Exploiting an Alternative CO2 Absorption Process ... - ACS Publications

May 21, 2015 - tetramethylammonium hydroxide (TAMH) has been devel- oped as an alternative solvent,12,13 which shows that its CO2 absorption product ...
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Exploiting an Alternative CO2 Absorption Process by Efficient Solvent Mixture Yun S. Yu,† Ting T. Zhang,†,‡ Xiao M. Wu,†,‡ De L. Mu,†,‡ Zao X. Zhang,*,†,‡ and Geoff G. X. Wang§ †

School of Chemical Engineering and Technology, Xi’an Jiaotong University, No. 28 Xianning West Road, Xi’an 710049, China State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China § School of Chemical Engineering, The University of Queensland, St Lucia, Queensland 4072, Australia ‡

S Supporting Information *

ABSTRACT: CO2 capture greatly helps with greenhouse gas mitigation. Chemical and physical absorption can control CO2 emission, but these methods are costly. To reduce the cost, an efficient solvent mixture of tetramethylammonium hydroxide (TAMH), tetramethylene sulfone (TMS), and ethylene glycol (EG) is assessed. Gas−liquid equilibrium, reaction kinetics, and mass transfer models are developed and validated by experiments. Henry’s constant, reaction kinetics, and mass transfer coefficients between CO2 and TAMH-TMS-EG are identified. CO2 loading and mass transfer coefficient are, respectively, obtained as 0.55 mol/molTAMH and 4.02kmol/m2/s/kPa, which are on average 25% and 34% higher than the typical MEA process. The theoretical energy consumption amount for desorption of TAMH-TMS-EG-CO2 solutions is identified as 1.11 GJ/t to 1.34GJ/t. Minimum mass transfer resistance is determined at 40% to 80% TMS fraction. A temperature bulge shift and improvement in the interface characteristics enhance mass transfer due to uniform temperature field and good gas and liquid countercurrent contact.

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INTRODUCTION Carbon dioxide (CO2) from power plants has produced a serious greenhouse gas emission problem in the world. CO2 capture is the feasible technique to solve the problem.1,2 The typical CO2 capture routes mainly include chemical and physical absorption, chemical looping, and membrane separation.3,4 During these routes, chemical absorption of CO2 by an amine solvent has been proven to be quite efficient for industrial CO2 capture.5−7 In the chemical absorption process, an advanced amine solvent is required to have high absorption capacity and low energy consumption amount of desorption. However, commercial amine and amino solvents still do not meet these requirements well.8 In order to achieve this, researchers attempt to develop new amine solvents, amine mixtures, or nonaqueous solvents for substituting the conventional monoethanolamine(MEA) solvent. 9−11 Recently, tetramethylammonium hydroxide (TAMH) has been developed as an alternative solvent,12,13 which shows that its CO2 absorption product acts as a photoresist and helps with etch residue removal. Hence, it is possible to utilize the TAMH to solve the CO2 capture and utilization problem to some extent. However, TAMH utilization results in the precipitation of tetramethylammonium carbamates, thus inhibiting the residue removal. Compared with chemical absorption of CO2, physical absorption of CO2 shows low energy consumption traits.8 The solvent is still the key to the physical absorption like the chemical absorption. The typical physical solvent (solvent with the ability to physically absorb CO2) is the tetramethylene sulfone (TMS), which shows great solubility for amine and CO2.14−16 Additionally, ethylene glycol (EG) is taken as a typical solvent for physical absorption of CO2.17,18 In these studies, TMS and EG provide good solubility for tetramethyl© XXXX American Chemical Society

ammonium carbamates, which helps to solve the precipitation problem for TAMH absorption of CO2. However, there is scarce research to integrate TMS and EG for CO2 absorption. Therefore, the objective of this work is to study the mixture solvent of TAMH-TMS-EG for CO2 absorption. It is expected that TAMH-TMS-EG will integrate the advantages of TAMH chemical absorption and TMS-EG physical absorption because MDEA-TMS has already shown improvements for CO2 absorption.19 Additionally, the absorption product shows promise to be used as a photoresist and in etch residue removal, which will certainly produce additional values for CO2 capture, as expected. More importantly, TAMH-TMS-EG may integrate the advantages of chemical absorption and physical absorption to simultaneously produce a higher mass transfer coefficient and lower energy consumption. Because TAMH-TMS-EG is being assessed as an alternative solvent, unknown key parameters, like solubility, reaction kinetics, and mass transfer coefficient between CO2 and TAMH-TMS-EG, should be determined first as they affect the CO2 absorption process.19,20 Therefore, the gas−liquid equilibrium model, reaction kinetics model, and mass transfer model are developed to characterize the CO2-TAMH-TMS-EG system. By using the model, parametric analysis is performed to determine CO2 loading and mass transfer coefficient under different operating parameters in the packed column. Finally, absorption performances are compared between the CO2TAMH-TMS-EG system and other typical CO2 absorption systems. Received: March 25, 2015 Revised: May 20, 2015 Accepted: May 21, 2015

A

DOI: 10.1021/acs.iecr.5b01113 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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MODEL In order to describe the CO2-TAMH-TMS-EG system, the gas−liquid equilibrium model, reaction kinetics model, and mass transfer model are developed first. These models provide the fundamental parameters for determining the performance of TAMH-TMS-EG with respect to absorption of CO2. During the model development process, the assumptions are the following: (1) The standard state for ionic solutes is the ideal, infinitely dilute aqueous solution at the system temperature and pressure. (2) The liquid is incompressible. (3) The liquid and gas phases are each assumed to be continuous. (4) The simulated flue gas is ideally mixed. Gas−Liquid Equilibrium Model. The gas and liquid equilibrium is calculated by the Henry’s constant method, which is given as fi = xiγiHi

which is calculated by the density and dielectric constant. These parameters are cited from the relevant literature.19,20 In solving the equations above, the mixing rules are referenced in the literature,22 which offer the adjusting parameters for integrating parameters obtained from different systems. The eqs 3 and 4 provide the activity coefficient, and eq 2 offers the Henry’s constant, which helps to determine the fugacity according to eq 1. Because the fugacity is also related to the system pressure, gas mole fraction, and fugacity coefficient,18 the gas−liquid equilibrium data between TAMH-TMS-EG and CO2 is finally obtained by the results calculated by eq 1. The gas−liquid equilibrium model provides the concentrations of species, which are used to help determine the CO2 loading together with the reaction kinetics model below. Reaction Kinetics Model. The unknown reaction kinetics of CO2-TAMH-TMS-EG is determined by experiments. TMS with concentration greater than 98% is offered by Chengdu Kelong Chemical Solvent Company, China. TAMH and EG are provided by Tianjin Letai Chemicals Co. Ltd. Differential scanning calorimetry DSC 204HP is used in the experiment. Its temperature ranges from 123.15 to 873.15 K, and its pressure can be operated at 0.1 MPa to 15 MPa. The DSC 204HP is used to measure the heat in the TAMH-TMS-EG absorption of CO2. The 10 wt % TAMH, 70 wt % TMS, and 20 wt % EG are mixed to form the solvent. Five cases are performed respectively at 298.15, 303.15, 308.15, 313.15, and 318.15 K. During the experiment, the DSC baseline is first determined before measuring the heat in the mixing solvent absorption of CO2. The solvent is packed in a ceramic crucible to prevent possible reaction between TAMH with metal. After the experiment, the reaction kinetics is obtained by the measured DSC profiles. The activation energy is determined as 200.59 kJ/mol, and the preexponential is 8.975 × 1035 with the 0.91 order of reaction. The correlation coefficient is 0.9965. The reaction is considered to follow the equation below

(1)

where f i is the fugacity of component i in the gas phase, kPa; xi is the mole fraction of component i in the liquid phase; γi is the activity coefficient of component i in the liquid phase; and Hi is Henry’s constant of component i in the liquid phase, kPa. The fugacity is derived from the Soave equation of the state, and the relevant binary interaction coefficients are found in the literature.14 Henry’s constant of CO2 in EG is 48 MPa to 120 MPa based on the literature data,17 which covers the temperature range from 300 to 450 K. Additionally, Henry’s constant of CO2 in TMS is given as 8.21 MPa to 22.39 MPa by the literature, which covers the temperature range from 303.15 to 373.15K.14 The other physical parameters are cited from the literature.20 The unknown Henry’s constant between CO2 and TAMH is developed as a function of the temperature Z2 + Z3 ln T + Z4T T

ln K = Z1 +

TMAH + CO2 → TMA+HCO3−

(2)

This Henry’s constant is determined by the mass balance methods ever used in the aqueous CO2-amine process.21 The activity coefficient is differentiated from the Clegg− Pitzer equation,19 which is provided as the short-range and long-range excess Gibbs free energy terms below. The short-range excess Gibbs free energy gS is developed as gS = x I ∑ xn ∑ ∑ FcFaWnca + RT n c a

As demonstrated by the previous work, there are two kinds of reactions between TAMH and CO2 in the solvent environment. Equation 5, which only refers to one kind of reaction, is reasonable because it ignores the other reaction producing tetramethylammonium carbonate. 12 Although tetramethylammonium bicarbonate (TMA+HCO3−) further reacts with CO2 to form tetramethylammonium carbonate,12 the TMS-EG provides good dissolution of tetramethylammonium carbonate (precipitate), which results in a rapid reaction between CO2 and tetramethylammonium bicarbonate. Thus, it is difficult for tetramethylammonium carbonate to exist in the TAMH-TMS-EG-CO2 system. This agrees with the lack of precipitate found in the experiment and also shows some agreement with the results in the literature.12 On the basis of these results, the reaction kinetics is subsequently determined as

∑ ∑ xnxn′(A n′ nxn + A n′ nxn′) n>n′

(3)

where xI is total mole fraction of ions; Fc and Fa are, respectively, the cation and anion fraction; An′n and Ann′ are interaction parameters between solvents; Wnca is the parameter between ion−solvent. As for the long-range excess Gibbs free energy, the term is developed as gL 4A I = − x x ln(1 + ζIx 0.5) RT ζ +

dα = 8.975 × 1035 × e−200590/ RT × (1 − α)0.91 dt

∑ ∑ xcxaBca{2[1 − (1 + αIx 0.5)exp(−αIx 0.5)]/(α 2Ix)} c

(5) 12

(6)

where α refers to the relative conversion of TAMH; R is gas constant, 8.314J/mol/K, and T is temperature, K. This reaction kinetics determines the chemical enhancement of CO2-TAMHTMS-EG, which shows greater enhancement effects than the typical aqueous CO2-MEA and CO2-MDEA from the stand-

a

(4)

where Ix is the mole fraction ionic strength; Ax is the Debye− Huckel parameter determined by mole fraction. ζ is the parameter correlated with the hard-core collision diameter, B

DOI: 10.1021/acs.iecr.5b01113 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research point of reaction kinetics.23 This suggests that TAMH-TMSEG shows some improvements for CO2 absorption. As for the desorption of CO2-TAMH-TMS-EG, the desorption experiment is performed at 375, 378, and 381 K by an electrical heater. The temperature range is chosen on the basis of the typical CO2 desorption temperature, and no tetramethylammonium carbonate is required to be desorbed. On the basis of recording CO2 desorption efficiency versus the different temperature (375, 378, and 381 K), the reaction kinetics for desorption is determined as k = 2.953 × 107 × e−4223.12/ T × C TMA+C HCO3‐

the column height, m; C0 is the initial concentration of TAMH, mol/m3; V is the volume, m3. In the previous research, the liquid film thickness τ is normally calculated by the empirical correlations.26 By using the correlations, the revision here is to correlate the liquid film thickness τ with the overall mass transfer coefficient as Kε 2πdhh =

(7)

where k represents the desorption rate, mol·s/m ; CTMA+and CHCO−3 , respectively, represent the concentration of TMA+ and HCO3−, mol/m3. The reaction heat is summarized in Table 1. Comparisons with the typical TMS-CO2, MEA-methanol-CO2, and TAMH-

r=

TAMH-TMS-EG-CO2 TMS-CO2 MEA-methanol-CO2 TAMH-methanol-CO2 a

physical dissolution heat

kJ/mol

kJ/mol

45 ± 3 -

9±2 12.4 66.54a 29.32a

(10)

εdc 2 (11)

Num

(12)

Deff is an effective diffusivity coefficient, which is estimated by porosity and diffusivity D.27 The concentration obtained by eq 12 provides the concentration between the film and phase, which helps to determine the boundary conditions for solving eq 10. The 2-D concentration distributions are determined to integrate the terms of (dC/db)|b=r−τ along the column height direction. The source term S refers to the CO2 absorbed in the liquid phase, which is developed as

this work literature data14 literature data13 literature data13

methanol-CO2 are provided in Table 1.13,14 The physical dissolution heat is obtained by performing the experiment of CO2 dissolution in TMS-EG solvent. It is found that TAMHTMS-EG-CO2 produces less heat against the MEA-methanolCO2 and TAMH-methanol-CO2, which will probably benefit the CO2 desorption process by consuming less energy. These results are used to calculate the energy consumption amount in the Discussion section. Mass Transfer Model. Because CO2 is usually absorbed by amine solutions in packed column, the mass transfer model is developed to characterize the mass transfer process in the packed column. The overall mass transfer equation is developed by considering the interface effects between gas and liquid phase,24 which is given as follows N p − p*

dC |b = r − τ dh db

∇·ρjUC = ∇·∇(ρjDeff C) + S

data source

Sum of reaction heat and physical heat.

K=

HCO2

l

In eq 11, dc refers to the column diameter, m; Num refers to the normalized number of packing along the radial direction. The concentration C is determined by the following equation

Table 1. Experimental Data for the TAMH-TMS-EG-CO2 System reaction heat

∫ l 0

where C is the CO2 concentration in the liquid phase, mol/m3; D is diffusivity, m2/s; HCO2 is the Henry’s constant of CO2 in the liquid, kPa; l is the liquid film length, m; b is the interface distance, m; r is the interpacking passage, which is determined by the packing distribution, giving

3

system

D

S = mC*N

ε 2 π d hh V

(13)

The terms m and C* respectively refer to molecular weight and equilibrium concentration. The interface between gas and liquid phase is obtained from the model below

∇·(jρ ·U ) = Scsf

(14)

where j is phase fraction; Scsf is the source term of the continuous surface force, which is cited from the literature.28 In this term, the interfaces between fluids are developed as transition regions of finite thickness, where the force density is proportional to the curvature of the surface. According to eq 14, the interface distance b is calculated as

(8) 2

where K is the overall mass transfer coefficient, mol/m /s/kPa; N is the CO2 mass flux, mol/m2/s; p and p* are, respectively, partial pressure and equilibrium partial pressure, kPa. The equilibrium partial pressure p* is obtained by the Henry’s constant in eq 2 and solutions loading.25 By considering the effects of the packing on mass transfer, it is required to revise the mass flux equation in the literature.24 Hence, the interface mass flux N (mol/m2/s) is developed by including the porosity caused by the packing 1 q 1 1 dα C0 V N= 2 = 2 ε π d hh ε π (dc + τ )h dt (9)

∇·(ρ ·U ) − ∇·(βρ·U ) = ρU /b

(15)

where β is the minimal value, which is used to substitute the extreme condition of the phase fraction equal to zero. This is developed by considering the mass conservation during the interface. On the basis of equations above, the interface area a (m2/ 3 m ) is finally deduced as a = τ [b(1 − jgas )]2 + l 2 /(bl τ 2 + b2 )

(16)

The jgas refers to the gas phase fraction. This interface area model is fundamentally developed and shows the clear mechanism between films and phases. Thus, it is improved to some extent against the empirical correlations.29 The liquid

where ε is the porosity; q is mass flux, mol/s; dh is the typical hydraulic diameter, which equals to the sum of column diameter dc and liquid film thickness τ on the packing, m; h is C

DOI: 10.1021/acs.iecr.5b01113 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research velocity U is obtained by the liquid phase momentum equation, which is typically used for absorption of CO2 in amine solutions.27,30 The typical energy equation is used to determine the temperature distribution,7,31 where the experiment reaction heat in Table 1 is used to revise the source term. The equations above help to determine the mass transfer coefficient accurately without referencing the empirical correlations, which usually depend on the specified operating conditions. By numerical solution, the mass transfer coefficient is obtained precisely.

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MODEL VALIDATION In order to validate the model above, 50 mL of TAMH-TMSEG solvent (with 15% TAMH and 65% TMS) is used to respectively absorb 10% and 15% (mole fraction) of CO2 in the CO2−N2 mixture gas. The absorption test is performed at atmosphere pressure. The CO2 absorption amount is measured by an IRME-S infrared gas analyzer, which covers the gas concentration range of 0−100%. By recording the time and CO2 concentration, the absorption rate is experimentally obtained as an input parameter, and CO2 absorption proceeds until the TAMH-TMS-EG-CO2 saturates gradually. Taking absorption rate and time as fit parameters, the absorption rate is used to validate the simulated results, which is given in Figure 1a. As shown in Figure 1a, the simulation results agree well with the experiment data with an acceptable maximum deviation of 5% during the 20 min absorption period, which is probably due to that there is time delay in the monitor system. The packed column experiment is performed in a column with a diameter of 0.1 m and a height of 0.8 m. Raschig rings are used as packings in the experiment. The CO2 mass flux and pressure are recorded to obtain the experiment mass transfer coefficient. Here, the experiment is carried out at the CO2 partial pressures of 5, 10, 15, 20, and 25 kPa as the input parameter. The parameter is determined by referencing the typical CO2 partial pressure in industrial flue gas. Taking mass transfer coefficient and CO2 partial pressure as fit parameters and CO2 partial pressure as input parameters, the comparisons between the experiment mass transfer coefficient and the simulation results are given in Figure 1b. As shown in Figure 1b, the simulation fits well with the experiment mass transfer coefficient. The experiment mass transfer coefficient is determined by titrating the sampling solution from the bottom to the top of the column and monitoring the gas pressure distribution by the pressure transducers. Additionally, it is found that TAMH-TMS-EG produces higher mass transfer coefficient against the typical MEA.24 This is possible because TAMH-TMS-EG provides greater enhancements according to reaction kinetics (eq 6) and wider CO2 loading ranges (details in Discussion section). Meanwhile, taking temperature and position in column as fit parameters, the axial temperature is compared with the experimental data, which is shown in Figure 1c. The experimental temperature is measured by five pairs of thermocouples installed in the axial direction of the column with 0.2 m axial space. These results still show good agreement, which ensures correct temperature fields and further produces precise velocity and concentration fields.

Figure 1. Absorption rate (a), mass transfer coefficient (b) and temperature distribution (c) in TAMH-TMS-EG absorption of CO2.

Supporting Information) and Tables 2 and 3. Table S1 provides Henry’s constant between CO2 and TAMH. As observed in Table S1, Henry’s constant between CO2 and TAMH are below the values of typical CO2-MEA,32,33 which suggests that TAMH improves the CO2 solubility to some extent. Additionally, fitted values of interaction parameters between EG, TAMH, TMS, TAMH+ and HCO3− are offered in Tables S2 and S3, which helps to determine the excess Gibbs

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RESULTS Gas−Liquid Equilibrium Results. In the TAMH-TMSEG-CO2 system, the missing parameters for gas−liquid equilibrium data are provided in Tables S1 to S3 (see D

DOI: 10.1021/acs.iecr.5b01113 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Table 2. Species Concentrations at 303.15 K in the TAMH-TMS-EG-CO2 System system MEA-methanol-CO211/kmol/kg

TAMH-TMS-EG-CO2/kmol/kg loading/mol/mol

TAMH

TMA+

HCO3−

MEA

MEAH+

MEACOO−

HCO3−

0 0.1 0.2 0.3 0.4 0.5

1094.806 868.26 688.62 538.92 449.1 309.38

0 109.78 249.5 329.34 459.08 568.86

0 109.78 249.5 329.34 459.08 568.86

4995 4215.78 2977.02 2217.78 1048.95 509.49

0 559.44 1348.65 1558.44 2017.98 2867.13

0 559.44 1348.65 1558.44 2017.98 2867.13

0 0 0 0 92.25 101.75

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free energy. In Tables S2 and S3, the typical CO2 absorption temperature of 303.15 K and 313.15 K are chosen. Additionally, species concentrations in the TAMH-TMS-EGCO2 system are determined at 303.15 K in Table 2. The species include TAMH, TAM+, and HCO3−. As shown in Table 2, it is found that HCO3− concentration increases from 0 to 568.86 kmol/kg as CO2 loading increases from 0 to 0.5 mol/mol. The TAM+ concentration equals the HCO3− concentration, which agrees with the reaction ratios. Table 2 also provides the comparison results with the typical MEA-methanol-CO2 system.11 It is found that high HCO3− concentration in the TAMH-TMS-EG-CO2 helps CO2 to be easily regenerated. On the contrary, it is difficult for a high concentration of MEACOO− to be regenerated to produce CO2 in the MEAmethanol-CO2. After the parameters above are obtained, the gas and liquid equilibrium data are predicted well. Here, the TAMH-TMSEG-CO2 system is considered to be operated at 313.15 and 373.15 K, which are typical absorption and desorption temperatures. The predicted results are given in Figure 2,

PARAMETRIC ANALYSIS RESULTS

In the CO2 absorption, CO2 loading and mass transfer coefficient are the key parameters to quantify the CO2 absorption performance. CO2 fraction, solvent composition (TAMH fraction and TMS fraction), and pressure normally influence CO2 absorption performance. Hence, the effects of CO2 fraction, both TAMH fraction and TMS fraction, and pressure on CO2 loading and mass transfer coefficient are given in Figures 3−6. All the CO2 loadings below are obtained at radial direction of 0.05 m and the axial direction position of bulge phenomenon occurring, which is an important point reflecting the CO2 absorption performance.

Figure 2. Solubility of CO2 in TAMH-TMS-EG solvent.

which shows that the partial pressure of CO2 varies from 2 to 25 kPa, corresponding to 0.15 mol/molTAMH to 0.453 mol/ molTAMH at 313.15 K. This suggests that the solubility of CO2 in TAMH-TMS-EG is high enough for CO2 absorption. Moreover, the solubility shows similar data to that of CO2 in the MDEA-TMS aqueous system.19 At 373.15K, the partial pressure of CO2 varies from 100 to 675 kPa, corresponding to 0.18 mol/molTAMH to 0.5 mol/molTAMH, which is a little less than that partial pressure of CO2 in MDEA-TMS aqueous system.19 Thus, this result provides evidence that is easier to regenerate CO2 in TAMH-TMS-EG.

Figure 3. Effects of CO2 fraction on CO2 loading (a) and mass transfer coefficient (b) (operating at 0.1 MPa of total pressure). E

DOI: 10.1021/acs.iecr.5b01113 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Effect of CO2 Fraction. CO2 fraction in flue gas usually varies with operating conditions in industry. Here, the CO2 mole fraction is set to vary from 5% to 25%, which are typical values in the industrial flue gas.2,27 The effects of this CO2 mole fraction on CO2 loading is given in Figure 3a, which shows that CO2 loading increases from 0.18 mol/molTAMH to 0.513 mol/molTAMH as CO2 mole fraction increases at 303.15, 308.15, and 313.15 K, respectively. This is due to the fact that CO2 partial pressure increases as CO2 mole fraction increases, which helps CO2 dissolution and reaction in the TAMH-TMSEG. Additionally, it is found that there is an average of 14% CO2 loading increase as temperature decreases from 313.15 to 303.15 K, which suggests that there is good CO2 absorption at 303.15 K. This is in agreement with the typical MEA absorption CO2 process,34 which is because the same exothermic reaction exists in the TAMH-TMS-EG absorption of CO2. As CO2 fraction varies, the mass transfer coefficient is plotted along the axial position of packed column in Figure 3b. As presented in Figure 3b, the mass transfer coefficient reaches the maximum at a position of 0.32 m, which agrees with the typical bulge phenomena existing in amines for absorption of CO2.35 The 25% CO2 fraction produces the largest mass transfer coefficient of 4.02kmol/m2/s/kPa, which is 34% higher typical 3kmol/m2/s/kPa of MEA absorption of CO2.24 As the CO2 fraction decreases to 5%, the mass transfer coefficient decreases by 10% against the 25% CO2 fraction circumstance. This is because a higher CO2 fraction provides a greater driving force for CO2 dissolution, which is generated by higher partial pressure. Effect of Solvent Composition. Weight fractions of TAMH and TMS influence the CO2 loading, and these data are summarized in Figures 4 and 5. As shown in Figure 4a, CO2 loading increases from 0.2 mol/molTAMH to 0.545 mol/ molTAMH as TAMH weight fraction increases from 5% to 25%. This CO2 loading is about 6% higher against that under the different CO2 fraction in Figure 3a. This is due to the fact that liquid TAMH is involved in the chemical reaction with CO2, which results in the effect that more liquid TAMH naturally absorbs more CO2 under EG as the solvent. In this sense, the average CO2 loading is decreased by 9% when the temperature increases from 303.15 to 313.15 K as a result of the exothermic heat during the TAMH absorption of CO2. Apparently, TAMH will also influence the mass transfer coefficient. The corresponding results are given in Figure 4b. It is clear that the mass transfer coefficient increases from 2.3 kmol/m2/s/kPa to 4.12 kmol/m2/s/kPa and then decreases to 2.95 kmol/m2/s/kPa from the top to the bottom of the column. As the TAMH fraction increases from 5% to 25%, the average mass transfer coefficient is increased by 15%. This is due to the fact that the reactant TAMH generally absorbs more CO2 when its concentration is increased according to the reaction mole ratios. TMS, as the physical solvent, influences the CO2 loading. Its effects on CO2 loading are presented in Figure 5a. As shown in Figure 5a, CO2 loading increases from 0.2 mol/molTAMH to 0.445 mol/molTAMH as TMS weight fraction increases from 55% to 75%. This CO2 loading is reduced by 18% compared to that under different TAMH fractions in Figure 4a. The reason is that TMS is a physical solvent, and its absorption capacity is not as strong as TAMH. This is further demonstrated by the fact that the CO2 loading is decreased by 24% when temperature increases from 303.15 to 313.15 K.

Figure 4. Effects of TAMH fraction on CO2 loading (a) and mass transfer coefficient (b) (operating at 0.1 MPa of total pressure and 15% CO2 fraction).

TMS obviously affects the mass transfer coefficient. The results are provided in Figure 5b. As shown in Figure 5b, the mass transfer coefficient profiles show the bulge phenomena,35 which produces the highest value of 3.98 kmol/m2/s/kPa at a column position of 0.32 m away from the top. As the TMS fraction increases from 55% to 75%, the average mass transfer coefficient is increased by 9%. This improvement is due to the fact that TMS has the ability to physically absorb CO2, and thus, a higher TMS fraction provides more TMS to absorb more CO2. Effect of Pressure. Because operating pressure affects the CO2 solubility in TAMH-TMS-EG and fluid flow in the packed column, it impacts CO2 loading, and these data are provided in Figure 6a. Here, by keeping TAMH and TMS weight fraction at 10% and 70%, respectively, the operating pressure is considered to be 0.08 MPa to 0.16 MPa, which is the pressure range that is easily implemented in industry.25 As shown in Figure 6a, CO2 loading increases from 0.32 mol/molTAMH to 0.436 mol/ molTAMH as the operating pressure increases from 0.08 to 0.16 MPa. This is probably due to the fact that high operating pressure provides the higher partial pressure of CO2, which results in great CO2 solubility in TAMH-TMS-EG. Meanwhile, F

DOI: 10.1021/acs.iecr.5b01113 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 6. Effects of pressure on CO2 loading (a) and mass transfer coefficient (b) (operating at 15% CO2 fraction).

Figure 5. Effects of TMS fraction on CO2 loading (a) and mass transfer coefficient (b) (operating at 0.1 MPa of total pressure and 15% CO2 fraction).

another factor that affects the CO2 absorption. Thus, it is important to determine the mass transfer resistance for the further solvent composition optimization. Usually, the Hatta number is used to determine the rate of the diffusion versus the chemical reaction.16,25 Here, the diffusivity of CO2 in the solvent is cited from the literature.16,36 The dimensionless solubility is used to indicate the ratio of the CO2 concentration in the liquid phase and gas phase at equilibrium state.16 The corresponding results are offered in Figure 7, which describes the situation at the radial position of 0.05 m and axial position of 0.4 m. As shown in Figure 7, the Hatta number is below 3 and thus agrees with the results in the literature for CO2 absorption in the solvents.23 It is found that the minimum mass transfer resistance corresponds to the TMS weight fraction from 40% to 80% due to higher Hatta number obtained accordingly. Additionally, the higher CO2 solubility above 1 is obtained in this TMS weight fraction range. This result may help with the future TAMH-TMS-EG solvent optimization design. In order to analyze the two phase effects, the interface area and interface distance are offered for the two systems of TAMH-TMS-EG and MEA absorption of CO2 (in Figure 8). The typical gas and liquid film flow regimes are chosen for comparison at the radial position of 0.05 m. As clearly shown, the interface area shows the similar bulge profiles versus the

high operating pressure enhances the fluid flow between gas and liquid phases, which improves CO2 loading. Additionally, the average CO2 loading is decreased by 8% when the temperature increases from 303.15 to 313.15 K because of higher dissolution at lower temperature. Pressure will also have some impact on the mass transfer coefficient. The results are given in Figure 6b. As clearly shown in Figure 6b, there is still the bulge phenomenon of the mass transfer coefficient, which produces the highest value at a column position of 0.32 m away from the top. As the pressure increases from 0.08 to 0.16 MPa, the mass transfer coefficient is increased by 14%. The reason is that higher pressure provides the higher external drive force for the CO2 dissolution and two phase flow (greater interface force) in the TAMH-TMS-EG absorption system.

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DISCUSSION On the basis of the results above, we can conclude that the chemical reaction and dissolution affect the TAMH-TMS-EG absorption of CO2. In order to offer more fundamental information for TAMH-TMS-EG absorption of CO2, the following investigation is performed at 303.15 K and 15% CO2 fraction in 0.1 MPa simulated flue gas. Here, the gas and liquid flux ratio is set as the typical value of 0.2. In fact, diffusion is G

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Industrial & Engineering Chemistry Research

Figure 7. Resistance in the TAMH-TMS-EG absorption of CO2. Figure 9. Temperature bulge shift in the TAMH-TMS-EG absorption of CO2.

between the CO2 and TAMH-TMS-EG, which suggests that high utilization efficiency of the solvent is achieved. Additionally, comparisons with typical MEA-CO2−H2O, MEA-CO2-TMS, and other solvents systems are made in Table 3 to assess the overall CO2 capture performance of TAMH-TMS-EG. As shown in Table 3, it is found that the average CO2 loading is 25% higher against the MEA-CO2− H2O and MEA-CO2-TMS.5,15,25 Additionally, the CO2 loading is, respectively, 31% and 87.5% higher than that of PZ-CO2 and AMP-CO2.37 The TAMH concentration is lower than MEA concentration, which results in a little greater solvent flux (physical solvent absorption of CO2 able to reduce the solvent flux) and thus may produce a little greater sensible heat consumption in desorption. However, the sensible heat consumption only takes a small portion in the overall energy consumption in desorption, and normally the reaction heat takes a great portion. Thus, the energy consumption is obtained as 1.11 GJ/t to 1.34 GJ/t for TAMH-TMS-EG absorption of CO2 (Table 3). This is simply calculated by only considering the reaction heat Hr and physical heat Hp in Table 1, giving

Figure 8. Interface characteristic in the TAMH-TMS-EG absorption of CO2.

interface distance in the TAMH-TMS-EG and MEA absorption of CO2. The typical advantage for TAMH-TMS-EG absorption of CO2 is that the average interface area is 250m2/m3, which is about 20% higher than that of MEA absorption of CO2. The reason is quite complicated because the interface area is affected by surface tension, viscosity, and fluid flow. TAMH-TMS-EG provides higher surface tension and steady film flow, which results in higher interface area, even though the higher viscosity of TAMH-TMS-EG system reduces interface area to some extent.12,16 Thus, TAMH-TMS-EG enhances mass transfer against the conventional MEA, which thus improves the CO2 absorption. Because it is influenced by the two phase effects, the temperature bulge of the TAMH-TMS-EG absorption of CO2 process shifts along the axial direction of the column, which favors the CO2 absorption process.35 As shown in Figure 9, it is found that the bulge temperature of the TAMH-TMS-EG absorption of CO2 is reduced from 322 to 315 K against the typical MEA absorption of CO2. This result suggests that the TAMH-TMS-EG provides more uniform temperature fields for absorption CO2, which ensures that more uniform absorption capacity is obtained along the packed column. More importantly, the bulge position shifts from 0.24 to 0.32 m. The bulge position of 0.32 m is close to the middle section of the column and thus offers good countercurrent contact

E=

Hr + H p

(17) m where E is energy consumption for desorption, GJ/t. This theoretical energy consumption amount shows a little less than the experiment data of 1.2GJ/t to 1.38 GJ/t obtained by just using the electrical heater to desorb the rich solution at 381 K with CO2 loading varying from 0.3 mol/mol to 0.5 mol/ mol. The deviation is due to the fact that there is a little sensible heat ignored in eq 17. By these results, it is quite clear that the desorption energy consumption for TAMH-TMS-EGCO2 is much less than the typical 3.0 GJ/t to 4.5 GJ/t in MEA solution absorption of CO2.5,25 The energy consumption is also below the typical 3.1 GJ/t to 3.72 GJ/t in PZ, AMP and BASF solvents absorption of CO2 (in Table 3).37−39 The reason may be that molecules of TMS have a higher dielectric constant (about 82.56 at 303.15K) compared with water,11,19 and hence, it is prone to act as a more active hydrogen atom donor and acceptor rather than keep intermolecular relations. Additionally, the evaporation heat is not existent in the TAMH-TMS-EGCO2 system due to no evaporation of TAMH, TMS, and EG H

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Industrial & Engineering Chemistry Research Table 3. Comparisons between the TAMH-TMS-EG-CO2 System and the Typical CO2 Absorption System system

CO2 loading/mol/mol

energy consumption/GJ/t

data source

TAMH-TMS-EG-CO2 MEA-CO2−H2O MEA-CO2-TMS PZ-CO2 AMP-CO2 BASF solvent-CO2

0.2−0.55 0.2−0.4 0.3 0.1−0.47 0.4 -

1.11−1.34 3.0−4.5 3.4 3.72 3.1−3.7

this work literature5,25 literature15 literature37,39 literature37,39 literature38

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occurring below 381 K. However, evaporation heat takes great portions in desorption of MEA-CO2 system. On the basis of the results above, TAMH-TMS-EG is identified as an alternative solvent for CO2 absorption. A full process simulation including the different unit operations will provide more useful information for industrial application and will be studied in the future. Moreover, the impurities of flue gas (oxygen, SOx and NOx) may influence the TAMH-TMSEG absorption of CO2 because there are some potential chemical reactions between the impurities and TAMH.40−42 The volatility of TAMH-TMS-EG is supposed to be lower than the typical MEA solutions due to good dissolution of TAMH in TMS and EG. However, the assessment of environmental impacts is still ongoing for the future industrial application of TAMH-TMS-EG to capture CO2. The risk of TAMH is almost the same as MEA. TMS may produce corrosion, and EG is flammable. Thus, the TAMH-TMS-EG may produce some safety problems compared with the typical MEA solutions. However, the lower volatility of TAMH-TMS-EG can control the risk together with sufficient measures to prevent the leak during the future application. In entirety, TAMH-TMS-EG has higher mass transfer coefficient, lower energy consumption, and controllable characteristics with respect to safety, and it shows great potential to capture CO2 from flue gas in power plants, nature gas purification factories, and in the cement industry.

ASSOCIATED CONTENT

S Supporting Information *

Henry’s constant and fitted parameters of the gas and liquid equilibrium. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.iecr.5b01113.

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support of the National Natural Science Foundation of China (No. 51276141) is gratefully acknowledged. This work is also supported by the China Postdoctoral Science Foundation funded project (No. 2013M530422), Natural Science Foundation of Shaanxi Province, China (2015JQ5192) and “Fundamental Research Funds for the Central Universities”.

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REFERENCES

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CONCLUSIONS In order to reduce the cost of CO2 capture, an efficient solvent mixture of tetramethylammonium hydroxide (TAMH), tetramethylene sulfone (TMS), and ethylene glycol (EG) was successfully assessed. Gas−liquid equilibrium, reaction kinetics, and mass transfer models were developed to determine CO2 absorption performance in packed column. Henry’s constant, reaction kinetics, and mass transfer coefficient between CO2 and TAMH-TMS-EG were provided. The parametric analysis identified that CO2 loadings were 0.2 mol/molTAMH to 0.55 mol/molTAMH, showing on average 25% higher values than typical MEA absorption of CO2. The mass transfer coefficient reached 4.02 kmol/m2/s/kPa, which is 34% higher than the MEA absorption system. Additionally, after the comparison analysis with typical 3.0 GJ/t to 4.5 GJ/t in MEA solution absorption of CO2, the theoretical energy consumption amount for desorption of CO2 in TAMH-TMS-EG was identified as 1.11 GJ/t to 1.34GJ/t. It was found that the minimum mass transfer resistance existed at the TMS weight fraction from 40% to 80% due to the corresponding higher Hatta number. The temperature bulge shift and interface characteristic improvement produced uniform temperature field and good countercurrent contact, which thus intensified CO2 mass transfer. Finally, the TAMH-TMS-EG was assessed as an alternative solvent for effective CO2 capture. I

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