Characterizing the Transport Properties of Multiamine Solutions for

Article pubs.acs.org/jced

Characterizing the Transport Properties of Multiamine Solutions for CO2 Capture by Molecular Dynamics Simulation Y. S. Yu,†,‡,§ H. F. Lu,† G. X. Wang,‡ Z. X. Zhang,*,†,§ and V. Rudolph‡ †

State Key Laboratory of Multiphase Flow in Power Engineering and §School of Chemical Engineering and Technology, Xi’an Jiaotong University, No. 28 Xianning West Road, Xi’an, 710049 China ‡ School of Chemical Engineering, The University of Queensland, St. Lucia, QLD 4072 Australia ABSTRACT: We performed molecular dynamics (MD) simulations on multiamine solutions of CO2 capture to optimize the conventional amine solvents (MEA, MDEA, DEA, AMP, and TEA). A synergy molecular dynamics model (SMD) was developed for the CO2 and amine solutions, in which the molecular synergy number (MSN) was introduced to quantify interactions between diffusion and molecule motion. The SMD model involved with the force field was validated with experimental data from CO2, amine, and water systems. The best synergy for the MDEA-DEA-TEA system was achieved at the ratio of 3:1:1, with a minimum MSN being 3.89. The overall synergy in ternary amine systems was found to be better than that in quaternary and quintuple amine systems. Several optimized multiamine solution systems were identified. The MDEA-DEA-TEA solution could simultaneously provide better diffusivity and less drag force with mass transfer coefficient being 33.33 % higher than that of AMP. The MEA-MDEA-DEA-TEA solution could improve CO2 transport by increasing 18 % of diffusivity when varying the ratio from 1:1:1:1 to 2:1:1:1. All of the results above provided a benchmark to optimize the amine mixing solution design for CO2 capture.

1. INTRODUCTION A solvent is an essential element in the chemical absorption method used for CO2 capture and sequestration (CCS) process which has received growing attention in the world recently.1−3 The characteristic of a solvent for the chemical absorption of CO2 has great impacts on capture performance.4 In CO2 capture by chemical absorption, amine is a basic and industrially promising solvent for tackling flue gas from power plants. Generally, there are five kinds of amines commonly employed as solvents to capture CO2. They are monoethanolamine (MEA), methyldiethanolamine (MDEA), triethanolamine (TEA), diethanolamine (DEA), and 2-amino-2-methyl-1propanol (AMP). However, there exist various drawbacks in the amine absorption of CO2. When MEA absorbs CO2 at a higher reaction rate, much more thermal energy is required to implement a regeneration process. Although MDEA has a high absorption capacity, its slow reactivity blocks its further development for capturing CO2.5 The slower reaction rate of TEA with CO2 is also confirmed. Another amine of DEA takes priorities of less corrosive and less required heat as a candidate solvent6 but exhibits ineffective CO2 capture performance. AMP, as a relatively new solvent, possesses advantages of larger loading capacity and less regeneration heat. However, its slower reaction rate than MEA limits its commercial application to control CO2 emission.7 In order to offset each amine’s problems above, a mixture of various amines can usually be used. Thus, understanding the transfer characteristics of the mixture such as mass transfer coefficient (MTC), diffusivity of © XXXX American Chemical Society

CO2 and viscosity of mixing amines become crucial as these are the basic and key parameters in optimizing the amine mixing solution design for CO2 capture.8,9 Mixtures of various amines have attracted a lot of attentions for a long period of time. Many previous studies have experimentally concentrated on densities and viscosities of the amine solutions such as MEA, MDEA, and AMP.10 However, these studies are limited in combination of two amines only, without considering mixing traits of multiple amines. A few studies are carried out experimentally by taking into account four kinds of amine solutions but still only dealing with mixture of two amines.11 The addition of AMP into a binary system of MDEA and DEA has supported the technical feasibility of a ternary amine solution for CO2 capture and provided the viscosity of the ternary solutions by experiment.12 Recently, mixed MDEA with AMP as a solvent has also been widely studied, for which relevant characteristic of the mixtures is calculated by Aspen plus.13 This provides a useful tool built on a theoretical basis for amine mixing design compared with previous studies. Moreover, attempts are made for reaction kinetics of MDEA and MEA through combined experimental and numerical analysis.14,15 The resultant kinetics parameters are also applied to investigate CO2 capture with the amine solution in a power plant. All of the previous contributions Received: July 5, 2011 Accepted: April 23, 2013

A

dx.doi.org/10.1021/je3005547 | J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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amines comprised of the five amines. This model could be of great help in the optimization of multiamine solutions within the synergy scope between molecule motion and diffusion. In the SMD model, potential energy and kinetic energy of amine molecules are taken into account as the cornerstone for obtaining transfer characteristics of amine mixtures. The aim of this article is to explain the trait of mixing amines from molecular theoretical views. This can be another way to investigate mixing amines besides classic experiment methods and empirical analysis, providing a further fundamental basis for the development of multisolvent absorption theory. For CO2 capture with amine, a mixture of the selected five amines can be used to realize a better performance of CO2 capture based on MTC. The SMD model can be used to optimize the theoretical design of amine solvents for CO2 capture.

above provide an optimistic sight to mix amines for CO2 absorption. However, it is still inconvenient and not accurate enough to use such conventional methods for amine mixing as it largely relies on the experimental information which is usually hard to obtain completely. To overcome the deficiency, molecular dynamics (MD) has also been employed to investigate physical and chemical properties of selected solutions used for CO2 capture. The MD method has been successfully used to compute reaction kinetics of solutions, in which so-called first principle theory16 is employed for validation. The MD method is also used to calculate the diffusivity of gaseous molecules in multicomponent diffusion process by means of the Green Kubo method.17 Combined with molecular orbital theory, the MD model has even been employed to design a new ionic liquid for CO2 capture.18 As for the chemical absorption of carbon dioxide, an attempt with MD simulation using a system of MEA and CO2 is made by Silva (2005)4 and the method is further developed to deduce the diffusivity and viscosity correlations.19 More recently, there are growing interests in applying MD simulation to relevant solvents for CO2 capture, such as numerically simulating the molecular dynamic characteristics of piperazine.20 Another application is MD simulation on a mixture of MDEA and ionic liquid for CO2 capture,1,2 in which the CO2−liquid equilibrium, angle bending force constant, and bond stretching parameters are discussed. The considerable work above significantly contributes to the development of MD simulation for amine solutions. Since the major works found in the literatures have always focused on the calculation of the transport properties, the optimization from a molecular view has been ignored for solvent mixing process. Hence, alternative solvents are still not optimized or discovered satisfactorily for CO2 capture. In order to search the optimized solvent for CO2 capture, two key questions arise: which solvent permits the capturing large amounts of CO2 industrially? what method can determine the optimized solvent from a fundamental point? In response to these questions, the present work introduces a multiamine solvent to improve CO2 capture efficiency. Another proposed tool namely synergy molecular dynamics clearly stands as an alternative optimization method to identify the multiamine composition based on molecule motion. The study may improve our understanding of transport properties of multiamine solutions for CO2 capture from molecular synergy of view. In this paper the commonly used five kinds of amines, i.e., MEA, DEA, MDEA, AMP, and TEA as mentioned above, are chosen as basic solvents for CO2 capture. Attempts to mix the five amines have been carried out to offset each amine’s disadvantages. Most crucially, transport properties of mixture amines are investigated by molecular dynamics, because those properties, including density, diffusivity, viscosity and solubility, play important roles in CO2 capture with amine.9 As the complexities of both reactions and diffusion occur in the capture process, diffusivity is hard to measure directly.9 Moreover, lower viscosity together with higher diffusivity is essentially required to design an absorber or a stripper in a plant.8,10,11 This is because MTC will become larger on condition of higher diffusivity and lower viscosity by Onda’s equation,21 which will enlarge gas flux in solution to improve the gas separation process. Therefore, a synergy molecular dynamics (SMD) model has been developed in this article to provide systematic analysis on the diffusivity of CO2 in solutions and viscosity of mixing

2. SYNERGY MOLECULAR DYNAMICS (SMD) MODEL In order to describe multiple amine mixtures and obtain their transport properties for CO2 capture, a SMD model has been developed by referencing molecular dynamics theory. In the SMD model, the total energy is assumed to be the sum of potential energy and kinetic energy. The potential energy is comprised of four parts, i.e., bond stretching, angle bending, out of plane bending, and dihedral motion potentials. Besides, van der Waals interactions and electrostatic interactions are also incorporated as Lennard-Jones potentials.22−24 After potential and kinetic energies are obtained, the diffusivity can be calculated based on Green Kubo expression.17 The transfer characteristic of viscosity can be induced by its correlation to pressure tensors.25,26 For the mixture of amines, the diffusivity can be obtained based on MD simulation results, giving 1 ∞ Dm = ∫ dt ∑ ⟨vi(0) ·vi(t )⟩ (1) 3 0 Mixing temperature is defined by Tm =

∑ E kin 1.5 ∑ NkB

(2)

The viscosity of mixture can be described by μm =

V kBT

∫0

∞

dt

∑ ⟨pij (0)·pij (t )⟩

(3)

Then transfer properties for mixtures can be introduced according to molecular dynamics. However, assessment of the overall transfer properties for mixture of amines requires using a uniform principle, which must satisfy the balance between diffusivity and viscosity as MTC described in eq 4.21 It indicates that higher diffusivity and lower viscosity will improve mass transfer process in gas separation. ⎛ g ⎞0.33⎛ L ⎞0.33 K = 0.0051μm−0.83Dm 0.5⎜⎜ ⎟⎟ ⎜ ⎟ ρm 0.5 (ad p)0.4 ⎝ ρm ⎠ ⎝ a w ⎠

(4)

Thus, a molecular synergy number is introduced below based on the macro synergy number from field synergy theory in order to quantify interactive effects between diffusion and molecule motion.27,28 The molecular synergy number is given by MSN = B

Dm 2k m Dm0 15Re 4/3

(5)



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Table 1. Intramolecular Force Field Parameters for Amines and Watera20,35,36,38,41 bond length

force constant

angle

force constant

type

bond

Å

kJ·mol−1·Å−2

angle type

degree

kJ·mol−1·rad−2

amine

NT-CT OH-HO OH-CT CT-CT CT-CT a CT-CT b CT-HO

1.47 0.97 1.42 1.54 1.56 1.57 1.10

1220.10 1100.48 1282.30 1052.63 1076.56 1052.63 1100.48

water

O−H

1.00 V1

2811.00

CT-NT-HC HO−OH-CT NT-CT-CT NT-CT-CT c OH-CT-CT OH-CT-HC CT-CT-HC CT-CT-HC a CT-CT-HC b HC-CT-HC H−O−H

109.98 107.77 107.89 109.83 111.95 107.99 110.78 115.78 115.56 107.25 109.47

119.62 83.73 136.36 119.37 167.46 129.19 86.12 96.12 86.12 76.56 191.50 V4

V2

V3

torsion20,38

kJ·mol−1

kJ·mol−1

kJ·mol−1

kJ·mol−1

NT-CT-CT-HC CT-NT-CT-CT CT-CT−OH-HO HC-CT-CT-HC HC-CT-CT−OH CT-CT-NT-CT HC-CT- OH-HO

−4.24 1.74 −1.49 0 0 1.74 0

−2.97 −0.54 −0.73 0 0 −0.54 0

1.98 2.91 2.06 1.26 1.96 2.91 1.47

0 0 0 0 0 0 0

force constant kJ·mol−1·rad

a

−2

angle

type

improper torsion

this work

experiment data38

degree

MDEA DEA TEA

C−C−N-R C−C−N-R C−C−N-R C−C−N-R

812.46 798.51 753.62 801.96

769.94 769.94 769.94 769.94

−176.6 −158.9 −139.4 72.71

R represents −C2H5OH. Superscripts a, b, and c denote AMP, DEA, and TEA, respectively.

Table 2. Intermolecular Force Field Lennard-Jones Parameters and Charges for Sites in Amines, CO2, and Watera2,35,38 charges/electron

a

site

MEA

DEA

MDEA

TEA

AMP

MDEA exp.38,41

H−(O) O −CH2−(O) −CH2−(N) −N− H−(N) CH3 HW OW CC OC

0.363 −0.665 0.285 0.250 −0.938 0.344

0.363 −0.665 0.285 0.116 −0.572 0.344

0.363 −0.665 0.285 0.030 −0.094

0.363 −0.665 0.285 −0.185 0.510

0.363 −0.665 0.285 0.265 −0.680 0.398 0.180

0.369 −0.66 0.291 0.030 −0.09

0.032

CO2

H2O

0.030 0.424 −0.848 21.2 −10.6

σ

ε/kB

Å

K

0.0 2.85 3.38 3.38 3.54 1.06 3.86 0.0 3.182 2.814 2.976

3.90 80.05 57.46 57.46 87.62 4.52 65.42 0.00 78.2 12.372 100.493

Subscripts of W and C denote water and carbon dioxide, respectively.

superposition method. This correlation may deviate away from the real reaction kinetics, but reaction kinetics is beyond of scope in this study.

The Reynolds number for molecule motion is introduced as follows. Re =

ρm v(t )r(t ) μm

k m = (∑ xiki)/k 0

(6)

where the particle position r(t) is set as the characteristic length, which is predicted by the SMD. This simulation accordingly permits the sampling of v(t) as the convergence reaches. The reaction kinetics described in eq 7 is established by literature correlations,29−32 by simply applying the linear

(7)

As for density, linear superposition is also applied to describe mixing of amines in the following equation based on literature data,10,29,33 giving ρm = C

∑ xiρi

(8)



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Figure 1. Geometry of Amines.

Table 3. Kinetic Energies and Pressure Tensors for Amines pressure tensor in i−i direction

kinetic energy −1

kJ·mol amine type MEA MDEA AMP DEA TEA

313 K 55.85 100.14 85.98 76.35 80.88

57.5637 87.2337

kPa 298 K

293 K

51.16 98.13 86.16 74.08 81.68

49.36 90.64 84.02 70.86 89.64

313 K 72.33 76.21 88.51 73.42 84.98

73.1534 74.3239 71.2640 78.5940

298 K

293 K

74.32 77.95 91.22 76.11 86.65

75.31 79.04 92.17 78.42 89.65

Interactions between different species are determined by the Lorentz−Berthelot mixing rules.38 Applying those force field parameters for amines to the SMD model above, potential energy and kinetic energy can be obtained numerically step-by-step as follows.42 (1) Establishing a simulation system by confirming atom types and its positions; (2) Maintaining constant volume and temperature (NVT ensemble); (3) Calculating the acceleration of each atom by using the force field parameters in Tables 1 and 2. Meanwhile, computing temperature, potential energy and total energy at current time step in the system; (4) Updating positions and velocities of atoms based on integrating molecule motion; (5) Increasing time step and returning to step 2; (6) Obtaining energy minimization and equilibration in a given temperature; (7) Ending while outputting the MD results at every 10 ps; (8) Calculating diffusivity and viscosity based on the output results. The numerical simulation above provides detailed information about the molecular characteristics of amines. The structures of the amines are presented in Figure 1. As a result, kinetic energies and pressure tensors of five basic amines are summarized in Table 3. These results are utilized to solve eqs 1 to 3. Thus the transfer properties of various amine mixtures are determined, which can be further used to compare with

The SMD model above attempts to realize excellent transport and fast kinetics for the CO2 capture process. Based on the SMD model, combinations of varieties of amines can be identified effectively, involving amine type and concentration. To solve the SMD model, first the molecular structures for primary amine and water are required, which affect force field parameters in MD simulation. This work utilizes all-atom version of OPLS (optimized potentials for liquid simulations)4 force field to perform the simulations. The key force field parameters including atomic charges used for amines and water are summarized in Tables 1 and 2.34−40 The conformers of MEA are incorporated with 14 nonequivalent conformers.4 The other conformers for the rest of amines are calculated with their optimal dihedral angles.4 The intramolecular hydrogen bonds show energy dependency of different dihedral angles and the strength of the hydrogen bonds are determined by radial distribution functions.4 Most of parameters for AMP, DEA and TEA are the same as that for MEA, with some exceptions denoted in Tables 1 and 2.32,35,37 The additional hydrogen bonds are found between amines and water besides those listed in Table 1,38 which give the radial distribution function of selective hydrogen bonds between amines and water. Since there are good models which have been utilized many times for different simulations, an extended simple point charge model (SPC/E) is used for water in this work.41 The rigid model is employed to describe the carbon dioxide molecule.2 The interactions between any two atoms is described by LennardJones and Coulomb forms.23,38 The detailed charge of atom and diameter of the united atom are given in Table 2. D



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peak appears around the minimum Lennard-Jones interaction distance, which agree well with literature data.20 In this regard, the force field provides the reasonable parameters for the amine absorption of CO2 system. By the force field, molecular dynamics simulations are performed to obtain the transport properties of three chosen systems: a system consisting of MEA and water, another system comprised of CO2, MEA, and water and the other system of MDEA, DEA, AMP, and water. Figure 4 shows the comparison of the simulation results from the SMD model with four experimental diffusivities of MEA in water.43 As anticipated, the SMD simulation result only shows little deviation from the experimental data, with the largest difference being 2 %. The simulated viscosity of MEA solution has produced the largest deviation of 3.8 % comparing with the empirical viscosities in Figure 4.44 In the system of CO2, MEA, and water, four simulated diffusivities of CO2 in MEA solution agree well with the literature data (Figure 4).21 As for viscosity, Figure 4 also shows less difference between simulation results and experimental data.45 In the MDEA-DEA-AMP system, the weight fraction of MDEA and DEA is 32.5 % and 12.5 %, respectively. The mixing solution viscosity as a function of AMP weight fraction is given in Figure 5, which has good agreement with experiment data.12 In short, comparison results above demonstrate that the SMD model can be employed effectively to simulate transfer characteristics of CO2−mine systems. Therefore, it is possible to apply the SMD model to three or more kinds of amine mixtures to identify their characteristics of CO2 absorption. Thus, diffusivity of CO2 and viscosity of the amine solution, two most important parameters for absorption process,8−11 can be determined by means of numerical simulation with the SMD model.

experimental data in literature for model validation. This will be discussed in detail in section 3.

3. MODEL VALIDATION The force field is validated by comparing the calculated torsional potential energy of N−C−C−O in MDEA against the literature data in Figure 2.20,38 The cutoff distance of 9.5 Å is

Figure 2. Torsional potential energy of N−C−C−O in MDEA solution.

employed to calculate the real part of the potential for all simulations.38 As shown in Figure 2, the simulation achieves a reasonable torsional potential energy in the range of the literature data. Moreover, the calculated kinetic energy of MEA and AMP in Table 3 is found to be comparable with deduced data from collisions between amines and CO2.37 The pressure tensors agreement with open information calculated by surface tension34,39,40 provide the accuracy of the force field. Furthermore, the observed affinity of CO2 and MDEA is compared by the pair correlation function shown in Figure 3,20 which gives affinity between nitrogen from the amine group in MDEA and carbon atom from carbon dioxide. The calculated pair correlation functions of N−C and O−C show that the first

4. RESULTS AND DISCUSSION Binary amine mixtures of MEA and MDEA have been identified to be promising multiamine mixtures for CO2 absorption.15,46 This paper will extend the study to more complex amine systems comprised of ternary, quaternary, and quintuple amine mixtures to investigate their transfer characteristics for CO2 capture by MD simulation. By the SMD model as described in eqs 1 to 8, the MD simulation is performed in a cubic box possessing dimensions 40 Å × 40 Å × 40 Å. The studied ensemble consists of 10 CO2 molecule and 460 water molecules. The number of amine molecule is determined by a mixture amine weight fraction of 30 % for all of the cases. Therefore, the periodic boundary condition together with Nose-Hoover thermostat is used to simulate the bulk environment by means of limited number of molecules.4,47 The simulation provides the mean squared displacement (MSD) profiles for various amine and water systems, as shown in Figure 6. The maximum time is almost 5600 ps for all of the amines at given 1 fs as time step. The total energy variation is within 1 %. According to the MSD profiles, diffusivity of CO2 and viscosity of the given multiamine mixtures can be calculated precisely. The transfer characteristics of the multiamine mixtures are discussed below. 4.1. Field Synergy Analysis. It has been demonstrated that there is always a challenge on how to keep a balance between diffusivity and viscosity of these systems to achieve the highly efficient and cost-effective performance for CO 2 capture.21 Field synergy analyses can provide a useful tool to

Figure 3. Pair correlation functions of N−C and O−C between MDEA and CO2. E



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Figure 4. Diffusivity of MEA in water and CO2 in MEA solution, viscosity of MEA solution, and CO2−MEA solution.

help achieve this objective28,48,49 and hence is introduced into the SMD in this study. In order to identify interactions between diffusion and molecule motion, macro synergy between diffusion and fluid flow has been introduced in the molecule system. In the SMD model developed in this paper, molecular synergy number (MSN) can be calculated for the multiple amine systems as discussed above to indicate the synergy effects between diffusion and molecule motion. The less the MSNs are, the better synergy can be identified for the system. This synergy principle can be applied to determine proper amine combination in terms of optimized balance between better diffusivity and lower drag force. The MSNs for various multiple amine systems (denoted by the amine weight fraction ratio) have been calculated and listed in Tables 4 to 6, respectively. Meanwhile, comparisons are made between multiamine systems and baseline of primary AMP system, which are both applied in amine absorption of CO2 at 313K in packed column. In the ternary amine system, mixtures of MEA-DEA-AMP, MEA-DEA-TEA, MDEA-DEA-AMP, and MDEA-DEA-TEA are studied in detail. As shown in Table 4, the synergy between

Figure 5. Viscosity of mixing MDEA-DEA-AMP solution.

Table 4. Field Synergy Effects of Ternary Amines Mixtures MTC 10−6

Figure 6. Mean squared displacement profiles for various amine and water systems.

F

mixture

weight fraction ratio

MSN

m·s−1

AMP MEA-DEA-AMP MEA-DEA-AMP MEA-DEA-AMP MEA-DEA-TEA MEA-DEA-TEA MEA-DEA-TEA MDEA-DEA-AMP MDEA-DEA-AMP MDEA-DEA-AMP MDEA-DEA-TEA MDEA-DEA-TEA MDEA-DEA-TEA

1:1:1 2:1:1 3:1:1 1:1:1 2:1:1 3:1:1 1:1:1 2:1:1 3:1:1 1:1: 1 2:1:1 3:1:1

12.28 7.95 5.43 7.95 6.28 5.55 14.78 8.87 6.13 5.38 4.51 3.89

1.08 1.15 1.61 1.75 1.32 1.64 1.81 1.39 1.46 1.52 1.28 1.41 1.44



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transport properties involved with CO2 are discussed in detail in section 4.2. 4.2. Transport Properties. When TEA is combined with MDEA and DEA, the diffusivity of CO2 in the solution is slightly changed as the MDEA concentration increases (Figure 7). Analysis of kinetic energy of MDEA and DEA demonstrates that there is 24 % difference between their kinetic energies but with only 3 % deviation of velocity autocorrelation function value. This contributes to only 6 % increase of diffusivity when increasing the MDEA concentration. As shown in Figure 7, the better diffusion behavior has been maintained at the ratio of 3:1:1 with maximum average diffusivity being 1.3 × 10−9 m2·s−1. The gradient of diffusivity is great, implying that temperature significantly affects diffusivity of CO2 in the solution due to kinetic energy of MDEA sensitive to temperature. The viscosity of the ternary amine MDEA-DEA-TEA solution has been simulated and described in Figure 7. The results show that the viscosity of the solution varies in the range from (2.54 to 2.9) mPa·s. The average viscosity is reduced by almost 5 % when increasing MDEA concentration from a ratio of 1:1:1 to 3:1:1. This result suggests that mixing MDEA, DEA and TEA is acceptable from the view of molecule motion. Although the viscosity is not reduced significantly, there is a slight diffusivity improvement in Figure 7. Likewise, the diffusivity of CO2 in the optimized quaternary amine MEA-MDEA-DEA-TEA solution is simulated (Figure 8). The results show that only 4 % of diffusivity drop has been found by comparing with that in MDEA-DEA-TEA solution (referring to Figure 7). The uniform enhancement trend for diffusivity is observed from the ratio 1:1:1:1 to 4:3:2:1 when increasing MEA and MDEA concentration. A maximum of 18 % diffusivity increment is determined when MEA concentration changed from the ratio 1:1:1:1 to 2:1:1:1, improving 13.74 % of MTC, which is attributed to greater velocity autocorrelation function value of MEA than that of DEA and TEA although with less kinetic energy. When concentrations of MEA and MDEA are both increased, the impact of weight fraction intensifies the diffusion process further. Thus, as for this quaternary amine system, the MEA-MDEA-DEA-TEA solution should be suitable enough for CO2 capture from the perspective of diffusion. The choice of the concentration increment between MEA and MDEA depends on overall effects of diffusivity, viscosity, chemical kinetics and absorption capacity. Figure 8 shows the viscosity profile of MEA-MDEA-DEATEA solution. As presented, the viscosity of this quaternary amine system has been decreased by 11 % compared with MDEA-DEA-TEA. Meanwhile, viscosity of the system is reduced as concentration of MEA and MDEA increases, with a maximum of 8.65 % drop from the ratio of 1:1:1:1 to 4:3:2:1,which is mainly caused by less pressure tensors of MEA than that of DEA. Moreover, increasing concentrations of MEA and MDEA forces the viscosity to drop further. As shown in Figure 8, increasing the MEA concentration results in a 7 % drop in viscosity, 3 % higher than that produced by the increase of MDEA concentration. Therefore, increasing the MEA concentration will be a better choice to achieve a lower viscosity of the solution against increasing the MDEA concentration. However, a balance should be provided between viscosity and CO2 absorption capacity by employing appropriate MEA and MDEA concentrations.

diffusion and molecule motion in the mixture of MEA-DEAAMP is improved as MEA concentration increased. This is because there is an almost a 50 % drop of MSN when the ratio of MEA, DEA, and AMP changes from 1:1:1 to 3:1:1, with a minimum MSN being 5.43 and a maximum MTC being 1.75 × 10−6 m·s−1 (62 % higher than that of AMP solution). For the system of MEA, DEA, and TEA, the best synergy is obtained at the ratio of 3:1:1 with a least MSN of 5.55. The same tendency for MDEA, DEA,and AMP system is confirmed as that for MEA, DEA, and AMP, with a little greater MSN at 6.13. Noticeably, mixture of MDEA, DEA, and TEA with a MSN being 3.89 is the best synergy of the four combinations, producing a MTC of 1.44 × 10−6 m·s−1, 33.33 % greater than that of AMP. Thus, in the ternary amine systems, MDEA-DEATEA can achieve the best synergy between diffusion and molecule motion. The quaternary amine system, comprising of MEA-MDEADEA-AMP and MEA-MDEA-DEA-TEA, shows similar trends as indicated in Table 5. Both of the simulated quaternary amine Table 5. Field Synergy Effects of Quaternary Amine Mixtures MTC 10−6 mixture


MSN

m·s−1

MEA-MDEA-DEA-AMP MEA-MDEA-DEA-AMP MEA-MDEA-DEA-AMP MEA-MDEA-DEA-TEA MEA-MDEA-DEA-TEA MEA-MDEA-DEA-TEA

1:1:1:1 2:1:1:1 4:3:2:1 1:1:1:1 2:1:1:1 4:3:2:1

14.72 9.03 7.76 6.17 5.01 4.36

1.34 1.42 1.46 1.31 1.49 1.52

mixtures achieve the best synergy at the ratio of 4:3:2:1. The MTC in MEA-MDEA-DEA-AMP (4:3:2:1) solution can be increased to 1.46 × 10−6 m·s−1, 35.2 % greater than that of AMP solution. For the MEA-MDEA-DEA-AMP mixture, about 37 % drop of the MSNs has been observed as MDEA and MEA concentration increases. For the MEA-MDEA-DEA-TEA, increasing MDEA concentration positively contributes to synergy, with 18 % drop of the MSNs. It is clearly that all the MSNs for the MEA-MDEA-DEA-TEA are below that for MEA-MDEA-DEA-AMP. Therefore, the MEA-MDEA-DEATEA produces better synergy between diffusion and molecule motion. As for the quintuple amine system, the synergy becomes better as MEA and MDEA concentration increases, as shown in Table 6. The best synergy reaches with a MSN being 8.14 at the Table 6. Field Synergy Effects of Quintuple Amine Mixtures MTC 10−6 mixture


MSN

m·s−1

MEA-MDEA-DEA-AMP-TEA MEA-MDEA-DEA-AMP-TEA MEA-MDEA-DEA-AMP-TEA

4:1:1:1:1 4:2:2:1:1 4:3:1:1:1

14.43 9.02 8.14

1.37 1.51 1.56

ratio of 4:3:1:1:1, increasing MTC by 13.86 %. Moreover, compared with results in Tables 4, 5, and 6, it can be noted that the overall synergy in ternary amine systems is better than that in quaternary and quintuple amine systems. The mixtures with better synergy effects, such as MDEA-DEA-TEA, MEA-MDEADEA-TEA, and MEA-MDEA-DEA-AMP-TEA, are accordingly determined as optimized combinations, and hence their G



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Figure 7. Diffusivity of CO2 in MDEA-DEA-TEA and viscosity of the ternary amine solution.

Figure 8. Diffusivity of CO2 in MEA-MDEA-DEA-TEA and viscosity of the quaternary amine solution.

Finally, the mixture of MEA-MDEA-DEA-TEA-AMP is studied. The result of the diffusivity of CO2 in such a complicated system is illustrated in Figure 9, which leads to almost the same diffusivity as that of MEA-MDEA-DEA-TEA. Under this circumstance, this phenomenon is attributed to greater kinetic energy TEA with higher velocity autocorrelation function value, which is taken as a dilute added into MEAMDEA-DEA-AMP systems. Increasing MDEA concentration and MEA concentration can improve the diffusivity. As a result, the diffusivity is increased by 5 % when the ratio of MEA, MDEA, DEA, AMP and TEA changes from 4:1:1:1:1 to 4:2:2:1:1. This is contributed by a higher velocity autocorrelation function value of MDEA. This phenomenon has also been observed when MDEA concentration is increased when the ratio changed from 4:2:2:1:1 to 4:3:1:1:1. However, its effect of increasing diffusivity is more obvious because DEA concentration with lower diffusivity is simultaneously reduced to 10 %. Thus, for this complicated system comprised of five

amines, increasing the MDEA concentration will improve the diffusion process for CO2 capture. The viscosity of the MEA-MDEA-DEA-TEA-AMP solution is clearly shown in Figure 9. The result demonstrates 2.89 % increment of viscosity compared with MEA-MDEA-DEA- TEA solution system. This is caused by the higher pressure tensor of AMP, which has induced great viscosity. As MDEA concentration is increased in terms of the weight fraction ratio from 1 to 3, an overall viscosity drop has been observed in Figure 9. The maximum viscosity drop is about 7 %. This is because lower viscosity of MEA takes the most portions over higher viscosities of AMP and TEA. However, the reduction is not substantial when the ratio changes from 4:2:2:1:1 to 4:3:1:1:1 (Figure 9). The reason is that pressure tensors of MDEA (77.73 kPa) and DEA (75.98 kPa) are almost the same (see Table 3), which results in nearly similar viscosities while the viscosities of the other three amines are kept constant. The results above demonstrates that the increment of MDEA can H



Article

Figure 9. Diffusivity of CO2 in MEA-MDEA-DEA-TEA-AMP and viscosity of the quintuple solution.

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reduce drag force for this quintuple amine system. Meanwhile, AMP has great impact on viscosity for the system. Therefore, it is possible to keep the AMP concentration lower to maintain better molecule motion in the quintuple amine system. On account of results above, increasing MDEA concentration in the quintuple amine system can simultaneously improve diffusivity and reduce viscosity. However, the value of diffusivity is not as great as that in the ternary and quaternary amine systems.

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-29-8266 0689. Fax: +86-29-8266 8566. E-mail: [email protected]. Funding

Financial support provided by the National Natural Science Foundation of China under Grants 51276141 and 20936004 is greatly acknowledged. Notes

The authors declare no competing financial interest.

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5. CONCLUSIONS

NOMENCLATURE a specific area, m2·m−3 aw wetted surface area, m2·m−3 D diffusivity, m2·s−1 dp packing diameter, m E energy, kJ·mol−1 g acceleration of gravity, m·s−2 K mass transfer coefficient, m·s−1 k reaction kinetic constant, m3·(mol·s)−1 kB Boltzmann constant, J·K−1 k0 reference reaction kinetic constant, m3·(mol·s)−1 L amine solution flux, kg·m−2·s−1 MSN molecular synergy number p pressure tensor, kPa r atom position, Å Re Reynolds number for molecule T temperature, K t time, s V volume, m3 v velocity, m·s−1 x weight fraction ρ density, kg·m−3 μ viscosity, Pa·s

In order to optimize the design of the solutions for CO2 capture, a SMD model has been developed for the multiamine solution systems in this paper. By means of a MSN, the micro synergy between diffusion and molecule motion in the multicomponent molecule systems was quantified. The overall synergy in ternary amine systems has been found better than that in quaternary and quintuple amine systems, particularly the MDEA-DEA-TEA solution with the ratio of 3:1:1 achieving the minimum synergy number of 3.89. By field synergy analyses on the transport properties of multicomponent systems, several optimized multiamine solution systems were identified. Among the ternary amine systems, MDEA-DEA-TEA solution could achieve 33.33 % higher MTC against AMP. As for the quaternary amine mixtures, 18 % of diffusivity increment was attributed to MEAMDEA-DEA-TEA solution with 13.74 % of MTC enhancement when MEA concentration increasing from 1:1:1:1 to 2:1:1:1. The quintuple amine solution consisted of MEA-MDEA-DEAAMP-TEA could apparently improve 5 % of CO2 diffusivity by increasing the MDEA concentration from 4:1:1:1:1 to 4:2:2:1:1. Noticeably, all the MTCs in the ternary amine system were increased for CO2 capture compared with primary AMP based process. In the future study, the simple and empirical reaction kinetics in the current SMD model could be substituted by more precise reaction kinetics to give more vigorous results for amine mixing.

Subscript

i,j kin m N I

index kinetic mixture number of particles dx.doi.org/10.1021/je3005547 | J. Chem. Eng. Data XXXX, XXX, XXX−XXX


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Characterizing the Transport Properties of Multiamine Solutions for

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