Design And Control Of Benzene-Cyclohexane Separation Azeotropic System Via Extractive Distillation

Separation process of benzene and cyclohexane is expensive and cumbersome by a conventional distillation process, considering the only 0.6 K difference in the boiling points of benzene and cyclohexane and the small interaction parameter between them. To aid the separation, furfural is used as the heavy boiling entrainer. And using the conventional extractive distillation, extractive dividing wall column and pressure-swing extractive with heat integration, the latter two have large energy-saving potential, to separate benzene and cyclohexane mixtures respectively. All processes optimal flow sheet with minimum energy requirements have been established using the multi-objective genetic algorithm with constrains. To test the control strategies of three processes, Aspen Dynamics software is applied. And two types of feed disturbances are used to test the proposed control structure performance, namely ± 20% changes in fresh feed flow rate and ± 10% changes in fresh feed benzene composition.Compared with the heat duty of the conventional configuration, EDWC and pressure-swing extractive with heat integration process with minimum heat duty of the reboiler processes presents savings of the total reboiler duty approximately 22% and 15.9%, respectively.The dynamics results reveal that all of the flowsheet show well controllable with appropriate control structure. And the feed-forward controller QR/F ratio can reduce the overshooting in product purity when subjected to disturbance in feed flow rate.Finally, the TAC of the conventional two-column system, the EDWC and pressure-swing extractive with heat integration are compared. The results show that the TAC of EDWC is 6.0% less than that of the conventional design, while the TAC of pressure-swing extractive with heat integrated is increase by 50.8%.Although the dividing-wall column and pressure-swing extractive with heat integration are promising technologies to save energy for separating mixture, the energy-saving potential is limited. This mainly because the high temperature of the combined reboiler which decreases the economic savings. Thus, the total reboiler duty and the TAC should be checked carefully in the design of a configuration. Needing only one column is the additional advantage of this EDWC design configuration, which reduces the space requirement. To a certain extent, the separation of benzene-cyclohexane mixture can benefit from the EDWC configuration.