Ind. Eng. Chem. Res. 2010, 49, 1123–1129
1123
Experimental Optimization of an Autonomous Scaled-Down Methane Membrane Reformer for Hydrogen Generation David S. A. Simakov* and Moshe Sheintuch* Department of Chemical Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel
Hydrogen generation by methane steam reforming in a thermally coupled membrane reformer-combustor has been experimentally studied. The concentric three-compartment reactor indirectly couples methane steam reforming with catalytic methane combustion and with a Pd-Ag membrane, to provide extrapure hydrogen. The reactor can be independently operated at steady state with the enthalpy required for the steam reforming and for heat losses provided by methane oxidation. The study focuses on the experimental demonstration of two approaches for the optimization of hydrogen generation in terms of power output and process efficiency: increasing the membrane separation ability and recycling reformer products to the combustion section. 1. Introduction Hydrogen as a fuel for energy-efficient and environmentally friendly polymer electrolyte fuel cells has attracted much attention during the last decade. However, the implementation of hydrogen-based fuel cell technologies is hindered by the drawbacks in its purification and transportation. Portable generation of ultrapure hydrogen from fossils by membrane reformers, where hydrogen production and purification steps are combined in one energetically efficient compact unit (as an alternative to a multistage reformer f water gas shift f preferential oxidation f hydrogen separation approach), has the potential to overcome these problems. Since there are large resourses of natural gas in the world, steam reforming of methane is of notable interest. The methane steam reforming (MSR) process is commonly described by the two independent reactions of methane conversion to hydrogen and carbon monoxide (eq 1a) and water gas shift (WGS, eq 1b) or by the dependent (overall) MSR reaction (eq 1c). CH4 + H2O ) CO + 3H2 ∆H1a ) 206 kJ/mol
(1a)
CO + H2O ) CO2 + H2 ∆H1b ) -41 kJ/mol
(1b)
CH4 + 2H2O ) CO2 + 4H2 ∆H1c ) 165 kJ/mol
(1c) Numerical1-6,12,13 and experimental7-15 investigation of hydrogen generation by MSR using membrane reformers has been reported in numerous works. A packed bed membrane reactor (PBMR)1-3,5-10 and a fluidized bed membrane reactor (FBMR)4,11-15 are commonly employed. The experimental verification of a PBMR for MSR that is independent of external energy sources (electricity) is still lacking and is the aim of the present project. Since the membrane reformer performance is limited by separation capability, optimization of membrane permeability is one of the important issues in developing the membrane reactor technologies. Membranes composed of a Pd-Ag thin layer on a ceramic or stainless steel porous supports8-10 are very promising, since they exhibit higher permeabilities than that of Pd-Ag foil membranes. Since the Pd cost is prohibitive, other substitutes with good selectivity for hydrogen separation, * To whom correspondence should be addressed. E-mail: simakov@ tx.technion.ac.il.
like carbon,17,18 silica,19,20 and zeolitic21,22 hydrogen-separation membranes have been extensively investigated, but they still cannot produce the desired selectivity and durability. The high endothermicity of the MSR process remains one of the major drawbacks in the MSR implementation for portable hydrogen generation. The membrane reformer has to be coupled to a heat source, which is commonly done in experimental works either completely7-12 or partially13-15 by electrical heaters. Gallucci et al.7 achieved 70% MSR methane conversion at 450 °C in an oven-heated PBMR equipped with a 50 µm thick Pd-Ag foil membrane. Other groups demonstrated complete MSR methane conversions at 550 °C in electrically heated PBMRs using highly permeable stainless steel(SS)-supported8 and Al2O3-supported9,10 Pd-Ag thin film (
