Water-gas-shift

The WGS reaction is of interest, both for the production of pure hydrogen for fuel cells and heavy chemical processing [143,144,211-218]. Moreover, the WGS is also one of the key steps involved in automobile exhaust processes, converting CO and water to hydrogen and CO2, and the presence of the hydrogen provides a very effective reductant for NOj, removal [211,212,215]. 

Water-gas was first obtained via the reaction of steam with coke. Catalysts are used to promote the following equilibrium in the direction of hydrogen [18,132]  

Moreover, the presence of 1% CO in the hydrogen causes major problems in Pt-based fuel cells because Pt is poisoned by CO at the operating temperatures of the fuel cell. Current commercial fuel processing systems rely on a subsequent CO clean-up system based on the reaction  

Another advantage for gold catalysts is that they can be used to selectively catalyse this oxidation, since the selectivity for the competing hydrogen/oxygen reaction is significantly lower at ambient temperatures (see Sect. 6.4.4). 

High catalytic activity for gold catalysts in the WGS was found by Andreeva and co-workers using Fe20s as a support [219,220,222]. Over this catalyst, CO conversion started at 393 K (see Fig. 6.5), and its activity exceeded that of the most efficient industrial WGS catalyst known at that time. 

Catalytic gas purification of reformate containing significantly more than 1% of carbon monoxide is performed by the water-gas shift reaction. Frequently, especially in the case of partial oxidation reactions, water is added to the reformate to shift the equilibrium of the reaction in the desired direction  

The equilibrium constant of the reaction may be easily calculated according to the following equation [78]  

The reaction is slightly exothermic, which is not favorable for the carbon monoxide equilibrium conversion when mnning shift reactors in the adiabatic mode. 

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 

Concentration polarization under WGS conditions was experimentally verified by Peters and co-workers for 2 pm Pd-23wt%Ag membranes at 400 °C and 20 bar. For this condition the authors reported that 65% of the reduction in driving force for the separation was caused by surface adsorption of gas components, while only 20% was due to concentration polarization. These figures also clearly demonstrate that the resistance of the 2 pm membrane was relatively unimportant, and that thicker, possibly more stable membranes could be used. 

Even though the hydrogen yield is improved in a membrane reactor, the authors concluded based on simulation that their employed 30 pm Pd-Ag membrane would need 20 times higher flux to generate hydrogen yields that match the DoE target. Table 11.4 shows experimentally reported results for membrane reactors employing ethanol as fuel. 

Co-feeding oxygen to provide heat, appears also to reduce the tendency of coking. The amount of oxygen co-fed with steam obviously has importance for the reaction path, with too little steam reforming will prevail. 

Depending on the operating conditions, three different types of WGS processes are applied [232] [303] [391]. High-temperature shift (300— 500°C) over a robust catalyst is used for primary conversion. Medium temperature shift (200—330 C) is used for special purposes. Low-temperature shift (185—250 C) is used to achieve maximum conversion. Sour gas shift (350 C) is used to operate under high sulphur eonditions and low H2/CO (raw coal gas, etc) 

The high-temperature shift process is typically carried out in adiabatic reactors at an inlet temperature above 300°C and with a temperature increase up to 500°C. The catalyst is a robust Fe-Cu-Cr catalyst [68], Chromium, which prevents sintering, is present as an iron chromium internal spinel [175] [361], The activity is improved by promotion with a low percent copper, which will be present as small metallic crystallites on the iron chromium spinel [232] [266], This will also inhibit the formation of hydrocarbons at low H2/CO ratios [96], It was shown [235] [391] that the activity for this reaction could be related to the phase transition into iron-carbide, which is a Fischer-Tropsch catalyst  

The transformation point can be determined by means of the principle of equilibrated gas [235] (refer to Section 5.2.3). 

The catalyst is robust towards sulphur [67] [68]. However, this can hardly be utilised as the product gas leaving the steam reformer is practically sulphur-free and because the raw syngas from a coal gasifier will have the potential for carbide formation due to the low H2/CO ratio (see below). 

Low-Temperature Shift (LTS) catalysts are copper-based catalysts which operate at temperatures as low as 185-225°C [232] [303] [391], They may in principle operate at even lower temperatures, but limited by the dew point of the process gas. The commercial catalysts are typically based on Cu/ZnO/A Os. The active phase is copper in close coimection with zinc oxide. Although the activity relates to the copper surface area, the role of the zinc oxide is still being discussed [216] [232], as is the reaction mechanism [207] [337], 

---

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]. 

Again with platinized Ti02, ultraviolet irradiation can lead to oxidation of aqueous CN [323] and to the water-gas shift reaction, CO + H2O = H2 + CO2 [324]. Some mechanistic aspects of the photooxidation of water (to O2) at the Ti02-aqueous interface are discussed by Bocarsly et al. [325]. 

Low pressure methanol carbonylation transformed the market because of lower cost raw materials, gender, lower cost operating conditions, and higher yields. Reaction temperatures are 150—200°C and the reaction is conducted at 3.3—6.6 MPa (33—65 atm). The chief efficiency loss is conversion of carbon monoxide to CO2 and H2 through a water-gas shift as shown. 

Methanol (qv) is one of the 10 largest volume organic chemicals produced in the wodd, with over 18 x 10 t of production in 1990. The reactions for the synthesis of methanol from CO, CO2, and H2 are shown below. The water gas shift reaction also is important in methanol synthesis. 

Study of the mechanism of this complex reduction-Hquefaction suggests that part of the mechanism involves formate production from carbonate, dehydration of the vicinal hydroxyl groups in the ceUulosic feed to carbonyl compounds via enols, reduction of the carbonyl group to an alcohol by formate and water, and regeneration of formate (46). In view of the complex nature of the reactants and products, it is likely that a complete understanding of all of the chemical reactions that occur will not be developed. However, the Hquefaction mechanism probably involves catalytic hydrogenation because carbon monoxide would be expected to form at least some hydrogen by the water-gas shift reaction. 

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. 

Prior to methanation, the gas product from the gasifier must be thoroughly purified, especially from sulfur compounds the precursors of which are widespread throughout coal (23) (see Sulfurremoval and recovery). Moreover, the composition of the gas must be adjusted, if required, to contain three parts hydrogen to one part carbon monoxide to fit the stoichiometry of methane production. This is accompHshed by appHcation of a catalytic water gas shift reaction. 

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) ... 

In the next step, the CO is converted to CO2 and hydrogen by the water gas shift reaction step ... 

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. ... 

The final composition of the reactor product gas is estabUshed by the water gas shift equiUbrium at the reactor outiet waste-heat exchanger inlet where rapid cooling begins. Some units quench instead of going directiy to heat exchanger. 

This reaction is commonly known as the CO or water gas shift reaction. The conversion of CO by this reaction is slightly exothermic and favored by lower temperatures. The lower practical operating temperature range for this reaction is between 180 and 200°C (8). 

Subtracting reaction 2 from reaction 1 gives the familiar water gas shift reaction (eq. 3). 

These reactions show that the synthesis gas stoichiometry is dependent on both the nature of the feedstock as well as the generation process. Reactions 4 and 5, together with the water gas shift reaction 3, serve to independently determine the equiUbrium composition of the synthesis gas. 

Reforming is completed in a secondary reformer, where air is added both to elevate the temperature by partial combustion of the gas stream and to produce the 3 1 H2 N2 ratio downstream of the shift converter as is required for ammonia synthesis. The water gas shift converter then produces more H2 from carbon monoxide and water. A low temperature shift process using a zinc—chromium—copper oxide catalyst has replaced the earlier iron oxide-catalyzed high temperature system. The majority of the CO2 is then removed. 

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. 

The overall process for producing a 1 1 CO to ratio by partial methane oxidation and the water gas shift reaction is represented by equation 5. 

Synthesis gas preparation consists of three steps ( /) feedstock conversion, (2) carbon monoxide conversion, and (2) gas purification. Table 4 gives the main processes for each of the feedstocks (qv) used. In each case, except for water electrolysis, concommitant to the reactions shown, the water-gas shift reaction occurs. 

Shift Conversion. Carbon oxides deactivate the ammonia synthesis catalyst and must be removed prior to the synthesis loop. The exothermic water-gas shift reaction (eq. 23) provides a convenient mechanism to maximize hydrogen production while converting CO to the more easily removable CO2. A two-stage adiabatic reactor sequence is normally employed to maximize this conversion. The bulk of the CO is shifted to CO2 in a high... 

HTS catalyst consists mainly of magnetite crystals stabilized using chromium oxide. Phosphoms, arsenic, and sulfur are poisons to the catalyst. Low reformer steam to carbon ratios give rise to conditions favoring the formation of iron carbides which catalyze the synthesis of hydrocarbons by the Fisher-Tropsch reaction. Modified iron and iron-free HTS catalysts have been developed to avoid these problems (49,50) and allow operation at steam to carbon ratios as low as 2.7. Kinetic and equiUbrium data for the water gas shift reaction are available in reference 51. 

Tubular Fixed-Bed Reactors. Bundles of downflow reactor tubes filled with catalyst and surrounded by heat-transfer media are tubular fixed-bed reactors. Such reactors are used most notably in steam reforming and phthaUc anhydride manufacture. Steam reforming is the reaction of light hydrocarbons, preferably natural gas or naphthas, with steam over a nickel-supported catalyst to form synthesis gas, which is primarily and CO with some CO2 and CH. Additional conversion to the primary products can be obtained by iron oxide-catalyzed water gas shift reactions, but these are carried out ia large-diameter, fixed-bed reactors rather than ia small-diameter tubes (65). The physical arrangement of a multitubular steam reformer ia a box-shaped furnace has been described (1). 

This is the reverse of the water-gas shift reaction in the production of hydrogen and ammonia (qv). Carbon dioxide may also be reduced catalyticaHy with various hydrocarbons and with carbon itself at elevated temperatures. The latter reaction occurs in almost all cases of combustion of carbonaceous fuels and is generally employed as a method of producing carbon monoxide. 

Conversion to Hydrogen (Water Gas Shift Reaction). Carbon monoxide reacts with water over a catalyst to produce hydrogen and carbon monoxide (25). This reaction is used to prepare high purity hydrogen or synthesis gas with a higher hydrogen-to-carbon monoxide ratio than the feed (eq. 3). 

---