Population balance equations (PBE) are often in the form of integro-partial differential due to complexities involved in nonconventional crystallization processes, especially gas antisolvent (GAS), since they include phenomena like primary nucleation, secondary nucleation, crystal growth, and agglomeration and/or breakage of crystals. Therefore the solution to such models has become rather difficult. Considering these difficulties a powerful numerical algorithm is adopted to treat the population balance model for the precipitation of aspirin by GAS process in this paper. This method is the combination of Lax-Wendroff and Crank-Nicholson numerical methods. It was used to investigate the effect of significant operating parameters, i.e., antisolvent addition rate, process temperature, and solute concentration on the final product properties for two solvents. The antisolvent addition rate was varied between 8 and 40 bar/min, the process temperature was kept constant at levels 37 and 42 ◦C, the solute concentration was variable at two levels, namely, 0.2 and 0.27 g solute/g solution, while methanol and acetone was used as the organic solvents. The results indicated the successful performance of the applied method to treat the PBE, since smooth particle size distributions were produced which were in an acceptable agreement with the experimental data of the investigated system.

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