Format And Type Fonts CCHHEEMMIICCAALL EENNGGIINNEEEERRIINNGG TTRRAANNSSAACCTTIIOONNSS VOL. 39, 2014 A publication of The Italian Association of Chemical Engineering www.aidic.it/cet Guest Editors: Petar Sabev Varbanov, Jiří Jaromír Klemeš, Peng Yen Liew, Jun Yow Yong Copyright © 2014, AIDIC Servizi S.r.l., ISBN 978-88-95608-30-3; ISSN 2283-9216 DOI: 10.3303/CET1439097 Please cite this article as: Arshad M., Wukovits W., Friedl A., 2014, Simulation of CO2 absorption using the system K2CO3- piperazine, Chemical Engineering Transactions, 39, 577-582 DOI:10.3303/CET1439097 577 Simulation of CO2 Absorption Using the System K2CO3- Piperazine Muazzam Arshad*, Walter Wukovits, Anton Friedl Vienna University of Technology, Institute of Chemical Engineering, Getreidemarkt 9/166-2, 1060 Vienna, Austria muazzam.arshad@tuwien.ac.at CO2 is one of the main sources of global warming affecting the environment. It is one of the biggest challenges of future to reduce CO2 emissions in addition to capture and disposal of CO2 produced as a result of burning of fossil fuels. In this work, CO2 removal from biogas by absorption process is studied. For CO2 absorption from biogas, aqueous K2CO3 and piperazine solvents are used independently and then ultimately in a combination. Equilibrium studies are done to calculate capabilities of simulation tool Aspen Plus® towards the component systems CO2/H2O/PZ, CO2/H2O/K2CO3 and finally CO2/H2O/PZ/K2CO3. Validation and comparisons of equilibrium studies were made with literature data. Furthermore, all the mentioned systems were then compared using typical parameter ranges for biogas composition and conditions. The effect on absorption loading capacity of CO2 was studied by varying parameters of temperature, pressure and biogas composition. 1. Introduction Main causes of greenhouse effect are emissions of harmful gases and oxides of different components such as oxides of carbon. Clean and sustainable energy is an important demand of future. Thus, worldwide a lot of effort is being put into production of clean energy. Investigations include improving already employed processes as well as developing new processes of producing clean and sustainable energy. Large emphasis is put in the use of biofuels to substitute fossil energy carriers. In the course of rising oil and gas prices as well as global warming, biofuels are gaining further attention. Producing biogas out of biomass and agricultural residues is a widespread and simple option of biofuel production. During the last years, a trend towards utilization of pure CH4 can be observed instead of production of just heat and power from biogas. To obtain pure CH4 from biogas, CO2 has to be removed. In this way purified CH4 can be obtained to be fed to an existing natural gas grid or to be used as chemical for synthesis or use as vehicle fuel (Niesner, 2013). Several processes are available for CH4/CO2 separation such as adsorption, membrane separation or absorption. In this work, removal of CO2 by absorption will be studied. There have been several works done in the development of processes to remove CO2 from exhaust gases of industries as well as from furnaces burning fossil fuels. Several solvents have been studied for the removal of CO2. Most important solvent for commercial use in absorption of CO2 on large scale is MEA or comparable amines. However, these solvents are not suitable for CO2 removal in small scale biogas facilities. Here, other solvents have been suggested, e.g. pressurized water or cold potassium carbonate solution. Unfortunately, potassium carbonate has a low CO2 absorption rate at low temperatures. However rate of absorption can be enhanced by piperazine as an activator (Cullinane, 2005). Several previous studies investigate the effect of piperazine on the solubility of CO2 for mixtures with other amines like MDEA. In this work, potassium carbonate and piperazine solutions are studied individually and later on in a combination of both for their performance in biogas upgrading. Based on experimental investigations, process simulation in the commercial simulation environment Aspen Plus® will be used to develop and design a dedicated absorption-desorption cycle for biogas upgrading in small scale. 578 In a first step, equilibrium studies are performed to investigate solubility of carbon dioxide in potassium carbonate, piperazine and the mixture of potassium carbonate and piperazine at equilibrium conditions as basis for process development. 2. Absorption systems 2.1 K2CO3 The use of K2CO3 as CO2 absorbent has been recognized since early 1900’s. Commonly absorption by potassium carbonate can be represented by the following overall reaction (Kothandaraman, 2010): 32322 2KHCOOHCOKCO  (1) The above reaction can be broken down in the following steps:   KCOCOK 2 2 332 (2)   KHCOKHCO 33 (3)   KOHKOH (4)   OHHCOOHCO 3322 2 (5) 2 3332   COOHHCOOH (6)   OHOHOH 32 2 (7) For treatment of gas at high temperatures, K2CO3 is effective. It has been used previously in hot potassium carbonate process (Benfield) for removal of CO2 and acid gases from ammonia synthesis gas and natural gas etc. (Cullinane, 2005). Solubility of KHCO3 can be increased by high temperature of absorber, which allows operation with highly concentrated solution. K2CO3 solutions offer advantages for absorption of CO2.of having low heat duty in reboiler as it requires less energy to regenerate. But low solubility and slow rate of absorption at a lower temperature, make the solvent unsuitable in cases where higher purity is required. However, these limitations can be solved by adding amines such as piperazine, which act as an activator or promoter to increase rate of CO2 absorption. 2.2 Piperazine Equilibrium reaction of piperazine with carbon dioxide can be described by the following mechanism (Hilliard, 2008):   HCOHOHCO 3222 (8)   HCOHCO 2 33 (9)   OHHOH 2 (10)   PZHHPZ (11) 2 2   PZHHPZH (12) OHPZCOOHCOPZ 23   (13) OHCOOPZHCOPZCOO 2 2 3 )(   (14)   COOPZHHPZCOO (15) Piperazine is a cyclic diamine (Bougie, 2011) making it capable of reacting twice with CO2 forming carbamate and bicarbamate species shown in reactions, Eq(12) and Eq(13). Reaction, Eq(13), formation of bicarbamate might be slower than former reaction, but it increases the capacity to capture CO2. Piperazine has a high rate of absorption and can thus be used as a promoter with K2CO3 to increase rate of absorption. 3. Model Simulation models were developed in Aspen Plus® based on Aspen example files using Aspen Plus example files for independent systems of piperazine (AspenTech, 2010) and K2CO3 (AspenTech, 2012), respectively, using ENRTL thermodynamic model with apparent component approach. Flash column was used for performing equilibrium calculations. For the combined system, piperazine model was taken as a base for manual modification since Aspen Plus® Electrolyte Wizard was not able to properly generate the electrolyte system for aqueous piperazine. Besides manual addition of components in the system, such as, 579 K2CO3, KHCO3 and KOH, additional reactions for K2CO3 were manually added to the chemistry section of the model. Furthermore, some binary interaction parameters such as Henry coefficients and electrolyte pair parameters were altered. These parameters were taken from K2CO3 example model (AspenTech, 2012). Design specifications were used to achieve a certain partial pressure of CO2 by varying total pressure of the flash column to observe corresponding CO2 loading in liquid stream. Also design specifications were included to maintain original concentration of piperazine and K2CO3 in liquid streams to ensure equilibrium conditions. 4. Results As prerequisite for a proper simulation of the upgrading process, correct representation of physical and thermodynamic properties in Aspen Plus® has to be ensured. In a first step, equilibrium studies are done to investigate solubility of carbon dioxide in potassium carbonate, piperazine and the mixture of potassium carbonate and piperazine at equilibrium conditions as basis for process development. Different data sets from literature were analysed and compared before selecting trustable data to validate Aspen Plus® simulations. In Figure 1, comparison of Aspen Plus® simulations with literature is done for a fixed concentration of 0.6 mol/kg piperazine at different temperatures to show the trend of equilibrium curves. It can be observed, that for a fixed concentration, higher loading capacity is obtained for lower temperatures. Similarly, in Figure 2, Aspen Plus® simulations are compared with literature data at a fixed temperature of 298 K for different concentrations of piperazine, showing a good fit, with exception of a few outliers at low partial pressure of CO2. Figure 1: Comparison of Aspen Plus® simulations with different literature data for 0.6 mol/kg aq. Piperazine solution; ■ Derk (298 K); ♦ Derk (313 K); ▲ Derk (343 K); + Bishnoi (343 K); ▬ Aspen Plus® (298 K,313 K,343 K) Figure 2: Comparison of Aspen Plus® simulations with literature data for different aq. Piperazine concentrations at 298 K; ♦ Derk (0.6 mol/kg); + Francis (0.6 mol/kg); ▲ Derk (0.2 mol/kg);■ Francis (0.1mol/kg); ▬ Aspen Plus® (0.1 mol/kg,0.2 mol/kg,0.6 mol/kg) Figure 3: Aspen Plus® simulations vs literature data for different concentrations of aq. K2CO3 solution at 343 K; ♦ Tosh (20 wt%); ■ Tosh (30 wt%); ▲ Tosh (40 wt%); ▬ Aspen Plus® (20 wt%,30 wt%,40 wt%) Figure 4: Comparison of Aspen Plus® simulations for 20 wt% aq. K2CO3 solution at different temperatures; ♦ 298 K; ■ 303 K; ▲ 343 K 580 Figure 5: Aspen Plus® simulation vs literature data of Hilliard NTNU for 3 mol/kg K2CO3/1.2 mol/kg PZ at different temperatures; ♦ Hilliard NTNU at 120 °C; ▲ Hilliard NTNU at 100 °C; ▬ Aspen Plus® (120 °C,100 °C) Figure 6: Aspen Plus® simulation vs literature data of Hilliard NTNU for different concentrations of solvents at 100 °C; ♦ 1.8 mol/kg K2CO3/0.6 mol/kg PZ; ▲ 3 mol/kg K2CO3/1.2 mol/kg PZ; ■ 2.5 mol/kg K2CO3/2.5 mol/kg PZ; ▬ Aspen Plus® (1.8 mol/kg K2CO3/0.6 mol/kg PZ, 3 mol/kg K2CO3/1.2 mol/kg PZ; 2.5 mol/kg K2CO3/2.5 mol/kg PZ) In Figure 3, calculated data show good agreement with literature data by Tosh et al (1959) for different concentrations of K2CO3 at 343 K. There are some outliers for 20 wt% K2CO3 but overall the match is quite reasonable. Comparisons were also made with literature data by Park et al (1997), not presented in this paper, showing reasonable fit except some deviation at 323 K. In Figure 4, Aspen simulations of equilibrium curves are shown for 20 wt% aqueous K2CO3 solution for different temperatures. For combined system of K2CO3+Piperazine, literature data were taken from Hilliard (2008), in which results from two different sets of experiments were given. One set of experiments was performed in University of Texas, Austin (UT) and the other set of experiments was performed in Norges Teknisk- Naturvitenshapelige Universitet (NTNU). The results of the system for the ENRTL model show a good fit, with exception of a few outliers at low partial pressure of CO2 as is observable in Figure 6. Though Aspen simulations match very well with the literature data, there are some deviations at low partial pressures of CO2 showing volatility of piperazine at low pressure. Furthermore, the combined model was validated by calculating equilibrium loadings for independent systems of K2CO3 and piperazine and verifying by literature (not shown in the paper). The equilibrium model simulations show a good agreement and can be used to predict the simulations at different process conditions. 4.1 Comparison of absorption systems Comparisons were then made to study the different solvent systems and analyze them. A random concentration of 3 mol/kg K2CO3 and 1.2 mol/kg piperazine were used for the study. Analysis was made by varying different parameters to see their effect on the absorption capacity at equilibrium. To study the systems, parameter ranges were chosen which could be interesting for biogas upgrading (Table 1). The parameters which were used for the study were temperature, composition of feed gas (amount of CO2 present in biogas), total pressure and equilibrium partial pressure of carbon dioxide. For instance, temperature range was taken considering the temperature range used for fermentation in production of biogas. Table 1: Typical ranges of parameters for biogas and parameter ranges chosen for simulation study Parameter Typical Range of biogas Range chosen Reference Temperature Thermophillic fermentation: upto 298 K 298-333 K Hilby (2014) Mesophillic fermentation: 305-315 K Hilby (2014) Thermophyllic fermentation: 323-331 K Hilby (2014) Composition CH4: 50-70 % CH4: 50-70 % Mursec (2009) CO2: 25-50 % CO2: 28-50 % Mursec (2009) N2: The rest Pressure 1-3 bar 581 Figure 7: Change in loading with varying temperature of flash column; ♦ 3 mol/kg K2CO3/1.2 mol/kg PZ; ■ 3 mol/kg K2CO3; ▲ 1.2 mol/kg PZ Figure 8: Change in loading with varying CO2 concentration in biogas; ♦ 3 mol/kg K2CO3/1.2 mol/kg PZ; ■ 3 mol/kg K2CO3; ▲ 1.2 mol/kg PZ Figure 9: Change in loading with varying pressure of flash column; ♦ 3 mol/kg K2CO3/1.2 mol/kg PZ; ■ 3 mol/kg K2CO3; ▲ 1.2 mol/kg PZ Figure 10: Change in loading with varying partial pressure of CO2; ♦ 3 mol/kg K2CO3/1.2 mol/kg PZ; ■ 3 mol/kg K2CO3; ▲ 1.2 mol/kg PZ Figure 7 represents changes in loading of carbon dioxide in liquid stream with change in temperature of flash column. We can observe that as expected, at constant pressure and biogas concentration, increasing temperature reduces carbon dioxide loading capacity. In Figure 8, it can also be seen that for a constant temperature and pressure, an increase in CO2 concentration in fed biogas leads to an increase in CO2 loading and hence its removal. Thus, higher initial concentration of CO2 results in higher loading. Furthermore, as expected, increase in equilibrium partial or total pressure leads to an increase in CO2 loading capacity for constant temperature and initial concentrations of CO2 in biogas as shown in Figure 9 and Figure 10. It can be seen that equilibrium curve for the combined system is always to the left showing lower loading and the piperazine curve is always to the right showing higher loading confirming the presence of carbamate formed by piperazine. The excess CO2 in piperazine curves can be explained by physically absorbed CO2 with piperazine. It is the usual trend seen for amines that first CO2 is absorbed chemically, but when all solvent is spent in chemical reactions, CO2 cannot be anymore absorbed chemically but has to be dissolved physically (Ermatchkov, 2006). This results in higher loadings in liquid stream. The comparison and behavior has to be studied and analyzed further. 5. Conclusion This work presents equilibrium data for CO2 absorption in aqueous solutions of piperazine and K2CO3. Literature data are compared with results from calculations with Aspen Plus® ENRTL property method. Calculations are performed in a flash unit at equilibrium conditions considering the involved electrolyte system. Adjustment of property parameters gives a proper representation of the component system CO2- K2CO3-Piperazine-H2O. Data from Aspen Plus® simulations match very well with literature data in case of solubility of piperazine solution with less than 4 % deviation. Deviations can however be observed at some temperatures with 582 some literature data probably related with experimental inaccuracies. Data calculated for K2CO3 match even better with less than 0.05 % deviation. For aqueous K2CO3-piperazine solutions, simulations show good match with literature data apart from some deviation at very low partial pressures of CO2 probably due to vapor pressure (volatility) of piperazine. But at higher pressures, the calculated data fit well with the literature data. This shows that the Aspen Plus® model can be used to predict CO2 absorption in equilibrium environment using K2CO3-piperazine as solvent. The presented equilibrium calculations will form the basis for rate based absorption studies in Aspen Plus® and thus for a detailed design of the absorption and desorption steps in the biogas upgrading process with aqueous K2CO3-piperazine solutions. 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