Catalytic shift reactors

The scheme of commercial methane synthesis includes a multistage reaction system and recycle of product gas. Adiabatic reactors connected with waste heat boilers are used to remove the heat in the form of high pressure steam. In designing the pilot plants, major emphasis was placed on the design of the catalytic reactor system. Thermodynamic parameters (composition of feed gas, temperature, temperature rise, pressure, etc.) as well as hydrodynamic parameters (bed depth, linear velocity, catalyst pellet size, etc.) are identical to those in a commercial methana-tion plant. This permits direct upscaling of test results to commercial size reactors because radial gradients are not present in an adiabatic shift reactor. 

When used in a PEFC system, the reformate must pass through a preferential CO catalytic oxidizer, even after being shifted in a shift reactor. Typically, the PEFC can tolerate a CO level of only 50 ppm. Work is being performed to increase the CO tolerance level in PEFC. At least two competing reactions can occur in the preferential catalytic oxidizer ... 

It is of interest to assess the process potential of methanol production by a direct partial oxidation of methane. This way the steam reformer and the shift reactor can be saved, and the catalytic methanol reactor can be replaced by a noncatalytic partial oxidation reactor. It is estimated that direct partial oxidation is competitive if a conversion of methane of at least 5.5% can be obtained with a methanol selectivity of at least 80%. 

Catalytic membrane reactors are not yet commercial. In fact, this is not surprising. When catalysis is coupled with separation in one vessel, compared to separate pieces of equipment, degrees of freedom are lost. The MECR is in that respect more promising for the short term. Examples are the dehydrogenation of alkanes in order to shift the equilibrium and the methane steam reforming for hydrogen production (29,30). An enzyme-based example is the hydrolysis of fats described in the following. 

Basile, A. Drioli, E. Santella, F., Violante, V. Capannelli, G, A Study on Catalytic Membrane Reactors for Water Gas Shift Reaction Gas Separation Purification 10(1) (1996b) 53-61. 

The particulate catalyst is composed of 90% Fe and 10% Cr. The Cr minimizes sintering of the active Fe phase. The catalytic reaction is limited by pore diffusion so small particles are used. The exit process gas contains about 2% CO as governed by the thermodynamics and kinetics of the reaction. This reaction is slightly exothermic and thermodynamics favor low temperatures that decrease the reaction rate. It is therefore necessary to further cool the mix to about 200°C where it is fed to a low-temperature shift reactor (LTS)... 

Itoh N., Shindo Y, Haraya T. and Hakuta T, A membrane reactor using microporous glass for shifting equilibrium of cyclohexane dehydrogenation, J. Chem. Eng. Jpn. 21 399 (1988). Tsotsis T.T., Champagnie A.M., Vasileiadis S.P., Ziaka E.D. and Minet R.G., Packed bed catalytic membrane reactors, Chem. Engng. ScL 47 2903 (1992). 

Violante, V., Basile, A., and Drioh, E., Composite catalytic membrane reactor analysis for water gas shift reaction in the tritium fusion fuel cycle, Fus. Eng. Design, 217-223, 30, 1995. 

Wanat, E.C., Venkataraman, K., and Schmidt, L.D. Steam reforming and water-gas shift of ethanol on Rh and Rh-Ce catalysts in a catalytic wall reactor. Applied Catalysis. A, General, 2004, 276 (1-2), 155. 

E. Gobina and R. Hughes, Equilibrium-shift in alkane dehydrogenation using a high temperature catalytic membrane reactor. Paper presented at the 1st International Workshop on Catalytic Membranes, September 1994, Lyon-Villeurbanne, France. 

Ettouney, H.M. Masiar, B. Bouhamra, S. Hughes, R. High temperature CO shift conversion using catalytic membrane reactors. Chem. Eng. Res. Des. 1996, 74, 649. 

The catalytic water-gas shift reactor has been demonstrated to reduce carbon monoxide (CO) to less than 0.5 %-vol. 

During the first year, most gasification trials were performed with the gasifier operated in conventional, air-blown mode. Experimental results included characterization of the gas obtained from gasification of switchgrass, preliminary evaluation of the performance of the steam reformer, and preliminary evaluation of the catalytic water-gas shift reactors. 

Evaluate alternate approaches for the major catalytic components (e.g., desulfurizer, reformer, shift reactor and selective oxidation reactor). 

Under normal operating conditions, in which the combustor is sufficiently warm and operated under fuel rich conditions, virtually no NOx is formed, although the formation of ammonia is possible. Most hydrocarbons are converted to carbon dioxide (or methane if the reaction is incomplete) however, trace levels of hydrocarbons can pass through the fuel processor and fuel cell. The shift reactors and the preferential oxidation (PrOx) reactor reduce CO in the product gas, with further reduction in the fuel cell. Thus, of the criteria pollutants (NOx, CO, and non-methane hydrocarbons [NMHC]), NOx CO levels are generally well below the most aggressive standards. NMOG concentrations, however, can exceed emission goals if these are not efficiently eliminated in the catalytic burner. 

A conventional FPS, shown in Fig. 14.2, includes a reformer, two WGS reactors, and two Preferential Oxidation (PrOx) reactors, located downstream of the WGS. For PEM fuel cells, it is a necessity to assure < 10 ppm of CO in the cell stack. These reactors form a considerable fraction of the FPS weight, volume, and cost. Replacing this train by an integrated hydrogen permeation selective membrane on the water gas shift reactor, shown in Fig. 14.3, results in a considerable reduction in the number of components, cost, and volume of the FPS. This will make fuel cell power plants practical and affordable for power generation in a wide range of applications, especially for residential and transportation. Numerous published works [8, 9] in the area of catalytic membrane reactors can be quoted in the experimental [10] and numerical [11, 12] domains. 

Based on the membrane properties, water-gas shift (WGS) membrane reactors are classified into two categories, namely, CO2 selective membrane reactors and H2 selective membrane reactors. In the CO2 selective membrane reactors, CO2 was removed from the catalytic membrane reactor and the reaction mixture becomes H2-rich steam. This may cause over reduction of Fe- or Cu-based catalysts. However, in the H2-selective membrane, CO2 will be present at a higher concentration in the reaction medium, affecting the reaction rate. 

A. Basile, E. DriolL, G. Vkullie, F. Santella, V. Violante, G. Capannellid, A study on catalytic membrane reactors for water gas shift reaction. Gas Sep. Purif. 10 (1996) 53-61. 

K. Hwang, S. J. King, J. S. Park, A catalytic membrane reactor for water-gas shift reaction, Korean J. Chem. Eng. 27 (2010) 816-821. 

---