High Temperature Carbon Monoxide Conversion

The gases from the furnace are cooled by the addition of condensate and steam, and then passed through a converter containing a high or low temperature shift catalyst, depending on the degree of carbon monoxide conversion desired. Carbon dioxide and hydrogen are produced by the reaction of the carbon monoxide with steam. 

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. 

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. 

Wanat et al. investigated methanol partial oxidation over various rhodium containing catalysts on ceramic monoliths, namely rhodium/alumina, rhodium/ceria, rhodium/ruthenium and rhodium/cobalt catalysts [195]. The rhodium/ceria sample performed best. Full methanol conversion was achieved at reaction temperatures exceeding 550 °C and with O/C ratios of from 0.66 to 1.0. Owing to the high reaction temperature, carbon monoxide selectivity was high, exceeding 70%. No by-products were observed except for methane. 

Pan and Wang switched four adiabatic preferential oxidation reactors in series downstream of the reformer/evaporator described in Section 7.1.3 [546]. Heat-exchangers were installed after each reactor. The four reactors were operated at the same inlet temperature of 150 °C, and the O/CO ratio in the feed increased from 1.6 to 3, to minimise the heat formation in the first reactors. Despite these measures, the temperatures rise in the first reactor was as high as 121 K, in the second reactor it was still at 82 K and decreased to 28 K in the third and 8 K in the last reactor. While only 50% carbon monoxide conversion could be achieved in the first reactor, conversion was complete after the last stage. The combined steam reformer/clean-up system was operated for 24 h. The carbon monoxide content ofthe reformate could be maintained below 40 ppm. 

An LTS catalyst should be sufficiently active to give a high conversion for a given volume of catalyst at the minimum practicable temperature. It should also be thermally stable and operate for the design period with maximum carbon monoxide conversion. With proper design and good upstream poisons removal, a typical catalyst lifetime is about three years. 

Because the synthesis reactions are exothermic with a net decrease in molar volume, equiUbrium conversions of the carbon oxides to methanol by reactions 1 and 2 are favored by high pressure and low temperature, as shown for the indicated reformed natural gas composition in Figure 1. The mechanism of methanol synthesis on the copper—zinc—alumina catalyst was elucidated as recentiy as 1990 (7). For a pure H2—CO mixture, carbon monoxide is adsorbed on the copper surface where it is hydrogenated to methanol. When CO2 is added to the reacting mixture, the copper surface becomes partially covered by adsorbed oxygen by the reaction C02 CO + O (ads). This results in a change in mechanism where CO reacts with the adsorbed oxygen to form CO2, which becomes the primary source of carbon for methanol. 

As is indicated in Figure 1, the heat liberated in the conversion of carbon monoxide to methane is 52,730 cal/mole CO under expected reaction conditions. Also, the heat liberated in the conversion of carbon dioxide is 43,680 cal/mole C02. Such high heat releases strongly affect the process design of the methanation plant since it is necessary to prevent excessively high temperatures in order to avoid catalyst deactivation and carbon laydown. Several approaches have been proposed. 

Insertion of carbon monoxide into Csp2—Zr bonds occurs readily at ambient temperatures or below to produce a,(5-unsaturated, reactive acyl zirconocene derivatives [27—29]. Early work by Schwartz demonstrated the potential of such intermediates in synthesis [5d], as they are highly susceptible to further conversions to a variety of carbonyl compounds depending upon manipulation. More recently, Huang has shown that HC1 converts 16 to an enal, that addition of a diaryl diselenide leads to selenoesters, and that exposure to a sulfenyl chloride gives thioesters (Scheme 4.11) [27,28]. All are obtained with (F)-stereochemistry, indicative of CO insertion with the expected retention of alkene geometry. 

After the reforming reaction, the gas is quickly cooled down to about 350 450 °C before it enters the (high-temperature) water-gas shift reaction (CO shift). Here, the exothermic catalytic conversion takes place of the carbon monoxide formed with steam to hydrogen (H2) and carbon dioxide (C02) in the following reaction ... 

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