Hydrogen separation membrane modules

Fig. 9.5 Hydrogen membrane module to separate 1.5 pounds (0.7 kg) of H2 per day from a simulated WGS feed stream...

The proposed scheme of Pre-combustion decarbonation based on the use of membranes for hydrogen separation is expected to have some advantages over the conventional and commercially ready technology, such as superior efiiciency and reduction in overall plant capital cost. Membrane-based systems may represent a real advancement in state-of-the-art H2 and CO2 capture in such power plants however, their real application is closely hnked to the realization of a membrane module that is economically built and maintained, as well as to the improvement of membrane permeance and selectivity. 

The product stream, containing H2S, H2, and S2, leaves through the top of the catalytic reactor and enter the first membrane module. Hydrogen is removed in the separation module and the retentate is cooled to the dew point temperature to separate sulfur. The decomposition gas leaving the third separator is recycled to the Claus reactor to treat the unconverted H2S and produce the required reaction heat through the Claus process. Hydrogen streams are cooled, compressed and further cooled before a final purification with a dedicated PSA. Since no natural gas is used the proposed scheme allows hydrogen production without CO2 emissions. 

In the best case where the permeate side pressure was 0.04 MPa and the temperature was 546.5°C, the product hydrogen flux was as high as 11.3 Nml/min/cm and the conversion was as high as 87.8%. This result shows that the MOC module has a potential similar to that of a conventional membrane reformer which is composed of a membrane module and separated fixed bed catalyst. 

An integrated proof-of-concept (POC) size fluidized-bed methane reformer with embedded palladium membrane modules for simultaneous hydrogen separation is being developed for demonstration (Tamhankar et al., 2007). The membrane modules will use two 6 in. X 11 in. Pd-alloy membrane foils, 25-pm thick, supported on a porous support. The developmental fluidized-bed reactor will house a total of five (5) membrane modules with a total membrane area of about 0.43 m2 and is scheduled for demonstration by September 2007. 

Capillary membrane modules are not as inexpensive or compact as hollow fine fiber modules, but are still very economical. Their principal drawback is the limited pressure differential the fibers can support, typically not more than 10 to 15 bar. This limitation means capillary modules cannot be used at the high pressures needed for hydrogen or natural-gas processing applications. However, capillary modules are ideally suited to lower-pressure separations, such as nitrogen from air or air dehydration. In these applications, capillary modules have essentially the entire market. 

Yoshino, Y., Suzuki, T., Nair, B.N., Taguchi, H., and Itoh, N., Development of tubular substrates, silica based membranes and membrane modules for hydrogen separation at high temperature, Journal of Membrane Science, 267, 8-17, 2005. 

Figure 8.1. Schematic of a hydrogen separation membrane and membrane module.

The heat in the exit gas is recovered downstream the tubular reformer, usually by the production of steam, preheating of boiler feed water, etc. The final separation into the desired product compositions depends on the application. Pressure swing adsorption (PSA) is in most cases used if pure hydrogen is desired. Pure carbon monoxide can be obtained by cryogenic separation in a cold box. Adjustment of the H2/CO ratio can be accomplished by polymer membrane modules with different selectivities for permeation of the two compounds. 

CA blend membranes for gas separation are commercially available from Envlrogenlcs Systems Co. (El Monte, CA), Separex Corp. (Anaheim, CA) and Grace Membrane Systems (Houston, TX), and are applied In spiral-wound modules for the separations of acidic gaseous components from natural gas, for the recovery of carbon dioxide In enhanced oil recovery processes for gas dehydration or the separation of hydrogen from carbon monoxide (21-23). 

A hydrogen-separation membrane module will not require any maintenance provided that... 

Gas permeation is used to separate gas mixtures, for example, hydrogen fixjm methane. High pressures on the order of 500 psia are used to force the molecules through a dense polymer membrane, which is packaged in pressure-vessel modules, each containing up to 4,000 ft of membrane surface area. Membrane modules cost approximately 35/ft of membrane surface area. Multiple modules are arranged in parallel to achieve the desired total membrane area. 

The Energy research Centre of the Netherlands (ECN), produces and offers a line of hydrogen separation modules (Hysep) on a pre-commercial basis for evaluation purposes. The technology is based on Palladium membranes which are capable of separating high purity hydrogen from a gas mixture. 

An essential element of the Hysep technology is the use of thin film palladium composite membranes to enable low cost and reliable hydrogen separation. The supported palladium layer in the Hysep module has a thickness as low as 3-9 pm, a substantial improvement over current commercial available palladium membranes, which are based on self supporting metal foils with a thickness of 20-100 pm. 

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