Carbon monoxide removal

Selective removal of CO is done over some catalysts that oxidize mainly CO and not hydrogen. Catalysts used for this purpose are usually alumina (Al203)-supported structures. Ru and Rh supported on alumina are among the most active catalysts. Near to complete CO conversion can be achieved 

In the methanahon process, CO is reacted with hydrogen to form methane and water. The amoxmt of hydrogen needed to carry out this reaction is, however, three hmes the amoxmt of CO removed. The reaction for this process is 

The CO2 present in the gas stream can react xvith the produced water in a reverse water shift reachon (Equation 13.4), thereby producing the unwanted CO. Methanahon can work for removal of CO but severe restrictions are necessary to make this process viable in a gas cleanup system. 

In 1915, the water-gas shift reaction has been used to remove carbon monoxide, by reaction with steam and to increase the potential hydrogen production since the first commercial ammonia plant began operation in 1913. The process was based on catalysts discovered by Wild of BASF in 1912 while following up 

Since then the basic formulation of high temperature shift catalyst (HTS) has been iron oxide, stabilized with chromium oxide, although the methods of stabilizing the catalysts and producing it in large quantities have been refined as operating conditions have changed 

The catalyst as supplied consists mainly of hematite and chromium oxide, together with a variable amount of the hexavalent chromium trioxide. Before use, the catalyst must be activated by reduction to magnetite using process gas, and at the same time, any chromium trioxide is also reduced to the trivalent state. Under the typical conditions for the shift reaction, magnetite is the ther-modynamically-stable state for the oxides of, and there is no further reduction to the metallic state. Some HTS catalysts may contain traces of sulfur, particularly 

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The authors developed a multi-layered microreactor system with a methanol reforma- to supply hydrogen for a small proton exchange membrane fiiel cell (PEMFC) to be used as a power source for portable electronic devices [6]. The microreactor consists of four units (a methanol reformer with catalytic combustor, a carbon monoxide remover, and two vaporizers), and was designed using thermal simulations to establish the rppropriate temperature distribution for each reaction, as shown in Fig. 3. 

Due to the operating requirements of PEM stack technology, shift reactors and a carbon monoxide removal step are required to produce reformate of sufficient quality. Similarly, the stack operating temperature and its humidity requirements require a water management system as well as radiators for heat rejection. Some developers are developing pressurized systems to the benefit from higher reactant partial pressures on both anode and cathode. Fuel processing for PEM APU systems is identical to that needed in residential power or propulsion applications. 

Figure 20. Fuel cell performance demonstration for the Battelle methanol processor and the carbon monoxide removal reactor.

Current efforts are focusing on optimizing the carbon monoxide removal reactor and developing a system prototype using commercially available pumps, blowers, fuel cells, valves, and controllers.In addition, durability testing along with thermal cycling needs to be done. 

Copper liquor scrubbing [737], for carbon monoxide removal, commonly employed in early plants has become obsolete and is now operated in only a few installations. Not only does it have a high energy demand, but it is also environmentally undesirable because of copper-containing wastewater. The choice today is methanation, which is by far the simplest method to reduce the concentrations of the carbon oxides well below 10 ppm and is widely used in steam reforming plants. It is actually the reverse reaction of steam reforming of methane ... 

Since most of the elementary steps in carbonylation reactions are reversible, it is not suiprising that transition metals and their complexes promote the decarbonylation of organic compounds in either a stoichiometric or a catalytic manner. In stoichiometric reactions carbon monoxide removed from the organic compound is retained by the metal complex, as in equation (68), whereas for catalytic behavior this CO must be released, a reaction that often occurs only at high temperatures (>200 C). 

Oh, S.H. and Sinkevitch, R.M. Carbon monoxide removal from hydrogen rich fuel cell feed streams by selective catalytic oxidation. Journal of Catalysis, 1993, 142, 254. 

The thermochemistry of the three vectors ranges from thermoneutral for water removal to 2,2 GJ/t endothermic for carbon dioxide removal and 4.5 GJ/t endothermic for carbon monoxide removal. The heat of combustion of the carbon monoxide is however 7.66 GJ so that it could be used either as a chemical reagent or as a heat source in an entirely biomass fueled process to produce fuels that most closely approximate todays preferred hydrocarbon fuels. 

Plants using ammoniacal cuprous solutions for carbon monoxide removal. 

Tlants using methanation for residual carbon dioxide and carbon monoxide removal ammonia-synthesis purge gas is burned as fuel. 

The reverse-flow chemical reactor (RFR) has been shown to be a potentially effective technique for many industrial chemical processes, including oxidation of volatile organic compounds such as propane, propylene, and carbon monoxide removal of nitrogen oxides sulfur dioxide oxidation or reduction production of synthesis gas methanol formation and ethylbenzene dehydration into styrene. An excellent introductory article in the topic is given by Eigenberger and Nieken on the effect of the kinetic reaction parameters, reactor size, and operating parameters on RFR performance. A detailed review that summarizes the applications and theory of RFR operation is given by Matros and Bunimovich. 

However, it can be stated that with a stoichiometric exhaust gas composition, the overall rate of carbon monoxide removal is one to two orders of magnitude higher than the overall rate of either hydrocarbon and nitrogen oxide removal. 

Reynolds, J.H. IV and M.N. Andrews Removal of carbon monoxide from cigarette smoke. II. Development and application of a rapid method for screening prospective carbon monoxide removal agents RDR, 1971, No. 16, July 29, see www.ijrtdocs.com 514902024 -2043. Reynolds, J.H. IV and B.R Hege Experiments in removal of carbon monoxide from cigarette smoke. IB. Successful catalytic removal of carbon monoxide from smoke RDM, 1973, No. 120, March 19, see www.ijrt-docs.com 508566003 -6012. 

The first of the above reactions is called methanation and is important in removing small quantities of carbon monoxide (or carbon dioxide) from a hydrogen stream. The reaction proceeds almost quantitatively so that the carbon monoxide removal is essentially complete in one pass. Since the reaction consumes 3 vol. of hydrogen and produces methane, it is not used with hydrogen containing more than a few tenths of a per cent of carbon monoxide. Carbon monoxide is an active poison for nickel catalysts used in hydrogenation and for this reason must be removed. Although synthesis gas can be converted to methane, the process is not economical at present and may not be until the demand for natural gas substitutes increases. 

Both chemical purification technologies for the carbon monoxide removal require hydrogen. The extra hydrogen demand leads to lower electrical efficiency of low temperature PEM fuel-cell heating appliance. 

More recently, the methanation reaction [Eq. (3.5), Section 3.1] has gained increasing attention as the final step in carbon monoxide removal. The main advantage of methanation is that - in contrast to preferential oxidation - no air addition is required to perform the reaction. Thus, fewer safety issues are involved. However, the reaction by itself consumes three moles of valuable hydrogen per mole of carbon monoxide converted, which is greater than the losses by preferential oxidation. 

Meyer et cd. described the development of a multi-fuel processor by International Fuel Cells, LLC [627]. Methanol and gasohne (quality California reformulated gasoline grade II) were the major fuel alternatives. The technology chosen consisted of feed desulfurisation, autothermal reforming and catalytic carbon monoxide removal by two water-gas shift stages and two preferential oxidation reactors. The system had a power equivalent of 50 kW. However, performance data were only provided with respect to the autothermal reformer Desulfurisation proved to increase the reformer conversion up to 98%. No residual heavy hydrocarbons then remained in the product. The hot spot of the autothermal reformer approached 1000 °C. 

Carbon monoxide removal for low temperature fuel cells... 

The mildly endothermic steam reforming of methanol is one of the reasons why methanol is finding favour with vehicle manufacturers as a possible fuel for FCVs. Little heat needs to be supplied to sustain the reaction, which will readily occur at modest temperatures (e.g. 250°C) over catalysts of mild activity such as copper supported on zinc oxide. Notice also that carbon monoxide does not feature as a principal product of methanol reforming. This makes the methanol reformate particularly suited to PEM fuel cells, where carbon monoxide, even at the ppm level, can cause substantial losses in performance because of poisoning of the platinum anode electrochemical catalyst. However, it is important to note that although carbon monoxide does not feature in reaction 8.7, this does not mean that it is not produced at all. The water-gas shift reaction of reaction 8.5 is reversible, and carbon monoxide in small quantities is produced. The result is that the carbon monoxide removal methods described in Section 8.4.9 are still needed with PEM fuel cells, though the CO levels are low enough for PAFC. 

Further fuel processing - carbon monoxide removal... 

For PEM fuel cells, further carbon monoxide removal is essential after the shift reactors. This is usually done in one of three ways ... 

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