Energy & Fuels, Vol.34, No.3, 3065-3086, 2020
Comprehensive Modeling of Sorption-Enhanced Steam Reforming of Coke Oven Gas in a Fluidized Bed Membrane Reactor
Integration of membrane hydrogen separation and carbon dioxide capture with fuel steam reforming efficiently promoted hydrogen production and feedstock conversion. In this study, catalytic steam reforming of coke oven gas in a sorptionenhanced fluidized bed membrane reactor (MA-SE-SRCOG) was simulated using a reactive three-fluid model under the Euler framework. The numerical studies provided insights into details about interactions of multiscale subprocesses, including hydrogen permeation, carbon dioxide adsorption, catalytic reforming, and multiphase flow dynamics, during MA-SE-SRCOG. Concentration polarization caused by hydrogen separation was also examined. Meanwhile, impacts of several operating parameters, such as reaction pressure, steam concentration, membrane position, and reactor scale, on the performance of MA-SE-SRCOG were evaluated in terms of CH4 conversion, CO selectivity, hydrogen recovery factor, and CO2 fix factor. The simulation results demonstrated that membrane hydrogen separation and carbon dioxide adsorption promoted reforming kinetics and reactant conversions in the fast reaction zone, and also extended the reactive zone, thus efficiently improving the overall reforming efficiencies. Fast CO2 adsorption kinetics showed more profound enhancements on reducing CO selectivity compared to membrane separation, which instead had greater potential to facilitate high CH4 conversion when membrane effectiveness (a parameter representing impacts of membrane area, layer thickness and composite on permeation rates) was sufficiently large, particularly at high operation pressures. With membrane effectiveness increasing from 1 to 5, a CH4 conversion of 91.3% was achieved by MA-SE-SRCOG at 0.33 MPa and 560 degrees C, while the H-2 permeation flux was augmented from 0.077 mol/(m(2).s) to 0.192 mol/(m(2).s). However, the increase of membrane effectiveness would cause more serious H-2 concentration polarization in FBMR, which inhibited the effective H-2 separation rates, so membrane effectiveness >10 has been observed to not remarkably benefit CH4 conversion and H-2 production further. High S/C (>6) enhanced the occurrence of larger bubbles, and decreased the driving force (hydrogen partial pressure) for hydrogen permeation, both of which degraded the reforming performances and reduced yield of high purity H-2. The more closely membrane units were installed to the fast reaction zone, the larger the enhancements of membrane expected to exert on COG-steam reforming reactions. With the given dimensions of the membrane tube, the decrease of reactor diameter helped to decrease concentration polarization, so higher CH4 conversion and H-2 separation factors were achieved.