Shift reaction, carbon monoxide

As shown in Figure 5.4 and Figure 5.17 and as described for each of the major processes that produce synthesis gas, the Water Gas Shift Conversion or the Carbon Monoxide Shift reaction is one of the traditional purification steps that will still be found in many ammonia plants. The CO must be removed because it acts as a poison to the catalyst that is used in ammonia synthesis. 

On account of the striking analogy with the selectivity of the alcohol decomposition, however, this does not seem so probable, but it is worth while to consider this possibility in the case of the decomposition of formic acid on those catalysts which are known to accelerate the carbon monoxide shift reaction i,e., Fe304 and MgO. 

In fact, moisture appears to play a more integral role in the combustion of hydrogen-deficient carbonaceous fuels (such as coal) than has been generally recognized. The carbon-steam reaction to produce carbon monoxide and hydrogen (which are then oxidized to the final products) is an important stage in the combustion sequence as is the carbon monoxide shift reaction to yield carbon dioxide and hydrogen ... 

Table 3.7 Equilibrium constant for the carbon monoxide shift reaction...

The reaction rate of the whole process in some reversible exothermic reactions, such as the carbon monoxide shift reaction, can be considerable. The heat needed to be extracted from unit volume of catalyst at the beginning and the ending stages of the reaction may vary by 10 times or more. These reactions require multi-stage heat exchange reactors. Other reactions, such as the ammonia synthesis... 

If the gas does not contain large quantities of sulfur and ash, the carbon monoxide-shift reaction is carried out directly after the synthesis operation. In some instances, it also is desirable to remove the carbon dioxide before the shift operation, as it has an adverse effect on the carbon monoxide-shift equilibrium. Carbon dioxide may be removed by scrubbing with water, monoethanolamine... 

It was shown in laboratory studies that methanation activity increases with increasing nickel content of the catalyst but decreases with increasing catalyst particle size. Increasing the steam-to-gas ratio of the feed gas results in increased carbon monoxide shift conversion but does not affect the rate of methanation. Trace impurities in the process gas such as H2S and HCl poison the catalyst. The poisoning mechanism differs because the sulfur remains on the catalyst while the chloride does not. Hydrocarbons at low concentrations do not affect methanation activity significantly, and they reform into methane at higher levels, hydrocarbons inhibit methanation and can result in carbon deposition. A pore diffusion kinetic system was adopted which correlates the laboratory data and defines the rate of reaction. 

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. 

The one example where the simple insertion product is a stable product is the carbon monoxide insertion reaction. The addition of carbon monoxide to alkylcobalt tetracarbonyls to form acylcobalt tetracarbonyls has already been discussed above because of its basic importance in alkylcobalt and acylcobalt carbonyl chemistry. The mechanism of this insertion is thought to involve a 1 2 shift of the alkyl group from cobalt to carbon followed by reaction of the intermediate acylcobalt tricarbonyl with another external carbon monoxide. Although there is no conclusive evidence for or against this mechanism in the carbonylation of cobalt compounds, there is evidence for it in the related carbonylation of alkylmanganese pentacarbonyls (2, 24). 

Kinetic studies produced some surprising results. The rate of carbonylation is independent not only of the concentration of methanol but also of the pressure of carbon monoxide. The reaction, however, is first order in each of the species rhodium and iodide (rate a [Rh][I ]). Bands at 2064 and 1984cm in the infrared spectrum, which correspond to the known ion [Rh(C0)2l2] shift to 2062 and 1711 cm" on addition of methyl iodide. The band at 1711 cm" suggests the formation of an acetyl complex by methyl migration (p. 225). Under... 

Heat exchange in multi-bed direct heat exchange reactor is by directly adding a cold gas to the reaction gas to lower the reaction temperature this is the so-called cold-quench . If the quenching gas is the feed gas, it is called feedgas quench , such as the Topspe S-100 type ammonia converter, and ICI cold-quencher ammonia synthesis converter. If the quenching gas is not the feed gas, it is called non-feedgas quench , such as the steam cold-quench carbon monoxide shift reactor. 

The hydrogen-to-carbon monoxide mole ratio for the product gas is usually 4 to 9 however, it can be increased if additional steam is used in the reaction. This will reduce the demand in the carbon monoxide-shift converters, which follow the secondary reformer. Steam sometimes is introduced after the reformer before the gas is fed to the carbon monoxide-shift converters. In the single-train ammonia plants, the natural gas is reformed in two steps. In the first step, the reaction takes place in the primary reformer in tubes suspended in a refractory-lined furnace. The large amount of endothermic heat is supplied by burning natural gas with air in the furnace. The heat flux in the tubes can be as high as 35,000 Btu/hr sq ft. The methane leakage is about 10 percent in the effluent dry gas, or about 60 to 65 percent of the feed methane is converted to synthesis gas. 

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... 

Low pressure methanol carbonylation transformed the market because of lower cost raw materials, gender, lower cost operating conditions, and higher yields. Reaction temperatures are 150—200°C and the reaction is conducted at 3.3—6.6 MPa (33—65 atm). The chief efficiency loss is conversion of carbon monoxide to CO2 and H2 through a water-gas shift as shown. 

Study of the mechanism of this complex reduction-Hquefaction suggests that part of the mechanism involves formate production from carbonate, dehydration of the vicinal hydroxyl groups in the ceUulosic feed to carbonyl compounds via enols, reduction of the carbonyl group to an alcohol by formate and water, and regeneration of formate (46). In view of the complex nature of the reactants and products, it is likely that a complete understanding of all of the chemical reactions that occur will not be developed. However, the Hquefaction mechanism probably involves catalytic hydrogenation because carbon monoxide would be expected to form at least some hydrogen by the water-gas shift reaction. 

The mixture of carbon monoxide and hydrogen is enriched with hydrogen from the water gas catalytic (Bosch) process, ie, water gas shift reaction, and passed over a cobalt—thoria catalyst to form straight-chain, ie, linear, paraffins, olefins, and alcohols in what is known as the Fisher-Tropsch synthesis. 

Prior to methanation, the gas product from the gasifier must be thoroughly purified, especially from sulfur compounds the precursors of which are widespread throughout coal (23) (see Sulfurremoval and recovery). Moreover, the composition of the gas must be adjusted, if required, to contain three parts hydrogen to one part carbon monoxide to fit the stoichiometry of methane production. This is accompHshed by appHcation of a catalytic water gas shift reaction. 

The ratio of hydrogen to carbon monoxide is controlled by shifting only part of the gas stream. After the shift, the carbon dioxide, which is formed in the gasifier and in the water gas reaction, and the sulfur compounds formed during gasification, are removed from the gas. 

Synthesis Gas Chemicals. Hydrocarbons are used to generate synthesis gas, a mixture of carbon monoxide and hydrogen, for conversion to other chemicals. The primary chemical made from synthesis gas is methanol, though acetic acid and acetic anhydride are also made by this route. Carbon monoxide (qv) is produced by partial oxidation of hydrocarbons or by the catalytic steam reforming of natural gas. About 96% of synthesis gas is made by steam reforming, followed by the water gas shift reaction to give the desired H2 /CO ratio. 

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