Low temperature shift

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. ... 

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). 

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 principal reactions are reversible and a mixture of products and reactants is found in the cmde sulfate. High propylene pressure, high sulfuric acid concentration, and low temperature shift the reaction toward diisopropyl sulfate. However, the reaction rate slows as products are formed, and practical reactors operate by using excess sulfuric acid. As the water content in the sulfuric acid feed is increased, more of the hydrolysis reaction (Step 2) occurs in the main reactor. At water concentrations near 20%, diisopropyl sulfate is not found in the reaction mixture. However, efforts to separate the isopropyl alcohol from the sulfuric acid suggest that it may be partially present in an ionic form (56,57). 

The catalyst should be the copper-based United Catalyst T-2370 in 3/16 , reduced and stabilized, in extrudate form. Initially, 26.5 g of this should be charged to the catalyst basket. This catalyst is not for methanol synthesis but for the low temperature shift reaction of converting CO to CO2 with steam. At the given conditions it will make methanol at commercial production rates. Somewhat smaller quantity of catalyst can also be used with proportionally cut feed rates to save feed gas. 

Formate Fractional Coverage 0formate at Steady-State Low-Temperature Shift Conditions at 225°C or 185°C... 

Table 95 Properties and CO conversion during low temperature shift using a feed containing 4.03%CO in Ar (dry basis) with a steam to gas ratio of 0.7, SV = 4000 h-1 with 0.5 cm3 catalyst465...

Table 96 CO conversion during low temperature shift using a feed containing 4.494%CO in... 

Jacobs, Davis, and coworkers—IR investigation shows Pt/thoria is an important analog to Pt/ceria for low temperature shift—formate mechanism advanced. 

E) Metal/a-Fe203 catalysts. Although Fe Cr and modified Fe-Cr-Cu catalysts have been used extensively in industry for high temperature shift applications, this section covers references only from 1993 to the present510-526 specifically dealing with metal/a-Fe203 catalysts for low temperature shift. 

Certainly, water-gas shift has in the past been carried out using zeolites as supports for Rh (e.g., Rh/Y Zeolite and Rh/NaY Zeolite539), ZnO (Na/mordenite540), and Fe oxide (Na/mordenite540) in high temperature shift catalyst studies. More recent investigations are aimed at applying zeolites and related materials for use in low temperature shift catalysts. 

Venugopal and Scurrell544 reported low temperature shift activity for 3%Ru/ Caio(P04)6(OH)2 and 3%Au/Ca10(PO4)6(OH)2, the support being hydroxyapatite, a material that is similar to the mineral component of bones. A comparison of their activities is provided in Table 132, along with the catalyst properties. 

The reformate gas contains up to 12% CO for SR and 6 to 8% CO for ATR, which can be converted to H2 through the WGS reaction. The shift reactions are thermodynamically favored at low temperatures. The equilibrium CO conversion is 100% at temperatures below 200°C. However, the kinetics is very slow, requiring space velocities less than 2000 hr1. The commercial Fe-Cr high-temperature shift (HTS) and Cu-Zn low-temperature shift (LTS) catalysts are pyrophoric and therefore impractical and dangerous for fuel cell applications. A Cu/CeOz catalyst was demonstrated to have better thermal stability than the commercial Cu-Zn LTS catalyst [37], However, it had lower activity and had to be operated at higher temperature. New catalysts are needed that will have higher activity and tolerance to flooding and sulfur. 

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. 

Downstream of the reformer the CO is converted into hydrogen by two subsequent water-gas shift sections a high-temperature shift (HTS) followed by a low-temperature shift (LTS). This is done because the equilibrium of the WGS reaction lies at the product side at lower temperatures (around 200 °C), but the reaction kinetics are faster at increasing temperature. Therefore, to reach high CO conversions, most of the CO is converted in a HTS section and the remainder is converted within a LTS section. 

For MCFC SOFC, no high temperature shift, low temperature shift, nor CO removal required. 

Halides in fuels such as naphtha have deleterious effects on steam reforming and low temperature shift, thus halogen guards need to be included in the fuel processing. 

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