Modeling of intensified glycerol steam reforming in a heat-exchange integrated microchannel reactor

Abstract

The aim of this study is to investigate glycerol steam reforming in a heat exchange integrated microchannel reactor involving solid wall separated, rectangular-shaped, parallel reaction and heating channels by means of mathematical modeling techniques. Heat needed for endothermic glycerol steam reforming taking place in Co-Ni/Al2O3 coated channels is supplied by steam flowing in the adjacent channels separated by solid walls. Simultaneous catalytic reaction and heat exchange within the microchannel reactor are modeled by 2D Navier-Stokes equations which were solved by the finite volume method in ANSYS 16.0 platform to study the effects of operational parameters, namely molar inlet steam-to-carbon ratio (S/C) and temperature of the reactive mixture, inlet temperature and velocity of steam, flow configurations of the reactive mixture and steam, and of structural parameters, i.e. reactor wall material and thickness of the wall between the reaction and heating channels on temperature distribution and on glycerol conversion. The results show that increasing S/C from 1 to 6 promotes H2 and CO2 selectivity, having average values of 64.7% and 21.2%, respectively. The same effect, however, suppresses CO selectivity from 12.8% to 10.8% and resulted in CH4 selectivity below 2.7%. Product distribution is found to be in alignment with those of experimental studies reported in the literature. Heating by steam has an obvious contribution to glycerol conversion, which is found to be 73.3% and 63.8% in the presence and absence of steam heating, respectively. Counter- and co-current flow of steam with respect to reactive mixture, however, do not lead to a notable difference in conversion. Increasing steam inlet temperature from 663K to 783K improved conversion by 10.0%, whereas changing steam inlet velocity from 2 to 8m/s did not change reforming channel temperature significantly. Reactor performance is significantly affected by increasing feed temperature of the reactive mixture from 673K to 773K which improved conversion by 11.7%. Higher glycerol conversions are also obtained by using thicker separating walls and thermally conductive wall materials, both of which favor heat flux into the reforming channel. Changing the wall thickness from 4×10−4m to 1.2×10−3m elevates conversion by 1.3%. A similar response is noted upon using silicon carbide instead of AISI steel as the material of construction of the microchannel reactor. In general, selectivity towards H2 is positively correlated with the degree of uniformity of reaction channel temperature, which is also found to dampen undesired CH4 selectivity.