(543b) Cyclic Hybrid Adsorbent-Membrane Reactor (HAMR) Studies for Hydrogen Production

As a result of stricter environmental regulations worldwide, hydrogen is progressively becoming an important clean energy source. For H2 to replace fossil fuels in mobile applications, it will require the creation of a production and delivery infrastructure equivalent to that currently existing for fossil fuels, which is an immense task. As an alternative, and as an interim step towards the new hydrogen economy, various groups are currently studying steam reforming of methane (SRM) for the on-board generation of hydrogen, or for on site production, in order to alleviate the need for compressed or liquid hydrogen gas storage[1-4]. Conventional technologies are, however, neither convenient nor economical to apply for small-scale (on site or on-board) hydrogen generation. Reactive separation processes have, as a result, been attracting renewed interest for application in H2 production through SRM. One such technology is the hybrid adsorbent-membrane reactor (HAMR) system, which couples reaction and membrane separation steps with adsorption on the reactor and/or membrane permeate side. The HAMR concept was originally proposed by our group[5, 6] for esterification reactions, and it was adapted recently for on-board or on-site hydrogen production applications. Our early studies involved the development of a mathematical model for the HAMR system (applied to hydrogen production through SRM[7]); recently experimental investigations with the water-gas shift reaction[8], using microporous membranes and CO2 hydrotalcite-type adsorbents, were carried out in order to validate the HAMR design models. Experimental data were compared with the model predictions, and found to be consistent. In this paper we focus on the practical process design aspects of the HAMR hydrogen production process. A continuous HAMR process scheme has been investigated, both experimentally and through modeling studies.

2. HAMR Cyclic Process

The steps involved in the proposed cyclic HAMR process for the direct production of pure H2 are described below. It consists of four steps:

1. Adsorption-reaction-membrane-separation step. The reactor is initially pre-saturated with H2 and steam at the desired reaction temperature and pressure. A mixture of steam and CH4 (or CO) at a prescribed ratio is then fed into the reactor, and an essentially pure H2 product is collected at the permeate side. The reaction step is continued up to the time needed for ?breakthrough? (i.e., when the H2 purity and recovery decrease to preset levels) to occur. This time depends upon adsorbent, membrane characteristics, and other reactor parameters. At the ?breakthrough? point the feed is diverted into a second identical reactor.

2. Blow-down Step. During this step, the reactor is depressurized to a lower pressure PL, countercurrently to the feed flow direction. The effluent gas stream contains all the components left in the reactor at the end of Step 1, and is either recycled as a feed to another reactor or to be used as fuel.

3. Purge Step. The reactor is counter-currently purged with a weakly adsorbing gas, such as steam or H2, to desorb the CO2. The desorption step operates at PL. The desorbed gas consists of CO, CH4, CO2, H2, and H2O, and is either separated for recycling or used as fuel.

4. Pressurization step. The reactor is countercurrently pressurized to the reaction pressure (for Step 1) with a mixture of steam and H2. At this point, regeneration of the reactor is completed, so that it is ready to undergo a new cycle.

In our studies, a 24 min, 4-bed/4 step cycle was investigated for the water-gas shift reaction. A H2-selective carbon molecular sieve membrane together with a CO2-selective hydrotalcite adsorbent, and a commercial Cu/Zn catalyst was used. Virtually 100% conversion is achieved during the reaction step, while simultaneously 100% of CO2 is being captured during this step. Since the membrane excludes CO, the hydrogen product in the permeate side is highly pure, and ready to use in a fuel cell. A more detailed description of the characteristics of the HAMR cyclic process will be discussed during the conference presentation.

Acknowledgement

The support of the US Department Of Energy and NASA is gratefully acknowledged.

References

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