Copper-promoted shift catalysts

After the development of pressure swing adsorption (PSA) units in the 1970 s, it became possible to operate the steam reformer in hydrogen plants at lower steam to carbon ratios. Before then, high steam to carbon ratios were required in order to limit the amount of methane in the product hydrogen. Decreasing the steam to carbon ratio below a certain level makes it impossible to use conventional iron-chromium high temperature shift catalysts due to the risk of the Fischer-Tropsch reaction (Hoejlund Nielsen et al., 1982). Instead, copper-based or copper-promoted shift catalysts may be used (Dybkjaer, 1981 and (Boegild Hansen et al., 1990). 

This reaction is first conducted on a chromium-promoted iron oxide catalyst in the high temperature shift (HTS) reactor at about 370°C at the inlet. This catalyst is usually in the form of 6 x 6-mm or 9.5 x 9.5-mm tablets, SV about 4000 h . Converted gases are cooled outside of the HTS by producing steam or heating boiler feed water and are sent to the low temperature shift (LTS) converter at about 200—215°C to complete the water gas shift reaction. The LTS catalyst is a copper—zinc oxide catalyst supported on alumina. CO content of the effluent gas is usually 0.1—0.25% on a dry gas basis and has a 14°C approach to equihbrium, ie, an equihbrium temperature 14°C higher than actual, and SV about 4000 h . Operating at as low a temperature as possible is advantageous because of the more favorable equihbrium constants. The product gas from this section contains about 77% H2, 18% CO2, 0.30% CO, and 4.7% CH. ... 

Adjustment of the C0 H2 ratio is effected by the shift reaction (iv) which proceeds over a chromium-promoted iron catalyst at 700-800°F (370-425°C) or over a reduced copper/zinc catalyst at 375" 50°F (190-230 C) and the fraction of crude gas sent through the shift reactor is calculated from the initial gas composition and specific downstream requirements. The latter are i1 lustrated by... 

More recent research efforts have focused on the development of other possible catalysts such as promoted Raney copper,371,403 catalysts prepared from intermetal-lic precursors,362,371 386 404-406 and catalysts that tolerate high C02 content.407 Catalyst modifications allowed to shift the selectivity to the formation of higher alcohols.208,408 110 For example, in a process developed by IFP, a multicomponent oxide catalyst is applied with copper and chromium as the main components 410 By this method, 70-75% total alcohol selectivities and 30-50% of C2 and higher alcohol selectivities can be achieved at 12-18% conversion levels (260-320°C, 60-100 atm). 

Kinetic evidence for synergic adsorption of carbon monoxide and water on the low-temperature shift catalyst Cu/ZnO/Fe203 was obtained by van Herwijnen and deJong (113), and IR spectra of surface formate were detected on several oxide catalysts, including CuO/MgO, at temperatures as low as 20 JC and pressures of 20 Torr, as reported by Davydov et al. (104). Decomposition of the surface formate to C02 and H2 occurred at 100-150°C over the Cu/MgO catalyst and at 250 300°C over the MgO catalyst, and the promotion effect of copper was attributed to the formation and decomposition of a labile surface formate (HCOO)2Cu. Ueno et al. (117) have shown earlier that surface formates are formed on zinc oxide, from CO and H20 as well as from C02 and H2, and hence an associative mechanism of the shift and reverse-shift reaction, involving formate intermediate, is believed to operate on many oxide catalysts. 

M. A. Edwards, D. M. Whittle, C. Rhodes, A. M. Ward, D. Rohan, M. D. Shanntm, G. J. Hutchings, C. J. Kiely, Microstructural studies of the copper promoted iron oxide/chromia water-gas shift catalyst, Phys. Chem. Chem. Phys. 4 (2002) 3902-3908. 

P. Kumar, R. Idem, A comparative study of copper-promoted water-gas-shift (WGS) catalysts. Energy Euel 21 (2007) 522-529. 

The first reactor (high-temperature shift) is loaded with high-temperature catalyst, generally chromium-promoted iron oxide, which operates at 623—673 K (Ledjeff-Hey, Roes, Wolters, 2000). The second reactor (low-temperature shift) is loaded with low-temperature catalyst of copper-promoted zinc oxide, which operates at 473 K (Ledjeff-Hey et al., 2000). 

The more active copper-promoted, high-temperature shift catalysts can be operated at somewhat lower temperatures. Haldor Tops0e (1992) recommends 610°-660°F as the optimum inlet temperature for their SK-201 copper-promoted iron/chromia-based catalyst and report that it can be operated at temperatures down to 570°F. 

Because of the pure performance of traditional Cu catalysts in the hydrogenation of C02, efforts have been made to find new, more effective catalysts for direct C02 hydrogenation. The problem is to improve selectivity, specifically, to find catalysts that display high selectivity toward methanol formation and, at the same time, show low selectivity in the reverse water-gas shift reaction, that is, in the formation of CO. It appears that copper is the metal of choice for methanol synthesis from C02 provided suitable promoters may be added. Special synthesis methods have also been described for the preparation of traditional three-component Cu catalysts (Cu-ZnO-A1203 and Cu-Zn0-Cr203) to improve catalytic performance for C02 reduction. 

Skeletal Cu-Zn catalysts show great potential as alternatives to coprecipitated Cu0-Zn0-Al203 catalysts used commercially for low temperature methanol synthesis and water gas shift (WGS) reactions. They can also be used for other reactions such as steam reforming of methanol, methyl formate production by dehydrogenation of methanol, and hydrogenolysis of alkyl formates to produce alcohols. In all these reactions zinc oxide-promoted skeletal copper catalysts have been found to have high activity and selectivity. 

It is the reprecipitation of Zn(OH)2 in the porous skeletal copper which provides promotion in methanol synthesis, water gas shift, and other reactions. The highly dispersed reprecipitated Zn(OH)2 decomposes at around 400 K to form ZnO which is an active promoter of copper catalysts. 

This makes the use of catalysts indispensable. Copper, nickel, zinc based systems are used in the vapor phase, promoted or not in metallic or oxide form, and possibly deposited on a support Tire lower temperature (S 150 Q reaction can be carried out in the liquid phase, in the presence of Raney nickel. Hie thermodynamic equilibrium in this case is shifted in the desired direction by the continuous removal of the hydrogen produced. It is also necessary to extract the acetone from the reaction medium upon its formation, by vaporization for example, owing to the inhibitory action it exerts on the activity of the catalyst... 

Tang X-J, Eei J-H, Hou Z-Y, Lou H, Zheng X-M (2008) Copper-zinc oxide and manganese promoted copper-zinc oxide as highly active catalysts for water-gas shift reaction. React Kinet Catal Lett 94 3-9... 

Cobalt-based catalysts are highly active, although iron may be more suitable for low-H2-content synthesis gases such as those derived from coal due to its promotion of the water-gas-shift reaction. In addition to the active metal, the catalysts typically contain a number of promoters, such as potassium and copper. 

Modern WGS catalysts can be divided into four classes, namely, HTS, LTS, sour gas shift, and precious-metal-containing catalysts. Iron oxide or iron-chromium mixed oxides promote the WGS in the 350-450 °C range and are modified by MgO or ZnO for good sulfur resistance and mechanical strength. The second type of WGS catalysts are mixed copper-zinc oxides which promote WGS in... 

Further improvement is expected from a pre-reformer positioned upstream of the primary reformer to allow reformer feed to be heated to a higher temperature at a low steamicarbon ratio without deposition of carbon. In addition, Topsoe has developed a shift conversion catalyst based entirely on copper, which does not promote a Fisher-Tropsch reaction at a low steam dry gas ratio. 

Chlorine, which will be present as HCl or organic chlorine compounds, is a poison, in particular for copper catalysts [391] and it may cause stress corrosion in the equipment. Chlorine can be removed by promoted alumina. Chlorine may be present in certain refinery offgases and in landfill gas. Chlorine may also originate from failure in the water purification system. If so, it will pass the guard bed and should be captured by a guard in the low-temperature shift reactor (see Section 1.5.2). 

The high-temperature shift process is typically carried out in adiabatic reactors at an inlet temperature above 300°C and with a temperature increase up to 500°C. The catalyst is a robust Fe-Cu-Cr catalyst [68], Chromium, which prevents sintering, is present as an iron chromium internal spinel [175] [361], The activity is improved by promotion with a low percent copper, which will be present as small metallic crystallites on the iron chromium spinel [232] [266], This will also inhibit the formation of hydrocarbons at low H2/CO ratios [96], It was shown [235] [391] that the activity for this reaction could be related to the phase transition into iron-carbide, which is a Fischer-Tropsch catalyst ... 

Chinchen, G., Spencer, M., Waugh, K., et al. (1987). Promotion of methanol synthesis and the water-gas shift reactions by adsorbed oxygen on supported copper catalysts, J. Chem. Soc. Faraday T, 83, pp. 2193-2212. 

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