High temperature shift reactor

A high temperature water-gas shift reactor 400°C) typically uses an iron oxide/chromia catalyst, while a low temperature shift reactor ( 200°C) uses a copper-based catalyst. Both low and high temperature shift reactors have superficial contact times (bas on the feed gases at STP) greater than 1 second (72). 

Generally, in a conventional WGS system a two-step shift is used to obtain high CO conversion rates. In the first high-temperature shift reactor the major part of the CO is converted at high activity, whereas in the second shift reactor the rest of the CO (closely up to the thermodynamic equilibrium) is converted at low temperature and also low activity. Steam to carbon monoxide ratios above the stoichiometric ratio (higher than 2) are generally being used to attain the desired carbon monoxide conversion, but also to suppress carbon formation on certain catalysts. 

Steam is added to the desulfurized feed to achieve the specified steam/ carbon ratio and the mixture is further preheated before entering the primary reformer. Methane is converted to hydrogen and carbon oxides in the primary reformer. The gas is cooled to about 340-455°C (645-850 F) and then enters the high temperature shift reactor [1]. 

After the raw syngas is cooled it is routed to a high temperature shift reactor, if necessary, to make the proper Hj/CO ratio. It then enters an acid gas removal system to remove H2S and COj. Following the removal of acid gases, the syngas is delivered to the battery limits of the plant for downstream use. 

HTS High-temperature shift (reactor for water gas shift carried out at high temperature)... 

The shift reaction is exothermic and thus the equilibrium is favored by low temperatures (Figure 6.2.4). Thus, the reaction temperature should be kept as low as possible, but is limited by the activity of the catalyst. The Fe-Cr shift catalyst is sufficiently active only above about 300 °C. Catalysts based on copper and zinc are active enough at about 200 °C but these catalysts are very sensitive to poisoning and require extremely pure gases, typically with less than Ippm H2S. In practice, the water-gas shift reaction is carried out in two adiabatic fixed-bed reactors with intermediate cooling between both converters. The first high-temperature shift reactor operates with a Fe-Cr catalyst, and the second low-temperature shift reactor contains the more active Cu-Zn system. At the exit of the second shift reactor, the CO2 present in the converted syngas is removed in a gas scrubber, usually by chemical absorption in aqueous amine solutions, for example, mono- or diethanolamine (Section 3.3.3). 

Several catalysts and processes are available for this reaction. In the classical process, the catalyst is magnetite, Fe304, promoted with chromia and in some cases with potassium or other promoters [221-223]. This catalyst requires temperatures above about 350 °C, and the exit carbon monoxide concentration is normally about 3 vol% in the dry gas. Conversion was in some cases improved by installing a two stage unit with carbon dioxide removal between two high temperature shift reactors [224]. However, this system was rather expensive and did not gain wide acceptance. 

Carbon monoxide leaving the secondary reformer is converted to useful hydrogen by the water gas shift reaction in a two-stage recovery section. The original high temperature shift reactor is now combined with a low temperature reactor filled with a copper catalyst. The removal of carbon monoxide is shown in Figure 9.4. Since copper catalysts are extremely prone to poisoning by sulfur and chlorine compounds, it is therefore essential that the concentration of these contaminants is reduced to an absolute minimum. 

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. ... 

A conceptual design and selection of an ATR biodiesel processor for a vehicle fuel cell auxiliary power unit were reported by Specchia et al. [81]. Three processor options were compared for H2 production with respect to efficiency, complexity, compactness, safety, controllability and emissions. The ATR with both high-temperature shift (HTS) and low-temperature shift (LTS) reactors showed the most promising results. 

A similar study reports the results of adding 100 ppm thiophene to As in the Palm et al. study,the catalyst is not described rather, it is identified only as a commercial naphtha reforming catalyst, presumably Pt-based. In their reactor, the reformate from the ATR step passes through separate high and low temperature shift reactors before being analyzed. Thus, it was not possible to determine the effect of sulfur on the reforming step alone, nor was any post-reaction characterization of the catalyst reported, for example to determine coke or sulfur content. Figure 16 shows the observed deactivation, as measured by a decrease in H2 and CO concentrations. 

The high activity of Rh compared to conventional Ni-based catalysts may also lead to a lower operating temperature of the reformer, eliminating high-and low-temperature shift reactors and minimizing the O/C. At 550°C (O/C = 1, S/C = 3.0, and GHSV = 179,290 h ), Newson et al. obtained a H2 yield of... 

In 2001 Hyprotech and Synetix announced an ammonia plant simulation that can be used for modeling, on-line monitoring and optimization of the plant. The simulation includes Synetix reactor models, customized thermodynamic data and information to simulate the performance of a range of catalysts. The reactor models in the simulation include Primary and Secondary Reformers, High Temperature Shift converter, Low Temperature Shift Converter, Methanator and Ammonia Synthesis Converter80. 

In the traditional plant concept, the gas from the secondary reformer, cooled by recovering the waste heat for raising and superheating steam, enters the high-temperature shift (HTS) reactor loaded with an iron - chromium catalyst at 320 - 350 °C. After a temperature increase of around 50 - 70 °C (depending on initial CO concentration) and with a residual CO content of around 3 % the gas is then cooled to 200-210 °C for the low-temperature shift (LTS), which is carried out on a copper - zinc - alumina catalyst in a downstream reaction vessel and achieves a carbon monoxide concentration of 0.1-0.3 vol%. 

WGS reactors are usually divided into two or more stages, including high-temperature shift (HTS) reactor and low-temperature shift (LTS) reactor. This multistage design is necessary due to the strong exothermic nature of the WGS reaction ... 

The concentration of CO leaving the low temperature shift reactor can be reduced to the range of 0.1-1%, depending on operating conditions. However, it is still too high to be used directly for PEMFC applications. 

Simultaneously in high- and low-temperature shift reactors, the so-called water gas shift reaction produces further H2 according to the exothermic equation ... 

For the production of hydrogen (ammonia, refinery purposes, petrochemistry, metallurgy, fuel cells), the carbon monoxide contained in the effluent stream is converted to additional hydrogen in high and low temperature shift reactors. The water gas-shift reaction (WGS) is ... 

The conventional ammonia production line consists of seven gas-solid catalytic reactors, namely desulfurization unit, primary reformer, secondary reformer, high temperature shift, low temperature shift, methanator and finally the ammonia converter. In addition the production line includes an absorption-stripping unit for the removal of CO2 from the gas stream leaving the low temperature shift converter. The ammonia converter is certainly the heart of the process with all the other units serving to prepare the gases for the ammonia synthesis reaction which takes place over an iron promoted catalyst under conditions of high temperature and pressure. 

Recent developments in shift catalyst formulations allow the combination of high temperature and low temperature shift reactors in a single medium temperature step. The medium temperature shift catalyst is a copper-based catalyst that operates in the range of 260-280°C (500-540 F). Carbon monoxide conversion is improved, resulting in an overall savings on feed and fuel of 0.3% to 0.8% [2]. The medium temperature shift reactor has been commercially proven with isothermal tubular reactors, however, the use of an adiabatic reactor with intercooling is also possible. 

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