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Study of formation of bicarbonate ions in CO2-loaded aqueous single 1DMA2P and MDEA tertiary amines and blended MEA-1DMA2P and MEA-MDEA amines for low heat of regeneration Rui Zhang, Zhiwu Liang, Helei Liu, Wichitpan Rongwong, Xiao Luo, Raphael O. Idem, and Qi Yang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b03097 • Publication Date (Web): 06 Mar 2016 Downloaded from http://pubs.acs.org on March 8, 2016

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Industrial & Engineering Chemistry Research

1

Study of formation of bicarbonate ions in CO2-loaded aqueous

2

single 1DMA2P and MDEA tertiary amines and blended

3

MEA-1DMA2P and MEA-MDEA amines for low heat of

4

regeneration

5

Rui Zhang1, Zhiwu Liang1 *, Helei Liu1, Wichitpan Rongwong1, Xiao Luo1,

6

Raphael Idem1,2, Qi Yang3 1

7

Joint International Center for CO2 Capture and Storage (iCCS), Provincial Key

8

Laboratory for Cost-effective Utilization of Fossil Fuel Aimed at Reducing

9

Carbon-dioxide Emissions, College of Chemistry and Chemical Engineering, Hunan

10

University, Changsha, Hunan, 410082, P.R. China

11 12 13

2

Clean Energy Technologies Institute (CETI), Faculty of Engineering and Applied Science, University of Regina, Regina, Saskatchewan, S4S 0A2, Canada 3

CSIRO Manufacturing Flagship, Clayton VIC 3168, Australia

14 15 16 17 18 19 20 21

*CORRESPONDING AUTHOR: Tel.: +86-13618481627; fax: +86-731-88573033;

22

E-mail address: [email protected] (Z. Liang).

23 24

ACS Paragon Plus Environment


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ABSTRACT

25 26

The formation of bicarbonate ions in an amine solution during CO2 absorption

27

results in lowering the heat duty for amine solvent regeneration in the CO2 capture

28

process because bicarbonate breakdown needs the lowest energy input to release CO2.

29

In this study, bicarbonate formation was conducted for two mixed solvents consisting

30

of tertiary amines (1DMA2P (1M) or MDEA (1M)) blended with MEA in order to

31

determine both formation rate and capacity of bicarbonate ions as compared to MEA

32

alone. The amines and concentrations used in the study were: MEA (5M),

33

MEA-MDEA (5:1 molar ratio, 6M total) and MEA-1DMA2P (5:1 molar ratio, 6M

34

total) at various CO2 loadings. The formation of bicarbonate ions was evaluated using

35

13

36

system, higher concentrations of bicarbonate ions were formed for MDEA than for

37

1DMA2P for the same CO2 loading. The results for the blended amine systems

38

showed that bicarbonate ions were generated at a lower CO2 loadings than MEA

39

alone, with MEA-1DMA2P generating bicarbonate ions at a lower CO2 loading (0.34

40

mol CO2/mol amine) than MEA-MDEA (0.38 mol CO2/mol amine). Thus, as an

41

additive in MEA, 1DMA2P has a better potential than MDEA to generate bicarbonate

42

ions at a leaner CO2 loading with the attendant lowering of the regeneration energy.

43

Keywords:

44

MEA-MDEA; MEA-1DMA2P, carbon dioxide capture.

C NMR technique at 293.15K. The results show that, for the single tertiary amine

Bicarbonate

formation,

NMR,

1-dimethylamino-2-propanol,

45 46


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47

1. INTRODUCTION

48

Climate change and global warming issues have caught the attention of

49

researchers and governments worldwide because of the risks they pose to human life

50

and the environment. Greenhouse gases (GHG), and in particular, carbon dioxide

51

(CO2) is blamed for this change. In recent years, various aqueous amine solutions and

52

their blends have been used as chemical absorbents in post combustion CO2 capture

53

technology to remove CO2 from a variety of sources such as flue gases, natural gas,

54

synthesis gas, and various refinery streams using various absorption apparatus.1-4

55

Precise knowledge of the reaction chemistry in amine based CO2 capture processes is

56

needed in order to make rational improvements to the efficiency of post-combustion

57

capture (PCC) using amines or other reactive solvents.

58

Primary amines such as monoethanolamine (MEA) and secondary amines such as

59

diethanolamine (DEA) have faster kinetics in their reaction with CO2 but have a lower

60

CO2 absorption capacity as compared to tertiary amines. On the contrary, tertiary

61

amines such as MDEA have higher CO2 absorption capacity and are more easily

62

regenerated but have lower reaction kinetics. Blends of primary amine and tertiary

63

amines are typically used in order to obtain a faster reaction rate, higher CO2 capacity,

64

and lower heat of solvent regeneration.5-8 To this end, a mixture of MEA and MDEA

65

has been tested to evaluate its CO2 capture performance in pilot plants and the results

66

showed that this mixture requires less energy for solvent regeneration than MEA

67

alone.9 1DMA2P (1-dimethylamino-2-propanol), a tertiary amine, has been

68

investigated for its kinetics in its reaction with CO2 using a stopped-flow apparatus in



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69

the temperature range of 293-313K and was found to react with CO2 faster than

70

MDEA.10 There has been no research reported in the literature on MEA-1DMA2P

71

mixture. In addition, it is known that the relatively high heat of formation of

72

carbamate increases the energy needed for amine regeneration. On the other hand,

73

there is no carbamate formation in the reaction of tertiary amines with CO2, implying

74

that the lower heat of formation of bicarbonate than carbamate in tertiary

75

amine-CO2-H2O system will result in a lower heat requirement in the regeneration of

76

tertiary amine systems. This further implies that, if the rich amine has more

77

bicarbonate in the CO2-loaded amine as it goes to the regeneration column and the

78

CO2-lean amine in the regeneration column can still generate bicarbonate ions at this

79

CO2 depleted stage, it will further decrease the energy costs for amine regeneration.

80

Barzagli et al.11 investigated the absorption efficiency as well as the amine

81

regeneration efficiency of AMP-MDEA, AMP-DEA, DEA and MDEA solvent

82

systems with the 13C NMR technique. The results showed that blends of amines have

83

enhanced absorption efficiency compared to single amines.11 Among the single amine

84

systems, MDEA performed the best in terms of regeneration efficiency while

85

AMP-MDEA

86

amine regeneration than AMP-DEA due to the fact that the DEA carbamate has a

87

higher thermal stability than AMP carbamate.11 Barzagli et al.12 investigated the

88

absorption efficiency of CO2 in a series of aqueous solutions of primary, secondary

89

and tertiary alkanolamines. The results showed that carbamate reduces the

90

CO2 absorption efficiency. In addition, they also found that the carbamate ion is not

displayed

better

performance

in

both


CO2 absorption

and

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91

only generated in CO2 absorption process, but also, in the solvent regeneration

92

process.12

93

MEA and MDEA have been widely used in commercial processes, and studies

94

have shown that the blend of MEA with tertiary amines leads to a reduction of energy

95

requirement and an increase in CO2 removal efficiency,13, 14 as well as absorption

96

ability and absorption rate15. The 5M solution of MEA in the amine mixture was

97

chosen for this study because it has been used widely in industrial plants. When the

98

tertiary amine reacts with CO2 it acts as a base that catalyzes the hydration of CO2 to

99

produce bicarbonate. It would be a significant development for the industry if the

100

tertiary amine can promote the 5M MEA to generate bicarbonate at the low CO2

101

loading because then the tertiary amine would have a positive catalytic effect on the

102

solvent regeneration. 1DMA2P, a novel solvent, could then be used to test for the

103

production of bicarbonate in 5M MEA solution compared to conventional MDEA

104

solvent. However, the bicarbonate ion as a factor in solvent regeneration has not been

105

investigated in detail in single and blended amines such as MDEA, 1DMA2P,

106

MEA-MDEA and MEA-1DMA2P. In this work, the 13C NMR technique was used to

107

investigate

108

1DMA2P-CO2-H2O, MDEA-CO2-H2O, MEA-CO2-H2O amine systems, as well as

109

blended binary MEA-MDEA-CO2-H2O and MEA-1DMA2P-CO2-H2O amine systems

110

at various CO2 loadings at 293.15K was investigated in order to determine their

111

potential to generate bicarbonate ions at lower CO2 loading for lower energy costs of

112

amine regeneration.

bicarbonate/carbonate

and

carbamate


formation

in

single


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113

To get useful data from our laboratory by NMR spectroscopy for 1DMA2P,

114

DEAB, which has a similar molecular structure to that of 1DMA2P, was used for

115

validation of the NMR spectroscopy and the experimental procedure. The results were

116

then compared with the literature (Figure 1).

117

2. THEORY

118

2.1. Chemistry of aqueous CO2-loaded 1DMA2P system

119

The reaction mechanism of CO2 with amines can be explained by several

120

theories based on different amine molecular structures. Caplow (1968) suggested a

121

two-step zwitterion mechanism,16 later reintroduced by Danckwerts (1979).17 The first

122

step is zwitterions formation while the second step is the deprotonation of the

123

zwitterions. Crooks and Donnellan35 proposed the termolecular mechanism which is

124

based on an amine reaction with CO2 in only one step with proton transfer taking

125

place simultaneously. Both mechanisms can only be used to explain the reaction

126

between CO2 and primary or secondary amines. 1DMA2P is a tertiary amine. Similar

127

to MDEA and DEAB, no hydrogen-atoms are directly connected to the

128

nitrogen-atoms in the molecular structure of 1DMA2P; as such, it cannot generate

129

carbamate ions.10, 18 Instead, 1DMA2P just like MDEA, acts as a base that catalyzes

130

the hydration of CO2. Consequently, the major component in the system is either

131

protonated 1DMA2P or protonated MDEA (in the case of MDEA) and their

132

bicarbonate ions. Reactions considered to be occurring when CO2 is introduced into

133

aqueous primary amine solutions and aqueous tertiary amine solutions are given in

134

Equations (1) to (13).


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135

MEA-CO2-H2O system.19

136

Dissociation of water:

137

2H 2O ↔ H 3O + + OH −

138

Dissociation of dissolved CO2 through carbonic acid:

139

CO 2 + 2H 2 O ↔ H 2 CO 3 + H 2 O ↔ H 3O + + HCO 3

140

Zwitterion formation for MEA:

141

MEA + CO 2 + H 2 O ↔ MEAH + COO − (zwitterio n) + H 2O

142

Carbamate formation from MEA zwitterion:

143

MEAH+ COO− (zwitterion) + H 2 O ↔ MEACOO− (carbamate) + H3O +

144

Formation of protonated amine:

145

MEA + H 3O + ↔ MEAH+ + H 2O

146

Formation of bicarbonate:

147

CO 2 + OH - ↔ HCO 3

148

Carbamate reversion to bicarbonate:

149

HO(CH2 )2 NHCOO- + H 2O ↔ HO(CH2 )2 NH 2 + HCO3

150

Dissociation of protonated amine:

151

MEAH+ + H 2O ↔ MEA + H 3O+

152

For tertiary amines, the following reactions are considered to occur in the

153

R1R2R3N-CO2-H2O system.20

154

Dissociation of water:

155

2H 2O ↔ H 3O + + OH −

156

Dissociation of dissolved CO2 through carbonic acid:

(1)

−

(2)

(3)

(4)

(5)

−

(6)

-

(7)

(8)

(9)



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157

CO 2 + 2H 2 O ↔ H 2 CO 3 + H 2 O ↔ H 3O + + HCO 3

158

Dissociation of bicarbonate:

159

CO 2 + OH - ↔ HCO 3

160

Formation of protonated amine:

161

R1R 2 R 3 N + H 3O + ↔ R1R 2 R 3 NH + + H 2O

162

Dissociation of bicarbonate:

163

HCO 3 + H 2 O ↔ CO 3

−

−

−

Page 8 of 33

(10)

(11)

2−

+ H 3O +

(12)

(13)

164

For blended amine systems such as MEA-MDEA or MEA-1DMA2P, 7 ions are

165

considered to exist in each of the loaded aqueous amine solutions, namely, MEA,

166

MDEA, MEAH+, MDEAH+, HCO3-, CO32-, MEACOO- (for the former amine system),

167

and MEA, 1DMA2P, MEAH+, 1DMA2PH+, HCO3-, CO32-, MEACOO- (for the latter

168

amine system).

169

2.2 The role of bicarbonate in solvent regeneration to strip CO2

170

As indicated, the ions in aqueous amine solutions are: free amine, protonated

171

amine, carbonates, bicarbonates and carbamates (only for primary and secondary

172

amine). The heat duty of solvent regeneration is controlled mainly from reactions (3),

173

(4) and (8) because these reactions are strongly endothermic.16, 21 Also, among these

174

ions, the bicarbonate ion is the most important as a result of its contribution to

175

lowering the heat duty for solvent regeneration. This is attributed to the bicarbonate

176

ion being able to act as a proton acceptor to release the proton from the MEAH+.

177

Consequently, the deprotonation process needs less energy than that with water.21 Yeh

178

et al.22 used the semibatch reactor to simulate the thermodynamics of aqueous


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179

ammonia reacting with CO2 at absorption/regeneration conditions. Their results

180

showed that, among the ammonium compounds, ammonium bicarbonate required the

181

least energy for regeneration.22 The breakdown of carbamate into CO2 by heating

182

needs a much larger quantity of heat and/or protons.21, 23 In a study, the concentration

183

of carbamate ions increased from 1.34 mol/L to 1.55 mol/L after heating the

184

CO2-loaded aqueous MEA solution, implying that carbamate was not decomposed to

185

release CO2 by heating under the conditions of their experiments.24 In addition, their

186

experimental results suggested that the generation of CO2 from the solution mainly

187

originated from bicarbonate/carbonate ions because the concentration of HCO3-/CO32-

188

was decreased significantly after heating the loaded aqueous MEA amine solution.24

189

This is because bicarbonates are relatively easy to decompose by heating. As well,

190

breaking C-O bonds requires less energy (in bicarbonate and carbonate breakdown)

191

than that for breaking C-N bonds (in carbamate breakdown).25

192 193

3. EXPERIMENTAL SECTION

194

3.1 Chemicals and apparatus

195

Reagent-grade 1-dimethylamino-2-propanol

(1DMA2P,

with

mass

196

purity of ≥99%) was purchased from Alfa Aesar, A Johnson Matthey Company,

197

China, and was then prepared to the desired concentration using deionized water.

198

N-methyldiethanolamine (MDEA, with purity of ≥99%) were purchased from Tianjin

199

Hengxing Chemical Preparation Co., Ltd., China.. DEAB was synthesized in the

200

solvent synthesis laboratory at the Joint International Center for CO2 Capture and



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Page 10 of 33

201

Storage (iCCS) according to the method described by Tontiwachwuthikul et al.26 The

202

purity of synthesized DEAB solvent was tested using NMR and found to be 97%. 1M

203

aqueous solutions of hydrochloric acid (HCl), NaCO3 (with mass purities ≥99.8%)

204

and NaHCO3 (with mass purities ≥99.6%) were prepared in iCCS laboratory, for use

205

in titration and NMR reference analysis. Deuterium oxide (D2O) and 1,4-dioxane

206

were purchased from InnoChem Science＆Technology Co. Ltd (Beijing, China) and

207

used for sample preparation. As well, commercial-grade CO2 (with purity ≥99%) was

208

supplied by Changsha Jingxiang Gas Co., Ltd., China.

209

In addition, a nuclear magnetic resonance (NMR) spectrometer (INOVA-400,

210

Varian company, USA) was used to test the samples. Several 5mm diameter NMR

211

tubes, produced by Wilmad Lab Glass and bought from Synthware glass Co. Ltd

212

(Beijing, China), were used for the NMR tests in this work.

213

3.2 Sample preparation

214

Each CO2-loaded amine solution was prepared by bubbling pure CO2 gas into

215

each

of

the

aqueous

1DMA2P

(1M),

MDEA

216

(nMEA:n1DMA2P=5:1, 6M), MEA-MDEA (nMEA:nMDEA=5:1, 6M) and MEA (5M)

217

solutions at 298K. The CO2 loading and total amine concentration of each sample was

218

ascertained by titration with 1M HCl solution. The titration apparatus and the

219

operating procedure are as described in the literature.27 The titration equipment in our

220

laboratory was validated in our previous work.28 A 0.5mL sample of each CO2-loaded

221

and unloaded amine solution was placed into a 5mm NMR tube, and then 10 mass%

222

of D2O was added into the sample to obtain a signal lock. A GVLab fixed-speed


(1M),

MEA-1DMA2P

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223

vortex was used to mix all contents in the NMR tubes in order to minimize resolution

224

error of the NMR spectra. About 0.015mL of pure 1,4-dioxane was also added into

225

the tubes to provide a sharp peak with a known chemical shift in the

226

spectrum as an internal reference standard. All the tested sample spectra analyzed

227

were calibrated based on the chemical shift of 67.19 ppm for 13C NMR.29

228

3.3 Determination of ion concentration in single/blended amines-CO2-H2O

229

system

13

C NMR

230

In the present work, a calibration method proposed by Shi et al.30 was used for

231

determination of ion concentration in single/blended amines-CO2-H2O system. In the

232

reference method, a

233

system for a VLE model as well as the liquid phase speciation in a

234

MEA-DEAB-CO2-H2O system at absorption and solvent regeneration conditions.30, 31

235

Equations (14)-(17) were used to calculate the ion concentrations,32 where the 168.03

236

ppm and 161.45 ppm are the chemical shift of pure carbonate and bicarbonate in 13C

237

NMR spectra (at 293.15K), respectively, and which is in agreement with the results

238

from Jakobsen et al.20 with an absolute deviation of 0.27% (at 293.15K).

239

[AmCOO− ] 2−

−

[CO3 ] + [HCO3 ] 240

[AmCOO− ] =

241

[CO 3 ] =

242

[HCO 3 ] =

2-

-

13

C NMR spectroscopy was used to study the DEAB-CO2-H2O

=

Scarbamate = R [Sbicarbonate + Scarbonate ]

R [CO2 ]0 1+ R

(δ - 161.45) [CO 2 ]0 (168.03 - 161.45)(1 + R) (168.03 - δ) [CO 2 ]0 (168.03 - 161.45)(1 + R)


(14)

(15)

(16)

(17)


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Page 12 of 33

243

[CO2]0 is the total amount of CO2 in the loaded amine aqueous solution. There

244

are three kinds of carbon compound in the loaded aqueous amine solution, namely,

245

carbamate, carbonate and bicarbonate. The [CO2]0 can be calculated based on the

246

Equation (18). Scarbamate and Scarbonate and Sbicarbonate represent the peak area in the

247

NMR spectra.

248

[CO 2 ]0 = Camine × loadingα

13

C

(18)

249

However, the calculation method of each ion concentration was performed using

250

the above equations in conjunction with the chemical shift and peak area obtained

251

from the 13C NMR spectra. Since the 13C signal of carbonate and bicarbonate was not

252

detected because no carbonate and bicarbonate ions were produced at low CO2

253

loading, Equations (19) and (20) were used to calculate the concentration of

254

carbamate, bicarbonate and carbonate in the blended system.

255

[AmH + ] + [H + ] = [HCO 3 ] + 2[CO 3 ] + [OH − ]

256

[MEACOO − ] + [HCO 3 ] + [CO 3 ] = [CO 2 ]0

257

3.4 Validation of NMR spectrometer using novel calibration method

2−

−

−

2−

(19) (20)

258

Because both DEAB and 1DMA2P contain a tertiary amino group and a

259

hydroxyl group, the only difference between these two amine molecular structures is

260

the size of the group which is connected to the tertiary amino group. In DEAB, the

261

tertiary amino group is connected to two ethyl groups but in 1DMA2P, the tertiary

262

amino group is connected to two methyl groups. Therefore, using DEAB as a

263

reference solvent to validate the NMR spectroscopy and experimental procedure was

264

done to increase the accuracy of the system. In this work, the NMR spectrometer was


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265

validated using aqueous solution of DEAB at concentration of 1.5M at 297.65K. The

266

calibration method was applied to test the accuracy of the apparatus for evaluating the

267

exact protonation ratio in the loaded amine. There were 10 samples tubes prepared

268

with protonation ratios of DEAB (nHCl:nDEAB = 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9,

269

1.0), respectively. The percentage H+ added into DEAB was calculated by titration

270

with 1M HCl. The 13C NMR analysis was performed on these 10 samples at 297.65K.

271

The reference data from the literature (Shi et al.33) were used to plot the curves of

272

chemical shifts against protonation ratio of DEAB. The experimental results obtained

273

in this work were then compared with those of Shi et al.33 as shown in Figure 1. It is

274

clearly shown from Figure 1 that the percentage of protonated amine based on the

275

chemical shift obtained from this work (at 297.65K) matched well with the

276

experimental results of Shi et al.33 at 297.65K. Therefore, the results obtained by the

277

NMR spectrometer in this work can be said to be accurate and reliable.

278 279

4. RESULTS AND DISCUSSION

280

4.1 Quantitative estimation of carbonate/bicarbonate ions in single aqueous

281

tertiary amine systems

282

The bicarbonate and carbonate concentration profiles of each of 1M aqueous

283

solutions of 1DMA2P and MDEA at various CO2 loadings, investigated by 13C NMR

284

technique at 293.15K, are plotted in Figure 2. It can be clearly seen from Figure 2 that

285

the concentration of bicarbonate increased as the CO2 loading increased. With the

286

CO2 introduced into both aqueous tertiary amine solutions, the hydration reaction of

287

CO2 with water occurs first, and then form H2CO3 (l) which then decomposes to



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288

bicarbonate and H+ ions as shown in reaction (10). The tertiary amine acts as a base

289

that catalyzes the hydration of CO2 to generate the bicarbonate and H+ ions.34 As a

290

result, the acidity of the solution increases due to the increase of the concentration of

291

H+ ions. In addition, it is also observed that the concentration of carbonate increases

292

until it reaches a maximum and then it starts to decrease as the CO2 loading increases.

293

At the low CO2 loading in the aqueous tertiary amine solution, the solution is strongly

294

basic (pH>10) because of the large portion of free tertiary amine that exists in the

295

solution, which results in the protons of the solution being accepted by free tertiary

296

amine, as described in reaction (12). This means that little HCO3- is converted to

297

CO32- based on reaction (13). The result is that the concentration of carbonate

298

increases as the CO2 loading increases. As more CO2 is introduced into the aqueous

299

tertiary amine solution (increased CO2 loading beyond a certain point), the

300

concentration of H+ ions increases resulting in basicity decrease. Then, the carbonate

301

ions start to accept protons and convert to bicarbonate ions as described in the reverse

302

of reaction (13). As a result, the concentration of carbonate has a maximum value, and

303

the concentration of bicarbonate increases monotonically as the CO2 loading increases.

304

The concentration profiles of carbonate/bicarbonate for the two tertiary amines

305

(MDEA and 1DMAP) are similar to that of DEAB.30

306

It can be seen from Figure 2 that the concentration of bicarbonate ions in the

307

single MDEA-CO2-H2O system is higher than that in the single 1DMA2P-CO2-H2O

308

system at the same CO2 loading, but conversely, its concentration of carbonate ions is

309

lower than that in 1DMA2P-CO2-H2O system. Rayer et al. (2014) investigated the


Page 14 of 33

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310

heats of absorption of CO2 in aqueous solution MDEA and 1DMA2P. The results

311

showed that the heat of absorption for the single MDEA system is lower than that for

312

1DMA2P when the CO2 loading is below 0.9 mole CO2/mole amine.18 They also

313

suggested that the formation of carbonate is the main contributor to the heat of

314

reaction in aqueous tertiary amines.18 Therefore, the higher heat of reaction means

315

that a larger quantity of carbonate ions exists in the aqueous amine solution. This is in

316

agreement with the results of this work for single tertiary amine systems, as shown in

317

Figure 2. In addition, the pKa of 1DMA2P10 is higher than that of MDEA35, which

318

leads to the pH in 1DMA2P-CO2-H2O system being higher than that of

319

MDEA-CO2-H2O system at the same concentration and CO2 loading. As a result,

320

more bicarbonate ions are generated in the MDEA-CO2-H2O system than in the

321

1DMA2P-CO2-H2O system at the same CO2 loading.

322

4.2 The effect of MEA on the production of bicarbonate/carbonate in blended

323

MEA-tertiary amine systems

324

The blended amine systems MEA-1DMA2P and MEA-MDEA at various CO2

325

loadings were tested for their carbonate and the bicarbonate ions concentrations at the

326

temperature of 293.15K. The results were compared with those for single tertiary

327

amine systems, as shown in Figures 3a and 3b for MEA-MDEA and MEA-1DMA2P,

328

respectively. In these blended primary-tertiary amines systems, there are three types

329

of carbon compounds: carbamate, carbonate and bicarbonate.31 Figures 3a and 3b

330

show that, in the blended amine MEA-MDEA and MEA-1DMA2P systems,

331

carbonate and bicarbonate were generated starting at the CO2 loading of 0.38 mole



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 33

332

CO2/mole amine (for MEA-MDEA) and 0.34 mole CO2/mole amine (for

333

MEA-1DMA2P). In the single tertiary amine system, however, the carbonate and

334

bicarbonate were generated immediately after CO2 was introduced into the aqueous

335

amine solution. Also, the concentration of bicarbonate increased as the CO2 loading

336

increased. It is clear that the blended amine system generates bicarbonate and

337

carbonate at a higher CO2 loading than the single tertiary amine system. This is

338

because the carbamate formation reaction as shown in reactions (3) and (4) is the

339

main reaction in the blended amine system at the low CO2 loading, and this is

340

attributed to the ability for MEA to react directly with CO2.16, 17 MEA reacts more

341

easily with CO2 than tertiary amines (such as MDEA and 1DMA2P). As such, if

342

MEA is added into a tertiary amine system, it delays the ability of the tertiary amine

343

in producing the carbonate and bicarbonate. As a result, the tertiary amine does not

344

generate bicarbonate ions at a low CO2 loading in the solution of blended amines.

345

With less bicabonate or even no bicarbonate in the rich amine, then the energy

346

consumption is higher when the rich blended amine solution is being regenerated.

347

4.3 The effect of tertiary amine on the production of carbamate in MEA

348

The

concentration

of

carbamate

profile

in

the

MEA-CO2-H2O,

349

MEA-1DMA2P-CO2-H2O and MEA-MDEA-CO2-H2O systems is plotted in Figure 4.

350

In the blended amine system, there are MEA-based compounds such as MEA

351

carbamate, MEAH+ and free MEA. MEA reacts with CO2 directly when CO2 is

352

introduced into aqueous MEA solution and forms MEA carbamate as shown in

353

reactions (3) and (4). Meanwhile, the MEAH+ is also generated by reaction (5). After


Page 17 of 33

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354

MEA is consumed completely by reacting with CO2, HCO3- is produced by reaction

355

(2) in the MEA-CO2-H2O system. However, for the dissociation reaction of MEAH+

356

as shown in reaction (8) in the blended amine system, tertiary amine replaces the H2O

357

as a base to release the MEAH+ so as to become free MEA. Because the basicity of

358

tertiary amine is stronger than H2O, MEAH+ more easily releases the proton to free

359

tertiary amine to form protonated tertiary amine. Thus, more MEAH+ is being

360

deprotonated by tertiary amine resulting in the release of more MEA. After that, the

361

free MEA reacts with CO2 as more CO2 is introduced into the solution leading to

362

more carbamate being generated. This is attributed to MEA, a primary amine, having

363

faster reaction kinetics relative to tertiary amine. The concentration of carbamate

364

increases till it reaches a maximum, and then it decreases by the hydrolyzation

365

process as shown in reaction (7), as the CO2 loading increases.

366

In the two blended amine system on the other hand, the concentration of

367

carbamate increases as the CO2 loading increases until the CO2 loading is over 0.38 or

368

0.34

369

MEA-1DMA2P-CO2-H2O systems respectively. This is because the same amount of

370

MEA (5M) in the original blended amines solutions and the MEA is available to react

371

first with CO2. However, after the original free MEA has been completely consumed,

372

(i.e. CO2 loading increased above 0.34 mole CO2/mole amine), the amount of

373

carbamate in MEA-1DMA2P-CO2-H2O system becomes less than that in

374

MEA-MDEA-CO2-H2O system as shown in Figure 4. This is because a larger amount

375

of free MEA is released from MEAH+ by MDEA in MEA-MDEA-CO2-H2O system

mole

CO2/mole

amine

for

MEA-MDEA-CO2-H2O


and


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

376

than that in MEA-1DMA2P-CO2-H2O system. More of the 1DMA2P is used to

377

generate bicarbonate rather than to release MEA from MEAH+ relative to MDEA.

378

This is explained in Figure 6 which shows that more bicarbonate was generated in

379

MEA-1DMA2P-CO2-H2O system when the CO2 loading was over 0.34 mole

380

CO2/mole amine implying that less 1DMA2P is used to release MEA from MEAH+.

381

When the MEA is consumed completely by CO2 at CO2 loadings over 0.34 mole

382

CO2/mole amine, the tertiary amine then reacts with CO2 to produce bicarbonate ions.

383

More 1DMA2P reacts with CO2 than MDEA because the 1DMA2P reacts faster with

384

CO2 than MDEA.10 Considering the above factors, more free MEA is released by

385

MDEA from MEAH+ than by 1DMA2P due to more 1DMA2P being consumed by

386

CO2 rather than by MEAH+. Consequently, there is more carbamate produced in

387

MEA-MDEA-CO2-H2O system than that in MEA-1DMA2P-CO2-H2O system. It

388

should be noted that less carbamate in the loaded amine solution leads to less energy

389

consumption in solvent regeneration.36 From Figure 4, it can be concluded that the

390

absorbent mixture of MEA-1DMA2P consumes less energy than MEA-MDEA in

391

solvent regeneration.

392

4.4 The effect of tertiary amine on the production of bicarbonate from MEA

393

The amounts of bicarbonate ions and carbonate ions obtained from aqueous

394

single MEA (5M) solution were respectively added to the ones obtained from aqueous

395

single MDEA (1M) solution. These sums were compared with the ones obtained in

396

aqueous blended amine solution (MEA-MDEA, nMEA:nMDEA=5:1, 6M), as shown in

397

Figure 5. In this process, Matlab software was used to calculate the total amounts of


Page 18 of 33

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398

bicarbonate and carbonate concentrations in single aqueous MEA (5M) solution and

399

single aqueous MDEA (1M) solution. It is clear that the total amounts of bicarbonate

400

or carbonate ions obtained by combining the individual amounts in the single aqueous

401

MEA and MDEA amine solutions is higher than those of the corresponding ions in

402

the blended amine solution. This is because when CO2 is added in the single MDEA

403

solution, bicarbonate and carbonate ions are produced right away due only to the

404

tertiary amine. However, in the single aqueous MEA solution, there is no bicarbonate

405

produced because CO2 reacts with MEA and forms carbamate preferentially.

406

Therefore, the results for the blended MEA-MDEA system shows that MEA does

407

suppress the production of bicarbonate of MDEA, as discussed previously.

408

It can also be seen from Figure 6 that the bicarbonate concentration in single

409

aqueous MEA solution increases significantly when the CO2 loading is higher than

410

0.45 mole CO2/mole amine. It is to be recalled that the concentration of

411

bicarbonate ions in a single aqueous MDEA solution increases nearly linearly as

412

shown in Figure 3. These two facts result in the total bicarbonate ion concentration

413

derived from the two single aqueous amine solutions increasing significantly when

414

the CO2 loading is higher than 0.45 mole CO2/mole amine as shown in Figure 5. After

415

this loading, the total concentration of bicarbonate ions in the two single aqueous

416

amine solutions comes mainly from the MEA reaction with CO2.

417

The situation for a blended amine solution is totally different; the MEA reacts

418

with CO2 and forms carbamate first in the blended amine system when CO2 is added

419

into the blended amine aqueous solution, and almost no carbonate or bicarbonate ions



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

420

are formed until the free MEA is reduced. This is seen in the blue circles, representing

421

the bicarbonate ion concentration in blended amine solution, which is almost zero

422

before the CO2 loading reaches 0.4 mole CO2/mole amine. However, it is also

423

apparent that the inflection point of concentration of bicarbonate is advanced in

424

aqueous blended amine solutions compared to the sum of both aqueous single primary

425

and aqueous single tertiary amine solutions. This can be explained using reaction (6),

426

which shows that tertiary amines exists in the aqueous blended amine solution, and

427

leads to more OH- being generated in the blended amine system because tertiary

428

amines have stronger basicity than H2O which is helpful for reaction (9). As a result,

429

the concentration of OH- is increased in the aqueous blended amine solution. In

430

addition, the reaction rate of the formation of carbamate, as shown in reaction (2) and

431

(3) in blended amine aqueous solution, is faster than that in aqueous single MEA

432

solution because more free MEA is released by tertiary amine, which leads to the

433

MEA of blended amine aqueous solution being consumed faster than that in single

434

aqueous MEA solution. Consequently, the bicarbonate is produced earlier in the

435

blended amine aqueous solution than that in the sum of both of single amine aqueous

436

solutions. Thus, after comparing the amounts of bicarbonate/carbonate ions between

437

blended amine system and the sum of the individual single amine systems, it can be

438

concluded that tertiary amine can promote MEA to produce bicarbonate/carbonate at

439

a lower CO2 loading.

440

A comparison of the generation of bicarbonate between the single aqueous MEA

441

solution system (5M, MEA) and aqueous blended amines solution system


Page 20 of 33

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442

(nMEA:nMDEA=5:1, 6M; nMEA:n1DMA2P=5:1, 6M) system was carried out and the results

443

are shown in Figure 6. It shows that the blended amine system also generates

444

bicarbonate at a lower CO2 loading than the single MEA system as discussed earlier.

445

It also shows that the bicarbonate appears in MEA-1DMA2P-CO2-H2O system at a

446

lower CO2 loading than that in MEA-MDEA-CO2-H2O system. This occurs because

447

more 1DMA2P reacts with CO2 when the MEA is completely consumed relative to

448

MDEA due to its faster reaction kinetics than MDEA as discussed earlier.10

449

However, rich amine of relatively low CO2 loading but with more bicarbonate

450

will not only lead to higher efficiency of CO2 removal but also to lower heat duty of

451

solvent regeneration. Based on these results, the heat duty of MEA-1DMA2P

452

regeneration is expected be lower than MEA-MDEA because it generates more

453

bicarbonate.

454 455

5. CONCLUSIONS In this work, the tertiary amines MDEA and 1DMA2P were investigated using

456

13

457

the

C NMR technique at 293.15K. In addition, the single amine MEA (5M) and

458

blends of MEA-MDEA (nMEA:nMDEA=5:1, 6M), MEA-1DMA2P (nMEA:n1DMA2P=5:1,

459

6M) were also tested. The results show that blended amines can generate bicarbonate

460

at a lower CO2 loading than MEA. MEA delays the tertiary amine producing

461

carbonate and bicarbonate. As well, the MEA-1DMA2P not only generates

462

bicarbonate at a lower CO2 loading than MEA-MDEA and MEA, but also, it

463

generates more bicarbonate than MEA-MDEA at the same CO2 loading. In addition,



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

464

less carbamate is generated in MEA-1DMA2P aqueous solution than that in

465

MEA-MDEA aqueous solution, which leads to MEA-1DMA2P needing less energy

466

for solvent regeneration.

467 468 469 470 471 472

ACKNOWLEDGMENTS

473

The support for this work provided by China's State “Project 985” in Hunan

474

University—Novel Technology Research & Development for CO2 Capture is

475

gratefully acknowledged. In addition, we would also like to acknowledge the research

476

support of Natural Sciences and Engineering Research Council of Canada (NSERC),

477

and Canada Foundation for Innovation (CFI).

478 479 480 481 482 483 484 485 486 487 488


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489

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capture processes. Carbon Management 2012, 3, (2), 185-200.

604

(34) Donaldson, T. L.; Nguyen, Y. N., Carbon dioxide reaction kinetics and transport

605

in

aqueous

amine

membranes.

606

Fundamentals 1980, 19, (3), 260-266.

Industrial

&

Engineering

Chemistry

607

(35) Rayer, A. V.; Sumon, K. Z.; Jaffari, L.; Henni, A., Dissociation Constants (pKa)

608

of Tertiary and Cyclic Amines: Structural and Temperature Dependences. Journal

609

of Chemical & Engineering Data 2014, 59, (11), 3805-3813.

610

(36) Song, H.-Y.; Jeon, S.-B.; Jang, S.-Y.; Lee, S.-S.; Kang, S.-K.; Oh, K.-J., Impact

611

of speciation on CO2 capture performance using blended absorbent containing

612

ammonia, triethanolamine and 2-amino-2-methyl-1-propanol. Korean Journal of

613

Chemical Engineering 2014, 1-9.

614 615


Page 29 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


616

FIGURE CAPTIONS

617

Figure 1. Validation of NMR apparatus for NMR test using 4-diethylamino-2-butanol

618

(DEAB).

619

Figure 2. The concentration of bicarbonate and carbonate plot in 1DMA2P-CO2-H2O

620

system and MDEA-CO2-H2O system at 293.15K.

621

Figure 3a. The effect of MEA on the production of bicarbonate/carbonate in single

622

MDEA system.

623

Figure 3b. The effect of MEA on the production of bicarbonate/carbonate in single

624

1DMA2P system.

625

Figure 4. The effect of tertiary amine on the production of carbamate in MEA.

626

Figure 5. A comparison of the total amounts of bicarbonate ions and carbonate ions

627

obtained from individual single MEA and MDEA amine systems with corresponding

628

ions from the blended MEA-MDEA amine system.

629

Figure 6. Effect of the tertiary amine on the production of bicarbonate in MEA.

630 631 632 633 634 635 636 637



638

FIGURES

639

640 641 642

Figure 1. Validation of NMR apparatus for NMR test using 4-diethylamino-2-butanol (DEAB).

643 644

0.8 Concentration (mole/L)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 33

CO32- (1DMA2P, 1M) HCO3- (1DMA2P, 1M) CO32- (MDEA, 1M)

0.6

HCO3- (MDEA, 1M)

0.4

0.2

0.0 0.0 645 646 647

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Loading (mole CO2/mole amine)

Figure 2. The concentration of bicarbonate and carbonate plot in 1DMA2P-CO2-H2O system and MDEA-CO2-H2O system at 293.15K.


Page 31 of 33

648 1.0 CO32- (MDEA, 1M) Concentration (mole/L)

0.8

0.6

HCO3- (MDEA, 1M) CO32- (5M MEA:1M MDEA, 6M) HCO3- (5M MEA:1M MDEA, 6M)

0.4

0.2

0.0 0.0

0.1

650 651 652

0.2

0.3

0.4

0.5

0.6


649

Figure 3a. The effect of MEA on the production of bicarbonate/carbonate in single MDEA system. 1.0 CO32- (1DMA2P, 1M) 0.8 Concentration (mole/L)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.6

HCO3- (1DMA2P, 1M) CO32- (5M MEA:1M 1DMA2P, 6M) HCO3- (5M MEA:1M 1DMA2P, 6M)

0.4

0.2

0.0 0.0 653 654 655

0.1

0.2

0.3

0.4

0.5

0.6


Figure 3b. The effect of MEA on the production of bicarbonate/carbonate in single 1DMA2P system.

656 657 658



Concentration of carbamate (mole/L)

659

3.0

Blend (5M MEA:1M MDEA, 6M) Blend (5M MEA:1M1DMA2P, 6M) Single amine (MEA, 5M)

2.5 2.0 1.5 1.0 0.5 0.0 0.0

0.1

661

0.2

0.3

0.4

0.5

0.6


660

Figure 4. The effect of tertiary amine on the production of carbamate in MEA.

662 2.5 CO2, Sum of single amine solutions 3 2 Concentrations (mole/L)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 33

HCO-3, Sum of single amine solutions CO2, Blended solution 3 HCO-3, Blended solution

1.5

1

0.5

0 0

663 664 665 666

0.1

0.2 0.3 0.4 Loading (mole CO2/mole amine)

0.5

0.6

Figure 5. A comparison of the total amounts of bicarbonate ions and carbonate ions obtained from individual single MEA and MDEA amine systems with corresponding ions from the blended MEA-MDEA amine system.

667


Page 33 of 33

1.0 Concentration of bicarbonate (mole/L)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.8

0.6

0.4

0.2

0.0 0.0 668 669

Blend (5M MEA:1M MDEA, 6M) Blend (5M MEA:1M 1DMA2P, 6M) Single amine (MEA, 5M)

0.1

0.2

0.3

0.4

0.5

0.6


Figure 6. Effect of the tertiary amine on the production of bicarbonate in MEA.

670


Study of Formation of Bicarbonate Ions in CO2 ... - ACS Publications

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