https://doi.org/10.3390/su14127095
“In CO2 absorption, the basic principle is that CO2 molecules are transferred from the gas phase into the liquid phase. Since the CO2-amine reaction occurs when gas comes into contact with liquid, in this study, the influence of the liquid flow rate on the process performance was investigated and was determined to range from 3.25 to 5.42 m3/m2∙h. The experiments were conducted using a CO2 concentration of 40% in the feed gas, which entered the column at a gas flow rate of 26.52 kmol/m2∙h. shows the variations in the liquid flow rate and Lamine/GCO2 ratio applied in the experimental work. The table shows that the Lamine/GCO2 ratio increases as the liquid flow rate increases.”
Table 4. Variations in the liquid flow rate and Lamine/GCO2 ratio.
| Total Liquid Flow Rate (m3/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) |
| 3.25 |
10.61 |
15.92 |
11.54 |
1.09 |
| 3.97 |
10.61 |
15.92 |
14.09 |
1.33 |
| 4.69 |
10.61 |
15.92 |
16.65 |
1.57 |
| 5.42 |
10.61 |
15.92 |
19.24 |
1.81 |
“As depicted in , increasing the liquid flow rate improved the CO2 removal performance from 70% to 100% at the column height of 2.04 m. Based on the trendline at 3.25 and 3.97 m3/m2∙h liquid flow rates, CO2 removal was gradually increased starting at the section of the column that was 0 to 2.04 m in height. However, both liquid flow rates showed insufficient CO2 removal, achieving rates of 70% and 83%, respectively. The most reactive section was also observed to be at the top of the column (1.36 to 2.04 m), achieving CO2 removal at 47% and 49% within this region, respectively. Such behaviour can be explained by the counter-current motion involved in the interaction between the fresh amine molecules in the liquid and the lower CO2 concentration in the gas phase in the top section of the column. The transfer of the CO2 molecules from the gas phase to the liquid phase increased the CO2 loading capacity in the solution.”
”
As the liquid travelled downwards to the middle section of the column (0.68 to 1.36 m), it continued to absorb more CO
2, increasing the CO
2 loading capacity [
7]. Approximately 18% to 25% of the CO
2 was removed within this region. At the bottom of the column, the absorbent came further into contact with the feed gas, which consisted of 40% of CO
2, as it was introduced into the column. While the absorbent was moving from the top to the middle section of the column, the CO
2 loading capacity began to gradually increase. The CO
2 loading capacity of the absorbent was almost saturated within the 0 to 0.68 m region since the absorbent was only able to remove less than 10% of the CO
2 during the process.
Based on the previous observations, a liquid flow rate of 4.69 m3/m2∙h (Lamine/GCO2= 1.57) was considered to be a sufficient process condition, achieving a maximum removal of 97% at the exit of the column. Referring to the trendline for the reaction at the bottom of the column (0 to 0.68 m), 28% of the CO2 removal occurred in this region. The gas came further into contact with the absorbent in the middle section of the column (0.68 to 1.36 m). This was the most reactive section in the column, with CO2 removal increasing from 28% to 75% (47% increment) as the gas counter-currently came into contact with the CO2-loaded amine from the top section. The CO2 concentration in the gas that was flowing upward was significantly decreased by approximately 10% of CO2 within the section that was 1.36 m in height. This gas then continued upwards and reacted with fresh amine, which has a low CO2 loading capacity. This resulted in the further removal of 22% of CO2 from the gas phase at the top region (1.36 to 2.04 m). It was expected that the reaction in the top region would be limited by the low CO2 concentration in the gas phase when there was an excess of amine molecules. Hence, a lower CO2 removal was observed in this section compared to in the middle section of the column.
At the liquid flow rate of 5.42 m3/m2∙h (Lamine/GCO2 = 1.81), a complete CO2 removal (100%) was achieved at the column height of 1.36 m. The most reactive section was at the bottom of the column (0 to 0.68 m), where 72% CO2 removal was achieved. This process subsequently removed an additional 28% of the CO2 in the middle section of the column (0.68 to 1.36 m). Beyond the column height of 1.36 m, no reaction was detected due to the absence of a reactant (CO2) in the gas phase. It was expected that the absorbent exiting at the bottom of the column was unsaturated in terms of the CO2 loading capacity. Hence, at this liquid flow rate, complete removal was able to be achieved within 67% of the column height. Thus, a shorter column can be designed for the biogas upgrading process, which will consequently reduce the capital cost of the equipment.
shows that the efficiency of CO
2 removal steadily increases from 70% to reach complete removal of CO
2 when the liquid flow rate is set from 3.25 to 5.42 m
3/m
2∙h. Additionally, the
KGav¯¯¯¯¯¯¯¯ values were improved by 5.9 times, from 0.066 to 0.389 kmol/m
3∙h∙kPa, when the
Lamine/GCO2 ratio increased from 1.09 to 1.81. This observation is in agreement with most studies [
23,
46,
50]. The possible reason for this behaviour is the presence of active amine molecules that increased as the flow rate increased, thus enhancing the reaction between CO
2 and the amine molecules [
29].”
”
Furthermore, spreading the liquid on the surface of the packing at a higher liquid flow rate increases the interfacial area per unit volume (
av) for the reaction between CO
2 and the amine molecules [
50,
59]. The liquid side mass transfer coefficient (
kL) was also increased at a higher liquid flow rate [
60,
61]. Both
kL and
av are proportional to the enhancement of mass transfer coefficient,
KGav¯¯¯¯¯¯¯¯. The enhancement factor (
E) is a significant factor that can influence mass transfer when there is a chemical reaction in the absorption process. According to Fu et al. [
62], a high liquid flow rate would increase the
E value and would eventually reduce the liquid film’s resistance
(HEk∘L). In addition, increasing the
Lamine/GCO2 ratio with a higher liquid flow rate would reduce the mass transfer resistance due to the boundary layer of the liquid phase becoming thinner, which consequently would accelerate the mass transfer performance [
63].
Although promising performances were observed at higher liquid flow rates in terms of the removal of CO
2 molecules from the gas phase, Liao et al. [
23] stated that a higher regeneration energy is needed to regenerate a larger volume of absorbent. Moreover, Gao et al. [
29] reported their concerns regarding a high circulation flow rate at a high liquid velocity, which may lead to large amounts of free active amine molecules that are unable to react with CO
2 molecules. This may increase the energy consumption for absorbent pumping and for the regeneration process. Thus, optimal parameters should be designed to achieve higher efficiency at reasonable operational costs for industrial operations.”
