https://doi.org/10.1016/j.ijggc.2022.103597
“The presence of HSS in an amine solution has direct and indirect effects on solution properties. The HSS directly alter the physical properties of the solution. For example, if weak acids with a low specific heat capacity accumulate, the specific heat capacity of the degraded solution becomes lower. An indirect effect of HSS is the change in solution properties that arises from the altered CO2 loading range. As shown in Table 7, both the lean and rich loading decreases due to the presence of HSS. When the CO2 partial pressure of the gas entering the absorber and the CO2 capture rate are fixed, the CO2 partial pressure (PCO2) at the top (∼0.4 kPa in this study) and bottom (∼4 kPa in this study) of the absorber are roughly constant. Furthermore, the equilibrium CO2 partial pressure (PCO2*) of the liquid must be always lower than PCO2 of the contacting gas to keep CO2 absorption flux from gas to liquid at each part of the absorber, that is, the CO2 concentration driving force of the flux must be maintained (i.e., PCO2−PCO2*>0). Therefore, the CO2 loading—which is relevant to the PCO2* — must be varied in accordance with VLE characteristics of solution to keep the CO2 concentration driving force of the absorption flux and thus the CO2 capture rate. Lean loading is adjusted by the reboiler duty in the stripper.
Figure 4 compared the VLE of the fresh and degraded solutions computed by Aspen Plus® model described in Section 2.1 along with experiment data. At a given temperature and CO2 loading, the PCO2* is higher for the degraded solution, which is consistent with the degraded MEA solution from a pilot plant (Aronu et al., 2014). This behavior can be explained by considering equilibrium constant of CO2 absorption reaction and increase in proton concentration due to weak acids (Weiland and Sivasubramanian, 2004) as follows. For amines absorbing CO2 in the form of carbamate (e.g., PZ), the chemical equilibrium constants of amine protonation (Ka) in Eq. (23) and carbamate formation (Kca) in Eq. (24) are determined by a given temperature. In addition, at a given temperature and PCO2, the concentration of CO2 ([CO2]) varies very little according to Henry’s law. In this situation, as shown in Eq. (24), the increase in the ratio of oxonium ion to water ([H3O+]/[H2O]) due to the presence of weak acids (cf. Eq. (12)) affects the decrease in the ratio of carbamate amine to free amine ([RNHCOO−]/[RNH2]) and results in the decrease in the CO2 loading. Likewise, for amines absorbing CO2 in the form of bicarbonate (e.g., AMP), the chemical equilibrium constant of bicarbonate formation (Kb) in Eq. (25) is determined by a given temperature, and [CO2] hardly varies at a given temperature and PCO2. Thereby, the increase in [H3O+]/[H2O] due to weak acids influences the decrease in the bicarbonate concentration ([HCO3−]) and leads to the reduction in the CO2 loading. Thus, the PCO2* increases at a given CO2 loading due to the presence of weak acids. When the PCO2* increases due to HSS, the CO2 loading range necessarily shifts to lower values (i.e., to the left in Fig. 4) so that the PCO2* is below the PCO2 and the concentration driving force of the absorption flux is maintained. An important consequence of this shift in the operational CO2 loading range is the alteration of most of the solution properties that affect process performance.
Fig. 4. VLE curves at 40 °C and 120 °C for fresh and degraded solutions computed by Aspen Plus® model (Experiments: *Brúder et al. (2011); **This study). The CO2 loading range for each solution in Table 7 are shown together. Approximate CO2 partial pressure at the top and bottom of the absorber in this simulation are also shown for explanation.
A concrete example of the CO2 loading dependence of solution properties is given by the viscosity. Figure 5 shows the dependence of viscosity on CO2 loading for fresh and degraded solutions. Though it is often stated that the solution viscosity increases due to the accumulation of HSS (Nielsen, 2018), this is true only if the CO2 loading is held constant. However, since the presence of HSS reduces the operational CO2 loading range, and resultant increase in solution viscosity is minor.(23)Ka=[RNH3+][H2O][RNH2][H3O+](24)Kca=[RNH3+][RNHCOO−][RNH2]2[CO2]=[RNH3+][RNH2][H3O+]*[H3O+][RNHCOO−][RNH2][CO2]=Ka*[H3O+][RNHCOO−][H2O][CO2][RNH2](25)Kb=[HCO3−][H3O+][CO2][H2O]2
Fig. 5. Viscosity at 40 °C for fresh and degraded solutions. The CO2 loading range for each solution in Table 7 are shown together.
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