Synergistic Effect on CO2 Capture by Binary Solvent System

CO2 absorption into a binary solvent system was studied using a batch‐mode gas/liquid absorption apparatus. The binary system composed of potassium carbonate (K2CO3) and piperazine (PZ) showed a strong synergistic effect, whereby the binary solvent performed better than either of the individual solvents for CO2 absorption. The other pairs of solvents tested (K2CO3/monoethanolamine (MEA) and K2CO3/NaOH) showed no synergistic effects. The results indicate that this synergistic effect only occurs with specific pairs of solvents. The mechanism for the synergistic effect is postulated that the activated CO2 on PZ migrates to K2CO3, or a more reactive intermediate complex between K2CO3 and PZ is formed.


Introduction
There has been a growing concern over greenhouse gas emissions as they are considered to be the direct cause of global warming [1,2]. Postcombustion capture technology is widely being studied for capturing CO 2 produced in power generation plants [3][4][5]. Compared with other CO 2 capture technologies such as oxy-fuel combustion and integrated gasification combined cycle (IGCC), postcombustion capture is regarded as the most probable technology to be first employed when carbon capture becomes a reality in the near future in terms of technology readiness level, flexibility, and economics [6]. Postcombustion capture technology uses liquid solvents to make efficient contact with CO 2 -containing flue gas, during which CO 2 interacts and reacts with the solvent and is removed from the flue gas stream. After absorption, the CO 2 -laden solvent undergoes a regeneration operation, releasing pure CO 2 which is then compressed, transported, and sequestered. The regenerated solvent, now at lean state, is returned to start the next cycle of CO 2 capture. The whole operation is a continuous process. The same or similar technologies have been applied for decades for natural gas purification and syngas CO 2 separation [7][8][9]. For greenhouse gas CO 2 mitigation applications, commercial solvents such as amine, potassium carbonate, and methanol are currently being tested, however, improved solvents are required to reduce the cost and increase the efficiency of postcombustion capture systems. At the moment, solvents that are being developed for CO 2 capture include nonconventional amines, aqueous ammonia, amino acids, ionic liquids, and mixtures of two or more solvents, i.e., hybrid systems [10,11].
Potassium carbonate is known to be used in industrial CO 2 separation processes, such as Benfield and Catcarb [12], as the main solvent with or without proprietary additives. It has advantages over amines: lower cost, lower heat of absorption, thermal stability, nonvolatile, less corrosiveness, low toxicity, and environmentally friendly. A major downside for using K 2 CO 3 , however, is its slow absorption rate and low CO 2 absorption capacity, resulting in poor CO 2 mass transfer rate relative to amines. The way to overcome the aforementioned shortcomings of K 2 CO 3 is to add promoter, i.e., a hybrid solvent. Hybrid solvent systems have the potential to perform better than the individual components alone. Physicochemical properties of different solvents can supplement each other. Synergistic effect or cooperative effect of hybrid solvents has been found in applications in other areas such as extraction and coal swelling [13,14]. The mechanisms of the synergistic effects are suggested to be engendered by thermodynamics and hydrogen bonding.
We have been studying CO 2 absorption using an aqueous potassium carbonate solvent solution with the addition of other solvents in an attempt to improve CO 2 absorption performance. In this chapter, we report results of a synergistic effect that became apparent during these studies. When small amount of piperazine (PZ) is added to the potassium carbonate solution, both CO 2 absorption rate and capacity are significantly enhanced, exceeding the mathematical sum of the CO 2 absorption rate and the capacity of the individual solvents.
Piperazine itself is an active absorbent for CO 2 [15]. For some engineering reasons, it has only been used as an additive or a promoter to other common CO 2 capture amines [16]. With amine solvents, piperazine has shown promotional effect. For instance, the CESAR-1 solvent is an aqueous blend of AMP (2-amino-2-methyl-1-propanol) and PZ which showed a reduction of about 20% in the regeneration energy and 45% in the solvent circulation rate compared to those of MEA-based CO 2 capture process under similar process condition [17].
There have been some reports on the promotional/synergistic effect on CO 2 capture by K 2 CO 3 and PZ [18]. This study builds upon previous achievements and provides convincing experimental evidence of the synergistic effect.

Experimental
A batch-mode liquid-gas absorption apparatus was constructed in CanmetENERGY, Ottawa. A schematic and a photo of the apparatus are shown in Figure 1. All of the connections within the system are vacuum-proof. The volume of the four-neck flask is 690 ml. The solute gas used in the experiment is a mixture of CO 2 and air (49 v% of CO 2 ). CO 2 absorption tests were carried out at 21℃ (room temperature). The flask was placed in a water bath to maintain a constant temperature (CO 2 absorption is exothermic). First, the flask was purged by the solute gas for 10 min. Then all of the valves of the flask were closed, leaving the gas in the flask at ambient pressure. After this, 10 ml of solvent was introduced into the flask by opening the two valves of the funnel, and then closing them quickly so that the flask becomes a closed system with gaseous solute in contact with liquid solvent. The liquid was agitated by a magnetic stirrer at 350 rpm (there was no difference on the CO 2 absorption results with rpm in the range of 300-400). When the CO 2 was absorbed, the pressure in the flask decreased. This pressure was monitored with a solid state pressure sensor/transducer (PX209-30V15GI) from Omega. A monotonous pressure declining curve was obtained, revealing the CO 2 absorption kinetics (rate of decline) as well as capacity (the final level-off of the decline). The solvents used and their concentrations in aqueous solution are shown in Table 1. In the test, the primary solvent was aqueous potassium carbonate, K 2 CO 3 . Other solvents were used as secondary promoters to see if there was a synergistic effect between the primary and secondary solvents. The hybrid solvents were obtained by mixing the individual solvents (shown in Table 1) with certain ratio (quantity in ml). Water was added to adjust the effective concentration and the final volume in a test.
Three test series were completed, one for each of the secondary solvents. These included:

Results and discussion
The CO 2 absorption results for test series 1 are shown in Figure 2. After the solvent was introduced into the flask filled with CO 2 /air, the chemisorption occurred as demonstrated by the pressure decrease. From the results in Figure 2, it can be seen that K 2 CO 3 showed a slow absorption rate and low absorption capacity. Piperazine's CO 2 absorption rate was faster and had higher capacity. When the two solvents were mixed, the binary solvent system absorbed more CO 2 at an even faster rate. The mathematical sum of the individual CO 2 absorption curves of the K 2 CO 3 and piperazine (the sum of the green curve and the light blue curve) is shown in Figure 2 as well (dark blue line). It is clear that the binary solvent system performed much better for CO 2 absorption than the mathematical sum of the individual solvents. The two curves (orange and purple in Figure 2) showing the CO 2 absorption results of the binary solvent system from two different tests under the same conditions indicate that the apparatus worked very well with a high degree of repeatability. The test results of the binary solvent system of K 2 CO 3 and MEA are shown in Figure 3. The component solvents of K 2 CO 3 and MEA were of similar effectiveness for CO 2 absorption. The binary solvent system showed only a slight synergistic effect.  In order to investigate the necessary and/or sufficient conditions for the synergistic effect of a stronger CO 2 solvent with a milder solvent (e.g., PZ with K 2 CO 3 ), the binary solvent system of K 2 CO 3 with NaOH was tested (Figure 4). It can be seen from Figure 4 that, although NaOH is a much stronger CO 2 solvent than K 2 CO 3 , the binary solvent system of K 2 CO 3 and NaOH does not show any synergistic effect.  Therefore, it is a necessary but not a sufficient condition for a binary solvent system with different CO 2 absorption capacities and kinetics to generate synergistic effect. Among the three pairs, only the binary solvent of K 2 CO 3 and PZ showed a positive synergistic effect on CO 2 absorption.
As shown by our experiment (Figure 2) and others [19], PZ is a stronger and faster CO 2 solvent than K 2 CO 3 . When the ratio of K 2 CO 3 and PZ was varied, the CO 2 absorption curves shifted from the curve of K 2 CO 3 to the curve of PZ, as shown in Figure 5. The binary solvent systems between the two pure solvents exhibit synergistic effect. Illustrated in Figure 6 is the synergistic performance of the binary solvent as well as the relationships with the two pure solvents (this is only a general illustration). Figure 6. Illustration of synergistic effect by a binary solvent system, e.g., K 2 CO 3 /PZ.
PZ is an expensive solvent. Whether or not it is suitable, alone, as a CO 2 capture solvent is still being explored in terms of thermal stability, corrosiveness and cost, etc. [19]. As shown by this study, it is promising to apply a binary solvent of K 2 CO 3 and PZ at a ratio that maximizes the synergistic effect on CO 2 capture. Savings from operating at this condition could be realized in terms of solvent cost, reduction of the absorber and regenerator sizes due to the improved CO 2 absorption rate and capacity. More effective solvents would require smaller absorbers and regenerators, leading to lower capital costs.
J. Tim Cullinane and Gary T. Rochelle have reported the promotional effect of K 2 CO 3 and PZ by kinetics [18]. They concluded that the promotional effect comes from the kinetics of the two individual solvents and that the two solvents absorb CO 2 independently. These cannot explain the observations of this study. The promotional or synergistic effect of PZ to K 2 CO 3 has been suggested to occur through an intermediate formed between CO 2 (aq) and PZ [20][21][22]. This hypothesis, however, still needs to be verified experimentally. Our results indicate that there may be a more interactive mechanism affecting the hybrid solvent performance. Having a binary solvent system with one solvent more effective than the other is a necessary condition for the synergistic effect (the pairs of K 2 CO 3 and PZ, K 2 CO 3 , and NaOH), but not a sufficient condition (K 2 CO 3 and NaOH). There must be other reasons behind the synergistic effect. Here we postulate two mechanisms: • CO 2 transition (or spill over or migration): CO 2 is reactivated by solvent B forming a labile state [[B] · [CO 2 ] (aq) ], then transfers or migrates to solvent A to finish CO 2 absorption (Figure 6). Likely hydrogen bonding is involved.
• Reactive complex intermediate structure between the two solvents: in the CO 2

Conclusion
The idea of combining solvents to improve absorption is effective for piperazine and K 2 CO 3 . These two solvents interact together and generate a greater absorption than each of the individual solvents. The other solvents, i.e., MEA and NaOH, when mixed with K 2 CO 3 did not improve CO 2 absorption, implying that the synergistic effect only occurs selectively between specific pairs of solvents. The solution of 3 ml piperazine with 7 ml potassium carbonate is the optimal ratio that increases CO 2 absorption using the least amount of piperazine. The results of these tests show the possibility of using piperazine and K 2 CO 3 solution at an industrial scale. If correctly implemented, it would result in savings in capital by reducing the absorber size compared to use K 2 CO 3 alone. The next step for this project is to apply these results within a larger system. The major conclusions from the tests conducted are summarized below: • A synergistic effect between K 2 CO 3 and piperazine was observed.
• This synergistic effect only happens between this specific pair of solvents and is not universal. Other than the thermodynamic reasons behind the effect, there seems to be some additional mechanism that enhances the reaction (potentially a labile [CO 2 ] formation followed by migration or some more reactive intermediate complex structure formed between the two solvent molecules).
• 3 ml piperazine/7 ml K 2 CO 3 ratio is the most effective (faster absorption rate and higher absorption capacity).