Hydrogen separation steam reforming with membranes

Integrating membranes into industrial chemical processes a case study of steam reforming with membranes for hydrogen separation... 

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. 

In Fig. 1.2, a draft of a reactor for methane steam reforming with multiple membrane tubes for hydrogen separation, operating in co-current mode, is shown. 

Palladium or its alloys are the most practical membrane materials, due to their high hydrogen permeability and stability at high temperatures. The membrane reformer is composed of a steam reformer equipped with palladium-based alloy modules in its catalyst bed, and can perform steam reforming reaction and hydrogen separation processes concurrently with no help from shift converter and PSA, as shown in Fig. 12.1. 

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. 

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 possible approach Natural gas can be converted at a high temperature into hydrogen, CO, C02 (syngas) in a steam reformer or partial-oxidation reactor, or autothermal reformer which is a combination of the first two. Most of the CO in the syngas is typically converted into carbon dioxide at a lower temperature in a water-gas shift reactor. The remaining small amount of CO must be removed to below 10 ppm level. This can be done using adsorption, or membrane separation, or catalytic preferential oxidation (at about 90°C with an air stream), or other practical means. Also, there are designs with membrane reformers in the literature. 

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