Crystallization of para-xylene in scraped-surface crystallizers

Crystallization kinetics of para-xylene in batch pilot-scale scraped-surface crystallizers were determined for commercial crystallization of a xylene mixture currently produced in Amoco plants. Dynamic mass and energy balances, coupled with the dynamic population balance, were used to model the scraped-surface crystallizer. The model assumes that crystal nucleation occurs at the walls of the crystallizer and crystal growth occurs in the bulk. The parameters in the kinetic models were estimated from online measurements of bulk temperature and slurry transmittance. Concentration measurements show that the xylene mixture is always saturated (supersaturation is essentially zero) during all crystallization runs. Therefore, a reduced, two-parameter, high growth rate model was developed to describe these crystallizers. The kinetic parameters were correlated highly for the reduced model. A new batch experiment with an alternative temperature profile was determined that removes the correlation and allows both parameters to be determined uniquely.