Converter shift

A shift converter is a catal5dic reactor which catalyzes the conversion of CO to H2 by the water gas shift reaction. This step can be used to increase the H2/CO ratio to a very high value. It is widely used in hydrogen plants. 

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Use of a low temperature shift converter in a PSA hydrogen plant is not needed it does, however, reduce the feed and fuel requirements for the same amount of hydrogen production. For large plants, the inclusion of a low temperature shift converter should be considered, as it increases the thermal efficiency by approximately 1% and reduces the unit cost of hydrogen production by approximately 0.70/1000 (20/1000 ft ) (140,141). 

The conversion of CO to CO2 can be conducted in two different ways. In the first, gases leaving the gas scmbber are heated to 260°C and passed over a cobalt—molybdenum catalyst. These catalysts typically contain 3—4% cobalt(II) oxide [1307-96-6] CoO 13—15% molybdenum oxide [1313-27-5] MoO and 76—80% alumina, JSifDy and are offered as 3-mm extmsions, SV about 1000 h . On these catalysts any COS and CS2 are converted to H2S. Operating temperatures are 260—450°C. The gases leaving this shift converter are then scmbbed with a solvent as in the desulfurization step. After the first removal of the acid gases, a second shift step reduces the CO content in the gas to 0.25—0.4%, on a dry gas basis. The catalyst for this step is usually Cu—Zn, which may be protected by a layer of ZnO. 

Reducing gas is generated from natural gas in a conventional steam reformer. The natural gas is preheated, desulfurized, mixed with steam, further heated, and reformed in catalyst-filled reformer tubes at 760°C. The reformed gas is cooled to 350°C in a waste heat boiler, passed through a shift converter to increase the content, mixed with clean recycled top gas, heated to 830°C in an indirect-fired heater, then injected into reactor 4. 

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. 

Carbon Dioxide Removal. The effluent gases from the shift converters contain about 17—19 vol % (dry) carbon dioxide (qv) which is ultimately reduced to a few ppm by bulk CO2 removal, followed by a final purification step. Commercial CO2 removal systems can be broadly classified as... 

Synthesis gas from the gasifier is first cleaned to remove gasifier tars and organic sulfur, and the composition of the gas is adjusted ia a catalytic shift converter to raise the hydrogen content... 

Recently, a new process has been developed to manufacture hydrogen by steam reforming methanol. In this process, an active catalyst is used to decompose methanol and shift convert carbon monoxide to carbon dioxide. The produced gas is cooled, and carbon dioxide is removed ... 

Figure 4.1. A process for producing hydrogen by steam reforming of hydrocarbons (1) reforming furnace (2,3) purification section, (4) shift converter, (5) pressure swing adsorption.

In the shift converter, carbon monoxide is reacted with steam to give carbon dioxide and hydrogen. The reaction is exothermic and independent of pressure ... 

The feed to the shift converter contains large amounts of carbon monoxide which should be oxidized. An iron catalyst promoted with chromium oxide is used at a temperature range of 425-500°C to enhance the oxidation. 

Figure 5-2. The ICI process for producing synthesis gas and ammonia (1) desulfurization, (2) feed gas saturator, (3) primary reformer, (4) secondary reformer, (5) shift converter, (6) methanator, (7) ammonia reactor.

J to this would be if the LT shift converter were partially by-passed, in... 

Feed gases to most, if not all, methanation systems for substitute natural gas (SNG) production are theoretically capable of forming carbon. This potential also exists for feed gases to all first-stage shift converters operating in ammonia plants and in hydrogen production plants. However, it has been demonstrated commercially over a period of many years that carbon formation at inlet temperatures in shift converters is a relatively slow reaction and that, once shifted, the gas loses its potential for carbon formation. Carbon formation has not been a common problem at the inlet to shift converters. It has been no problem at all in our bench-scale work, and it is not expected to be a problem in our pilot plant operations. 

For a clearer understanding of the behavior of syngases in a shift converter, we established another set of carbon isotherms when considering the shift reaction only (without methanation) in addition to the carbon-forming reactions. Figure 6 shows isotherms at a partial pressure of 270 psia for all components of a gas mixture, but excluding methane. 

Methanol can be considered as a hydrogen carrier in a fuel cell. Conventionally, methanol has been reformed/shift converted to produce hydrogen. A low concentration of carbon monoxide formed during this process leads to a strong poisoning of the anode, and even after cleaning of the... 

In the production of hydrogen by the steam reforming of hydrocarbons, the classic water-gas reaction is used to convert CO in the gases leaving the reforming furnace to hydrogen, in a shift converter. 

The products include H2, C02, CO, and steam, which are then processed in a shift converter, resulting in the reaction... 

The synthesis gas enters the shift converter, where the exothermic water-gas shift reaction occurs at 475 to 675 K as follows ... 

Figure 3.5 A shift converter reacts carbon monoxide and water to give carbon dioxide and more hydrogen. (Courtesy of Solutia Inc., Luling, LA)...

Process waste-heat boilers then cool the reformed gas to about 371°C while generating high-pressure steam. The cooled gas-stream mixture enters a two-stage shift converter. The purpose of shift conversion is to convert CO to C02 and produce an equivalent amount of H2 by the reaction CO+H20 C02 + H2. Since the reaction rate in the shift converter is favored by high temperatures, but equilibrium is favored by low temperatures, two conversion stages, each with a different catalyst provide the optimum conditions for maximum CO shift. Gas from the shift converter is rhe raw synthesis gas, which, after purification, becomes the feed to the NH3 synthesis section. 

Tile partially purilied synthesis gas leaves the C02 absorber containing approximately 0.1% CO2 and 0.5% CO. This gas is preheated at the methanator inlet by heat exchange with the synthesis-gas compressor interstage cooler and the primary-shift converter effluent and reacted over a nickel oxide catalyst bed in the methanator. The methanation reactions are highly exothermic and are equilibrium favored by low temperatures and high pressures. 

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