Reverse-shift reaction

However, it was found that the effect on the equilibrium formation of aromatics is not substantial due to thermodynamic considerations. A more favorable effect was found for the reaction between ethylene (formed via cracking during aromatization of propane) and hydrogen. The reverse shift reaction consumes hydrogen and decreases the chances for the reduction of ethylene to ethane byproduct. 

It is expected that the actual rate of CO methanation will always be high, at least under industrial conditions, whereas the C02 methanation rate will vary from about the same as that for CO down to zero, depending on operating pressure, temperature, CO content of the gas, and catalyst particle size. Meanwhile a water-gas shift (or reverse shift) reaction will be occurring at all times at a fairly high rate. 

Steel/chemical industry Nuclear iron making by recirculation of CO formed from the reverse-shift reaction of effluent C02 with nuclear produced hydrogen by the water-splitting. 

Kinetic evidence for synergic adsorption of carbon monoxide and water on the low-temperature shift catalyst Cu/ZnO/Fe203 was obtained by van Herwijnen and deJong (113), and IR spectra of surface formate were detected on several oxide catalysts, including CuO/MgO, at temperatures as low as 20 JC and pressures of 20 Torr, as reported by Davydov et al. (104). Decomposition of the surface formate to C02 and H2 occurred at 100-150°C over the Cu/MgO catalyst and at 250 300°C over the MgO catalyst, and the promotion effect of copper was attributed to the formation and decomposition of a labile surface formate (HCOO)2Cu. Ueno et al. (117) have shown earlier that surface formates are formed on zinc oxide, from CO and H20 as well as from C02 and H2, and hence an associative mechanism of the shift and reverse-shift reaction, involving formate intermediate, is believed to operate on many oxide catalysts. 

Equation 3.5.3 is called the reverse-shift reaction because it occurs opposite to the normal direction. Carbon dioxide will react with hydrogen in the converter according to Equation 3.5.4 to form methanol. 

In the presence of excess hydrogen, the rate of methane formation through the methanation reaction increases by increasing the operating pressure, while at the same time carbon dioxide will react with hydrogen to produce carbon monoxide through the reversed shift reaction. The same trend is observed for the release rate of ethylene and... 

Curiously, in spite of the hydrogen atom coverage being reduced by a factor of 10 and in spite of the detailed kinetics of reaction (A) remaining unchanged, the rates of reverse shift reaction predicted by this model are higher at all temperatures than those predicted by the non-activated... 

The calculations described above clearly show that, at steady state, during the reverse shift reaction, part of the surface of the copper which is catalysing the reaction is covered with oxygen atoms to an extent which is dependent on the detailed kinetics of the adsorption of hydrogen. The adsorption CO2 on to this area of surface oxidised copper will produce a surface carbonate... 

A reverse shift reaction parallel to the reactions already mentioned takes place H2 + CO2 CO + H2O... 

It has to be added that CO2 cannot be considered to be an inert gas due to in-situ CO-formation in the presence of hydrogen by the so called reverse shift reaction [42]. 

Rates of formation of methanol and water at 230°C depend strongly on hydrogen pressure. Rate of methanol synthesis is almost independent of partial pressure of carbon monoxide and carbon dioxide at low conversions. At high carbon dioxide content in the feed gas the rate depends strongly on conversion at low carbon dioxide content it is almost independent of conversion. This indicates that the methanol formation is inhibited by water but not by methanol. Initial rates of formation of methanol and water are equal indicating that methanol is formed by hydrogenation of carbon dioxide. Rate of reverse shift reaction is low compared to rate of methanol synthesis. Rate of shift reaction is higher when favoured by equilibrium. 

The ensemble control allows operation at (H2O + C02)/CH4 ratios close to stoichiometry. Operation on mixtures of CO2 and methane without steam is also possible except for the steam required for prereforming of the higher hydrocarbons in the feed. No steam was needed when operating on pure methane (CO2/CH4 = 1.2) in a laboratory reactor. Conversion of CO2 to CO - by reaction (4) and by the reverse shift reaction (reaction(2)) -of up to 75% may be achieved. 

With CO2 recycle or CO2 import, more CO can be produced essentially by the reverse shift reaction (reaction (2)). However, COj-conversion is feasible only when aiming at low H2/CO-ratios. As an example, an overall conversion of CO2 of ca. 30% is achieved at a ratio CO2/CH4 of about 1 in the feed resulting in a H2/CO in the product gas of about 1.0. This is much lower than the conversion of CO2 obtained in SPARG reforming. 

---