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 dividingwall 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.

Owing to the great degree of mass integration and energy integration between the prefractionator and the main distillation column, it is usually infeasible to achieve a four-point composition control policy (i.e., the control of the main compositions of the three products and the ratio of the two impurities in the intermediate product) in a dividing-wall distillation column (DWDC), and this restricts, to a certain extent, process applicability and flexibility. The issue is referred to as the blackhole problem in the current work and has received very little attention so far. In this paper, an attempt is made to tackle this intricate problem, and the number of stages in each section of the DWDC is used as adjustment variables to coordinate the relationship between the prefractionator and the main distillation column involved. A simple and yet effective procedure is devised to guide the structural modifications of a given DWDC and the 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 its feasibility and effectiveness. Through steady-state analysis and closed-loop operation studies, it is demonstrated that the black-hole problem can be completely circumvented with the careful adjustment of the number of stages in each section of the DWDC. This outcome gives evidence to the feasibility and effectiveness of the proposed philosophy and indicates a reasonable way to enhance the applicability and flexibilities of the DWDC.

A simplified temperature difference control (STDC) scheme, consisting of two temperature and two temperature difference control loops, is proposed for the operation of dividing-wall columns (DWCs). The two temperature control loops are located, respectively, in the rectifying and stripping sections of the main distillation column and work to maintain the top and bottom product qualities. The two temperature difference control loops are arranged, respectively, in each side of the dividing wall with their temperature measurements arranged above and below the locations for introducing feed to the prefractionator and for withdrawing the intermediate product from the main distillation column. These special arrangements serve to keep a certain degree of separation of the fed mixture in the prefractionator and the purity of the intermediate product, tightening product quality control and rendering the STDC scheme with high robustness to the changes in the thermodynamic properties of the ternary mixtures separated. Separation of three ideal ternary mixtures of hypothetical components, A, B, and C, with different ease of separation indexes (ESIs) is chosen as illustrative examples to evaluate the proposed STDC scheme and thorough comparison is conducted with the currently available temperature control (TC) and temperature difference control (TDC) schemes. It is found that the STDC scheme is much superior to the TC scheme irrespective of the great changes in the ESIs. Although with relatively lower instrumentation cost and smaller engineering effort, it can generally lead to performance comparable with the TDC scheme. Application of the STDC scheme to the DWC separating a ternary mixture of ethanol, propanol, and butanol is also attempted and similar outcomes are obtained. Since the STDC scheme behaves in good accordance with the working principle of the DWC, it should be viewed as a general and effective strategy for the operation of the DWC.

Because of the strong coupling between the prefractionator and the main distillation column involved, the dividing-wall distillation column (DWDC) may exhibit the so-called black-hole problem in the operating range of interest when four product compositions (i.e., the main compositions in the three products and the ratio between the two impurities in the intermediate product) have been specified. In this paper, a novel strategy is proposed to solve the black-hole problem by the arrangements of multiple intermediate products to the main distillation column of the DWDC. The number, locations, and flow rates of the multiple intermediate products are designated as decision variables to coordinate the relationship between the prefractionator and the main distillation column involved, and a simple procedure is developed for the determination of their values effectively. The separations of two ternary mixtures of hypothetical components, A, B, and C and ethanol, propanol, and butanol, are chosen as illustrative examples to evaluate the feasibility and effectiveness of the proposed strategy. In terms of steady-state analysis and dynamic operation studies, it is demonstrated that the black-hole problem can be completely circumvented by the arrangements of multiple intermediate products to the DWDC. The proposed strategy is considered to be of general significance and can be applicable and effective to the synthesis and design of the DWDC separating ternary mixtures with widely different thermodynamic properties.