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Energies 2019, 12, 469 8 of 15 1. 2. The rate of RWGS can be estimated and compared with the measured CO2 rate based on the (easily) measurable rate of the reverse WGS reaction and some thermodynamic considerations. The rate of CO2 conversion must equal the rate of RWGS, if (hypothetically) only indirect CO2 methanation via CO takes place. The RWGS is then the rate determining step followed by fast CO methanation. A calculation based on the CO and CO2 rate equations (as determined in this work) lead to the concentration of the intermediate CO, which should correspond to the observed values. If the calculated CO yield is much higher than the one observed, this is an indication of the dominance of direct CO2 methanation and vice versa. Approach 1: Unfortunately, the rate of the RWGS could not be directly measured here, because methanation of CO is fast hindering the decision whether direct CO2 methanation or the RWGS followed by CO methanation takes place. However, the rate of RWGS can at least be estimated based on the measurable rate of the WGS, Equation (8), and the equilibrium constant Kc. The rate of the WGS was investigated with 20% CO and 20% H2O in N2 (mcat = 2 g, VSTP = 8 l h−1, 200–260 ◦C). The conversion of CO was less than 4%. (Remark: The equilibrium CO conversion of the WGS for these conditions is higher than 90%; hence, only the WGS without any influence of the RWGS was measured.) The simplifying assumption used here is that the WGS is first order both with respect to CO and H2O. It has to be noted that this assumption was not experimentally proved and verified. Nevertheless, the rate of the WGS obtained by this assumption is still a rough but good estimation to show (see below) whether CO2 is most probably directly converted (hydrogenated) to methane or indirectly via CO. Using this assumption, the rate constant kWGS was determined, yielding a pre-exponential factor, kWGS,0, of 5.18×104 m3 mol−1·kg−1·s−1 and an activation energy, EA,WGS, of 97 kJ/mol. The equilibrium constant KC of the WGS is approximately the ratio of the rate constants of WGS and RWGS, and thus the rate constant of the RWGS was calculated (kRWGS = kWGS/KC). CO+H2O↔CO2+H2 ∆H0298 =−41kJmol−1 (8) According to [31], the equilibrium constant KC of the WGS can be determined by: K (T) = cCO2 cH2 = 0.0147 e 4577.8 (9) cccT CO H2O So finally, the rate of the RWGS is given by rRWGS = kWGS/KC·cCO2 ·cH2 . For 190 ◦C and a feed gas containing 20% CO2, 55% H2, and 25 vol% N2, and the conditions used here (mcat = 2 g, VSTP = 8 l h−1), rRWGS is only 0.0005 mmol CO2 kg−1·s−1 compared to the much higher measured reaction rate of CO2, rCO2 , of 0.29 mmol CO2 kg−1·s−1. Hence, CO2 is most probably not consumed by the RWGS followed by CO methanation, but only by direct methanation. Approach 2: If the hypothesis would be correct that CO2 methanation is just an indirect reaction and only occurs via RWGS and subsequent CO methanation, then the measured reaction rate of CO2 should only reflect the rate of RWGS. The concentration of the intermediate CO during the conversion of CO2 with H2 can then be calculated based on the CO and CO2 reaction rates as determined in this work, Equation (4) and Equation (6). Figure 5a,b show the results both of the calculation and of two experiments for a feed gas with 6 vol% CO2 and 55% H2 (rest N2), if either the temperature (140–252 ◦C) or the residence time (for 223 ◦C and 234 ◦C) is increased. The calculated volumetric content of the “intermediate” CO can now be compared with experimental data: The calculation leads to relative high values of the CO content of up to 400 ppm, although no CO was experimentally detected for T < 200 ◦C. Even for 223 ◦C and 9% CO2 conversion (12% measured), the simulation leads to 240 ppm CO, whereas the experimental value is only 20 ppm. For 234 ◦C, XCO2,calculated is 12% (still in acceptable agreement with 16% measured) and 190 ppm CO are calculated compared to the experimental value of 25 ppm CO. These results again lead to the conclusion that the indirect pathway of CO2 methanation is not or only to a very low extent taking place, and that CO2 is directly hydrogenated to CH4 on the used Ru catalyst.PDF Image | Selective Methanation of CO over a Ru-y-AI2O3 Catalyst in CO2 H2
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