Kinetics, Thermodynamics, and Mechanism of a Novel Biphasic

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Kinetics, thermodynamics, and mechanism of a novel biphasic solvent for CO2 capture from flue gas Shihan Zhang, Yao Shen, Peijing Shao, Jianmeng Chen, and Lidong Wang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05936 • Publication Date (Web): 19 Feb 2018 Downloaded from http://pubs.acs.org on February 25, 2018

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Environmental Science & Technology

1

Kinetics, thermodynamics, and mechanism of a novel

2

biphasic solvent for CO2 capture from flue gas

3

Shihan Zhanga,*, Yao Shena, Peijing Shaoa, Jianmeng Chena, Lidong Wangb,*

4

a

5

China

6

b

7

University, Baoding 071003, China

8

ABSTRACT: The main issue related to the deployment of amine-based absorption

9

process for CO2 capture from flue gas is its intensive energy penalty. Therefore, this

10

study screened a novel biphasic solvent, comprising a primary amine e.g.,

11

triethylenetetramine (TETA) and a tertiary amine e.g., N, N-dimethylcyclohexylamine

12

(DMCA), to reduce the energy consumption. The TETA-DMCA blend exhibited high

13

cyclic capacity of CO2 absorption, favorable phase separation behavior, and low

14

regeneration heat. Kinetic analysis showed that the gas- and liquid-side mass transfer

15

resistances were comparable in the lean solution of TETA-DMCA at 40oC, whereas

16

the liquid-side mass transfer resistance became dominant in the rich solution. The rate

17

of CO2 absorption into TETA-DMCA (4M, 1:3) solution was comparable to 5M

18

benchmark monoethanolamine (MEA) solution. Based on a preliminary estimation,

19

the regeneration heat with TETA-DMCA could be reduced by approximately 40%

20

compared with that of MEA. 13C NMR analysis revealed that the CO2 absorption into

21

TETA-DMCA was initiated by the reaction between CO2 and TETA via the zwitterion

22

mechanism, and DMCA served as a CO2 sinker to regenerate TETA, resulting in the

23

transfer of DMCA from the upper to lower phase. The proposed TETA-DMCA

24

solvent may be a suitable candidate for CO2 capture.

College of Environment, Zhejiang University of Technology, Hangzhou, 310014,

School of Environmental Science and Engineering, North China Electric Power

1

ACS Paragon Plus Environment


25

1 INTRODUCTION

26

Carbon dioxide (CO2) is a major contributor to the global warming, which incurs

27

severely environmental issues such as weather extremes and a rise in sea level. The

28

Paris Agreement, ratified by 159 nations as of summer 2017, aims to mitigate the

29

global warming. Because CO2 accounts for over 77% of greenhouse gas emission, the

30

development of the advanced technologies for CO2 capture is critical.1-4

31 32

The monoethanolamine (MEA)-based absorption process is regarded as a state-of-art

33

technology for near-term CO2 capture,5-7 but costly due to the high energy

34

requirement. As estimated by Department of Energy of the United States, the

35

MEA-based process incurs an increase in the cost of electricity by approximately

36

80%.8 The cost of the steam usage for the solvent regeneration and required CO2

37

compression accounts for 60-70% of the total.8-10 Therefore, it is crucial to develop

38

novel solvents and tailored absorption processes to reduce the energy penalty of CO2

39

capture. 11-15

40 41

Currently, the biphasic solvents are proposed to capture CO2.16 Upon CO2 absorption

42

or temperature swing, the CO2-laden solvent undergoes liquid-liquid or liquid-solid

43

phase transition, and over 90% of the absorbed CO2 is concentrated in a CO2-rich

44

phase. Therefore, only the CO2-rich phase is sent to the stripper for the solvent

45

regeneration, and the CO2-lean phase is directly sent back to the absorber. As a result,

46

the sensible heat is expected to be reduced due to a smaller amount of solution for

47

regeneration.17 Moreove, because a more concentrated CO2-rich phase is used for

48

solvent regeneration, a higher CO2 partial pressure in the stripper can be achieved,

49

resulting in a lower stripping heat and less compression work. On the other hand, the 2


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absorption heat may also be reduced by optimizing composition of biphasic solvent.

51

Therefore, the biphasic solvent-based process possesses a great potential to

52

substantially lower the energy consumption of CO2 capture.17

53 54

Recent studies on biphasic solvents predominantly focused on liquid-liquid phase

55

transition solvents, because their corresponding processes are straightforward to

56

operate.18 Most of the reported biphasic solvents are amine blends, comprising a

57

primary or secondary amine as a CO2 absorption accelerator and a tertiary amine as a

58

CO2 sinker. For example, the N-methyl-1,3-propane-diamine (MAPA) and

59

2-(diethylamino)-ethanol (DEEA) blend,19,

60

blend,21 diethylenetriamine (DETA) and pentamethyldiethylenetriamine (PMDETA)

61

blend,13

62

(DMCA) blend,22 and DMXTM solvent have been proposed as biphasic solvents for

63

CO2 absorption.23, 24 MAPA-DEEA, BDA-DEEA, and DETA-PMDETA solvents are

64

CO2-triggered biphasic solvents, whereas MCA-DMCA and DMXTM solvent are

65

temperature swing-triggered biphasic solvents. Compared with the CO2-triggered

66

biphasic

67

solvent-based process requires a phase separator to be positioned downstream of the

68

heat exchanger which may result in higher sensible heat.25 In addition, the temperature

69

swing-triggered biphasic solvent is regenerated at approximately 90 oC which results

70

in a relatively low CO2 partial pressure before the compression and thus increases the

71

required compression work.25 However, the critical issue related to CO2-triggered

72

biphasic solvents is their phase separation behavior. The liquid-liquid phase

73

separation has been reported to disappear with an increase in CO2 loading.19-21 As a

74

result, a tradeoff of the phase separation behavior and cyclic absorption capacity

N-methylcyclohexylamine

solvent-based

process,

20

1,4-butanediamine (BDA) and DEEA

(MCA) and

the

N,N-dimethylcyclohexylamine

temperature

3


swing-triggered

biphasic


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75

should be considered to develop a novel CO2-triggered biphasic solvent. Moreover,

76

most of the previous researches on biphasic solvent only focused on absorption

77

capacity and phase transitional behavior. Therefore, the energy penalty of the

78

developed biphasic solvents requires further investigation.

79 80

The phase transition of CO2-triggered biphasic solvents is due to a sharp increase in

81

ionic strength in the CO2-rich phase after the formation of carbamate or bicarbonate.26

82

Therefore, to achieve phase separation, selection of a primary or secondary amine

83

with multiple protonatable sites (e.g., diamine and triamine) is preferred to increase

84

the ionic strength upon CO2 absorption. Additionally, a tertiary amine with suitable

85

hydrophobicity is beneficial to ensure phase separation even at high CO2 loading.

86

Therefore, the triethylenetetramine (TETA) with two primary amino groups and two

87

secondary amino groups was used as the CO2 absorption accelerator in this study,

88

which possesses a fast absorption rate and high CO2 absorption capacity as reported in

89

the literature.27 Five tertiary amines with different hydrophobicity such as DEEA,

90

DMCA,

91

bis(2-dimethylaminoethyl)ether (BDMAEE) were analyzed as CO2 sinkers to achieve

92

excellent phase separation behavior.

PMDETA,

3-(Diethylamino)-1,2-propanediol

(DEAPD),

and

93 94

The aim of this study was to develop a novel biphasic solvent with high CO2

95

absorption capacity, fast absorption rate, desirable phase separation behavior, and low

96

energy penalty. The kinetics of CO2 absorption into the novel biphasic solvent was

97

analyzed using a double-stirred cell reactor. The vapor-liquid equilibrium (VLE) data

98

were determined, and the energy penalty of the biphasic solvent was evaluated using

99

an estimation method reported in the literature.28, 29 Furthermore, the mechanism of 4


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CO2 absorption into the developed biphasic solvent was investigated through

101

speciation analysis. This study provides insight into development of an energy-saving

102

biphasic solvent.

103 104

2 EXPERIMENTAL METHODOLOGY

105

2.1 Chemicals

106

Monoethanolamine (MEA, purity ≥99%), triethylenetetramine (TETA, purity ≥68%),

107

2-(diethylamino)ethanol

108

(DMCA, purity≥98%) and N,N,N′,N′′,N′′-pentamethyldiethylenetriamine (PMDETA,

109

purity ≥99%) were obtained from Aladdin Industrial Corporation, China.

110

Bis(2-dimethylaminoethyl)ether (BDMAEE, purity ≥98%) was obtained from Nine

111

Ding Chemistry, China. 3-(Diethylamino)-1,2-propanediol (DEAPD, purity ≥98%)

112

was obtained from Dibo Chemistry, China. CO2 (purity ≥99.99 vol%) and N2 (purity

113

≥99.99 vol%) were supplied by Jingong Gas Co., China. The hydrophobicity (LogP)

114

and alkalinity (pKa) of the tested amines were provided in Table S1.

(DEEA,

purity≥99%),

N,N-dimethylcyclohexylamine

115 116

2.2 Experimental procedure

117

The CO2 absorption capacity and phase separation behavior of the TETA-DEEA,

118

TETA-DMCA, TETA-PMDETA, TETA-BDMAEE, and TETA-DEAPD solvents

119

were tested in a 50 mL glass bubbler reactor. In a typical test, 13vol% of CO2

120

balanced with N2, at a flow rate of 200 mL min-1, was bubbled into 30 ml of the tested

121

solvents with a total amine concentration of 4 M at a desired blend ratio for 120 min

122

at 40oC and atmospheric pressure. The effect of the ratio of primary to tertiary amines

123

(0.5:3.5, 1.0:3.0, and 1.5:2.5) on absorption capacity and phase separation behavior

124

was also investigated. The solvent with the promising absorption capacity and phase 5



125

separation behavior was selected to determine the VLE data at the different

126

temperatures (40, 50, and 60oC) and inlet CO2 concentrations (0.09-13vol%).

127

Equilibrium was achieved and indicated by an equal concentration of CO2 in the inlet

128

and outlet. After reaching equilibrium, both of the two liquid phases, if formed, were

129

sampled to determine the CO2 loading by the Chittick apparatus.30

130 131

The absorption rates of CO2 into the desired solvents were tested in a 500 mL

132

double-stirred cell reactor (DSCR) with an internal cross-section of 28.3cm2 (Figure

133

S1). Two different concentrations of CO2 balanced by N2 (1.3 and 13vol%), at a flow

134

rate of 1 L min-1, were introduced to 250 mL of the promising biphasic solvent at

135

different CO2 loadings (0.25 and 0.75 mol mol-1) and temperatures (40, 50, and 60oC).

136

The stirring rate of the gas phase was 250 rpm and that of the liquid phase was varied

137

from 250 to 400 rpm to ensure a smooth gas-liquid interface and homogeneous

138

mixing.

139 140

The CO2 absorption rate in DSCR was determined via the difference between the inlet

141

and outlet gas flow rate, measured by a soap-film flowmeter, and was calculated as

142

follows: 31 =−

143 144 145 146 147

( − ) (1) ,

where is the CO2 absorption rate, kmol m-2 s-1; and are the standard and

room temperatures, respectively, K; and are the gas flow rates in the outlet and inlet, respectively, m3 s-1; is the interfacial area between the gas and liquid

phase, m2; and , is the molar volume of the gas at the standard temperature and atmospheric pressure, m3 kmol-1.

148 6


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149 150

2.3 Data interpretation

151

2.3.1 Mass transfer resistance determination

152

According to the two-film theory of gas absorption with reaction, the overall

153

liquid-side mass transfer resistance can be determined as follow:32

154

where is the overal l liquid-side mass transfer coefficient, m s-1; is the

155 156 157 158

1 1 = + (2)

Henry’s law constant of CO2, kmol m-3 kPa-1; is the temperature of the solution,

K; is the universal gas constant, kPa m3 kmol-1 K-1; is the individual gas-side

mass transfer coefficient, m s-1; is the enhancement factor; and is the

individual liquid-side mass transfer coefficient, m s-1.

159 160

161

The enhancement factor can be determined by:33, 34 ∗ ( − ) ! − !"# = − = (3) 1 1 , + ∗ where ! is the partial pressure of CO2 in the gas phase, kPa; !"# is the partial

162

pressure of CO2 in equilibrium which can be obtained from the VLE data, kPa. The

163

estimation of the physiochemical parameters in the gas and liquid phases was

164

provided in Supporting Information. The estimated physiochemical parameters used

165

in this study were summarized in Table S2.

166 167

For a pseudo-first-order reaction, the enhanced factor can also be calculated as: ≈ ' =

168

() ,*+ , (4)

where ' is the Hatta number; ) ,*+ , is the diffusion coefficient of CO2 in the 7



169

amine solution, m2 s-1; - is the overall first-order rate constant, s-1. The chemistry

170

of CO2 absorption into the biphasic solvent is provided in Supporting Information.

171

Therefore, - can be calculated.

172 173

To meet the pseudo-first-order reaction assumption, this general criterion should be

174

satisfied: 35 3 < ' ≪ 1 (5) 1 = 1 +

' = 175 176

34*+ , )*+ , (6) 4 ) ,*+ ,

() ,*+ , (7)

where 1 is the infinite enhancement factor; 3 is the stoichiometric reaction ratio

of CO2 to amines; 4*+ , is the concentration of amines, kmol m-3; )*+ , is the

177

diffusion coefficient of amines in the solution, m2 s-1; and The values of )*+ , in

178

the fresh amine solution were determined according to the approach reported by

179

Snijder et al.36 Additionally, the values in the CO2-laden amine solution were

180

estimated based on a modified Stokes–Einstein relation.37 The values of ) ,*+ ,

181

were estimated based on the method developed by Sada et al.38 Under the tested

182

conditions in this work, the criterion (Eq. 5) of the pseudo-first-order assumption was

183

satisified.

184 185

2.3.2 Thermodynamic data evaluation

186

Regeneration heat (7,8, ) was estimated using the method reported by Kim et al..28

187

A schematic diagram of the biphasic solvent-based process is provided in Figure S2,

188

and the selected temperatures at the top ( 9) and bottom (: ) of the stripper for the

189

estimation are presented in Table S3. 8


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190 191 192

7,8, comprises heat of reaction (7; ), sensible heat (

TETA-DMCA

>

TETA-BDMAEE

>

246 247

The phase separation behavior of the blends with high CO2 absorption capacity was

248

also investigated. Experimental results revealed that the phase transition behavior

249

depended upon the hydrophobicity (LogP in Table S1) of the tertiary amines. The

250

more hydrophobic the tertiary amine was, the greater the liquid-liquid phase

251

separation appeared (Figure S3). For example, since DEAPD has a low 11



252

hydrophobicity, the TETA-DEAPD blend did not undergo a phase transition

253

throughout the entire absorption process (data not shown). Although the phase

254

separation occurred in TETA-DEEA blend during the CO2 absorption, the phase

255

separation disappeared and a homogenous solvent was formed at a high CO2 loading

256

of 0.95 mol mol-1 (Figure S3). Similar phase separation behavior was also obtained in

257

the reported biphasic solvent.41 Although the fresh TETA-DMCA separated into

258

two-liquid phases, its phase separation behavior was improved compared with the

259

previous reported biphasic solvent.41 For instance, the CO2-laden phase accounted for

260

65vol% even once it had reached its maximum CO2 loading (Figure S3). Notably, lots

261

of the reported biphasic solvents at their lean loadings had two-liquid phases even

262

though their fresh solutions were one homogeneous phase.13, 21, 27, 42 Therefore, the

263

TETA-DMCA blend can be regarded as a promising biphasic solvent because two

264

distinct liquid phases formed at the maximum CO2 loading. Furthermore, over 98% of

265

the absorbed CO2 was concentrated into the lower phase.

266 267

The composition of the TETA-DMCA solvent was optimized in terms of its phase

268

separation behavior and absorption capacity. With an increase in TETA concentration,

269

the absorption capacity increased from 0.58 to 0.88 mol mol-1 (Figure S4). However,

270

with a high TETA: DMCA ratio (e.g., 1.5:2.5), a liquid-solid phase separation

271

occurred rather than a two-liquid phase separation. Therefore, the TETA-DMCA

272

solvent with a ratio of 1:3 was selected as the optimum composition for CO2

273

absorption and used in the further investigation.

274

12


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3.2 Kinetic analysis

276

CO2 absorption into TETA-DMCA solvent with normalized lean and rich solution

277

loadings of 0.25 and 0.75 mol mol-1 was performed to simulate the solvent

278

composition at the top and bottom of an absorber, respectively. Figure 2 revealed that

279

the absorption rates into both solutions increased with the absorption temperatures

280

despite their CO2 loadings. The overall absorption rate of CO2 into lean and rich MEA

281

solution was of the same order of magnitude as the reported data in the previous

282

work,32, 43 indicating that the method used in this study for determining the CO2

283

absorption rate was relevant. The CO2 absorption rates into the TETA-DMCA

284

solvents were slightly slower than those into MEA solutions (Figure 2). For example,

285

at 50oC, the absorption rates into the lean and rich TETA-DMCA solutions were 16%

286

and 8% slower than those into MEA solutions, respectively. The slower absorption

287

rates into the TETA-DMCA blend were due to their higher CO2 loadings compared

288

with MEA solutions. As the absorption rates into the TETA-DMCA and MEA

289

solvents were comparable, the sizes of the absorbers with TETA-DMCA and MEA as

290

solvents for CO2 capture would be comparable. However, it should be noted that the

291

capacity of the TETA-DMCA solvent is 70% higher than MEA, which will reduce the

292

energy penalty of CO2 capture.

293 294

A Kinetic analysis was conducted to determine the enhancement factor when the

295

TETA-DMCA blend was used to identify the main mass transfer resistance during

296

CO2 absorption in a DSCR. The total mass transfer resistance (1/KL) was calculated

297

according to Eqs. 2 and 3, and the gas- ( ⁄ ) and liquid-side (1⁄ ) mass

298

transfer resistances were also determined.

299 13



300

As depicted in Table 1, for the lean TETA-DMCA solution, the individual gas-side

301

and liquid-side mass transfer resistance were comparable at 40oC, indicating that the

302

absorption rate was controlled by both gas- and liquid-side mass transfer. These

303

results were in good agreement with the lean MEA solvent reported by Dang.44 For

304

the rich TETA-DMCA solution, the liquid-side mass transfer resistance was dominant

305

at 40 and 50 oC, revealing that the overall absorption rate was limited by the

306

liquid-side mass transfer (including reactions). Notably, the enhancement factors of

307

the lean and rich solution increased with the absorption temperature, resulting in a

308

decrease of the liquid-side mass transfer resistance. For example, when the absorption

309

temperature was 60oC, the gas-side mass transfer resistance was dominant for the

310

TETA-DMCA lean solution. The total mass transfer resistances of the lean solutions

311

were considerably smaller than those of the rich solutions, suggesting that the average

312

absorption rate in the absorber was limited by the rich solution. However, within the

313

tested absorption temperatures, the total mass transfer resistances of the rich solution

314

decreased as the absorption temperature was increased, indicating that a high

315

absorption temperature (e.g., 60oC) may be beneficial for CO2 absorption.

316 317

3.3 Thermodynamics and regeneration heat estimation

318

In this study, the VLE data of TETA-DMCA blends with a total amine concentration

319

of 4 M and ratio of 1:3 were measured at 40, 50, and 60oC, respectively. Moreover,

320

the VLE data of the 5M benchmark MEA were also determined at 40oC. As shown in

321

Figure 3, the measured VLE data of MEA solution were in good accordance with the

322

data reported in the literature,45 indicating that the set-up used in this study for VLE

323

data determination was relevant. For the TETA-DMCA solvent, the VLE curves as a

14


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324

function of CO2 loading were reverse S-shaped. The CO2 partial pressure in

325

equilibrium gradually increased with the CO2 loadings.

326 327

The regeneration heat of the benchmark MEA solution was estimated according to

328

Eqs. 8-11. The result was in good agreement with the reported data (Table S4),

329

suggesting that the method used in this work is relevant to estimate the regeneration

330

heat. For the estimation of regeneration heat of TETA-DMCA solvent, the main

331

parameters, such as reboiler temperatures, CO2 loadings of the rich and lean solutions,

332

CO2 partial pressures at the top of the stripper, ?*+ , and ?P are listed in Table S3.

333

To calculate the sensible heat, approximately 72wt% of the solution (lower phase)

334

was assumed for the regeneration because over 98% of the absorbed CO2 was

335

concentrated in the lower phase. The preliminary estimation results indicated that the

336

regeneration heat of the TETA-DMCA solvent was dependent upon the normalized

337

lean loading (the loading at the top of the absorber calculated based on the total amine

338

amount) at a stripping temperature of 120oC (Figure 4a). When the lean loading was

339

increased from 0.25 to 0.45 mol mol-1, the regeneration heat substantially decreased

340

from 3.92 to 2.07 GJ t-1 CO2. As depicted in Figure 4a, the decrease in regeneration

341

heat was attributed to a sharp drop in the latent heat due to the high CO2 partial

342

pressure at the top of the stripper (Table S3). Conversely, the sensible heat gradually

343

increased with an increase in the lean loading because of the decrease in the cyclic

344

capacity. The regeneration heat did not noticeably vary when the stripping

345

temperature was increased from 90 to 130oC (Figure 4b), indicating that a relatively

346

low stripping temperature (90-110oC) can be used for TETA-DMCA solvent

347

regeneration.

348 15



349

As expected, lower sensible and latent heat were achieved using a biphasic solvent

350

compared with the MEA solution (Figure 4a). The lowest regeneration heat of the

351

TETA-DMCA solvent (2.07 GJ t-1 CO2) was achieved with rich and lean loadings of

352

0.75 and 0.45 mol mol-1, and this regeneration heat was approximately 40% lower

353

than that of MEA (3.7 GJ t-1 CO2)46. Furthermore, the regeneration heat determined in

354

this study was comparable to that of the reported biphasic solvents as shown in Figure

355

4a and Table S4. Therefore, the TETA-DMCA solvent may be a promising candidate

356

for CO2 absorption.

357 358

3.4 Mechanism

359

Three pairs of the upper and lower phases of the solvents with different CO2 loadings

360

(0, 0.20, and 0.55 mol mol-1) were collected for 13C NMR analysis to investigate the

361

mechanism of CO2 absorption into the TETA-DMCA blend. Becasue TETA

362

possesses two primary amine groups and two secondary amine groups, four types of

363

carbamate can be formed during CO2 absorption (Figure S5).

364 365

As shown in Figure 5, for the neat DMCA solution, characteristic peaks

366

corresponding to the carbons of DMCA were detected at the signals of 27.2, 28.0,

367

30.5, 42.8, and 65.1 ppm. Interestingly, the upper phases of the solutions with

368

different CO2 loadings exhibited the same characteristic peaks as neat DMCA,

369

indicating that the component of the upper phase was DMCA/DMCA+.

370

Distinguishing signals of amine and its protonated one using 13C NMR is difficult due

371

to the rapid proton exchange between the amines and water.21 In the lower phase,

372

characteristic peaks assigned to both DMCA and TETA were detected. Moreover, the

373

intensity of peaks corresponding to DMCA gradually increased with CO2 loadings, 16


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374

suggesting that DMCA was gradually transferred from the upper to lower phase

375

during CO2 absorption. Furthermore, at the CO2 loading of 0.2 mol mol-1, the primary

376

carbamate formed through the reaction between CO2 and TETA was detected at the

377

resonance of 161.12 ppm according to the ACD-LAB 6.0 analysis. With further

378

increase in the CO2 loading to 0.55 mol mol-1, the characteristic peaks of both

379

secondary carbamate and CO32-/HCO3- were detected at 157.94 and 161.30 ppm,

380

respectively, revealing that the reaction between CO2 and the amines followed the

381

order of primary amines, secondary amines, and tertiary amines, as expected.27, 41

382

Moreover, as the CO2 loading was increased, the acidity of the solvents increased,

383

resulting in a slight change in the chemical shift of the characteristic peaks of DMCA

384

and TETA as shown in Table S5.

385 13

386

Based on the

387

TETA-DMCA was proposed (Figure 6). Overall, the main reactions can be described

388

as follows:

389 390 391 392 393 394 395 396

C NMR analysis, the mechanism of CO2 absorption into

+ 4gh ↔ []l 4ggm

[]l 4ggm + ↔ []l + []4ggm

[]l 4ggm + )]4 → [)]4] l + []4ggm

[]l 4ggm + h g → []l + 4gom

(R1) (R2) (R3) (R4)

397

As shown in Figure 6, CO2 absorption was initiated by the reaction between CO2 and

398

the primary amine groups in TETA through the zwitterion mechanism. With the

399

consumption of primary amine groups, the secondary amine groups participated into

400

CO2 absorption, as indicated by the 13C NMR spectra. Furthermore, with the depletion

401

of TETA, DMCA acted as a “sinker” to regenerate TETA for sustaining continuous

402

CO2 absorption. Therefore, DMCA gradually transferred from the upper to lower 17



403

phase, resulting in a volume expansion of the lower phase.

404 405

In summary, the biphasic solvent, TETA-DMCA (4M, 1:3), exhibited an absorption

406

rate comparable with that of MEA, a high cyclic absorption capacity, and a

407

considerably low heat regeneration. Kinetic analysis revealed that the absorption rate

408

into lean solution was limited by both the gas- and liquid-side mass transfer, whereas

409

the liquid -side mass transfer rate determined the absorption rate into the rich solution.

410

Furthermore, the CO2 absorption into TETA-DMCA occurred with TETA as an

411

accelerator and DMCA as a sinker. The DMCA in the upper phase gradually

412

transferred to the lower phase to regenerate TETA, resulting in a high absorption rate

413

even at a high CO2 loading. The regeneration heat of the TETA-DMCA blend was

414

approximately 40% lower than that of 5M benchmark MEA solution. Therefore, this

415

novel TETA-DMCA solvent is a promising candidate for CO2 capture from flue gas.

416 417

AUTHOR INFORMATION

418

Corresponding Authors

419

*(S.H.Z.) Tel: +86 571 8832 0853; E-mail address: [email protected]

420

*(L.D.W.) Tel: +86 312 752 5511; E-mail address: [email protected]

421

Note

422

The authors declare no competing financial interest.

423

18


Page 18 of 35

Page 19 of 35


424

ACKNOWLEDGMENTS

425

We appreciate the financial support from National Natural Science Foundation of

426

China (No. 21606204) and Zhejiang University of Technology Initial Research

427

Foundation (No. 2017129000729).

428 429

SUPPORTING INFORMATION

430

Additional details on physiochemical parameters evaluation and chemistry of CO2

431

absorption into biphasic solvent. Additional tables giving hydrophobicity and the

432

alkalify of the tested amines, physiochemical parameters, thermodynamic data,

433

comparison of regeneration heat of TETA-DMCA with other reported biphasic

434

solvents, and carbon chemical shift of the carbons in TETA and DMCA.

435

Additional figures depicting schematic diagram of the double-stirred cell reactor

436

and typical phase change absorption process, influence of TETA: DMCA ratios on

437

the performance, and the potential carbamates formed in TETA-DMCA solvent.

438 439

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440

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441

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442

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DMXTM CO2 capture process in steel industry. Energy Procedia 2017, 114,

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25. Tan, Y. H. Study of CO2-absorption into thermomorphic lipophilic amine

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energy-efficient post-combustion CO2 capture. Int. J. Greenhouse Gas Control 2015,

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28. Kim, H.; Hwang, S. J.; Lee, K. S., Novel Shortcut Estimation Method for

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Regeneration Energy of Amine Solvents in an Absorption-Based Carbon Capture

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carbonate solution promoted with the enzyme carbonic anhydrase: Comparison with a

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absorption into aqueous monoethanolamine/1-hydroxyethy-3-methyl imidazolium

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glycinate solution. Chem. Eng. J. 2015, 280, 695-702.

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reaction between CO2 and Alkanolamines in aqueous solutions. Chem. Eng. Sci. 1984,

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36. Snijder, E. D.; Riele, M. J. M. t.; Versteeg, G. F.; Swaaij, W. P. M. v., Diffusion

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coefficients of several aqueous alkanolamine solutions. J. Chem. Eng. Data 1993, 38,

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37. Versteeg, G. F.; Vanswaaij, W. P. M., Solubility and diffusivity of acid gases

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38. Sada, E.; Kumazawa, H.; Butt, M. A., Solubility and diffusivity of gases in

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W.; Al-Marri, M. J.; Benamor, A., Heat duty, heat of absorption, sensible heat and

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heat of vaporization of 2-Amino-2-Methyl-1-Propanol (AMP), Piperazine (PZ) and

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potassium carbonate promoted by piperazine. Int. J. Greenhouse Gas Control 2009, 3,

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energy efficient regeneration. Ph.D. Dissertation, University of Dortmund, 2014.

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absorption in aqueous blends of N,N-diethylethanolamine (DEEA) and

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N-methyl-1,3-propane-diamine (MAPA). Chem. Eng. Sci. 2015, 129, 145-155.

563

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reactive absorption of carbon dioxide in high CO2-loaded, concentrated aqueous

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monoethanolamine solutions. Chem. Eng. Sci. 2003, 58, (23), 5195-5210.

566

44. Dang, H. Y.; Rochelle, G. T., CO2 absorption rate and solubility in

567

monoethanolamine/piperazine/water. Sep. Sci. Technol. 2003, 38, (2), 337-357.

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568

45. Zhang, Y.; Que, H.; Chen, C.-C., Thermodynamic modeling for CO2 absorption

569

in aqueous MEA solution with electrolyte NRTL model. Fluid Phase Equilib. 2011,

570

311, 67-75.

571

46. Pinto, D. D. D.; Knuutila, H.; Fytianos, G.; Haugen, G.; Mejdell, T.; Svendsen, H.

572

F., CO2 post combustion capture with a phase change solvent. Pilot plant campaign.

573


574

47. Raynal, L.; Alix, P.; Bouillon, P.-A.; Gomez, A.; de Nailly, M. l. F.; Jacquin, M.;

575

Kittel, J.; di Lella, A.; Mougin, P.; Trapy, J., The DMXTM process : an original

576

solution for lowering the cost of post-combustion carbon capture. Energy Procedia

577

2011, 4, 779-786.

578

26


Page 26 of 35

Page 27 of 35


579

Table 1. Kinetic analysis of TETA-DMCA under different temperatures in a DSCR.

T o

( C)

40 50 60

CO2 loading (mol mol-1)

0.25

pqrs

(kPa) 1.30

tuv

N (kmol m-2

E

-1

(s )

s-1)

1.34E-06

82

763

Liquid-sid

Total

Gas-side

resistance

(w⁄xy)

resistance

288

46.5

53.5

13.8 62.7 23.6 74.0 47.0

86.2 37.3 76.4 26.0 53.0

0.75

13.0

2.05E-06

18

84

841

0.25

1.30

1.63E-06

124

1407

274

0.75

13.0

2.36E-06

26

139

633

0.25

1.30

1.8E-06

167

2110

294

0.75

13.0

2.53E-06

60

554

402

580 581

27


(%)

e resistance (%)


582

Figure captions:

583

Figure 1. CO2 absorption capacity of the tested five blends and 5M benchmark MEA.

584

(total amine concentration of the blends: 4M, amine ratio: 1:3, 40oC, 13vol% CO2,

585

and 200 mL min-1)

586

Figure 2. Comparison of the absorption rates between TETA-DMCA and MEA under

587

the simulated conditions corresponding to the top and bottom of an absorber (1 atm,

588

300 rpm, and 1 L min-1). The blue solid and dash lines represent the absorption rates

589

into the rich and lean MEA solutions at 50oC reported in the literature, respectively.32

590

Figure 3. Vapor-liquid equilibrium (VLE) of the TETA-DMCA blend and MEA

591

solution. MEA 40 oC represents VLE data of MEA solution measured at 40oC.45

592

Figure 4. Regeneration heat estimation of TETA-DMCA from the rigorous

593

simulation (a) under different lean loadings (Treboiler=393 K, ∆T=10oC, normalized

594

rich loading=0.75 mol mol-1); (b) under various temperature at the reboiler (∆T=10oC,

595

normalized rich loading=0.75 mol mol-1). The lean loading represents the loading of

596

the lean solution at the top of the absorber; the data of MEA obtained from Kim et al.;

597

28

598

reported biphasic solvent such as DEEA-MAPA and DMX-A solvent.46, 47

599

Figure 5. Quantitative

600

loading levels. (a) the upper phase and neat DMCA; (b) the lower phase and neat

601

TETA.

602

Figure 6. Schematic diagram of the mechanism of CO2 absorption into TETA-DMCA

603

solution.

the red dashed bar showed in Fig. 4a representing the regeneration heat of the

13

C NMR spectra of TETA-DMCA solvent with various CO2

604 605

28


Page 28 of 35

Page 29 of 35


CO2 loading (mol/mol)

1.0 0.8 0.6 0.4 0.2 0.0

TETA/ TETA- TETA- TETAMEA TETADEAPD DEEA DMCA BDMAEE PMDETA 606 607

Figure 1. CO2 absorption capacity of the tested five blends and 5M benchmark MEA.

608

(total amine concentration of the blends: 4M, amine ratio: 1:3, 40oC, 13vol% CO2,

609

and 200 mL min-1)

610

29


Absorption rate ×108 (kmol·m-2·s-1)


350 300

TETA-DMCA 0.25 TETA-DMCA 0.75 MEA 0.2 MEA 0.45

Lean solvent TETA-DMCA (0.25 mol CO2 mol-1 amine)

250

Page 30 of 35

Cleaned gas 1.30% CO2

200 150 100 50

Rich solvent TETA-DMCA (0.75 mol CO2 mol-1 amine)

0 40

50

60 o

Temperature ( C)

Flue gas 13.0% CO2

611 612 613

Figure 2. Comparison of the absorption rates between TETA-DMCA and MEA under

614

the simulated conditions corresponding to the top and bottom of an absorber (1 atm,

615

300 rpm, and 1 L min-1). The blue solid and dash lines represent the absorption rates

616

into the rich and lean MEA solutions at 50oC reported in the literature, respectively.32

617

30


Page 31 of 35


CO2 partial pressure (kPa)

618

10

1

0.1 40oC 50oC 60oC MEA 40oC MEA 40oC lit.

0.01

0.001 0.2

0.3

0.4

0.5

0.6

40oC 50oC 60oC MEA 40oC MEA 40oC lit.

0.7

0.8

0.9

CO2 loading (mol/mol)

619 620

Figure 3. Vapor-liquid equilibrium (VLE) of the TETA-DMCA blend and MEA

621

solution. MEA 40 oC represents VLE data of MEA solution measured at 40oC.45

622

31



Page 32 of 35

Regeneration heat (GJ/t CO2)

5.0 4.5

(a)

Qlatent Qsens Qrxn

4.0 3.5

Reported regeneration heat

3.0 2.5 2.0 1.5 1.0 0.5 0.0

MEA lit. 0.25

0.3

0.35

0.4

0.42

0.45

Lean loading (mol/mol)

623

Regeneration heat (GJ/t CO2)

4.0 3.5

(b)

3.0 2.5 2.0 1.5 Lean loading=0.35mol mol-1 Lean loading=0.40mol mol-1 Lean loading=0.45mol mol-1

1.0 0.5 0.0 360

370

380

390

400

410

Temperature (K)

624 625

Figure 4. Regeneration heat estimation of TETA-DMCA from the rigorous

626

simulation (a) under different lean loadings (7,:=,7 =393 K, ∆T=10oC, normalized

627

rich loading=0.75 mol mol-1); (b) under various temperature at the reboiler (∆T=10oC,

628

normalized rich loading=0.75 mol mol-1). The lean loading represents the loading of

629

the lean solution at the top of the absorber; the data of MEA obtained from Kim et al.;

630

28

631

reported biphasic solvent such as DEEA-MAPA and DMX-A solvent.46, 47

the red dashed bar showed in Fig. 4a representing the regeneration heat of the

32


Page 33 of 35


2'

3* 2* 3^ 2" 4^

1* 6' 1" 6^ 1'

5' 4' 5^2^ 1

9 α=0.55

10

11 13 12

7" 7* 147^7' α=0.55 8" 8'

2

10

5^

4^ 3^

2^

9 10

11 13 12

α=0.20 7^7*

12

9 3" 3' 1^

11

1 1* 6^

23

2* 3*

α=0.20

3

13 11

13 12

1^ 9

10

1

2 3

10

11 13 12

10

11 1213

9 α=0.00

α=0.00

10

9 2 3

9 DMCA

12 11 13

1

TETA

170 165 70 65 60 55 50 45 40 35 30 25

170 165 70 65 60 55 50 45 40 35 30 25

f1 (ppm)

f1 (ppm)

(a)

(b)

632 633 634

Figure 5. Quantitative

635

loading levels. (a) the upper phase and neat DMCA; (b) the lower phase and neat

636

TETA.

13

C NMR spectra of TETA-DMCA solvent with various CO2

637

33



638 639 640

Figure 6. Schematic diagram of the mechanism of CO2 absorption into TETA-DMCA

641

solution.

642

34


Page 34 of 35

Page 35 of 35


643 644

TOC art: CO2

Clean Gas

CO2

645 646 647

Flue Gas

Heat Exchanger

Qrenge=2.07 GJ/t CO2

DMCA TETA Carbamate H2O HCO3-

Absorber

Stripper

35


Kinetics, Thermodynamics, and Mechanism of a Novel Biphasic

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