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The conventional method for configuring double temperature difference control (DTDC) schemes relies on sensitivity analysis (SA) and singular value decomposition (SVD) analysis, respectively, to determine sensitive and reference stages. Since no considerations are given at all to the interactions between the synthesized double temperature differences (DTDs) and to the coordination between the upper and lower temperature differences (TDs) subtracted in each synthesized DTD, the conventional method may lead to DTDC schemes that fail to secure tight product quality control of distillation columns. In this article, a novel method is proposed that employs a newly defined metric in our recent work, the averaged absolute variation magnitudes (ASVM), to determine the two reference stages in each DTDC loop. The ASVM measures the variations of TDs between the sensitive stage and the remaining ones with the assumption of complete rejection of all disturbances concerned and can thus reflect the inherent characteristics of coupling between the controlled product qualities. For each DTDC loop, while the first reference stage should be chosen to cope with its coupling with the other control loops, the second reference stage should be to coordinate the two TDs involved, thereby yielding a favorable effect to the inference of the controlled product qualities. Four example systems, including one conventional distillation column separating a binary mixture of ethanol and butanol, two conventional distillation columns separating a ternary mixture of ethanol, propanol, and butanol, and one dividing-wall distillation column separating a ternary mixture of ethanol, propanol, and butanol, are used to assess the proposed method by means of in-depth comparison with the conventional method. While they display comparable dynamic performances, the former leads to considerably smaller steady-state deviations in the controlled product qualities than the latter. These striking outcomes demonstrate evidently that the proposed method can be a promising alternative for the pursuit of tight temperature inferential control of various distillation columns. [ABSTRACT FROM AUTHOR]

Reactive distillation columns with double reactive sections (RDCs-DRS) at the top and bottom are especially favorable for the separations of reacting mixtures featuring the most unfavorable ranking of relative volatilities. In this article, their dynamics and control are studied in great detail, with special attention given to the influences of feed splitting on process dynamics and controllability. Because of the totally refluxed and totally reboiled operation mode plus an intermediate-product withdrawal, the RDC-DRS is generally characterized by severe under-dampness between the intermediate-product compositions and the reboiler heat duty (i.e., the regulation path) and by severe asymmetry between the intermediate-product compositions and the heaviest reactant feed flow rate (i.e., the disturbance path), consequently posing a great challenge to tight product quality control. With feed splitting of the lightest and heaviest reactants in the RDC-DRS (RDC-DRSFS), the simultaneous suppression of the under-dampness in the regulation path and the asymmetry in the disturbance path is attained, thereby presenting favorable influences to process dynamics and controllability. Two reactive distillation systems, executing a hypothetical reversible reaction, A + B ↔ C + D (αA > αC > αD > αB), and the lactic acid esterification with methanol, are employed to inspect the dynamics and controllability of the RDC-DRS and RDC-DRSFS. Although the RDC-DRS can be maintained as stable, rather sluggish tracking responses are noticed because the under-damped and asymmetrical behaviors restrict tight controller tuning. The RDC-DRSFS, on the other hand, shows greatly improved tracking performance with even enhanced disturbance rejection capabilities. The comparison endorses the great significance of feed splitting for the RDC-DRS, namely, not only an effective strategy for process retrofitting but also a potential booster for process dynamics and controllability. [ABSTRACT FROM AUTHOR]

With reference to the disproportionation of trichlorosilane to silane, a three-stage consecutive reversible reaction with rather unfavorable reaction kinetics of a near zero thermodynamic conversion, an in-depth comparison in steady-state performance is performed between the reactive distillation column with a single reactive section (RDC-SRS) and those with multiple reactive sections (RDC-MRS), under the assumptions of the same total number of stages and the same total amount of catalyst employed. With the incremental arrangement of reactive sections, the RDC-MRS shows a steady improvement in steady-state performance with considerably reduced operating cost and capital investment as compared with the RDC-SRS. The great advantages originate essentially from the additional degrees of freedom resulting from the arrangement of multiple reactive sections in process synthesis and design. Apart from the coordination effect to the three-stage consecutive reversible reactions processed, the additional degrees of freedom serve also to reinforce internal mass integration and internal energy interaction between the reaction operations and the separation operations involved. Arrangement of side-condensers is also examined toward the RDC-MRS, and the outcomes reveal the thermodynamic rationale to adopt multiple reactive sections in process development. Although these findings are derived from the specific case study chosen, it should be considered to be of general significance for the synthesis and design of reactive distillation columns separating complicated reacting mixtures involving multiple reversible reactions. [ABSTRACT FROM AUTHOR]