describe the crystal growth rates of NH4Cl on the basis of the difference of chemical potentials of NH4Cl at solid–liquid interface in aqueous NH4Cl, NH4Cl-NaCl, and NH4Cl-MgCl2 solutions. The solid–liquid equilibrium and activity coefficient of NH4Cl are calculated by the newly developed accurate Pitzer model with aid of Aspen Plus™ platform. The predictions of the resulting model are in good agreement with the experimental data published in literature and determined in this work at 283.15–333.15 K within the supersaturation up to 0.1. The kinetic model was subsequently used to analyze the effect of several operation variables, including temperature (283.15–333.15 K), supersaturation (up to 0.1), and NaCl or MgCl2 concentration (0∼2.5 mol kg−1), on the crystal growth rate of NH4Cl. The crystal growth rate of NH4Cl, with activation energy of 39 kJ mol−1, is strongly temperature-dependent and increases with increasing temperature in the three systems investigated. The advantage of MgCl2 over NaCl on the recovery of NH4Cl is theoretically and experimentally illustrated from the thermodynamic and kinetic perspectives with the aid of the established model. © 2011 American Institute of Chemical Engineers AIChE J, 2012