Kinetic analysis for crystal growth rate of NH₄Cl in the NaCl-MgCl₂-H₂O system with a thermodynamic approach

Abstract

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