High temperature shift catalysts

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. ... 

Andreev and coworkers—promoting effect of Cu addition on Fe-Cr shift catalysts, and Raney Cu/Zn catalysis described by an associative mechanism. Andreev and coworkers240 added promoters to traditional high temperature shift Fe-Cr catalyst, and found that Cu and Co moved the maximum in the CO conversion curve to lower temperatures. No explanation was provided for the effect. 

Querino, do Carmo Rangel, and coworkers—improvement in catalytic activity by doping Pt/zirconia with Ce. Querino et al.469 reported that doping of a fraction of approximately 0.1 Ce to Pt/Zr02 led to enhanced activity for the high temperature shift reaction. The catalyst was prepared using a sol-gel synthesis... 

Table 100 CO conversion rates as a function of steam/gas (S/G) ratio under high temperature shift conditions. Feed (dry basis) 10% CO, 10%CO2, 50% H2, 30% N2. WHSV = 42 h l. 33 mg of catalyst employed469...

E) Metal/a-Fe203 catalysts. Although Fe Cr and modified Fe-Cr-Cu catalysts have been used extensively in industry for high temperature shift applications, this section covers references only from 1993 to the present510-526 specifically dealing with metal/a-Fe203 catalysts for low temperature shift. 

Certainly, water-gas shift has in the past been carried out using zeolites as supports for Rh (e.g., Rh/Y Zeolite and Rh/NaY Zeolite539), ZnO (Na/mordenite540), and Fe oxide (Na/mordenite540) in high temperature shift catalyst studies. More recent investigations are aimed at applying zeolites and related materials for use in low temperature shift catalysts. 

The reformate gas contains up to 12% CO for SR and 6 to 8% CO for ATR, which can be converted to H2 through the WGS reaction. The shift reactions are thermodynamically favored at low temperatures. The equilibrium CO conversion is 100% at temperatures below 200°C. However, the kinetics is very slow, requiring space velocities less than 2000 hr1. The commercial Fe-Cr high-temperature shift (HTS) and Cu-Zn low-temperature shift (LTS) catalysts are pyrophoric and therefore impractical and dangerous for fuel cell applications. A Cu/CeOz catalyst was demonstrated to have better thermal stability than the commercial Cu-Zn LTS catalyst [37], However, it had lower activity and had to be operated at higher temperature. New catalysts are needed that will have higher activity and tolerance to flooding and sulfur. 

K-HTC is used more successfully for sorption-enhanced water-gas shift [27]. Breakthrough of CO occurs at the same time as breakthrough of CO2 [37] and the amount of steam necessary for desorption can be kept relatively low [27]. A commercial high-temperature shift catalyst can be used. During desorption in steam, the catalyst can be oxidized, so it is necessary to add some hydrogen to the purge steam [27]. 

A high temperature water-gas shift reactor 400°C) typically uses an iron oxide/chromia catalyst, while a low temperature shift reactor ( 200°C) uses a copper-based catalyst. Both low and high temperature shift reactors have superficial contact times (bas on the feed gases at STP) greater than 1 second (72). 

M.-S. Xu, Z.-M. Du, X.-F. Gao, False transient method for determining effectiveness factors of high-temperature shift reaction catalyst B109, J. Chem. Ind. Eng. (China) 44 (1993) 465. 

Conventionally the reaction is performed in two stages, the so-called high- and low-temperature water-gas shift. In large-scale industrial processes, Fe203/Cr203 catalysts are applied for high-temperature shift (which is then performed between... 

Temperature ramps were applied for testing, which were set to 300, 325 and 350 °C and held for 1 h each for low-temperature shift. For high-temperature shift testing, the temperature ramps were set to 350,375 and 400 °C for the same duration. These low reaction temperatures compared with industrial conditions for high-temperature shift (up to 450 °C) were applied because mostly precious metal catalysts were tested in the screening protocol, which are subject to coke formation at higher reaction temperatures. 

The shift from carbon monoxide to carbon dioxide generally occurs in two steps - first a High Temperature Shift Conversion and then a Low Temperature shift conversion. In some cases the two steps may be combined in one isothermal or adiabatic step called Medium Temperature Shift Conversion. When the feed gas to the CO conversion is not desulfurized, the CO conversion is called Sour Gas Shift and a special type of sulfur-resistant catalyst is used166. 

As shown above, Eq. (5.2) is exothermic, and high temperatures are unfavorable for complete conversion. In the High Temperature Shift (HTS) conversion, the synthesis gas is passed through a bed of iron oxide/chromium oxide catalyst at around 400°C and a pressure of 25 to 28 bar. The CO content of the gas is reduced to about 3% (on a dry gas basis), and this is limited by the shift equilibrium at the actual operating temperature. The catalyst is not... 

In 2001 Hyprotech and Synetix announced an ammonia plant simulation that can be used for modeling, on-line monitoring and optimization of the plant. The simulation includes Synetix reactor models, customized thermodynamic data and information to simulate the performance of a range of catalysts. The reactor models in the simulation include Primary and Secondary Reformers, High Temperature Shift converter, Low Temperature Shift Converter, Methanator and Ammonia Synthesis Converter80. 

This is called the water gas shift reaction. We discuss this reaction in some detail in Chapter 4 (see Section 4.3). The heterogeneous catalysts used for the water gas shift reaction are of two types. The high-temperature shift catalyst is a mixture of Fe304 and Cr203 and operates at about 500°C. The low-temperature shift catalyst contains copper and zinc oxide on alumina, operates at about 230°C, and is more widely used in industry. 

In the traditional plant concept, the gas from the secondary reformer, cooled by recovering the waste heat for raising and superheating steam, enters the high-temperature shift (HTS) reactor loaded with an iron - chromium catalyst at 320 - 350 °C. After a temperature increase of around 50 - 70 °C (depending on initial CO concentration) and with a residual CO content of around 3 % the gas is then cooled to 200-210 °C for the low-temperature shift (LTS), which is carried out on a copper - zinc - alumina catalyst in a downstream reaction vessel and achieves a carbon monoxide concentration of 0.1-0.3 vol%. 

Generally, in a conventional WGS system a two-step shift is used to obtain high CO conversion rates. In the first high-temperature shift reactor the major part of the CO is converted at high activity, whereas in the second shift reactor the rest of the CO (closely up to the thermodynamic equilibrium) is converted at low temperature and also low activity. Steam to carbon monoxide ratios above the stoichiometric ratio (higher than 2) are generally being used to attain the desired carbon monoxide conversion, but also to suppress carbon formation on certain catalysts. 

Huang, D.C. Braden, J.L. High temperature shift catalyst. EU Patent 0353453, 1990. 

---