## 4 Results and Discussion
### Scenario 2: Servo Performance Test  
Following the disturbance rejection test, a set point change (servo performance) test was conducted to observe the process behavior when the set point (capture rate in this case) would be changed from 85% to 90% instantaneously. It is possible to cause the change in a specified time interval, but the worst-case scenario was tested. The capture rate was controlled through the lean amine flow rate while the other parameters were kept constant. The temperature of the absorber stabilized in about 3 min while the pressure was maintained at the operating point. The desired capture rate was observed after 12 min and was achieved accurately, indicating a good controller servo performance tracking the changes in set points induced by the operator. The resulting plots from the servo performance test are shown in Figures 6 and 7. When the capture rate was altered, the solvent flow rate was adjusted by the flow controllers accordingly, affecting the liquid level in the sump. Any increase in the amine flow rate would increase the sump liquid level. When a 5% increase was set, the sump level rises, as shown in Figure 8, but it is still within the allowable limit. If the level rises higher than the maximum allowable height, there will be a risk of tower flooding, thus an intermediate tank would ensure flexible operation. The tanks could store the solvent when the absorption operation is maintained, but the desorption operation is paused or decreased due to an increase in electricity prices, for instance. Another approach would be to vent any extra flue gas and only allow the required amount of feedstock to enter into the absorber for CO\({}_{2}\) capture. Figure 5: Disturbance rejection in the desorber unit for change in flue gas flow rate. An extended run was simulated that spanned 14 h of simulation time to observe the stabilizing time required for the sum level to return back to the set point. The results of the run can be seen in Figures 9-11. Figure 8: Sum level rising in response to 5% increase in capture rate. Figure 6: Absorber parameters in response to change in capture rate. Figure 7: (Left) capture rate % as the set point is induced. (right) Controller servo performance of tracking the set point. The first 4 h no disturbance was initiated, and the model was left to run until the level was at the set point and the process was stable. A decrease in the capture rate caused the solvent flow rate to decrease and hence the sum level dropped instantaneously. The sum level returned to the set point after about 6 h, meaning that if we would like to transition from one steady-state condition to the next the interval of a few hours would be necessary to ensure that the process stabilized, and would transition to the next steady-state condition. Similar behavior was noticed from the reboiler as the disturbance was caused at the fourth simulation hour, the duty decreased in response to the decreased solvent flow rate and stabilized in around 6 h back to a stable duty. Nevertheless, in the transition phase, the process was still operating, and the CO\({}_{2}\) stream was being captured in the absorber and stripped in the desorber and the sum level would be fluctuating within the allowable range as per the sum design. [PERSON] et al. [46] observed similar behavior where the authors were investigating the dominating dynamics of the process and recorded a settling time of 606.26 min in the reboiler outlet in the case of a 10% decrease in the flue gas flow rate. Figure 10: Stripper sum stabilizing time. Figure 9: Sump level stabilizing time. It is clear that controlling the product CO\({}_{2}\) flow rate through altering lean amine flow rate results in a relatively slower response than if controlled by altering the reboiler duty. Keeping in mind that by altering the reboiler duty, the stripper temperature changes and, in turn, the lean loading. Similar behavior was also observed in work published by [PERSON] et al.