The size distributions of calcium sulfate dihydrate crystals formed during the industrial phosphoric acid production process are critical to the acid filtration efficiency. In this work, a thermodynamically consistent definition of supersaturation is derived and modeled using the mixed-solvent-electrolyte framework in OLI software for gypsum in an acid mixture. Continuous reactive crystallization experiments are carried out to estimate the gypsum growth and nucleation kinetics at different temperatures. A population balance model is implemented using the method of characteristics and integrated with the thermodynamic model platform to accurately simulate the dynamic propagation of the solution concentration and particle size distribution. The experimentally fitted kinetic parameters are verified through a comparison with the predicted crystal size distribution (CSD) and supersaturation. The comparison shows a good agreement between the predicted and measured CSD and supersaturation at a temperature range from 25 to 60 degrees C.