Hydrogen separation performances

Wang, H., Lin, Y. S. (2012). Synthesis and modification of ZSM-5/silicalite bilayer membrane with improved hydrogen separation performance. Journal of Membrane Science, 396, 128-137. 

Hopkins, S., High-Performance, Durable, Palladium-Alloy Membrane for Hydrogen Separation and Purification, Proceedings of 2007 U.S. DOE Hydrogen Annual Merit Review Meeting, Arlington, VA, May 2007. 

Much research has been performed on silica membranes produced by Chemical Vapour Infiltration for hydrogen separation purposes. In chapter 7, CVI experiments are described and a concise literature review is provided as well. Below some highlights will be presented briefly. 

Because Pd-based metal membranes, commonly used for hydrogen separation [11] are not resistant towards sulphur, not much research has been performed on the use of such membranes in H2S dehydrogenation reactors. Some success has, however, been reported by Edlund and Pledger [12], They developed a platinum-based layered metal membrane that could resist irreversible attack by H2S at 700°C. At this temperature a conversion of 99.4% was achieved in the membrane reactor. Without hydrogen removal the conversion was only 13%. No permeance data is provided, but platinum-based metal membranes are known for their low hydrogen permeance [14], Johnson-Matthey developed palladium composite membranes with a hydrogen permeance of about 1 10 mol/m sPa [14], but these are most probably not resis-... 

Cheng Y.S. Performance of alumina, zeolite, palladium, Pd-Ag alloy membranes for hydrogen separation from Towngas mixture. J.Membr.Sci 2002 204 329-340. 

Separation of the polar gases such as carbon dioxide, hydrogen sulfide and sulfur dioxide behaves in many respects differently from other nonpolar gases. Their transport mechanisms through porous membranes are often different and therefore their separation performances can also be markedly different. This has been observed for separating carbon dioxide from other nonpolar, non-hydrocarbon gases. 

The selection of adsorbents is critical for determining the overall separation performance of the above-described PSA processes for hydrogen purification. The separation of the impurities from hydrogen by the adsorbents used in these processes is generally based on their thermodynamic selectivities of adsorption over H2. Thus, the multicomponent adsorption equilibrium capacities and selectivities, the multi-component isosteric heats of adsorption, and the multicomponent equilibrium-controlled desorption characteristics of the feed gas impurities under the conditions of operation of the ad(de)sorption steps of the PSA processes are the key properties for the selection of the adsorbents. The adsorbents are generally chosen to have fast kinetics of adsorption. Nonetheless, the impact of improved mass transfer coefficients for adsorption cannot be ignored, especially for rapid PSA (RPSA) cycles. 

From this we can conclude that membranes with a selectivity higher than Knudsen diffusion are needed the process conditions should be changed in order to increase the membrane separation performance, and the dehydrogenation reaction kinetics seem fast enough to react on the hydrogen removal, at the chosen residence time of 0.5 s. 

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 separator using 1 mil thickness low-density polyethylene membrane is to be designed for concentrating hydrogen in a hydrogen-methane-carbon monoxide gas mixture. The separator performance may be approximated by a perfect mixing model. The feed flow rate is 1.0 x 10" cnf (STP)/s, and its composition and component permeabilities in polyethylene membrane are given below ... 

Hydrogen separation experiments were carried out from hydrogen/nitfogen mixtures that clearly showed that partial pressure/concentration driven Hi separation can be achieved through our membranes. The experiments were performed using 500 p.m thick composite disks and supported 35 xm thick membranes similar to the ones shown in Fig. 4.6. The active surface area of these membranes typically ranged from 1 to 1.5 cm. The feed side gas was at ambient pressure and the product side was swept with nitrogen at a known flow-rate. The gas mixture from the product... 

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