(449b) Steady-State Behavior and Control of Four-Product Divided-Wall Columns for Alcohols Separation

Kaibel divided-wall column (KDWC) and extended Petlyuk divided-wall column (PDWC) are capable of separating mixtures of four components into high purity products. This work studies the separation of a four-component mixture of methanol, ethanol, 1-propanol and 1-butanol (MEPB) in KDWC and PDWC, respectively. The rigorous calculation models of KDWC and PDWC were set up on the basis of short-cut calculation at first. Simulation showed that KDWC could save 36.46% energy consumption compared to the conventional direct separation configuration, while PDWC could save up to 45.95% energy. The prefractionator of KDWC performs a difficult ME/PB split, and then the main column splits M/E and P/B in the different parts of the column. The prefractionator of PDWC splits M/B, and then the upper and lower parts of middle column in PDWC splits M/P and E/B, respectively. After that, the main column of PDWC splits M/E and P/B at the same time. The intermediate compounds donâ??t remix in the main column of KDWC and PDWC, but remix in the other parts of the two DWCs. The composition control structure were proposed in Aspen Dynamic, investigating the effect of feed flow rate and composition disturbances. Dynamic control results show that KDWC can handle ±10% disturbances of flow rate and feed composition. PDWC can handle ±10% and ±20% disturbances of flow rate and feed composition. However, a larger deviations of ethanol product can be found when the disturbances of P and M disturbances happened. Combined with the control results and V_min diagrams for composition disturbances, it is found that the concentration of ethanol product in upper side stream will decrease when the disturbance make M/E split harder. Otherwise, an easier M/E split will increase ethanol purity.