Modeling of oxygen permeation through a LSCF ion transport membrane

Abstract

The oxyfuel combustion characteristics have been investigated numerically for stagnation flow conditions inside a membrane reactor. The effect of combustion in the permeate side on the oxygen permeation through a La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF-6428) ionic ceramic membrane is presented. The membrane reactor has a simple symmetric design allowing the reduction of the number of coordinates to 2D without reducing the accuracy of the calculations. The results of the present model were validated through comparison with the available literature data. The membrane separates oxygen from oxygen containing stream (typically air). The oxygen transports to a downstream permeate side containing methane, with CO2 as an inert carrier gas. The membrane area is 8cm×8cm and both streams are at 800°C. The numerical simulations were performed using CFD software FLUENT 6.3 with the aid of Gambit 2.2 to construct the mesh. A source sink term has been added to the conservation equations through a series of user defined functions compiled and incorporated to fluent in order to account for the transfer of oxygen across the membrane surface. The simulations were carried out over a wide range of mass fluxes of feed side and permeate side considering the n-type flux equation transport mechanisms and oxidation reaction kinetics (real finite rate combustion of hydrocarbon versus no reaction or cold combustion). It was found that the oxygen permeation flux increases as the permeate side flux increases as a result of reduced partial pressure of O2. Also, it was found that the combustion in the permeate side has a great effect on the oxygen permeation flux. This is due to the increase in the temperature in the permeate side due to combustion which enhances oxygen diffusion and the reduction in the oxygen partial pressure in the permeation side due to the consumption of oxygen in the combustion process. In addition, increasing the partial pressure of oxygen in the air side has a great effect on oxygen permeation flux and overall combustion process. The membrane temperature in all simulations was found to be very close to the inlet flow temperature which should be controlled within the operating temperature of the membrane.