Modeling of a combined ion transport and porous membrane reactor for oxy-combustion

Abstract

Ion transport membranes (ITMs) offer promising technology for high purity oxygen production (up to 99%) without adversely affecting the efficiency of the oxy-fired plants. The permeation rate of such ITMs can be increased by replacing the conventional inert sweep gas with a reactant/diluent mixture (e.g. CO2, CH4) as this reduces the permeate partial pressure on the permeate side of the membrane, which, along with the temperature, governs the permeation flux. The significant limitation of this approach is that an uncontrolled, exothermic consumption of the permeated specie, can lead to membrane damage, and thus limits the potential of ITMs using reactive sweep gases (i.e. ITM reactors). By using a multichannel ITM reactor (isothermal reactor), it is proposed to operate the ITM reactor at, or near to isothermal conditions (i.e. a spatially uniform temperature). This may be achieved by introducing a reactant into the permeate stream uniformly across the entire ITM reactor length from an adjacent channel with porous walls. The present work is aimed at modeling a nearly isothermal reactor using an ITM and a porous membrane to achieve uniform stoichiometric ratio of fuel/O2 in order to have a uniform combustion all along the length of the membrane. A two-dimensional, computational fluid dynamics (CFD) model is solved to study the characteristics. The simulations are based on the numerical solution of the conservation of mass, momentum, energy and species transport equations of two dimensional flows. For the CFD calculations, the commercial solver FLUENT has been used. The models used have been validated against the experimental results found in the literature and are found to be in good agreement. The influence on the performance of oxygen separation through the ITM has been studied by varying the flow conditions at the permeate side. Results show that an approximate uniform combustion can be achieved along with effective thermal management of the ITM using the present isothermal reactor. It is also observed that for a constant mass flow rate of fuel mixture, the permeation rate of oxygen through ITM increases with the increase in CH4/CO2 ratio. The results indicate that the oxygen permeation flux increases by approximately 3 times for reactive case compared to separation only case. Moreover it is shown that using the present reactor model the reaction zone can be controlled.