High and Low Temperature Water-Gas Shift Reactions

The reaction is highly endothermic and favoured at lower pressures. The traditional SMR process essentially consists of feed gas preheating and pretreatment, reforming, high and low temperature water gas shift reactions, CO removal and methanation. 

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... 

Wei-Hsin Chen, M.-R. Lin, T.L. Jiang, M.-H. Chen, Modeling and simulation of hydrogen generation from high temperatureand low temperature water gas shift reaction. Inter. J. Hydro. Ener. 33 (2008) 6644-6656. 

A breadboard gasoline fuel processor was assembled by Moon et d. [67]. Fixed bed reactors served for reforming by steam supported partial oxidation (see Section 7.1.1), followed by high and low temperature water-gas shift Commercial iron oxide/ chromium oxide catalyst was applied for high temperature shift at a 4200 h gas hourly space velocity and 450 °C reaction temperature, while the copper/zinc oxide low temperature water-gas shift catalyst was operated at 250 °C and 5600 h gas hourly space velocity. 

The low temperature water-gas shift reaction is well described by a micro-kinetic model [C.V. Ovesen, B.S. Clausen, B.S. Hammershoj, G. Sreffensen, T. Askgaard, I. Chorkendorffi J.K. Norskov, P.B. Rasmussen, P. Stoltze and P.J. Taylor,/. Catal. 158 (1996) 170] and follows to a large extent the scheme in Eqs. (23-31). The analysis revealed that formate may actually be present in nonvanishing amounts at high pressure (Fig. 8.18). 

Figure 3.28 shows the adiabatic temperature rise, which is the increase in gas temperature in a perfectly insulated reactor, for a typical reformate for increasing conversion of carbon monoxide by a water-gas shift [57]. Owing to thermodynamic limitations and its exothermicity, the reaction is divided into two consecutive steps, known as high temperature and low temperature water-gas shifts on the industrial scale. However, two water-gas shift stages are only mandatory for fixed bed or monolithic reactors, which will be discussed in Section 5.2.1. High temperature... 

Catalysts based on CuO-ZnO are of great industrial interest because they exhibit high activity for the low temperature-pressure methanol synthesis and the water-gas-shift reactions. It is known that the activity and useful life of catalysts depend mainly on the activation process and the thermal history they experience during the operation. In the low temperature water gas shift (LTWGS) process, prior to reaction, the catalyst is activated by gas reduction to convert copper oxide into metallic copper [1]. It has been observed that reduction conditions affect the activity and the stability of Cu-ZnO catalysts. For instance, sintering and formation of alloys must be avoided in the reduction step because they deactivate the catalyst [2-3] for the water-gas-shift reaction. 

The water-gas shift reaction usually suffers from mass transfer limitations similar to the reforming process when fixed-bed catalysts are applied. Levent found catalyst effectiveness factors for high temperature iron/chromium water-gas shift catalysts in the range between 10 and 20% [391]. Giuntaet al. calculated catalyst effectiveness factors lower than 10% for a fixed low temperature water-gas shift catalyst bed [392]. 

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. ... 

Low-temperature (<500 °C) reforming technologies are also under investigation. The advantages of low-temperature technologies are reduced energy intensity, compatibility with membrane separation, favorable conditions for the water-gas shift reaction, and minimization of undesirable decomposition reactions typically encountered when carbohydrates are heated to high temperatures [44]. 

Directly supported clusters of type Os3H(CO)10(O—metal oxide) break down at quite low temperatures to give species which have a high selectivity to methane from CO and H2 (381,400). Similar behavior has been reported for Os3(CO)12 itself (401), but it is difficult to rule out metal as the catalyst. Os3(CO)12 also leads to methanol, methyl and ethyl formate, and acetone by reaction with CO and H 2 (190° C, 180 atm) in glyme solvents (402). The water-gas-shift reaction is catalyzed by Os3(CO)12, using KOH or even sodium sulfide in methanol as the base (403), although ruthenium catalysts are better (404). 

Followed by water-gas shift reactions at high (HTWGS) and low temperature (LTWGS)... 

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