| Although conventional reactive distillation column (CRDC) with reactive section between rectifying section and stripping section represents a promising technology of process intensification for combining reaction operation and separation operation in one unit, it is not always advantageous than conventional multi-unit flow-sheet involved reactors followed by a number of distillation columns for all kinds of reaction systems. For all the quaternary reversible reactions, A+B(?)C+D, they can be reasonably divided into four categories in terms of the ranking of relative volatilities:the most favorable category (αC>αA>αB>αD), somewhat favorable category (αA>αB>αC>αD and αC>aD>αA>αB), somewhat unfavorable category (αC>αA>αD>αB and αA>αc>αB>αD), and the most unfavorable category (αA>αC>αD>αB). CRDC is especially difficult to be used for the separation of the reacting mixtures belonged to the most unfavorable and somewhat unfavorable categories, confining terribly their applicability and flexibility. These drawbacks stem essentially from the structural limitation of CRDC, in which the unconverted reactants are hardly to be recycled to the reaction section. Even for the separation of reacting mixtures belonged to the most favorable and somewhat favorable categories, CRDC still has its drawbacks and should be further improved by elaborate structural modification.In order to overcome these problems, the author proposed a novel strategy of introducing external recycle into the single RDC, leading to the creation of a novel reactive distillation column with external recycle (RDC-ER). The flow rate and location of the external recycle are important decision variables for process development and can help to enhance significantly internal mass integration and internal energy integration between the reaction operation and the separation operation involved. Therefore, it is worthwhile to study the effects of the external recycle on the steady-state and dynamic performances of the RDC-ER. The purpose of this dissertation is to conduct systemic studies on the RDC-ER for all the categories of reaction system. The major works and innovations are described as follows.The steady-state and dynamic models were developed for the RDC-ER. These models were developed with equilibrium stage model and conducted with commercial software, Mathematica and Aspen Plus. It was found that these models were rigorous and reliable. Thus, they can be utilized for the studies on the design and control of the RDC-ER.A novel reactive distillation column with external recycle placed between the top and the bottom (RDC-TBER) was proposed for the separation of the reacting mixtures belonged to the most unfavorable category. The external recycle should be placed between the top and the bottom of RDC-TBER and its direction arranged according to the thermodynamic characteristics of the reaction involved. More especially, while it should be directed from the top to the bottom of the RDC-TBER for the separation of exothermic reactions, it should be directed from the bottom to the top for the separation of endothermic reactions.A simple and effective procedure was devised for the synthesis and design of the RDC-TBER. Four structural variables (i.e., the total number of stages, the number of stages in the reactive section, the location of the side product, and the location of the external recycle) are searched with a trial and error method and four operating variables (i.e., the operating pressure, the total kinetic holdups, the flow rate of the external recycle, and the reboiler heat duty) are found with a modified steepest gradient method. Four illustrative examples, a hypothetical exothermic reaction, a hypothetical endothermic reaction, the esterification of lactic acid with methanol, and the esterification of palmitic acid with isopropanol, were employed to evaluate the proposed procedure and the proposed configuration. It was found that the proposed procedure can be convergent after a few iterative computations. The obtained results showed that RDC-TBERs were definitely superior to the conventional schemes comprising of a continuous stirred tank reactor plus a conventional distillation column (CSTR-CDC), reactive distillation column with two reactive sections at its both ends (RDC-TRS), and reactive distillation column with one reactive sections at its either end (RDC-ORS) in the aspect of capital investment (CI), operating cost (OC), and total annual cost (TAC).A novel reactive distillation column with two external recycle (RDC-TER) was proposed for the separation of the reacting mixtures belonged to the most unfavorable category to further examine the effects of adding more external recycles. One external recycle should be placed between the top and the bottom of RDC-TER and its direction should be arranged according to the thermodynamic characteristics of the reaction involved. The other external recycle should be arranged between the bottom/top and the reactive section. It directs from bottom to reactive section for exothermic reactions, while it from top to reactive section for endothermic reaction. Two illustrative examples, including the separation of a hypothetical exothermic reaction and a hypothetical endothermic reaction, were chosen to evaluate the performance of the RDC-TER. The results showed that the RDC-TER had better economic performance than the CSTR-CDC, the RDC-TRS, and the RDC-TBER.A reactive distillation column with external recycle placed between the top/bottom and the reactive section (RDC-ERTR/RDC-ERBR) was proposed for the separation of the reacting mixtures belonged to the somewhat unfavorable category. The external recycle should be directed from the top to the reactive section for the separation of reacting mixtures with the relative volatilities ranking of αA>αc>αB>αD, and it should be from the bottom to the reactive section for the separation of reacting mixtures with the relative volatilities ranking of αc>αA>αD>αB.A simple and effective procedure was developed for the synthesis and design of the RDC-ERBR. Three illustrative examples, a hypothetical exothermic reaction, esterification of acetate acid with methanol, and transterification of butyl acetate with ethanol, were chosen to evaluate the impact of the external recycle. Through strict comparison with RDC, it was demonstrated that the inclusion of such an external recycle was beneficial to enhance system performance and should be taken into account in process synthesis and design. The procedure for the synthesis and design of the RDC-ERTR can also be developed with analogous method.The synthesis and design strategy of control system of the RDC-TBER was devised after the process dynamics and controllability have been studied. The chemical reactions belonged to the most unfavorable category would arouse complex dynamics characteristics and the phenomenon of multiple steady-states is found. The process is open-loop unstable while high product purity is required. The unexpected phenomenon would be relieved with a relatively large flow rate of external recycle. Since RDC-TBER is designed to work in totally refluxed and totally reboiled operation modes, the reflux drum and sump interact strongly with the reaction operation involved, retarding considerably the dynamics of the reaction operation; therefore, they should be tightly controlled. The flow rate of external recycle can be utilized to control one composition in the side product and reboiler heat duty can be chosen to control the other composition in the side product. The close-loop responses to the disturbances of feed flow rate, feed composition, and product specifications showed that the RDC-TBER can be well controlled even if they have relatively low controllability because of its highly intensified feature.The synthesis and design of plant-wide reactive distillation processes with top-bottom external recycle (RDC-TBER flow-sheet) were studied. Two real complex reactions, esterification of adipic acid, and esterification of glutaric acid, were chosen as illustrate examples to evaluate the performance of the RDC-TBER flow-sheet. The results showed that the RDC-TBER flow-sheet was better than the conventional multi-unit flow-sheet with a conventional reactive distillation column in terms of CI, OC, and TAC.The dynamics and control of the RDC-TBER flow-sheet were studied. A hypothetical ideal exothermic reaction was chosen as illustrate example to evaluate the control performance of the RDC-TBER flow-sheet proposed. The outcomes showed that the RDC-TBER flow-sheet can be well controlled. |
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