MgO) and

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Comparative Kinetic studies of solid absorber catalyst (K/MgO) and solid desorber catalyst (HZSM-5)-aided CO2 absorption and desorption from aqueous solutions of MEA and blended solutions of BEA-AMP and MEA-MDEA Daniel B Afari, James Coker, Jessica Narku-Tetteh, and Raphael O. Idem Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02931 • Publication Date (Web): 26 Oct 2018 Downloaded from http://pubs.acs.org on November 5, 2018

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Industrial & Engineering Chemistry Research

1 2 3

Comparative Kinetic studies of solid absorber catalyst (K/MgO) and solid desorber catalyst (HZSM-5)-aided CO2 absorption and desorption from aqueous solutions of MEA and blended solutions of BEA-AMP and MEA-MDEA

4

Daniel B. Afari1, James Coker1, Jessica Narku-Tetteh1, Raphael Idem1*

5 6 7

1Clean

Energy Technologies Research Institute, Faculty of Engineering and Applied Science, University of Regina, Regina, Saskatchewan S4S 0A2, Canada

8

Abstract

9

The kinetic performance of a novel amine solvent blend BEA-AMP was compared with MEA and

10

blended MEA-MDEA in the presence and absence of a solid acid catalyst (HZSM-5) in the

11

desorber column of a bench-scale pilot plant. In addition, a total of seven solid base catalysts were

12

screened using a semi-batch reactor to select the one that is most suitable as catalyst for amine-

13

based CO2 absorption. The selected solid base catalyst, K/MgO, was placed in the absorber of the

14

pilot plant. Overall, three absorber-desorber catalytic scenarios were evaluated: blank-blank,

15

blank-HZSM-5 and K/MgO-HZSM-5. For the blank-blank and blank-HZSM-5 scenarios, the

16

novel solvent (4M BEA-AMP) outperformed 5M MEA and 7M MEA-MDEA blend despite BEA-

17

AMP having the lowest molarity. The rates of absorption and desorption for the blank-blank (non-

18

catalytic) scenario for BEA-AMP were 14.8 and 38.4 mol/m3min, respectively. For the blank-

19

HZSM-5 system, the rates were 18.1 and 45.6 mol/m3min, respectively. Absorption and desorption

20

rates of 29.7 and 62.4 mol/m3min, respectively were obtained for the K/MgO-HZSM-5 system.

21

These results reveal higher rates of absorption and desorption with the inclusion of solid base and

22

solid acid catalysts to the amine-based CO2 capture process. The results show that in the presence

23

of the amine, the electron-rich anion species in K/MgO easily attack the dissolved CO2. This

24

interaction ties the CO2 molecules to the surface of the catalyst, making them readily available for

25

nitrogen atom of the amine in the CO2 absorption process. This process is facilitated because of

26

the large pore size of K/MgO. With the desorber catalyst, easier proton donation by HZSM-5

27

results in weakening the N-C bond in carbamate thereby causing CO2 to break away.

28 29 30

Corresponding Author: R. Idem; Email: [email protected]; Tel: 1-306-585-4470; Fax: 1-

31

306-585-4855 1 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 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

32

Nomenclature

33

AMP- 2-Amino-2-methyl-1-propanol

34

[𝐴] – Concentration of species, mol/dm3

35

BEA – Butyl ethanolamine

36

C – Concentration of species, mol/dm3

37

CO2 in – CO2 composition in the inlet gas

38

CO2 out – CO2 composition in outlet gas

39

𝐹i – Molar flow rate of species i mol min−1

40

MEA – monoethanolamine

41

MDEA – Methyldiethanolamine

42

MW – molecular weight

43

ri – rate of reaction based on a particular species, mol gcat−1 min−1

44

V – Volume of reactor, m3

45

V – Volumetric flowrate, slpm

46

W – Weight of catalyst, g

47

(𝑊/𝐹io) – Contact time, min

48

Xi – conversion of component i

49 50

Greek Letters

51

∆ - gradient

52

α - CO2 loading

53 54 2 ACS Paragon Plus Environment

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55

Subscripts

56

Am – amine

57

Lean – solution lean in CO2

58

rich – solution rich in CO2

59

In – entering the reactor

60

Out – exiting the reactor

3 ACS Paragon Plus Environment


62

1. Introduction

63

Despite its huge contribution as a major source of energy, the use of fossil fuels could be

64

limited by fact that large quantities of greenhouse gases (GHGs), with CO2 being a major

65

contributor, are emitted. Among several existing technologies for mitigating global warming by

66

CO2 capture, post-combustion capture (PCC) emerges as the most advanced technology in use.1

67

The use of amine-based solvents in PCC has been a great area of research and has seen progress

68

to a very large extent. Nonetheless, its merits tend to be stifled by the huge energy penalty

69

associated with the capture process. Thus, current studies are focused more on reducing this energy

70

penalty than in other areas of challenge. The two main areas that are being understudied are process

71

optimization and solvent enhancement. Energy requirement is classified under the former.

72

There exist many amine-based solvents (primary, secondary and tertiary) used for CO2

73

capture. However, the most widely used is the primary amine, monoethanolamine (MEA) due to

74

its high kinetics and moderate absorption capacity. Its limitation is associated with the huge energy

75

penalty which has necessitated the development of many other solvents with better absorption and

76

desorption characteristics. Works done by Chakraborty et al.2, Liao et al.3, Mandal et al.4, Sun et

77

al.5, Choi et al.6, and Sutar et al.7 show a better performance of these amine-based solvents (single

78

and blended) over the conventional MEA. The selection of these amine solvents has been based

79

more on a trial and error approach and not on any rational scientific approach.

80

Recently, a more rational criterion for selection of amine-based solvent was developed by

81

Narku-Tetteh et al.8 and Muchan et al.9. A group of primary, secondary and tertiary amines were

82

studied to check for the effect of their side chain structures and number of hydroxyl groups on CO2

83

absorption and desorption.8 A combination of solvent properties and performance estimation

84

parameters such as CO2 absorption and desorption kinetics, equilibrium loading, heat duty, cyclic

85

capacity, pKa and heat of absorption were grouped into Absorption and Desorption parameters. A

86

novel bi-solvent aqueous amine blend constituting 2-butyl-aminoethanol (BEA) and 2-amino-2-

87

methyl-1-propanol (AMP) of equimolar concentration (2M each) outperformed other potential

88

solvents, including MEA and MDEA (known for its excellent desorption characteristics) based on

89

newly developed “Absorption and Desorption parameters”. A comparative kinetic study between

90

the novel solvent and conventional MEA as well as MEA-MDEA blend is studied in this work.


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91

Catalysts account for faster kinetics in many reactions. The use of catalysts to enhance the

92

CO2 capture system is in its incipient stages and has not yet attained commercial implementation.

93

A number of catalysts have been employed in enhancing both CO2 absorption and desorption. The

94

use of inorganic catalysts to enhance CO2 absorption into aqueous solutions have been reported by

95

Sharma et al.10, Bandyopadhyay et al.11, Ghosh et al.12, Guo et al.13, Nicholas et al.14, and Phan et

96

al.15. Sivanesan et al.16 used tertiary amine nitrate salts in the presence of an aqueous tertiary amine

97

medium to enhance the CO2 absorption rate using the stopped-flow technique. The application of

98

solid acid catalysts in the desorber unit was introduced recently by Idem et al.17 This was in view

99

of reducing the conventional regeneration temperatures of about 120-140oC to temperatures below

100

100oC (between 85oC and 95oC).18 Shi et al.18 studied single MEA and blends of MEA-MDEA

101

and MEA-DEAB over two solid acid catalysts (HZSM-5 and γ-Al2O3) on a batch scale and

102

achieved a substantial reduction in heat duty with the introduction of these catalysts. He explained

103

his findings using the duplicative role of γ-Al2O3 as a HCO3- ion and the donation of protons by

104

HZSM-5 aiding in the breakdown of carbamate. This study was taken a step further to the bench-

105

scale pilot plant level by Akachuku19, who performed kinetic studies on MEA and MEA-MDEA

106

over these two solid acid catalysts and thus proved the results obtained by Shi et al.18 Other

107

researchers 20-22 have also reported on significant heat duty reduction by applying different solid

108

acid catalysts (SAPO-34, SO42-/TiO2, SO42-/ZrO2, MCM-41) to the CO2 desorption process.

109

Catalytic studies involving the use of solid mineral catalysts solely in the CO2 absorption process

110

is almost nonexistent in the literature. Recently, Shi et al.23 studied the addition of solid base

111

catalysts to DEA solvent to enhance the absorption process using CaCO3 and MgCO3 on a batch

112

scale. A reduction in overall reaction time by up to 14-28% and 11-28% were obtained for CaCO3

113

and MgCO3, respectively.

114

The present study evaluates the kinetic performance of a novel solvent blend, butyl (amino)-

115

ethanolamine

and

2-amino-2-methyl-1-propanol

(BEA/AMP)

116

Monoethanolamine (MEA) and blended monoethanolamine and n-Methyldiethanolamine

117

(MEA/MDEA) solvents and compares their kinetic performance with the addition of a solid acid

118

catalyst (HZSM-5) in the desorber section of a bench-scale pilot plant. Also, the performance of a

119

total of 7 solid base catalysts (Hydrotalcite, BaCO3, CaCO3, Cs2O/α-Al2O3, Cs2O/Al2O3, Ca(OH)2

120

and K/MgO) are evaluated and screened on a semi-batch scale with an equimolar mixture of the

121

novel solvent 4M BEA-AMP blend. The criterion for selecting the catalysts for screening was 5 ACS Paragon Plus Environment

with

conventional


Page 6 of 44

122

based on the H – scale, a measure of identifying solid base strengths as proposed by Tanabe et

123

al.24. With respect to this definition, they classified numerous solid base catalysts as either strong

124

or weak. Solid bases with H– values greater than +26 were classified as superbases. The selected

125

solid base catalysts for screening all had H– values greater than +26. These superbases have been

126

proven to be very reactive for most organic reactions including alkene isomerization and toluene

127

side-chain alkylation 25. Out of these, seven catalysts were selected and screened for amine-based

128

CO2 capture. They are BaCO3, CaCO3, Ca(OH)2, Cs2O/α-Al2O3 (hydrothermally treated),

129

Cs2O/Al2O3, K/MgO and Hydrotalcite. After screening, the selected solid base catalyst was

130

incorporated into the absorber section of the bench-scale pilot plant and its performance was

131

compared with two other configurations of (i) a blank case having no catalysts in both columns

132

(non-catalytic) and (ii) a semi-blank case with no solid base catalyst in the absorber but with solid

133

acid catalyst, HZSM-5 present in the desorber. The results are presented and discussed in this

134

paper.

135 136

2. Theory 2.1 Amine based CO2 reaction (Zwitterion mechanism).

137

Typical primary and secondary amines react rapidly with CO2 in a 2:1 mole ratio to form

138

carbamates and protonated amines via the zwitterion mechanism.26 This mechanism involves a

139

two-stage process where there is the formation of a ‘zwitterion’ intermediate (step 1) and

140

subsequent breakdown of this intermediate to form carbamate (step 2). BEA, which is a secondary

141

amine, reacts via this process to yield carbamate while AMP, a sterically hindered primary amine,

142

hydrolyzes to bicarbonate after the formation of its carbamate.27 These reactions are summarized

143

as follows:

144 145

BEA/AMP + CO2 → BEA + COO-/AMP+COO-

(1)

146

BEA+COO-/ AMP+COO- + BEA/AMP → BEACOO-/AMPCOO- + BEAH+/AMPH+

(2)

147

The carbamate reversion reaction for AMP to yield bicarbonate proceeds as:

148

AMPCOO- + H2O → AMP + HCO3-

149

2.2 Solid base catalysts

(3)


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150

The reaction between aqueous amine solvents and CO2 is exothermic leading to the release

151

of large amount of heat. These reactions can be viewed as acid-base reactions since CO2 is an acid

152

gas and amines are generally basic. With CO2 being an acid gas, there is the need for a basic

153

medium to enhance its rate of absorption into an aqueous solvent. One classification of a basic

154

medium is the introduction of solid base sorbents (solid base catalysts). The use of these base

155

sorbents (catalysts) can facilitate the rate of the transfer process by both physically increasing the

156

interfacial area for mass transfer and chemically by providing another reaction pathway with a

157

lower activation energy allowing for faster chemical absorption of CO2.28

158 159

3. Experimental Section

160

3.1 Chemicals and Materials

161

The

following

chemicals:

BaCO3(≥99%),

CH3COOCs(≥95%),

CaCO3

(≥99%),

162

KOH(≥99.99%),

163

Na2CO3(≥99%),

164

Butylethanolamine (≥98%) and Ludox HS 40 colloidal silica (40wt% suspension in H2O) were all

165

purchased from Sigma Aldrich Co. Canada. 2-amino-2-methyl-1-propanol (≥99%) was purchased

166

from Acros Organics. Ca(OH)2(≥99%) and NaOH (≥99%) were purchased from Fisher Scientific

167

Co. Canada. γ-Al2O3 and α-Al2O3 were purchased from Zeochem Inc., US while HZSM-5 was

168

purchased from Zibo Yinghe Chemical Company Limited, China. Pure CO2 and N2 gas tanks were

169

supplied by Praxair Inc., Regina, Canada. 15% CO2 (N2 balance) gas tanks for the Gas analyzer

170

calibrations was also purchased from Praxair Inc., Regina, Canada. Standard 1N Hydrochloric acid

171

used in titration experiments was purchased from Sigma Aldrich Co., Canada.

Monoethanolamine

Al(NO3)3.9H2O(≥98%),

Mg(OH)2(≥95%),

(≥98%),

Methyl

Mg(NO3)2.6H2O(≥98%),

diethanolamine

(≥98%),

2-

172 173

3.2 Catalyst Preparation

174

The preparation procedure from Cavani et al.29, Climent et al.30, Diez et al.31, and

175

Gorzawski et al.32 were followed for the preparation of each catalyst with slight modifications.

176

The preparation of Hydrotalcite was done following the procedure outlined by Cavani et al.29 using

177

the co-precipitation method.



178

Cs2O/α-Al2O3 and Cs2O/Al2O3 were prepared by the procedure outlined in Gorzawski et

179

al.32 with little modifications. α-Al2O3 and Al2O3 beads were crushed and impregnated with

180

prepared solutions of caesium acetate. For α-Al2O3, prior to impregnation with caesium acetate, it

181

was hydrothermally treated in a Parr reactor at 500oC for 2 hours and dried for 12 hours. Upon

182

impregnation with Caesium acetate, both α-Al2O3 and Al2O3 products were stirred for

183

approximately 2 hours and finally calcined at 900oC for 2 hours to decompose the caesium acetate.

184

The steps outlined by Diez et al.31 were employed in the preparation of K/MgO. All

185

catalysts were pelletized to the desired size by passing through appropriate sieves after being

186

pressed using a 4 cm internal diameter die set under a hydraulic press.

187 188

3.3 Catalyst Characterization

189

The Brunauer-Emmett-Teller (BET) Surface Area, Pore Volume, and Average Pore Size

190

measurements and X-ray Diffraction (XRD) characterization experiments were performed at the

191

Chemical and Biological Engineering Department Laboratory at the University of Saskatchewan,

192

Saskatoon. The Temperature Programmed Desorption (TPD) and Scanning Electron Microscope

193

(SEM) experiments were conducted at the Clean Energy Technologies and Research Institute

194

(CETRI) laboratory at the University of Regina. ICP analysis was conducted at the Environmental

195

Analytical Laboratories of the Saskatchewan Research Council in Saskatoon.

196

For the BET analysis, the instrument used was BET ASAP 2020 from Micromeritics,

197

(Georgia, USA). The sample was degassed at 150oC for 5 hours. Nitrogen gas was used during

198

analysis. BJH method was employed to calculate the surface area, pore volume and pore size

199

obtained from the adsorption and desorption isotherms. 46 relative pressure points were recorded

200

to give the Isotherm plots.

201

Powder X-Ray Diffraction (XRD) experiments were performed on a Rigaku Ultima IV X-

202

Ray diffractometer, equipped with a Cu source (1.54056 Å), a CBO optical, and a scintillation

203

counter detector available at the Saskatchewan Structural Science Centre of the University of

204

Saskatchewan, Saskatoon. The measurements were carried out on the multipurpose attachment in

205

the parafocusing mode, with a Kβ filter (Ni foils) inserted into the receiving slit box. The intensity

206

data were obtained over a 2θ scan range from 5° to 80°, with a scan rate of 5° per minute and a 8 ACS Paragon Plus Environment

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207

step size of 0.02. The generator voltage and current were set to 40 kV and 40 mA, respectively.

208

Identification of the crystalline phases were done using the reference data from International

209

Center for Diffraction Data (ICDD) and literature.

210

TPD measurements were done using a CHEMBET-3000 analyser with a TCD Detector

211

from Quantachrome Instruments. The catalyst sample was initially degassed by being exposed to

212

100% helium gas accompanied by heating the furnace gradually to a temperature of 250oC at

213

10oC/min ramping. The system was kept at this temperature for 60 minutes after which the

214

temperature was reduced to 30oC. A 3% CO2 gas (balance nitrogen gas) was introduced for 60

215

minutes at a flowrate of 30 ml/min for adsorption to take place. The temperature was then increased

216

to 700oC in a constant flow of helium gas.

217

The surface morphology of the prepared catalyst samples was also investigated by scanning

218

electron microscopy (SEM) using a JEOL 5600 132-10 electron probe micro analyser with an

219

active area of 10 mm2available at CETRI, University of Regina. The sample was first crushed to

220

obtain polished flat surfaces and was then loaded into the specimen chamber. Beams were

221

generated based on the accelerating voltage of 25 kV. The positioning of the beam was controlled

222

by the computer software and micrographs were finally acquired.

223

ICP analysis was conducted to obtain the actual metal content of catalysts. A mass of 0.2g

224

of the catalyst sample was digested in 5ml concentrated HNO3. It was then mixed and left standing

225

for 2 hours. Digestion was done by using a Mars 6 (CEM Corporation) extraction system. The

226

sample was then microwave digested for half an hour by heating at 1200 W and pressure

227

increments from 80 to 150 psi. Dilution of the sample digests was done to 50 ml volume and

228

digests and blanks were then analysed by Agilent 8800 ICP-MS.

229 230

Catalyst Screening

231

Absorption experiments for the catalyst screening were carried out at a temperature of 45±1oC

232

at atmospheric pressure of 1 atm. For a fair basis of comparison, a catalyst weight of 50g for each

233

catalyst was used. This is about 10% of the mixture. It is known that a higher catalyst percentage

234

may tend to overwhelm the reaction itself (i.e. reflects inactivity of catalyst) and causes it to reach

235

maximum conversion. This would mean that the addition of more catalyst would not change the

236

conversion. However, as will be later shown in the results section, considerable and varying 9 ACS Paragon Plus Environment


237

performance (increase in conversion) among the various catalysts was evident. A 4M BEA-AMP

238

solvent concentration, solvent volume of 500 ml and a constant stirring speed of 600 rpm were

239

maintained throughout all runs. The concentration was confirmed by titration with a 1N HCl

240

solution. The apparatus consisted of a 1000 ml three-necked round bottom flask immersed in a

241

preheated oil bath. The catalyst (3.5-4 mm particle size) was carefully placed in a stainless-steel

242

basket and fully immersed into the solution by being suspended with the aid of stainless steel wires

243

to ensure no contact with the magnetic stirrer or bottom of the flask as shown in Figure 1. A non-

244

catalytic (blank) run was also performed and used as the baseline for comparing the performance

245

of the various catalysts. Table 1 shows the experimental conditions used in catalyst screening

246

experiment.

247

Each experiment started with a known volume of solvent (500 ml) introduced into the flask

248

and the filled flask was immersed into an oil bath which was heated to the desired absorption

249

temperature. Via the dispersion tube, the solvent was then bubbled through with a pre-mixed gas

250

of 15%CO2 (balance N2) at a flowrate of 650±5 ml/min. After reaching the desired temperature,

251

samples were taken at regular intervals of 5 minutes during the first hour and subsequently at

252

intervals of 30 minutes. This was done to measure the CO2 loadings at those time intervals with

253

the aid of the Chittick apparatus as described by Ji et al.33 Sampling continued until the solution

254

attained equilibrium at 10 hours (600 minutes) of run time. Solutions were filtered prior to

255

measuring their CO2 loadings in order to eliminate catalyst particle interference with loading

256

measurements. CO2 loading (mol/mol) versus time (minutes) curves were generated and slopes of

257

the linear portion of these curves gave the initial rate of absorption. Following the selection of the

258

catalyst, γ-Al2O3 and colloidal silica (40 wt. %) were employed as binders for the selected catalyst

259

in order for it to be used in the absorption unit of the bench-scale pilot plant. Their effect on the

260

overall performance of the selected catalyst was also tested under the same experimental

261

conditions of the absorption experiments. This was to ensure that there was no added contribution

262

or adverse effect whatsoever from the binders.


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263

Figure 1 Experimental set-up of semi-batch run for catalyst screening

264 265 266

Table 1. Operating conditions of semi-batch catalyst screening experiments Parameter

Value

Gas flowrate

650 ml/min

Liquid volume

500 ml

Absorption temperature

45oC

CO2 in feed gas

15%

Catalyst weight

50g

267 268

3.4 Pilot plant

269

The system consisted of two lagged stainless-steel columns each with dimensions of 1.067

270

m in height and an internal diameter of 0.0508m. The absorption column was designed with 4

271

ports, being the gas inlet, off-gas outlet, lean-solvent inlet and rich-solvent outlet. However, the

272

desorber column had 3 ports namely; rich-solvent inlet, lean-solvent outlet and CO2 product gas

273

outlet. Both columns were equipped with 5.08 cm LDX sulzer structured packing arrangement

274

with the solid base catalyst beds interspersed between them for that of the absorption column. The

275

absorber column had the desired solid base catalyst weight evenly distributed between any two 11 ACS Paragon Plus Environment


276

structured packing. Also, the desorber column packing arrangement enclosed a solid acid catalyst

277

bed (HZSM-5) mixed with 3 mm inert marbles. Between the structured packing arrangement and

278

the catalyst bed-3mm inert marbles section, were 6 mm inert marbles which acted as support for

279

the catalyst bed-3mm inert marbles section. The bench-scale pilot plant and absorber and desorber

280

bed arrangements are shown in Figures 2 and 3 respectively. Table 2 shows the operating

281

conditions employed in the bench-scale pilot plant experiment.

282

The key feature of the experimental set-up is the replacement of the reboiler in conventional

283

systems with a rich amine heater. A typical experimental run began with the lean amine solvent,

284

with the desired concentration and flowrate, fed from the storage tank to the top of the absorber

285

column via a variable-speed gear pump. At the same time the heating bath was set to the desired

286

temperature to heat up the rich amine prior to entering the desorber. Once amine solvent circulation

287

was set, a mixture of CO2 and N2 gas at the appropriate CO2 concentration of 15% was introduced

288

to the bottom of the absorber column via a gas flow meter where it was met by a counter-current

289

flow of lean amine solvent from the top of the column. Here, a three-phase system was set-up

290

comprising the amine solvent, CO2-N2 gas and solid base catalyst. Treated gas exited the top of

291

the absorber column while the rich amine solvent exited at the absorber bottom and exchanged

292

heat with the hot lean amine stream coming from the bottom of the desorber. The rich amine stream

293

was further heated, through a heat-exchanger network, by the heating medium. The heated rich

294

amine stream was fed to the top of the desorber column. Upon contacting the catalytic desorber

295

bed, further desorption was enhanced by the catalyst bed and the lean amine leaves the bottom of

296

the desorber, is cooled, and fed into the absorber column making a complete cycle. A condenser

297

is employed to cool the CO2 product gas leaving the top of the desorber column to remove any

298

entrained water and amine, and the product gas is dried prior to being measured by the rotameter.

299

The absorber column temperature profile was consistently monitored to check for

300

attainment of equilibrium. At equilibrium, the CO2 concentrations in the gas phase along the

301

absorber and temperature profiles in both columns were measured using an Infra-Red (IR) gas

302

analyzer from Nova Analytical Systems Inc., Canada and J-type thermocouples from Cole Parmer

303

Inc., Canada respectively. Also, lean and rich amine samples were taken from the bottom of both

304

columns to determine the rich and lean CO2 loadings (liquid phase CO2 concentrations). The CO2

305

loadings were determined using a Chittick apparatus as described by Ji et al.33 Loading 12 ACS Paragon Plus Environment

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306

measurements were done thrice to ensure repeatability and accuracy. The average deviation was

307

less than 1%. Gas phase CO2 concentrations at the inlet and outlet of the absorber were taken twice

308

also to ensure accuracy in measurements. Prior to this, the IR gas analyzer was calibrated with a

309

15% CO2 premixed gas. A mass balance error calculation was done to determine the validity of

310

each run. The calculation compared the quantity of CO2 removed from the gas phase to the CO2

311

quantity absorbed by the liquid phase. A value of ≤10% was considered a valid run. Table S1

312

(Supporting information) shows the details.

313 314

Table 2. Operating conditions of bench-scale pilot plant Parameter

Value

Feed Gas flowrate

15 standard litres per minute (slpm)

CO2 concentration in feed gas

15%

Solvent flowrate

60 ml/min

Absorber and Desorber Catalyst weight

150 g

Average desorber temperature

85 oC

Pressure in both columns

1 atm

315 316



317 318

Figure 2. Schematic representation of bench-scale pilot plant experimental set-up34


Page 14 of 44

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319

Figure 3. Absorber and desorber columns packing and catalyst bed arrangement

320 321

3.5 Calculations

322

For the screening experiments, the CO2 loading was taken at various time intervals for each

323 324

sample. This was used to determine the initial absorption rate according to equation 7: 𝐶𝑂2𝑙𝑜𝑎𝑑𝑖𝑛𝑔

325

𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝑎𝑏𝑠𝑜𝑟𝑝𝑡𝑖𝑜𝑛 𝑟𝑎𝑡𝑒 =

(

𝑚𝑜𝑙 𝐶𝑂2 𝑚𝑜𝑙 𝑎𝑚𝑖𝑛𝑒

) × 𝑎𝑚𝑖𝑛𝑒 𝐶𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 (

𝑡𝑖𝑚𝑒 (𝑚𝑖𝑛𝑢𝑡𝑒𝑠)

𝑚𝑜𝑙 𝑎𝑚𝑖𝑛𝑒 𝐿

)

(7)

326

The Kinetic analysis using the bench-scale CO2 capture pilot plant was evaluated in terms

327

of CO2 conversion and rate at which this conversion occurs. The kinetic data were obtained based

328

on the assumption of an integral type (plug-flow) reactor. The rate of reaction was determined by

329

using the differential method of analysis. CO2 conversion and rate of reaction are expressed in

330

equations 8 and 9 respectively, as:

331

𝑋

𝐶𝑂2 =

(8)

𝐶𝑂2, 𝑖𝑛 ― 𝐶𝑂2, 𝑜𝑢𝑡 𝐶𝑂2, 𝑖𝑛



332

𝑑𝑋𝐴

Page 16 of 44

𝑑𝑋𝐶𝑂2

(9)

― 𝑟𝐴 = 𝑑(𝑉/𝐹𝐴0) = 𝑑(𝑉/𝐹𝐶𝑂 0) 2

333

The absorption rates were obtained by taking the slopes of 𝑋𝐶𝑂2versus 𝑉/𝐹𝐶𝑂20 curves and

334

evaluating them at different points on the reactor. The average rate of absorption was determined

335

by taking the logarithmic mean of the rates at specific points along the column. For the absorber,

336

gas phase CO2 concentrations were obtained along the column. The desorber is not equipped with

337

sampling points along the column, hence CO2 loading (liquid phase CO2 concentration) at the top

338

and bottom of the column were used in determining CO2 conversion and finally the rate of

339

desorption. Thus, for the rate of desorption, the derivative in the above equation becomes a finite

340

difference, ∆. Other performance parameters evaluated were cyclic capacity, which represents the

341

quantity of CO2 absorbed in the liquid phase, and CO2 removal efficiency (absorber efficiency)

342

which represents the quantity of CO2 removed from the gas phase. They are represented in the

343

form:

344

Cyclic capacity (kg/hr) = 60 × 106 × FAm × MWCO2 × (αrichCAm,rich ― αleanCAm,lean)

345

Removal efficiency (%) =

VinXCO2,in ― VoutXCO2,out VinXCO2,in

×

60 × 103MWCO2 Vm,CO2

× 100%

(10) (11)

346 347

4

Results and Discussion

348

4.1 Characterization

349

The X-ray diffraction spectra for all catalysts studied are shown in S2 (Supporting

350

information). The XRD peaks of crystallized powders of BaCO3 agree with the reflection of a pure

351

orthorhombic structure and single phase of BaCO3 (witherite). This was also evident in the works of

352

Zelati et al.35 and Salehizadeh et al.36 That of CaCO3 revealed the presence of a single

353

rombohedrical crystal structure corresponding to a single calcite phase which is the most

354

thermodynamically stable form of CaCO3 predominant at room temperature. Won et al.37, Harris

355

et al.38 and Render et al.39 also reported similar phase appearance corresponding to calcite. The

356

XRD pattern of the commercially obtained Ca(OH)2 revealed a predominant portlandite phase and

357

a peak corresponding to CaCO3 which corroborates findings in the literature.40,41 The sharp distinct 16 ACS Paragon Plus Environment

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358

peaks present in the hydrotalcite sample reflect the presence of a layered double hydroxide which

359

is characteristic of magnesium-aluminum hydrotalcites.42-45 Other smaller peaks obtained revealed

360

the presence of gibbsite, brucite and MgO periclase phases.42 The diffraction patterns obtained for

361

Cs2O/Al2O3 and Cs2O/α-Al2O3 are also shown in S2. They both showed phases corresponding to

362

their constituents.46,47 XRD pattern of K/MgO revealed distinct phases of MgO periclase, brucite

363

(Mg(OH)2), and very subtle KOH phases. The results are in close agreement with those in

364

literature.48,49

365

SEM images of the catalysts studied are shown in S3 (Supporting information), where their

366

surface morphology and distribution of active species can be seen. CaCO3 particles show a very

367

large degree of agglomeration while that of BaCO3 displays a rather smooth appearance. It can

368

also be observed that both hydrotalcite and Cs2O/Al2O3 show a somewhat flaky appearance. The

369

surface morphology of K/MgO and Cs2O/α-Al2O3 catalysts are seen to be very porous with the

370

distribution of the active site K clearly evident for the K/MgO catalyst. The hydrothermal treatment

371

of the Cs2O/α-Al2O3 may have resulted in improving the surface characteristics of the catalyst50,51.

372

The TPD profiles of the catalysts are shown in S4 (Supporting information) except for

373

BaCO3 and CaCO3 since they do not desorb CO2 but rather undergo decomposition at very high

374

temperatures52,53. ICP analysis on K/MgO, Cs2O/Al2O3 and Cs2O/α-Al2O3 and calculated weight

375

compositions of the other catalysts are summarised in S5 (Supporting information).

376 377

4.2 Catalyst Screening Results - Initial CO2 absorption rates

378

The CO2 absorption profiles for all catalysts studied are presented in Figures 6(a) and 6(b).

379

As stated earlier, the slopes of the linear portion of these profiles were extracted to represent the

380

initial CO2 absorption rates into the solvent in the presence of the various catalysts studied.

381

Detailed plots showing the linear portions used for the initial absorption rate calculations are

382

shown in S6 (Supplementary information) and the results are summarised in Table 3.

383



0.60

Loading (mol CO2/mol amine)

0.50 blank BaCO3

0.40

K/Mgo Hydrotalcite

0.30

Ca(OH)2 Cs2O/g-Al2O3

0.20

CaCO3 Cs2O/a-alumina

0.10

K/MgO+g-alumina K/MgO + CS

0.00 0

100

200

300

400

500

600

700

800

time (minutes)

384

Fig. 6 (a) CO2 absorption profiles of various catalysts understudied

385 386

0.45 0.40

loading (mol/mol)

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 18 of 44

blank

0.35

BaCO3

0.30

K/MgO

0.25

Hydrotalcite Ca(OH)2

0.20

Cs2O/g-alumina

0.15

CaCO3

0.10

Cs2O/a-alumina

0.05

K/MgO+g-alumina K/MgO + CS

0.00 -0.05

0

50

100

150

200

250

time (min)

387 388

Fig. 6 (b) linear portion of CO2 absorption profiles

389 18 ACS Paragon Plus Environment

300

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390


Table. 3 Summary of Initial rates of absorption of various catalysts studied Catalyst

Initial rate of absorption (mol/L.min) × 10

Blank (non-catalytic)

5.2

BaCO

5.6

CaCO

4.8

Hydrotalcite

5.2

Ca(OH)

6.0

3 3

2

Cs O/Al O 2

2

2

391

5.6

3

Cs O/α-Al O 2

3

3

6.0

K/MgO

6.0

K/MgO + *CS binder

6.0

K/MgO + γ-alumina binder

5.6

*CS = Colloidal Silica

392

As is well known, the catalysts behavior (Lewis or Bronsted) is dependent on the type of

393

reaction they take part in. According to the well accepted Zwitterion mechanism, the first step is

394

rate determining and involves an electron transfer26. Therefore, the behavior of interest to promote

395

this reaction is the catalyst that exhibit Lewis behavior in this first reaction step. Hence, in this

396

work, the classification of catalysts as being either Lewis or Bronsted is in relation to this reaction

397

step in the Zwitterion mechanism. Based on the above, Lewis base catalysts included K/MgO,

398

Ca(OH)2, Cs2O/Al2O3, BaCO3, Cs2O/α-Al2O3 and CaCO3 whereas Hydrotalcite was classified as

399

a Bronsted base catalyst.

400

From Figure 6 and Table 3, it was observed that K/MgO, Ca(OH)2 and Cs2O/α-Al2O3

401

recorded the fastest rates of absorption, followed by Cs2O/Al2O3, BaCO3, hydrotalcite, blank and

402

CaCO3 in decreasing order of absorption rates. The trend observed can be summarised as: K/MgO

403

~ Ca(OH)2 ~ Cs2O/α-Al2O3 > Cs2O/Al2O3 ~ BaCO3 > Hydrotalcite ~ blank > CaCO3. The reason

404

for the observed trend can be explained based on the mechanism for the CO2 reactions with amines.

405

According to Caplow26, the zwitterion formation step (first step) in the zwitterion

406

mechanism happens to be the rate determining step. As already stated, this step involves the 19 ACS Paragon Plus Environment


407

transfer of electrons. Therefore, it is imperative that an electron transfer process will be enhanced

408

in the presence of electrons. The presence of O2- anions of unsaturated co-ordination accounts for

409

the basic sites in Lewis solid base catalysts thereby possessing the ability to release their electrons

410

ahead of the amine solvents to enhance the reactivity of CO2 with amines, and later recover them

411

from these amines. This results in faster kinetics as can be seen when the initial rate of absorption

412

of the blank (no catalyst) is compared to that of the catalysts.

413

The poor performance in initial absorption rate for CaCO3 catalyst when compared with

414

the blank is due to its very low surface area which was less than 0.1 m2/g. This is supported by the

415

SEM results (S3) which revealed CaCO3 surface had a large degree of agglomeration which may

416

also have resulted in its rather poor performance. In a long run, this may have adversely affected

417

the rate of CO2 absorption thereby resulting in lower rates when compared to the blank. The

418

physical and chemical properties of the catalysts studied are shown in Table 4.

419

K/MgO recorded one of the fastest rates of absorption. Its relatively higher surface area

420

and pore volume in comparison with the other catalysts is a possible reason for its good

421

performance. Also, TPD results (S4 and table 4) showed K/MgO possessed the largest number of

422

basic sites. According to Chen et al.54, the generation of super basic sites was related to the

423

existence of O2- anion vacancies in MgO and its accompanying electrical induction effect. Jimenez

424

et al.48 reported on the interaction between K and Mg in MgO resulting in the weakening of the

425

Mg-O bonds and therefore aiding in the easy migration of the O2- anion species. In the presence

426

of the amine, these electron-rich anion species easily attack dissolved CO2, and this interaction ties

427

the CO2 molecules to the surface of the catalyst, making them readily available for the nitrogen

428

(N) atom of the amine. In this way, a greater contact time is available between the amine solvent

429

and CO2 thereby enhancing the rate of reaction. It is important to note that immediately the catalyst

430

comes into contact with aqueous amine, a small fraction of MgO phase of the calcined catalyst

431

changes to Mg(OH)2. Thus, both MgO and Mg(OH)2 are present during reaction as shown in the

432

XRD pattern of the used catalyst (Figure 7b). This is supported completely by the catalyst XRD

433

results of both the before use and after use catalysts. Another important information from the XRD

434

results of the used catalyst was the absence of carbonates (CO32-) which shows that the catalyst is

435

indeed not a reactant. This type of result has been reported by other authors48, where the absence

436

of MgCO3 was attributed to the presence of the K-promoter which easily decomposes any formed 20 ACS Paragon Plus Environment

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437

carbonates on the catalyst surface. The very porous nature of the K/MgO catalyst may have also

438

contributed to its superior performance (S3).

439

Apart from O2- anion species, OH- ions also account for basicity. This was the case for

440

Ca(OH)2 which had comparable results with K/MgO. Its good performance could be due to the

441

two OH- ions which might have provided twice the effect of one OH- ion. Also, its somewhat large

442

number of basic sites and porous surface is a possible reason for its good performance as well, as

443

compared to CaCO3, BaCO3, Cs2O/Al2O3 and Hydrotalcite. There is the possibility that Ca(OH)2

444

can form carbonates. However, this was not tested and the catalyst was not selected for further use

445

in the pilot plant. BaCO3 and Cs2O/Al2O3 exhibited moderate activities which may be due to their

446

very low surface area and pore volume. BaCO3, being a carbonate, had a higher initial rate of

447

absorption than the blank (non-catalytic case). This observation nullifies any view that the

448

formation of a carbonate could stifle the reaction. The above results confirm the catalytic

449

contribution of the materials used.

450

Cs2O/α-Al2O3 performance was at par with K/MgO and Ca(OH)2. Its surface area was very

451

close to that of Ca(OH)2 (Table 4). Also, SEM image of Cs2O/α-Al2O3 indicates the possession of

452

a porous structure, which may have resulted in its good performance. The reason for this can be

453

due to the hydrothermal treatment of the α-Al2O3 which improved upon the physical properties of

454

the catalyst. Several studies have shown an improvement in the physical and chemical properties

455

of catalysts when they undergo hydrothermal treatments. This is evident in the work of Kovanda

456

et al.50 where an increase in pore size, crystallite size as well as an improvement in thermal stability

457

was observed when Ni-Al layered double hydroxides and other mixed oxides were hydrothermally

458

treated. Jung et al.51 also reported on an enhancement in the chemical stability of CuO-CeO2 where

459

cuprous ion was shown to have migrated to the surface of catalyst leading to an increase in surface

460

concentration of copper and the subsequent formation of cupric oxide on the surface of catalyst.

461

It should be noted that the catalysts are in the solid form (heterogeneous) and are insoluble

462

in the amine solvent. Therefore, being insoluble, the OH- ions are not free to react. A typical

463

example is the K/MgO-catalyzed reaction, where it is evident that the K/MgO is not reacting

464

chemically, and hence carbonates are not formed. Ca(OH)2 and Al2O3 can form carbonates if they

465

are able to dissolve. However, these catalysts were used for screening and were not selected for

466

further use in the pilot plant. 21 ACS Paragon Plus Environment


Page 22 of 44

467

Hydrotalcite barely showed any activity despite its relatively large number of basic sites.

468

It was expected that hydrotalcite should have improved upon the reaction since it is a basic catalyst

469

and considering that the rate determining step (RDS) requires an electron transfer. However, its

470

rather unappreciable performance for this reaction can be attributed to the ineffectiveness of the

471

surface oxygen anions, O2- (Lewis sites) as compared to the interlayer hydroxyl ions, OH-

472

(Bronsted sites) which are predominant in the hydrotalcite structure. 55 Also, the presence of Lewis

473

acid sites (Al3+) present may have hindered the performance of the Hydrotalcite catalyst.56

474

Despite their high activities, Ca(OH)2 and Cs2O/α-Al2O3 exhibited very poor mechanical

475

stability (i.e. ability to withstand crushing upon agitation) and were found to disintegrate easily in

476

the amine solvent even after pelletizing. On the other hand, K/MgO was found to possess good

477

mechanical stability. Consequently, K/MgO was selected and its application was transferred to a

478

bench-scale CO2 capture pilot plant. Prior to its application on the bench-scale pilot plant (which

479

is subject to agitations from process equipment), it was imperative to improve upon the mechanical

480

stability of the K/MgO catalyst without altering its activity or performance. Thus, binders such as

481

40wt% colloidal silica (CS) solution and γ-alumina were added. No change in activity was seen

482

with the CS binder but a drop in activity was observed with the γ-alumina binder (Table 3). Hence

483

the CS binder was selected for use with K/MgO.

484 485

Table 4. Physical and Chemical properties of catalysts studied Catalyst

BET surface Pore area (m2/g)

volume Pore

(cm3/g)

size Number

(nm)

basic

of Basic sites strength (oC)

*(a.u) K/MgO

63.33

0.270

17.08

58785

237

Ca(OH)2

8.43

0.042

19.89368

15565

862

Cs2O/α-Al2O3

7.61

0.041

21.44

8619

132

BaCO3

4.66

0.008

6.46

-

-

Cs2O/Al2O3

3.33

0.009

10.33

818

555

Hydrotalcite

0.46

0.0001

0.86

28258

179

CaCO3

-

-

-

-

-


Page 23 of 44 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

486


*arbitrary units

a

b

487 488

Figure 7. XRD pattern of K/MgO (a) before use (b) after use

489 490

4.3 Statistical Analysis

491

A correlation to relate the catalyst physical properties with initial absorption rate was

492

developed. The variables employed were BET surface area, pore volume and Pore size. These

493

were all normalized to values between 0 and 1. The statistical analysis was conducted using a Non-

494

linear regression tool, NLREG® version 6.3. The R2 and R2-adjusted of the correlation were

495

0.9582 and 0.9163, respectively. The relationship between Initial absorption rate, R, and the

496

physical properties is represented as shown in equation 12:

497

Initial absorption rate, R = (4.941×10-3) + (5.929×10-3 × Surface area) - (5.853×10-3 × Pore

498

volume) + (1.236×10-3 × Pore size)

(12)

499

Another correlation to relate both physical and chemical properties of the catalysts,

500

including surface area, pore size, number of basic sites and basic strength, with the initial

501

absorption rate was also developed. Again, the independent parameters were normalized to values

502

between 0 and 1. The correlation obtained is shown in equation 13:

503

Initial absorption rate, R = (5.243×10-3) + (1.289×10-4 × Surface area × Basic Strength × Number

504

of basic sites) + (7.896×10-4 × Pore size)

(13)



505

The R2 and R2-adjusted of the correlation were 0.9594 and 0.9323 respectively. The

506

coefficients of each parameter were used to determine their impact on the initial absorption rate.

507

From the results, the most critical parameter was shown to be the Pore size followed by the

508

combined parameter involving number of basic sites, basic strength and surface area. It is observed

509

that the catalysts with the larger pore sizes are K/MgO, Ca(OH)2 and Cs2O/α-Al2O3. The results

510

indicate that, despite the combination of large number of sites, high basic strength and large surface

511

area, the catalytic performance is hinged on how well the reacting molecules have access to these

512

basic sites. This means the greater the access to the basic sites, which is potentially provided by

513

the pore size, the better the catalytic absorption performance. This accounts for the poor catalytic

514

performance of hydrotalcite, BaCO3 and Cs2O/Al2O3.

515 516

4.4 Solvent Performance and effect of solid acid catalyst in Pilot plant

517

The novel solvent, BEA-AMP was tested based on comparative runs with the conventional

518

MEA and MEA-MDEA blend at the bench-scale pilot plant level. Akachuku19, Osei et al.28,

519

Srisang et al.34 and Decardi-Nelson57 studied the use of solid acid catalyst (HZSM-5) on the latter

520

two solvents. Therefore, in this work we compared the novel solvent with these solvents on the

521

same basis of employing their solid acid catalyst in the desorber. Figures 8 and 9 show the

522

absorption and desorption rates of the 3 solvents respectively with and without desorber catalyst

523

(HZSM-5) while figures 10 and 11 show their cyclic capacities and absorption efficiencies in the

524

absence and presence of the solid acid catalyst (HZSM-5). From these figures, it can be observed

525

that the novel solvent, BEA-AMP outperformed the other two, both in the absence and presence

526

of the catalyst. For the blank case (non-catalytic run), BEA-AMP recorded a percentage increase

527

of 73.3% over MEA in absorption rate and 67.4% over blended MEA-MDEA. With the addition

528

of the desorber catalyst (HZSM-5) to all three solvent systems, values of 92.6% and 85.7% increase

529

in absorption rate for BEA-AMP over MEA and blended MEA-MDEA were respectively

530

observed.

531

The temperature profile in the absorber is shown in Figure 12. The trend is proven by these

532

charts where it is observed that the largest bulge in the absorber temperature profile is evident for

533

BEA-AMP signifying higher reactivity of this solvent over the other two in both cases of blank


Page 24 of 44

Page 25 of 44

534

and the inclusion of HZSM-5 in the desorber. Comparatively, with HZSM-5 addition, larger

535

temperature bulges in the absorber were observed over that of the blank case for all three solvents.

536

reaction rate (mol/L.min)

0.02

0.0181

0.018 0.016

0.0149

0.014

0.0118

0.012 0.01

0.0128 0.0089

0.0086

0.008 0.006 0.004 0.002 0 MEA

MEA-MDEA

BEA-AMP

solvent blank

537 538 539

HZSM-5

Figure 8. CO2 absorption rates of MEA, MEA- MDEA and BEA-AMP with and without desorber catalyst HZSM-5

540 0.045 reaction rate (mol/L.min)

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


0.041

0.04 0.034

0.035

0.035

0.03 0.025 0.02

0.023

0.024

0.0167

0.015 0.01 0.005 0 MEA

MEA-MDEA solvent

541

blank

HZSM-5


BEA-AMP


542 543

Figure 9. CO2 desorption rates of MEA, MEA- MDEA and BEA-AMP with and without desorber catalyst HZSM-5

544 0.14 0.12 cyclic capacity (kg/hr)

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 26 of 44

0.1 0.08 0.06 0.04 0.02 0 MEA

MEA-MDEA

BEA-AMP

Solvent

545

blank

HZSM-5

546

Fig. 10. CO2 cyclic capacity of MEA, MEA- MDEA and BEA-AMP with and without

547

HZSM-5 in desorber


Page 27 of 44

60 50 40

20

}

30

HZSM-5 blank

}

absorption effiecieny (%)

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


10 0 MEA

MEA-MDEA

BEA-AMP

SOLVENT

548

by CO2 mass flow

by CO2%

by loading

549

Fig. 11. CO2 absorption efficiency of MEA, MEA- MDEA and BEA-AMP with and without

550

HZSM-5 in desorber

551

4.4.1 Absorption Performance

552

For the blank run, the 4M BEA-AMP blend had the highest CO2 absorption rate, followed

553

by 7M MEA-MDEA, with 5M MEA being the slowest. This could be explained on the basis of

554

their structural properties and lean loadings, which were developed into Absorption Parameter and

555

Desorption Parameter by Narku-Tetteh et al.8 in which BEA and AMP exhibited higher Absorption

556

Parameters and Desorption Parameter than MEA and MDEA. That explains why the BEA-AMP

557

blend outperforms MEA in the rate of CO2 absorption. The BEA-AMP bi-blend similarly

558

outperformed MEA-MDEA blend.

559

Similarly, with the incorporation of the solid acid catalyst, HZSM-5 in the desorber, an

560

identical trend was observed with BEA-AMP being the fastest while MEA was the least reactive.

561

This is because the inclusion of the catalyst in the desorber provided an alternative pathway where

562

there was a greater weakening of the N-C bond in carbamate, leading to a faster release of CO2

563

from the solvents. Hence, a considerable drop in solvent lean loadings led to higher reactivity in

564

the absorber as compared to the non-catalytic run in both columns. Lean loadings of the three

565

solvent systems are shown in table 5. The loading values reported are the sum total of all the CO2 27 ACS Paragon Plus Environment


566

groups present in each system. The speciation of the aqueous CO2-amine system for all the amines

567

studied – MEA, MDEA, BEA and AMP – are existent in literature.58-61 From these works, it is

568

clear the CO2 groups present are carbonates, bicarbonates, carbamates and free CO2. Of a certainty,

569

quantifying reaction products is important. In this work, reaction products were represented in the

570

form of the solvents’ CO2 loading, and was enough to perform calculations. An increase in

571

absorption rate of 26% was observed for BEA-AMP with the inclusion of the solid acid catalyst,

572

HZSM-5, while MEA-MDEA blend and MEA respectively had a 14% and 12% increase in the

573

absorption rate.

574 575

4.4.2 Desorption Performance

576

From Figure 9, it can be observed that for the non-catalytic case (blank), BEA-AMP blend

577

had the highest desorption rate, followed by MEA-MDEA blend, with MEA being the slowest.

578

This can be explained from the results obtained in previous studies by Narku-Tetteh et al.8, where

579

the absorption and desorption parameters were compared for several single amine solvents (Figure

580

13). It is observed that the single solvents comprising the novel bi-blend, BEA and AMP had

581

considerably higher absorption and desorption parameter values than MEA. The BEA-AMP bi-

582

blend having the highest desorption rate can be attributed to the steric hindrance effect of AMP as

583

it forms a highly unstable carbamate which easily breaks down to form bicarbonate ions and

584

enhances desorption of CO2. Also, the longer alkyl group (butyl) in the BEA structure forms a

585

highly unstable carbamate hence increasing CO2 desorption. As stated earlier, MEA has an H in

586

place of the alkyl group hence it forms a stable carbamate making it difficult to release CO2.

587

MDEA, being a tertiary amine, forms bicarbonate ions which accepts protons to form carbonic

588

acid and finally releases CO2. Hence, MDEA blended with MEA increased the desorption rate of

589

CO2 as compared to single MEA solvent. Also, BEA-AMP had a higher desorption rate than MEA-

590

MDEA.

591

Aside the effect of the sterically-hindered AMP, the secondary amine, BEA has an electron

592

donating group (butyl) attached to N, whereas MDEA has two electron-withdrawing groups (–OH

593

groups) attached to N as explained earlier. Due to this, a higher electron density is generated around

594

the N in BEA-AMP making the amine more reactive than the lower electron density-N in MEA-

595

MDEA. With the addition of the solid acid HZSM-5 catalyst, a similar trend resulted but at faster

596

desorption rates. The effect of the catalyst is seen with an increase in desorption rates for all the 28 ACS Paragon Plus Environment

Page 28 of 44

Page 29 of 44

597

three solvents. BEA-AMP recorded an increase of 17% in the CO2 desorption rate, while MEA-

598

MDEA blend and MEA had an increase of 41% and 35%, respectively.

599

The presence of the catalyst lowers the activation energy by providing an alternative

600

catalytic pathway. According to Akachuku19, HZSM-5 has both Lewis and Bronsted acid sites

601

which play an important role in increasing the rate of CO2 desorption. For a Bronsted acid site, a

602

proton is donated to the carbamate ion converting it to carbamic acid. Chemisorption on the Al

603

site weakens the N-C bond causing CO2 to break away. Also, a proton can be transferred to

604

bicarbonate ion which eventually leads to the release of CO2. For the Lewis acid site (which lacks

605

electrons), an attack is made on the high electron density Nitrogen (N), again weakening the N-C

606

bond. Consequently, CO2 breaks away. The novel solvent thus exhibited better performance than

607

the conventional solvents for the full cycle operation of the pilot plant in both cases of non-catalytic

608

and catalyst inclusion in the desorber.

609 610 611 1

Height from bottom (m)

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


0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 6

612 613

11

16

21

26

31

Temperature

(oC)

36

41

46

MEA blank

MEA-MDEA blank

BEA-AMP blank

MEA HZSM-5

MEA-MDEA HZSM-5

BEA-AMP HZSM-5

Figure 12. Temperature profile along absorber

614 29 ACS Paragon Plus Environment

51

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

Absorption parameter,*10-2 (mol CO2 absorbed)2/(mol amine.min.Lsoltn)


0.900 0.800 MEA

0.700

AMP

0.600

BEA

0.500

tBEA

0.400

BDEA

0.300

tBDEA 4-A-1B

0.200

MDEA

0.100 0.000 0.0000

0.0200

0.0400

0.0600

0.0800

Desorption Parameter, *10-2 (mol CO2 desorbed)3/ (kJ.(Lsoltn)2.min

615

Figure 13. Amine Selection Chart8

616 617

Page 30 of 44

Table 5. Solvent lean loading of the three solvents studied Lean loading (mol

5M MEA

5/2M MEA-MDEA

2/2M BEA-AMP

0.42

0.35

0.33

0.41

0.32

0.30

CO2/mol amine) no catalyst HZSM-5

catalyst

(Si/Al = 19) 618 619

4.5 Effect of Solid base catalyst addition in Pilot plant

620

The selected solid base catalyst (K/MgO) from the screening results was transferred to the

621

absorption column of the bench-scale pilot plant and was tested on the novel solvent 4M BEA-

622

AMP. A catalyst weight of 150g and average desorber bed temperature of 85oC were used. This

623

selected catalyst weight was based on previous studies on this same set-up where 150g of a solid

624

acid catalyst (HZSM-5) for the desorption column was the optimum weight after performing a

625

sensitivity analysis.19 Absorption and desorption rates were determined for three configurations

626

shown in Table 6. The absorber temperature profiles obtained from the experiments are shown in 30 ACS Paragon Plus Environment

Page 31 of 44 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


627

Figure 14. The highest reactivity in the absorber is seen with configuration 3 (catalysts in both

628

columns) where its temperature bulge is largest. Configuration 2, the case of only HZSM-5 in the

629

desorber and no absorber catalyst, comes next, and finally, configuration 1 (solvent only) exhibits

630

the smallest bulge in temperature along the absorber.

631

From Figure 15, it can be observed that the addition of HZSM-5 to the desorber system

632

resulted in an increase in the absorption rate from about 0.015 to 0.018 mol/L.min which

633

corresponds to a 22% increment. Such observations can be linked to the lean loading of the solvent.

634

It was observed that the lean loading dropped when HZSM-5 was incorporated into the desorber

635

system. HZSM-5 contributes to better desorption performance, as explained earlier. This translates

636

into enhancing CO2 absorption since the process is cyclic. The leaner solvent means more active

637

free amines were available to react thereby resulting in an increase in the reaction rate. Upon the

638

addition of the solid base-catalyst (K/MgO) into the absorber (configuration 3), a huge

639

improvement is seen in the rate of CO2 absorption. Faster kinetics occurs resulting in a higher rich

640

loading of the amine. When compared to the case of only HZSM-5 in desorber (configuration 2),

641

an increase of 61% is made when K/MgO is placed in the absorber. A synergistic increase in

642

absorption rate of about 99% is observed with the addition of K/MgO and HZSM-5 (configuration

643

3) using configuration 1 as basis of comparison. The explanation for the observed trend is based

644

on established proof that CO2 reactions with amines proceeds through the Zwitterion mechanism

645

and that the rate determining step is the Zwitterion formation.26 As highlighted earlier, this step

646

involves the transfer of electrons. Therefore, any enhancement in electron transfer will speed up

647

the Zwitterion formation. Since K/MgO is a Lewis base catalyst (electron donor), it facilitates the

648

easy transfer of electrons which accelerates the rate limiting step, hence improving upon the overall

649

rate of reaction. Hence, a faster rate of CO2 absorption is observed. With no solid base catalyst in

650

the absorber, the electron transfer process is highly hinged on the inherent solvent characteristics

651

which results in relatively slower reaction rates as compared to when an easier electron transfer

652

facilitator (solid base catalyst) is present in the process.

653

The desorption rate had a similar trend with configuration 1 having the slowest desorption

654

rate, followed by configuration 2, and the fastest being configuration 3. The performance in both

655

columns are linked in that whatever transpires in the absorber is translated to the desorber. Since

656

the fastest rate of CO2 absorption was seen for configuration 3, it implies more CO2 was absorbed 31 ACS Paragon Plus Environment


Page 32 of 44

657

into the amine solvent, hence any little application of heat coupled with the presence of solid acid

658

catalyst (HZSM-5) in the desorber will lead to faster release of CO2. An increase of about 37% is

659

seen in desorption rate for configuration 3 when compared to configuration 2 (only HZSM-5 in

660

desorber). Using configuration 1 (non-catalytic) as basis, a synergistic improvement in desorption

661

rate of 63% is realized with the addition of both K/MgO and HZSM-5 in the absorber and desorber

662

respectively. Desorption rates are displayed in Figure 16. Table 7 shows the lean and rich loadings

663

for the three configurations studied.

664 665 666 667 668 669 670

Table 6. Absorber and desorber configurations System Configuration

Absorber

Desorber

1

Solvent

Solvent

2

Solvent

Solvent + solid acid catalyst (HZSM-5)

3

Solvent + solid alkaline catalyst Solvent + solid acid catalyst (K/MgO)

(HZSM-5)

671 672 673


Page 33 of 44

1

Height from bottom (m)

0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 6

11

16

21

26

31

36

41

46

Temperature (oC) 1

674

2

3

Figure 14. Temperature profile along absorber

675 0.035

Absorption rate (mol/L.min)

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


0.03 0.025 0.02 0.015 0.01 0.005 0 1

2

3

System Configuration

676 677

Figure 15. CO2 absorption rates of different system configurations

678


51


0.07

Desorption rate (mol/L.min)

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 34 of 44

0.06 0.05 0.04 0.03 0.02 0.01 0 1

2

3


679 680

Figure 16. CO2 desorption rates of different system configurations

681 682

Table 7. Lean and Rich loadings of the different system configurations studied. Configuration

1

2

3

Lean loading (mol

0.33

0.3

0.32

0.49

0.49

0.58

CO2/mol amine) Rich loading (mol CO2/mol amine) 683 684

4.6 Effect of solid base catalyst weight

685

To determine the optimum solid base catalyst weight for the process, the catalyst weight

686

was varied while keeping the CO2 flowrate constant. The pilot plant represents a much larger scale

687

than the semi-batch absorption experiments, therefore a greater catalyst weight was required for

688

the testing. In view of that, the catalyst weights were varied from 50g to 170g to clearly observe

689

any changes in performance. Accordingly, it was evident that there was an increase in conversion

690

as the catalyst weight was increased, signifying catalytic activity until it tapered off at 170g.

691

Figures 17 and 18 display the effect of catalyst weight on the CO2 conversion in the absorber as 34 ACS Paragon Plus Environment

Page 35 of 44

692

well as the rate of CO2 absorption. It can be observed that as the catalyst weight was increased,

693

CO2 conversion also increased. More catalyst means higher availability of active surface area and

694

the physical presence of greater number of porous surfaces, thereby allowing for a greater number

695

of reacting species to have access to these additional sites, hence resulting in an increase in

696

conversion. This resulted in an increased rate of absorption, as expected, introduced by increasing

697

the weight of the catalyst. A percentage increase in conversion of about 8.5% and 5.9% was seen

698

with catalyst increments from 50 to 100g and 100 to 150g, respectively. However, generally, after

699

a weight of 150g, the conversion in the absorber is seen to be fairly constant. This might be the

700

result of the reaction having attained its thermodynamic limit and as such no increase is seen with

701

further addition of catalyst.

702 1

fractional CO2 Conversion

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


0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0g

50g

100g

150g

170g

catalyst weight

703 704

Figure 17. Effect of catalyst weight on CO2 conversion (Desorber bed temperature: 85oC; Amine

705

flowrate: 60 ml/min)

706


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

rate of absorption (mol/L.min)


Page 36 of 44

0.04 0.035 0.03 0.025 0.02 0.015 0.01 0.005 0 0g

50g

100g

150g

170g

catalyst weight

707 708

Figure 18. Effect of catalyst weight on Absorption rate (Desorber bed temperature: 85oC; Amine

709

flowrate: 60 ml/min)

710 711 712 713

5

Conclusions

714

Out of the seven solid basic catalysts screened on a semi-batch level to select the most

715

suitable on the basis of initial rate of absorption and mechanical stability, K/MgO was shown to

716

be the most suitable as absorber catalyst in a bench-scale pilot plant.

717

Comparative solvent studies between the novel 4M BEA-AMP bi-solvent blend and

718

conventional solvents 5M MEA and blended 7M MEA-MDEA revealed better carbon capture

719

characteristics of the former over the two latter solvents. Absorption and desorption kinetics were

720

both highest for BEA-AMP blend followed by MEA-MDEA with MEA being least for both cases

721

with catalyst and without catalyst in the desorber. The solvent structural properties and lowest lean

722

loading of BEA-AMP resulted in faster reaction rate as compared to single MEA and MEA-MDEA

723

blend. For the blank case (non-catalytic run), BEA-AMP recorded a percentage increase of 73.3%

724

over MEA in absorption rate and 67.4% over blended MEA-MDEA.


Page 37 of 44 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


725

With the addition of the desorber catalyst (HZSM-5) to all three solvent systems, values of

726

92.6% and 85.7% increase in absorption rate for BEA-AMP over MEA and blended MEA-MDEA

727

respectively were observed. The inclusion of HZSM-5 resulted in leaner CO2 concentrations hence

728

resulting in higher rates of absorption. As with absorption rates, the desorption rates for the blank

729

case (non-catalytic run), followed the same trend with BEA-AMP recording a percentage increase

730

of about 110% over MEA and 45.8% over blended MEA-MDEA. With the addition of the desorber

731

catalyst (HZSM-5) to all three solvent systems, there were increments of 78.3% and 20.6% in

732

desorption rates for BEA-AMP over MEA and blended MEA-MDEA respectively. The steric

733

effect of AMP in the blend contributed to the fastest CO2 desorption rate for the BEA-AMP blend.

734

The novel blend thus was proven to be superior over the conventional MEA and MEA-MDEA

735

solvents. HZSM-5 increased desorption rates by providing an alternative pathway where

736

bicarbonate ions were produced hence resulting in the faster release of CO2.

737

Upon the addition of the solid base-catalyst (K/MgO) into the absorber, a huge

738

improvement was seen in the rate of CO2 absorption. When compared to the case of only HZSM-

739

5 in desorber (no absorber catalyst), an increase of 61% is made when K/MgO is added. A

740

synergistic increase in absorption rate of about 99% is observed with the addition of K/MgO and

741

HZSM-5 using the blank case of no catalyst in both columns (non-catalytic case) as basis of

742

comparison. K/MgO exhibited excellent absorption performance due to its superior electron

743

donating ability. Using the non-catalytic case as basis, there was a synergistic improvement in

744

desorption rate of 63% with the addition of both K/MgO and HZSM-5 catalysts in the absorber

745

and desorber, respectively.

746

The configuration of K/MgO in the absorber and HZSM-5 in the desorber greatly enhanced

747

the reactivity of the amine blend with CO2. Consequently, higher absorption and desorption rates

748

with the addition of these catalysts to the post combustion capture process implies shorter columns

749

hence huge reduction in capital costs.

750 751

Acknowledgements

752

The financial support provided by Natural Science and Engineering Research Council of Canada

753

(NSERC), Government of Saskatchewan, Clean Energy Technologies Research Institute (CETRI) 37 ACS Paragon Plus Environment


754

and Faculty of Graduate Studies and Research (FGSR), University of Regina is greatly

755

acknowledged.

756 757 758 759 760 761

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3. Liao C. H., Li M. H. Kinetics of absorption of carbon dioxide into aqueous solutions of

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4. Mandal B. P., Biswas A. K., Bandyopadhyay S. S. Absorption of carbon dioxide into aqueous

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at high ionic strengths. Indian J. Technol. 1980, 18, 475–477.

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12. Ghosh, U.K., Kentish, S., Stevens, G.W. Absorption of carbon dioxide into aqueous potassium carbonate promoted by boric acid. Energy Procedia 2009, 1 1075–1081.

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13. Guo, D., Thee, H., da Silva, G., Chen, J., Fei, W., Kentish, S., Stevens, G.W. Borate-catalyzed

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14. Nicholas, N.J., da Silva, G., Kentish, S., Stevens, G.W., (2014). Use of vanadium (V) oxide

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15. Phan D. T., Maeder M., Burns R. C., Puxty G. Catalysis of CO2 absorption in aqueous

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16. Sivanesan D., Kim Y. E., Youn M. H., Park K. T., Kim H. J., Grace A. N., Jeong S. K. The

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925 926 927 928 929 930 931 932 933 934 935 43 ACS Paragon Plus Environment


936

Graphical Abstract - TOC

937 0.035

Absorption rate (mol/L.min)

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

0.03 0.025 0.02 0.015 0.01 0.005 0 no catalyst in both columns

solid acid catalyst in desorber only

solid acid catalyst (desorber)+solid base catalyst (absorber)


938 939


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MgO) and

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