Influence of Inlet Liquid Temperature using PZ + AMP

In the CO₂ absorption process, the inlet liquid temperature is one of the critical parameters. In the current research, the absorption behaviour was observed at four different inlet liquid temperatures: 30 ± 2, 35 ± 2, 40 ± 2, and 45 ± 2 °C. The experimental work was conducted using a blended solution containing 7 wt.% PZ + 23 wt.% AMP and constant gas and liquid flow rates of 26.52 kmol/m²∙h and 4.69 m³/m²∙h, respectively. Table 6 shows the variations in inlet liquid temperature with a Lamine/GCO2 ratio value of 1.57 for these studies.

At the inlet liquid temperature of 40 ± 2 °C, approximately 36% of the CO₂ was removed from the gas phase at the bottom section of the column (0 to 0.68 m). The remaining 25.6% of CO₂ in the gas that was flowing upward from this section continued to the middle section (0.68 to 1.36 m) and reacted with the absorbent that was flowing downwards. Consequently, 55% more CO₂ was removed, reducing the CO₂ concentration to less than 4% in the gas phase. Beyond the 1.36 m section, approximately 9% of the CO₂ was further eliminated to achieve complete removal at the column height of 1.70 m. Analysing this trend, a decreased CO₂ removal was able to be observed in the top section (10% of CO₂) compared to in the middle section (55% of CO₂) of the column. This was due to the low availability of CO₂ in the top section, which limited the reaction in this section.

CO₂ removal also steadily increased along the column when the inlet liquid temperature was 30 ± 2 °C. Based on the trendline at the bottom section of the column (0 to 0.68 m), the amount of CO₂ was reduced by 28% in this region. It was found that the most reactive section was in the middle of the column (0.68 to 1.36 m), in which approximately 48% of the CO₂ was eliminated from the gas stream as the gas continued upwards against the counter current of the CO₂-loaded amine from the top section. The CO₂ concentration in the gas that was flowing upward significantly decreased to approximately 10% at the 1.36 m section. This gas flow continued upwards and reacted with fresh/low CO₂-loading-capacity amines. An additional 22% of CO₂ was removed from the gas phase in the top region (1.36 to 2.04 m), with 97% CO₂ removal being observed at the exit of the column. The reaction in the top region was expected to be limited by the low CO₂ concentration in the gas phase, while the available amines were in excess. Hence, a lower CO₂ removal performance was observed in this section (22% of CO₂) compared to in the middle section of the column (48% of CO₂).

The lowest performance was observed when the inlet liquid temperature was set at 45 ± 2 °C, in which the trendline for this temperature setting was constantly lower than at other inlet liquid temperatures. Approximately 11% of the CO₂ was absorbed in the bottom section of the column (0 to 0.68 m), while a better removal performance was observed in the middle section (0.68 to 1.36 m), where approximately 37% of the CO₂ was eliminated. In the top section (1.36 to 2.04 m) of the column, an additional 44% of the CO₂ was absorbed, resulting in 92% CO₂ removal from the process. However, this condition was insufficient for sale gas specifications.

As depicted in Figure 12, the CO₂ removal efficiency at the liquid inlet temperature of 30 ± 2 °C was 97% with a KGav¯¯¯¯¯¯¯¯ value of 0.18 kmol/m³∙h∙kPa. The performance was slightly increased and reached complete removal at 35 ± 2 and 40 ± 2 °C. The increased CO₂ absorption performance at 30 ± 2 to 40 ± 2 °C may be described by the decreasing the viscosity of the solution at a higher temperature, allowing more liquid to spread on the surface of the packing. Consequently, this phenomenon resulted in the enhancement of the interfacial areas for the reaction between CO₂ and the amine molecules [55].

Although complete CO₂ absorption (100%) was achieved at both 35 ± 2 °C and 40 ± 2 °C, the mass transfer performance at 40 ± 2 °C was slightly reduced compared to at 35 ± 2 °C. A reduction of approximately 19% in the mass transfer coefficient was observed between the operation at 35 ± 2 °C and at 40 ± 2 °C. The KGav¯¯¯¯¯¯¯¯ values at 35 ± 2 °C and 40 ± 2 °C were 0.36 kmol/m³∙h∙kPa and 0.29 kmol/m³∙h∙kPa, respectively. These experimental findings reveal that the CO₂-amine reactions were dominated by a forward reaction at temperatures lower than 35 ± 2 °C. Additionally, increasing the inlet liquid temperature to be over 35 ± 2 °C shifted the reaction mechanisms towards reverse reactions. Therefore, this phenomenon resulted in a significant decline in the removal performance at 45 ± 2 °C, with only 92% of the CO₂ being removed from the feed gas. The KGav¯¯¯¯¯¯¯¯ value was also significantly reduced at 40 ± 2 °C and 45 ± 2 °C.

A similar trend was reported by Zeng et al. [65], in which the forward CO₂-amine reactions were dominant at 20 °C to 35 °C. However, inlet liquid temperatures higher than 35 °C shifted the reaction to a reverse controlling mechanism. The decreased absorption performance at higher inlet liquid temperatures could also be explained by the decreased CO₂ solubility, which consequently increased the mass transfer resistance in the liquid film. Similarly, the reduction in the mass transfer performance was observed in this study, as the inlet liquid temperature was increased to beyond 35 ± 2 °C. Hence, it was proven that the inlet liquid temperature can highly affect process performance.”