Permeation carbon monoxide

M REC, as the TREC, does not depend on the reaction path. In addition, there is no dependence on the membrane-permeation properties (related to the time required for species permeation).1 In any case, the final value reached depends on the extractive capacity of the system, for example, the pressure and composition on the permeate side. The composition on the permeate side, similarly to the feed molar ratio, can be expressed by considering the ratio (named sweep factor) between the initial molar number of nonpermeating species (present on the permeate side) and the initial molar number of the reference reactant, for example, methane for methane steam reforming, or carbon monoxide for water gas shift). The sweep factor was defined for a closed M Ras ... 

Wu et al. (1993] have developed a mathematical model based on Knudsen diffusion and intermolecular momentum transfer. Their model applies the permeability values of single components (i.e., pure gases) to determine two parameters related to the morphology of the microporous membranes and the reflection behavior of the gas molecules. The parameters are then used in the model to predict the separation performance. The model predicts that the permeability of carbon monoxide deviates substantially from that based on Knudsen diffusion alone. Their model calculations are able to explain the low gas separation efficiency. Under the transport regimes considered in their study, the feed side pressure and pressure ratio (permeate to feed pressures) are found to exert stronger influences on the separation factor than other factors. A low feed side pressure and a tow pressure ratio provide a maximum separation efficiency. 

In some cases it is actually advantageous to deposit the catalyst (e.g., Pt) near the ends of the (alumina) membrane pores on the low pressure (permeate) side for the oxidation of carbon monoxide to form carbon dioxide [Fumeaux, 1987]. 

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. 

B.A. Van Hassel, J.E. ten Elshof and H.J.M. Bouwmeester, Oxygen permeation flux through Lai-ySryFeOs-e limited by carbon monoxide oxidation rate. Appl. Catal. A General, 119 (1994) 279-291. 

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. 

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 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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