Kinetics of CO2 Absorption in Aqueous Hexamethylenediamine

Article pubs.acs.org/IECR

Cite This: Ind. Eng. Chem. Res. 2017, 56, 14902−14913

Kinetics of CO2 Absorption in Aqueous Hexamethylenediamine Blended N‑Methyldiethanolamine Bikash K. Mondal,† Syamalendu S. Bandyopadhyay,‡ and Amar N. Samanta*,† †

Chemical Engineering Department, Indian Institute of Technology Kharagpur, Kharagpur 721302, India Cryogenic Engineering Centre, Indian Institute of Technology Kharagpur, Kharagpur 721302, India

‡

ABSTRACT: Carbon dioxide (CO2) absorption kinetics in the aqueous blend of hexamethylenediamine (HMDA) and Nmethyldiethanolamine (MDEA) is studied at the temperatures varying from 303 to 333 K using the pressure decay technique in a reaction calorimeter setup. For the experimental study, HMDA concentration is varied in the range of 5−15 mass % keeping total amine (HMDA + MDEA) concentration at 30 mass %. Overall rate constant for this reaction system is estimated assuming pseudo-first-order reaction condition. CO2 absorption kinetics are significantly enhanced due to the presence of HMDA as compared to single MDEA solvent. The individual rate contribution of CO2−HMDA and CO2−MDEA reaction systems are combined to represent the overall CO2 absorption rate in this mixed amine solvent. The kinetic models (I and II) developed using the zwitterion and termolecular mechanism for HMDA− CO2 reaction system and base catalyzed hydration mechanism for MDEA-CO2 reaction are able to predict the kinetic data with good accuracy.

1. INTRODUCTION The major global challenge in the coming years is to reduce the carbon dioxide emission in view of its apparent contribution to the global warming. Fossil fuel-based power plants release major amounts of CO2 in the atmosphere; 40% of the world’s electricity is generated by burning coal which contributes about 44% of the total CO2 emission worldwide.1 To mitigate the atmospheric CO2 emission, immediate implementation of the postcombustion capture and clean coal technology is essential. For the rapid capture of CO2 from large scale industrial gas streams, amine scrubbing is the most developed and widely used technology. Aqueous solutions of monoethanolamine (MEA) and diethanolamine (DEA) are the conventional amines used for the CO2 capture.2 But, the use of these amines for CO2 capture from power plant flue gases leads to a substantial energy penalty due to high regeneration energy requirement to handle large volumetric flow rates of the flue gas stream having 10−15% (v/v) CO2 content.3 So the CO2 capture research is mainly aimed at developing a novel solvent with higher CO2 absorption rate and capacity and lower enthalpy of absorption. The higher loading potential and low regeneration energy requirement of noncarbamate forming amine N-methyldiethanolamine (MDEA),4−7 has made it an attractive solvent for CO2 capture. But the CO2 absorption rate with aqueous MDEA is very slow. So, this amine is mixed with another amine of higher CO2 absorption rate to improve the kinetic characteristics of the solvent. Much literature is available on the MDEA blended conventional amine solvent such as (MDEA + MEA),8,9 (MDEA + DEA)10 to make the solvent more efficient in terms of kinetics © 2017 American Chemical Society

as well as regeneration energy. It is found from the recent literature that aqueous diamine has a high CO2 absorption rate and capacity compared to the conventional amines. Some of the diamines which are found in the recent literature are piperazine (PZ)11 and its derivatives,12,13 2-((2-aminoethyl)amino)ethanol (AEEA),14 hexamethylenediamine (HMDA),15−17 1,4-butanediamine (BDA), 1 8 ethylenediamine (EDA), 1 9 and 3(methylamino)propylamine (MAPA).20 These diamines can potentially improve the solvent characteristics to a greater extent compared to the conventional MDEA blends because of their superior kinetic and loading capacity. Literatures are available on the PZ promoted MDEA solvent21−27 which show enhanced CO2 absorption kinetics and loading capacity of this activated solvent. CO2 absorption study in the aqueous (MDEA + MAPA)28 and (MDEA + AEEA)29 also indicate potential of the diamine as better solvent activator. In our earlier works16,30 it is shown that HMDA has very high CO2 absorption rate (k2,HMDA: 59190 m3 kmol−1 s−1 at 313 K) and loading capacity (CO2 loading: 1.143 mol/mol at 15 kPa CO2 partial pressure and 313 K temperature). These superior solvent properties of HMDA and dearth of literature data motivated us to study the kinetic characteristics of aqueous (HMDA + MDEA) solvent. High CO2 loading potential31 and low absorption enthalpy32 of this blended solvent is already presented in our previous works. In this work, Received: Revised: Accepted: Published: 14902

July 5, 2017 November 20, 2017 November 21, 2017 November 21, 2017 DOI: 10.1021/acs.iecr.7b02744 Ind. Eng. Chem. Res. 2017, 56, 14902−14913

Article

Industrial & Engineering Chemistry Research

Figure 1. N2O solubility measurement setup: (1) Stirred-cell contactor; (2) buffer cell for N2O storage; (3) gas phase stirrer; (4) liquid phase stirrer (magnetic bar); (5) magnetic stirrer speed controller; (6) pressure transducers; (7) circulator temperature controller; (8) N2O cylinder; (9) temperature sensors; (10) liquid solution inlet; (11) external temperature sensor of circulator temperature controller; (12) thermostated water bath.

solvent to obtain dynamic viscosity. Each measurement is quintuplicated, and the standard relative uncertainty is calculated to be within ±1.5%. 2.4. N2O Solubility and Liquid Phase Mass Transfer Coefficient Measurement. Liquid phase mass transfer coefficient and physical solubility data are required to analyze the kinetic behavior of the CO2−amine system. Because of the reactive characteristics of CO2 with the amine solution, the physical solubility is measured using N2O which does not react with aqueous amine and also has a similar molecular and electronic configuration as CO2.33,34 First, equilibrium N2O solubility in the aqueous amine solution is measured in the experimental temperature range and then it is converted to physical CO2 solubility using the method of N2O-analogy. 2.4.1. Experimental Procedure. Experimental setup (Figure 1) and procedure for the solubility measurement is presented in our previous work16 and it is described here briefly. N2O solubility is measured in a stirred cell contactor (500 × 10−6 m3) connected to a buffer cell (650 × 10−6 m3) through a needle valve. The temperature of the cells are maintained using a thermostated water bath (JULABO F 32 HL, FRG, accuracy: ±0.1 K). For each experimental run, 200 × 10−6 m3 amine solution at the desired temperature is kept in the stirred cell under vacuum condition for at least 1 h to record the vapor pressure of the solvent. Then N2O gas from the buffer vessel at the desired temperature is transferred to the stirred cell (PN2O ≈ 100 kPa) and stirrers (both gas phase and liquid phase) are started. Pressure decrease in the stirred cell indicated by the pressure transmitter (model, Rosemount 3051TA; range, 0−50 psia; accuracy, ±0.04% of the range) is recorded continuously. Equilibrium condition is indicated when there is no change of cell pressure for at least 1 h. 2.4.2. Estimation of N2O Solubility. N2O solubility in aqueous amine solvent is estimated using the following expression.33

the kinetics of CO2 absorption in aqueous (HMDA + MDEA) solvent is studied in the temperature range of 303−333 K for its potential application as CO2 capture solvent.

2. EXPERIMENTAL SECTION 2.1. Materials. Reagent grade HMDA (minimum purity 98% by mass) and MDEA (minimum purity 99% by mass) are purchased from Sigma-Aldrich India. Nitrogen, nitrous oxide, and carbon dioxide with minimum purity of 99.99% (by volume) are supplied by Linde India Limited. Aqueous amine solutions are prepared on a mass percentage basis without further purification of the chemicals using a precision balance (CITIZEN, CX-301 model, accuracy: ±0.001g). 2.2. Density. The density of aqueous (HMDA + MDEA) is required to estimate molar concentration of the solvent from the mass % concentration. It is measured using a standard GayLussac pycnometer (∼25 × 10−6 m3 at 298 K). Before the experiment, the volume of the pycnometer is standardized using double distilled water. The density of the solvent is measured dividing the mass of the solvent contained in the pycnometer by the standard volume of the pycnometer at constant temperature. To maintain the experimental temperature, a thermostated water bath (JULABO F32 HL, FRG) with a precision of ±0.1 K is used. Each experiment is repeated for at least three times and relative standard uncertainty is calculated to be ±0.1%. 2.3. Measurement of Viscosity. Viscosity data is required to estimate diffusivity of CO2 in the solvent. It is also an important property for solvent pumping cost estimation. A Cannon-Fenske viscometer (size, 50; viscometer constant (VC), 0.004 cSt·s−1) is used to measure the kinematic viscosity (KV) of the solvent. KV (cSt) is estimated by multiplying the efflux time of a specific volume of the solvent through the capillary of the viscometer with VC (0.004 cSt·s−1) at constant temperature. A temperature controlled water bath (JULABO F32 HL, FRG) with a precision of ±0.1 K is used to maintain experimental conditions. Then KV is multiplied by the density (g·mL−1) of the 14903

DOI: 10.1021/acs.iecr.7b02744 Ind. Eng. Chem. Res. 2017, 56, 14902−14913

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Industrial & Engineering Chemistry Research m N2O, m =

(PNi 2O − PNe 2O) Vg RT = . HN2O, m PNe 2O Vl

So eq 3 simplifies to the following. (1)

NCO2 = mCO2EkL

Here mN2O,m is the dimensionless physical solubility defined as the ratio of equilibrium N2O concentration in the liquid to the gas phase. HN2O,m is Henry constant in the aqueous mixed solvent, PNi 2O is initial N2O partial pressure in the cell, PeN2O is the equilibrium partial pressure of N2O in the cell, Vg and Vl are gas phase and liquid phase volume in the cell. 2.4.3. Estimation of Liquid Phase Mass Transfer Coefficient. Liquid phase mass transfer coefficient (kL) (in absence of chemical reaction) is estimated using the expression35 given below. ln PN2O|t = t = −

m N2OAkL Vg

NCO2 = −

NCO2 = K G(PCO2

dPCO2

+

RT mCO2EkL

=−

mCO2EkLA VG

DCO2

Ei =

PCO2

(7)

DAm

DAm [Am]RT DCO2 νCO2PCO2mCO2

+

(8)

In this equation DCO2 and DAm are the diffusivities of CO2 and amine in the aqueous solution, [Am] is amine solution concentration, νCO2 is stoichiometric coefficient of CO2 in the reaction. Ha is the ratio of maximum reactive conversion rate of CO2 in the film to the maximum diffusional transport through the film.35 According to the film theory of mass transfer, E and Ha are related as follows. E=

Ha tanh(Ha)

(9)

The terms involved in eq 7 can be converted to measurable quantity by assuming pseudo-first-order (PFO) reaction conditions. Criteria for the PFO reaction conditions is as follows.36 3 < Ha ≪ E i

(10)

With this assumption, E becomes equal to Ha. Then E can be expressed as follows.

(3)

E = Ha =

with 1 1 RT = + KG kG mCO2EkL

(6)

In this expression, E is a function of infinite enhancement factor (Ei) and Hatta number (Ha). Ei is the enhancement factor corresponding to the infinitely fast reaction of CO2 in the liquid film when the absorption rate is limited by the diffusion process only. Ei for a single irreversible reaction is given by,

(2)

* ) (PCO2 − PCO 2 1 kG

V dPCO2 1 dnCO2 =− G A dt ART dt

Here, A, VG, T, and R are the mass transfer area, gas phase volume, temperature of the system, and universal gas constant, respectively. Then, combining and rearranging eq 5 and eq 6 gives

where Vg and A are the gas phase volume and interfacial area of gas−liquid interaction, respectively. PN2O and mN2O,m denote N2O partial pressure in the stirred cell and dimensionless physical solubility parameter in the mixed solvent, respectively. From the slope of the “ln PN2O versus time” plot, kL can be calculated. Since the physical absorption process (without chemical reaction) is slow, slope is calculated using data up to 100 s. 2.5. Carbon Dioxide Absorption Rate Measurement. The rate of CO2 absorption in aqueous (HMDA + MDEA) solvent is measured using the “pressure decay method”. Formulation of the pressure decay expression and kinetic measurement technique are as follows. 2.5.1. Derivation of the Pressure Decay Expression. Absorption of CO2 in aqueous amine solution is a complex phenomenon involving phase equilibria and reaction kinetics. During the absorption process CO2 is dissolved in the liquid phase physically and then reacts with amine components forming different ions. Because of the chemical reaction, the rate in the chemical absorption is much faster compared to the purely physical absorption process. This improved absorption rate is characterized by the enhancement factor (E) which is the ratio of the chemical absorption flux to the physical absorption flux under same driving force. Assuming film theory, CO2 absorption flux (NCO2) in an aqueous amine solution can be presented as follows. * )= − PCO 2

(5)

RT

Again, the flux can also be expressed as

dt

t + ln PN2O|t = 0

PCO2

k OVDCO2 kL

(11)

Replacing the enhancement factor with Hatta number expression, eq 7 becomes as follows:

(4)

where PCO2 is the CO2 partial pressure in the system, P*CO2 is the equilibrium CO2 partial pressure corresponding to the CO2 concentration in the bulk liquid, KG is the overall mass transfer coefficient and mCO2 is the physical solubility of CO2 in aqueous amine solvent. kG and kL are the gas phase and liquid phase mass transfer coefficient, respectively. The terms involved in the above equation can be simplified using the following assumption. • negligible CO2 concentration in the bulk liquid (and hence * 2 = 0) PCO • negligible gas phase resistant (and hence 1 = 0).

dPCO2 dt

=−

mCO2A k OVDCO2 VG

PCO2

(12)

This is the differential form of pressure decay expression When eq 12 is integrated, the integral form of the pressure decay expression is obtained as shown below.37 ln PCO2|t = t = −

mCO2A k OVDCO2 VG

t + ln PCO2|t = 0

(13)

where VG and A are the gas phase volume and interfacial area of gas−liquid reaction, respectively. PCO2, mCO2, DCO2, and kOV

kG

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Figure 2. Schematic of CO2 absorption rate measurement set up. (1) double-jacketed glass reactor (reaction calorimeter, model, RC1e); (2) anchor type impeller; (3) reactor temperature sensor; (4) calibrated heater (25W); (5) Rosemount pressure transmitter (model, 3051TA); (6) magnetically coupled stirrer; (7) solvent inlet; (8) (a) N2 cylinder, (b) CO2 cylinder; (9) (a) N2 gas regulator, (b) CO2 gas regulator; (10) thermostated water bath; (11) temperature sensor to control water bath temperature; (12) CO2 feed coil.

Figure 3. Pressure decay plot for CO2 absorption rate measurement (for 5% HMDA + 25%MDEA at 303 K).

calorimeter setup (Mettler Toledo; model, RC1e). The reactor of the calorimeter setup is a thermostated glass vessel (volume, 1.2 × 10−3m3) equipped with an impeller (anchor type), a digital pressure transducer (model, Rosemount 3051TA; range, 0−15 psia; accuracy, ±0.04% of the pressure range), a calibrated heater (25 W) and a temperature sensor (Pt 100). The reactor is connected to a coil dipped in a temperature controlled water bath for feeding CO2 into the reactor at the desired temperature. The entire system is tested to make it leak-proof. For rate measurement, 500 g of aqueous solution is heated to the desired temperature under vacuumed condition for at least 1

denote the partial pressure of CO2, dimensionless physical solubility, diffusivity of CO2, and overall reaction rate constant, respectively. From the slope of the “ln PCO2 versus t” (eq 13), the overall rate constant for the CO2 absorption kinetics can be evaluated since all other parameters are known. 2.5.2. Absorption Kinetics Measurement Setup and Procedure. Details of the CO2 absorption rate measurement setup (Figure 2) and experimental procedure are published in our earlier work16 which is described here briefly. CO 2 absorption rate in this work is measured using a reaction 14905


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Here ηi, xi, and Gij denote dynamic viscosity of pure component i, mole fraction of component ‘i’ in the mixture, and interaction parameter between components i and j, respectively. For the ternary mixed solvent system viscosity is given by the following expression

h and then vapor pressure (Pv) of the solvent is recorded. Once the reactor content attains thermal equilibrium, a small amount of CO2 heated to the desired temperature is transferred into the reactor. After feeding CO2 gas, the reactor content is stirred at 150 rpm and real time data of the pressure decay inside the reactor due to chemical absorption is logged continuously. This pressure decay data is used to estimate the overall rate constant (kOV) by applying the pressure decay expression (eq 13). A typical plot of “ln (PCO2/Pa) versus t/s” is shown in Figure 3. From the initial slope of the plots, kOV is calculated. Initial CO2 partial pressures for all the experimental runs are kept low to uphold the PFO condition.

ln ηm = x1 ln η1 + x 2 ln η2 + x3 ln η3 + x1x 2G12 + x1x3G13 + x 2x3G23

3.3. Physical Solubility Model. The physical solubility of N2O in aqueous (HMDA + MDEA) solution is modeled using excess Henry’s coefficient model developed by Wang et al.40 The excess Henry coefficient is defined as the excess quantity over the sum of the product of the volume fraction and logarithm of individual Henry’s coefficient in the mixed solvent. Expression for the excess Henry’s coefficient as defined by Wang is shown below.

3. MODEL 3.1. Density Model. For modeling the density data of aqueous (HMDA + MDEA), the excess molar volume expression proposed by Redlich and Kister38,39 is used. It is presented as follows. V E = Vm −

∑ xiVio

n

R = ln HN2O, m −

(14)

(22)

n

∑i xiMi ρm

φi = xiV io/∑ xiV io

(15)

xi and are the mole fraction and molar volume of component i in the mixture. Again, for a ternary mixed solvent system, the excess Henry coefficient is given by the following equation.

n E V12 /m 3·kmol−1 = x1x 2 ∑ Ai (x1 − x 2)i i=0

R = φφα + φφ α + φφ α + φφφ α i j ij i k ik j k jk i j k ijk

(17)

where Ai are interaction parameters. These are assumed to be the function of temperature as given below. a Ai = i (18) T

αij = k1 +

∑ xi ln ηi + ∑ ∑ xixjGij i

j

(19)

with

b2 (b3 + T )

k2 k3 + T

(25)

Equation 22 and 24 are combined to form the model expression for the Henry’s coefficient (HN2O,m) which is fitted to the experimental data by regressing the coefficients of αij. 3.4. Rate Model. The overall rate of CO2 absorption in the (HMDA + MDEA + H2O) mixed solvent is the combined rate contribution of HMDA−CO2, MDEA−CO2, and water−CO2 reaction system. For the amine−CO2 reaction, there is three generally accepted mechanisms to analyze the kinetic behavior of CO2 absorption. These are the zwitterion, termolecular, and base catalyzed hydration mechanisms. The rate contribution and reaction mechanism of the individual reaction systems are presented in the following section. 3.4.1. Reaction Rate Dependence on HMDA. In the HMDA molecule, two primary amine groups are connected through a six carbon straight chain. It forms a carbamate ion upon reaction with CO2 in the aqueous medium. In this work, both zwitterion

3.2. Viscosity Model. The Grunberg and Nissan (1949) model38,39 is used in this work to correlate the dynamic viscosity of the ternary (HMDA + MDEA + H2O) solvent with that of pure component (HMDA, MDEA, and H2O) viscosity. They introduced a temperature-dependent interaction parameter to account for the deviation in the viscosity due to nonideal mixing. The expression for this model is shown below. i

(24)

Here φi, φj, and φk are the volume fraction of HMDA, water, and MDEA in the mixed solvent, respectively. αijk is a three-body interaction parameter which is set to be constant for the ternary system studied. αij, αik, and αjk are two-body interaction parameters which are assumed to be a function of temperature. The temperature dependency of these parameters is expressed as follows.

(16)

Excess molar volume expression for the ternary mixed solvent system (HMDA + MDEA + H2O) is given by the following equation. E E E V E = V12 + V13 + V23

(23)

i=1

Voi

here, Mi and xi are molecular weight and mole fraction of component i and ρm is the measured density of mixed solvent. Excess molar volume for a binary mixed solvent system, can be expressed as given below.

Gij = b1 +

2

where HN2O,m, HN2O,i and φi denote the Henry coefficient of N2O in the mixed solvent, Henry coefficient of N2O in pure component i ,and volume fraction of the component i in the mixed solvent, respectively. Volume fraction is calculated as given below.

where Vm and Voi denote the molar volume of the mixed solvent and the molar volume of the pure component respectively at the system temperature. Mixed solvent molar volume is calculated as follows.

ln ηm /mPa·s =

∑ φi ln HN O,i i=2

i

Vm =

(21)

(20) 14906


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Industrial & Engineering Chemistry Research and termolecular mechanisms are used to analyze the kinetic characteristics of the CO2−HMDA reaction. Zwitterion Mechanism. The reaction steps using this mechanism are as follows.

rCO2 − HMDA = (k H2O[H 2O] + kMDEA[MDEA] + kHMDA[HMDA])[HMDA][CO2 ]

3.4.2. Reaction Rate Dependence on MDEA. Since MDEA is a tertiary amine, it undergoes base-catalyzed hydration reaction with CO2. The mechanism for this reaction is shown below.

It is reported by many authors37,41,42 that the reaction of CO2 with aqueous MDEA follows a pseudo-first-order reaction kinetics. Rate expression for this reaction is as follows.

According to this mechanism, the reaction rate can be written as given below. rCO2 − HMDA = =

k 2,HMDA[CO2][HMDA] 1+

rCO2 − MDEA = k 2,MDEA[MDEA][CO2 ]

k−1 ∑ k b[B]

+

1 k2,HMDAk H O 2 [H O] + k2,HMDAk MDEA [MDEA] + k2,HMDAk HMDA [HMDA] 2 k−1 k−1 k−1

(27)

CO2 + H 2O ↔ H 2CO3

where Σkb[B] is the contribution of all bases (HMDA, MDEA, OH−, H2O) for the deprotonation reaction. k2,HMDA and k−1 are the forward and reverse rate constants for the zwitterion formation reaction. kHMDA, kMDEA, and kH2O are the rate constants for the deprotonation reaction. In this reaction system H2O, MDEA, and HMDA are taken as deprotonating bases. The contribution of OH− ion is neglected because of low concentration. Termolecular Mechanism. The reaction in the termolecular mechanism proceeds through the formation of a loosely bound encounter complex in a single step as shown below.

−

CO2 + OH ↔

(28)

[HMDA][CO2 ] 1 k 2,HMDA

+

+

(35)

k OV = k 2,MDEA[MDEA] + (k H2O[H 2O] + kMDEA[MDEA]

[HMDA] k 2,HMDA

+

k2,HMDAk HMDA [HMDA] k−1

where, kOV, the overall rate constant for (HMDA + MDEA + H2O + CO2) reaction system, is given by the following equation.

k OV = k 2,MDEA[MDEA] 1

= k OV[CO2 ]

1 k2,HMDAk H O k2,HMDAk MDEA 2 [H 2O] + [MDEA] k−1 k−1

where, kOV is the overall rate constant for (HMDA + MDEA + H2O + CO2) reaction system and it is given by the following expression.

+

(33)

Kinetic Model I. On the basis of the zwitterion mechanism for HMDA−CO2 reaction system and base catalyzed hydration mechanism for MDEA−CO2 reaction system, the overall rate expression becomes,

The rate expression using this mechanism where H2O, MDEA, and HMDA are the dominating bases (B), can be presented as follows. rOV = k 2,MDEA[MDEA][CO2 ] +

(32)

HCO−3

Since the rate constant of the CO2 hydration reaction (eq 32) is very small (kH2O = 0.026 s−1 at 298 K),43 the contribution of this reaction to the overall rate is generally neglected. The contribution of the bicarbonate formation reaction (eq 33) is also neglected without significant loss of the accuracy in the reaction rate due to the low concentration of hydroxyl ion.37,41,42 3.4.4. Overall Reaction Rate of CO2 with (HMDA + MDEA + H2O). The overall CO2 absorption rate in the (HMDA + MDEA + H2O) solvent, is given below. rOV = rCO2 − HMDA + rCO2 − MDEA (34)

HMDA + CO2 + B ↔ [encounter complex] → (HMDA)COO− + BH+

(31)

This second-order rate equation is used to represent the rate of the CO2−MDEA reaction system. 3.4.3. Reaction Rate Dependence on Water. Water interacts with the CO2 by the following reactions

[HMDA][CO2 ] 1 k 2,HMDA

(29)

1

+ kHMDA[HMDA])[HMDA]

k2,HMDAk H O 2 [H O] + k2,HMDAk MDEA [MDEA] + k2,HMDAk HMDA [HMDA] 2 k−1 k−1 k−1

(38)

(36)

4. RESULTS AND DISCUSSION Physico-chemical properties and CO2 absorption rate in the HMDA-blended MDEA solvent is studied in this work at the temperatures 303, 313, 323 and 333 K. Solvent compositions used for the physicochemical property study are (5% HMDA + 25% MDEA + 70% H2O), (10% HMDA + 20% MDEA + 70% H2O), (15% HMDA + 15% MDEA + 70% H2O), and (20% HMDA + 10% MDEA + 70% H2O), and for the rate measurement HMDA composition varied in the concentration

Kinetic Model II. Using a termolecular mechanism for the HMDA−CO2 reaction system and a base-catalyzed hydration mechanism for the MDEA−CO2 reaction system, the rate expression takes the following form. rOV = k 2,MDEA[MDEA][CO2 ] + (k H2O[H 2O] + kMDEA[MDEA] + kHMDA[HMDA])[HMDA][CO2 ] = k OV[CO2 ]

(37) 14907


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Industrial & Engineering Chemistry Research Table 1. Physico-Chemical Properties of Aqueous (HMDA + MDEA) composition

temperature

density

viscosity

diffusivity ×109

dimensionless N2O solubility

physical CO2 solubility

%HMDA + %MDEA

K

g/mL

cP

m2/s

mol/mol

mol/mol

5% + 25%

303 313 323 333 303 313 323 333 303 313 323 333 303 313 323 333

1.0150 1.0101 1.0043 0.9976 1.0067 1.0013 0.9951 0.9884 0.9983 0.9925 0.9862 0.9795 0.9895 0.9837 0.9772 0.9700

2.89 2.10 1.59 1.24 2.80 2.02 1.54 1.20 2.73 1.98 1.51 1.19 2.72 1.98 1.50 1.18

0.836 1.139 1.508 1.961 0.857 1.173 1.548 1.999 0.872 1.192 1.571 2.022 0.875 1.193 1.579 2.030

0.490 0.436 0.393 0.361 0.498 0.442 0.407 0.374 0.507 0.455 0.417 0.390 0.514 0.463 0.428 0.405

0.675 0.614 0.569 0.534 0.686 0.625 0.588 0.554 0.699 0.643 0.604 0.576 0.708 0.654 0.620 0.599

10% + 20%

15% + 15%

20% + 10%

Figure 4. Density of aqueous (HMDA + MDEA) solvent: Experimental data and model predictions (AAD, 0.05%).

range of 5−15% keeping the total amine concentration at 30%. Experimental data and modeling results are as follows. 4.1. Density. The density of unloaded aqueous (HMDA + MDEA) solvent measured at different temperatures and solvent compositions is tabulated in Table 1 and shown in Figure 4. As seen from the figure, the density of the solvent linearly decreases with the increase in temperature as well as with the increase in the HMDA concentration in the solvent (total amine concentration being 30 mass %). The solvent density data is correlated using the Redlich−Kister excess molar volume model. For the estimation of excess molar volume of the aqueous ternary mixed solvent, pure component density data is required. Molar volume data of pure water and MDEA are taken from the work of Hsu and Li44 and that for HMDA is taken from our previous work.16 Binary parameters of the Redlich−Kister model are regressed to fit experimental molar volume data to that of the model expression using the least-square method. Regressed model parameters of the aqueous (HMDA + MDEA) solvent system are given in Table 2. Using these parameters, the model predicted density

Table 2. Binary Parameters of the Excess Molar Volume Model for [(HMDA (3) + MDEA (2) + H2O (1)] Solvent binary pair parameter

H2O−MDEA

H2O−HMDA

MDEA−HMDA

a0 a1 a2

−565.27 −878.16 −1117.29

−698.35 −1037.48 −1296.68

−430.82 −4.29 4.14

values are compared with the experimental density data in Figure 4. This figure reveals a good correlation between experimental data and model predicted results with an average absolute deviation (AAD = (1/n)Σin= 1 |(MExp − MMod )/MExp i i i |) of 0.05%. Density data obtained in this work is used to estimate the molar concentration of the solvent and dynamic viscosity data. 4.2. Viscosity. Dynamic viscosity of the unloaded solvent is estimated from the measured kinematic viscosity and density data. Dynamic viscosity data obtained in this work is given in Table 1 and shown in Figure 5. It is seen from the figure that 14908


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Figure 5. Viscosity of aqueous (HMDA + MDEA) solvent: Experimental data and model predictions (AAD: 0.31%).

dynamic viscosity of the solvent decreases exponentially with temperature. It also decreases with the increase in HMDA concentration (mass%) in the solvent keeping total amine concentration (30 mass%) constant. Grunberg-Nissan model is used to correlate the mixed solvent dynamic viscosity data. For this model, the viscosity of pure water and MDEA are taken from the literature.39 The viscosity of pure HMDA is taken from our previous work.16 Binary parameters of the Grunberg−Nissan model are regressed to fit experimental viscosity data to that of model predicted values using the leastsquare method and are presented in Table 3. The model

4.3. Physical Solubility. The N2O solubility (mN2O,m) data (Table 1) measured in this work decreases exponentially with the increase in temperature and with the decrease in HMDA concentration (Figure 6) in the solvent having constant total amine concentration (30 mass %). N2O solubility data in aqueous (HMDA + MDEA) is correlated using excess Henry’s coefficient model.40 For this model mN2O,m data is converted in the form of Henry’s coefficient (HN2O,m = RT/mN2O,m) using eq 1. Henry’s coefficient data for N2O in pure water,33 MDEA,40 and HMDA16 are taken from the literature. The least-squares optimization method is used to fit the experimental HN2O,m data with that of model predicted HN2O,m values by regressing the two body (αij, αik, αjk) and three body (αijk) interaction parameters. Regressed model parameters are presented in Table 4. The model predicted HN2O,m values converted to mN2O,m are compared with that of experimental mN2O,m values in Figure 6. This figure indicates good prediction capability of the model with 0.38% AAD. Experimentally measured N2O solubility data is used to estimate physical CO2 solubility (mCO2) in the studied solvent using N2O analogy. In aqueous amine solvent, physical CO2 solubility is proportional to the N2O solubility at constant temperature and the proportionality constant is the ratio of the solubility of CO2 and N2O in pure water. The expression for the N2O analogy is given below.

Table 3. Interaction Parameters of the Viscosity Model for [(HMDA (1) + MDEA (2) + H2O (3)] Solvent interaction parameters parameter

G12

G13

G23

b1 b2 b3

62.91 −5800.83 −266.95

0.939 1371.64 −232.14

−6.88 3122.34 −183.48

predicted dynamic viscosity data for aqueous (HMDA + MDEA) are compared with the corresponding experimental data in Figure 5. This work shows good model predictions with 0.31% AAD between experimental data and model results. The dynamic viscosity data obtained in this work is used to estimate the diffusivity of the CO2 in an aqueous mixed amine solvent using the modified Stokes−Einstein relation15,45 as given below. (DCO2, Lη0.74)|water = (DCO2, Lη0.74)|Am

⎛ mCO ⎞ 2 ⎜⎜ ⎟⎟ ⎝ m N2O ⎠

(39)

water

(41)

Physical CO2 and N2O solubility in pure water are taken from the literature.33 Estimated mCO2 data is given in Table 1 which is used for the evaluation of the rate constant. 4.4. Overall Rate Constant. Rate of CO2 absorption in aqueous (HMDA + MDEA) is measured using the “pressure decay technique”. Overall rate constant (kOV) for different solvent composition and temperature is estimated using eq 13.

Diffusivity of CO2 in water is taken from the work of Versteeg and van Swaaij.33 ⎛ −2119 ⎞ ⎟ DCO2,water = 2.35 × 10−6 exp⎜ ⎝ T ⎠

aminesolution

⎛ mCO ⎞ 2 ⎟⎟ = ⎜⎜ ⎝ m N2O ⎠

(40)

Estimated diffusivity values are given in Table 1. 14909


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Figure 6. Dimensionless N2O solubility (mN2O,m) in aqueous (HMDA + MDEA) solvent: Experimental data and model predictions (AAD: 0.38%).

higher than that in 30 mass % MDEA alone. At a higher HMDA concentration the enhancement of the rate is more pronounced. Kinetic Model I. In the kinetic model based on the zwitterion mechanism (for HMDA−CO2) and base catalyzed hydration mechanism (for MDEA−CO2), kOV is a function of kinetic constants k2,HMDA, k2,MDEA, k2,HMDA kH2O/k−1, k2,HMDA kHMDA/k−1, and k2,HMDA kMDEA/k−1. Using the values of k2,MDEA taken from the literature4 as shown in eq 42, the values of k2,HMDA, k2,HMDA kH2O/k−1, k2,HMDA kHMDA/k−1, and k2,HMDA kMDEA/k−1 are regressed in the form of Arrhenius expression by fitting the experimental kOV data to the kinetic model.

Table 4. Interaction Parameter of Excess Henry Coefficient Model for [(HMDA (1) + MDEA (2) + H2O (3)] Solvent two-body interaction parameter

three-body interaction parameter

parameter

α12

α13

α23

α123

k1 k2 k3

11.49 8344.62 −77.38

−2.59 371.03 −218.51

−4.89 2162.02 20.95

−66.57

For the estimation of the kOV value, required diffusivity and physical CO2 solubility data are evaluated in the previous sections. Estimated kOV values are given in Table 5 and shown in

⎛ 5400 ⎞ ⎟ k 2,MDEA /(m 3·kmol−1·s−1) = 4.01 × 108 exp⎜ − ⎝ T /K ⎠

Table 5. Kinetic Data for the Absorption of CO2 in Aqueous (HMDA + MDEA) concentration mass% HMDA

mass% MDEA

5

25

10

15

20

15

T

kL

kOV

K

105 −1

s−1

Ha

Ei

1.37 1.95 3.01 3.90 1.67 2.28 3.28 4.13 1.99 2.62 3.55 4.31

1489.9 3093.2 5865.9 8738.1 5465.7 7706.2 12423.1 17204.9 10771.5 13928.2 19806.6 23911.2

81 96 99 106 130 132 134 142 153 155 157 162

1345 1520 1677 1815 1304 1472 1596 1725 1264 1409 1534 1633

303 313 323 333 303 313 323 333 303 313 323 333

ms

(42)

To evaluate the kinetic constants, the following objective function is minimized using the generalized reduced gradient optimization technique.

F=

1 N

N

∑ i=1

exp mod k OV, i − k OV, i exp k OV, i

(43)

Mod where, N is the total number of data points, kExp OV,i and kOV,i are the experimental and model predicted values of the overall rate constant, respectively. The temperature dependent form of the regressed kinetic constants obtained in this work are given below.

⎛ 478 ⎞ ⎟ k 2,HMDA /(m 3·kmol−1·s−1) = 8.42 × 104 exp⎜ − ⎝ T /K ⎠ (44)

Figures 7 and 8. It can be seen from the plot that kOV increases with the increase in temperature as well as HMDA concentration in the solution. In this figure, the overall rate constant for CO2 absorption in 30 mass % (2.5 M) aqueous MDEA4 is also compared with that of aqueous (HMDA + MDEA) mixed amine solvent. At 313 K, the overall rate constant for CO2 absorption in aqueous (5%HMDA + 25%MDEA) is found to be 41 times

k 2,HMDAk H2O k −1

/(m 6·kmol−2·s−1)

⎛ 11032 ⎞ ⎟ = 7.66 × 1013 exp⎜ − ⎝ T /K ⎠ 14910

(45)


Article


Figure 7. Comparison of experimental kOV data with model prediction based on Kinetic Model I.

Figure 8. Comparison of experimental kOV data with model prediction based on Kinetic Model II.

k 2,HMDAkHMDA k −1

⎛ 10589 ⎞ ⎟ = 1.36 × 1019 exp⎜ − ⎝ T /K ⎠

k 2,HMDAkMDEA k −1

to the kinetic model eq 38 developed based on the termolecular mechanism for HMDA−CO2 reaction system and base-catalyzed hydration mechanism for (MDEA−CO2) reaction system using the generalized reduced gradient nonlinear optimization technique using the same objective function as shown in eq 43. But no meaningful results are obtained when all the bases (H2O, MDEA, HMDA) are considered in the termolecular expression. So, to fit the kinetic model with the experimental data, the contribution of MDEA in the termolecular expression of HMDA−CO2 reaction is neglected. This type of approach is also reported by Ramachandran et al.46 for the aqueous (MDEA + MEA) system. In view of this, the modified rate expression which is fitted to the experimental data, is shown below.

/(m 6·kmol−2·s−1)

(46)

/(m 6·kmol−2·s−1)

⎛ 12668 ⎞ ⎟ = 2.28 × 1018 exp⎜ − ⎝ T /K ⎠

(47)

Using these kinetic constants, predicted kOV values are compared with the experimental data in Figure 7. The AAD between the model predicted and experimental kOV values is estimated to be 6.5%. Kinetic Model II. In this model, kOV is a function of kinetic constants kHMDA, kMDEA, and kH2O. Experimental kOV is also fitted

k OV = k 2,MDEA[MDEA] + (k H2O[H 2O] + kHMDA[HMDA])[HMDA] 14911

(48)



■

The temperature dependent kinetic constants kHMDA and kH2O regressed is this work are shown below.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: + 91-3222-283948.

⎛ 417 ⎞ ⎟ kHMDA /(m 3·kmol−1·s−1) = 1.61 × 104 exp⎜ − ⎝ T /K ⎠

ORCID

Amar N. Samanta: 0000-0002-0290-7461

(49)

⎛ 6511 ⎞ ⎟ k H2O/(m 3·kmol−1·s−1) = 1.38 × 1011 exp⎜ − ⎝ T /K ⎠

Article

Notes

The authors declare no competing financial interest.

■

(50)

Experimental data and predicted results based on this model are compared in Figure 8 which shows low average absolute deviation of 8.24%. The infinite enhancement factor (Ei) and Hatta number (Ha) for this reaction system are calculated to check the PFO reaction criteria. Estimated Ei and Ha values are listed in Table 5. For all the runs, Ha values are higher than 3 and Ei values are much higher than the Ha values. This indicates that the criteria for the pseudo-first-order reaction is satisfied and eq 13 is valid for the interpretation of CO2 absorption kinetics in aqueous (HMDA + MDEA).

REFERENCES

(1) Mills, S. Coal-Fired CCS Demonstration Plants; IEA Clean Coal Centre, 2012. (2) Kohl, A. L.; Nielsen, R. B. Gas Purification, 5th ed.; Gulf Publishing Company, TX, 1997. (3) D’Alessandro, D. M.; Smit, B.; Long, J. R. Carbon Dioxide Capture: Prospects for New Materials. Angew. Chem., Int. Ed. 2010, 49, 6058. (4) Ko, J.; Li, M. Kinetics of Absorption of Carbon Dioxide into Solutions of N-Methyldiethanolamine + water. Chem. Eng. Sci. 2000, 55, 4139. (5) Shen, K. P.; Li, M. H. Solubility of Carbon Dioxide in Aqueous Mixtures of Monoethanolamine with Methyldiethanolamine. J. Chem. Eng. Data 1992, 37, 96. (6) Kamps, Á . P.-S.; Xia, J.; Maurer, G. Solubility of CO2 in (H2O + piperazine) and in (H2O + MDEA + piperazine). AIChE J. 2003, 49, 2662. (7) Böttger, A.; Ermatchkov, V.; Maurer, G. Solubility of Carbon Dioxide in Aqueous Solutions of N -Methyldiethanolamine and Piperazine in the High Gas Loading Region. J. Chem. Eng. Data 2009, 54, 1905. (8) Mandal, B. P.; Guha, M.; Biswas, A. K.; Bandyopadhyay, S. S. Removal of Carbon Dioxide by Absorption in Mixed Amines: Modelling of Absorption in Aqueous MDEA/MEA and AMP/MEA Solutions. Chem. Eng. Sci. 2001, 56, 6217. (9) Liao, C. H.; Li, M. H. Kinetics of Absorption of Carbon Dioxide into Aqueous Solutions of monoethanolamine + N-Methyldiethanolamine. Chem. Eng. Sci. 2002, 57, 4569. (10) Rinker, E. B.; Ashour, S. S.; Sandall, O. C. Absorption of Carbon Dioxide into Aqueous Blends of Diethanolamine and Methyldiethanolamine. Ind. Eng. Chem. Res. 2000, 39, 4346. (11) Derks, P. W. J.; Kleingeld, T.; van Aken, C.; Hogendoorn, J. A.; Versteeg, G. F. Kinetics of Absorption of Carbon Dioxide in Aqueous Piperazine Solutions. Chem. Eng. Sci. 2006, 61, 6837. (12) Chen, X.; Rochelle, G. T. Aqueous Piperazine Derivatives for CO2 Capture: Accurate Screening by a Wetted Wall Column. Chem. Eng. Res. Des. 2011, 89, 1693. (13) Ume, C. S.; Ozturk, M. C.; Alper, E. Kinetics of CO2 Absorption by a Blended Aqueous Amine Solution. Chem. Eng. Technol. 2012, 35, 464. (14) Ma’mun, S.; Dindore, V. Y.; Svendsen, H. F. Kinetics of the Reaction of Carbon Dioxide with Aqueous Solutions of 2-((2Aminoethyl)amino)ethanol. Ind. Eng. Chem. Res. 2007, 46, 385. (15) Singh, P.; van Swaaij, W. P. M.; Brilman, D. W. F. Kinetics Study of Carbon Dioxide Absorption in Aqueous Solutions of 1,6Hexamethyldiamine (HMDA) and 1,6-Hexamethyldiamine, N,N′ DiMethyl (HMDA, N,N′). Chem. Eng. Sci. 2011, 66, 4521. (16) Mondal, B. K.; Bandyopadhyay, S. S.; Samanta, A. N. Kinetics of CO2 Absorption in Aqueous Hexamethylenediamine. Int. J. Greenhouse Gas Control 2017, 56, 116. (17) Dinda, S.; Goud, V. V.; Patwardhan, A. V.; Pradhan, N. C. Kinetics of Reactive Absorption of Carbon Dioxide with Solutions of 1,6Hexamethylenediamine in Polar Protic Solvents. Sep. Purif. Technol. 2010, 75, 1. (18) Xu, Z.; Wang, S.; Chen, C. Kinetics Study on CO2 Absorption with Aqueous Solutions of 1,4-Butanediamine, 2-(Diethylamino)Ethanol, and Their Mixtures. Ind. Eng. Chem. Res. 2013, 52, 9790. (19) Li, J.; Henni, A.; Tontiwachwuthikul, P. Reaction Kinetics of CO2 in Aqueous Ethylenediamine, Ethyl Ethanolamine, and Diethyl Monoethanolamine Solutions in the Temperature Range of 298−313

5. CONCLUSION Kinetics of CO2 absorption in the aqueous blend of HMDA and MDEA is studied at 303, 313, 323, and 333 K in a reaction calorimeter setup using the pressure decay technique. Density, viscosity, and physical solubility of the unloaded solvent are also measured in this work. Diffusivity and physical solubility of CO2 in the solvent are evaluated using the modified Stokes−Einstein relation and N2O-analogy, respectively. The overall rate constant for the CO2 absorption in aqueous (HMDA + MDEA) is measured in the PFO reaction regime. Kinetic data shows significant enhancement of the CO2 absorption rate in aqueous (HMDA + MDEA) solvent due to the addition of a small amount of HMDA in aqueous MDEA. At 313 K, overall CO2 absorption rate constant in aqueous (5%HMDA + 25%MDEA) is 41 times higher than that of aqueous 30% MDEA. Again, absorption rate of this solvent (19.7 × 10−6 kmol·m−2·s−1 at 313 K) is 14% higher than that of aqueous (5%PZ + 25%MDEA) (16.9 × 10−6 kmol· m−2·s−1 at 313 K).21 It is also indicated in our previous work that at 15 kPa CO2 partial pressure, loading capacity of this solvent (αCO2 = 0.584 mol·mol−1 at 313 K) is 24.5% and 16.7% higher than that of aqueous 30% MDEA (αCO2 = 0.0.441 mol·mol−1 at 313 K)47 and (5%PZ + 25%MDEA) (αCO2 = 0.486 mol·mol −1 at 313 K)48 respectively. Low CO2 absorption enthalpy of this solvent is also reported in our previous work.32 At 313 K and 0.5 mol·mol −1 CO2 loading, absorption enthalpy of aqueous (5% HMDA + 25%MDEA) is 67 kJ·mol −1 CO2. All these properties indicate the superior solvent characteristics of aqueous (HMDA + MDEA) for CO2 capture. To evaluate the kinetic characteristics of this mixed amine solvent, the overall rate of CO2 absorption is assumed to be the combined effect of rate contributions of CO2−HMDA and CO2−MDEA reaction system. The rate contribution of CO2− HMDA reaction system is analyzed using both zwitterion (Kinetic Model I) and termolecular (Kinetic Model II) mechanisms, whereas the CO2−MDEA reaction system is analyzed by the base catalyzed hydration mechanism. Both kinetic models developed in this work are able to predict the kinetic data with good accuracy. Kinetic Model I is found to represent the experimental data better with an AAD of 6.5% compared to the Kinetic Model II (AAD: 8.3%). 14912


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