https://doi.org/10.3390/su14127095
“CO2 partial pressure is related to the feed gas composition and total gas pressure in the column. Since biogas consists of a higher CO2 concentration, a CO2 concentration range from 10% to 55% was used as the base concentration in this study, which was conducted at 200 kPa of the total operating pressure in the column. CO2 absorption experiments were conducted using the best performed blended solution, i.e., 7 wt.% PZ + 23 wt.% AMP. The solution was continuously pumped to the top of the column at 3.97 m3/m2∙h. The total gas flow rate was supplied at 26.52 kmol/m2∙h. shows the variations in the CO2 partial pressure and Lamine/GCO2 ratio for these runs. The ratio of Lamine/GCO2 is the main driving force in this chemical absorption process. It indicates the availability of CO2 and amines molecules for the reaction at the initial condition of the experiment. As the CO2 partial pressure in the gas stream increased (refer to ), the Lamine/GCO2 ratio decreased.”
Table 2. Variations in CO2 partial pressure and Lamine/GCO2 ratio.
| PCO2(kPa) |
CO2 in NG (%) |
CO2 Flow Rate (GCO2) (kmol/m2 h) |
Amine Flow Rate (Lamine) (kmol/m2 h) |
Lamine/GCO2(kmol/kmol) |
| 20 |
10 |
2.65 |
14.09 |
5.32 |
| 50 |
25 |
6.63 |
14.09 |
2.13 |
| 80 |
40 |
10.61 |
14.09 |
1.33 |
| 110 |
55 |
14.59 |
14.09 |
0.97 |
“ depicts the CO2 removal efficiency profiles along the column. At the column height of 2.04 m, the CO2 removal performance decreased from 100% to 58% when the CO2 partial pressure increased from 20 to 110 kPa.”
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For a CO2 partial pressure at 20 and 50 kPa, the CO2 was effectively absorbed from the gas stream, achieving a 100% CO2 removal at a column height of 2.04 m. This result indicates that an Lamine/GCO2 value higher than 2.13 was sufficient for complete CO2 removal. It was observed that, at 20 kPa, this system was able to achieve the fastest complete removal at the column height of 1.36 m. This behaviour was mainly due to the high Lamine/GCO2 value of 5.32, which resulted in higher CO2-amine reactions in the column. The removal performance at 20 kPa indicated that the reaction was able to take place when there was an excess supply of free amine molecules during operation. Consequently, all of the CO2 molecules in the gas phase were completely absorbed into the liquid phase. Then, the removal performance reached a plateau beyond 1.36 m of the column height.
Based on the trendline observed at the CO2 partial pressure of 50 kPa, the CO2 removal was almost linear along the column compared to at 20 kPa. This behaviour was expected due to the lower Lamine/GCO2 value supplied to the system at 50 kPa (2.13), which was less than half of the Lamine/GCO2 value at 20 kPa (5.32). It was observed that the CO2 molecules were constantly removed along the column and reached complete removal at the column height of 2.04 m without any excess active amine being discharged during the regeneration process.
On the other hand, by increasing the CO2 partial pressure from 50 kPa to 80 kPa and 110 kPa, the Lamine/GCO2 values decreased to 1.33 and 0.97, respectively. Without a sufficient Lamine/GCO2 ratio, the system was unable to completely remove CO2. This was observed even in the most reactive area for CO2-amine reactions, which was within the top section of the column (1.36 to 2.04 m). Within this section, this system was only able to remove 49% and 44% of CO2 at 80 and 110 kPa, respectively. In this section, CO2 was aggressively absorbed due to the presence of fresh amines with zero CO2 loading when it first came into contact with the gas in the top section of the column. As liquid travels downward, CO2 loading would gradually increase in the liquid phase, resulting in a reduction in amine molecules available for CO2-amine reactions when the column height ranges from 0 to 1.36 m. Therefore, incomplete CO2 removal can be observed due to there being insufficient amines for the reaction to take place.
shows the CO2 removal efficiency and mass transfer performance at different CO2 partial pressures upon entry to the packed column. As discussed in the earlier part of this section, 100% CO2 removal was observed at the CO2 partial pressures of 20 and 50 kPa due to the low concentration of CO2 molecules reacting with a sufficient amount of amine molecules in the liquid phase. By further increasing the CO2 partial pressure to 110 kPa, the CO2 removal efficiency was reduced to 58%.
”
”
In terms of mass transfer performance, a significant decrease in KGav¯¯¯¯¯¯¯¯ from 0.630 to 0.039 kmol/m3∙h∙kPa was observed when the CO2 partial pressure was increased from 20 to 110 kPa. The increased CO2 partial pressure led to a reduction in the mass transfer performance of approximately 94%. Generally, the reaction between CO2 and the amines would occur instantaneously at the gas–liquid interface, which would lead to a steeper CO2 gradient while enhancing the mass transfer process in the liquid. These reactions can reduce the equilibrium partial pressure of CO2 (P∗CO2) throughout the solution, which can greatly increase the driving force for mass transfer. This reaction is strongly influenced by the amine (reactant) concentration, in which the availability of amine molecules in a continuous process is represented by the amine’s molar flow rate (Lamine) as it is being pumped into the column.
As listed in , the
Lamine/GCO2 is reduced as the CO
2 partial pressure increases. As expected, the mass transfer performance (
KGav¯¯¯¯¯¯¯¯) was also reduced in this condition due to the decreasing driving force for mass transfer in the packed column. Furthermore, when the CO
2 partial pressure is higher, more CO
2 molecules can react with the limited number of amine molecules in the liquid film, which could reduce the enhancement factor (
E) in the process [
55].
E is one of the factors contributing to the resistance in the liquid film
(HEk∘L). Consequently, this condition would increase the resistance of the liquid film, reducing the mass transfer performance when the CO
2 partial pressure is higher.”
