A multi-stage crystallization separation process operated under three-phase conditions to obtain high-purity and high-yield para-xylene from xylene mixtures

A crystallization separation technique for xylene purification was proposed, which is a multi-stage process that is operated under three-phase (solid, liquid, and gas) coexisting conditions with decreasing temperatures and pressures. In contrast to conventional crystallization processes, centrifugation, filtration, and washing steps are not required. The proposed technique also differs from batch stripping crystallization, insomuch as para-xylene is recovered as much as possible via purifying the vapor fraction further and keeping the operating conditions close to the eutectic point. A series of models were developed to describe the ternary phase equilibrium and the mass and heat balances. Then, binary parameters between xylene isomers were fitted from experimental data, and a flowchart was proposed featuring the means to determine the phase compositions and flowrates. These were subsequently validated by experiments. This contributed to developing a practical multi-stage process that, when applied to a practical xylene mixture, resulted in a yield of more than 80% and purity in excess of 99.5%.