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1.INTRODUCTIONThe widespread application of fossil fuels such as petroleum, natural gas, and coal is gradually increasing with the development of global industrialization1-3. This has caused lots of CO2 emission into the atmosphere in modern society. It is urgent to decrease the emission of CO2 owing to its contribution on global warming. Several technologies such as chemical absorption, adsorption and membrane separation are usually used to separate CO2 from mixed gas in chemical industry4-6. Among these technologies, the adsorption method has a series of advantages such as simple operation, easy control and low cost, but it also has drawbacks such as poor adsorption capacity and low selectivity7. Gas membrane separation technology has the advantages of low energy consumption, simple operation, and easy control. However, conventional membrane materials are limited by the upper limit of “Robeson”, making it difficult to obtain both high selectivity and high permeability8,9. Chemical absorption method has become the main method for carbon capture due to its good absorption capacity and rate. However, there are still some problems such as high energy consumption, severe material loss, and volatility. Hence, improving the existing absorption solution and developing new low energy CO2 absorbents have become a major direction for future solution absorption methods10-13. Alcoholamine, as a commonly used CO2 absorbent, mainly includes N-methyldiethanolamine (MDEA), monoethanolamine (MEA), di-2-propanolamine (DIPA), diethanolamine (DEA) etc.14-17. Compared with other alcohol amines, although MEA has a high absorption rate, its high volatility and high energy consumption also affect its overall capture performance. Although DEA has a relatively low absorption rate, its low desorption temperature and high absorption capacity make it one of the main CO2 absorbents. If a certain accelerator is added to increase the absorption rate of DEA, it may greatly improve the CO2 absorption performance of DEA. In recent years, piperazine (PZ) has been reported as an absorption promoter in the field of capturing CO2 18-20. It can be regarded that adding suitable piperazine to the alcohol amine will help improve the CO2 absorption rate. In our work, considering the shortcomings of DEA in absorption rate, a certain amount of piperazine was added in the preparation process of the absorbent to improve the absorption effect. As the main gas absorption equipment, packed towers play a crucial role in the CO2 capture industry. Therefore, it is necessary to investigate the absorption performance of CO2 into the mixed solution of PZ and DEA with packed column. In our experiment, the mass transfer coefficient of CO2 into DEA solution activated by piperazine was calculated, and the effect of PZ on gas-liquid mass transfer was also discussed. 2.THEORY2.1Reaction mechanismReferred to References21-23, lots of reactions can take place as CO2 is absorbed by aqueous solution of piperazine and diethanolamine, as follows. The absorption mechanism of CO2 into the mixed solution includes two zwitterion mechanism: (1) the formation of zwitterion; (2) The base (DEA, PZ and PZCOO-) occurred PZ-monocarbamate, PZ-dicarbamate and protonated base in the deprotonation of the zwitterion24-26. The key deprotonation of the zwitterion in an aqueous blend of PZ and DEA mainly comes from DEA, PZ and a smaller amount of H2O. As usual, deprotonation is a rapid reaction as it only involves proton transfer, and the formation of zwitterion is considered a rate-controlled step. The addition of PZ helps to form more substrates, which is conducive to the formation of DEA zwitterions and thus facilitates absorption. 2.2Absorption setupAs described in our previous work27, CO2 is absorbed by a packed column, and the gas-liquid reaction occurs at the packing surface in the column, as shown in Figure 1. The device mainly includes the parts: N2 cylinder, CO2 cylinder, buffer bottle, packed column, tank of absorbent recovery, rotameter, absorbent vessel, super constant temperature water bath, drying bottle and portable infrared gas analyzer. Figure 1.Absorption setup. (1): cylinder of N2 gas; (2): cylinder of CO2 gas; (3): buffer bottle; (4): packing column; (5): absorbent recovery tank; (6): temperature water bath; (7): constant flow pump; (8): absorbent vessel; (9): drying bottle; (10): infrared gas analyzer; (11)-(13): rotameters.  At first, N2 and CO2 are mixed in a buffer bottle, and then enter the absorption tower. After CO2 is absorbed by the absorbent, the gas is discharged from the top of the packed column. After dried in a dryer, the CO2 content in the mixed gas is measured by a flue gas analyzer. The absorption temperature of gas and liquid is controlled by constant temperature water bath. The absorption experiment begins as the CO2 content in the inlet gas remains stable. At this moment, the CO2 content in the mixed gas is recorded, and the absorption liquid is pumped from the tank into the packed column. Under normal circumstances, KG which means mass transfer coefficient was obtained by reaction kettle and wetted wall column that based on the theory of gas-liquid two films reported in some literatures28,29. In fact, this apparatus is just one unit of our packed column. In order to obtain the relationship between CO2 mass transfer and PZ concentration in industrial applications, the volumetric mass transfer coefficient (KGaV) of CO2 in packed tower absorption can be obtained according to equation (8)30-33. In the equation, G-inert gas flow rate (mol·m-2 ·s-1); P-operational pressure (Pa); aV-interfacial area per unit volume of packing (m2.m-3); yA-CO2 mole fraction in bulk; 3.RESULT AND DISCUSSION3.1The relationship between PZ/DEA mass ratio and the value of KGaVThe CO2 absorption into the mixed solution of PZ and DEA was performed at the condition of 303 K absorption temperature, 200 mL.min-1N2 flow rate, 50 mL.min-1 CO2 flow rate and 2 rpm.min-1 constant flow pump. Four aqueous amine blends of DEA and PZ were used to absorb CO2 with 30 wt% total amine concentration, and the mass ratio of PZ/DEA were respectively 3/27, 6/24, 9/21, 12/18, the relation of KGaV values and PZ/DEA mass ratio was shown in Figure 2. Based on Equation (12). It is clear that the KGaV values increases with the increasing of PZ/DEA mass ratio. This is mainly due to the higher concentration of piperazine promoting the rate of diethanolamine formation into carbamate, which is beneficial for accelerating gas-liquid mass transfer. 3.2The relationship between temperature and the value of KGaVThe absorption of carbon dioxide by amine aqueous solution is a reversible reaction. Temperature is also a major factor affecting gas-liquid mass transfer. This article measured the total mass transfer coefficient of PZ-DEA aqueous solution absorbing CO2 at different ambient temperatures (operational conditions: 6/24 PZ/DEA mass ratio, 200 mL.min-1 N2 flow rate, 50 mL.min-1 N2 flow rate and 2 rpm.min-1 liquid constant flow pump), the result was shown in Figure 3. It shows that the KGaV value decreases with the increasing of absorption temperature as the other operational condition remains unchanged. This result just indicates that the absorption process is an exothermic reaction, which is consistent with the conclusion that the absorption of CO2 by alkanolamine is an exothermic process. Increasing the temperature is not conducive to absorption and mass transfer at 293-323 K, and 293 K is chosen as the optimal absorption temperature. 3.3The relationship between liquid flow rate and KGaV valueIn order to study the effect of CO2 absorbent flow rate, the KGaV values was determined as the rotation speed of constant pump increases from 0.5 rpm.min-1 to 2.0 rpm.min-1 (operational conditions: 6/24 PZ/DEA mass ratio, 303 K absorption temperature, 100 mL.min-1 N2 flow rate, 25 mL.min-1 N2 flow rate), the result was shown in Figure 4. It shows that KGaV values increases with the increasing of pump speed, which means increasing liquid flow rate is helpful to gas-liquid mass transfer. This is mainly because more liquid will facilitate the formation of more liquid films on the surface of the packing, thereby increasing the effective gas-liquid contact area and enhancing the mass transfer performance. 3.4The relationship between gas flow rate and the value of KGaVAn appropriate gas flow rate is not only beneficial for improving mass transfer efficiency, but also for the smooth operation of the packed column. The KGaV values of CO2 into PZ-DEA solution were determined for different inlet gas flow in this experiment, the result was shown in Figure 5. It clearly indicates that increasing inlet gas flow can effectively improve the mass transfer coefficient. The results indicate that within a certain range of gas flow rate, increasing the gas flow rate is beneficial for full gas-liquid contact and improving mass transfer. Therefore, an appropriate liquid to gas ratio is a means to improve column efficiency. 3.5The relationship between CO2 content and the value of KGaVThe effect of CO2 Content in inlet gas on mass transfer of CO2 in the packed tower was measured within the range of 8% to 20%, the result was shown in Figure 6. From the figure, it can be seen that increasing the CO2 content in inlet gas on the contrary reduces the overall volumetric mass transfer coefficient, and high CO2 content is not conducive to CO2 mass transfer. This may be because a higher gas content will gradually reduce the effective components in the liquid of the packing section, reducing the mass transfer driving force. Therefore, it leads to an increasing of CO2 content in the exhaust gas and reduces mass transfer efficiency. 4.CONCLUSIONThe effect of PZ/DEA mass ratio, absorption temperature, absorbent flow rate, the CO2 content, gas flow rate on the overall volumetric mass transfer coefficient were investigated at normal pressure. It is observed that the performance of CO2 mass transfer in packed column significantly increases as PZ concentration increases. Piperazine has a certain promoting effect on the mass transfer of CO2 in solution. As the temperature of absorption increases, the decreasing of KGaV value prove that the absorption process is an exothermic reaction. The total volumetric mass transfer coefficient increases with the increasing of gas and liquid flow rates at the inlet, but decreases with the increasing of CO2 content in the inlet gas. Through the research in this article, it can conclude that improving existing alkanolamine absorbers and developing new composite absorbers will have a certain positive effect on improving CO2 capture performance. In the future, research on new absorbents will also be a major direction for CO2 capture using solution methods. ACKNOWLEDGEMENTSThe authors are grateful for the financial support fromthe Open Project of hubei Key Laboratory of Industrial Fume & Dust Pollution Control (HBIK-2021-01), the Open Project of Key Laboratory of Novel Reactor and Chemical Technology of Hubei Province (NRG202105), Principal fund for Students of Wuhan Institute of Technology in China (XZJJ2020020) and the Training Program of College Students Innovation and Entrepreneurship in Hubei province (S202010490003, S202110490002). REFERENCESZhou, Y., Rao, Y. H., Wang, T. L. and Jens K. J.,
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