(449e) Internal Heat Integration of Double Feed Reactive Distillation Processes Involving Kinetically Controlled Reactions

Authors: 
Cárdenas-Guerra, J. C., Universidad de Guanajuato
Marmolejo-Correa, D., University of Guanajuato
Hernández, S., Universidad de Guanajuato

Distillation is the most extensively used separation technology in chemical industry; however, its main disadvantages lie on a large demand of energy and a very low thermodynamic efficiency. To deal with these aspects, a considerable amount of research has been undertaken since the beginning of the last century, and, to date, a number of new designs have been proposed. In this context, the heat pump assisted distillation schemes, the distillation columns with intermediate heat exchangers, the internally heat integrated distillation columns and the thermally coupled distillation sequences have shown to achieve lower energy requirements and higher thermodynamic efficiencies. The common principle behind these configurations is the minimization of the irreversibilities in the separation operation in terms of thermodynamic analyses, i.e., a rectifying section is usually a potential heat source and the stripping section, a potential heat sink. For a reactive distillation process (RDP) involving reactions with a highly thermal effect, the thermodynamic efficiency could sometimes be improved by seeking further internal heat integration between the reaction and separation operations. The heat of reactions might be used effectively as either a heat source (in case of exothermic reactions) or a heat sink (in case of endothermic reactions) to drive the separation operation in the stripping section or in the rectifying section, accordingly. In this work, the feasibility and effectiveness of seeking further internal heat integration between the reaction and separation operations in the design of reactive distillation processes involving kinetically controlled reactions is studied. Basically, we analyze two conventional reactive distillation columns (RDC´s) for two reactive systems, one of which is the reversible etherification of isobutene and methanol for the synthesis of MTBE, while the other represents the irreversible hydrodesulfurization (HDS) of thiophene, benzothiophene, dibenzothiophene and 4,6-dimethyldibenzothiophene with n-hexadecane as solvent for the production of ultra-low sulfur diesel. These complex reaction-separation structures are generated in a process simulator (ASPEN PLUS) using the equilibrium stage model and the reaction rates are calculated assuming a pseudohomogenous model. One procedure that can be effective for the reinforcement of internal heat integration in the design of the conventional RDCâ??s is the variation of feed stage allocation mainly in the reactive zone. Therefore, steady-state simulations are carried out until the minimum energy consumption is reached. Then, the new energy-efficient designs of the RDCâ??s are subjected to a thermodynamic analysis to evaluate the proposed procedure. This analysis emphasizes the use of the second law (exergy) of thermodynamics beside the first law. Increased exergy loss over an intensified system is possible because process intensification is founded on increasing the driving thermodynamic forces and molecular rate processes as well as increasing the entropy production in the system. The results indicate that the heat duties of the RDC´s with the consideration of further internal heat integration can reduce the energy requirements by feed stage rearrangement, achieving significant energy savings (e.g., the HDS case can reach energy savings up to 10%). Regarding thermodynamic efficiency, the MTBE-RDP presented a high value while the HDS-RDP showed a low value. The reason is that the energy needed in HDS reactive system is supplied to the highest temperature level.
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