Design concepts of a scaled-down autothermal membrane reformer for on board hydrogen production

摘要

The design of an on-board autothermal membrane reactor producing pure hydrogen at atmospheric pressure, while recuperating heat, is analyzed mathematically. The suggested design incorporates two reactors exchanging heat; an endothermic methane steam-reforming (MSR) reactor embedding Pd membranes to separate pure H-2, and an exothermic methane oxidation (MOx) reactor fed by the MSR effluent. The analysis is conducted at three levels of details: (i) Thermodynamics reveals that the minimum operating temperature for high thermal efficiency under adiabatic conditions with full recycle of MSR effluents is 550 degrees C. Feeding the oxidation reactor with added fuel does not improve efficiency. (ii) A kinetic model that accounts for the permeance of the Pd membranes suggests even higher temperatures should be considered when operating with limited membrane area. The effect of catalytic kinetics is small. (iii) A transient detailed one-dimensional model considering heat exchange between the reactors, heat losses to surroundings and axial distribution of the MOx feed is used to study the performance of a 1.3 L system in terms of thermal efficiency and permeate flow rate. Efficiency and H-2 output are favored by higher flow rates, which result in higher temperatures. Combustion of recycled effluent produces hot spots, but distributing the feed axially mitigates the non-uniformity, and improves efficiency and permeate flow rate. Some distinct dynamical aspects are presented.