Optimization of a Simulated Moving Bed Unit within an Existing and Revamped Aromatics Complex with Crystallization and Toluene Methylation Units

The aromatics complex is a petrochemical facility where the main products are benzene and p-xylene. The xylene isomer is produced through an energy-intensive cycle loop in which the p-xylene separation unit plays a significant role. Many works have focused on improving the efficiency of the separation unit; however, optimizations aiming to reduce the energy consumption of the whole complex are very scarce. On the basis of the aforementioned, an existing aromatics complex was studied and the p-xylene separation unit, that is the simulated moving bed (SMB) unit, was optimized in the framework of several modifications to the complex. The changes consisted of the addition of a crystallization unit, operating at two different temperatures, and a toluene methylation unit. The objective function included the revenue from the final products and the major operating costs from the units of the complex. Because of the complexity of the optimization problem, simplified models were used but validated against reported data. For almost all the cases studied, the optimum SMB consisted of more columns in zones I and IV and switching times of 180 s; when the feed of the SMB increased around 40%, the optimum switching time was 112 s. The crystallization unit alone did not improve the efficiency of the complex; however, p-xylene production doubled with the addition of the toluene methylation unit, which constituted the best proposal among the cases studied. Optimum toluene conversion and p-xylene selectivity were about 20 and 95%, respectively. The results also showed that it is more profitable to operate the crystallization unit at higher temperatures and send the mother liquor, with a p-xylene fraction above the thermodynamic equilibrium, back to the SMB.