Available online at www.sciencedirect.com Energy Procedia 37 (2013) 455 – 460 GHGT-11 Mixed Alkanolamines with Low Regeneration Energy for CO2 Capture in a Rotating Packed Bed Cheng-Hsiu Yu and Chung-Sung Tan* Department of Chemical Engineering, National Tsing Hua University, Hsinchu, Taiwan, R. O. C Abstract In this study, the concentrated piperazine (PZ) and mixed PZ + diethylenetriamine aqueous solutions were used as the absorbents to capture CO2 from a nitrogen gas stream containing 10% of CO2 in a rotating packed bed (RPB). To reduce the heat of vaporization of absorbent in regeneration, diethylene glycol was added into the concentrated PZ aqueous solution. The higher gas-liquid contact area and mass transfer rate as well as the smaller size were observed in a RPB as compared with a packed bed to achieve the same CO2 capture efficiency. © 2013 Elsevier Ltd.Ltd. 2013 The TheAuthors. Authors.Published Publishedbyby Elsevier Selection of GHGT Selectionand/or and/orpeer-review peer-reviewunder underresponsibility responsibility of GHGT Keywords: CO2 Capture, Chemical absorption, Alkanolamine, Diethylenetriamine, Piperazine, Diethylene glycol, Rotating packed bed 1. Intoduction The global warming issue which resulted from greenhouse gases has received widespread concern nowadays. Among the greenhouse gases, the contribution of carbon dioxide (CO2) to global warming is more than 60%. Coal supplies 25% world energy and 40% of carbon emission, and therefore it is essential to capture and storage CO2, especially from fossil fuel power generation plants to cope with the global demanding of CO2 reduction [1, 2]. Chemical absorption has been used to treat acid gases, such as SO2 and CO2 in packed bed. However, the drawback of using packed bed is that large volume is required to treat a huge amount of the exhausted gas ffrom power plant and significant mass transfer limitations exist in the gas-liquid interface. To enhance mass transfer rate between gas and liquid, a RPB device was suggested. Not like a packed bed, liquid contacts with gas on the surface of packing under high centrifugal condition in a RPB, and hence the gas-liquid contact area as well as the mass transfer efficiency is increased owing to liquid is split into small droplets and liquid film by passing through the packing. Many studies have been reported to use various aqueous alkanolamine solutions to capture CO2 in RPB. The results indicated that the size of a * Corresponding author. Tel.: +886-3-572-1189 ; fax: +886-3-572-1684 . E-mail address: [email protected] . 1876-6102 © 2013 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of GHGT doi:10.1016/j.egypro.2013.05.131 456 Cheng-Hsiu Yu and Chung-Sung Tan / Energy Procedia 37 (2013) 455 – 460 RPB could be reduced significantly as compared with a packed bed when the same CO2 capture efficiency was desired to achieve [3-8]. Alkanolamine-based absorbents such as monoethanolamine, diethanolamine, methyldiethanolamine and their mixtures as well are commonly used to capture CO2. From our previous work, it can be seen that diethylenetriamine (DETA) and piperazine (PZ) are preferred to be used in a RPB to capture CO2 because of their high reaction rate. Since there is the solubility limitation for PZ in water, 14% at 293 K, PZ is usually used as the promoter to mix with other alkanolamines for the enhancement of reaction rate with CO2. In a recent report by Freeman et al. , the concentrated PZ solution was suggested owing to its effective resistant to oxygen degradation and thermal degradation. However, research related to the use of the concentrated PZ aqueous solution for CO2 capture in a RPB is limited. In this study, the concentrated PZ solution was used as the absorbent to capture CO2 because of its fast reaction rate with CO2. A mixture of DETA and PZ, which had been proposed to be an effective absorbent in our previous study, was also used for the comparison of CO2 capture efficiency purpose. To reduce the latent heat of vaporization of absorbents, diethylene glycol (DEG) was proposed to add into the concentrated PZ aqueous solution. The energy required in the regeneration process was found to reduce because the reduction of vapor pressure after the addition of DEG. Nomenclature ri Inside radius of the RPB (cm) ro Outside radius of the RPB (cm) Yi CO2 Concentration in the feed stream (%) Yo CO2 Concentration in the discharged stream (%) 2. Experimental PZ, DETA, and DEG with a purity of 99% were purchased from Seedchem, Aldrich, and Tedia, respectively. N2 with a purity of 99.99% and CO2 with a purity of 99.5% were purchased from Boclh Industrial Gases Co (Taiwan). All the chemicals and gases were used as received. The schematic diagram of the RPB used in this study is shown in Fig. 1.The inner, outer diameters and height of the packing were 2.5, 12.5 and 2.3 cm, respectively. The total volume of the packing in the RPB was of 270.9 cm3. The stainless wire mesh was used as the packing with a surface area of 887.6 m2/m3 and a void fraction of 0.96. The gas stream flowed from the outer side into the RPB. The absorbent was pumped from the central axis tube into the RPB. Both gas and liquid were heated by the heaters equipped with temperature controller before entering the RPB. The CO2 gas contacted with the absorbent counter-currently in the RPB. After absorption, the gas stream left from the axis of the RPB and CO2-rich absorbent was discharged from the outer side of the RPB. The CO2 concentrations of both the feed and discharged gas stream were measured by a NDIR CO2 analyzer (Drager, Polytron IR CO2). The CO2 capture efficiency Cheng-Hsiu Yu and Chung-Sung Tan / Energy Procedia 37 (2013) 455 – 460 and height of transfer unit (HTU) were used as the indicators for the absorbent comparison, and they were calculated by the Eqs (1) and (2): CO2 Capture efficiency HTU Yi - Y Yi 100% (1) ro ri (2) Yi ln( ) Yo where ro and ri are the outside and inside radiuses of the RPB, respectively. Yi and Yo are CO2 concentration in the feed and discharged stream, respectively. A smaller HTU value represents a smaller reactor volume needed for capturing CO2. Fig. 1. Experimental apparatus for CO2 capture in a rotating packed bed. 3. Results and Discussion In our previous work, it had been found that the CO2 capture efficiency of DETA was superior to that of MEA in a RPB. Besides, the absorbent 15% DETA/15% PZ/70% H2O was an effective absorbent to achieve a high CO2 capture efficiency in a RPB . This absorbent was therefore chosen for the comparison purpose. Table 1 shows the CO2 capture efficiencies of various absorbents at the operation conditions as a temperature of 323 K, a gas flow rate of 30 L/min, a liquid flow rate of 100 mL/min, and a rotating speed of 1300 rpm. It is seen that the CO2 capture efficiency of the solution 40.8% PZ/59.2% H2O was higher than that of the mixed alkanolamine aqueous solution 15% DETA/15% PZ/70% H2O, and the CO2 capture efficiency of the solution 40.8% PZ/47.36% H2O/11.84% DEG was slightly lower than that of the solution 15% DETA/15% PZ/70% H2O. The difference of CO2 capture efficiency among the used absorbents was believed mainly due to the different reaction rates with CO2. Because of the contact time between gas and liquid was very short in a RPB, the absorbents possessing higher reaction 457 458 Cheng-Hsiu Yu and Chung-Sung Tan / Energy Procedia 37 (2013) 455 – 460 Table 1. CO2 capture efficiency and HTU for various absorbents Absorbent Content (%) CO2 Capture efficiency (%) HTU (cm) DETA/PZ/H2O 15/15/70 80.0 3.1 PZ/H2O 40.8/59.2 85.0 2.6 PZ/H2O/DEG 40.8/47.36/11.84 77.5 3.4 rates with CO2 would result in higher CO2 capture efficiencies. Because the reaction rate of PZ with CO2 is higher than that of DETA, the CO2 capture efficiency of 40.8% PZ/59.2% H2O was therefore higher than the mixed solution 15% DETA/15% PZ/70% H2O. As shown in Table 1 that the CO2 capture efficiency of 40.8% PZ/47.36% H2O/11.84% DEG was lower than that of 40.8% PZ/59.2% H2O. This was probably due to that the reaction rate of absorbent with CO2 is proportional to the solubility parameter of solvent . As can be seen from table 2, the solubility parameter of DEG is lower than water, and hence, the reaction rate of 40.8% PZ/47.36% H2O/11.84% DEG was therefore lower than that of 40.8% PZ/59.2% H2O, leading to a lower CO2 capture efficiency as compared to 40.8% PZ/59.2% H2O. Table 2. Solubility parameter of various solvents  Solvent Water DEG Solubility parameter (J/m3)1/2 48.1 24.8 It can also be seen from Table 1 that the HTU values of the absorbents used in this study were all lower than 3.4 cm, lower than that of the conventional packed bed where the HTU value is generally found to be about 340 cm . The significant decrease of HTU using RPB was believed due to the increased gas-liquid contact area and mass transfer rate in a RPB as compared with a packed bed, and hence, the smaller size of a RPB was therefore required to achieve the same CO2 capture efficiency. The regeneration energy estimated in this study was based on the following assumptions: (1) ideal mixing, (2) CO2-rich loading as 0.82 mol of CO2/mol of amine, (3) regeneration efficiency of 50%, (4) at 393 K, and (6) regeneration temperature approach as pressure at 2 atm. The thermal properties including heat of reaction with CO2 and heat capacity of DETA and PZ are listed in Table 3. These properties were used for estimation of regeneration energy required. Fig. 2 shows the regeneration energy required for the solution 40.8% PZ/59.2% H2O was lower than that of 15% DETA/15% PZ/70% H2O. This was due to that the heat of reaction of PZ with CO2 is lower than DETA and the PZ concentration in the concentrated PZ solution was higher than that of the solution 15% DETA/15% PZ/70% H2O, resulting in the lower heat of vaporization of water. In Table 1, the regeneration energy of 40.8% PZ/47.36% H2O/11.84% DEG was found to be lower than that of 40.8% PZ/59.2% H2O. Obviously, the heat of vaporization of water could be reduced with the addition of DEG into the solution. Hence the addition of the low vapor pressure solvents, such as DEG, might be a remedy to reduce the energy required in the regeneration process. Cheng-Hsiu Yu and Chung-Sung Tan / Energy Procedia 37 (2013) 455 – 460 Table 3. Thermal properties of DETA and PZ [13-15]. Absorbent Heat of reaction with CO2 (kJ/mol) Heat capacity (J/g/K) 4.0 PZ 73 70 2.83 2.51 Regeneration temperature = 393 K Regeneration pressure = 2 atm 3.5 Regeneration energy (GJ/tonne CO2) DETA 3.0 2.5 2.0 1.5 1.0 0.5 0.0 15% DETA/15% PZ/70% H2O 40.8% PZ/59.2% H2O 40.8% PZ/47.36% H2O/11.84% DEG Fig. 2. Regeneration energy of various absorbents. 4. Conclusions The concentrated PZ solution was verified to be a very powerful absorbent to capture CO2 in a rotating packed bed absorber because of its fast reaction rate with CO2. However, the absorption should be carried out at high temperatures because its solubility in water is limited. The mixed alkanolamine solution containing PZ and DETA was found to be an appropriate absorbent for CO2 capture in RPB regarding both CO2 capture efficiency and HTU. The HTU obtained using the proposed alkanolamine aqueous solutions in a RPB were found to be significantly smaller than those in the conventional packed bed, indicating the volume of an absorber could be reduced with great extent when RPB instead of packed bed is employed. 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