The separations of quaternary reversible reactions with the most unfavorable ranking of relative volatilities (i.e., the two reactants are the lightest and heaviest components and the two products in between) require a reactive distillation column, followed by one or two conventional distillation columns. The multiple-column structure reflects not only a high energy intensity in the reaction operation and separation operation involved but also a great potential of process intensification in process development. With reference to a two-column system involving a reactive distillation column with an external recycle and a conventional distillation column (ER-RDC + CDC), a kind of reactive double dividing-wall distillation column (R-DDWDC) is derived via careful coordination and mass and thermal coupling between the ER-RDC and CDC. Closely dependent upon the magnitude of the reaction thermal effect, the left dividing wall is located either at the top or at the bottom. In addition to allowing the mass and thermal coupling between the ER-RDC and CDC, it facilitates primarily the reaction operation in the reactive section. The right dividing wall is located at the middle to fully strengthen the mass and thermal coupling not only between the ER-RDC and CDC but also between the separation operations included. The derived R-DDWDC features essentially the greatest degree of process intensification and becomes consequently the most thermodynamically efficient scheme in the separations of quaternary reversible reactions with the most unfavorable ranking of relative volatilities. The derived R-DDWDC is evaluated in terms of two examples, involving, respectively, the separations of ideal exothermic and endothermic quaternary reversible reactions with the most unfavorable ranking of relative volatilities. The derived R-DDWDC is found to be much more energy efficient than the ER-RDC + CDC, the reactive single dividing-wall distillation column, and those R-DDWDCs with different configurations. These outcomes demonstrate that the derived R-DDWDC is a much competitive alternative for the separations of quaternary reversible reactions with the most unfavorable ranking of relative volatilities. They also highlight the feasibility and effectiveness of the proposed strategy for process synthesis and design, that is, when the reaction processed is exothermic, the optimum R-DDWDC must come from one of R-DDWDC2 and R-DDWDC4; otherwise, the R-DDWDC4 remains to be the only possibility.
Although Kaibel distillation columns are superior to conventional distillation sequences owing to smaller equipment investment and operation cost, they display high nonlinearity and this greatly increases the difficulty of achieving their tight control. To overcome this problem, four decentralized composition control structures, i.e., the CSR/QR, CSR/B, CSD/QR, and CSD/B structures, are proposed and compared based on the control of a Kaibel distillation column fractionating a methanol/ethanol/propanol/butanol quaternary mixture. These four composition control structures all include five composition control loops. While the four of them are employed to maintain the purity of the top, upper sidestream, lower sidestream, and bottom products, the remaining one is employed to minimize the energy consumption of the Kaibel distillation column by maintaining the composition of propanol at the first stage of the prefractionator. Dynamic simulation results show the CSR/QR and CSR/B structures can tightly maintain the purity of the controlled products with a small overshoot and short settling time after facing various disturbances in feed conditions, but the CSD/QR and CSD/B structures lead to oscillatory responses (the latter even shows divergent responses under individual disturbances). At the end of the article, some effective guides for developing composition control systems are given.
The potential of adopting asymmetrical temperature control (ATC) schemes to tightly control dividing-wall distillation columns (DWDCs) has yet to be fully explored due to the following two problems. First, the inference accuracy toward the controlled product purity of the candidate controlled variables (CCVs) synthesized for each control loop was not high enough. Second, the determination of the most appropriate CCV for each control loop lacked a sufficient theory basis. To alleviate these two problems, two measures are specially adopted in the current work, thereby forming a systematic method for deriving the ATC schemes. First, the creations of a double temperature difference (DTD) and temperature difference (TD) are reformed with the aid of a recently defined performance metric, the averaged absolute variation magnitudes (AAVMs). Namely, DTD is generated according to a novel method based on the AAVMs, rather than the conventional method based on the sensitivity analysis and SVD analysis, and TD is modified in form through multiplying the temperature of the reference stage by a coefficient that equals the ratio of the AAVMs of the sensitive stage and the reference stage. Then, a sequential search procedure is proposed to help determine the most appropriate CCV for each control loop from all of the synthesized CCVs with a combined consideration of the inference accuracy to the controlled product purity and the interaction between control loops. In terms of the operation of a benzene–toluene–o-xylene DWDC, the proposed method is assessed by means of a thorough comparison between the derived ATC scheme and the double temperature difference control scheme. In comparison with the latter, the former displays not only relatively better transient responses but also smaller steady-state deviations in the three controlled product qualities. These findings demonstrate the effectiveness and feasibility of the method proposed for deriving ATC schemes.
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