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Kinetics of CO2 Absorption into Aqueous Basic Amino Acid Salt: Potassium Salt of Lysine Solution Shufeng Shen,* Ya-nan Yang, Yangyang Bian, and Yue Zhao School of Chemical and Pharmaceutical Engineering, Hebei University of Science and Technology, Shijiazhuang 050018, P.R. China S Supporting Information *

ABSTRACT: Aqueous amino acid salts are considered as an attractive alternative to alkanolamine solvents (e.g., MEA) for carbon dioxide (CO2) absorption. The kinetics of CO2 into unloaded aqueous solutions of potassium lysinate (LysK) was studied using a wetted wall column at concentrations ranging from 0.25 to 2.0 M and temperatures from 298 to 333 K. Physicochemical properties of aqueous LysK solutions such as density, viscosity, and physical solubility of CO2 were measured to evaluate the reaction rate constants. The reaction pathway is described using zwitterion mechanism taking into account the effect of ionic strength on the reaction rate. Under the fast pseudo-first-order regime, the reaction rate parameters were obtained and correlated in a power-law reaction rate expression. LysK shows higher chemical reactivity toward CO2 than the industrial standard MEA and most of amino acid salts. Its reaction rate constants increase considerably with concentration and temperature. The reaction order is found to be an average value of 1.58 with respect to LysK. The forward second-order kinetic rate constant, k02, are obtained as 31615 and 84822 m3 kmol−1 s−1 at 298 and 313 K, respectively with activation energy of 51.0 kJ mol−1. The contribution of water to the zwitterion deprotonation seems to be more significant than that of LysK for the above-mentioned kinetic conditions.

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comparable with alkanolamines.7,8 Majchrowicz et al. and Paul et al. studied the absorption rate of CO2 in aqueous potassium prolinate (ProK) at temperatures of 290−323 K with concentration range of 0.5−3 mol L−1 (M). They found that the zwtterion deprotonation is the rate-determining step and the reaction order was between 1.36 and 1.44 for ProK.16,17 The effect of counterion on overall reaction rate was also addressed. It was concluded that potassium-based absorbents showed higher reactivity toward CO2 than sodium-based absorbents.17 However, a reaction order of approximately 1.00 and activation energy of 12 kJ mol−1 were found by Sodiq et al. for sodium prolinate (ProNa) at concentrations below 0.01 mol L−1.9 The absorption rate of CO2 was also studied in aqueous potassium sarcosinate (SarK) solutions by several researchers and a significant difference was observed between their works.8,12,18,19 Kumar and co-workers studied the reaction kinetics of CO2 in potassium taurinate (TauK) at temperatures of 285−305 K and over the concentration ranging from 0.1 to 4 mol L−1.6 The partial reaction order changes from 1.0 to 1.5 with the increasing concentration of TauK. Sodiq et al. researched the pseudo-first-order reaction rate constants for sodium taurate (TauNa) using a stopped-flow technique and low reaction rate of CO2 absorption was also found for both

INTRODUCTION Carbon dioxide (CO2) is one of the major contributors to global warming. Post combustion CO2 capture technology has the potential to reduce CO2 emissions from main sources such as fossil fuel-fired power plants. The leading technology today for postcombustion CO2 capture is based on the reversible chemical absorption of CO2 in amine-based solvents particularly monoethanolamine (MEA).1−4 However, these solvents exhibit several drawbacks such as solvent losses due to evaporation, high energy consumption for regeneration, limited stability in highly oxygenated environments.3 Aqueous alkaline salts of amino acids have been recognized as interesting alternative for CO2 removal. Due to having identical functional group as alkanolamines, amino acid salts (AAS) are expected to have high reactivity toward CO2. They are also characterized by several favorable properties such as low volatility due to their ionic nature in alkaline solutions, resistance to oxidative degradation and high surface tension.5−8 In recent years, there have been reported regarding CO2 absorption into several amino acid salt solutions using different experimental techniques such as stopped-flow, stirred cell reactor, and wetted-wall column.5−25 Knowledge of the kinetics between AAS and CO2 is essential for evaluation or simulation and accurate design of their CO2 absorption processes. Several researchers have already studied the kinetics of CO2 absorption in potassium glysinate (GlyK) and sodium glysinate.5,6,10−15 Van Holst et al. investigated the overall kinetic constants for several amino acid salts and concluded that they were © XXXX American Chemical Society

Received: September 15, 2015 Revised: November 21, 2015 Accepted: January 11, 2016

A

DOI: 10.1021/acs.est.5b04515 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology salts of taurine.9,20 Portugal et al. and Hwang et al. determined the reaction kinetics of CO2 in aqueous potassium threonate (ThrK) solution in a stirred cell reactor at concentrations ranging from 0.1 to 3.0 M and temperatures from 293 to 313 K.21,22 Kim et al. and Lim et al. studied the absorption of CO2 in aqueous potassium alaninate (AlaK) solution.23,24 Shen et al. reported the potassium arginate as a rate promoter in carbonate solutions and second-order reaction rate constant as a function of temperature over the experimental range was obtained.25,26 A comprehensive list of aqueous amino acid salts proposed for CO2 removal and the kinetic data for each salt with CO2 available in the literature are briefly summarized in Table 1. Aqueous solution of potassium lysinate (LysK, as shown in Figure 1) has been recongnized as a candidate solvent with high absorption rate of CO2 in our previous screening work.25,28 LysK possesses a primarily amino group (pKa 9.16) and an aliphatic amino group in its side chain (pKa 10.67), which involves different chemical structure from the reported amino acid salts. For aqueous lysine by neutralized with equimolar KOH, the pH of solution is usually in the range of 11 − 13, anionic state of lysine is expected. Apart from the primarily amino group, the amino group in side chain could also participate in reactive absorption in these basic environments. However, kinetic information for LysK absorbent is rare in literature. Hence, a comprehensive study on the reaction kinetics and mechanism appears desirable. In the present study, the kinetics of CO2 into aqueous LysK solutions were conducted at concentrations (0.25−2.0 mol L−1) and temperatures (298−333 K) using a wetted wall column. The reaction rate parameters were correlated from the experimental data.

asymptotic cases, the total reaction order changes between two and three, which has been reported for several amino acid salts as shown in Table 1.6,12,16−19 The absorption rate must be expressed in terms of activities rather than concentrations for thermodynamic consistency. An effective kinetic constant, k2,eff, can be applied to account for the effect of ionic strength in the nonideal solutions.11,19,21,23 k 2,eff = k 20exp(bI )

where b is a constant and I is the ionic strength given by I = 0.5 ∑Ci z2i , where Ci and zi are the molar concentration and the charge if ions in the solution, respectively. For LysK solution at low CO2 loadings, I becomes similar to the salt concentration, CLysK. For aqueous AAS, the contribution of CO2 hydration to the overall absorption rate is usually neglected.6,16−19 If the CO2 reaction with OH− is considered then the overall reaction rate constant can be expressed by eq 6,16,25 where kapp is the apparent rate constant for CO2 absorption. kapp = k OV − k OH−COH−

n kapp = k 2C LysK

* )= NCO2 = K G(PCO2,b − PCO 2

(1)

(2)

Assuming quasi-steady-state conditions for the zwitterion concentration and a first-order behavior of CO2, the overall reaction rate can be generally expressed as follows: −rCO2 = k OVCCO2 =

k 2C LysKCCO2 1+

k −1 ∑ kBC B

(3)

Ha =

where ∑ kB CB indicates the contribution of the bases present in solution for proton removal. If the deprotonation of the zwitterion is relatively fast when compared to the reverse rate of CO2 and amines, hence k−1 /∑ kB CB ≪1, a simple secondorder kinetics, eq 4, is obtained. ‐rCO2 = k 2C LysKCCO2

HCO2

+

1 kg

(8)

where NCO2 is the molar flow of CO2 entering the liquid solution. kL is the physical mass transfer coefficient. kg is the gas-side mass transfer coefficient. PCO2 is the CO2 partial pressure in the gas phase, which can be representative by the log mean pressure (Pln) at the top and bottom of the wettedwall column.25 P*CO2 is the equilibrium partial pressure and will have a negligible effect on mass transfer driving force by using fresh unloaded LysK solutions for present work.28 HCO2 is the Henry constant of CO2 in solution. A is the interfacial area between the gas and the liquid phases and E is the enhancement factor. The enhancement factor represents the ratio between the rate of absorption in the presence of the chemical reaction and the physical rate of absorption, which is the function of Hatta number, Ha, and the infinite enhancement factor, E∞.

C−OO+H 2N−CHR′ − COO− K+ + B kB

* ) A(PCO2,b − PCO 2 EkL

k2 −

→ −COOHN−CHR′−COO− K+ + BH+

(7)

Mass Transfer. The absorption flux of CO2 into an unloaded LysK solution can be described by the following equation:11,25

THEORETICAL BACKGROUND Reaction Mechanism. The reaction of CO2 with LysK can be described based on zwitterion mechanism that originally proposed by Caplow29 to model the CO2 absorption in the aqueous solutions of alkanolamines and amino acid salts.14−25 It involves the formation of a zwitterion as an intermediate (eq 1) that is deprotonated by bases (B)such as H2O, OH− or amine groups in AAS, thereby resulting in carbamate formation (eq 2). k−1

(6)

The reaction rate expression can also be evaluated assuming power-law kinetics with respect to the concentration of the AAS to hold:

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CO2 + H 2N−CHR′−COOK+ XoooY COO+H 2N−CHR′−COO‐K+

(5)

E∞ =

kovDCO2 kL

(9)

DCO2 ⎛ H C ⎞ D ⎜⎜1 + AAS × CO2 AAS ⎟⎟ DAAS ⎝ DCO2 νAASPCO2 ⎠

(10)

where DCO2 and DAAS are the diffusion coefficients of CO2 and amino acid salt in solution, respectively. ν AAS is the stoichiometric coefficient for amino acid salt. The reaction kinetics can be determined from the absorption rate in the LysK solution when measuring in the fast pseudo-

(4)

If k−1 /∑ kB CB ≫1, this results in a complex reaction rate expression which is the same to the case using a termolecular mechanism.9,19 In the transition region between the two B


C

taurine

sarcosine

proline

potassium

9.06 stirred cell reactor

stopped flow

stirred cell reactor

potassium

C2H7NO3S

string of dicks contactor

potassium

sodium

wetted-wall column

potassium



potassium 9.97

stopped flow

sodium

C3H7NO2

wetted-wall column

potassium

potassium

wetted-wall column


potassium

10.47

wetted-wall column

sodium

C5H9NO2

stirred cell reactor stirred cell reactor

potassium sodium

potassium

stopped flow

potassium


stopped flow

9.58

experimental technique

potassium

C2H5NO2

pKaa

wetted-wall column

potassium

glysine

mol. form

potassium

counter ion

amino acid

285−305

298−313

298−308

298−335

315−355

298

290−303

298−313

315−355

303−323

298

303−323

285−303 298−318

278−303

298−313

298−335

293−303

temperature (K)

1.0−1.5

1.00

1.66

1.25−1.81

1.39

1.40−1.44

1.00

1.17

1.36−1.40

1.08

1.0

1.0 1.0

1.0

1.0

1.0

1.0

reaction orderb

Penny and Ritter (1983)

⎛ −5508 ⎞ ⎟ k 2 ‐ Gly = 8.51 × 1011exp⎜ ⎝ T ⎠

Sodiq et al. (2014)

Majchrowicz et al. (2014)

⎛ −1440 ⎞ ⎟ k 2 ‐ Pro = 5.28 × 105exp⎜ ⎝ T ⎠ ⎛ −5211 ⎞ ⎟ k 2 ‐ Pro = 3.69 × 1012exp⎜ ⎝ T ⎠

Thee et al. (2014)

Aronu et al. (2011)

Simons et al. (2010)

⎛ −1699 ⎞ ⎟ k 2 ‐ Sar = 6.24 × 1010exp⎜ ⎝ T ⎠ ⎛ −915.8 ⎞ ⎟ k 2 ‐ Sar = 2.6198 × 109exp⎜ ⎝ T ⎠ ⎛ −3127 ⎞ ⎟ k 2 ‐ Sar = 8.67 × 108exp⎜ ⎝ T ⎠

Kumar et al. (2003)

⎛ −5700 ⎞ ⎟ k 2 ‐ Tau = 3.23 × 1012exp⎜ ⎝ T ⎠

9

Sodiq et al. (2014)

⎛ −5780 ⎞ ⎟ k 2 ‐ Tau = 5.44 × 1011exp⎜ ⎝ T ⎠

6

18

19

12

Holst et al. (2009)

kov = 9.77 × 103C1.41 AAS

8

9

Thee et al. (2014)

⎛ −2168 ⎞ ⎟ k 2 ‐ Pro = 1.02 × 1011exp⎜ ⎝ T ⎠

12

Paul and Thomsen (2012)

8

⎛ −4384 ⎞ ⎟ k 2 ‐ Pro = 2.42 × 1011exp⎜ ⎝ T ⎠

14

Holst et al. (2009)

Lee et al. (2007)

Vaidya et al. (2010) 5 Park et al. (2008) 15

16

17

10

1.08 kov = 2.09 × 104CAAS

⎛ −7670 ⎞ ⎟ k 2 ‐ Gly = 1.95 × 1013exp⎜ ⎝ T ⎠

⎛ −7188 ⎞ ⎟ k 2 ‐ Gly = 3.82 × 1012exp⎜ ⎝ T ⎠

data available

Guo et al. (2013)

⎛ −5459 ⎞ ⎟ k 2 ‐ Gly = 1.24 × 1012exp⎜ ⎝ T ⎠

13

12

Thee et al. (2014)

⎛ −5434 ⎞ ⎟ k 2 ‐ Gly = 1.22 × 1012exp⎜ ⎝ T ⎠

11

Portugal et al. (2007)

11

⎛ −5800 ⎞ ⎟ k 2 ‐ Gly = 2.81 × 1013exp⎜ ⎝ T ⎠

reference Portugal et al. (2007)

kinetic expressionc

⎛ −8544 ⎞ ⎟exp(0.44C kov = 2.42 × 1016exp⎜ AAS)CAAS ⎝ T ⎠

Table 1. Kinetic Data Available in the Literature on the Reaction Between CO2 and Amino Acid Salt

Environmental Science & Technology Article


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first-order reaction regime. This case is true and the Ha equals the E when the following condition is fulfilled:

a The listed pKa is for the most basic amine group in the amino acids.27 bReaction order is with respect to amino acid salt. cZwitterion reaction mechanism was used for calculating the second-order reaction rate constants (k2, m3 kmol−1 s−1).

25

Shen et al. (2013)

⎛ −8645 ⎞ ⎟ k 2 ‐ Arg = 2.58 × 1016exp⎜ ⎝ T ⎠ 1.0 313−342 wetted-wall column 12.10 potassium arginine

C6H14N4O2

potassium alanine

potassium

C3H7NO2

9.71


293−313

1.0

⎛ −3845 ⎞ ⎟exp(0.5706C kov = 4.518 × 108exp⎜ AAS)CAAS ⎝ T ⎠

Kim et al. (2012)

23

22

Hwang et al. (2010)

⎛ −4883 ⎞ ⎟ k 2 ‐ Thr = 3.95 × 109exp⎜ ⎝ T ⎠ 1.0 293−313 stirred cell reactor

Portugal et al. (2008)

⎛ −3580 ⎞ ⎟exp(0.90C kov = 4.13 × 108exp⎜ AAS)CAAS ⎝ T ⎠ 1.0 293−313 8.96 potassium threonine

potassium

C4H9NO3

mol. form counter ion amino acid

Table 1. continued


20

Wei et al. (2014)

⎛ −6074 ⎞ ⎟ k 2 ‐ Tau = 2.7 × 1012exp⎜ ⎝ T ⎠ 1.0 323−353 wetted-wall column

pKaa

experimental technique

temperature (K)

reaction orderb

kinetic expressionc

reference

21

Environmental Science & Technology

3 < Ha < E∞

(11)

Physicochemical properties such as diffusion coefficients and physical solubility of CO2 need to be estimated indirectly by analogy with a widely used nonreactive gas, N2O. CO2 diffusion coefficient in aqueous LysK solution can be estimated using a modified Stokes−Einstein relation:16−18 0.6 DCO2μAAS = DCO2,H2OμH0.6O

(12)

2

where DCO2,H2O and μH2O are the diffusion coefficient and viscosity of CO2 in water, respectively, which can be taken from literature.25,30 The solubility of CO2 in aqueous LysK solution can be estimated by N2O/CO2 analogy and Schumpe model for ionic solutions:7 HCO2 = (HCO2,H 2O/HN2O,H2O)HN2O

(13)

⎛ HCO ⎞ 2 ⎟⎟ = log⎜⎜ H ⎝ CO2,H2O ⎠

(14)

∑ (hi + hg)Ci,solution

where HCO2 and HN2O are, respectively, the Henry constants of gases in the LysK solution. HCO2,H2O and HN2O,H2O are, respectively, the Henry constants of gases in water. hi is the ion-specific parameter for either the cation (h+) or the anion (h−), hg is the gas specific parameter, and Ci is the concentration of ion i in the solution.

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EXPERIMENTAL SECTION Materials. L-Lysine (Lys, ≥ 98% purity), potassium hydroxide (KOH, Semiconductor grade, 50% aqueous solution) and 0.319 M sulfuric acid standard solution were purchased from Aladdin reagent, China. The aqueous LysK solutions were prepared by neutralizing the amino acid dissolved in deionized water with an equimolar amount of KOH in a volumetric flask at 293 K ± 1.0 K. pH was determined by a Hanna HI2111 pH Meter (Hanna Instruments, Italy). An electronic analytical balance (OHAUS, CP214) was used for weight measurements with a precision of ± 0.1 mg. The CO2 loading (α) of solutions after absorption was determined by measuring the volumes of CO2 released from the titration with sulfuric acid using the Chittick CO2 apparatus with errors less than 0.01.25,26,28 N2 (99.99%, v/v), CO2 (99.995%, v/v) and N2O (99.9%, v/v) were obtained commercially. Standard N2/CO2 mixed gases, S1 (10.02% CO2) and S2 (19.98% CO2) were purchased from Nanjing Special Gas Factory Co., Ltd. and used for calibration of two nondispersive infrared (NDIR) CO2 gas analyzers with a resolution of 0.01% (H3860, 0−20%, Beijing Huahe Tiandi Pty. Ltd. and GXH-3011N, 0 − 20%, Institute of Beijing HUAYUN Analytical Instrument). Density and Viscosity. Density measurements were performed using a digital oscillating tube densimeter (Anton Paar, DMA-4100M) having stated precision of ± 1.0 × 10−4 g cm−3. Viscosity measurements were done using a digital rolling ball microviscometer (Anton Paar, Lovis 2000M/ME) with the precision up to 0.5%. Physical Solubility. The physical solubility of N2O was determined experimentally in a thermostatted vessel similar to that of Hartono et al.31 The experimental setup and procedure can be found in the Supporting Information (SI). The lysinate D


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Figure 1. Different dissociation states of lysine (pKa:27 2.15(α-carboxyl group), 9.16 (α-amino group), 10.67 (amino group in side chain)) as a function of solution pH.

Figure 2. Schematic diagram of the wetted wall experimental apparatus.

The liquid film physical mass transfer coefficient, kL, was calculated using penetration theory to estimate Ha.38 CO2 Absorption Kinetic Measurements. The absorption rates of CO2 into unloaded aqueous LysK solutions were investigated for LysK concentrations of 0.25−2.0 M and temperatures of 298−333K using a wetted wall column (WWC) apparatus, as shown in Figure 2. The experimental procedure was described in detail in our previous work.25 The measurable counter-current contact area for mass transfer is 56.4 cm2. Experiments start by daily calibration of the CO2 gas analyzers using pure N2 and standard gases. The liquid rate was set at 52 mL min−1, which was the condition required for the assumptions of the fast pseudo-first-order kinetics to be valid. When the setup attained the required temperature, a mixture of CO2 and N2 passed through the column. The partial pressure of CO2 was obtained typically between 6 and 12 kPa. A steady state was recorded by the CO2 analyzer. Then, the fresh unloaded LysK solution was pumped through the WWC until a homogeneous liquid film was obtained. The once-through operation mode can be applied for all runs. Pressure in the

ion-specific parameters (hLys-) were obtained from the measured N2O solubility in the LysK solutions studied, see the SI. The potassium ion and gas specific parameters were taken from literature.32 Physical Mass Transfer Coefficient. The kinetics of reaction of CO2 with MEA solutions is well studied and reported as overall second order reaction.33−35 The reaction is fast enough to make the contribution of gas side resistance significant. In this work, the gas phase mass transfer coefficient was also estimated in the wetted wall column by CO2 absorption in 2.0 M unloaded aqueous MEA solutions. This method was reported in literature.36,37 Under consideration that the contribution of reaction between CO2 and OH− on overall reaction can be neglected, the gas phase mass transfer coefficient can be calculated as follows: * )A (PCO2,b − PCO 1 2 = − kg NCO2

HCO2,MEA k 2,MEADCO2CMEA

(15) E


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Environmental Science & Technology Table 2. Density and Viscosity of Aqueous LysK Solutions and CO2 Solubility into Their Solutions. CLysK (M)

T (K)

ρ (kg m−3)

η × 103 (Pa s)

HN2O (kPa m3 kmol−1)

HCO2 (kPa m3 kmol−1)

DCO2,LysK × 109 (m2 s−1)

0.26

298 303 308 313 318 323 328 333

1015.5 1013.9 1012.1 1010.2 1008.1 1005.8 1003.4 1000.8

1.0506 0.9411 0.8470 0.7675 0.7007 0.6435 0.5935 0.5507

4488 5128 5745 6362 6990 7580 8170 8682

3317 3739 4134 4522 4910 5262 5607 5895

1.64 1.84

298 303 308 313 318 323 328 333

1028.1 1026.4 1024.5 1022.5 1020.3 1018.0 1015.5 1012.9

1.1936 1.0681 0.9619 0.8727 0.7976 0.7310 0.6732 0.6226

5061 5673 6329 6973 7591 8225 8832 9439

3738 4136 4556 4957 5331 5709 6061 6409

1.43 1.58

298 303 308 313 318 323 328 333

1043.4 1041.5 1039.5 1037.4 1035.1 1032.7 1030.2 1027.5

1.4078 1.2510 1.1195 1.0099 0.9160 0.8358 0.7669 0.7064

5574 6209 6897 7573 8271 8923 9546 10171

4117 4526 4965 5384 5810 6195 6554 6906

1.24 1.38

298 303 308 313 318 323 328 333

1058.8 1056.9 1054.7 1052.4 1050.1 1047.6 1044.9 1042.2

1.7010 1.5097 1.3496 1.2145 1.0981 1.0017 0.9159 0.8390

6056 6687 7371 8088 8755 9440 10065 10670

4473 4876 5305 5750 6150 6554 6910 7245

2.40

1.54

298 303

1087.5 1085.3

2.4082 2.1081

6990 7650

5163 5578

0.75 0.86

1.98

298 303

1116.8 1114.3

3.5570 3.0707

8040 8792

5938 6410

0.56 0.65

0.51

0.76

1.01

2.85 3.52

2.01 2.52 3.12

1.77 2.20 2.73 1.05 1.19 1.53 1.92

The experimental solubility of N2O and the calculated Henry’s constants of CO2 in aqueous LysK solutions are given in Table 2. The solubility of N2O in water was used to validate the experimental apparatus and method. The results are shown in SI Figure S2. There is good agreement between the solubilities obtained in this work and those by Hartono et al.31 and Versteeg and Van Swaaij.39 The relative errors are within 1.4% and 3.0%, respectively. The values of lysinate anion specific parameter (hLys−) for Schumpe model were also estimated, as presented in Table 3. A value of 0.1035 was obtained for hLys− at 298 K. Parameters hLys− are expected to be essentially constant with the temperature from the original model.32 However, the values obtained for hLys− in present work show slight temperature dependence. The same phenomenon was also reported for the N2O solubility in aqueous potassium prolinate and potassium threonate solutions.16,17,21 Therefore, the Henry constants of CO2 were estimated by eq 13 from the original values in this work.

WWC was also recorded using pressure transducer with an uncertainty of 0.6 kPa (GS4200-USB, 0−600 kPa, ESI, UK). The CO2 content in the outlet gas stream was continuously recorded until another steady state was obtained. A liquid sample was then withdrawn from liquid outlet to be analyzed.

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2.30

RESULTS AND DISCUSSION

Physicochemical Properties. The densities and viscosities of aqueous LysK solutions at temperatures from 298 to 333 K and concentrations from 0.25 to 2.00 M are presented in Table 2. Diffusivity of CO2 in these solutions were also estimated. Both densities and viscosities of the investigated solutions increase with the increase in LysK concentration in the solutions at a constant temperature. The measurements in this study matched well with the reported data.28 The average absolute deviations (AAD) are within 0.2% and 3.9% for density and viscosity, respectively. F


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experimental conditions, the gas side resistance took 50−85% of the total resistance. The values of α and β were obtained as 0.78 and 1.02, respectively. Kinetic Measurements. In this work, all the LysK solutions were operated in a once-through mode. The liquids contacted with gas mixtures in the WWC were pumped to reservoir 2 (Figure 2). Absorption rates of CO2 were measured at temperatures of 298−333 K and over the concentrations of 0.25−2.0 M. Gas flowrate was in the range of 230−320 L h−1 with CO2 concentration of about 9.80 v/v% in inlet gas stream. Under these conditions, the contribution of gas film resistance to the total resistance was in the range of 10 − 25%. The effects of chemical reaction dominate the physical mass transfer inside the liquid film. The kinetic results are presented in Table 4. The kov and kapp are calculated using eqs 6, 8, and 9. The deviation between them, with a maximum of 8.5% and less than 1% in most cases, shows a small contribution of the reaction of CO2 with OH− to the overall reaction. For all the experiments, the values of CO2 loadings (the moles of CO2 per mole of amino acid salt) after absorption are less than 0.15. As presented in Table 4, the Hatta numbers are all greater than 3 and much less than E∞, which indicates under the fast pseudo-first-order reaction condition for all the runs. The comparison of kov values obtained in this work with those reported for other amino acid salts at 303 K is shown in Figure 3. For MEA and SarK, the kinetic data are presented at 298 K. Among amino acid salts, LysK show higher reactivity toward CO2 than ProK and SarK which were selected as

Table 3. Ion and Gas Specific Parameters for Schumpe Model in Aqueous LysK Solutions

a

T (K)

hK+ (m3 kmol−1)a

hCO2 (m3 kmol−1)a

hN2O (m3 kmol−1)a

hLys− (m3 kmol−1)

298 303 308 313

0.0922 0.0922 0.0922 0.0922

−0.01714 −0.01908 −0.02102 −0.02296

−0.00843 −0.01082 −0.01322 −0.01561

0.1035 0.1020 0.0950 0.0832

Values taken from literature.32

Gas Side Mass Transfer Coefficient. The gas side mass transfer coefficient is dependent on the hydrodynamic conditions. It basically depends on gas flow velocity and its pathway. The influence of these conditions is well described in the mass transfer coefficient model of Hobler.40 The model comprises the Sherwood (Sh), Schmidt (Sc), and Reynolds (Re) numbers and is valid specifically for laminar-flow regime in wetted-wall columns. In this work, an empirical 2-parameter model was used developed by Pacheco:37 β ⎛ d ⎞ Sh = α⎜Re × Sh × h ⎟ ⎝ h⎠

(16)

where α and β are the parameters to experimental results of CO2 absorption in The absorption flux has been measured for MEA solutions at 298, 318, and 333 K. presented in SI Table S1 and Figure

be fitted using MEA solutions. 2.0 M unloaded The results are S3. Under the

Table 4. Kinetic Data for the Absorption of CO2 into Aqueous LysK Solutions at Different Temperature.

a

CLysK (M)

T (K)

NCO2 × 103, (mol m−2 s−1)

Pln, (kPa)

kg′ × 106,a (mol m−2 s−1Pa−1)

Ha

E∞

kov (s‑1)

kapp (s‑1)

0.26

298 303 313 323 333

7.10 7.58 7.67 8.17 8.90

9.13 9.08 8.84 8.63 7.89

0.89 0.97 1.01 1.11 1.34

61 69 76 84 100

885 1007 1255 1509 1849

5331 7093 8982 11 865 17 676

5316 7063 8870 11 500 16 168

0.51

298 303 313 323 333

9.28 9.71 10.40 11.16 11.67

8.71 8.65 8.57 8.41 7.70

1.29 1.37 1.50 1.67 1.93

111 122 137 151 172

1860 2081 2541 2948 3624

16 292 20 330 27 613 35 931 49 063

16 252 20 260 27 348 34 948 45 930

0.76

298 303 313 323 333

10.48 10.81 11.55 12.93 13.23

8.75 8.61 8.39 8.32 7.95

1.49 1.58 1.76 2.04 2.20

157 170 193 223 232

2655 2986 3688 4351 5104

30 224 36 744 50 917 72 979 84 178

30 148 36 622 50 377 70 985 77 203

1.01

298 303 313 323 333

11.04 11.69 12.51 13.26 13.87

8.57 8.51 8.35 8.14 7.58

1.63 1.77 1.97 2.18 2.48

211 230 256 279 304

3481 3857 4713 5580 6681

50 576 62 315 83 740 105 870 134 457

50 485 62 126 82 961 103 209 126 357

1.54

298 303

13.49 13.99

8.92 8.74

1.84 1.97

355 372

4467 5005

120 814 139 007

120 557 138 428

1.98

298 303

13.46 14.02

8.61 8.54

1.91 2.03

529 550

5351 5938

230 258 261 226

229 804 260 330

kg′ is the normalized flux, a partial pressure driving force across the liquid film.25 G


Article

Environmental Science & Technology ⎛A⎞ kapp = k 20exp⎜ ⎟exp(bC LysK )C LysK ⎝T ⎠

(17)

The fitting model (eq 18, R2 = 0.9928) was obtained by minimizing the sum of the relative residues and is shown in Figure 4 along with the experimental data. The model matches well with the experimental results at the LysK concentrations over 0.5−2.0 M, whereas the difference is intensified at the low concentration of 0.26 M. ⎛ −2513 ⎞ ⎟exp(0.807C kapp = 1.075 × 108exp⎜ LysK )C LysK ⎝ T ⎠ (18)

A “log−log plot” of the apparent rate constant versus the concentration is generally used to identify the partial reaction order of the reaction between amino acid salt and CO2.6,8,16,17,19 There are two limiting cases for the zwitterion mechanism. When the formation of the zwitterion (eq 1) is rate determining, the reaction rate appears to be first-order for both the AAS and CO2 concentrations. On the other hand, when the zwitterion deprotonation steps are rate determining, the reaction rate appears to have a fractional order between 1 and 2 for the AAS concentration. In the present work, the reaction order n with respect to LysK were obtained in the range of 1.54−1.69 and with an average value of 1.58. The reaction order for ProK is in the range of 1.36−1.44 reported by Majchrowicz et al. and Paul and Thomsen.16,17 The n value of 1.25 − 1.81 for SarK was found in the literature.8,12,18,19 These suggest that the zwitterion deprotonation steps are not much faster than zwitterion formation step. The kapp data were nonlinearly regressed to the reaction rate expression (eqs 3 and 5) by means of a Levenberg−Marquardt fitting procedure. It is noted that only H2O and LysK are considered as bases to the contribution of zwitterion deprotonation. The rate constants were fitted (R2 = 0.9953) as a function of temperature by the Arrhenius equations as follows:

Figure 3. Overall kinetic constants for aqueous LysK compared with other absorbents at 303 K

promising absorbents reported by van Holst et al. (2009) in their screening study. The reaction kinetics is the lowest for TauK. AlaK and ThrK have comparable kinetics with MEA. It is noted that a significant difference is observed for SarK results between Aronu et al. and Simons et al. In the earlier study, Penny and Ritter proposed a Brønsted relationship to relate the kinetic constant with the basicity of amine group (pKa). LysK possesses a primarily amino group (pKa 9.16) and an aliphatic amino group in its side chain (pKa 10.67). Fast deprotonation from the zwitterion by basic amino group is possible. ProK and SarK also possess high pKa values, 10.47 and 9.97, respectively. Low pKa and steric hindrance can also be explained the low kinetics for other AAS. In Figure 4, kapp is shown as the function of LysK concentration and temperature. A clear temperature and

⎛ −6138 ⎞ ⎟ k 20 = 2.778 × 1013exp⎜ ⎝ T ⎠ kLysK k −1 k H 2O k −1

(19)

⎛ 11718 ⎞ ⎟ = 8.204 × 10−18exp⎜ ⎝ T ⎠

(20)

⎛ 4965 ⎞ ⎟ = 1.632 × 10−9exp⎜ ⎝ T ⎠

(21)

A good agreement between the experimental and the predicted from this model is shown in Figure 5. The activation energy for k2 is calculated to be 51.0 kJ mol−1, which is reasonably in line with the values for MEA, ProK, GlyK, and TauK.9,11,16,17,24 Arrhenius plots for reaction rate constants and the comparison with those for other amino acid salts are presented in SI Figure S4 and Table S2. It was found that water contributes significantly to the zwitterion deprotonation even at moderately high LysK concentrations. For examples, at 298 K, the contribution of water to the deprotonation is found to be 45% and 38%, respectively, for 1.54 and 1.98 M LysK solutions. The same phenomena were also reported by Kumar et al. for aqueous TauK and by Majchrowicz et al. for aqueous ProK solution.

Figure 4. Apparent reaction rate constants as a function of LysK concentrations and at different temperatures: experimental data and model lines with the consideration of ionic strength effect.

concentration dependence of the kapp can be observed. Under the assumption that the kinetic constant follows the Arrhenius law and the deprotonation of zwitterion is relatively fast when compared to the rate of the reverse reaction in eq 1, it is possible to make a fit of kapp as a function of temperature and LysK concentration: H


Article

Environmental Science & Technology E∞ hi,hg H HCO2 HN2O, H2O Ha I k−1 k02 k2 kapp k2,eff Figure 5. Apparent reaction rate constants as a function of LysK concentrations and at different temperatures: experimental data and zwitterion model (eqs 19−21) lines with the effect of ionic strength.

■

kg KG kL kOH‑

ASSOCIATED CONTENT

S Supporting Information *

kov NCO2 PCO2 P*CO2

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.5b04515. The solubility of N2O in aqueous LysK solutions was measured in a thermostatted stirred cell. The experimental setup and validation by measuring the solubility of N2O in water can be found in Figures S1 and S2. The correlation of gas side mass transfer coefficient in the wetted wall column was determined by measuring the rate of absorption of CO2 into an unloaded 2.0 M MEA solution at different temperatures. The results are shown in Figure S3 and Table S1. Tables S2 report the comparison of reaction rate constants for LysK and other amino acid salts. Arrhenius plots for reaction rate constants of aqueous LysK solution are also presented in Figure S4 (PDF)

■

Pln rCO2 R Re Sc Sh T

Greek Letters

μ solution viscosity, Pa s ρ solution density, kg m−3 ν the stoichiometric coefficient for LysK

AUTHOR INFORMATION

Corresponding Author

Subscripts

Notes

AAS CO2 g H2O LysK

*Phone: +86 311 88632183; fax: +86 311 88632183; e-mail: [email protected]. The authors declare no competing financial interest.

■

■

ACKNOWLEDGMENTS We acknowledge National Natural Science Foundation of China (No.21206029), Hebei Provincial Natural Science Foundation for Distinguished Young Scholars of China (B2015208067), Hebei Provincial Science and Technology Research Project of College and University (QN2015070) and Hebei Provincial Scientific Research Foundation for the Returned Overseas Chinese Scholars (2013-2015) for financial support.

■ A B C D E

infinite enhancement factor in instantaneous reaction regime, dimensionless ion and gas specific constants in the Shumpe equation, m3 kmol−1 Henry constant, kPa m3 kmol−1 Henry constant of CO2 in the LysK solution, kPa m3 kmol−1 Henry constant of N2O in the water, kPa m3 kmol−1 Hatta number, dimensionless ionic strength reverse reaction rate constant, L mol−1 s−1 second order rate constant without salt effect, L mol−1 s−1 second order rate constant, L mol−1 s−1 apparent reaction rate constant, s−1 effective kinetic constant with consideration of ionic strength effect in the nonideal solutions, L mol−1 s−1 gas film mass transfer coefficient, m s−1 overall gas phase mass transfer coefficient, mol m−2 s−1 Pa−1 liquid phase physical mass transfer coefficient, m s−1 reaction rate constant with hydroxide ion, L mol−1 s−1 overall kinetic constant, s−1 CO2 absorption flow, mol m−2 s−1 CO2 partial pressure, kPa equilibrium partial pressure over the absorbent solution, kPa log mean pressure (Pln) at the top and bottom of the wetted-wall column, kPa rate of reaction, mol s−1 universal gas constant, J mol−1 K−1 Reynolds number Schmidt number Sherwood number temperature, K

amino acid salts carbon dioxide gas phase water potassium salt of lysine

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NOMENCLATURE gas−liquid interfacial area, m2 base species in aqueous amino acid salt solution concentration, M or kmol m−3 diffusion coefficient, m2 s−1 enhancement factor, dimensionless I


Article

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