https://doi.org/10.3390/su14127095
“The effect of the gas flow rate on the removal performance was observed in the range from 22.10 to 35.36 kmol/m2∙h. The blended solution screened from Section 3.1 that performed the best, i.e., 7 wt.% PZ + 23 wt.% AMP, flowed into the column at a liquid flow rate of 4.69 m3/m2∙h. Table 3 shows the variations in the gas flow rate and in the Lamine/GCO2 ratio for these experiments. The Lamine/GCO2 values decreased as the gas flow rate increased.”
Table 3. Variations in the gas flow rate and Lamine/GCO2 ratio.
| Gas Flow Rate (kmol/m2∙h) | CO2 Flow Rate (GCO2) (kmol/m2 h) |
NG Flow Rate (kmol/m2∙h) | Amine Flow Rate (Lamine) (kmol/m2 h) | Lamine/GCO2(kmol/kmol) |
|---|---|---|---|---|
| 22.10 | 8.84 | 13.26 | 16.65 | 1.88 |
| 26.52 | 10.61 | 15.91 | 16.65 | 1.57 |
| 30.94 | 12.38 | 18.56 | 16.65 | 1.35 |
| 35.36 | 14.14 | 21.22 | 16.65 | 1.18 |
“Figure 5 shows the CO2 removal profiles along the absorption column over gas flow rates ranging between 22.10 and 35.36 kmol/m2·h. At 2.04 m of the column height, the experimental findings indicated that the increased gas flow rate caused the CO2 removal percentage to decrease from 100% to 72%.”
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The trendline for the gas flow rate at 22.10 kmol/m2∙h shows that complete CO2 removal can be achieved in the middle section of the column. A significant reduction in the CO2 concentration from 40% to 0% was achieved at column heights ranging from 0 to 1.36 m. These results indicate that a shorter column height is adequate for full CO2 removal from the gas stream when the system is conducted with the Lamine/GCO2 value of 1.88. This behaviour shows that the absorption process operates with excessive free amine molecules in the system, which could increase operational costs due to the expensive cost of absorbents and energy for regeneration.
Moreover, increasing the gas flow rate from 22.10 to 26.52 kmol/m2∙h reduced CO2 removal to 97%. The removal trend was almost linear along the column, with the most reactive region being observed in the middle section of the column, within a height range from 0.68 to 1.36 m. Within this section, approximately 48% of the CO2 was successfully removed during the process. Using the designated column, this behaviour indicates that setting the Lamine/GCO2 value at 1.57 is sufficient for sale gas specifications (less than 3% of CO2 exists in the clean gas).
Additionally, further increasing the gas flow rates to 30.94 and 35.36 kmol/m2∙h contributed to lower Lamine/GCO2 values at 1.35 and 1.18, respectively. Both of these gas flow rates showed a similar increasing trendline for the CO2 removal performance along the column. Although these process parameters were able to remove CO2 at 89% and 72%, respectively, the CO2 molecules were not fully absorbed at the outlet of the column. This phenomenon was expected due to a higher number of CO2 molecules entering the column as the gas flow rate increased, whereby the reaction was limited due to insufficient free amine molecules. This observation is in line with the expectation that the Lamine/GCO2 ratio decreases at higher gas flow rates, which would adversely reduce the absorption efficiency [56].
At the gas flow rates of 30.94 and 35.36 kmol/m2∙h, the figure shows that the most reactive section is located at the top section of the column (1.36 to 2.04 m). This was where active amine molecules flowing down from the top of the column were able to remove 54% and 48% of the CO2 molecules, from the gas phase, respectively. The ability to remove CO2 molecules was slightly decreased (23% and 17%, respectively) in the middle section of the column (at 0.68 to 1.36 m), indicating that the number of free active amine molecules decreased as the liquid travelled down the column. The CO2 loading capacity of the absorbents was further increased and almost saturated at column heights ranging from 0 to 0.68 m, with less than 12% CO2 removal within this region.
As presented in Figure 6, a decrease in the CO2 removal efficiency from 100% to 72% can be observed when the gas flow rate increased from 22.10 to 35.36 kmol/m2∙h. A reduction in the KGav¯¯¯¯¯¯¯¯ values from 0.317 to 0.092 kmol/m3∙h∙kPa was also observed during the process. The significant 71% reduction in the mass transfer performance was expected due to the higher concentration of CO2 entering the column, and this reaction was restricted by an insufficient amount of amine molecules [57]. This expectation is confirmed by the values of Lamine/GCO2 presented in Table 3, which show that the initial Lamine/GCO2 value of 1.88 decreased to 1.18.”
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Thus, it was concluded that the driving force for mass transfer with the chemical reaction in the absorption column was strongly influenced by Lamine/GCO2. The high mass transfer coefficient (KGav¯¯¯¯¯¯¯¯) observed when the Lamine/GCO2 value was 1.88 indicated a greater gas–liquid contact surface area that can maximise the mass transfer with the chemical reaction taking place during CO2 absorption. The results also show that the absorption was faster when the driving force for mass transfer (y − y*) was increased when the Lamine/GCO2 was higher. Such information is useful when designing a packed column and consequently for reducing the operational costs.
It should also be noted that the gas residence time in the column began to decrease at higher gas flow rates, leading to the respective decrease in the effective interfacial areas for CO2-amine reactions [7,23]. Hence, increasing the gas flow rate would result in less contact time between the CO2 and amine molecules for the reaction to occur [31,58]. This condition would consequently affect the process performance in terms of the removal efficiency and mass transfer in the packed column.”