Process intensification is connected to the analysis and design of equipments and processing techniques with better efficiency and sustainability. In this sense, the coupling of exothermic with endothermic reactions in multifunctional reactors can be a potential improvement, optimizing capital and operational costs.
The integration of reactions in a multifunctional reactor with recuperative coupling is similar to shell and tube heat exchangers. One set of reactions flows in the tubes region and the other in the shell side, typically in a fixed bed configuration. The heat transfer area enables thermal coupling whereas the possible presence of a membrane leads to mass coupling, usually in equilibrium reactions. In this work the coupling of ethanol dehydration and the catalytic combustion of methane in a multifunctional reactor was modeled and simulated.
Ethylene is an important raw material in petrochemical plants and can be produced through ethanol dehydration, an alternative route when compared to steam cracking of hydrocarbons. Besides, ethanol dehydration is an endothermic reaction which comprises several reaction subsets and is carried out in a fixed bed of alumina, in a temperature range from 513 K to 723 K. In this work it was considered that the heating of the endothermic reaction system was supplied by the combustion of methane in a palladium / platinum bed.
The multifunctional reactor was modeled by considering molar balances for each of the components in the reactions and also by taking into account energy balances for the exothermic and endothermic systems, in steady state. Pressure drop was also considered in the model, through Ergun equation and the kinetics of the reactions were taken from the literature. The following assumptions were made: ideal gas behavior; radial temperature and radial concentration variations were neglected; the mass transfer resistance in the catalyst was disregarded and a constant overall heat transfer coefficient in the process was assumed. The system of differential equations of the proposed model was solved in Matlab.
The operational conditions of the multifunctional reactor were mapped by considering parallel flow and stream feed temperatures in the range from 550 K to 750 K for the endothermic reaction and from 750 K to 950 K for the exothermic reaction, with spatial times of 0.14 s for the former and 1.67 s for the latter. In these mapped regions, ethanol yield varied from 55% to 82% and ethylene selectivity, defined as ethylene produced in relation to ethanol consumed, ranged from 0.30 to 0.90. It was shown that both ethanol yield and ethylene selectivity can be further improved with different spatial times. When a spatial time of 0.24 s for the endothermic reaction is considered, for instance, ethanol yield and ethylene selectivity are respectively equal to 86% and 0.93. On the other hand, pressure drop didn’t have a meaningful effect in the results.
In addition to the parallel flow, different configurations in the multifunctional reactor were also studied - countercurrent flow between the streams and partitioned ethanol feed into the equipment. These configurations didn’t improve the performance of the reactor, the best one was still obtained in parallel flow. If a spatial time of 0.24 s is considered once more for the endothermic reaction, in parallel flow, but now the ethanol feed is symmetrically divided in two, ethanol yield and ethylene selectivity will be equal to 83% and 0.89, respectively.