Mass-Transfer Performance of CO2 Absorption with Aqueous

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Mass-transfer performance of CO2 absorption with aqueous diethylenetriamine (DETA)-based solutions in a packed column with Dixon rings Miaopeng Sheng, Chenguang Liu, Chunyuan Ge, Moses Arowo, Yang Xiang, Baochang Sun, Guang-wen Chu, and Haikui Zou Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 19 Sep 2016 Downloaded from http://pubs.acs.org on September 25, 2016

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Industrial & Engineering Chemistry Research

1

Mass-transfer performance of CO2 absorption with aqueous

2

diethylenetriamine (DETA)-based solutions in a packed column with

3

Dixon rings

4 5

Miaopeng Sheng,† Chenguang Liu, † Chunyuan Ge,§ Moses Arowo,†,‡ Yang

6

Xiang,†,‡ Baochang Sun, †,‡,* Guangwen Chu, †,‡ Haikui Zou,†,‡,*

7

†

8

Beijing University of Chemical Technology, Beijing 100029, PR China

9

‡

Research Center of the Ministry of Education for High Gravity Engineering and Technology,

State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical

10

Technology, Beijing, 100029, PR China

11

§

Troops 92609 of People’s Liberation Army, Beijing 100077, China

12

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13

Abstract

14

This

study

investigated

the

(DETA)-based

absorption solution

performance

containing

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of

piperazine

CO2

into

15

diethylenetriamine

(PZ)

or

16

1-(2-Aminoethyl) piperazine (AEPZ) as an activator in a packed column with Dixon

17

rings. The effects of various operation conditions such as the activator concentration,

18

gas flow rate, liquid flow rate, CO2 partial pressure and solution temperature on

19

overall gas-phase volumetric mass-transfer coefficient (KGav) were explored. Results

20

indicate that the presence of PZ in DETA solution yields better enhancement effect on

21

KGav than AEPZ , and thus, a combination of 5%PZ+25%DETA solution is expected

22

to be a promising absorbent for CO2 absorption. The results further show that KGav

23

increases with an increase in liquid flow rate and a decrease in CO2 partial pressure,

24

and firstly increases and then decreases with an increase in solution temperature. On

25

the other hand, the gas flow rate has insignificant effect on KGav. A simplified

26

empirical correlation for KGav as a function of the operation parameters has been

27

proposed and the most of the calculated values are in agreement with the experimental

28

data with a deviation within ±15%.

29 30

Keywords: CO2 absorption, Diethylenetriamine, Piperazine, 1-(2-Aminoethyl)

31

piperazine, Packed column, Overall gas-phase volumetric mass-transfer coefficient

32


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33

1. Introduction

34

CO2 is considered as a major greenhouse gas (GHG) and a main contributor to

35

global warming which consequently results into a series of serious environmental

36

problems.1 Its continued rise in atmospheric concentration largely due to emissions

37

from combustion of fossil fuels is thus a major concern all over the world, and hence

38

the need to promptly carry out significant and sustainable mitigation measures.2 Currently, the end-of-pipe treatment process of chemical absorption is one of the

39

feasible

and

matured

technologies

for

reducing

CO2

emission.3,4

40

most

41

Monoethanolamine (MEA) solution is a typical chemical absorbent that is the most

42

commonly used for CO2 absorption owing to its fast reaction rate with CO2.

43

Nonetheless, its limitations of low capacity, high regeneration energy, corrosion and

44

degradation significantly expand the operation costs.5 As a result, efforts in

45

developing new amine absorbents such as piperazine (PZ),5 diethylenetriamine

46

(DETA),6 1-(2-Aminoethyl) piperazine (AEPZ)7 and 4-diethylamino-2-butanol

47

(DEAB)8 have been made in attempt to accelerate the absorption rate, improve CO2

48

capacity or lower the regeneration loss.

49

DETA containing two primary amine groups and one secondary amine group

50

exhibits high reaction rate and CO2 capacity. A study on reaction kinetics of CO2 with

51

DETA solution shows that DETA has a much higher third-order rate constant (k3) than

52

MEA but lower than PZ.6 As a result, PZ has been adopted as activator in aqueous or

53

non-aqueous DETA solutions to enhance CO2 absorption performance.9-11 However,

54

the application of PZ is somewhat limited due to its low solubility in water.5 AEPZ, 3 ACS Paragon Plus Environment


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which is one of PZ derivatives, also acts as a good activator in MDEA solution, and

56

has been proposed for CO2 absorption owing to its high reaction rate with CO2 and

57

better solubility in water as compared to PZ7,12. A comparison of CO2 absorption

58

performance between DETA and MEA in a packed column with Dixon rings reveals

59

higher overall gas-phase volumetric mass-transfer coefficient (KGav) of DETA than

60

that of MEA.13 Although some studies previously performed in a rotating packed bed

61

or stirred tank reactor indicate that PZ+DETA solution is a promising absorbent and

62

exhibits good performance for CO2 absorption,9-11 to the best of our knowledge, there

63

is limited report on the study of CO2 absorption into PZ+DETA solution in a packed

64

column with Dixon rings. Moreover, there is inadequate information on the

65

application of AEPZ+DETA solution for CO2 absorption.

66

To investigate the absorption performance of CO2 into DETA-based solution in

67

the presence of PZ or AEPZ as an activator, this work presents a study on the

68

absorption of CO2 into PZ+DETA or AEPZ+DETA solution in a packed column with

69

Dixon rings. Experiments are performed to investigate the effects of various operation

70

conditions including activator concentration, gas flow rate, liquid flow rate, CO2

71

partial pressure and solution temperature on the overall gas-phase volumetric

72

mass-transfer coefficient (KGav). A simplified empirical correlation to predict KGav in

73

the CO2 absorption process with DETA-based solution in a packed column with

74

Dixon rings is also developed.

75 76 77

2. Mass Transfer in a Packed Column Based on the two film theory, at a steady state, the absorption rate of CO2 4 ACS Paragon Plus Environment

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78

( N CO2 av ) into a liquid solution can be expressed as:14

79

* N CO2 av = K G av P ( yCO2 − yCO ) 2

(1)

80

where KGav is the overall gas-phase volumetric mass transfer coefficient; P is total

81

* pressure; yCO2 and yCO are mole fraction and equilibrium mole fraction of CO2 in 2

82

gas phase, respectively.

83

Owing to the small size of the packed column as well as relatively low gas flow

84

rates adopted in this study, there were no sampling points installed along the packing

85

section of the column in order to avoid interference with mass-transfer process. Also,

86

mass transfer coefficient varies vertically along the packed column since the total gas

87

flow rate constantly reduces along the column due to chemical absorption of CO2.

88

However, during the chemical absorption process, mass-transfer resistance mainly

89

exists in liquid film and hence the variation of total gas flow rate has insignificant

90

effect on mass transfer when yCO2 of the inlet gas stream is relatively low.

91

Furthermore, owing to very low CO2 loading of solution, the amount of DETA-based

92

solution is sufficient during the whole absorption process as long as liquid flow rate

93

remains unchanged. Consequently, it is reasonable to assume that the variation of

94

mass transfer coefficient along the column is small and can be neglected.15,16

95 96

97

98 99

Considering an element of packing with height of dh, the mass balance equation can be expressed as:

 yCO2 N CO2 av dh = GI d   1 − yCO  2

  

where GI is inert gas flow rate and is unchanged along the column. Then, dh can be derived from eqs (1) and (2) as:


(2)


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100

dh =

 yCO2 GI 1 d  * K G av P yCO2 − yCO 2  1 − yCO2

  

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(3)

101

* According to the fast reaction regime between CO2 and amine, yCO in eq (3) is 2

102

very low and is close to zero, which is commonly assumed to be neglected.13-15

103

* Although Zhao16 suggested that yCO should not be neglected because of different 2

104

reaction mechanisms between CO2 and absorbents in mixed absorbents solution. Zeng

105

et al.15 assumed that CO2 is completely exhausted in the liquid film during the

106

* absorption process and therefore yCO could be neglected in the calculation of KGav. 2

107

The reaction kinetics of CO2 absorption into individual PZ, AEPZ or DETA

108

solutions have been reported,6,7,17-21 and the comparison of reaction kinetics constants

109

(second-order rate constant k2 or third-order rate constant k3) is shown in Figure 1. It

110

can be seen that k2 of PZ and AEPZ are much higher than that of DETA, MEA and

111

NH3. Also, k3 of DETA is much higher than that of MEA and NH3. Therefore, the

112

reaction rate of CO2 with PZ+DETA or AEPZ+DETA solution can be assumed to be

113

much higher than that with MEA or ammonia solution. According to Bishnoi and

114

Rochelle13 and Chang et al.23, the equilibrium CO2 partial pressure of low-loaded

115

PZ+DETA solution used in this work is much less than 0.8 kPa (5%PZ+25%DETA

116

with a loading of 0.84) and 0.042 kPa (0.6 mol L-1 PZ with a loading of 0.32), further

117

* * indicating that yCO is very small relative to yCO2 . In addition, research on yCO of 2 2

118

the two absorption systems is still scarce. Therefore, it is reasonable and acceptable

119

* that yCO is assumed to be negligible and equals zero for convenience of calculations. 2

120

Therefore, eq (3) can be rewritten as:


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dh =

121

122

  

(4)

By integral calculation, packing height h can be expressed as

h=

123

124

 yCO2 GI 1 d K G av P yCO2  1 − yCO2

 yCO ,in (1 − yCO ,out )  yCO ,in yCO2 ,out GI 2 2 2 ×  ln + − K G av P  yCO2 ,out (1 − yCO2 ,in )  1 − yCO2 ,in 1 − yCO2 ,out

    

(5)

    

(6)

Thus, KGav can be derived as:

K G av =

125

yCO2 ,out GI  yCO2 ,in (1 − yCO2 ,out )  yCO2 ,in ×  ln + − Ph  yCO2 ,out (1 − yCO2 ,in )  1 − yCO2 ,in 1 − yCO2 ,out

126 127 128

3. Experimental Section

129

3.1 Materials

130

CO2 (purity ≥99.9%) was supplied by Beijing Ruyuanruquan Technology Co. Ltd

131

while air was obtained through an oil free air compressor (TYW-1, Suzhou Tongyi

132

Electrical and Mechanical Co. Ltd). A mixture of the CO2 and air made up the feed

133

gas. Diethylenetriamine (DETA, purity≥98.0%) was purchased from Tianjin Fuchen

134

Chemical

135

1-(2-Aminoethyl) piperazine (AEPZ, purity≥99.0%) were supplied by Tianjin

136

Guangfu Chemical Research Institute and Aladdin Industrial Corporation, respectively.

137

All of the aqueous solutions were prepared with deionized water. All the chemicals

138

were used as supplied without further purification.

Reagents

Factory,

while

piperazine

(PZ,

purity≥99.0%)

and

139 140

3.2 Experimental Apparatus and Procedure

141

Figure 2 shows a schematic diagram of the experimental setup for CO2

142

absorption. The experiments were conducted in a packed column containing Ф5×5 7 ACS Paragon Plus Environment


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143

Dixon rings as packing and the column was wrapped by heating band and heat

144

insulating material to control the temperature. Air and CO2 were firstly mixed in a

145

buffer tank and then introduced into the column from the bottom section. When CO2

146

concentration in the gas outlet reached a desired steady value, the DETA-based

147

solution in the liquid stock tank was introduced into the column from the top section.

148

The liquid stream contacted countercurrently with the gas stream inside the column,

149

leading to absorption of CO2 into the DETA-based solution and its subsequent

150

reaction with the amines. Finally, the liquid stream and gas stream exited the column

151

via the liquid outlet and gas outlet respectively. During the experiment, the difference

152

in temperature between the inlet and outlet solution was less than 2 oC, and the

153

average temperature was used.

154

During each experimental run, CO2 concentration in the gas stream at the top of

155

column was monitored by an infrared gas analyzer (GXH-3010F, Beijing Huayun

156

Analytical Instrument Institution, detect range from 0 to 30%). DETA-based solution

157

was used as soon as it was prepared. The initial CO2 loading of the solution, as

158

determined by chemical analysis,14 revealed a low initial CO2 loading (α ≤0.04 mol

159

CO2 mol-1 amine). All of the experiments were conducted under atmospheric pressure

160

and the corresponding data were obtained only when the system reached a steady state.

161

Details on the specifications of the packed column and operation conditions for the

162

CO2 absorption process are shown in Table 1.

163 164


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165

4 Results and Discussions

166

4.1 Effect of activator concentration

167

Figure 3 shows the effect of activator concentration on KGav. It is evident that

168

KGav increased with an increase in PZ or AEPZ concentration, and the activation

169

effect of PZ in DETA was higher than that of AEPZ with the same mass fraction.

170

Since both PZ and AEPZ possess a higher reaction rate with CO2 than DETA,6,7,9,20 a

171

small addition of PZ or AEPZ into individual DETA solution can enhance the reaction

172

rate between CO2 and absorbents, and thus lead to higher KGav. However, KGav

173

slightly decreased when AEPZ concentration in the aqueous AEPZ+DETA solution

174

exceeded 5%. This was attributed to that more AEPZ increased the viscosity of the

175

solution and consequently weakened the liquid-phase mass transfer process.24,25 Based

176

on the above discussion and considering the price of PZ and AEPZ as well as the low

177

solubility of PZ, the activator concentration was maintained at 5% in the ensuing

178

studies in this work.

179 180

4.2 Effect of gas flow rate

181

Figure 4 shows the effect of gas flow rate on KGav. It is evident that varying gas

182

flow rate had little effect on KGav. This observation is in agreement with that of both

183

Fu et al.13 and Aroonwilas et al.14, who also noted that gas flow rate has insignificant

184

effect on KGav. This means that mass transfer resistance mainly exits in liquid film.

185

The results confirm that the liquid-phase mass-transfer process dominates in the CO2

186

absorption into aqueous DETA-based solution in a packed column, and thus varying

187

gas flow rate has insignificant effect on KGav.

188



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189

4.3 Effect of Liquid Flow Rate

190

Figure 5 shows the effect of liquid flow rate on KGav. It is clear that KGav

191

increased with an increase in liquid flow rate. As aforementioned, the mass-transfer

192

resistance mainly exists in the liquid film in the CO2 absorption process. Therefore,

193

increasing liquid flow rate means increasing the turbulence of liquid on the packing

194

surface which consequently reduces liquid-phase mass-transfer resistance. Increasing

195

liquid flow rate also leads to an increase in wetted surface area of packing, and thus

196

provides more effective gas-liquid interfacial area. Furthermore, higher liquid flow

197

rate means bringing in additional fresh amine solution, leading to a lower CO2 loading

198

of the solution and consequently an increase in the liquid-phase mass-transfer driving

199

force. All of these factors favor an increase in KGav with increasing liquid flow rate.

200 201

4.4 Effect of CO2 partial pressure

202

Figure 6 shows the effect of CO2 partial pressure on KGav. It is evident that KGav

203

decreased with an increase in CO2 partial pressure. Although increasing CO2 partial

204

pressure can increase gas-phase mass transfer drive force and thereby enhance

205

mass-transfer process, the mass-transfer resistance mainly exists in liquid film in the

206

two absorption systems employed in this work, and therefore an enhancement in

207

gas-phase driving force has a limited effect on KGav. Also, an increase in CO2 partial

208

pressure means more CO2 per unit of absorption solution. However, the restricted

209

diffusivity of CO2 and amine molecules in the liquid phase means that the solution

210

may only hold a relatively steady amount of CO2.26 This suggests that the term of

211

* P( yCO2 − yCO ) increases while NAav remains constant in eq (1). As a result, 2


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212

increasing CO2 partial pressure results in a drop in KGav.

213 214

4.5 Effect of solution temperature

215

Figure 7 shows the effect of solution temperature on KGav. It is evident that KGav

216

firstly increased and then decreased with an increase in solution temperature. Higher

217

solution temperature accelerates the reaction between CO2 and amines and

218

consequently yields a higher reaction rate constant as is illustrated in Figure 1. Also,

219

higher temperature leads to lower viscosity of solution, which is favorable to CO2 and

220

amine molecules travelling in the liquid phase. Both of the two factors favor a higher

221

value of KGav. On the other hand, higher solution temperature also results in a

222

decrease in CO2 solubility and a growing volatilization loss of absorbent, which is

223

unfavorable to CO2 absorption.5,23 The latter factor was more predominant when the

224

solution temperature exceeded 323.15 K in this work and hence the observed

225

reduction in KGav.

226 227

4.6. Empirical Correlation for KGav

228

The overall gas-phase volumetric mass-transfer coefficient (KGav) is an

229

important parameter in designing a packed column for CO2 absorption. Many

230

empirical correlations for KGav with relation to operation parameters have been

231

developed on the basis of experimental data.13,15-17,27,28 According to previous

232

studies,13,15,28 KGav is related to amine concentration (Camine), CO2 loading (α) and

233

equilibrium CO2 loading (αeq) of absorbent, i.e. (αeq-α)Camine. However, to the best of

234

our knowledge, there is no published data on equilibrium CO2 loading (αeq) of 11 ACS Paragon Plus Environment


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235

AEPZ+DETA solution. Thus, a simplified empirical correlation of KGav for CO2

236

absorption into aqueous DETA-based solution in the packed column with Dixon rings

237

was developed on the basis of previous study28 and the experimental data, and it can

238

be expressed as: K G av =m1 Lm 2 G m 3 e m 4 (WA /WB )Wtotal e

239

m 5 PCO 2 + m 6 / T

(7)

240

where m1-m6 are the coefficients for the respective parameters in the eq (7), WA, WB

241

and Wtotal are the mass fraction of activator (PZ or AEPZ), DETA and total amine in

242

the solution, respectively. An average absolute relative deviation (AARD) was used to

243

assess the deviation between the experimental data and the calculated values as:

AARD=

244

245

1 N

N

K G av,cal − K G av,exp

i =1

K G av,exp

∑

× 100%

(8)

where N is the number of experimental data.

246

By trial and error, the optimum coefficients of m1-m6 were obtained and the

247

specific correlations for PZ+DETA and AEPZ+DETA solutions are shown

248

respectively as: -0.047PCO 2 -1303/ T

249

K G av-PZ+DETA =12.658 L0.557 G 0.118 e1.576(WPZ / WDETA ) Wtotal e

250

K G av-AEPZ+DETA =4.191 L0.726 G 0.140 e 0.0234(WAEPZ / WDETA ) Wtotal e

-0.035 PCO 2 -1337/ T

(9) (10)

251

Comparisons between the experimental data and the calculated values of KGav by

252

eqs (9) and (10) are shown in figure 8. It is evident that most of the calculated values

253

of KGav are in agreement with the experimental data with a deviation within ±15%,

254

and the corresponding AARDs calculated by eqs (9) and (10) are 5.35% and 7.32%

255

respectively.

256

However, due to the lack of physicochemical property data, further studies are 12 ACS Paragon Plus Environment

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257

needed for more precise correlations for the two absorption systems. Also, more

258

research is needed to examine whether the established correlation in this work can be

259

applied to other absorption columns with different packing.

260 261

5. Conclusions

262

This work separately employed two aqueous solutions of PZ+DETA and

263

AEPZ+DETA as absorbents for CO2 absorption in a packed column with Dixon rings.

264

The overall gas-phase volumetric mass-transfer coefficient KGav under various

265

operation conditions including activator concentration, gas flow rate, liquid flow rate,

266

CO2 partial pressure and solution temperature was evaluated in each of the absorption

267

systems. The results indicate that both PZ and AEPZ can enhance CO2 absorption

268

performance of DETA solution, with PZ displaying better enhancement effect than

269

AEPZ. This suggests that a combination of 5%PZ+25%DETA solution can be a

270

promising absorbent for CO2 absorption. The results further show that KGav increased

271

with an increase in liquid flow rate and a decrease in CO2 partial pressure. Also, KGav

272

firstly increased and then decreased with an increase in solution temperature whereas

273

it was insignificantly affected by gas flow rate. A simplified empirical correlation was

274

also developed for predicting KGav and the results show that most of the calculated

275

values are in agreement with the experimental data with a deviation within ±15%.

276 277

AUTHOR INFORMATION

278

Corresponding Authors

279

*Tel.: +86 10 64443134. Fax: +86 10 64434784. E-mail: [email protected] 13 ACS Paragon Plus Environment


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280

(Baochang Sun). P.O. Box 35, No. 15 Bei San Huan Dong Road, Beijing, China

281

100029.

282

*Tel.: +86 10 64449453. E-mail: [email protected] (Haikui Zou). P.O. Box 35,

283

No. 15 Bei San Huan Dong Road, Beijing, China 100029.

284 285

ACKNOWLEDGEMENTS

286

This work was supported by National Key Technology R&D Program of China

287

(No. 2008BAE65B02) and the National Natural Science Foundation of China (No.

288

21406009).

289 290

Nomenclature

291

α

292

AARD average absolute relative deviation

293

G

294

GI inert gas molar flow rate (kmol m-2 h-1)

295

h

296

KGav overall gas-phase volumetric mass-transfer coefficient (kmol m-3 h-1 kPa-1)

297

L

298

N CO2 av absorption rate of CO2 into solution (kmol m-3 h-1)

299

P

300

PCO 2

301

T

302

W mass fraction (%)

CO2 loading of solution (mol mol-1)

gas flow rate (kmol m-2 h-1)

height of the packing (cm)

liquid flow rate (m3 m-2 h-1)

total pressure (kPa) partial pressure of CO2 (kPa)

temperature (K)


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303

yCO2

mole fraction of CO2 in gas phase (%)

304

* yCO 2

equilibrium mole fraction of CO2 in gas phase (%)

305

yCO2 ,in

inlet mole fraction of CO2 in gas phase (%)

306

yCO2 ,out outlet mole fraction of CO2 in gas phase (%)



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307

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Tabular material: Table 1. Specifications of the packed column and operation conditions packed column diameter of the column (mm)

39

packing type

Ф5×5 Dixon ring

packing height (mm)

700

packing surface area (m2 m-3)

1700

absorption solutions

PZ+DETA, AEPZ+DETA

concentration of absorption solution

30% (PZ or AEPZ: 0-10%)

temperature of absorption solution (K)

303.15-333.15

inlet CO2 concentration (%)

6.1-14.1

liquid flow rate (m3 m-2 h-1)

6.10-12.36

inlet gas flow rate (kmol m-2 h-1)

27.39-65.48

pressure (atm)

1.03


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Figures 2-8:

Figure 1. Comparison of the reaction constant between CO2 and different absorbents in the literature

Figure 2. Experimental setup for the absorption of CO2 in the packed column



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Figure 3. Effect of activator concentration on KGav

Figure 4. Effect of gas flow rate on KGav


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Figure 5. Effect of liquid flow rate on KGav

Figure 6. Effect of CO2 partial pressure on KGav



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Figure 7. Effect of solution temperature on KGav

Figure 8. Comparisons between the experimental KGav and the calculated KGav as determined by eqs (9) and (10)


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Abstract graphic:


Mass-Transfer Performance of CO2 Absorption with Aqueous

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