Ferrochrome Catalyst

Based on analysis of isotope exchange data and stoichiometric number analysis, Oki and Me/aki12 14 corroborated two most probable mechanisms as shown below, involving the elementary steps in Equations 6.3-6.7 and 6.8-6.13, respectively, with the steps labeled I (CO adsorption) and V (hydrogen desorption) being rate limiting  

From transient response experiments, Hakkarainen et al.n found a probable mechanism involving the elementary steps (Eqs. 6.14-6.18) below  

Newsome3 analyzed the suitability of several rate equations for the HTS reaction on ferrochrome catalysts. The information analyzed pointed to a power rate law-type equation given in Equations 6.19 and 6.20, 

More recently, Lloyd et al.15 pointed out that more theoretically derived Langmuir-Hinshelwood expressions similar to Equation 6.21 provide the best fit of experimental data  

In conjunction with this equation form, an intrinsic activation energy value of 122kJ/mol is listed.5 Kinetic control is most prevalent in small catalyst pellets, tablets, or rings having approximate volumes up to -0.3 cm3 each and internal surface areas of 60-80 m2/g.5 

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Under certain conditions, especially when feeds derived from coal gasification are used, irreversible arsenic poisoning of the ferrochrome catalyst is likely to occur. When operated within normal parameters, expected life for this catalyst ranges between 1 and 3 years. 

While the commercial catalysts and technologies described above are successfully applied in the industry, some major drawbacks exist with these catalysts such as the low activity of the otherwise robust ferrochrome catalyst at low temperatures, and the susceptibility to poisoning and sintering of the CuZn shift catalyst. Additionally, both classes of catalysts are pyrophoric, generating serious safety problems in the case of accidental air exposure. Furthermore, both catalysts require a special, carefully controlled activation treatment in order to achieve the optimal active phase configuration, with the CuZn catalyst being particularly sensitive to accidental shutdowns, accidental water condensation, or temperature or concentration transients. 

Despite the extensive list of potential drawbacks with existing commercial catalysts, few new WGS catalyst formulations have been commercialized in the past decades. Based on the data compiled by Armor,20,21 no new WGS catalysts were commercialized during the 1980s, while only two new catalysts were developed during the 1990s by the Siid-Chemie Group a ferrochrome catalyst promoted with Cu was developed for enhanced activity with suppressed Fischer-Tropsch by-product... 

Also notable in the direction of classical catalyst improvement is the research conducted by Rhodes et al.22 A series of coprecipitated promoters were evaluated on ferrochrome catalyst activity at temperatures between 350 and 440 °C. It was found that activity decreases in the following order Hg > Ag > Ba > Cu > Pb > unpromoted > B. A noticeable compensation effect observed in the correlation between preexponential factors and apparent activation energies led the authors to conclude that these promoters might only influence the CO adsorption on catalyst rather than the course of surface reactions. 

A form of empirical rate expression which appears to be extremely successful in correlating water-gas shift rate data over ferrochrome catalysts is the power type expression. This equation is of the following form ... 

Bohlboro (1961,1969) studied the shift reaction at atmospheric pressure over a commercial ferrochrome catalyst of 0.8 to 1.2 mm. Fitting of the experimental data with the power type expression was accomplished by determining each individual exponent by varying the concentration of one of the species while keeping the concentrations of all other species constant. The values of the rate constants were determined by choosing the value of k which gave the closest agreement between the experimental and calculated conversions. 

The chapter focuses on a HTS ferrochrome catalyst industrial reactor modeling. 

An empirical rate expression succesfully used to describe the WGSR in ferrochrome catalysts is a power law type (Newsome, 1980)... 

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