Low temperature shift reactor

To reach a better CO conversion, it is possible to add a low-temperature shift reactor, which increases the CO2 capture rate (see also Fig. 10.3). If both clean CO2 for storage and clean hydrogen for fuel cell applications are required, a combination of a C02-capture plant (e.g., absorption with Rectisol) and a PSA plant is necessary. If only pure hydrogen is required, a PSA unit would be sufficient (and is standard practice), but the C02 stream would be contaminated by impurities, such as H2, N2 or CO, which have to be removed for geological storage. 

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). 

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... 

The particulate catalyst is composed of 90% Fe and 10% Cr. The Cr minimizes sintering of the active Fe phase. The catalytic reaction is limited by pore diffusion so small particles are used. The exit process gas contains about 2% CO as governed by the thermodynamics and kinetics of the reaction. This reaction is slightly exothermic and thermodynamics favor low temperatures that decrease the reaction rate. It is therefore necessary to further cool the mix to about 200°C where it is fed to a low-temperature shift reactor (LTS)... 

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 ... 

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. 

Moon et al. [59] presented a breadboard fixed-bed fuel processor for isooctane, which was composed of an ATR and high-and low-temperature shift reactors. The fuel processor was applied for testing different catalysts. A NiO/CaO/Al203 catalyst performed equivalent to a Ni/Fe/MgO/AlaOs catalyst for the autothermal reforming reaction. 

Chlorine, which will be present as HCl or organic chlorine compounds, is a poison, in particular for copper catalysts [391] and it may cause stress corrosion in the equipment. Chlorine can be removed by promoted alumina. Chlorine may be present in certain refinery offgases and in landfill gas. Chlorine may also originate from failure in the water purification system. If so, it will pass the guard bed and should be captured by a guard in the low-temperature shift reactor (see Section 1.5.2). 

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). 

The residual sulfates in precursor of catalyst react with hydrogen to produce H2S, and it releases sulfur during the reduction. The produced H2S may poison the low-temperature shift catalysts in the following step. Therefore, the process gases can be sent to the low-temperature shift reactor only after the release of sulfur is finished. 

Ersoza et al. (2005) studied a 100 kW net electrical power PEM fuel cell system consisting of an autothermal reformer, high and low temperature shift reactors, a preferential oxidation reactor, a PEM fuel cell, a combustor and an expander. Intensive heat integration within the PEM fuel cell system was necessary to achieve acceptable net electrical efficiency levels. The fuel cell stack efficiency has been calculated as a function of the number of cells (500-1250 cells). The obtained net electrical efficiency levels are between 30% (500 cells) and 37% (1250 cells) and they are comparable with the conventional gasoline based internal combustion engine systems, in terms of the mechanical power efficiency. 

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