CO low-temperature shift catalysts

It is possible to use Cu-based CO shift catalysts since sulfur content in synthesis gas can be reduced to below 0.1 ml m with the changes of industrial feedstock for ammonia s3mthesis and development of gas piu ification technology. Low-temperature shift process was first commercialized in the United States in 1963. The same process was also industrialized in China in 1965. 

After high-temperature shift reaction, CO content is about 3%-4% in the process gas, and can be reduced to about 0.2%-0.4% after low-temperature shift reaction. Both theoretical calculation and practical production have proved that the yield of hydrogen or ammonia can be increased by about 1.1%-1.6% if CO content is decreased by about 0.1% after low-temperature shift reaction. Therefore, low-temperature shift catalyst is one of key economic catalysts in the production processes of hydrogen and ammonia. 

Cu crystallites at operating temperatures, and are the best promoters. At present, Cu Zn0-Al203, Cu Zn0-Cr20s, Cu-ZnO catalysts and Cu-Zn0-Al203 catalysts are widely applied. 

The reduction reactions release a great deal of heat, while the Cu catalysts are very sensitive to heat. Therefore, the shift catalysts are usually reduced by hydrogen at below 250°C although the reduction of CuO is easy either by H2 or CO. The reduced catalysts will spontaneously combust when it is exposed to air because there are a lot of H2 and CO adsorbed on the internal surface. Thus, the reduced catalysts must be carefully removed from the reactor when reduction process is finished. 

It is inevitable that Cu based catalysts are poisoned. Self-protection is usually adopted to assure CO conversion. The choice of catalysts and their replacement periods are directly based on the efficiency and economy of an ammonia plant. A little increase in CO conversion will compensate the fee of replacement of catalysts in short term. 

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