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Appl. Sci. 2020, 10, 4692 8 of 19 Figure 4. CH4 and CO2 concentrations as a function of time. According to the parametric analysis results, it is found that the modelling results are fitted very well with the experimental data, and the mass transfer coefficient of CO2 is 0.027 s−1, which is larger than that for CH4 of 0.0008 s−1. It can be seen that there is a significant difference between these two kinetic parameters. The reason is that when the biogas is injected in the adsorption bed, most of the CH4 is released through the outlet without adsorption. This is due to the low selectivity of the adsorbent, which is related to a very low mass transfer coefficient. However, in the case of CO2, the large amount of CO2 was adsorbed due to fast adsorption kinetics. 3.2. Validation of the Model To verify the correctness of the assumptions explained in Section 3.1, it is useful to simulate a simple breakthrough curve of a CO2/CH4 mixture with 45% vol./55% vol. proportions, in order to reproduce the data obtained from an experiment by Cavenati et al. involving a fixed bed track [8]. An inlet pressure of 3.2 bar and a temperature of 303 K have been set, with a total flow rate equal to 1 standard liter per minute (SLPM). The same dimensions with Cavenati’s study were maintained for the adsorbent bed, i.e., a height of 0.83 m and a diameter of 0.021 m. In order to function, Aspen Adsorption must initialize the gas content inside the adsorbent bed. Since it was not possible to set the column to be empty at the initial conditions, it was decided to assume that the bed was completely filled with N2, requiring that it could not be adsorbed (isotherm parameters = 0) and did not offer any resistance to diffusion (KN2 = 0). In this way, a curve quite similar to the experimental data was obtained in Figure 5, but with an excessive slope regarding the CO2 breakthrough curve. This is related to the fact that, while the kinetic parameters of methane and carbon dioxide have regressed to 308 K, the experimental temperatures were obtained at 303 K. This is because of the fact that the slope of the curve is not only changed due to the adsorption limit of adsorbent but also the temperature increase during the adsorption step, resulting in the non- symmetric S-shape curve due to a faster diffusion. However, the isothermal operation is presented in this study, so the thermal effects are lumped into the mass transfer coefficient, which is helpful in obtaining a faster solution. For this reason, manually decreasing the kinetic parameter of CO2 allowed us to obtain KCO2 = 0.01 s−1, which was a better fitting of the curve. The results of the two simulations, first keeping the KCO2 regressed to 308 K and then correcting it to 303 K, are shown in Figure 5 for (K0.01) and (K0.027).PDF Image | Biogas Six-Step Pressure Swing Adsorption
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