Solubility of CO2 in Amino-Acid-Based Solutions of (Potassium

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Solubility of CO2 in Amino-Acid-Based Solutions of (Potassium Sarcosinate), (Potassium Alaninate + Piperazine), and (Potassium Serinate + Piperazine) Dongwoo Kang,† Sangwon Park,† Hoyong Jo,† Jaehong Min,† and Jinwon Park*,†,‡ †

Department of Chemical & Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, Republic of Korea ‡ R&D Planning Team, Biomass and Waste Energy, Korea Institute of Energy Technology Evaluation and Planning (KETEP), Teheran-ro 114gil 14, Gangnam-gu, Seoul 135-502, Republic of Korea

ABSTRACT: Amino acid salt solutions are useful as carbon dioxide absorbents. In this study, vapor−liquid equilibrium (VLE) experiments were conducted using 4 M K-SAR, 1.5 M K-ALA-PZ, and 1.5 M K-SER-PZ solutions. Partial pressures and solubilities were measured for each absorbent solution at temperatures of 313.15 K, 333.15 K, and 353.15 K. The range of CO2 partial pressure measured at 313.15 K over 4 M K-SAR was from 0 kPa to 812.5 kPa with loadings from 0.06 to 0.98. The partial pressures of CO2 ranged from 0.2 kPa to 1041.7 kPa with loadings from 0.1 to 1.09 at 313.15 K over 1.5 M K-ALA-PZ absorbent solution. The partial pressures of CO2 ranged from 0 kPa to 665.5 kPa with loadings from 0.1 to 1.14 at 313.15 K over 1.5 M KSER-PZ absorbent solution.

1. INTRODUCTION The amount of energy used by human beings has drastically increased since the mid-18th century when the Industrial Revolution started and energy was obtained by burning fossil fuels like coal, petroleum, and natural gas. In the present day, global industries also rely on fossil fuels to get power and generate electricity, and about 80 % of worldwide energy demand is supplied by fossil fuels.1 There is significant amount of greenhouse gases contained in the flue gases discharged after combustion of fossil fuels. Greenhouse gases have a great impact on global warming. The representative substances of greenhouse gases are water vapor, carbon dioxide, chlorofluorocarbons (CFCs), methane, nitrogen oxides, and sulfur oxides. About 80 % of greenhouse gases emitted are produced when fossil fuels combusted,2and it is known that their level of contribution to global warming is approximately 61 %.3 As industries are developed to higher degrees, the amount of energy consumption increases continuously, and therefore, the amount of greenhouse gas emitted is increasing. This results in the increase of concentration of greenhouse gas in the atmosphere. So arguments for environmental problems such as alteration of ecosystem and coastline due to the global warming have been animated by degrees. For instance, there are the Climatic Change Convention which is a global argument for reduction of greenhouse gases and Kyoto Protocol which is the detailed method. In the Kyoto Protocol, the desired value of reduction of greenhouse gases for © 2013 American Chemical Society

developed countries is stipulated, and the member countries directly involved should take steps to follow the policies including improvement of energy efficiency, protection of greenhouse gas sinks and storing places, and research for new regeneration energy. Although the consumption of fossil fuels has to be restrained so as to reduce the production of greenhouse gases by the roots, it is practically difficult considering economic feasibility since most of power plants are based on fossil fuels and operated continuously. However, if the greenhouse gases contained in flue gas can be captured and stored effectively, the level of contribution of greenhouse gases to global warming will be much decreased. For the part of these efforts, research for reduction of carbon dioxide is conducted globally. The reactive absorption technique using liquid absorbent is in progress among several techniques to reduce carbon dioxide. Its merits are that a great deal of research for liquid absorbent has been conducted and its applicability to the current operating facilities. Up to the present, research for the liquid absorbent affiliated with alkanolamines such as MEA (monoethanolamine), DEA (diethanolamine), and MDEA (methyldiethanolamine) has been conducted. Although a primary alkanolamine such as Received: February 25, 2013 Accepted: May 3, 2013 Published: May 14, 2013 1787

dx.doi.org/10.1021/je4001813 | J. Chem. Eng. Data 2013, 58, 1787−1791

Journal of Chemical & Engineering Data

Article

MEA shows fast reaction kinetics with CO2, the regeneration energy is very high (4 GJ/ton CO2). Also, for secondary and tertiary alkanolamines such as DEA and MDEA, respectively, it is hard to apply in practice since the absorption rate is slow. Above this, there are more reasons that make using an alkanolamine absorbent in real processes hard. To begin with, these substances are toxic, and they can cause environmental problems. Second, even though absorbents affiliated with alkanolamines shows low volatility to some extent, they are required to have lower volatility to prevent absorbent loss so as to be applied to real processes. Third, when these absorbents are contacted with flue gas stream repeatedly, degeneration can easily occur by heat, oxygen, and sulfur dioxide contained in it. Also, these substances are highly corrosive and can corrode equipment if they are used as carbon dioxide absorbents in large-scale facilities. Therefore, research about new carbon dioxide absorbents has been conducted, and among them, amino acid salt solutions are attracting interest.4 First of all, amino acids as substances which exist in human body are nontoxic and eco-friendly. Second, when potassium hydroxide or lithium hydroxide is added to amino acids, the carboxylic group is neutralized, making solutions have an ionic structure with lower volatility than the alkanolamine type absorbents. This leads to a carbon dioxide absorption system which has better economic feasibility because it prevents losing absorbent when s desorption reaction occurs at high temperature and low pressure when applied to industrial facilities such as power plants. Also, the absorption performances of amino acid salts are similar to those of alkanolamines since amino acid salts and alkanolamines have same functional group. It is also known that, when promoters or enhancers such as piperazine (PZ) is added to the solution, their heat-resisting and antioxidation properties are improved.5 Also, amino acid salt absorbents are suitable for industrial application because they are less erosive than the alkanolamine absorbents. Rao and Rubin showed that an absorption system based on an amine is appropriate for combustion power plants through economical evaluation.6 For sterically hindered amino acid salts which have bulky substituents, initial absorption occurs slowly because it is hard for carbon dioxide molecules to approach amino groups and react with it. However, when a small amount of piperazine is added, it acts as a rate promoter and improves the initial absorption rate.4 Since amines which have bulky substituents requiring less regeneration energy at the desorption process, it is economical and applicable to industry. Another merit of amino acid salt solutions as carbon dioxide absorbents is that they have higher surface tension than that of MEA. One of the methods of removing acidic gases is using porous hollow fiber membrane. At the system, there exist absorbents at one side and flowing acidic gas at another side divided by the membrane which is porous and has large surface area. A gas is removed by mass transfer through the gas−liquid interface at the membrane. When absorbents which have lower surface tension are used, a wetting problem may occur as absorbents permeate into membrane, disturbing mass transfer.7 It is reported that, when amino acid salt solutions are used as an absorbent, it resulted in a drier polypropylene microporous membrane.8 Also, it is known that surface tension does not decrease significantly when a small amount of piperazine is added to water.9 Also, amino acid salt solution shows better absorption performance compared to MEA and MDEA at specific conditions.10

In this way, amino acid salt has potential as carbon dioxide absorbent since it has lots of merits not only on performance but also in the view of environmental, economical, and practical aspects. In our previous research, screening tests for 16 common amino acids mixed with PZ were conducted in which sarcosine, alanine, and serine showed good absorption characteristics.4 In this study, vapor−liquid equilibrium (VLE) experiments were conducted using aqueous solutions of potassium sarcosinate (K-SAR), potassium alaninate mixed with piperazine (K-ALA-PZ), and potassium serinate mixed with piperazine (K-SER+PZ) at 313.15 K, 333.15 K, and 353.15 K, respectively. However, sterically hindered amino acids like ALA and SER were first mixed with PZ to increase CO2 absorption reactivity.13 The data set obtained from this experiment can help develop a carbon dioxide reduction system and simulate it.

2. EXPERIMENTAL SECTION 2.1. Materials. Serine (purity > 0.99 mass fraction, CAS 5645-1) and sarcosine (purity > 0.98 mass fraction, CAS 107-971)

Figure 1. Molecular structures of serine, alanine, and sarcosine.

Table 1. Physical Properties of Absorbent Solutions solution

Ta/K

densityb/(g·cm−3)

mol aminec/mol

4 M K-SAR 1.5 M K-ALA-PZ 1.5 M K-ALA 1.5 M K-SER-PZ 1.5 M K-SER

303.15 303.15 303.15 298.15 298.15

1.1852 1.0826 1.0784 1.1060 1.0687

0.8610 0.5855 0.3499 0.5731 0.3425

The uncertainty in the temperature is ± 0.1 K. bThe uncertainty in the density is 0.1 % cThe uncertainty in the moles of amine is 0.001. a

used in this experiment were prepared by Acros Organics and used without further purification. Alanine (purity > 0.99 mass fraction, CAS 302-72-7) was prepared by Alfa-Aesar and used without further purification. Figure 1 shows the molecular structures of serine, alanine, and sarcosine. A 8 N KOH solution used for making absorbent solutions was prepared by Acros-Organics. PZ (purity > 0.99 mass fraction, CAS 110-85-0) was purchased from Acros-Organics and used as a rate promoter to increase the initial absorption rate of alanine and serine, which are sterically hindered amines with bulky substituents. For the experiments, 4 M K-SAR, 1.5 M K-ALA-PZ, and 1.5 M K-SER-PZ absorbent solutions were prepared. When preparing the absorbent solutions, amino acid was dissolved in a small amount of deionized distilled water, and 8 N KOH solution was added dropwise using a buret to achieve a 1:1 stoichiometric ratio of amino acid and KOH to convert the amino acid into a potassium salt. 2.2. Experimental Setup and Procedure. Overall, nine experiments were conducted for three kinds of absorbent 1788



Article

Figure 2. Schematic of experimental apparatus.

With the P−T data obtained from each experiment and the Pitzer correlations for the second virial coefficient equation, the solubility (α) and partial pressure of CO2 was determined or calculated using correlations suggested below and calculating pressure difference.

solutions (K-SAR, K-ALA-PZ, K-SER-PZ) at three temperatures (313.15 K, 333.15 K, 353.15 K). Detailed procedures are given below. First, 250 g of absorbent solution was injected into the reactor through an absorbent inlet line. Then, the air in the reactor was removed using a vacuum pump. The pressure and temperature inside the reactor were recorded. The temperature of the air bath was set, and the stirring speed was set to 70 rpm. After temperature inside a reactor has reached a set value, the pressure inside the reactor at this particular temperature represents the vapor pressure which is considered while measuring solubility and partial pressure of CO2. Then an appropriate amount of carbon dioxide gas was injected into the cylinder. Over time, the reactor reached vapor−liquid equilibrium and stabilized. The interior of the cylinder also reached equilibrium and stabilized. When the reactor and cylinder reached equilibrium, the pressure and temperature remained constant unless the system was disturbed. The pressure and temperature inside the reactor and cylinder were recorded. Stirring was paused, and the valve connecting the cylinder and reactor was opened to inject carbon dioxide gas from the cylinder into the reactor. The valve was closed to stop the injection of gas, and the pressure and temperature inside the cylinder were recorded. To promote CO2 absorption, the stirring speed was set to 400 rpm. Since the reaction between amino acid salts and CO2 is exothermic, the temperature inside the reactor increased after injecting CO2. However, over time, the system inside the reactor again reached equilibrium at the set temperature and new pressure. If pressure and temperature remained constant, the system was considered to have reached a new equilibrium. The stirring speed was then set to 70 rpm. Once the system stabilized, the pressure and temperature inside the reactor were recorded. This procedure was repeated to obtain data of the temperature and pressure changes inside the reactor and cylinder. Using the same procedure, analyses of other absorbent solutions at other temperatures were conducted.

α=

mol of CO2 absorbed mol of amino acid salt absorbent

(1)

Z=

BP P P PV BP =1+ = 1 + B̂ r = 1 + c r RT RT Tr RTc Tr

(2)

B̂ = B0 + ωB1 B0 = 0.083 − B1 = 0.139 −

(3)

0.422 Tr1.6

(4)

0.172 Tr 4.2

(5)

where α is the solubility, Z is a compressibility factor, and B0 and B1 are parameters for the second virial coefficient equation. Tr is the reduced temperature calculated by Tr = T/Tc, where Tc is the critical temperature. Pr is the critical pressure, which is calculated by Pr = P/Pc, where Pc is the critical pressure and ω is an acentric factor. For carbon dioxide, the acentric factor ω is 0.224, the critical temperature Tc is 304.2 K, and the critical pressure Pc is 73.83 bar.11 Each absorbent solution was prepared at the temperature indicated in Table 1. Densities were measured using a Gay− Lussac pycnometer (36.940 cm3) and calibrated using degassed distilled water. The temperature was maintained by using the thermostatic water bath (C-WBA3, Chang Shin Scientific Co.) with an accuracy of ± 0.1 K. The precision of the balance used was 1·10−4 g. The density measurement was performed with a 0.1 % accuracy. 1789



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The moles of amine suggested in the Table 1 was estimated by the summation of the moles of amino acid salt and moles of PZ (only moles of amino acid were estimated for 4 M K-SAR solution since it contained no PZ). The uncertainty of the moles of amine was 0.001. The number of moles of amine contained in 250 g of absorbent solution was calculated using the mass, concentration, and density of solution. Also, the apparatus (Figure 2) was similar to that used by Song et al.12 The apparatus which was used to measure carbon dioxide solubility in the absorbent solutions consists of gas cylinder and reactor. The volumes of gas cylinder and reactor were 3960 cm3 and 503 cm3, respectively. The pressure inside the cylinder and reactor was measured by Wallace and Tiernan precision mercury manometer with an accuracy of ± 0.1 kPa. The temperature was measured by using a Pt-100 temperature probe with an accuracy of ± 0.1 K. The pressure and temperature of the reactor was continuously recorded by computer connected to the apparatus. The experimental apparatus and measurement methods are the same with our previous study.12 The uncertainty of the CO2 solubility measurement was 2.5 %.

Figure 3. Solubility of CO2 in 4 M potassium sarcosinate solution at temperatures of 313.15 K, 333.15 K, and 353.15 K.

Table 3. Solubility of CO2 in 1.5 M K-ALA-PZ(aq) solubility of CO2 in 1.5 M K-ALA-PZ(aq) 313.15 K

3. RESULTS The solubility of CO2 measured in 4 M potassium sarcosinate solution is shown in Table 2 and Figure 3. As seen in Figure 3,

α

a

0.1033 0.2090 0.3306 0.4462 0.5869 0.7238 0.8652 0.9752 1.0391 1.0717 1.0903

Table 2. Solubility of CO2 in K-SAR(aq) solubility of CO2 in K-SAR(aq) 313.15 K α

a

0.0915 0.1835 0.2732 0.3581 0.4386 0.5140 0.5907 0.6691 0.7488 0.8264 0.8910 0.9330 0.9583

333.15 K b

pCO2 /kPa

0 0.2 0.2 0.2 0.4 1.6 8.8 31.7 69.2 145.2 298.2 517.4 716.1

α

a

0.0554 0.1106 0.1649 0.2187 0.2726 0.3206 0.3681 0.4124 0.4590 0.5079 0.5651 0.6243 0.6835 0.7392 0.7868 0.8243 0.8519 0.8740

353.15 K

pCO2 /kPa

α

pCO2b/kPa

0 0 0 0 0 0 0 0.4 1.2 3.9 12.9 36.5 83.4 165.6 284.1 423.6 567.6 676.8

0.0629 0.1091 0.1644 0.2225 0.2791 0.3394 0.4073 0.4754 0.5431 0.6072 0.6667 0.7170 0.7556 0.7854 0.8114 0.8428

0.6 0.8 0.8 1.6 1.6 2.4 3.8 8.8 24.6 68.6 153.9 281.1 429.4 577.8 731.5 938.4

b

a

333.15 K

pCO2 /kPa

α

0.2 0.3 0.3 0.8 1.8 9.6 65.9 312.9 690.0 917.4 1041.7

0.0878 0.1865 0.2864 0.3852 0.4823 0.5826 0.6849 0.7764 0.8583 0.9213 0.9612 0.9887 1.0339

b

a

353.15 K

pCO2 /kPa

α

0 0 0 0 0.8 4.5 17.2 53.3 142.7 298.3 462.5 619.0 775.5

0.0784 0.1555 0.2330 0.3124 0.3945 0.4769 0.5633 0.6482 0.7279 0.8000 0.8579 0.8993 0.9298 0.9540

b

a

pCO2b/kPa 0.6 0.8 1 1 1 2.8 10.8 28.9 71.5 156.5 288.9 434.4 581.7 709.8

The uncertainty in the solubility α is 0.001. bThe uncertainty in the pCO2 is 0.2 kPa. a


as temperature increased, the solubility of CO2 decreased at a constant CO2 partial pressure. The solubility of CO2 measured in 1.5 M potassium alaninate + 1 M PZ solution is shown in Table 3 and Figure 4. The solubility of CO2 measured in 1.5 M potassium serinate + 1 M PZ solution is shown in Table 4 and Figure 5. Also, comparison for CO2 solubility of each absorbent was performed at 15 kPa which is the partial pressure of carbon dioxide gas produced at normal power plant.

Figure 4. Solubility of CO2 in 1.5 M potassium alaninate + 1 M PZ solution at temperatures of 313.15 K, 333.15 K, and 353.15 K.

At temperatures of 313.15 K and 333.15 K, (K-ALA+PZ) absorbent solution showed highest solubility among three types 1790



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Table 4. Solubility of CO2 in 1.5 M K-SER-PZ(aq) 333.15 K

AUTHOR INFORMATION

Corresponding Author

solubility of CO2 in 1.5 M K-SER-PZ(aq) 313.15 K

Article

*Tel.: +82-2-364-1807. Fax: +82-2-312-6401. E-mail: jwpark@ yonsei.ac.kr.

353.15 K

αa

pCO2b/kPa

αa

pCO2b/kPa

αa

pCO2b/kPa

0.1010 0.2065 0.3184 0.4332 0.6031 0.7345 0.8606 0.9588 1.0210 1.0624

0 0.2 0.2 0.6 2 13.3 77.4 265.1 521.5 702.2

0.0927 0.1898 0.2957 0.4105 0.5216 0.6368 0.7472 0.8379 0.9029 0.9438 0.9733

0 0 0.6 1.6 4.7 18.4 70.1 205.3 409.0 602.6 721.7

0.1028 0.1877 0.2750 0.3603 0.4500 0.5422 0.6352 0.7191 0.7881 0.8399 0.8800 0.9228

0 1.6 2.4 4.1 8.4 19.2 50.2 123.7 256.5 423.5 577.7 783.4

Funding

This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST) (No. 2011-0029161) and a Korea Carbon Capture & Sequestration (KCRC) of Korea (NRF) grant funded by the Korean government (MEST). Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Abu, Z.; Mohammad, R. M.; Niederer; John, P. M.; Feron; Paul, H. M.; Versteeg Geert, F. CO2 capture from power plants: Part I. A parametric study of the technical performance based on monoethanolamine. Int. J. Greenhouse Gas Control 2007, 1 (2), 135−142. (2) Lim, J. A.; Kim, D. H.; Yoon, Y.; Jeong, S. K.; Park, K. T.; Nam, S. C. Absorption of CO2 into Aqueous Potassium Salt Solutions of LAlanine and L-Proline. Energy Fuels 2012, 26, 3910−3918. (3) Houghton, J. T.; Jenkins, G. J.; Ephraums, J. J., Eds. Climate Change: The IPCC Scientific Assessment; Cambridge University Press: Cambridge, U.K., 1990. (4) Song, H. J.; Park, S. W.; Kim, H.; Gaur, A.; Park, J. W.; Lee, S. J. Carbon dioxide absorption characteristics of aqueous amino acid salt solutions. Int. J. Greenhouse Gas Control 2012, 11, 64−72. (5) Freeman, S. A.; Dugas, R.; Wagener, D. V.; Nguyen, T.; Rochelle, G. T. Carbon dioxide capture with concentrated, aqueous piperazine. Energy Proc. 2009, 1 (1), 1489−1496. (6) Rao, A. B.; Rubin, E. S. A Technical, Economic, and Environmental Assessment of Amine-Based CO2 Capture Technology for Power Plant Greenhouse Gas Control. Sci. Technol. 2002, 36 (20), 4467−4475. (7) Mansourizadeh, A.; Ismail, A. F. Hollow fiber gas-liquid membrane contactors for acid gas capture: A review. J. Hazard. Mater. 2009, 171, 38−53. (8) Kumar, P. S.; Hogendoorn, J. A.; Feron, P. H. M.; Versteeg, G. F. New absorption liquids for the removal of CO2 from dilute gas streams using membrane contactors. Chem. Eng. Sci. 2002, 57, 1639−1651. (9) Derks, P. W.; Hogendoorn, K. J.; Versteeg, G. F. Solubility of N2O in and Density, Viscosity, and Surface Tension of Aqueous Piperazine Solutions. J. Chem. Eng. Data 2005, 50 (6), 1947−1950. (10) Yan, S.; Fang, M. X.; Zhang, W. F.; Wang, S. Y.; Xu, Z. K.; Luo, Z. Y.; Cen, K. F. Experimental study on the separation of CO2 from flue gas using hollow fiber membrane contactors without wetting. Fuel Process. Technol. 2007, 88 (5), 501−511. (11) Smith, J. M. Introduction to Chemical Engineering Thermodynamics; McGraw-Hill: New York, 1987. (12) Song, H. J.; Lee, M. G.; Kim, H. T.; Gaur, A.; Park, J. W. Density, Viscosity, Heat Capacity, Surface Tension, and Solubility of CO2 in Aqueous Solutions of Potassium Serinate. J. Chem. Eng. Data 2011, 56, 1371−1377. (13) Bishnoi, S.; Rochelle, G. T. Absorption of carbon dioxide into aqueous piperazine: reaction kinetics, mass transfer and solubility. Chem. Eng. Sci. 2000, 55, 5531−5543.


Figure 5. Solubility of CO2 in 1.5 M potassium serinate + 1 M PZ solution at temperatures of 313.15 K, 333.15 K, and 353.15 K.

of absorbent solutions. At a temperature of 353.15 K, K-ALA +PZ and K-SER+PZ absorbent solutions showed higher solubility compared to that of K-SAR solution. (K-SAR) absorbent solution showed lowest solubility at 15 kPa for 313.15 K, 333.15 K, and 353.15 K. This reduced absorption capacity for (K-SAR) solution was because of the lowest basicity of sarcosine (pKb = 11.64) at amino group in the molecule compared to that of alanine (pKb = 9.69) and serine (pKb = 9.15) while all of them showed similar acidity at carboxyl group (for sarcosine pKa = 2.36, for alanine pKa = 2.35, for serine pKa = 2.21). This data set will be useful for developing CO2 absorption systems.

4. CONCLUSION In this work, the solubilities of CO2 of 4 M potassium sarcosinate solution, 1.5 M potassium alaninate + 1 M PZ solution, and 1.5 M potassium serinate + 1 M PZ solution were measured at temperatures of 313.15 K, 333.15 K, and 353.15 K. For each absorbent solution, the solubility of CO2 decreased as temperature increased at a constant CO2 partial pressure (Figures 3 to 5). 1791


Solubility of CO2 in Amino-Acid-Based Solutions of (Potassium

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