SOLUBILITY OF CARBON DIOXIDE IN AMINE BLEND: EFFECT OF CYCLICS AND AROMATICS

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1 SOLUBILITY OF CARBON DIOXIDE IN AMINE BLEND: EFFECT OF CYCLICS AND AROMATICS Channarong Wongboonma a, Raphael Idem b, Teeradet Supap b, Uthaiporn Suriyapraphadilok a,c, Chintana Saiwan* a a The Petroleum and Petrochemical College, Chulalongkorn University, Bangkok, Thailand b Clean Energy Technologies Research Institute, University of Regina, Canada c Center of Excellence on Petrochemical and Materials Technology, Bangkok, Thailand Keywords : CO 2 capture, Blended amines, CO 2 absorption rate, CO 2 desorption rate, and Heat duty ABSTRACT Nowadays, capture of CO 2 through chemical absorption using amine solution is the most reliable and economical post-combustion processes; however, the absorption performance is depended on the amine structure used. In this work, the effect of different structure of amine was studied for screening the amine in a blended system. 2M of selected cyclic (piperazine, 2-(1-piperazinyl)ethylamine or PZEA, 1-methylpiperazine, and 2-(1- piperazinyl)ethanol) and aromatic amines (pyridine, 2-aminopyridine, and 2- methylpyridine) were investigated for the CO 2 capture activities including equilibrium solubility, initial absorption rate, initial desorption rate, heat duty, heat of absorption, and basicity of amine (pka). The results show that cyclic amines have a good potential as a promoter for blended amine. However, aromatic amines were incapable of absorbing CO 2 due to the resonance stability. The effect of different substitution group (-OH, -CH 3, -NH 2 ) on CO 2 absorption was also investigated. Cyclic amine substituted by -NH 2 group shows the highest potential for promoter. Moreover, the selected cyclic amine PZEA was blended with conventional amine MDEA to improve the CO 2 capture performance. The results of the blended system help improve CO 2 capture performance compared with the single MDEA. *chintana.sa@chula.ac.th INTRODUCTION Global warming has high impact on environmental crisis and climate changes. Carbon dioxide (CO 2 ) is one of the greenhouse gases (GHGs) and the most unavoidable anthropogenic GHG contributing to the global warming, especially in sections of electricity, heat, and industry. Therefore, technologies for CO 2 capture and storage have been developed to reduce large amount of CO 2 emission from flue gases, such as oxyfuel combustion, pre-combustion, and post-combustion. Capture of CO 2 through chemical absorption using aqueous amine solution is known as one of the most reliable and economical post-combustion processes. Commercial amine solution used in absorption process can be primary, secondary and tertiary amines. For example, monoethanolamine (MEA) provides fast absorption rate; however, it is oxidative and degraded easily and the degradation products are corrosive. Other important disadvantages are solution losses through vaporization and high energy of regeneration. On the other hand, tertiary amines Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 1

2 require less heat than primary and secondary amines for regeneration (Filburn et al., 2005), lower corrosion, and lower vaporization loss; however, the reaction rate of tertiary amines is slow. Next development of chemical absorption is to use blended amines by combining advantages of each amine to compensate weaknesses of the others. Since properties of amine are related to their structures, it is interesting to study the effect of different structure of amine on CO 2 absorption activities, which can be later on used as a guide for selecting amine in a blended system. The purpose of this research was to investigate the effects of cyclic and aromatic amines and the selected cyclic or aromatic blended with conventional amine on the solubility of carbon dioxide and other CO 2 capture activities. Moreover, the effects of different substitution groups in cyclic and aromatic amines were also investigated. EXPERIMENTAL A. Chemicals and equipment Single amine solution, piperazine (99%), 1-methylpiperazine ( 99%), 2-(1- piperazinyl)ethanol ( 98%), 2-(1-piperazinyl)ethylamine (PZEA, 98%), pyridine (99.5%), 2-aminopyridine ( 98%), 2-methylpyridine ( 98%) was prepared with a total concentration of 2.0 M using deionized water. Standard hydrochloric acid (HCl) of 1 M with methyl orange indicator was used to determine the exact concentration of amine solution. Piperazine was obtained from ACROS ORGANICS while the other amines were obtained from Merck. For the absorption experiment, high purity grade carbon dioxide (99.99% CO 2 from Praxair Inc.) and nitrogen (99.99% N 2 from Praxair Inc.) were used as a feed gas. A water bath was used for heating the solution. Mass flow controllers (Electronic AALBORG GFC-17) were used for controlling the flow rates of the mixed CO 2 and N 2 gases. The concentration of CO 2 in the mixed gas stream was verified by a portable infrared (IR) CO 2 gas analyzer model 906 (Quantek Instruments, Inc.). The electrical hot plate Model no. C- MAGHS 10 was used for desorption experiment. AMTAST ph meter model PH900 was used to determine the ph of each amine solvent in pka determination experiment. B. Single amine study B.1 Absorption experiment Absorption experimental set-up is shown in Figure 1 for determining the initial CO 2 absorption rate and CO 2 solubility. The experiment was performed at 40 C and atmospheric pressure. The mixed gas line was connected to the saturation cell and absorption reactor, respectively. 15%vol CO 2 with N 2 balance was verified by a portable infrared (IR) CO 2 gas analyzer. The mixed gas was fed to the saturation cell then to the absorption reactor, respectively. During the absorption experiment, the amine solution was periodically sampled (1 ml) at 10-minute interval to analyze the CO 2 loading using Chittick apparatus. An absorption profile was plotted by CO 2 loading values against time. The absorption experiment was also used for investigating the heat of absorption (H abs ) of amine solution. The CO 2 solubility was determined at the temperature of 25, 40, and 60 C and the CO 2 partial pressure was varied from 3 to 100 kpa for each of the temperature condition. The solubility of CO 2 at various conditions was determined for estimating the heat of absorption using the Gibbs-Helmholz equation (Muchan et al., 2017a). Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 2

3 Figure 1 Schematic diagram of absorption experimental set-up. B.2 Desorption experiment Desorption experiment was used to obtain the desorption profile and to determine the initial desorption rate and heat duty. The desorption experiment was performed at 90 C immediately after the absorption experiment was done. 50 ml of equilibrium CO 2 absorption solution from the absorption experiment was added in desorption reactor then the desorption experiment was started. At the 4-minute interval, 1 ml of amine solution sample was taken to determine the CO 2 absorbed. Desorption profile is a plotted of CO 2 loading values against time. B.3 Amine dissociation constant (pka) The pka was determined by a titration technique. 100 ml of 0.05 M amine solution was titrated with standardized 1 M HCl and 0.10% methyl orange as indicator. Each 0.5 ml of standardized HCl solution was dropped into the amine solution and the ph of solution is determined by a ph meter until the end point is reached when the solution turns red. Finally, the recorded data were plotted and calculated the pka of amine solution following the half equivalence point method and Henderson Hasselbalch equation (Muchan et al., 2017b). C. Blended amine study The highest potential of cyclic or aromatic amines from single amine study was selected to use as a promoter in blended amine study. In this case, 2-(1-piperazinyl)ethylamine (PZEA) was selected to use as a promoter for blending with conventional amine N- methyldiethanolamine (MDEA, 99% purchased from Sigma-Aldrich). 30%wt was a total concentration which was selected based on a real process and some of previous works (El Hadri et al., 2014). The blended amines were studied with different ratio of MDEA/PZEA including 25%wt/5%wt, 20%wt/10%wt, and 15%wt/15%wt. All of experiments were performed using the same condition as the single amine study. The CO 2 absorption activities study included CO 2 solubility, initial absorption rate, heat duty, initial desorption rate, and heat of absorption. Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 3

4 RESULTS AND DISCUSSION A. Single amine study In this section, 2 M solution of the selected cyclic (piperazine, 2-(1-piperazinyl)ethylamine, 1-methylpiperazine, and 2-(1-piperazinyl)ethanol) and aromatic amines (pyridine, 2- aminopyridine, and 2-methylpyridine) were investigated for all of CO 2 capture activities (equilibrium CO 2 solubility, initial absorption rate, initial desorption rate, heat duty, heat of absorption, and pka). The results are summarized in Table 1. The results obviously show different CO 2 capture activities between cyclics and aromatics. The cyclics showed better CO 2 capture performance than the aromatics. The aromatic amines show low pka because of resonance stability of aromatic structure. The resonance effect delocalizes electron in the aromatic ring, thus, reduces alkalinity of aromatics and CO 2 solubility in the aromatic amines. As reported by Muchan et al., (2017a), a suitable pka value for CO 2 absorption should be higher than 7.5. The results indicated that the aromatics were not suitable to be a promoter. The cyclics showed good potential for use as a promoter. The different of CO 2 capture activities among the cyclics could be explained by different substitution group as shown in Figure 2. From the results in Table 1, PZEA showed the best performance among cyclic group in terms of initial desorption rate, heat duty, equilibrium CO 2 solubility, and initial absorption rate due to the substitution of -NH 2 in piperazine structure that provided more reactive site for CO 2. However, substituted in piperazine structure by -CH 3 can improve only desorption terms. Finally, substituted in piperazine structure by -OH decrease the CO 2 capture activities compared with piperazine because -OH group is considered as an electron withdrawing group (EWG), resulting in the reduction of the electron at the nitrogen atom. Therefore, PZEA was selected to use as a promoter in the blended amine study. Figure 2 Structure of cyclic and aromatic amines. Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 4

5 Table 1 Results of single amine study at 40 C, 15%vol CO 2 with N 2 balance in absorption experiment and 90 C in desorption experiment. Amine Solubility (mol CO 2 /mol amine) absorption rate desorption rate Heat duty Heat of absorption pka piperazine methylpiperazine (1-piperazinyl)ethylamine (1-piperazinyl)ethanol pyridine No absorption methylpyridine No absorption aminopyridine B. Blended amine study From the single amine study, PZEA was selected due to the best performance in initial desorption rate, heat duty, equilibrium CO 2 solubility, and initial absorption rate. However, PZEA is corrosive. To improve a conventional amine by using as a promoter, it reasonable to blended PZEA with conventional tertiary amine MDEA. MDEA is considered as a tertiary amine which require less heat than primary and secondary amines for regeneration, lower corrosion, and lower vaporization loss; however, the absorption rate of tertiary amines is slow due to its structure. MDEA doesn t have hydrogen atom connected with the nitrogen atom, resulting in slowly react with CO 2. Therefore, MDEA and PZEA were selected for blended amine study to improve the absorption capability in MDEA, still keeps good performance of desorption, and cope with corrosive problem. Table 2 The result of blend amine study at 40 C, 15%vol CO 2 with N 2 balance in absorption experiment and 90 C in desorption experiment. Amine Solubility (mol CO 2 /mol amine) absorption rate Desorption rate Heat duty Heat of absorption 30 %wt MDEA %wt PZEA %wt MDEA + 5 %wt PZEA 20 %wt MDEA + 10 %wt PZEA Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 5

6 15 %wt MDEA + 15 %wt PZEA The results of all activities of blended amine are shown in Table 2. The results indicated that the increase of PZEA ratio in the blended amine helps improve all of CO 2 absorption activities of MDEA, especially in CO 2 solubility and initial absorption rate which are the weak point of tertiary amine. Moreover, the initial desorption rate and heat duty which are the important factors for minimizing the operating cost of absorption technology are also improved. CONCLUSIONS By investigation of CO 2 capture activities in single and blend amine study, the aromatic amines were not suitable to be a promoter in blended amine while cyclic amines showed better results. PZEA showed the highest potential among cyclic amines. Moreover, using PZEA as a promoter in blended amine helped improve CO 2 capture performance in conventional amine MDEA. ACKNOWLEDGEMENTS The authors would like to thank the Petroleum and Petrochemical College, Chulalongkorn University for the research support and Clean Energy Technologies Research Institute, University of Regina, Canada. REFERENCES El Hadri, N., Quang, D.V., Rayer, A.V. and Abu-Zahra, M.R.M. (2015). Development of Amine-blend Systems for CO 2 Post-Combustion Capture Filburn, T., Helble, J.J., and Weiss, R.A. (2005). Development of supported ethanolamines and modified ethanolamines for CO 2 capture. Industrial & Engineering Chemistry Research, 44, Muchan, P., Saiwan, C., Narku-Tetteh, J., Idem, R., Supap, T. and Tontiwachwuthikul, P. (2017a). Screening tests of aqueous alkanolamine solutions based on primary, secondary, and tertiary structure for blended aqueous amine solution selection in post combustion CO 2 capture. Submitted to Chemical Engineering Science. Muchan, P., Narku-Tetteh, J., Saiwan, C., Idem, R. and Supap, T. (2017b). Effect of number of amine groups in aqueous polyamine solution on carbon dioxide (CO 2 ) capture activities. Separation and Purification Technology 184, Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 6