Entropy generation minimization for CO2 hydrogenation to light olefins

Abstract

An optimization model is established for the reaction process of CO2 hydrogenation to light olefins in a fixed-bed tubular reactor based on finite time thermodynamics or entropy generation minimization theory. In the present study, the specific generation rate (entropy generation rate averaged by the production rate of the target product) is proposed as an optimization objective function and the optimal design parameters which minimize the objective function have been investigated. The model is developed based on the reversible kenetic models and their cooresponding kinetic parameters, which are obtained by fitting the experimental data. The irreversibilities due to heat transfer, chemical reactions and viscous flow are considered and the local entropy generation rate of each term is calculated according to the irreversible thermodynamics. The analyses of the performance characteristics are conducted as well. The results show that the CO2 hydrogenation to light olefins accords with a two-step reaction mechanism, and Fischer-Tropsch reaction is the rate-controlling step. The irreversibility mainly located in the front of the reactor, which most contributions are caused by chemical reactions. The reductions of the specific entropy generation up to 24.78% and 10.04% can be achieved for optimal reactor inner diameter and optimal catalyst bed density, respectively.