Water-gas shift mixtures

Before describing variations in the critical points in the four-component water-gas shift mixture it is instructive to examine the critical points in the various binary mixtures. There are six binary pairs to consider. 

In general, oxides of Pd are not thermodynamically stable in the partially reducing atmospheres of water-gas shift mixtures, and pure Pd is not poisoned by high-pressure steam alone [50]. However, if impurities are present in Pd, which segregate to the gas-Pd interface and react with steam to form stable oxides, then the Pd can be poisoned. Likewise, hydrothermal transport of silicon and metals by high-pressure steam via volatile oxyhydroxides, originating from ill-chosen reactor wall materials, can also poison the catalytic activity of Pd. 

Figure 4.7 shows hydrogen flux data for membranes of Group IVB-VB material tested at Eltron Research Inc. using feeds containing various components of a water-gas shift mixture. The membranes were tested with a total pressure in the feed up to 450 psi (31.0 bar) and with argon sweep gas at ambient pressure. In an... 

Figure 4.7 Hydrogen flux upon exposure to various components of a high-temperature water-gas shift mixture. Steam and CO do not affect flux or catalysts relative to an ideal H / He mixture. Some loss in hydrogen flux occurs with addition of CO, but a flux of 150 mL min" cm" (STP) was achieved with over 1 bar CO in the mix at 693 K (420°C).

Figure 4.8 shows the results of a long-term study at Eltron Research Inc. in a water-gas shift mixture containing 41.4 mol% H2, 3.3 mol% CO, 17.8 mol% CO2, and 37.3 mol% steam with a balance of inert gases. The study was conducted at ambient pressure for over 2800 h (4 months) under continuous operation in the water-gas shift mixture. Membrane catalysts were protected by guard bed adsorbents. After four months, a permeabUily of 1.0 x 10 mol m s" Pa" was maintained. 

Figure 4.8 Long-term test in a water-gas shift mixture at 613 K (340 °C) and ambient pressure. A permeability of over 1x10 mol m s" Pa" - remained after over 2800 h (4 months) continuous operation in the mixture. Guard bed adsorbents were used to protect the membrane catalysts.

Bredesen R., Peters T. A., Kaleta T., Stange M. and Lovvik O. M., Performance of Pd-alloy membranes in water gas shift mixtures containing sulphur, Proc. of ICCMR10, St. Petersburgh (Ru), June 20-24,2011. 

Peters T. A., Stange M., Klette H. and Bredesen R., High pressure performance of thin Pd-23%Ag/stainless steel composite membranes in water gas shift mixture influence of dilution, mass transfer and surface effects on hydrogen flux,/ Membrane 5 a.,316 (2008) 119-127. 

The Fischer-Tropsch reaction is essentially that of Eq. XVIII-54 and is of great importance partly by itself and also as part of a coupled set of processes whereby steam or oxygen plus coal or coke is transformed into methane, olefins, alcohols, and gasolines. The first step is to produce a mixture of CO and H2 (called water-gas or synthesis gas ) by the high-temperature treatment of coal or coke with steam. The water-gas shift reaction CO + H2O = CO2 + H2 is then used to adjust the CO/H2 ratio for the feed to the Fischer-Tropsch or synthesis reactor. This last process was disclosed in 1913 and was extensively developed around 1925 by Fischer and Tropsch [268]. 

The mixture of carbon monoxide and hydrogen is enriched with hydrogen from the water gas catalytic (Bosch) process, ie, water gas shift reaction, and passed over a cobalt—thoria catalyst to form straight-chain, ie, linear, paraffins, olefins, and alcohols in what is known as the Fisher-Tropsch synthesis. 

Synthesis Gas Chemicals. Hydrocarbons are used to generate synthesis gas, a mixture of carbon monoxide and hydrogen, for conversion to other chemicals. The primary chemical made from synthesis gas is methanol, though acetic acid and acetic anhydride are also made by this route. Carbon monoxide (qv) is produced by partial oxidation of hydrocarbons or by the catalytic steam reforming of natural gas. About 96% of synthesis gas is made by steam reforming, followed by the water gas shift reaction to give the desired H2 /CO ratio. 

Reactions of Synthesis Gas. The main hydrogen manufacturing processes produce synthesis gas, a mixture of H2 and CO. Synthesis gas can have a variety of H2-to-CO ratios, and the water gas shift reaction is used to reduce the CO level and produce additional hydrogen, or to adjust the H2 to-CO ratio to one more beneficial to subsequent processing (69) ... 

The partial-oxidation process differs only in the initial stages before the water gas shift converter. Because it is a noncatalyzed process, desulfurization can be carried out further downstream. The proportions of a mixture of heavy oil or coal, etc, O2, and steam, at very high temperature, are so adjusted that the exit gases contain a substantial proportion of H2 and carbon monoxide. 

Synthesis gas, a mixture of CO and o known as syngas, is produced for the oxo process by partial oxidation (eq. 2) or steam reforming (eq. 3) of a carbonaceous feedstock, typically methane or naphtha. The ratio of CO to may be adjusted by cofeeding carbon dioxide (qv), CO2, as illustrated in equation 4, the water gas shift reaction. 

In a typical PAFC system, methane passes through a reformer with steam from the coolant loop of the water-cooled fuel cell. Heat for the reforming reaction is generated by combusting the depleted fuel. The reformed natural gas contains typically 60 percent H9, 20 percent CO, and 20 percent H9O. Because the platinum catalyst in the PAFC can tolerate only about 0.5 percent CO, this fuel mixture is passed through a water gas shift reactor before being fed to the fuel cell. 

Figure 6.4. Examples for the four types of global classical promotion behaviour. Work function increases with the x-axis. (a) Steady-state (low conversion) rates of ethylene oxide (EtO) and C02 production from a mixture of 20 torr of ethylene and 150 torr of 02 for various Cs predosed coverages on Ag(lll) at 563 K19 (b) Rate of water-gas shift reaction over Cu(l 11) as a function of sulphur coverage at 612 K, 26 Torr CO and 10 Torr H202° (c) Effect of sodium loading on NO reduction to N2 by C3H6 on Pd supported on YSZ21 at T=380°C (d) Effect of sodium loading on the rate of NO reduction by CO on Na-promoted 0.5 wt% Rh supported on Ti02(4% W03).22...

Some deactivation processes are reversible. Deactivation by physical adsorption occurs whenever there is a gas-phase impurity that is below its critical point. It can be reversed by eliminating the impurity from the feed stream. This form of deactivation is better modeled using a site-competition model that includes the impurities—e.g., any of Equations (10.18)-(10.21)— rather than using the effectiveness factor. Water may be included in the reaction mixture so that the water-gas shift reaction will minimize the formation of coke. Off-line decoking can be... 

Furthermore, the application of the SOD membrane in a FT reaction has been investigated. The advantages of water removal in a FT reaction are threefold (i) reduction of H20-promoted catalyst deactivation, (ii) increased reactor productivity, and (iii) displaced water gas shift (WGS) equilibrium to enhance the conversion of CO2 to hydrocarbons [53]. Khajavi etal. report a mixture of H2O/H2 separation factors 10000 and water fluxes of 2.3 kg m h under... 

CO in the synthesis gas mixture for the methanol synthesis does not seem to take part directly in the reaction, but it does influence the process through two effects First the water-gas shift reaction and, secondly, through its effect on the surface morphology (and possibly also composition). For thermodynamic reasons, however, it would be desirable if CO could be hydrogenated directly via Eq (18) instead of going through two coupled equations (3) and (19), since it would yield a higher equilibrium concentration of methanol at the reactor exit. 

When methanol is produced from a mixture of CO2, CO and H2, the reverse water-gas shift reaction complicates the system, since it competes with the methanol synthesis. 

Figure 8.18. Surface coverages ofthe various intermediates on a copper surface during the water-gas shift reaction at 200 °C in a gas mixture of33% HjO, 52% Hj, 13%C02, and 1 % CO. Note the high coverage of formate at...

Yoshida and Otsuka found that platinum(O) complexes [PtLj] (26) (a L = PEtj b L = P Prj) and rhodium hydrido complexes such as [RhHLj] (L = P Prs 33, PEts), [RhiHidr-NJlPCyslJ (34), tram-[RhH(N2)(PPh Bu2)J, and [RhH(P Bu3)J, all of which carry electron-donating alkylphosphine ligands, can catalyze the water gas shift reaction under fairly mild conditions (100-150°C CO 20 kg/cm ) (Eq. 6.32) [23, 60]. Among these complexes, [RhH(P Pr3)3] (33) was the most active catalyst precursor. Several complexes were isolated from or detected in the reaction mixture... 

Additional utilization of the water gas shift reaction also allows ethylene or methanol to be produced in a second synthesis step, which was developed around 1925 by Fischer and Tropsch [2], The catalyst for this heterogeneous process consists of Co-Th02-MgO mixtures supported on kieselgur. 

The system is not limited to the use of synthesis gas as feed. Mixtures of carbon dioxide and hydrogen also give rise to the formation of polyhydric alcohols, and it is also claimed that the reaction mixture can consist of steam and carbon monoxide (62). This latter claim is consistent with the presence of C02 in the reaction mixture when CO/H2 is used as feed [infrared data (62)], and suggests that these ionic rhodium systems are also active catalysts for the water gas-shift reaction (vide infra). 

The water gas shift reactions were also run in methanol-water mixtures of varying compositions using KOH as the base. 

These optimum methanol-water mixtures as solvents for the water gas shift reactions represent compromises between a high concentration of the reactant water and a high concentration of methanol to solubilize the CO and metal carbonyls. Furthermore, all of the solvent mixtures used in this work contain amounts of water which are large relative to that consumed in the water gas shift reaction. Therefore, the concentration of water may be regarded as a constant during the water gas shift reactions conducted in this research project. 

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