ARTICLE pubs.acs.org/IECR
Kinetics of Carbon Dioxide Absorption into Aqueous Amino Acid Salt: Potassium Salt of Sarcosine Solution Ugochukwu E. Aronu,† Ardi Hartono,† Karl A. Hoff,‡ and Hallvard F. Svendsen*,† † ‡
Department of Chemical Engineering, Norwegian University of Science and Technology, N-7491 Trondheim, Norway SINTEF Materials and Chemistry, N-7465 Trondheim, Norway ABSTRACT: The kinetics of CO2 absorption into an unloaded aqueous solution of the potassium salt of sarcosine (KSAR) was studied using a string-of-disks contactor for the concentration range of 1.04.0 kmol m3 and temperatures of 2562 °C. The zwitterion and termolecular mechanisms were reinterpreted to account for nonidealities in salt systems due to the effect of ionic strength through the introduction of an ionic factor, kion. Both models adequately represent the experimental data with no practical difference. The reaction rate constant for KSAR is high but comparable to values for amines. It increases considerably with temperature and concentration. The reaction order varies with KSAR concentration from 1.25 to 1.81, with an overall average value of 1.58. The experimentally derived kinetic model was combined with a thermodynamic model of aqueous KSAR from Aronu et al. (Chem. Eng. Sci. 2011, 66, 21912198) to predict the loading dependency of the overall mass-transfer coefficient, KG, determined in pilot-plant experiment. The predicted results show that the experimental data agree well with the pilot-plant results.
1. INTRODUCTION Removal of CO2 from gas streams by chemical absorption is an important process operation for industries such as natural gas production, refineries, ammonia plants, and others. Earlier, this operation was used to meet product specifications and to enable improved process efficiency. Recently, the focus has also been to reduce greenhouse gas emissions from coal- and natural-gas-fired power plants in which CO2 is a major contributor. CO2 absorption by chemical solvents is common, and the reaction kinetics of the solvent is critical in determining absorber performance and enabling proper design. The overall capture efficiency could therefore be improved by proper understanding of the fundamental mechanisms that affect the reaction rate. Researchers have developed several amine solvents including blended systems for CO2 removal applications. The amines include primary amines such as monoethanolamine (MEA), secondary amines such as diethanolamine (DEA), and tertiary amines such as triethanolamine (TEA),1,2 as well as blended amine systems.3,4 Apart from amine-based absorbents, other absorbents such as carbonate systems,5,6 amino acid salt (AAS) systems,79 and more recently amine amino acid salt systems10,11 have been developed for CO2 capture purposes. Amino-acid-based solvent systems are of particular interest because of the benign nature of most amino acids and because the ionic nature of the solvents makes them practically nonvolatile.12 Recently, Jockenh€ovel and Schneider13 reported a reduced energy requirement (2.7 MJ/kg) for a CO2 capture process using an amino acid salt system. A further understanding of the fundamentals of reaction mechanisms for amino acid salt solvent systems is of interest in further improvements in the efficiency of such processes as the design of acid-gas-treating processes requires information on mass transfer, reaction kinetics, and physiochemical properties. In the study of the kinetics of amino acid salt systems, the zwitterion mechanism14 has been widely used in the interpretation of experimental data.79,15 Only Kumar et al.7 have applied another r 2011 American Chemical Society
model, namely, the termolecular mechanism,16,17 to interpret data for the reaction kinetics of amino acid salts. The objectives of this work were to characterize the kinetics and mechanisms of CO2 absorption into an aqueous solution of an amino acid salt, namely, the potassium salt of sarcosine (KSAR), using experimental data obtained with a string-of-disks contactor (SDC) for different concentrations in the range of 1.04.0 kmol m3 and for temperatures of 2562 °C. Both the zwitterion and termolecular mechanisms were applied in the interpretation of the experimental data, and the kinetic results were compared with pilot-plant results.
2. THEORY Amino acids such as sarcosine can exist in three states, the acidic state (eq 1), the unstable zwitterion state (eq 2), and the deprotonated zwitterion state (eq 3). In water, the amino acids exist in the zwitterion state. The acidic state and the zwitterion state of amino acids are not active in reactions with CO2. For reaction to occur, the zwitterion sarcosine must be deprotonated. Deprotonation is usually achieved by the use of a strong base such as potassium hydroxide, KOH, which dissociates completely in water (eq 4). Acid-state sarcosine K1SAR
ð1Þ
K2SAR
ð2Þ
NHR 1 R 2 COOH þ H3 Oþ h NH2 þ R 1 R 2 COOH þ H2 O
Zwitterion state NH2 þ R 1 R 2 COOH þ H2 O h NH2 þ R 1 R 2 COO þ H3 Oþ
Received: March 24, 2011 Accepted: August 8, 2011 Revised: July 15, 2011 Published: August 08, 2011 10465
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ð3Þ
If the deprotonation of the zwitterion (eq 10) is the ratedetermining step, then k1/∑kb[B] g 1, and eq 11 becomes Z ¼ k2 kb ½B=k1 ½KSAR½CO2 ð13Þ rCO 2
ð4Þ
If the dominating bases for the reactions are the aqueous amino acid salt (KSAR), water, and hydroxyl (OH) group, then eq 11 becomes
Deprotonated zwitterion state K3SAR
NH2 þ R 1 R 2 COO þ H2 O h NHR 1 R 2 COO þ H3 Oþ
Dissociation of KOH KKOH
KOH h K þ þ OH
In the above equations, R1 = CH3, and R2 = CH2. The overall reaction for the formation of the amino acid salt, the potassium salt of sarcosine (KSAR), can thus be written as
∑
½KSAR½CO2 Z rCO ¼ 1 2 1 þ Z k2 kKSAR ½KSAR þ kZH2 O ½H2 O þ kZOH ½OH
ð14Þ where the secondary amine group in the amino acid salt in the product is now free to react with CO2. Absorption of CO2 into a KSAR solution, could, as for amines, occur through a set of parallel reactions forming carbamate, bicarbonate, and carbonic acid18 CO2 þ 2KSAR h KSARCOO
þ KSARH
þ
ð6Þ
CO2 þ OH h HCO3
ð7Þ
CO2 þ 2H2 O h HCO3 þ H3 Oþ
ð8Þ
19
NMR speciation studies on KSAR reactions with CO2 show that, at low loadings, the main contribution to the KSAR reaction with CO2 is carbamate formation (eq 6). The mechanism for this reaction could be described using either the zwitterion or termolecular mechanism.7 2.1. Zwitterion Mechanism. This is a two-step mechanism originally proposed by Caplow14 and used by Danckwerts20 to explain the reactions of CO2 with amines. The first step is the reaction of CO2 with the amino acid salt to form a zwitterion (reaction 9), followed by deprotonation of the zwitterion by a base B (base catalysis) to form carbamate as in reaction 10.
and eq 13 becomes Z rCO ¼ kZKSAR ½KSAR þ kZH2 O ½H2 O 2 þ kZOH ½OH ½KSAR½CO2
ð15Þ
where = = k2kH2O/k1, and kOHZ = k2kOH/k1. Equation 12 shows an overall second-order kinetics that is first-order with respect to each KSAR and CO2, whereas eq 14, in which the AAS is one of the deprotonating bases, shows a fractional-order rate dependency on the AAS concentration varying from 1 to 2. 2.2. Termolecular Mechanism. Crooks and Donnellan16 described the reaction between CO2 and amines in aqueous solution as a single-step termolecular mechanism. They argued that the zwitterion mechanism proposed by Danckwerts20 is unlikely, as proton transfer has to be rate-determining to have fractional-order kinetics. The termolecular mechanism suggests that amine bonding to CO2 and proton transfer take place more or less simultaneously. This mechanism was assessed by da Silva and Svendsen17 using ab initio calculations and a solvation model, with results supporting the mechanism originally proposed by Crooks and Donnellan.16 They pointed out that any zwitterion formed will have a very short lifetime, so that a singlestep mechanism should better describe the reaction taking place. da Silva and Svendsen17 further used this mechanism to describe the fractional- and higher-order kinetics. Earlier works7,21 argued that the termolecular mechanism cannot describe the fractionaland higher-order kinetics. The single-step termolecular mechanism applied to AAS is given by the equation kZKSAR
Z k2kKSAR/k1, kH 2O
At a quasisteady state, Danckwerts20 showed that the forward reaction rate for the zwitterion formation in reaction 6 can be expressed as Z rCO 2
k2 ½KSAR½CO2 ¼ k1 1 þ kb ½B
ð11Þ
∑
where B indicates the contribution to proton removal in reaction 10 by all bases present in solution. If zwitterion formation (eq 9) is the rate-determining step, then 1 g k1/∑kb[B], and eq 11 simplifies to Z rCO ¼ k2 ½KSAR½CO2 2
ð12Þ
When KSAR, water, and hydroxyl (OH) group are dominating bases, the reaction rate for the forward reaction becomes T rCO ¼ kTKSAR ½KSAR þ kTH2 O ½H2 O 2 þ kTOH ½OH ½KSAR½CO2 ð17Þ which is the same as the limiting case of k1/∑kb[B] g 1 in the zwitterion mechanism. 2.3. Overall Observed Kinetic Rate Constant. From eqs 14 and 17, the overall observed kinetic rate constants can be 10466
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expressed as T Z rCO ¼ rCO ¼ kobs ½CO2 2 2
ð18Þ
Using the termolecular mechanism for example kobs ¼ kTKSAR ½KSAR þ kTH2 O ½H2 O þ kTOH ½OH ½KSAR ð19Þ The apparent kinetic rate constant, kapp, is calculated when contributions to the reaction with CO2 from bases other than the amine/amino acid salt of interest are subtracted from the overall observed kinetic constant. 2.4. Ionic Strength Effect. The reaction mechanisms described in the preceding subsections are suitable when treating amine systems that are nonelectrolyte systems before CO2 is added. For ionic systems such as amino acids salts, it is necessary for the reaction mechanism models to take into account nonidealities resulting from the effect of ionic strength on the reaction rate at increasing salt concentrations. The potential of ionic strength to alter reaction rates has been recognized in many types of reactions.9,2228 A thermodynamically sound model expresses the kinetic constant in term of activities rather than concentrations.29 The nonidealities can be lumped into a salt effect that can be expressed as24,26 ln k ¼ ln k∞ þ bI k ¼ k∞ expðbIÞ
ð20Þ
where k and k∞ are the reaction rate constants with and without salt effects, respectively. I is the ionic strength (I = 1/2∑cizi2), representing the salt concentration, and b is a constant that depends on the nature of the salt. The expression thus implies that limIf0k = k∞ = kobs. To account for this effect, the zwitterion and termolecular mechanisms (i.e., eqs 14 and 17, respectively) were reinterpreted through the introduction of an ionic factor, kion, representing the contribution of ionic strength to the reaction rate when the contribution from the hydroxyl group is negligible, as follows kion ¼ expðbIÞ kapp ¼
Z rCO kion ½KSAR 2 ¼ 1 1 ½CO2 þ Z k2 kKSAR ½KSAR þ kZH2 O ½H2 O
T rCO 2 ½CO2 ¼ kion kTKSAR ½KSAR þ kTH2 O ½H2 O ½KSAR
ð21Þ ð22Þ
kapp ¼
acid salt (AAS), the potassium salt of sarcosine (KSAR), was prepared by neutralizing sarcosine with equimolar amounts potassium hydroxide (KOH). The experimentally derived expression for preparing aqueous KSAR solutions can be found in ref 30. The absorption rates of CO2 into unloaded aqueous KSAR solutions were measured for KSAR concentrations of 1.04.0 kmol m3 and temperatures of 2562 °C using a string-of-disks contactor (SDC) apparatus, as shown in Figure 1. The SDC apparatus has been used for several kinetics measurements.5,31,32 Experiments start by daily calibration of the analyzers using calibration mixtures of CO2 and N2. The SDC column was run in countercurrent flow of liquid from the top and gas from the bottom. The liquid rate was set at ∼50 mL/min, which was above the minimum value required to ensure that the mass-transfer flux was independent of the liquid flow rate, a condition required for the assumptions of the fast reaction regime and pseudo-first-order kinetics to be valid. The liquid flow rate for which flux became independent of liquid rate was determined by the procedure described by Hartono et al.31 When the column attained the required temperature, a mixture of CO2 and N2 was circulated through the column, with makeup gas added to maintain the CO2 level in the gas constant. Readings were taken when a steady state was achieved, as indicated by constant readings by the CO2 analyzer and thermometer. A FisherRosemount BINOS 100 NDIR gas analyzer records the online gas-phase CO2 composition. The analyzer comprises two channels, 02000 ppm and 01% ( 0.01%; it thus measures low gas-phase CO2 compositions with good accuracy. A detailed description of the apparatus and experimental procedure can be found in Ma’mun et al.32
4. DETERMINATION OF KINETIC RATE CONSTANT 4.1. Mass-Transfer Properties. CO2 absorption in an AAS is by reactive chemical absorption: the mass transfer is enhanced by the chemical reaction. The flux expression can be written as
NA ¼
1 ðC CA, b Þ 1 RT A þ EA kl Hkg
ð24Þ
The enhancement factor, EA, is the ratio of the liquid-side masstransfer coefficient with chemical reaction to the liquid-side mass-transfer coefficient without the chemical reaction under the same driving force.33 The enhancement factor can be calculated using different mass-transfer models.34 For reaction kinetics to be determined in an absorption experiment or gasliquid reaction, the reaction must be in the fast pseudofirst-order reaction regime. The pseudo-first-order condition is fulfilled when 3 < Ha , E∞
ð23Þ
ð25Þ
From the expression for kion (eq 21), it is clear that, for the limiting case where ionic strength is zero, kion = 1. The kapp expressions then become the same as the expressions for amines.
The Hatta number, Ha, is given by pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi kobs DCO2 Ha ¼ kl
3. KINETICS EXPERIMENT The chemicals were obtained as follows: sarcosine (Fluka, purity g 98%), potassium hydroxide (KOH) (Merck KGaA, purity g85%). The gases CO2 (purity > 99.999 mol %) and N2 (purity > 99.999 mol %) were supplied by Yara Praxair AS. All solution samples were prepared with deionized water. The amino
where E∞ is the infinite-dilution enhancement factor, which can be estimated, according to penetration theory, by the equation35 sffiffiffiffiffiffiffiffiffiffiffi rffiffiffiffiffiffiffiffiffiffiffi DCO2 ½AmA DAmA þ ð27Þ E∞ ¼ P CO2 DAmA DCO2 vCO2 HCO2 10467
ð26Þ
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Figure 1. String-of-disks contactor kinetics apparatus.31
According to the penetration model, the enhancement factor for an irreversible reaction can be written as34,35 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð28Þ EA ¼ 1 þ Ha2 In the fast pseudo-first-order regime, diffusion and reaction occur in parallel in the liquid film; the enhancement factor can thus be assumed to be equal to the Hatta number (Ha = EA), and the absorption rate can be assumed to be independent of the physical mass-transfer coefficient (i.e., the liquid flow rate). Substituting eqs 26 and 28 into eq 24 and noting that the experiments were performed in the fast reaction regime with very low CO2 loadings, the concentrations of CO2 in the bulk were practically zero. Then, CA,b ≈ 0, and the expression for average CO2 absorption flux for the system (eq 24) can be written as CA NA ¼ ð29Þ 1 RT pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi þ Hkg kobs DCO2 The experimental CO2 flux (NA) was calculated using the same procedure as Ma’mun et al.32 by taking a CO2 balance over the entire system, that is, the difference between the flow of CO2 into the system, as measured by a mass flow meter, and that going out of the system into the analyzer. The string-of-disks contactor was characterized by Hartono et al.,31 for the liquid-film mass-transfer coefficient using the procedure of Ma’mun et al.32 The correlation based on the original Stephens and Morris36 was reinterpreted and is given by kl =D ¼ 17:92ð4Γ=μÞ1:0 ðμ=FDÞ0:5
ð30Þ
whereas the gas-side mass-transfer coefficient (eq 31) is given by32 kg d=D ¼ 0:12ðFvd=μÞ0:79 ðμ=FDÞ0:44
ð31Þ
4.2. Physiochemical Properties. Physiochemical properties such as density, viscosity, and N2O solubility are required to enable kinetics calculations. The physiochemical property data required in this work were reported in earlier work.37 The diffusivity of N2O into aqueous KSAR solution was estimated from viscosity using a modified StokesEinstein relation38 as !0:8 μH2 O DCO2 KSAR ¼ DCO2 H2 O ð32Þ μKSAR
The exponent R = 0.8 for the relation was obtained using experimental values of the diffusion coefficient for N2O. Brilman et al.39 concluded that the ionic strength of salt solution does not influence the diffusion coefficient, and Portugal et al.9 found a lower than 5% error between using R = 0.8 or the R = 0.74 value obtained by Kumar et al.,40 for aqueous potassium taurate. This is less than the general experimental error for the determination of diffusion coefficients. Therefore, R = 0.8 was used in this work. Diffusion of CO2 into salt solution was determined using the N2O analogy.41 Portugal et al.9 earlier argued that the so-called N2O analogy cannot be directly applied to estimate the solubility of CO2 in amino acid salt solutions because such solutions are ionic, but van Holst et al.8 showed that estimation of the physical solubility of CO2 using either the N2O analogy1,42 or the Weisenberger and Schumpe43 method yielded the same result. This work thus estimated the physical solubility of CO2 (Henry’s law constant) in aqueous KSAR solution using the N2O analogy. 4.3. Observed Kinetic Rate Constant. It is now possible to calculate the overall observed kinetic constant (kobs). For primary and secondary alkanolamines, the hydration reaction of CO2 (eq 8) is a very slow reaction, and its contribution to the overall reaction can be disregarded,18 whereas the bicarbonate formation reaction (eq 7) can be substantial at low concentrations of 10468
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secondary amines and is essential for tertiary amines. The forward reaction rate can be expressed as44 rCO2 ¼ kOH ½CO2 ½OH
ð33Þ
Pohorecki and Moniuk45 showed that kOH is a strong function of ionic strength and defined an expression for the rate constant for the CO2 reaction with OH. This expression has been widely used in the literature, but it has strictly been shown to be valid only for 1841 °C. Similarly, Astarita et al.33 defined an expression for kOH as a function of ionic strength that is valid from 0 to 110 °C, namely log kOH ¼ 13:635 2895=T þ 0:08I
5. RESULTS AND DISCUSSION The kinetics of the reaction between CO2 and aqueous KSAR solution was determined using a string-of-disk contactor for concentrations of 14 kmol m3 and temperatures of 2562 °C. The experimental results are summarized in Table 1. All experiments were conducted at practically zero loadings; thus, ionic strength, I, equals the initial salt concentration, Cs. Water concentrations were determined experimentally. Two different models (zwitterion and termolecular) were used to correlate the experimental results. Correlations for the rate constant for the two different models were derived from the experimental data as follows kion ¼ expð0:38Cs Þ
ð34Þ
This work calculated kOH using the expression from Astarita et al.33 because measurements were carried out up to 62 °C. It should be pointed out, however, that use of either expression gave the same result. The hydroxyl ion concentration for loadings close to zero can be estimated as31,33 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð35Þ ½OH ¼ Kw ½KSAR=Ka
For the termolecular mechanism, the results are kTKSAR ¼ 6:3494 106 exp½1589:6=T ðKÞ m6 kmol2 s1 kTH2 O ¼ 3:9805 108 exp½3924:4=T ðKÞ m6 kmol2 s1 ð41Þ For the zwitterion mechanism, the results are
The dissociation constant for water (Kw) was taken from Olofsson and Hepler,46 and that for aqueous KSAR (Ka) (eq 3) was taken from Aronu et al.30 ln Ka ¼ 5:9752 5185:1=T
kapp ¼ kobs k
OH
½OH
ð37Þ
Calculations from this work indicate that the contribution from OH to the overall reaction rate is very small and negligible; thus, the carbamate formation reaction (eq 6) dominates. van Holst et al.8 earlier observed a negligible contribution of OH for amino acid salts. In this work, the experimental apparent kinetic rate constant was represented using the zwitterion mechanism (eq 22) and termolecular mechanism (eq 23). These mechanisms take into account the effect of ionic strength on the reaction rate of the amino acid salt solutions using eq 21. The model parameters—three in the zwitterion mechanism and two in the termolecular mechanism—were determined using a MATLABbased parameter estimation tool, Modfit.47 The objective function used minimized the relative error between the calculated and experimental values F ¼
n
calc kexp app kapp
i¼1
kapp
∑
!2 ð38Þ
exp
The deviations between the model results and the experimental data were determined as absolute average relative deviations (AARDs) by AARD ¼ 100%
1 n
∑n
jkmodel kexp j kexp
k2 ¼ 2:6198 109 exp½915:8=T ðKÞ m3 kmol1 s1 kZKSAR ¼ 6:3494 106 exp½1589:6=T ðKÞ m6 kmol2 s1 kZH2 O ¼ 3:9805 108 exp½3924:4=T ðKÞ m6 kmol2 s1
ð36Þ
The apparent kinetic rate constant can thus be expressed as
ð39Þ
ð40Þ
ð42Þ Cullinane and Rochelle27 reported that a rigorous model used to interpret the effect of ionic strength gave an average value of b in eq 21 of 0.45 ( 0.10. In this work, a value of b = 0.38 was found. This is within the accuracy expected for b. The termolecular mechanism has two parameters, whereas the zwitterion mechanism has three. The results from this work show that the two models give identical results. The models for the kinetic constants kKSAR and kH2O for the two models are the same, indicating that the k2 parameter of the zwitterion mechanism makes no significant contribution to the model. This agrees with the findings of Hartono et al.,31 who reported that, in fitting their results to the zwitterion mechanism, noise was introduced by the fitting of k2. The large values of k2 found in this work indicate in eq 13 that the deprotonation of the zwitterion is the ratedetermining step; therefore, the zwitterion and termolecular mechanism expressions become identical. kapp does not vary significantly (
