CO2 absorption into aqueous potassium salts of lysine and proline:Density, viscosity and solubility of CO2

Shufeng Shen a,b,*, Ya-nan Yang a, Yong Wang a, Shaofeng Ren a,b, Jiangze Han a,Aibing Chen a

a School of Chemical and Pharmaceutical Engineering, Hebei University of Science and Technology, Shijiazhuang 050018, ChinabHebei Research Center of Pharmaceutical and Chemical Engineering, Shijiazhuang 050018, China

A R T I C L E I N F O

Article history:Received 13 February 2015Received in revised form 15 April 2015Accepted 24 April 2015Available online 25 April 2015

Keywords:DensityViscosityCO2 solubilityPotassium lysinatePotassium prolinateVapor–liquid equilibrium

A B S T R A C T

Amino acid salts have potential use as alternative absorbents to alkanolamines for post combustion CO2

capture, because of their negligible volatility, resistance to oxidative degradation and fast absorptionrates. In this work, the density and viscosity of CO2-free and CO2-loaded aqueous solutions of potassiumlysinate (LysK) and potassium prolinate (ProK) weremeasured at temperatures from (298.15 to 348.15)Kand for their concentrations from (0.2 to 3.0)mol L�1. The datawere correlated accurately (R2�0.99) as afunction of temperature, concentration and CO2 loading by nonlinear regression analysis. The solubilityof CO2 in 2.5mol L�1 amino acid salt solutions was also determined in vapor–liquid equilibriumvessels attemperatures (313 and 333) K and CO2 partial pressures relevant to flue gas conditions. It was found that,the CO2 absorption capacity of LysK is higher than that of industrially usedmonoethanolamine (MEA) andthe CO2 loadings vary as LysK>ProK>MEA at given CO2 partial pressures. Moreover, LysK may offerhigher cyclic loading than MEA for CO2 capture.

ã 2015 Elsevier B.V. All rights reserved.

1. Introduction

One of the most disturbing global environmental problems isglobal warming and climate change. This problem is most likelycaused by the increasing carbon dioxide (CO2) concentration inatmosphere from a variety of emission sources such as thecombustion of fossil fuels (coal and natural gas) for powergeneration, chemical industrial processes as well as humanactivities. CO2 capture by chemical absorption remains the mostpromising technology to reduce CO2 emissions. This technologyemploys the chemical solvents which can reversibly react withCO2. Amine-based solvents particularly monoethanolamine (MEA)have been the most studied absorbents for this capture process[1–3]. However, these solvents also exhibit several drawbacks suchas high energy consumption for regeneration, limited CO2 capacity,thermal and oxidative degradation [4]. Improvements of energyefficiency and cost reduction are needed but these should at the

same time improve the environmental aspects of the process. Onesolution to enhance process performance is to apply attractivealternative absorbents such as amino acid based solvents andpromoted carbonate-based solvents [2,5–8]. Aqueous alkaline saltsof amino acid have received particular interest because manyamino acids are benign chemicals and have more favorableproperties such as low volatility (due to their ionic nature), lowtoxicity, high level of biodegradability. They have the samefunctional group like amines and can react with CO2 in a similarway. Several potassium salts of amino acids such as glycine,proline, taurine, alanine, threonine, sarcosine and arginine areunder extensive investigation on kinetics of absorption and havebeen found good potential as solvents for CO2 capture [6–18].

Physicochemical properties of absorbents such as density,viscosity and CO2 solubility are necessary for detailed characteri-zation of the solvent for industrial application and they arerequired in the process modeling, simulation and design ofgas–liquid contactor columns for CO2 absorption and regeneration[19–21]. These data are also necessary to deduce chemical reactionkinetics fromCO2 absorption rate experiments. Kumar et al. [22,23]andWei et al. [24] reported the densities and viscosities of 0.4–6Mpotassium taurate and CO2 solubility in 2–6M potassium tauratesolution over the temperature range of 293–353K. The properties

* Corresponding author at: Hebei University of Science and Technology, School ofChemical and Pharmaceutical Engineering, No.70 Yuhua East Road, Shijiazhuang050018, China. Tel.: +86 311 88632183; fax: +86 311 88632183.

E-mail addresses: sfshen@hebust.edu.cn, shufengshen@gmail.com (S. Shen).

http://dx.doi.org/10.1016/j.fluid.2015.04.0210378-3812/ã 2015 Elsevier B.V. All rights reserved.

Fluid Phase Equilibria 399 (2015) 40–49

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of CO2-free and CO2-loaded taurate solution and 30% MEA werealso compared. Shaikh et al. [25] recently reported the propertiessuch as density, viscosity and refractive index of aqueous solutionsof sodium prolinate (SP) with 0.05–0.40 mass fractions attemperatures from 298.15K to 343.15K. Majchrowicz et al. [26]and Chang et al. [27] measured the solubility of CO2 in aqueouspotassium salt of L-proline solutions. A beneficial effect of theprecipitation formation on the overall CO2 solubility wasdiscussed. Holst et al. [19] and Titona et al. [28] examinedphysicochemical properties such as density, viscosity, refractiveindex and electrical conductivities of aqueous alkali salts of alanineat temperatures from (303.15 to 343.15) K and concentrationsranging from (0.5 to 3.5)M. Aronu et al. [29] and Song et al. [30]studied the physicochemical properties of aqueous potassium saltsof sarcosine and serine. To the best of our knowledge, the data ofsuch properties of potassium lysinate have not yet been reported inthe open literature. Moreover, the reported data for aqueouspotassium prolinate solutions are rare and scattered. Effect of CO2

loading on these properties is also unavailable. Therefore, in thepresent work, we presented experimental data on density,viscosity and CO2 solubility of CO2-free and CO2-loaded aqueouspotassium salts of lysine and proline. Measurements were madefor potassium lysinate concentrations from (0.2 to 3.0) M andtemperatures from (298.15 to 348.15) K and for potassiumprolinate concentrations from (0.5 to 3.0) M and temperaturesfrom (298.15 to 348.15) K. Correlations of experimental values as afunction of temperature, concentration and CO2 loading were alsodeveloped, which can be useful in calculations for the kinetics andthe design of capture processes.

2. Materials and methods

2.1. Materials and amino acid salt preparation

The amino acids, L-Lysine (Lys CAS No. 56-87-1, 98% purity) andL-Proline (Pro CAS No. 147-85-3, 99% purity), and potassiumhydroxide (KOH, CAS No. 1310-58-3, GR, 95% purity) werepurchased from Aladdin Industrial Inc., China. Sulfuric acid(H2SO4, CAS No. 7664-93-9, 98% purity) was supplied from Tianjin

Yongda reagent, China. N2 (99.99%, v/v) and CO2 (99.995%, v/v)were obtained commercially. Standard N2/CO2 mixed gases, S1(10.02% CO2) and S2 (19.98% CO2) were purchased from NanjingSpecial Gas Factory Co., Ltd. and used for calibration of a CO2 gasanalyzer with a resolution of 0.01% (GXH-3011N, 0�20%, Institute

Nomenclature

AAD Average absolute deviation, dimensionlessAAS Amino acid saltC The molar concentration of the solution, (mol L�1)ki The fitting polynomial coefficients, dimensionlessKi The fitting polynomial coefficients, dimensionlessP Total pressure in the equilibrium vessel, (kPa)PCO2 CO2 partial pressure in gas bulk phase, (kPa)P�CO2

Equilibrium partial pressure, (kPa)PH2O

T The vapor pressure of water in the equilibrium vessel,(kPa)

R The gas constant, 8.3145, (m3 PaK�1mol�1)wt% Weight percentage, dimensionlessT Absolute temperature, (K)yCO2

IR mol fraction of CO2 measured by gas analyzer,dimensionless

Greek lettersa CO2 loading, the moles of CO2 per mol of absorbent (mol

CO2 /mol absorbent)r The density, (g cm�3)h The viscosity, (mPa s�1)

Table 1Description of chemical samples used in this study.

Chemicalname

Molarmass

Source Purity(massfraction)

Purificationmethod

L-Lysine 146.19 Aladdin Industrial Inc. 98.0% NoneL-Proline 115.13 Aladdin Industrial Inc. 99.0% NoneMEA 61.08 Tianjin Yongda reagent �99.0% NoneKOH 56.11 Aladdin Industrial Inc. 95.0% NoneK2CO3 (anhydrous)

138.21TianjinYongdareagent

�99.0% NoneH2SO4 98.08 Tianjin Yongda reagent 98.0% NoneNaOH Standard

solutionTianjin Yongda reagentAladdin Industrial Inc.

0.5051 M0.5000 N

None

CO2 44.01 Shijiazhuang Xisanjiaooxygen generationstation

99.995%(v/v)

None

N2 28.01 Shijiazhuang Xisanjiaooxygen generationstation

99.999%(v/v)

None

Densitystandard

Anton Paar GmbH,Austria

Ultra purewater

None

Standardgas S1

Nanjing Special GasFactory Co., Ltd.

10.02%CO2/89.98% N2

None

Standardgas S2

Nanjing Special GasFactory Co., Ltd.

19.98%CO2/80.02% N2

None

Water 18.02 Merck-Millipore Aquelix5

15MV RO-Elix

Table 2Comparison of experimental data of density (r) and viscosity (h) of purecompounds with literature values.a

T(K) r (g cm�3) h (mPa s�1)

This work Ref. [32,33] This work Ref. [32,33]

Water 293.15 0.9982 0.99821 1.0016 1.0016298.15 0.9971 0.99704 0.8908 0.89002303.15 0.9957 0.99565 0.7990 0.79722308.15 0.9940 0.99403 0.7220 0.7230313.15 0.9922 0.99221 0.6564 0.65273318.15 0.9902 0.99021 0.6009 0.5990

MEA 303.15 1.0077 1.0098 [34] 14.6050 15.1058 [35]1.0091 [36] 15.1940 [36]1.0084 [37] 14.88 [37]

313.15 0.9998 1.0022 [34] 9.7244 10.0284 [35]1.0013 [36] 10.0283 [36]1.0001 [37] 9.93 [37]

323.15 0.9917 0.9944 [34] 6.8067 6.9715 [35]0.9934 [36] 6.9463 [36]0.9919 [37] 6.89 [37]

333.15 0.9837 0.9862 [34] 4.9695 5.0473 [35]0.9854 [36] 5.0454 [36]0.9836 [37] 4.97 [37]

343.15 0.9755 0.9774 [34] 3.7838 3.7793 [35]0.9771 [36] 3.8050 [36]

a Measurements were performed at 101.3�0.2 kPa. The measurement uncer-tainties u are u(T) =�0.02K, u(r) = 0.0001g cm�3,u(h) =� 0. 5%.

S. Shen et al. / Fluid Phase Equilibria 399 (2015) 40–49 41

of Beijing HUAYUN Analytical Instrument). Titration was per-formed using an automatic potentiometric titrator (ZDJ-5, INESAScientific Instrument Co., Ltd.) with a precision of �0.1mV. All thereagents were used without further purification. A chemicalsample description is given in Table 1.

The aqueous amino acid salt solutions were prepared byneutralizing the amino acid dissolved in deionized water with anequimolar amount of KOH in a volumetric flask at 293K�1.0K. An

electronic analytical balance (OHAUS, CP214) was used for weightmeasurementswith a precision of�0.1mg. CO2-loaded amino acidsalt solutionswere prepared by bubbling CO2 froma gas cylinder ina flask. The exact concentrations of amino acid salt weredetermined by potentiometric titration during vapor–liquidequilibrium experiments and CO2 loading (a) of solutions wasmeasured by the Chittick CO2 apparatus [31]. a is defined as themoles of CO2 per mole of amino acid salt (AAS), mol CO2/mole AAS.

[(Fig._1)TD$FIG]

Fig. 1. A schematic diagram of experimental apparatus for vapor–liquid equilibrium measurement.

Table 3Density of CO2-free and CO2-loaded aqueous potassium lysinate or prolinate solutions at 101.3�0.2 kPa.a

C (mol L�1) ab T (K)

298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15 348.15r (g cm�3)

Potassium salt of lysine0.2001 0 1.0105 1.0090 1.0073 1.0054 1.0033 1.0011 0.9986 0.9956 0.9934 0.9906 0.98770.2500 0 1.0175 1.0160 1.0142 1.0123 1.0102 1.0079 1.0054 1.0028 1.0028 0.9973 0.99430.5000 0 1.0282 1.0271 1.0252 1.0232 1.0210 1.0186 1.0162 1.0137 1.0110 1.0081 1.00520.7500 0 1.0432 1.0414 1.0395 1.0373 1.0351 1.0327 1.0301 1.0275 1.0247 1.0218 1.01871.0082 0 1.0560 1.0541 1.0520 1.0498 1.0475 1.0450 1.0424 1.0397 1.0369 1.0339 1.03091.5291 0 1.0848 1.0827 1.0804 1.0779 1.0754 1.0728 1.0701 1.0672 1.0643 1.0613 1.05821.7900 0 1.1110 1.1086 1.1060 1.1034 1.1008 1.0981 1.0952 1.0923 1.0893 1.0863 1.08321.9999 0 1.1216 1.1191 1.1165 1.1138 1.1110 1.1081 1.1052 1.1022 1.0991 1.0960 1.09272.5000 0 1.1419 1.1391 1.1364 1.1335 1.1306 1.1276 1.1246 1.1215 1.1183 1.1150 1.11183.0000 0 1.1510 1.1483 1.1455 1.1426 1.1397 1.1367 1.1337 1.1306 1.1275 1.1243 1.12112.5000 0.3466 1.1620 1.1594 1.1567 1.1540 1.1513 1.1486 1.1458 1.1429 1.1399 1.1368 1.1336

0.4276 1.1692 1.1666 1.1641 1.1614 1.1587 1.1560 1.1532 1.1503 1.1473 1.1442 1.14110.5091 1.1708 1.1682 1.1656 1.1630 1.1603 1.1577 1.1550 1.1523 1.1493 1.1462 1.14300.6008 1.1816 1.1790 1.1765 1.1740 1.1713 1.1686 1.1658 1.1629 1.1599 1.1568 1.15370.6999 1.1960 1.1934 1.1908 1.1882 1.1855 1.1828 1.1799 1.1770 1.1740 1.1710 1.16800.7103 1.1958 1.1933 1.1908 1.1883 1.1857 1.1813 1.1787 1.1758 1.1728 1.1698 1.16670.8760 1.2029 1.2004 1.1979 1.1954 1.1927 1.1899 1.1870 1.1841 1.1812 1.1782 1.17511.0431 1.2130 1.2104 1.2067 1.2041 1.2013 1.1985 1.1956 1.1927 1.1898 1.1867 1.18371.1089 1.2206 1.2180 1.2154 1.2127 1.2100 1.2072 1.2043 1.2014 1.1985 1.1954 1.1924

Potassium salt of proline0.5001 0 1.0266 1.0252 1.0235 1.0216 1.0196 1.0176 1.0152 1.0128 1.0104 1.0080 1.00561.0000 0 1.0547 1.0530 1.0510 1.0490 1.0468 1.0445 1.0422 1.0399 1.0374 1.0355 1.03571.5000 0 1.0880 1.0864 1.0845 1.0822 1.0812 1.0788 1.0766 1.0747 1.0737 1.0719 1.07052.0002 0 1.1114 1.1092 1.1070 1.1047 1.1025 1.1004 1.0985 1.0958 1.0936 1.0943 1.09242.5000 0 1.1458 1.1432 1.1406 1.1379 1.1351 1.1322 1.1292 1.1262 1.1230 1.1199 1.11663.0000 0 1.1669 1.1641 1.1613 1.1584 1.1555 1.1525 1.1494 1.1463 1.1431 1.1398 1.13653.0000 0.0915 1.1715 1.1688 1.1660 1.1632 1.1602 1.1572 1.1511 1.1542 1.1479 1.1446 1.1413

0.2725 1.1974 1.1947 1.1919 1.1891 1.1862 1.1832 1.1802 1.1771 1.1739 1.1707 1.16430.5198 1.2131 1.2104 1.2076 1.2047 1.2017 1.1987 1.1956 1.1924 1.1892 1.1859 1.18250.6399 1.2218 1.2191 1.2163 1.2135 1.2106 1.2076 1.2046 1.2015 1.1983 1.1950 1.1917

a The measurement uncertainties u are u(T) =�0.02K, u(r) = 0.0001g cm�3, u(a) =�0.002.b a is defined as the moles of CO2 per mole of amino acid salt (AAS), mol CO2 /mole AAS.

42 S. Shen et al. / Fluid Phase Equilibria 399 (2015) 40–49

2.2. Density measurement

Density measurements were performed using a digitaloscillating tube densimeter (Anton Paar, DMA-4100M) havingstated precision of �1.0�10�4 g cm�3. It had an integratedthermostat with Pt-100 thermometer and a low-thermal massmeasuring cell (a U-shaped borosilicate glass tube), whichenabled rapid changes and exact adjustments to the measuringtemperature. The temperature was controlled to �0.01K of theset value, and the precision of the measured value was �0.02 K.The densimeter was calibrated using dry air and distilleddeionized water as standard fluids. The calibration wasvalidated by measuring the water and pure MEA at themeasuring range considered in this work. The validated resultsare given in Table 2.

2.3. Viscosity measurement

Viscosity measurements were done using a digital rollingball microviscometer (Anton Paar, Lovis 2000M/ME) with theprecision up to 0.5%. The viscometer had a built-in Pt-100 temperature sensor for temperature control of capillaryand measurement (uncertainty of �0.02K). In the present work,measurements were performed using a capillary with diameterd =1.59mm, which allowed measurements for a wide range ofviscosities (0.3–90)mPas�1. Before and after each measurement,the viscometer was carefully calibrated with deionized water.The calibration was validated by measuring the viscosity of thedensity standard water and pure MEA, which the viscosity of oursamples is within the viscosity range in the investigatedtemperature range. The results of the validation are also givenin Table 2. Six measurements were carried out to report theviscosity data in average.

2.4. Vapor–liquid equilibrium experiments

CO2 absorption capacity in aqueous solutions of differentabsorbents was determined in an absorption apparatus asdescribed in our previous work [5]. CO2 was bubbled throughthe solutions at an operating temperature of 293K until the CO2

loading became constant or precipitation occurred. The absorptiontime to reach the points for these absorbents was various. The CO2

absorption capacity expressed in Kg CO2 per liter solvent wasobtained.

Vapor–liquid equilibrium for amino acid salt solutions withknown CO2 loading was measured from 313K to 343K and atnear atmospheric pressure. A schematic diagram of theexperimental set-up was shown in Fig. 1. Absorbent samples,120 cm3 each, were placed in three successive vessels sub-merged in a thermally regulated water bath (�0.1 K). N2 wasfirst flushed through the devices to purge out the gases withinthe system, and then the gas phase was circulated and analyzedonline until steady values of CO2 concentration were recordedby the online GXH-3011N gas analyzer. Total pressure in thevessel was measured using pressure transducer with a full scaleuncertainty of �0.27 kPa (PX409-050AUSBH, 0�3.4 bar, OmegaEngineering Inc.). Equilibrium partial pressures of CO2, P

�CO2

, ingas phase were calculated as follows:

P�CO2

¼ ðP � PTH2OÞyIRCO2(2)

where P and PTH2Oare the total pressure in the vessel and the vapor

pressure of water in the equilibrium vessel at the equilibriumtemperature, respectively. yIRCO2

is mol fraction of CO2 measured bygas analyzer with drying tube.

[(Fig._2)TD$FIG]

290 300 310 320 330 340 350 360

1.00

1.05

1.10

1.15

ρ, g

.cm

-3

T, K

0.2001 M 0.2500 M 0.5000 M 0.7500 M 1.0082 M 1.5291 M 1.7900 M 1.9999 M 2.5000 M

Fig. 2. Density of aqueous potassium lysinate solutions at different temperaturesand concentrations. Line: correlation results from Eq. (2).

[(Fig._3)TD$FIG]

Fig. 3. Effect of CO2 loading on density of 2.5M aqueous potassium lysinatesolutions at various temperatures. Line: correlation results from Eq. (3).

[(Fig._4)TD$FIG]

290 300 310 320 330 340 350 360

1.00

1.05

1.10

1.15

1.20

ρ, g

.cm

-3

T, K

0.5001 M 1.0000 M 1.5000 M 2.0002 M 2.5000 M 3.0000 M

Fig. 4. Density of aqueous potassium prolinate solutions at different temperaturesand concentrations. Line: correlation results from Eq. (2).

S. Shen et al. / Fluid Phase Equilibria 399 (2015) 40–49 43

3. Results and discussion

To validate the experimental methods and results, experimen-tal measurement of density and viscosity of water standard andpure MEA were measured at temperatures ranging from 293.15Kto 343.15K and compared with values from published work inTable 2. The validity was evaluated by the average absolutedeviation (AAD) between experimental and published values.

AAD ¼ 1n

Xni¼1

jyexpi � ylitiyliti

j � 100% (1)

where n is the number of experimental points andyexpi andyliti represent the experimental and literature values. Itwas found from Table 2 that the measured data were in goodagreement with the data in the literature and the average absolutedeviation values are 0.015–0.108% for density and 0.647–1.064% forviscosity, respectively. The deviation could be probably due to thedifference in purity and source of chemicals.

3.1. Density

The measured density results for aqueous solutions ofpotassium lysinate and potassium prolinate at concentrationsup to 3.0M and temperatures from (298.15 to 348.15) K arepresented in Table 3. The effect of temperature and CO2 loadingon density are shown in Figs. 2–5. It can be found that, withincreasing the concentration of amino acid salt in the solution,the density increases; with increasing the CO2 loading, thedensity increases greatly; however, the density decreases withthe rise of temperature for two amino acid salt solutions at agiven concentration and CO2 loading.

An additional goal of this work is to fit suitable correlations forphysical properties (as a function of temperature, concentrationand/or CO2 loading), which could be applied to calculate theirproperties in kinetic study and process simulation on CO2 capture.

The experimental densities were correlated to a polynomialfunction that is temperature, concentration and/or CO2 loadingdependent, according to Eqs. (2) and (3), respectively.

[(Fig._5)TD$FIG]

Fig. 5. Effect of CO2 loading on density of 3.0M aqueous potassium prolinatesolutions at various temperatures. Line: correlation results from Eq. (3).

Table 4Viscosity of CO2-free and CO2-loaded aqueous potassium lysinate or prolinate solutions at 101.3�0.2 kPa.a

C (mol L)�1 ab T (K)

298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15 348.15h (mPa s�1)

Potassium salt of lysine0.2001 0 1.0031 0.8978 0.8095 0.7344 0.6711 0.6168 0.5697 0.5292 0.4937 0.4633 0.43590.2500 0 1.0079 0.9023 0.8125 0.7377 0.6737 0.6185 0.5705 0.5294 0.4939 0.4630 0.43580.5000 0 1.2899 1.1581 1.0454 0.9512 0.8660 0.7960 0.7351 0.6833 0.6409 0.6042 0.57150.7500 0 1.3479 1.1996 1.0745 0.9697 0.8813 0.8040 0.7388 0.6798 0.6290 0.5855 0.54721.0082 0 1.5747 1.3969 1.2473 1.1214 1.0138 0.9223 0.8431 0.7748 0.7155 0.6637 0.61811.5291 0 2.2856 2.0071 1.7745 1.5815 1.4184 1.2798 1.1621 1.0606 0.9729 0.8961 0.83061.7900 0 3.2362 2.8050 2.4540 2.1732 1.9344 1.7354 1.5660 1.4211 1.2960 1.1884 1.09451.9999 0 3.5636 3.0545 2.6762 2.3578 2.1164 1.9159 1.7567 1.6208 1.5011 1.4351 1.34582.5000 0 5.4062 4.5967 3.9504 3.4328 3.0085 2.6594 2.3683 2.1358 1.9272 1.7485 1.59433.0000 0 8.4071 7.0267 5.9463 5.0939 4.4075 3.8517 3.3936 3.0120 2.6926 2.4232 2.20792.5000 0.3466 6.2327 5.3107 4.5779 3.9852 3.5015 3.0939 2.7595 2.4755 2.2358 2.0437 1.8602

0.4276 6.8486 5.8179 5.0005 4.3377 3.8009 3.3574 2.9864 2.6763 2.4098 2.1815 2.01200.5091 6.6842 5.7203 4.9261 4.2971 3.7787 3.3361 2.9879 2.6957 2.4351 2.2177 2.04220.6008 7.7466 6.5530 5.6141 4.8710 4.2625 3.7546 3.3372 2.9864 2.6872 2.4458 2.23790.6999 8.9513 7.6101 6.5429 5.6988 4.9789 4.3755 3.8796 3.4619 3.1042 2.8051 2.55120.7103 8.6686 7.4006 6.3762 5.5291 4.8646 3.9953 3.5802 3.2248 2.9042 2.6378 2.40640.8760 8.8287 7.5606 6.5201 5.6532 4.9671 4.3690 3.8970 3.4684 3.1194 2.8214 2.56221.0431 9.3274 7.9124 6.9613 6.0767 5.3010 4.6739 4.1409 3.7020 3.3311 2.9941 2.71371.1089 9.7656 8.2537 7.0592 6.1033 5.3343 4.6952 4.1611 3.7203 3.3438 3.0155 2.7379

Potassium salt of proline0.5001 0 1.1047 0.9857 0.8913 0.8086 0.7351 0.6741 0.6219 0.5764 0.5373 0.5039 0.47331.0000 0 1.3800 1.2318 1.1032 0.9949 0.9035 0.8251 0.7569 0.7002 0.6493 0.6080 0.57921.5000 0 1.8613 1.6423 1.4652 1.3105 1.2000 1.0847 0.9922 0.9175 0.8552 0.7966 0.74862.0002 0 2.2927 2.0282 1.7944 1.6012 1.4391 1.2961 1.1957 1.0930 1.0086 0.9587 0.89722.5000 0 3.1809 2.7552 2.4092 2.1360 1.9028 1.7068 1.5415 1.4001 1.2781 1.1731 1.08183.0000 0 4.0539 3.4909 3.0383 2.6618 2.3539 2.1018 1.8866 1.7041 1.5493 1.4140 1.29933.0000 0.0915 4.1610 3.5826 3.1140 2.7313 2.4155 2.1633 1.7549 1.9424 1.5946 1.4562 1.3370

0.2725 4.8399 4.1514 3.5984 3.1510 2.7833 2.4770 2.2179 2.0110 1.8236 1.6629 1.46680.5198 4.9079 4.2133 3.6591 3.2061 2.8350 2.5254 2.2646 2.0573 1.8685 1.7065 1.56660.6399 5.1655 4.4409 3.8558 3.3802 2.9899 2.6630 2.3899 2.1575 1.9663 1.7958 1.6473

a The measurement uncertainties u are u(T) =� 0.02K, u(a) =�0.002, u(h) =�0.5%.b a is defined as the moles of CO2 per mole of amino acid salt (AAS), mol CO2/mole AAS.

44 S. Shen et al. / Fluid Phase Equilibria 399 (2015) 40–49

r ¼ k0 þ k1T þ k2C (2)

r ¼ K0 þ K1T þ K2C þ K3a (3)

where r is the density in g cm3; T is the absolute temperature in K;C is the molar concentration of the solution in mol L�1 (M); a is theCO2 loading of the solution, mol CO2/mole AAS; ki (i = 0 to 2) and Ki

(i = 0 to 3) are the polynomial coefficients obtained by fitting theequations to the experimental data by the nonlinear least squaresregression. The optimized coefficients were determined byminimizing the objective function r in term of the AAD, whichare presented in Table 5. The values of R2 of both equations areabove 0.99 and the maximum AAD for both amino acid salts islower than 0.29%. The curved lines in Figs. 2–5 correspond to thecalculated values from Eq. (2) or (3). As shown, the lines fromEq. (2) are in fairly good agreement with the experimental dataespecially for potassium prolinate and potassium lysinate in a lowconcentration range.

3.2. Viscosity

The measured viscosity results for aqueous solutions ofpotassium lysinate and potassium prolinate at different concen-trations and temperatures are given in Table 4, and the viscosityversus temperature, concentration and CO2 loading is shown inFigs. 6–9.

As known, the viscosity of absorbents can affect the CO2

absorption rate. Solvents with low viscosity can increase the

diffusion coefficient of CO2 in the liquid phase, which results in lowmass transfer resistance. As can be seen that for both systems, theviscosity decreases with an increase in temperature at allconcentrations studied. However, with increasing the concentra-tion of absorbents, the viscosity tends to increase. The similartrends of variation in viscosity with concentration and tempera-ture are found for other amino acid salts in the reported work[25,28–30]. After analysis of the experimental data, it was foundthat the viscosities of lysinate solutions are higher than those ofprolinate solutions at same conditions. The viscosities of bothamino acid salt solutions increase after loadingwith CO2. ThemoreCO2 absorbed, the more viscous the solutions. The effect oftemperature on viscosity of both absorbents at high concentrationsfrom (1.5 to 3.0) M is greater than that at concentrations blew1.0M. For example, the viscosity of 2.5M potassium lysinatedecreases from 5.40 cP at 298.15K to 2.37 cP at 328.15K. but for0.5M potassium lysinate, it decreases from (1.29 to 0.73) cP in thesame temperature range. It is also noted that the viscosities of1.5M lysinate solutions and 2.0M prolinate solutions are similar tothe commonly used 30% MEA (4.97M) at temperatures of 298–343K. This indicates the diffusion coefficients of CO2 in the highlyconcentrated solutionswill be lower than or similar to those in 30%MEA solution.

Based on the common description that the effect of tempera-ture on viscosity of fluids can be expressed by the Arrheniusrelationship, two temperature-, concentration- and/or CO2 load-ing- dependent exponential-type equations are correlated to Eqs.(4) and (5) by fitting the experimental data.

Table 5Fitting parameters of Eq. (2)a and (3)b for density of aqueous potassium salt solution of amino acids.

Amino acid No. of data points CO2 loaded solution data included ki (i = 0, 1, 2); Ki (i = 0, 1, 2, 3) R2 AADc (%)

0 1 2 3

Lysine 99 NO 1.15704 �5.16283�10�4 0.05652 – 0.9929 0.287187 YES 1.16568 �5.39647�10�4 0.05501 0.06789 0.9971 0.270

Proline 66 NO 1.14167 �4.64606�10�4 0.05486 – 0.9943 0.269110 YES 1.16121 �5.22855�10�4 0.05425 0.08356 0.9960 0.283

a r ¼ k0 þ k1T þ k2C.b r ¼ K0 þ K1T þ K2C þ K3a.

c AAD ¼ 1n

Xni¼1

jrexpi

�rcalci

rexpi

j.

[(Fig._6)TD$FIG]

290 300 310 320 330 340 3500.00

1.00

2.00

3.00

4.00

5.00

6.00

η, m

Pa.s

T, K

0.2001 M0.2500 M0.5000 M0.7500 M1.0082 M1.5291 M1.7900 M1.9999 M2.5000 M

Fig. 6. Viscosity of aqueous potassium lysinate solutions at different temperaturesand concentrations. Line: correlation results from Eq. (4).

[(Fig._7)TD$FIG]

Fig. 7. Effect of CO2 loading on viscosity of 2.5M aqueous potassium lysinatesolutions at various temperatures. Line: correlation results from Eq. (5).

S. Shen et al. / Fluid Phase Equilibria 399 (2015) 40–49 45

h ¼ k0expk1expðk2CÞ

RT

� �expðk3CÞ (4)

h ¼ K0expK1expðK2CÞ

RT

� �expðK3CÞexpðK4aÞ (5)

where h is the viscosity in mPas; the activation energy of flow (Ea)is described by an exponential-type relation dependent on the

concentration; T is the absolute temperature in K; R is the gasconstant, 8.3145 m3PaK�1mol�1; C is the molar concentration ofthe solution in mol L�1; a is the CO2 loading of the solution, molCO2 / mole AAS; ki (i = 0 to 3) and Ki (i = 0 to 4) are the fittingcoefficients by the nonlinear regression. The optimized coefficientsare listed in Table 6. The calculated values from Eq. (4) or (5) arealso presented as curved lines in Fig. 6–9. The values of R2 (�0.99)of fitting Eqs. (4) and (5) and the maximum AAD betweenexperimental and the predicted data are in good agreement witheach other. Hence, the proposed correlations along with theparameters could be used to obtain reliable prediction of theviscosities of the aqueous amino acid salt solutions investigated inthe range of concentrations and temperatures in the present work.

3.3. CO2 solubility

3.3.1. Validation of experimental methodTo validate the experimental method used in this work, a series

of experiments on CO2 solubility in 30wt% (4.97M) MEA solutionat T =313.2K and 35 wt% potassium carbonate (K2CO3) atT =343.2K were performed. Before entering the CO2 gas analyzer,the gas stream is required to cool down to eliminate the effect ofwater vapor. Values of 7.36, 19.95 and 31.25 kPawere used for PT

H2O

over absorbent solutions at 313, 333 and 343K respectively.Variation of these values are neglected at the investigated loadings.The vapor–liquid equilibrium results are listed in Table 7. Theseresults from the present work are also presented in Fig. 10alongside those reported from Endo et al. [38], the default ASPENplus E-NRTL values [39] and Aronu et al. [40]. A reasonableagreement can be observed for both systems in the temperaturerange investigated. Therefore, the method and apparatus used arereasonable and reliable.

3.3.2. CO2 solubility in aqueous solutions of potassium lysinate andpotassium prolinate

Comparison of CO2 absorption capacity in the 2.5M amino acidsalt systems studied at 293K with 30wt% MEA solution (4.97M) isshown in Fig. 11. The maximum absorption capacity by MEAsolvent is approximately 0.114Kg CO2/L, which is similar to thereported (0.36–0.38Kg CO2 per Kg MEA) [41–43]. The absorptioncapacity in 2.5M LysK and 2.5M ProK is 0.129 and 0.09Kg CO2/L,respectively. Potassium lysinate presents a higher absorptioncapacity at the same solution concentration. The maximum CO2

loading in 2.5M LysK and 2.5M ProK is 1.20 and 0.81mol CO2/molAAS, respectively. Hence, LysKwas found superior compared to thetwo others on the basis of absorption capacity. It could be apromising alterative to alkanolamine absorbents (say, MEA).

Equilibrium CO2 solubility in aqueous solutions of 2.5M LysKand 2.5M ProK at 313 and 333K are given in Table 8. In the presentwork, the experimental VLE data will be focused on situationswhere no precipitation occurred. The measurements were

[(Fig._8)TD$FIG]

290 300 310 320 330 340 3500.00

1.00

2.00

3.00

4.00

5.00

η, m

Pa.s

T, K

0.5001 M 1.0000 M 1.5000 M 1.9999 M 2.5000 M 3.0000 M

Fig. 8. Viscosity of aqueous potassiumprolinate solutions at different temperaturesand concentrations. Line: correlation results from Eq. (4).

[(Fig._9)TD$FIG]

Fig. 9. Effect of CO2 loading on viscosity of 3.0M aqueous potassium prolinatesolutions at various temperatures. Line: correlation results from Eq. (5).

Table 6Fitting parameters of Eq. (4)a and (5)b for viscosity of aqueous potassium salt solution of amino acids.

Amino acid No. of data points CO2 loaded solution data included ki (i = 0, 1, 2, 3); Ki (i = 0, 1, 2, 3, 4) R2 AADc (%)

0 1 2 3 4

Lysine 99 NO 1.140�10�3 16445.8 0.09225 0.03955 – 0.9912 5.23187 YES 8.889�10�4 17144.5 0.09753 �0.04627 0.53089 0.9947 4.52

Proline 66 NO 1.710�10�3 15558.2 0.07385 �0.02585 0.9964 2.89110 YES 1.790�10�3 15481.4 0.08667 �0.12272 0.34943 0.9948 3.20

a h ¼ k0exp½k1expðk2CÞRT �expðk3CÞ.b h ¼ K0exp½K1expðK2CÞ

RT �expðK3CÞexpðK4aÞ.c AAD ¼ 1

n

Xni¼1

jhexpi

�hcalci

hexpi

j.

46 S. Shen et al. / Fluid Phase Equilibria 399 (2015) 40–49

performed in the range of partial pressures of CO2 varying between0.05 and 17.5 kPa, which is the range typically encountered in CO2

removal from flue gas at fossil fuel – fired power generation. Fig.12also presents the VLE results for the two amino acid salt systemsalongwith those for 30wt%MEA solution, which is one of themostwidely used CO2 absorbents nowadays.

It can be observed that the CO2 partial pressures increase withincreasing the CO2 loading for all absorbents investigated. For agiven absorbent and CO2 partial pressure, the CO2 loading tends todecrease as temperature increases. The results suggest that the CO2

solubility decreases with increasing temperature. Similar trendscan also be found for other amino acid salts: glycinate and taurate[21,24]. This behavior can be attributed to the exothermic nature ofCO2 absorption.

The CO2 loading ranges of three absorbents showed quitedifferent in the range of CO2 partial pressure between 0.1 and10kPa. At the CO2 partial pressure of 10 kPa and 313K, which is themost used operating conditions in the CO2 capture industrialprocesses using amines, the equilibrium loadings can reach 0.5 forMEA, 0.7 for ProK and 1.0 for LysK. From the relative positions ofequilibrium curves, absorption capacity is the highest for 2.5MLysK and the lowest for 30wt% MEA. Of the two amino acid salts,LysK has smaller slope of equilibrium curves than ProK, whichmayoffer higher cyclic loading for CO2 capture. It should also be noted

that, the equilibriumCO2 partial pressures are nomore than 0.1 kPafor 2.5M LysK in the loading range of 0.2–0.6. These benefit fordriving force ðPCO2 � P�CO2

Þ and are the favorite features of apromising absorbent for CO2 removal from flue gas streams.

Different equilibrium curves for these absorbents could beattributed to their structures and solution pH. Two amino acid saltshaving a-amino functional group can react with CO2 in the similar

Table 7Solubility of CO2 in aqueous solutions of 4.97M MEA and 35wt% potassium carbonate at 313 and 343K.a

35wt% potassium carbonate 30wt% MEA (4.97M)

a (mol CO2/mole potassium ion) PCO2* (kPa) at 313Kb a (mol CO2/mole AAS) PCO2

* (kPa) at 343K

0.114 0.160 0.518 13.9400.145 0.270 0.505 11.6310.192 0.535 0.492 7.6210.234 0.970 0.493 5.3770.287 1.977 0.490 4.3140.330 2.506 0.491 4.199

0.488 3.5940.475 2.5720.473 1.9920.423 0.3800.409 0.313

a The measurement uncertainties u are u(T) = 0.1 K, u(a) = 0.002, u(PCO2*) = 0.02kPa.b PCO2

*: equilibrium partial pressure of CO2.

[(Fig._10)TD$FIG]

Fig. 10. Comparison of experimental and literature data of CO2 equilibrium partialpressure over 35wt% equivalent K2CO3 solution at 343K and 30wt% MEA at 313K.PC30 and PC35 represent 30wt% and 35wt% equivalent K2CO3 solution,respectively. Default ASPEN: VLE data from default ASPEN plus E-NRTL model.

[(Fig._11)TD$FIG]

4.97M MEA 2.5M LysK 2.5M ProK

0.06

0.09

0.12

CO

2 a

bsor

ptio

n ca

paci

ty (K

g/L)

Absorbents

(a)

T = 293K

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0

0.4

0.6

0.8

1.0

1.2

Elapsed absorption time, h

2.5M LysK 2.5M ProK

α, m

olC

O2 /m

ol A

AS

(b)

T= 293K

Fig.11. CO2 absorption capacity of absorbents (a) and CO2 loading variation in 2.5Maqueous solution of potassium lysinate and potassium prolinate with the elapsedabsorption time (b) at 293K and atmospheric pressure.

S. Shen et al. / Fluid Phase Equilibria 399 (2015) 40–49 47

way as amines [2,6–8]. The amino group in ProK is involved in twocarbon–nitrogen bond, so it is a secondary amino group. It isexpected that the carbamate can be formed in these solutionsduring CO2 absorption. Lysinate also contains a basic aliphatice-amino group in its side chain with pKa 10.53 [44], which will bepositively charged with the increasing CO2 loading. The positivecharge may participate in hydrogen bond to associate with water,which would be favorable for deprotonation from the CO2-aminoacid complex and facilitate the carbamate formation. Moreover,the deprotonated e-amino group can react with CO2 as a primaryamino group in the low CO2 loading range where pH above 11.0.Carbamate may undergo hydrolysis to bicarbonate and deproto-nated amino group if the pH is suitablewhere OH� ionmight play a

central role [45,46]. 2.5M LysK has a pH of 10.0–11.8 in the loadingrange investigated whereas a pH of 9.0-10.6 for 2.5M ProK.Solution of LysK is more basic than ProK at a specific loading.Meanwhile, the stability of carbamates decrease with decreasingbasicity of a-amino group [22]. Therefore, the higher CO2 loadingfor LysK can be observed. However, for MEA and ProK, only oneamino group is involved in the reaction. Two molecules arerequired: one acting as a base accepting a proton and the otherforming a carbamate. Thus, the lower loadings are expected at agiven CO2 partial pressure.

4. Conclusions

Aqueous solution of potassium lysinate (LysK) was proposed asa novel candidate absorbent for CO2 absorption. Physicochemicalproperties of aqueous solutions of LysK and ProKweremeasured attemperatures from (298.15 to 348.15) K and for their concen-trations from (0.2 to 3.0) mol L�1. Experimental results werecorrelated well with empirical correlations including the param-eters such as temperature, concentration and CO2 loading. Thesolubility of CO2 in 2.5 mol L�1 amino acid salt solutions wasmeasured at temperatures (313 and 333) K and CO2 partialpressures from (0.1 to 17.5) kPa. LysK has higher CO2 absorptioncapacity than monoethanolamine (MEA) and has high cyclicloading for CO2 capture. The CO2 loadings vary as LysK >ProK>MEA at given CO2 partial pressures. In summary, our results havedemonstrated that 2.5M LysK are superior to the widely used30wt% MEA in CO2 solubility and absorption.

Acknowledgements

The authors would like to acknowledge National NaturalScience Foundation of China (No.21206029), Hebei ProvincialNatural Science Foundation for Distinguished Young Scholars ofChina (No.B2015208067), Hebei Provincial Natural Science Foun-dation of China (No.2012208022) and Hebei Provincial Scientific

Table 8Equilibrium solubility of CO2 in aqueous solutions of 2.5M potassium lysinate and 2.5M potassium prolinate at 313 and 333K.a

Equilibrium partial pressure of CO2, PCO2* (kPa)

a (mol CO2/mole AAS) 313K a (mol CO2/mole AAS) 333K

2.5M Potassium lysinate 1.024 17.444 0.936 16.0880.998 14.235 0.873 3.3590.995 12.354 0.865 2.1211.001 10.082 0.830 1.8070.996 8.770 0.815 0.8120.991 6.657 0.754 0.4330.966 4.962 0.703 0.3200.955 3.353 0.710 0.2540.932 2.412 0.675 0.1820.925 2.156 0.676 0.1400.906 1.169 0.646 0.1080.828 0.209 0.618 0.0690.818 0.1520.803 0.104

2.5M Potassium prolinate 0.674 6.954 0.594 15.7280.608 2.341 0.589 9.9300.571 1.203 0.585 7.0060.559 0.649 0.557 5.0150.534 0.320 0.547 3.9850.522 0.180 0.512 1.9500.478 0.107 0.486 1.259

0.485 0.4930.448 0.2470.386 0.0650.373 0.037

a The measurement uncertainties u are u(T) = 0.1K, u(a) = 0.002, u(PCO2*) = 0.02 kPa. Measurements were performed at 103.3�2.0 kPa.

[(Fig._12)TD$FIG]

Fig. 12. CO2 equilibrium partial pressures over 2.5M amino acid salt solutions and30wt% MEA at 313K and 333K.

48 S. Shen et al. / Fluid Phase Equilibria 399 (2015) 40–49

Research Foundation for the Returned Overseas Chinese Scholars(2013-2015) for financial support and thank Minjie Zhang and YunLi for assistance in this study.

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