Operation of Dividing-Wall Distillation Columns. 2. A Double Temperature Difference Control Scheme

In the first paper of this series, it has been demonstrated that, although the temperature difference control (TDC) and simple temperature difference control (STDC) schemes render stricter control of the three product qualities of the dividing-wall distillation column (DWDC) than the temperature control (TC) scheme, they still exhibit somewhat great steady-state offsets, which deteriorate system performance. To deal with this drawback, we devise a double temperature difference control (DTDC) scheme to facilitate the operation of the DWDC in the current work. The proposed DTDC scheme consists of four double temperature difference control loops, with one in the prefractionator, whose objective is to maintain a certain degree of separation of the fed mixture, and the remainder in the main distillation column, whose objective is to maintain the purities of top, intermediate, and bottom products. In particular, the three temperature measurements in the control loops for the prefractionator and intermediate product are required to be in the each side of the dividing wall, and this makes the DTDC scheme behave in good accordance with the working principle of the DWDC. Separation of two ternary mixtures of hypothetical components A, B, and C and benzene, toluene, and o-xylene are chosen as illustrative examples to evaluate the performance of the DTDC scheme, and thorough comparison is conducted with the currently available TDC scheme. The obtained results indicate that the proposed DTDC scheme is much superior to the TDC scheme in the operation of the DWDC with substantially reduced steady-state discrepancies. This permits the DTDC scheme to be capable of handling severe disturbances as great as 30% in feed compositions with acceptable steady-state deviations in the three product qualities. The great advantages of the DTDC scheme justify the employment of four additional temperature measurements in the operation of the DWDC, compared with the currently available TDC scheme.