Separation of para-xylene from xylene mixture via crystallization

Abstract

Crystallization kinetics of para-xylene from xylene isomers mixture using a lab-scale cooling batch crystallizer were determined. The cooling batch crystallizer type is simple, flexible and requires less process development. Dynamic mass and population balances were used to model the batch crystallizer. The model equations were solved using the numerical method of lines; a new proposed solution method. The kinetic parameters of nucleation and growth rates were estimated by measuring the concentration and the total mass of para-xylene suspended crystals during the process time. A nonlinear optimization technique was then applied to estimate the parameters. The effect of the cooling strategy on the estimated parameters was studied. It was found that model predictions using the optimum estimated parameters were in good agreement with the experimental results under various cooling strategies. The optimal kinetic parameters were then used to find the optimum cooling strategy to maximize the yield of para-xylene crystals which have an average size greater than 0.5 mm. A new objective function was formulated and also, a nonlinear optimization technique was applied to find the optimum cooling strategy to achieve this product characterization. The optimization technique converged successfully and the proposed objective function was found to be effective to optimize the para-xylene crystallization process.