Water-gas shift converter

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. 

Water gas shift converters in industry are largely unchanged from their original design adiabatic fixed bed reactors with particulate catalysts. There are only two kinds of these reactors in use in industry today HTS and LTS. They differ in the operating temperatures and catalysts which they use. Two reactors are needed to convert the majority of CO, because of the equilibrium limitations of the process (see previous section). The inlet temperature of the HTS reactor is typically around 320°C. The outlet temperature rises to about 400-450° C because of the reaction exotherm. The gases are cooled to about 200°C before entering the LTS reactor, where the final 2-3% CO is partially converted to CO2 and H2. 

Mathematical Modelling of the High Temperature Water-Gas Shift Converter Mathematical Modelling of the Low Temperature Water-Gas Shift Converter Ammonia Converters... 

Elnashaie, S.S.E.H. and Alhabdan, F.M, Mathematical Modelling and Computer Simulation of Industrial Water-Gas Shift Converters. Math, and Comput. Modi, Vol, 12, pp. 1017-1034, 1989. 

Mathematical Modelling of the High Temperature Water-Gas Shift Converter... 

The hot syngas produced, composed by CH4, H2O, H2, CO, and CO2, is used to generate very high pressure (VHP) steam for mixing with the feed (internal use) and for export outside the unit. The cooled syngas is sent to a water gas shift converter generally, the operating temperature is about 400-450°C to support the reaction kinetics over the iron oxides catalyst. 

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. ... 

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... 

The process begins with a gasification process that converts coal into carbon monoxide and hydrogen. Part of this gas is sent to a water-gas shift reactor to increase its hydrogen content. The purified syngas is then cryogenically separated into a carbon monoxide feed for the acetic anhydride plant and a hydrogen-rich stream for the synthesis of methanol. 

In conventional production of hydrogen from coal, as described earlier, coal is converted to hydrogen and carbon monoxide (CO) through the water-carbon reaction as shown in reactions 3.9 through 3.11. CO is then converted to hydrogen and carbon dioxide by the water-gas shift reaction as shown in reaction 3.12. 

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. 

Gary Jacobs and Burt Davis (University of Kentucky) review catalysts used for low-temperature water gas shift, one of the key steps in fuel processors designed to convert liquid fuels into hydrogen-rich gas streams for fuel cells. These catalysts must closely approach the favorable equilibrium associated with low temperatures, but be active enough to minimize reactor volume. The authors discuss both heterogeneous and homogeneous catalysts for this reaction, with the latter including bases and metal carbonyls. 

Pettit and coworkers—metal hydride intermediates by weak base attack over Fe carbonyl catalysts. Pettit et al.ls approached the use of metal carbonyl catalysts for the homogeneous water-gas shift reaction from the standpoint of hydroformyla-tion by the Reppe modification.7 In the typical hydroformylation reaction, an alkene is converted to the next higher aldehyde or alcohol through reaction of CO and H2 with the use of a cobalt or rhodium carbonyl catalyst. However, in the Reppe modification, the reduction is carried out with CO and H20 in lieu of H2 (Scheme 6) ... 

Fiolitakis and Hofmann—wavefront analysis supports Eley-Rideal/redox mechanisms. In 1982 and 1983, Fiolitakis and Hofmann231,232 carried out wavefront analysis to analyze the dependence of the microkinetics of the water-gas shift reaction on the oxidation state of CuO/ZnO. They observed three important mechanisms after treatment of the catalyst surface with different H20/H2 ratios. These included two Eley-Rideal mechanisms which converted the reactants via adsorbed intermediates, and a redox mechanism that regulated the oxygen activity, as shown in Scheme 56. The authors indicated that different mechanisms could be dominating at different sections along the length of the fixed bed reactor. 

The resulting synthesis gas can subsequently be converted into methanol (Reaction 3) or polymerized to a mixture of hydrocarbons via the Fischer-Tropsch synthesis (Reaction 4) [37, 38]. These conversions usually require a H2/CO molar ratio close to 2 (Reactions 3 and 4), which contrasts with the H2/CO ratio of 0.5 that is delivered upon biomass gasification (Reaction 2). It can therefore be suitable to adjust the H2/CO ratio through the water-gas shift reaction (Reaction 5) ... 

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