Hydrogen permeation carbon monoxide

Other gas permeation applications include separation of hydrogen from methane, hydrogen from carbon monoxide, and removal of components such as carbon dioxide, helium, moisture, and organic solvents from gas streams. Gas permeation for such operations may provide a more economical and more practical alternative than conventional separation processes such as cryogenic distillation, absorption, or adsorption. 

Even if the results are better than the ones obtained with a traditional packed bed reactor, there are many variables that can be used for optimizing the performance of a silica membrane reactor. One of these is the operative mean pressure that creates obstacles for the thermodynamic, but help for the permeation (Lee, Hacarlioglu, Oyama, 2004). This explains why a maximum can be found in the hydrogen and carbon monoxide yield as a function of pressure (Lee et al., 2004), as visible in Figure 4.5. 

In the same work, a 9% NiO—La203/Y-Al203 catalyst was prepared to compare with a traditional arrangement. As usual, the permeation capacities were preliminarily tested. The experimental data showed a higher permeance for hydrogen and carbon monoxide. Increasing the temperature causes the permeances of these two components to decrease, while those of methane and carbon dioxide remain constant. It was stated that the permeance is the result of a combined mechanism of diffusion and adsorption (Liu Au, 2001). 

This chapter first considers the complex mix of attributes required of SOFC anodes, including matching of thermal expansion coefficients, chemical compatibility with the electrolyte and the interconnect, porous structure to allow gas permeation, and corrosion resistance to the fuel and impurities therein. Then the nickel cermet anode is described in detail, especially its fabrication processes. Steady-state anode reactions of hydrogen and carbon monoxide are analysed, followed by a description of transient effects. Finally, behaviour under current load and operation on different fuels are discussed. The details of the anode reactions and polarisations are described in Chapter 9. 

Hasegawa Y, Kusakabe K, and Morooka S. Selective oxidation of carbon monoxide in hydrogen-rich mixtures by permeation through a platinum-loaded Y-type zeolite membrane. J Membr Sci 2001 190 1-8. 

The influence of the Peclet number is shown in Fig. 14.14. Pe is reciprocally proportional to the membrane surface. Decreasing the Pe number increases hydrogen recovery and as a consequence the CO conversion. When more membrane surface is available, also more carbon dioxide and carbon monoxide permeates through the membrane and the carbon recovery decreases. 

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. 

Even small amounts of carbon monoxide or other contaminants can poison a fuel cell. Sandia National Laboratories (SNL) is working on developing gas-selective thin film membranes to improve and lower the cost for hydrogen purification. Defect-free aluminosilicate and silicalite zeolite thin films supported on commercially available alpha and gamma alumina disk substrates were developed. In tests using SNL s permeation unit, which can test both pure and mixed gases from room temperature to 250°C, excellent separation values for hydrogen were achieved. 

F. Sakamoto, Effect of carbon monoxide on hydrogen permeation in some palladium-based alloy membranes, Int. J. Hydrogen Energy 1996, 21(11/12), 1017-1024. 

Criscuoli et al. compared Pd membrane reactor with mesoporous membrane reactor and fixed-bed reactor [5]. Figure 6.5 shows the effect of space velocity on the CO conversion for the three reaction systems. As expected both membrane reactors exhibit better CO conversion than traditional reactor. Between the two membrane reactors Pd membrane reactor exhibits much better CO conversion compared to mesoporous membrane reactor. At the highest time factor, Pd membrane reactor exhibits 100% CO conversion. By increasing the Pd membrane thickness, the hydrogen permeation rate decreases and lower conversions of carbon monoxide are achieved. When they compared experimental results with simulation results the model fits well with the experimental points. 

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