Heat of vaporization from process simulation

According to Eq. (22), the ratio of partial pressure of H2O and CO2 at the top of the stripper (PH2O/PCO2) determines the heat of vaporization (qvap). As PH2O/PCO2 increases, the qvap increases. As shown in Table 7PH2O/PCO2 of the degraded solution was slightly larger than that of the fresh solution.

As the PCO2*of the solution becomes high at a given CO2 loading,PH2O/PCO2 in the stripper becomes low. As mentioned in Section 3.2, the presence of HSS increases the PCO2* at a given CO2 loading, but it also reduces the operational CO2 loading range. These two changes cancel one another out, and PH2O/PCO2 for the simulated degraded solution slightly increased.

As mentioned in Section 3.2, under a constant PCO2 at the absorber inlet, the CO2 concentration driving force (PCO2−PCO2*) of the absorption flux is maintained by varying the CO2 loading to keep a constant CO2 capture rate. If the liquid side mass transfer coefficient with gas side units (kg’) decreases (assuming a constant gas-liquid contact area), a higher CO2 concentration driving force of the flux is required in the absorber, meaning that the CO2 loading range must be decreased. As the CO2 loading range is reduced, the corresponding PCO2* in the stripper decreases, leading to a decrease in PCO2 in the stripper and an increase in qvap. Therefore, the kg’ in the absorber affects qvap indirectly. Figure 7 shows the kg’ in the absorber for the fresh and degraded solutions. The kg’ of the degraded solution is roughly equivalent to that of the fresh solution. In amine-based PCC system, the kg’ in the absorber is affected by the kinetics of the CO2 absorption reaction and the diffusivity of reactants and products in solution. The free-amine concentration influences the reaction kinetics while the viscosity influences the diffusivity (Dugas, 2009). The sum of the mole fraction of PZ and PZCOO is considered as an index of free-amine concentration because the chemical reaction of PZ with CO2 (e.g., 5f or 7f in Table 3) has a higher reaction rate compared to other kinetically controlled chemical reactions (e.g., 2f or 9f in Table 3) (Ashraf et al., 2020), making its impact on the kg’ dominant. Figure 8 shows the profile of the solution viscosity and the free-amine concentration index in the absorber. As shown in Fig. 8, both the solution viscosity and the mole fraction of free amine of degraded solution are roughly equivalent to those of fresh solution. At a given CO2 loading, the free-amine concentration decreases, and the viscosity increases due to the presence of HSS; both changes generally lead to a decrease in the kg’. On the other hand, the presence of HSS reduces the CO2 loading range. This shift decreases the viscosity and increases the free-amine concentration. Thus, although HSS alter the viscosity, free amine concentration, and CO2 loading range, these changes largely cancel one another out in terms of impact on the kg’.

Fig 7

Fig. 7. The vertical profile of the liquid side mass transfer coefficient in the absorber for fresh and degraded solutions.

Fig 8

Fig. 8. The vertical profile of the solution viscosity and the free amine mole fraction in the absorber for fresh and degraded solutions.

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