Membrane reactors hydrogen separation

De Lorenzo L, Kreutz TG, Chiesa P, Williams RH (2008) Carbon-free hydrogen and electricity from coal options for syngas cooling in systems using a hydrogen separation membrane reactor. J Eng Gas Turbines Power 130 031401-1... 

Key words hydrogen production, power generation, COj capture, membrane integration, oxygen membranes for oxy-combustion, hydrogen separation membrane reactors. 

Fig. 9.2 Scenario for integrating hydrogen separation membranes with coal gasifiers and WGS reactors for producing hydrogen at high purity while retaining CO2 at high pressure and concentration for economic sequestration...

General criteria for selection of materials for the processing of hydrogen separation membranes are discussed. Performance and stability standards required for applications in high temperature membrane reactors have been focused. The correlations between pore structure and stability issues of membranes made of amorphous materials, specifically silica membranes are discussed in detail. 

Since the feasibility and durability of the hydrogen separation membranes and the feasibility of membrane reactor has been already demonstrated, the main obstacle for implementation is the cost added by the membrane. The cost contribution to the membrane can be gauged as follows as stated previously... 

In plants implementing pre-combustion CO2 removal, the integration of hydrogen separation membranes in steam reformers or WGS reactors promotes carbon conversion to CO2 and simultaneously increases the COj concentration by removing Hj-The retentate stream is therefore mainly composed of CO2 but also contains significant traces of other species such as CH4,... 

As an example the use of ceramic membranes for ethane dehydrogenation has been discussed (91). The constmction of a commercial reactor, however, is difficult, and a sweep gas is requited to shift the product composition away from equiUbrium values. The achievable conversion also depends on the permeabihty of the membrane. Figure 7 shows the equiUbrium conversion and the conversion that can be obtained from a membrane reactor by selectively removing 80% of the hydrogen produced. Another way to use membranes is only for separation and not for reaction. In this method, a conventional, multiple, fixed-bed catalytic reactor is used for the dehydrogenation. After each bed, the hydrogen is partially separated using membranes to shift the equihbrium. Since separation is independent of reaction, reaction temperature can be optimized for superior performance. Both concepts have been proven in bench-scale units, but are yet to be demonstrated in commercial reactors. 

A special version of the membrane reactor using Pd was made for separating hydrogen and oxygen and their controlled reaction. 

GP 11] ]R 20] Investigations with a Pd membrane reactor relied on reaction of streams separated via a membrane (to prevent complete mixing of reactants, not to enhance conversion) [11]. A hydrogen/nitrogen stream is guided parallel to an oxygen stream, both separated by the membrane and water is thereby formed. The membranes, made by thin-film processes, can sustain a pressure up to 5 bar. 

Because Pd-alloy membranes operate at high temperatures in the range of WGS reaction and on the lower end of methane reforming reaction, they can be used in a membrane reactor configuration for the simultaneous separation of hydrogen. As discussed earlier,... 

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. 

Damle, A.S., Separation of Hydrogen and Carbon Dioxide in Advanced Fossil Energy Conversion Processes using a Membrane Reactor, 2002 Pittsburgh Coal Conference, Pittsburgh, PA, September 2002. 

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