CO shift reactions

Steam-Moderated Process. The basic idea behind this approach is to limit the extent of conversion of the methanation reaction, Reaction 1, by adding steam to the feed gases. This process simultaneously provides for (46) elimination of the CO shift, Reaction 2, to get a 3 1 H2 CO ratio from the make-up gas ratio of about 1.5 1 and avoidance of carbon laydown by operation under conditions in which carbon is not a thermodynamically stable phase (see Chemistry and Thermodynamics section above). 

In this energy chain, coal is gasified to generate synthesis gas. The H2 CO ratio required for an optimum efficiency is adjusted via the CO shift reaction of a part of the carbon monoxide (CO) contained in the synthesis gas. The remaining synthesis gas is converted to liquid hydrocarbons via Fischer-Tropsch synthesis or via methanol synthesis with a downstream MtSynfuels (trademark by Lurgi) process (see beginning of Section 7.3.4). The liquid hydrocarbon yield amounts to about 0.40 MJ per MJ of hard coal, which is of the same order of magnitude as in the case of BTL ( 0.40 MJ/MJ) to calculate the thermal process efficiency, the electricity export must also be taken into account (see Table 7.12). 

By proper adjustment of the oxygen-to-carbon and steam-to-carbon ratios, the partial combustion in the thermal zone [reaction (9.8)] supplies the heat for the subsequent endothermic SR reaction (9.1) [24]. The CO shift reaction (9.2) also takes place in the catalytic zone. 

The success of the correlation of catalytic behavior with bulk Mossbauer parameters by Skalkina et al. is also reflected in the work of Tops0e and Boudart (96). As discussed earlier, these authors found a decrease in the isomer shift of the octahedral iron ions in a lead-promoted Cr-Fe304 carbon monoxide shift catalyst, indicative of an increased covalency of these ions. Schwab et al. (203) have proposed a correlation of the activity for CO oxidation by ferrites with the octahedral ions in these materials, and the electron transfer required for this catalytic process may be facilitated by an increased covalency of the metal ions (204). In view of these suggestions, the lead-promoted catalyst is expected to possess a higher catalytic activity for the CO shift reaction than an unpromoted catalyst, as evidenced by the Mossbauer parameters of these two samples. This has in fact been shown experimentally to be the case (96). For the reverse CO shift reaction over supported europium (176), the success of the correlation between catalytic activity and the Mossbauer parameters (in this case the reducibility) has already been noted in Section III, A, 4. 

Reformed gas is cooled to the shift inlet temperature of 250°C by generating steam (4). The CO shift reaction is carried out in a single stage in the isothermal shift reactor (5), internally cooled by a spiral wound tube bundle. To generate MP steam in the reactor, de-aerated and reheated process condensate is recycled. 

A potential way to use CO2 as the carbon source for the synthesis of organic compounds is the hydrogenation of CO2 via Fischer-Tropsch (FT) synthesis using a CO2 rich synthesis gas. Some indication can be found in the literature that the hydrogenation of CO2 to hydrocarbons proceeds via CO as an intermediate [1], which means that the catalyst must have a high activity for the reverse CO shift reaction (1) together with good properties for the FT reaction (2). 

Based on an experimental study the present investigation addresses for two different types of catalysts the effect of CO2 concentration in the reaction gas on carbon conversion rates, yields of organic products and selectivity in the carbon number range Cj to 20- Two catalysts on Fe- and Co-basis with significantly different CO shift reaction activity were characterized by parameters according to the previously developed model of non trivial surface polymerisation , based on extended Anderson-Schulz-Flory kinetics [2]. 

With increasing CO2 content, only a slight decrease of the yield of hydrocarbons was found for Fe, whereas with Co a strong decrease was observed. Due to its high activity for the reverse CO shift reaction, a negative CO2 conversion was found with Fe for low CO2 concentrations in the reaction gas, in contrast to Co (Fig. 2). 

Allen et al. derived separate equations for CO and CO2 formation based on a mechanism involving reaction between CH4 gas and adsorbed H2O, where the overall rate was controlled by the desorption of the two products. The CO shift reaction took place between adsorbed CO and H2O in the gas phase. The rate expression for CO formation was that given in (15). The numerator relates... 

D. Summary of Mechanism Studies.—The recent research does not solve old problems. There are areas of disagreement in the findings, no doubt due to different catalysts and experimental conditions. No single primary product can be identified. Carbon oxides are often the major products leaving the catalyst surface, in relative concentrations dependent upon the efficiency of the catalyst for the CO shift reaction (4). On Rh the CO content tends to be high and the CO 2 low, probably indicative of lower activity for the shift reactions compared with Ni, where CO tends to be lower. Methane however can be formed directly from the higher hydrocarbons at 600 °C and above, as well as by methanation of carbon oxides. 

Elucidate the genetic system of the CO shift reaction to (1) identify the various components involved and (2) allow manipulation of gene expression. 

Characterize the physiological and biochemical mechanisms in the CO shift reaction by which Rx. gelatinosus CBS produces H2 from CO. 

Completed physiological/biochemical study of the CO shift reaction and determined that CO acts as an inducer of the CO shift reaction at the gene transcription level and that at least 10% (v/v) CO is required to maximally induce the H2 production activity. 

Determine if the CO shift reaction can serve as an energy-yielding step in darkness. If verified, this energy can be used to sustain cell growth and to induce new CO shift enzymes to support H2 production for a longer duration without the need for light input. 

To fully utilize the CO shift reaction as a renewable energy resource, it is necessary to understand the mechanism of how Rx. gelatinosus... 

Figure 3. Effect of CO Feedings on Rates and Longevity of the CO Shift Reaction...

CO serves as an inducer for the CO shift reaction in Rx. gelatinosus CBS, and its constant presence is required to maintain this reaction both at a higher rate and for a longer duration. 

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