WGS reaction

The WGS reaction is a reversible reaction, that is, it attains equilibrium with reverse WGS reaction. Thus the fact that the WGS reaction is promoted by H20(a reactant), in turn, implies that the reverse WGS reaction may also be promoted by a reactant, H2 or CO2. In fact the decomposition of the surface formates produced from H2+CO2 is promoted 8-10 times by gas-phase hydrogen. The WGS and reverse WGS reactions can conceivably proceed on different formate sites of the ZnO surface unlike usual catalytic reaction kinetics, while the occurrence of the reactant-promoted reactions does not violate the principle of microscopic reversibility[63]. 

The optimization of heat transfer in a heat-exchanger reactor was also the objective of the work of TeGrotenhuis et al. [165]. Specifically, the exothermic water-gas shift (WGS) reaction ... 

TeGrotenhuis et al. studied a counter-current heat-exchanger reactor for the WGS reaction with integrated cooling gas channels for removal of the reaction heat. The computational domain of their 2-D model on the basis of the finite-element method... 

The WGS reaction was limited by thermodynamics under the experimental conditions employed in this study, since the value of the reaction constant K (based on the asymptotic concentration of reactants and products) was close to the value of the equilibrium... 

Step 4 Side reaction such as CO dissociation (Eqn. 6), the Boudouard reaction (Eqn. 7) or the water gas shift (WGS) reaction (Eqn. 8, with surface OH species) may also occur ... 

The promoter role of ceria in WGS reaction is demonstrated in Figure 8.6. Temperature-programmed conversion of CO was carried out in an oxygen-deficient medium. The oxygen content was adjusted to obtain a maximum conversion of 40% by... 

Once particulate matter is removed, the syngas passes through a two stages catalytic reactor, where CO reacts with steam to produce C02 and further yield H2 water-gas-shift (WGS) reaction. 

Figure 2.19 provides the thermodynamic equilibrium data for methane decomposition reaction. At temperatures above 800°C, molar fractions of hydrogen and carbon products approach their maximum equilibrium value. The effect of pressure on the molar fraction of H2 at different temperatures is shown in Figure 2.20. It is evident that the H2 production yield is favored by low pressure. The energy requirement per mole of hydrogen produced (37.8 kj/mol H2) is significantly less than that for the SMR reaction (68.7 kj/mol H2). Owing to a relatively low endothermicity of the process, <10% of the heat of methane combustion is needed to drive the process. In addition to hydrogen as a major product, the process produces a very important by-product clean carbon. Because no CO is formed in the reaction, there is no need for the WGS reaction and energy-intensive gas separation stages.

Following reaction 8.1, equilibrium concentrations of CO, H2, CH4, C02, and H20 are established according to the reaction equilibria for the methanation and WGS reactions ... 

The solid sorbent material must also be heated and cooled for regeneration and absorption cycles. Because the solid C02 sorbents operate at higher temperatures than the liquid absorption systems, they provide opportunities for simultaneous WGS reaction with C02 removal improving the CO conversion in sorbent-enhanced reaction process (SERP). A number of studies have been reported on the SERP concept and are currently underway as discussed in later sections. 

Because Pd-alloy membranes operate at high temperatures in the range of WGS reaction and on the lower end of methane reforming reaction, they can be used in a membrane reactor configuration for the simultaneous separation of hydrogen. As discussed earlier,... 

Figure 17.6 illustrates a gasification process integrated with the calcium looping process. Once the water gas mixture is formed at the exit of the gasifier, calcium oxide fines are injected into the fuel gas stream. As the fuel gas flows past the WGS catalyst, the WGS reaction takes place and forms additional C02. The injected CaO sorbent particles react with C02 and H2S in the gas stream, thereby allowing further catalytic WGS reaction to occur. The reactions involved in the calcium looping scheme are... 

Figure 3.4 and Table 3.3 show that C02 selectivity over the 15% Co/Si02 catalyst is quite low, less than 0.4% regardless of the calcination procedure used, indicating a small extent of the water-gas shift (WGS) reaction over the catalysts. However, as shown in Table 3.3, a slightly lower average C02 selectivity was observed over the NO calcined 15% Co/Si02 catalyst compared to the air calcined one (0.19 vs. 0.29%), another indication that the NO calcination benefited FTS performance. 

CO can be converted into either hydrocarbon products and water (via FTS) or C02 and Fl2 via the water-gas shift (WGS) reaction. The reversible WGS reaction accompanies FTS over the iron-based catalyst only at high temperature conditions. The individual rates of FTS (rFTS) and the WGS reaction (rWGS) can be calculated from experimental results as rWGS = r(,and rFTS = rco-rc02, where rCo2 is the rate of C02 formation and rco is the rate of CO conversion. 

In this chapter a two a selectivity model is proposed that is based on the premise that the total product distribution from an Fe-low-temperature Fischer-Tropsch (LIFT) process is a combination of two separate product spectrums that are produced on two different surfaces of the catalyst. A carbide surface is proposed for the production of hydrocarbons (including n- and iso-paraffins and internal olefins), and an oxide surface is proposed for the production of light hydrocarbons (including n-paraffins, 1-olefins, and oxygenates) and the water-gas shift (WGS) reaction. This model was tested against a number of Fe-catalyzed FT runs with full selectivity data available and with catalyst age up to 1,000 h. In all cases the experimental observations could be justified in terms of the model proposed. 

For the development of a selectivity model it is helpful to have a picture of the surface of the catalyst to ht the explanation of how the product spectrum is formed. The fundamental question regarding the nature of the active phase for the FT and water-gas shift (WGS) reactions is still a controversial and complex topic that has not been resolved.8 Two very popular models to describe the correlations between carbide phase and activity are the carbide9 and competition models.10 There are also proposals that magnetite and metallic iron are both active for the FT reaction and carbides are not active11. These proposals will not be discussed in detail and are only mentioned to highlight the uncertainty that is still present on the exact phase or active site responsible for the FT and WGS reactions. 

For a precipitated iron catalyst, several authors propose that the WGS reaction occurs on an iron oxide (magnetite) surface,1213 and there are also some reports that the FT reaction occurs on a carbide surface.14 There seems to be a general consensus that the FT and WGS reactions occur on different active sites,13 and some strong evidence indicates that iron carbide is active for the FT reaction and that an iron oxide is active for the WGS reaction,15 and this is the process we propose in this report. The most widely accepted mechanism for the FT reaction is surface polymerization on a carbide surface by CH2 insertion.16 The most widely accepted mechanism for the WGS reaction is the direct oxidation of CO with surface 0 (from water dissociation).17 Analysis done on a precipitated iron catalyst using bulk characterization techniques always shows iron oxides and iron carbides, and the question of whether there can be a sensible correlation made between the bulk composition and activity or selectivity is still a contentious issue.18... 

One of these surfaces is also responsible for the WGS reaction (polar or oxide surface). 

The WGS reaction as well as n-parafflns, 1-olefins, and oxygenates are formed on the polar surface. 

Traditionally, iron-based catalysts have been used for FT synthesis when the syngas is coal derived, because of their activity in both FTS and WGS reactions. Complex mixtures of straight-chain paraffins, olefins, and oxygenate (in substantial proportions) compounds are known to be formed during iron-based FTS. Olefin selectivity of iron catalysts is typically greater than 50% of the hydrocarbon products at low carbon numbers, and more than 60% of the produced olefins are a-olefins.13 For iron-based catalysts, the olefin selectivity decreases asymptotically with increasing carbon number. 

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