Microsoft Word - 164.docx CHEMICAL ENGINEERING TRANSACTIONS VOL. 56, 2017 A publication of The Italian Association of Chemical Engineering Online at www.aidic.it/cet Guest Editors: Jiří Jaromír Klemeš, Peng Yen Liew, Wai Shin Ho, Jeng Shiun Lim Copyright © 2017, AIDIC Servizi S.r.l., ISBN 978-88-95608-47-1; ISSN 2283-9216 Effect of Solution Height on Carbon Dioxide Absorption using High Frequency Ultrasonic Irradiation Wee Horng Tay, Ali Abdullah Baqhimh Barashaid, Kok Keong Lau*, Azmi Mohd Shariff Department of Chemical Engineering, Universiti Teknologi PETRONAS, 32610 Bandar Seri Iskandar, Perak, Malaysia laukokkeong@petronas.com.my Technology for capturing CO2 is gaining attention due to their potential in reducing the amount of CO2 in the atmosphere and opportunities to reuse or transform the captured CO2 into other valuable resources. This technology is commonly used during the welling for natural gas where the gas reservoir contains high concentration of CO2. High frequency ultrasonic assisted absorption can be used to generate ultrasonic fountain that provides abundant fine droplets for CO2 absorption. The height of the solution will affect the ultrasonic streaming velocity, which will then affect the amount of the generated droplet. In this paper, the high frequency ultrasonic assisted absorption was studied using different height of monoethanolamine (MEA) solution at varied ultrasonic power. The experiment was conducted using a pressurised batch process. The absorption rate was determined from the pressure drop profile. Based on the experiment results, the height of the solution has significantly affected the performance of the ultrasonic assisted absorption. The CO2 absorption coefficient has been increased from 0.0388 to 0.0470 kmol.m-3.h-1.kPa-1 by decreasing the height of solution from 50 mm to 25 mm under the ultrasonic power of 13.2 W. The thresholds of the ultrasonic fountain have been decreased from approximate 3 W to 1 W by decreasing the height of solution from 50 mm to 25 mm. Overall, the height of the solution in the ultrasonic reactor is one of the important factors to be considered for the ultrasonic assisted absorption process for CO2. 1. Introduction CO2 capture technology plays a crucial role in reducing the amount of greenhouse gas (GHG) released to the atmosphere. CO2 capture technology is also essential for the development of natural gas resource. Most of the undeveloped world gas reservoir contains high concentration of CO2. The gas separation for CO2 and natural gas is thus required to be applied at offshore site in order to inject the CO2 back to the deep underground. Absorption is one of the most established technologies for industrial application. Chemical absorption method is widely used to capture CO2 attributed to its high selectivity towards CO2. Amine-based solvent, such as monoethanolamide (MEA), has been commercialised in industry due to their high absorption rate and high capacity for CO2. The absorption process is usually conducted using packed bed (Tan et al., 2016) and bubble column method (Elhajj et al., 2014). These existed technologies involve huge unit operations and are not appropriate to be installed at the offshore site. Several research studies have been conducted on the ultrasonic-assisted mass transfer process using low frequency ultrasonic irradiation, such as vapour-liquid mass transfer (Laugier et al., 2008), adsorption, (Schueller and Yang, 2001), and degassing (Gondrexon et al., 1997). In our previous research, high frequency ultrasonic irradiation is proposed to be one of the potential approaches in order to enhance the CO2 absorption rate (Tay et al. 2016). The reported results show a great enhancement for the volumetric absorption rate due to the ultrasonic-assisted physical enhancement effect and this has largely reduce the size of absorption column required (Tay et al. 2016). Figure 1 shows physical enhancement effect using ultrasonic irradiation in the formation of ultrasonic streaming force (Rudenko, 1998) and ultrasonic fountain (Yasuda et al., 2005). The process involves the use of high pressure and an ultrasonic fountain caused by the ultrasonic irradiation. It is a form of physical absorption of CO2 in a water or solution based system under elevated pressure. DOI: 10.3303/CET1756205 Please cite this article as: Tay W.H., Barashaid A.A.B., Lau K.K., Shariff A.M., 2017, Effect of solution height on carbon dioxide absorption using high frequency ultrasonic irradiation, Chemical Engineering Transactions, 56, 1225-1230 DOI:10.3303/CET1756205 1225 Figure 1: Schematic diagram of ultrasonic assisted absorption It is noted that effects of the operating parameters on the performance of ultrasonic assisted absorption are still required to be studied and optimised. Based on our previous research (Tay et al. 2016), ultrasonic fountain is the dominated enhancement effect for the absorption process. The ultrasonic fountain can only be generated under a sufficient ultrasonic intensity and streaming momentum on the liquid surface. The intensity of ultrasonic irradiation will be diverged and decreased by increasing the distance from ultrasonic transducer. The height of the solution is suggested to be one of the important factors that can affect the absorption performance. The objective of this work is to study the effect of solution height on the ultrasonic fountain threshold using different ultrasonic power, which ranged from 1.5 W to 13.2 W. The study of CO2-MEA reaction system has been well established by Danckwerts (1979), and further improved by Astarita et al. (1983). The overall reaction for the carbamate formation in CO2-MEA system can be expressed as in Eq(1): CO2 + 2RNH2 ↔ RNH3 + + RNHCOO− (1) In the CO2-MEA system, the CO2 reacted with MEA to form the carbamate. This reaction can be described by the zwitterion mechanism. The zwitterion mechanism was first proposed by Caplow (1968) and reintroduced by Danckwerts (1979). It involves two step of reactions, firstly is the formation of complex ion, called zwitterion, followed by its deprotonation. The deprotonation of zwitterion is relatively faster than the carbomate reversion rate. Therefore, the chemical reaction is considered as irreversible. The CO2 absorption rate can be expressed as in Eq(2): �̇�𝑐𝑜2 = 𝐸𝑀𝐸𝐴 𝑘𝑙 𝑜𝑎𝑒 (𝐶𝐶𝑂2 ∗ − 𝐶𝐶𝑂2) (2) where 𝑘𝑙 𝑜 = the mass transfer coefficient without chemical reaction, which can be determined from the experiment using physical solvent; 𝑎𝑒 = the effective surface area; 𝐸𝑀𝐸𝐴 = the chemical enhancement factor; 𝐶𝐶𝑂2 ∗ = the concentration of CO2 in the liquid interfacial and 𝐶𝐶𝑂2 = the concentration of CO2 in the liquid bulk. 2. Methodology The experiment was set up in order to determine the absorption rate based on pressure drop profile (Tay et al., 2016). Figure 2 shows the schematic diagram of the experiment setup. Firstly, the operating temperature was maintained at 30 °C by placing ultrasonic assisted reactor in a temperature control bath. Then, the ultrasonic reactor was filled with different amount of MEA solvent (50 mL and 100 mL) with 30 wt% concentration. Subsequently, pure CO2 gas was compressed into the gas storage vessel. Following this, the ultrasonic assisted reactor was pressurised by opening valve V1 and valve V2. The pressure of the ultrasonic assisted reactor was governed by a back-pressure regulator. The valve for the ultrasonic vessel was immediately closed after achieving the designed initial absorption pressure. Finally, the pressure of the CO2 was recorded for every second using a data acquisition system. Ultrasonic fountain Ultrasonic irradiation Solution Height 1226 Figure 2: Experiment setup for the ultrasonic-assisted chemical absorption study For an ultrasonic-assisted mass transfer experiment, the absorption rate (�̇�𝑐𝑜2) was measured using the expression as in Eq(3): �̇�𝑐𝑜2 = 𝐾𝑔 𝑎𝑒 (𝑝 − 𝑝 ∗) = 𝑉 𝑍𝑅𝑇 𝑑𝑝 𝑑𝑡 (3) where, 𝑉 = vapour volume in the vessel; 𝑍 = compressibility and 𝑝∗ = the CO2 vapour pressure on the interfacial surface, which can be considered to be zero at initial stage. The overall mass transfer coefficient can be expressed as in Eq(4): 1 𝐾𝑔 = 1 𝑘𝑔 + 𝐻𝐶𝑂2 𝐸𝑀𝐸𝐴𝑘𝑙 𝑜 (4) If the gas phase resistance is too small, the first term for Eq(4) can be omitted. For 𝑘𝑙 𝑜 determination, 𝑘𝑙 𝑜 was measured based on physical absorption results by using water as the solvent. The ultrasonic enhancement can be obtained using the expression as in Eq(5): 𝐸𝑈𝑆 = (�̇�𝑐𝑜2)𝑢𝑠 (�̇�𝑐𝑜2)𝑛𝑢𝑠 (5) Where, (�̇�𝑐𝑜2)𝑢𝑠 = absorption rate with ultrasonic irradiation; (�̇�𝑐𝑜2)𝑛𝑢𝑠 = the absorption rate without ultrasonic irradiation and 𝐸𝑈𝑆 = the targeted parameter, which is required to be determined from the experiment. 3. Result and discussion The relationship between CO2 absorption with different ultrasonic irradiation was plotted. Figure 3 shows the pressure drop profiles using different ultrasonic power up to 13.2 W at initial pressure of 11 bar for the solution with 25 mm height. By referring to Figure 3, the pressure drop is significantly faster at the higher ultrasonic power. The results have demonstrated that the absorption rate is significantly higher at higher ultrasonic power. Ultrasonic irradiation at high power of 13.2 W, 9.2 W, 5.5 W and 3.3 W all showed significant drop of pressure along the 200 s time. The drop in pressure occurred the fastest when an ultrasonic irradiation power of 13.2 W was used. The CO2 pressure dropped significantly when the reaction begins. Low ultrasonic 1227 irradiation power at 1.5 W showed constant CO2 pressure where the CO2 pressure decrease gradually along the reaction time of 200 s. At the end of experiment, the CO2 pressure at the ultrasonic irradiation power of 1.5 W was significantly higher than the CO2 pressure obtained for ultrasonic irradiation employing a high power of 13.2 W, 9.2 W, 5.5 W and 3.3 W. Figure 3: The time dependent pressure profiles during the CO2 absorption processes with 11 bar and 25 mm height of solution as the initial pressure under different ultrasonic irradiation. Figure 4 shows the mass transfer coefficient and ultrasonic enhancement using solution with 25 mm height. It can be seen from Figure 4 that the intercept of ultrasonic power indicates the thresholds of the ultrasonic power for absorption process. The ultrasonic fountain might not be formed below the thresholds of ultrasonic power. It is attributed to the insufficient ultrasonic streaming velocity in order to overcome the surface tension of the solution for the formation of the ultrasonic fountain. For the case in which ultrasonic power higher than the threshold, ultrasonic fountain can be generated, and thus, a large amount of the liquid droplet can be produced. Subsequently, the absorption rate can be greatly enhanced. In order to determine the effect of the height of the solution on the efficiency of CO2 absorption rate, a comparison of the CO2 absorption rate between a 25 mm and 50 mm of height of solution was carried out. Figure 5 shows the comparison result of the absorption coefficient between the solution with the height of 25 mm and 50 mm. Under the ultrasonic power of 13.2 W, an absorption coefficient of 0.0470 kmol.m-3.h-1.kPa-1 is obtained using 25 mm height of solution. Lower absorption coefficient of 0.0388 kmol.m-3.h-1.kPa-1 is obtained using a higher height of solution (50 mm). This shows that lower height of solution can be more effective in the absorption of CO2 due to the shorter distance travelled by the ultrasonic irradiation. This is mainly due to the increasing ultrasonic threshold with the height of solution. The ultrasonic power threshold can be determined from the intercept of the ultrasonic power as shown in Figure 5. Referring to Figure 5, the ultrasonic fountain thresholds for the solution height of 25 mm and 50 mm are approximate to 1 W and 3 W. The increase in ultrasonic threshold with the increasing solution height is due to the reduction of the ultrasonic irradiation intensity, which is caused by the longer distance between ultrasonic transducer and the liquid surface. Therefore, the ultrasonic streaming velocity is reduced. This has resulted in a requirement of higher ultrasonic power in order to generate the ultrasonic fountain on the surface of solution. Less intensity of ultrasonic power is required when the solution height is lower due to efficient transduction of the ultrasonic power. 1 2 3 4 5 6 7 8 9 10 11 0 50 100 150 200 C O 2 P re ss ur e, b ar Time, s 13.2 W 9.2 W 5.9 W 3.3 W 1.5 W 1228 Figure 4: Mass transfer coefficient and ultrasonic enhancement of 25 mm height of solution at varied ultrasonic power. Figure 5: Comparison of absorption coefficient for 25 mm and 50 mm height of solution 4. Conclusion This study investigated the effect of ultrasonic irradiation under different intensity on the efficiency of CO2 absorption. The study also compared the efficiency of CO2 absorption between a high and low solution height. In conclusion, the effect of the solution height on the CO2 absorption has been investigated using 25 mm and 50 mm height of solution. The absorption rate was determined from the pressure drop profile. Based on the 0 10 20 30 40 50 0 10 20 30 40 50 60 0 2 4 6 8 10 12 14 E u s k g , m ol /m 3 . h. kP a Ultrasonic power, W 0 5 10 15 20 25 30 35 40 45 50 0 2 4 6 8 10 12 14 K g, m ol /m 3. h. kP a Ultrasonic power, W 25 mm 50 mm 1229 results, the CO2 absorption coefficient has been increased from 0.0388 to 0.0470 kmol.m-3.h-1.kPa-1 by decreasing the height of solution from 50 mm to 25 mm. This shows that the CO2 absorption coefficient is significantly affected by the height of solution of the ultrasonic fountain. The thresholds of the ultrasonic fountain have been decreased from approximate 3 W to 1 W by decreasing the height of solution from 50 mm to 25 mm. Overall, the height of the solution in the ultrasonic reactor is essential to be considered for efficient ultrasonic assisted absorption process. This study provides a better insight on the relationship among CO2 absorption efficiency, intensity of ultrasonic irradiation and the height of solution of the ultrasonic fountain. More systematically study is still required in order to optimise the CO2 absorption performance based on the effect of the solution height. Acknowledgments The financial and technical supports provided by Fundamental Research Grant Scheme (FRGS/1/2014/TK05/UTP/02/1) are duly acknowledged. Reference Astarita G., Savage D.W., Bistro A., 1983, Gas Treating with Chemical Solvents, John Wiley & Sons, New York, United States of America. Caplow M., 1968, Kinetics of carbamate formation and breakdown, Journal of The american Chemical Society 90, 6795-6803. Danckwerts P.V., 1979, The reaction of CO2 with ethanolamines, Chemical Engineering Science 34, 443-446. Elhajj J., Al-Hindi M., Azizi F., 2014, A Review of the Absorption and Desorption Processes of Carbon Dioxide in Water Systems Industrial & Engineering Chemistry Research 53, 2-22. Gondrexon N., Renaudin V., Boldo P., Gonthier Y., Bernis A., Petrier C., 1997, Degassing Effect and Gas-liquid Transfer in a high Frequency Sonochemical Reactor, Chemical Engineering Journal 66, 21-26. Laugier F., Andriantsiferana C., Wilhelm A.M., Delmas H., 2008, Ultrasound in gas–liquid systems: Effects on solubility and mass transfer, Ultrasonics Sonochemistry 15, 965-972. Rudenko O.V.a.S., A.A., 1998, Nonstationary Eckart Streaming and Pumping of Liquid in an Ultrasonic Field, Acoustical Physics 44, 565-570. Schueller B.S., Yang R.T., 2001, Ultrasound Enhanced Adsorption and Desorption of Phenol on Activated Carbon and Polymeric Resin, Industrial & Engineering Chemistry Research 40, 4912-4918. Tan L.S., Shariff A.M., Tay W.H., Lau K.K., Halim H.N.A., 2016, Integrated mathematical modeling for prediction of rich CO2 absorption in structured packed column at elevated pressure conditions, Journal of Natural Gas Science and Engineering 28, 737-745. Tay W.H., Lau K.K., Shariff A.M., 2016, High frequency ultrasonic-assisted CO2 absorption in a high pressure water batch system, Ultrasonics Sonochemistry 33, 190–196 Yasuda K., Bando Y., Yamaguchi S., Nakamura M., Oda A., Kawase Y., 2005, Analysis of concentration characteristics in ultrasonic atomization by droplet diameter distribution, Ultrasonics Sonochemistry 12, 37- 41. 1230