https://doi.org/10.1016/j.ijggc.2022.103771
“In addition to the slope of a drop of pressure plot, which can preliminary represent the rate of CO2 absorption, the reaction kinetics constant (k) is a typical kinetics parameter that well represents the kinetics performance of CO2 absorption [6]. Based on the fast pseudo-first order reaction of CO2 and amine, the important kinetics parameter, which is kov, can be obtained from a plot of lnPCO2 and time as shown in Eq. (1). Regarding the well agitated stirred tank reactor and the smooth gas–liquid interfacial area, the kov can be calculated from a slope of the plot as presented in Eq. (2) [6].(2)Slope=−RTAVGHeCO2kovDCO2
It should be pointed out that the Henry’s constant of CO2 (HeCO2) of 5% wt. AMP-25% wt. MPDL, 10% wt. AMP-20% wt. MPDL, and 15% wt. AMP-15% wt. MPDL were estimated to be that of 5% wt. AMP-25% wt. DEA, 10% wt. AMP-20% wt. DEA, and 15% wt. AMP-15% wt. DEA, respectively. This is because HeCO2 or physical solubility of CO2 in amine solvent depends on the solvent density in that the physical solubility of CO2 increased as solvent density decreased or the HeCO2 decreased as solvent density decreased [7]. Based on our previous study [5], it was found that the density of 30% wt. AMP-MPDL solvent (0.9819–0.9961 g/cm3) are close to that of 30% wt. AMP-DEA solvent (0.9825–1.019 g/cm3) [8] over ranges of studied concentration and temperature. Therefore, it is reasonable to estimate HeCO2 of AMP-MPDL to be that of AMP-DEA. The HeCO2 of AMP-DEA over ranges of concentration and temperature were reported by Penttila et al. [9]. The calculated HeCO2 values are given in Table 2.
Due to chemical reaction between CO2 and amine, the physical diffusivity of CO2 into AMP-MPDL solvent or DCO2 cannot be measured directly. Thus, in the present work, the DCO2 of AMP-MPDL was then estimated by the modified Stokes–Einstein equation as of Eq. (3) [10]. This equation has been widely accepted as an accurate and a reliable correlation for determining the DCO2 of amine solvent [6]. As presented in Eq. (3), DCO2 can be calculated based on the dynamic viscosity of AMP-MPDL solvent, which was experimentally measured and reported in our previous study [5]. The values of DCO2−water and μwater were obtained from the literature [10]. As a result, the DCO2 of AMP-MPDL over ranges of concentration and temperature are presented in Table 2.(3)DCO2−amineμamine0.8=DCO2−waterμwater0.8where DCO2−amine is physical diffusivity of CO2 in amine solvent (m2/s), DCO2−water is physical diffusivity of CO2 in water (m2/s), μamine is dynamic viscosity of amine solvent (kPa s), and μwater is dynamic viscosity of water (kPa s).
Based on Eq. (2), the kov of AMP-MPDL solvent at various concentrations and temperatures can then be calculated. The results are presented in Table 2 and plotted in Fig. 2. It was found that the kov increased as temperature increased over a temperature range of 313–333 K. This observation well corresponds with the slope of a drop of pressure plot (shown in Figs. 1(a)–1(c)) and the literature [2], [6]. The data also showed that an increasing of kov is exponential function with the temperature. This is in good agreement with the Arrhenius relationship in that the reaction kinetics constant is an exponential function with the temperature [4]. Additionally, Table 2 and Fig. 2 show that the kov increased as concentration of AMP in the 30% wt. AMP-MPDL increased. It is because AMP reacts faster with CO2 than MPDL. In other words, the reaction kinetics constant of AMP is higher than MPDL (at 313 K, k2,AMP is 1,276 m3/kmol s, while k2,MPDL is 90 m3/kmol s) [4]. By comparing the kov of the three AMP-MPDL concentrations, it was found that 15% wt. AMP-15% wt. MPDL is much faster than 10% wt. AMP-20% wt. MPDL and 5% wt. AMP-25% wt. MPDL, respectively. Thus, the results presented in this section (shown in Figs. 1(a)–1(c) and Fig. 2) confirm the hypothesis of the present work that the overall reaction kinetics of AMP-MPDL solvent can be improved by an addition of AMP. Therefore, according to the overall reaction kinetics constant (kov) and the slope of drop of pressure plot, it can be concluded that 15% wt. AMP-15% wt. MPDL showed the best performance in terms of reaction kinetics within the range of studied concentration.”
“Fig. 2. Overall reaction rate constant (kov) of 30% wt. AMP-MPDL at various concentrations and temperatures.”