Pressure-driven hydrogen separation

Elangovan, S., B. Nair, J. Hartvigsen, and T. Small, Mixed Conducting Membranes for Pressure Driven Hydrogen Separation from Syngas, 225th American Chemical Society National Meeting, Fuels Division, New Orleans, LA, March 2003. 

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... 

Facilitated transport membranes can be used to separate gases membrane transport is then driven by a difference in the gas partial pressure across the membrane. Metal ions can also be selectively transported across a membrane driven by a flow of hydrogen or hydroxyl ions in the other direction. This process is sometimes called coupled transport. 

For separation of colloidal particles and for breaking down emulsions, the ultra-centrifuge is used. This operates at speeds up to 30 rpm (1600 Hz) and produces a force of 100,000 times the force of gravity for a continuous liquid flow machine, and as high as 500,000 times for gas phase separation, although these machines are very small. The bowl is usually driven by means of a small air turbine. The ultra-centrifuge is often run either at low pressures or in an atmosphere of hydrogen in order to reduce frictional losses, and a fivefold increase in the maximum speed can be attained by this means. 

Metallic membranes, (Pd-Ag) alloys, are typically used for separation of H2, either as an unsupported foil or a supported thin film. In these membranes, the hydrogen transport is by adsorption and atomic dissociation on one side of the membrane, dissolution in the membrane, followed by diffusion, and finally desorption (on the other side). Due to the H2 dissociation step, H2 separation is driven by a transmembrane difference of the square roots of the hydrogen partial pressures. The preparation technologies of both unsupported and supported Pd-Ag membranes are well developed and such membranes are commercially available. Since the membrane reformer performance is limited by separation capability, optimization of membrane permeability is one of the important issues. 

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