Steam methane reforming membrane separation

In recent years, new concepts to produce hydrogen by methane SR have been proposed to improve the performance in terms of capital costs reducing with respect to the conventional process. In particular, different forms of in situ hydrogen separation, coupled to reaction system, have been studied to improve reactant conversion and/or product selectivity by shifting of thermodynamic positions of reversible reactions towards a more favourable equilibrium of the overall reaction under conventional conditions, even at lower temperatures. Several membrane reactors have been investigated for methane SR in particular based on thin palladium membranes [14]. More recently, the sorption-enhanced steam methane reforming (Se-SMR) has been proposed as innovative method able to separate CO2 in situ by addition of selective sorbents and simultaneously enhance the reforming reaction [15]. 

A membrane reformer equipped with palladium membrane modules for in situ hydrogen separation is a compact, simple and highly efficient hydrogen production system, and an improvement in these respects on the conventional steam methane reformer. In addition, CO2 in the off-gas of a membrane reformer can be easily separated and captured by direct liquefaction, owing to the high concentration of COj. 

Li A, Lim C J and Grace J R (2008), Staged-separation membrane reactor for steam methane reforming , Chem EngJ, 138,452-459. 

Northwest Power Systems obtained U.S. Patent 5,997,594 in 1999 for a Steam Reformer with Internal Hydrogen Purification. This process is based on a steam reformer that has a membrane separation system and a methanation system in close proximity to the area in which the reforming reaction occurs. Several potential equipment configurations are described in the patent. 

The application of membrane reactors to methane reforming has also been evaluated in two recent studies. A technical and economic evaluation of the use of dense Pd-membrane in methane steam reforming has been presented by Aasberg-Petersen et aL [6.10]. They assumed a thin (2 Lim thick) Pd membrane, which exhibited perfect separation and, as a result, the pure hydrogen product was taken from the permeate side of the membrane. No sweep gas was used on the permeate side of the reactor. This necessitated compression of the low pressure hydrogen product. The authors concluded that membrane-based reforming using a dense film membrane became attractive only in the cases where electrical costs were low. 

Chapter 5 will report a detailed assessment of methane steam reforming MR performance. Moreover, it has to be cited the test plant fabricated by Tecnimont KT and described in Chap. 10, which is composed by a series of reformer reactors and Pd-based membrane separators and has a capacity of 20 Nm /h of pure H2 production. 

While steam reforming of methane is suppressed at high pressure and is a reversible reaction, this is not so for other fuels. Thus methane steam reforming at pressures of 10-20 bar (1 bar = 10 Pa) suffers from low methane conversion due to the thermodynamic equilibrium [48]. Such high pressure is required, for example, if the reforming process is combined with membrane separation using conventional palladium membranes (see Section 5.2.4). Industrial steam reformers work with... 

Catalytic membrane reactors are not yet commercial. In fact, this is not surprising. When catalysis is coupled with separation in one vessel, compared to separate pieces of equipment, degrees of freedom are lost. The MECR is in that respect more promising for the short term. Examples are the dehydrogenation of alkanes in order to shift the equilibrium and the methane steam reforming for hydrogen production (29,30). An enzyme-based example is the hydrolysis of fats described in the following. 

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