Low-Concentration CO2 Capture from Natural Gas Power Plants

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Environmental and Carbon Dioxide Issues

Low Concentration CO2 Capture from Natural Gas Power Plants Using Rotating Packed Bed Reactor Chenxia Xie, Yuning Dong, Liangliang Zhang, Guang-Wen Chu, Yong Luo, Bao-Chang Sun, Xiaofei Zeng, and Jian-Feng Chen Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02780 • Publication Date (Web): 18 Oct 2018 Downloaded from http://pubs.acs.org on October 19, 2018

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Energy & Fuels

1

Low Concentration CO2 Capture from Natural Gas Power Plants Using

2

Rotating Packed Bed Reactor

3 4

Chenxia Xie1, Yuning Dong1, Liangliang Zhang1 *, Guangwen Chu1, Yong Luo1,

5

Baochang Sun1, Xiaofei Zeng1, 2 *, Jianfeng Chen1, 2

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1Research

Beijing University of Chemical Technology, Beijing, 100029, PR China

8

9

2State

Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing, 100029, PR China

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

Abstract:

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A rotating packed bed (RPB) was employed as a highly effective reactor to intensify

13

CO2 capture in a green and natural amino acid salt absorbent, Potassium Sarcosine

14

(KSAR), from the flue gas containing low CO2 concentration. Experimental results show

15

that a good CO2 capture performance, presented in terms of CO2 capture efficiency and

16

overall volumetric mass-transfer coefficient (KGa), can be obtained at low CO2

17

concentration of 2%~6%. CO2 capture efficiency could reach higher than 80% at

18

relatively high gas-liquid ratio with CO2 loading up to 0.17 mol CO2/mol KSAR.

19

Comparison results with packed column show that RPB can obtain higher CO2 capture

20

performance with a smaller device size. Moreover, a mathematical model was developed

21

to describe the mass transfer process in RPB. Calculated values of KGa well agreed with

22

experimental data with a deviation within ±25% and the tendency of the CO2

23

concentration at outlet of RPB can be well predicted under high liquid flow rate and high 1

ACS Paragon Plus Environment

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rotation speed. The model provides a view on the mass transfer process of CO2 capture in

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the RPB and offers the theoretical basis for design and application of RPB in the future.

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Keywords: CO2 Capture; Rotating Packed Bed; Mass Transfer; Amino acid salt;

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Chemical Absorption.

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1 INTRODUCTION

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Over the past century, human activities have caused a sharp increase in carbon

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dioxide (CO2) concentration in the atmosphere, which leads to unpredictable impact on

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the climate system.1 The most important impact is the global warming. Since 1900, the

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global temperature has increased more than 1 oC,2 which is very close to the dangerous

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level. The historical Paris Agreement calls for limiting the global temperature rise “well

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below 2 oC”. In order to meet this objective, in the near term, before 2050, the CO2

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emissions must be strictly controlled to slow down the global warming trend. In the long

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term, large-scale CO2 must be captured and stored to reduce the total amount of CO2 in

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the atmosphere. Therefore, it is urgent and necessary to develop stable, safe, and

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environmentally acceptable approaches and technologies for CO2 capture and storage

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(CCS).3

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The major CO2 emission comes from the combustion process of fossil fuels,

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including natural gas and coal, in power plants. Chemical absorption method with

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aqueous

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diethanolamine (DEA), is considered to be the most technologically mature and

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commercially viable option to capture CO2 from coal-fired power plants.4 Although these

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amines have already been in industrial use for a long time, the use of these amines still

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faces lots of challenges: high energy consumption for absorbent regeneration, toxicity,

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corrosion of the equipments, easy thermal and oxidation degradation, and high

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volatility.5-7 New CO2 capture technology should be developed to overcome these

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drawbacks of the present processes. Meanwhile, new technology should coordinate with

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the feature of future power plants. Natural gas is considered as a clean fuel due to its

amine-based

absorbents,

typically

monoethanolamine

3


(MEA)

and

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clean nature and will play more important role in power generation in the foreseeable

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future. However, considering the fact that CO2 concentration of flue gas from natural gas

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power plants is in a low range of 3~5%,8 CO2 capture from flue gas containing such low

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CO2 partial pressure is more difficult. Thus, chemical absorbents used in the new

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technology should have stronger affinity and reactivity to CO2.9

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Amino acid salts (AASs) are a class of alkaline salts of amino acid which contain

58

both amine group (–NH2) and carboxylic acid group (–COOH). AASs exhibit potential

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benefits over amines in CO2 capture and have been regarded as possible alternative to the

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conventional absorbents.10-13 Due to the salt nature, AASs offer negligible vapor pressure

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and good resistance to oxidative and thermal degradation. Thus they can effectively

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minimize the volatilization and degradation during absorption and regeneration processes.

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Unlike amine-based absorbents, the most AASs are natural, and they possess features of

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low toxicity and good biodegradability, which make the disposal of them and their related

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products simpler with less impact on the environment.14,15 Therefore, AASs are regarded

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as green and environmentally-friendly absorbents for CO2 capture.

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Lots attention have been paid on using AASs for CO2 capture in recent years.

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Muñoz et al.16 studied CO2 absorption with the potassium salts of threonine, proline,

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serine, arginine, ornithine, histidine, glycine, and taurine at normal temperature and

70

pressure. They concluded that these AASs have a comparable CO2 loading with MEA.

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Wei et al.17 investigated the CO2 absorption with potassium taurinate at high temperatures,

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and found that the overall mass transfer coefficients (KGa) of CO2 in potassium taurinate

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solutions are increased with increasing temperature up to 353 K. Aronu et al.18 studied

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the kinetic of CO2 absorption in aqueous KSAR solution. The kinetic rate constant of

4


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CO2 with KSAR is 30.7×103 m/kmol/s at 25 oC, which is about 30 times higher than it

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with MEA. Simons et al.19 also found that the reaction rate constants of KSAR with CO2

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is about 14360~50263 L/mol/s at 298K with absorbent concentration of 0.5~3.8 mol/L.

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He et al.20 conducted a screening test of amino acid salts for CO2 absorption at flue gas

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temperature in a membrane contactor. They found that potassium sarcosine (KSAR) can

80

be a better absorbent for high-temperature CO2 absorption compared with MEA and other

81

amino acid salts. Holst et al.21 investigated the kinetics of CO2 absorption in AASs

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solution at 298 K and found that KSAR and potassium proline exhibited high reaction

83

rate. These aforementioned researches indicated that AASs appear to be the promising

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alternative absorbents for chemical absorption due to their high CO2 capacity and better

85

affinity toward CO2 than amine-based absorbents.

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Commonly-used gas-liquid contactors for CO2 absorption include packed tower and

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spray tower. A major concern about these equipments is the huge equipment size because

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the mass transfer efficiency is restricted by local gravity and is hard to be improved to a

89

large extent. Rotating packed bed (RPB), as a high-efficiency process intensification

90

device, has been proven to be suitable for acid gas treatment due to its enhancement on

91

gas-liquid mass transfer process.22 By the strong centrifugal force created by highly

92

rotating packing, liquid is spilt or spread into tiny liquid elements (including films,

93

droplets and threads) when passing through the RPB, and these tiny elements can provide

94

huge gas-liquid contact area. Moreover, the fast coalescence and redispersion of liquid

95

elements as well as the collision between liquid and packing result in a high surface

96

renewable rate, and thus a high value of mass transfer coefficient.23 Therefore, the

97

equipment size of RPB can be greatly reduced as compared to conventional towers,

5


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which has been proven by both experimental works and simulation studies. 24-26

99

The objective of this work is to evaluate the feasibility of enhancing CO2 capture in

100

AASs solution by RPB, and KSAR was adopted as a representative absorbent in this

101

work due to its low volatility, thermal stability and high reaction rate with CO2 at high

102

temperature. Our work presents the results of CO2 capture performance from the

103

simulated flue gas from natural gas power plant which contains low CO2 content. CO2

104

capture performance was evaluated in terms of the CO2 capture efficiency and the overall

105

volumetric mass-transfer coefficient (KGa). The dependences of CO2 capture performance

106

on various operation conditions, such as absorbent concentration, CO2 loading (α),

107

rotation speed of RPB(N), gas-liquid ratio(G/L), reaction temperature (T) and CO2 inlet

108

concentration (yin), were investigated. A mass transfer model was also proposed to

109

describe the mass transfer process and the results can provide the theoretical basis for

110

design and application of RPB in the future.

111 112 113 114 115 116 117

2 REACTIONS OF CO2 ABSORPTION INTO KSAR SOLUTION Reaction between CO2 and KSAR can be described by zwitterion mechanism, and the following reactions occur during the absorption process: CO2 +2KSAR⇌KSARCOO - + KSARH + CO2 + OH - ⇌HCO3CO2 +2H2O⇌HCO3- + H3O +

(1) (2) (3)

118

According to the zwitterion mechanism,18,19 reaction (1) is considered as a

119

combination of reaction (4) (the formation of a zwitterion) and reaction (5) (the

120

deprotonation of the zwitterion by a base catalysis (B, such as KSAR, H2O, OH-)).

6


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122 123 124

Energy & Fuels

k2 ⇌ CO2 +CH3NHCH2COO CH3N + HCOO - CH2COO k -1 -

k CH3N + HCOO - CH2COO - +B ⇌BCH3NCOO - CH2COO - + BH +

(5)

Assuming a quasi-steady-state condition for the zwitterion and a pseudo-first-order regime of CO2, the overall forward reaction rate can be expressed by Eqs. (6) and (7).

rCO2  kov [CO 2 ]

125

126

(4)

kov 

kion [KSAR] 1 1  Z Z Z k2 kKSAR [KSAR]  kH2O [H 2 O]  kOH [OH - ]

(6) (7)

127

According to Aronu et al.18 the effect of OH- on the deprotonation of the zwitterion

128

can be negligible and the effect of ionic strength in the solution system needs to be

129

considered. Therefore, Eq. (7) can be simplified by Eq. (8), and the rate constants in Eq.

130

(8) are given by Eqs (9) to (12).

131

132

kov 

kion [KSAR] 1 1  Z k2 kKSAR [KSAR]  kHZ2O [H 2 O]

kion  exp[0.38I ]

(8)

(9)

133

k2  2.6198 109 exp[915.8 / T ]

(10)

134

Z kKSAR  6.3494 106 exp[1589.6 / T ]

(11)

135

kHZ2O  3.9805 108 exp[3924.4 / T ]

(12)

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2 where I is the ionic strength ( I  1/2  ci zi ).

137 138

3 EXPERIMENTAL SECTION 7


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3.1 Materials

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Sarcosine (purity≥98%) was purchased from Beijing HWRK Chem Co., LTD.,

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China. KOH (analytical grade) was obtained from Beijing Chemical Works. Aqueous

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KSAR solutions were prepared by the reaction of Sarcosine with equimolar amounts of

143

KOH. CO2 (purity≥99.9%) was purchased from Beijing Ruyuanruquan Technology Co.,

144

LTD., China. Concentrated H2SO4 (>95 wt%) was purchased from Beijing Chemical

145

Works to prepare dilute H2SO4 solution (∼1mol L−1). All chemicals were used without

146

further purification.

147 148

3.2 Experimental procedure

149 150

Figure 1. Schematic diagram of the absorption apparatus

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Experimental setup is presented in Figure 1. Stainless wire mesh with a wire

152

diameter of 0.25 mm was filled as packing of the RPB. The outer diameter, inner

153

diameter and axial height of packing were 15 cm, 5 cm and 5.3 cm, respectively. Mixed 8


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gas of CO2 and air was used to simulate the flue gas from natural gas power plant or

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boiler. The CO2 concentration in the mixture gas was ranged from 2% to 6%. Prior to

156

each experiment, the RPB was pre-heat and pre-wet through circulating KSAR solution.

157

When temperature of the outlet absorbent reached a steady value, the mixed gas was then

158

introduced into RPB. At the same time, a fresh lean absorbent was also pumped into RPB

159

to capture CO2. The liquid flow rate was varied at the range of 12~34 L/h and the gas

160

flow rate was fixed at 3.5 m3/h with a gas retention time in the packing of 0.9 s.

161

Absorbent temperature ranged from 313 to 353 K, and KSAR concentration was in the

162

range of 1~3 mol/L. All experiments were conducted under atmospheric pressure, and all

163

data were recorded when the system reached a steady state.

164

CO2 concentration in gaseous mixture was monitored by infrared CO2 analyzer

165

(GXH-3010F, Beijing Huayun Analytical Instrument Institution), and CO2 loading in

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solvent was analyzed by measuring the volume of released CO2 from solvent through

167

adding an excess amount of dilute H2SO4 solution.

168 169

4 RESULTS AND DISCUSSION

170

Overall volumetric mass-transfer coefficient (KGa) is an important parameter to

171

evaluate the mass-transfer performance of an equipment, and a deep understanding of

172

KGa can help engineers make an accurate design of the reactor and absorption process.

173

According to the two-film theory, the absorption rate of CO2 ( N CO2 a ) can be expressed

174

by Eq. (13) under a steady operating condition. And the mass balance equation (i.e. Eq.

175

(14)) can be deduced when considering a ring micro-size element of packing with an

176

axial height of Z and radial thickness of dr in the RPB. 9


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* N CO2 a  K G a  P ( yCO2  yCO ) 2

177

 yCO2 N CO2 a  2 rZ  dr  GI d   1  yCO 2 

178

(13)

  

(14)

179 180

* where P is total pressure; yCO2 and yCO are mole fraction and equilibrium mole fraction 2

181

of CO2 in the gas phase, respectively; GI is the inert gas molar flow rate.

182

Combining and rearranging Eqs. (13) and (14) yields the following equation:

184

 (15)   * In Eq.(15), yCO2 can be assumed to be zero due to the fast reaction rate between CO2

185

and KSAR. Therefore, KGa can be calculated by Eq.(16) through the integration of

186

Eq.(15).

183

187

KG a 

KG a 

 yCO2 GI 1 d  2 *  PZ (rout )  rin2 )  ( yCO2  yCO 2  1  yCO2

  y y (1  yout ) y GI  ln in   in  out 2 2  PZ (rout  rin )  yCO2 ,out (1  yin )  1  yin 1  yout

   

(16)

188

It should be noted that the calculated KGa by Eq.(16) represents the average values

189

of KGa in RPB because KGa varies along the radial path of the packing. Additionally, the

190

CO2 capture efficiency can be calculated by the following equations:

191

 y (1  yin )  Cpature efficiency  1  out   100%  yin (1  yout ) 

192 193

4.1 Effect of KSAR concentration on CO2 capture efficiency and KGa

10


(17)

Page 11 of 31

100

8

85

6

3

70 4 55 2

G/L=300 G/L=175

40 25

KGa (kmol/h/m /kPa)

194

Capture Efficiency (%)

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

Energy & Fuels

1.0

1.5

2.0

2.5

3.0

0

CKSAR(mol/L)

195

Figure 2. Effect of KSAR concentration on KGa and CO2 capture efficiency

196

(Conditions: G=3.5 m3/h, yin=4%, T=313K, N=1000 r/min)

197

Figure 2 shows the effect of KSAR concentration on KGa and CO2 capture

198

efficiency, and the values of KGa and CO2 capture efficiency increased with KSAR

199

concentration increasing. Increasing KSAR concentration can accelerate the reaction rate

200

between CO2 and KSAR according to Eq. (8), and inevitably improve the absorption

201

process of CO2. High concentration of absorbents can improve mass transfer process in

202

liquid phase according to two-film theory, and enhance the entire mass transfer between

203

gas and liquid. On the other hand, increasing KSAR concentration will also lead to an

204

increase in liquid viscosity, and then affect the diffusivities of CO2 and KSAR, which

205

hinders the liquid-phase mass transfer process. As a result, a rapid increase in CO2

206

capture efficiency was founded when KSAR concentration increased from 1mol/L to

207

1.5mol/L, while the increasing trend became gentle with a further increase in KSAR

208

concentration. In this work, 2mol/L and 3mol/L KSAR solutions were selected for the

209

reprehensive absorbents for the further investigation. 11


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4.2 Effect of rotation speed on CO2 capture efficiency and KGa

10

100

212

8

6 80 4 70

60

3 mol/L KSAR 2 mol/L KSAR 600

800

1000

1200

1400

3

90

KGa (kmol/h/m /kPa)

211


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2

0

N (r/min)

213

Figure 3. Effect of rotation speed on CO2 capture efficiency and KGa

214

(Condition: G=3.5 m3/h, L=20 L/h, T=313 K, yin=4%)

215

Figure 3 shows the effect of rotation speed of RPB on CO2 capture efficiency and

216

KGa. It can been found that both KGa and CO2 capture efficiency increased with

217

increasing rotation speed from 600 r/min to 1000 r/min, but the growing trend was

218

limited when rotation speed was above 1000 r/min. Increasing rotation speed from 600 to

219

1400 r/min can provide a centrifugal acceleration from 220 to 1200 m/s2, which greatly

220

accelerates the liquid flow in the packing and consequently enhances the turbulence on

221

the gas-liquid interface. Moreover, the liquid will be split into very tiny liquid elements

222

when passing through the high-speed shearing packing. Previous studies27,

223

observed that the form of tiny liquid elements in RPB includes films, droplets and threads.

224

Increasing rotation speed is beneficial to forming smaller size of these tiny liquid

225

elements, leading to larger gas-liquid contact area.29 These factors enhance the absorption

226

performance of CO2 into KSAR solution, and result in higher values of KGa and CO2 12


28

have

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227

capture efficiency. But the liquid retention time in RPB could also be reduced as rotation

228

speed increases. This is unfavorable to CO2 capture, and can partly offset the

229

enhancement on mass transfer process. As a result, the increase in KGa and capture

230

efficiency is limited at higher rotation speed.

231

233

4.3 Effect of gas-liquid ratio on CO2 capture efficiency and KGa

15

85

12

70

9

3mol/L KSAR 2mol/L KSAR 1mol/L KSAR

55

6

40

3

25

100

150

200

250

300

3

100

KGa/(kmol/h/m /kPa)

232


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Energy & Fuels

0

G/L

234

Figure 4. Effect of gas-liquid ratio on CO2 capture efficiency and KGa

235

(Condition: G=3.5 m3/h, N=1000 r/min, T=313 K, yin=4%)

236

Figure 4 presents the effect of the gas-liquid ratio on CO2 capture efficiency and

237

KGa. Both CO2 capture efficiency and KGa decreased with an increase in gas-liquid ratio,

238

and the trend became more obvious when 1 mol/L KSAR solution was used. In this work,

239

the gas flow rate was fixed at 3.5 m3/h with a gas retention time of 0.9 s and gas-liquid

240

ratio was adjusted by changing the liquid flow rate. Increasing gas-liquid ratio, i.e.

241

decreasing liquid flow rate, can increase the residence time of KSAR solution but reduce

242

the amount of reactant supply in RPB. The former allows the absorption process to 13


Energy & Fuels

243

proceed more completely, but the latter leads to a drop in value of effective gas-liquid

244

contact area (a), which has a negative effect on CO2 capture efficiency. From Figure 4,

245

the negative effect caused by increasing gas-liquid ratio was more significant, which

246

resulted in a drop in both CO2 capture efficiency and KGa.

247

But it also can be found that the CO2 capture efficiency and KGa still maintains an

248

acceptable level with the increasing of gas-liquid ratio when the absorbent concentration

249

is higher than 2 mol/L. Even the gas-liquid ratio enlarged to 300, higher than 85% capture

250

efficiency was obtained as shown in Figure 4. This is very important for CO2 capture

251

from natural gas power plant. Due to low CO2 content in the flue gas, the CO2 capture

252

process for natural gas power plant must be conducted at high gas-liquid ratio in order to

253

reduce the circulating amount of absorbent, thereby reducing the energy consumption of

254

capture process.

255

257

4.4 Effect of inlet CO2 concentration on KGa and CO2 capture efficiency

5

90

4

80

3

3

100

KGa (kmol/h/m /kPa)

256


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70

60

2


2

3

4

5

6

7

1

yin (%)

258

Figure 5. Effect of inlet CO2 concentration on KGa and CO2 capture efficiency

259

(Condition: G=3.5 m3/h, L=20 L/h, T=313 K, N=1000 r/min) 14


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Figure 5 shows the dependence of CO2 capture efficiency and KGa on CO2 inlet

261

concentration at different KSAR concentrations. Higher than 90% capture efficiency was

262

achieved by 3 mol/L KSAR solution when inlet CO2 concentration ranged from 2% to

263

6%, and the CO2 capture efficiency could reach more than 87% when 2 mol/L KSAR

264

solution was used. Moreover, when the inlet CO2 concentration was decreased, there was

265

a little enhanced effect on the CO2 capture efficiency and gas-liquid mass transfer.

266

According to two-film theory, decreasing CO2 inlet concentration can decrease the

267

gas-phase mass transfer driving force, which is unbeneficial to the mass transfer process

268

of CO2 from gas phase to liquid phase. But it is known that this chemical absorption

269

process of CO2 in RPB is mainly controlled by the liquid-phase mass transfer process

270

because of the high reaction rate between CO2 and KSAR according to previous

271

studies.30,31 Decreasing the CO2 inlet concentration will lead to more free KSAR existing

272

in the liquid bulk, which is beneficial to the CO2 reaction with KSAR molecular. Hence,

273

the CO2 diffusion in the liquid film becomes the limiting resistance for the CO2 removal.

274

From Figure 5, the KGa value can be obtained in a range of 2.9~5.6 kmol/h/m3/kPa at the

275

CO2 concentration of 2~6%. Zhou et al.30 had conducted the CO2-KSAR absorption

276

process in wetted wall column, which was widely regarded as an excellent reactor for the

277

gas liquid absorption process owing to the thin liquid film in the reactor wall that can

278

greatly decrease the mass transfer resistance. They found that the KGa of CO2-KSAR

279

system is about 2.2~2.8 kmol/h/m3/kPa at 313K with KSAR concentration of 2~4 mol/L

280

and CO2 concentration of 2~10%, which is much lower than that in RPB. This result fully

281

demonstrates that RPB has a strong ability to enhance mass transfer in liquid phase, even

282

at very low carbon dioxide concentration. In addition, decreasing CO2 inlet concentration

15


Energy & Fuels

283

supplies relatively more amount of free KSAR for per unit volume of CO2. Therefore,

284

both capture efficiency and KGa slightly increased when decreasing CO2 inlet

285

concentration.

286

288

4.5 Effect of temperature on CO2 capture efficiency and KGa

10

95

8

90

6

85

4

3

100

KGa (kmol/h/m /kPa)

287


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

Page 16 of 31


300

310

320

330

340

350

2

T/K

289 290

Figure 6. Effect of temperature on CO2 capture efficiency and KGa (Condition: G=3.5 m3/h, L=20 L/h, N=1000 r/min, yin=4%)

291

The effects of temperature on CO2 capture efficiency and KGa are shown in Figure 6.

292

It can be seen that both CO2 capture efficiency and KGa increased with elevating

293

temperature. Increasing temperature will simultaneously affect reaction process and

294

gas-liquid equilibrium. On the one hand, elevating temperature can accelerate the reaction

295

rate with CO2 and lower the viscosity of solution, thereby improving the liquid phase

296

mass transfer process and the absorption of CO2. On the other hand, elevating

297

temperature will lead to the drop of CO2 solubility in the aqueous solution, which is

298

unbeneficial to absorption process. In this work, it is found that elevating temperature

299

slightly improve the CO2 capture performance, which means the aforementioned positive 16


Page 17 of 31

300

effects for reaction are more effective. Our results are consistent with those by Sheng et

301

al.32, in which diethylenetriamine (DETA) was adopted as absorbent.

302

From Figure 6, CO2 capture efficiency reached higher than 90% even when the

303

temperature was heated up to 343 K. This is an encouraging result. Because there is

304

usually no flue gas desulfurization (FGD) process for natural gas power plant, this leads

305

to the high temperature of the flue gas without water washing. The additional cooling

306

systems and equipment are required to cool flue gases increases the cost, water and

307

energy consumption of CCS. Furthermore, for traditional amine-based absorbents, high

308

temperature means the large volatilization of absorbents, which will cause high loss of

309

absorbents and secondary pollution. Obviously, our new method has potential to be used

310

as high temperature CO2 capture process as compared to traditional amine-based method,

311

because of its excellent CO2 capture performance enhanced by RPB and low volatility of

312

AASs.

313

4.6 Effect of lean CO2 loading on CO2 capture efficiency and KGa

100

10

G/L=300 G/L=210

90

8

6

70

4

60

2

3

80

KGa (kmol/h/m /kPa)

314


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

Energy & Fuels

50

0.00

0.05

0.10

0.15

0.20

0.25

0

 (mol CO2/mol KSAR)

315 316

Figure 7. Effect of lean CO2 loading on KGa and CO2 capture efficiency (Condition: G=3.5 m3/h, yin=4%, T=313 K, N=1000 r/min) 17


Energy & Fuels 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 31

317

Figure 7 shows the effect of lean CO2 loading on KGa and CO2 capture efficiency.

318

As shown in Figure 7, both CO2 capture efficiency and KGa decreased with lean CO2

319

loading increased. Increasing lean CO2 loading means less amount of free KASR in

320

solution, thereby leading to a decrease in overall reaction rate between KASR and CO2.

321

From Figure 7, RPB achieved more than 93% CO2 capture efficiency when fresh KASR

322

solution was used. When CO2 loading was increased to 0.17 mol CO2/ mol KSAR, CO2

323

capture efficiency was still higher than 80% at relatively high gas-liquid ratio. Owing to

324

the very short contact time in RPB, these results indicate that the reaction rate between

325

CO2 and KSAR at a loading up to 0.17 is still fast enough to effectively capture CO2 in an

326

RPB.

327 328

4.7 Comparison between RPB and packed column

329

Table 1. Comparisons of specifications and CO2 capture performance between RPB and

330

Packed Column Packing type Packing height (mm) Diameter (mm) Packing volume (cm3) KSAR concentration G (m3/h) L (L/h) Inlet CO2 concentration (%) Temperature (K) Rotation speed (r/min) Capture efficiency (%) KGa (kmol/h/m3/kPa)

Packed Column θ-ring 1100 50 2160 2~3 mol/L 1~3.5 2.28~33.4 4% 313 -73.5~95.2% 0.2~2.1

RPB Stainless wire mesh 53 ID=50; OD=150 833 2~3 mol/L 3.5 11.4~33.4 2%~6% 303~343 600~1400 88.0~97.1% 3.3~5.8

331 332

Table 2. Experimental results by RPB and Packed Column under gas flow rate of 3.5

333

m3/h* 18


Page 19 of 31 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

Energy & Fuels

Packed Column RPB CO2 capture CO2 capture KGa KGa efficiency efficiency (kmol/h/m3/kPa) (kmol/h/m3/kPa) (%) (%) 2 mol/L KSAR solution 11.4 83.6 1.3 84.2 3.1 16.7 88.7 1.5 89.5 3.7 21.1 89.4 1.6 91.1 4.0 26.3 90.4 1.6 92.9 4.3 33.4 91.3 1.7 95.0 4.9 3 mol/L KSAR solution 11.7 89.9 1.6 93.9 4.6 16.7 93.5 1.9 95.7 5.2 21.7 94.0 2.0 96.6 5.6 26.7 94.5 2.0 97.1 5.8 33.3 95.2 2.1 97.1 5.8 *Conditions: T=313 K, yin=4%, Rotation speed of RPB was 1000 r/min liquid flow rate (L/h)

334 335

To compare the CO2 absorption performance between the RPB and the conventional

336

packed column, absorption experiments using KSAR as an absorbent were also

337

conducted in a lab-scale packed column fulfilling with ⌀7×7 θ-ring packing. The packing

338

volume in packed column was about 2.6 times of that in RPB. Comparison results are list

339

in Table 1 and Table 2, respectively. It can be seen that RPB with smaller size can

340

obtained better CO2 capture performance as compared to packed column under the

341

similar operation conditions. From Table 2, the capture efficiency by RPB was

342

comparable to that by packed column under the same gas flow rate of 3.5m3/h. This

343

means RPB can be used for larger flue gas treatment with a smaller device size due to its

344

intensification on mass transfer process.

345 346

5 MODEL DEVELOPMENT

347

Establishing an accurate model to calculate KGa is very important for predicting the

348

effect of operation parameters on the CO2 absorption process in RPB and assisting the

19


Energy & Fuels 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

349

Page 20 of 31

design of RPB.

350

The liquid movement in the packing of RPB is found to be very complex, but it is

351

widely accepted that the intensification of gas-liquid absorption is partly achieved by a

352

larger gas-liquid effective interfacial area. In RPB, low-viscosity liquid tends to be split

353

in to the form of droplets when the rotation speed is relatively high.33 According to visual

354

study by Burns and Ramshaw,27 the main form of liquid flow in RPB was droplet flow

355

when the rotation speed was approximately above 800 r/min, and high rotation speed is

356

more beneficial to the formation of liquid droplets. In this work, the rotation speed was

357

mainly maintained over 800 r/min. Thus liquid passing through the packing in the RPB is

358

assumed to be existed as the form of spheral droplets. Gas-liquid mass transfer process

359

takes place mainly at the interface of the spheral droplets. In order to analyze the mass

360

transfer process in the droplet, the following assumptions are proposed:

361 362 363 364 365 366 367 368 369

370

1. The droplets remain spherical when passing the packing of RPB, and the diameter of the droplet is a constant; 2. Liquid back mixing along the radial path of the packing is negligible, and thus plug flow is employed for both gas and liquid flows; 3. The internal motion of the droplet is ignored and only the radial mass transfer process of CO2 is considered; 4. Gas-liquid equilibrium state at the interface (i.e. the surface of droplets) obeys Henry’s law. The mass balance of CO2 in a spheral droplet can be expressed as

CCO2 t



DL   2 CCO2  R   kov CCO2  CCO2 ,e R 2 R  R 



20




(18)

Page 21 of 31 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

Energy & Fuels

371

Considering that the reaction between CO2 and KSAR is very fast, it is believed that

372

the equilibrium concentration of CO2 in the droplet is zero (CCO2,e=0) when lean CO2

373

loading is low.34 Then Eq.(18) can be simplified as:

CCO2

374

DL   2 CCO2  R   kovCCO2 t R 2 R  R  dCCO2 d  t  0 B.C. CCO2    C0 ; 0 dR R 0 2 

(19)

375

where CCO2,0, determined by (P0*yCO2)/(He), is the concentration of CO2 at the gas-liquid

376

interface; kov can be calculated by Eqs.(8) to (12).

377

The diameter of a droplet in the RPB can be estimated by35 1/2

378

379 380

   d  0.7284  2    r L 

The mean residence time of the droplet can be described as Eq.(21), and u in Eq.(21) is the average radial flow rate of the liquid flow and can be calculated by Eq.(22).36

t

381

382

383 384

387

(21) 0.5448

(22)

Liquid-phase mass transfer coefficient, kL, can be calculated by Eq.(23) when the distribution of CO2 concentration in droplets is known. t

0

386

rout  rin u

u  0.0217 L0.2279  2 r 

 385

(20)

kL 

DL

CCO2 R

dt R

d 2

tCCO2 ,0

(23)

Previous study has reported that gas-phase mass transfer coefficient, kG, of RPB is similar to that of conventional packed bed,37 and its value can be estimated by Eq.(24).38 21


Energy & Fuels

388

2.0 kG RT  2 ReG 0.7 ScG1/3  at d p  at DG

389

When the values of kL and kG are known, the KGa of RPB can be obtained by the

390

(24)

following equation:

1 1 He   K G a kG a k L a

391

(25)

392

The effective gas-liquid contact area (a) was determined by the operating condition

393

of the RPB. Based on previous study, 39 the values of a were measured to be 458 m2/m3 to

394

811 m2/m3 through chemical method using NaOH-CO2 system.

395

Density and viscosity of KSAR solution were taken from the data reported by Holst

396

et al.40,41 Diffusion coefficient and Henry’s law constant were estimated from the

397

references data.18, 19 ,41 Surface tension of KSAR solutions was obtained from He et al.20

398

The partial differential equation describing the diffusion of CO2 into liquid droplet

399

was solved with the aid of MATLAB software by using “pdepe” and “pdeval” functions.

400

The distribution of CO2 concentration in the liquid droplet is shown in Figure 8. It can be

401

seen that CO2 was quickly decreased in the region close to the surface of liquid droplet.

402

Obviously, most of CO2 is consumed in the liquid film close to the interface.

1.0 0.8 CCO2/CCO2,0

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 22 of 31

(a).

0.6 0.4 0.2 0.0 0.80

403 404

(b).

0.85

0.990

0.995

1.000

0.85

0.990

0.995

R/(d/2)

Figure 8. Distribution of CO2 concentration in liquid droplets 22


1.000

Page 23 of 31

405

(Condition: T=313 K, yin=4%, N=1000 r/min, G=3.5 m3/h, L=20 L/h (a) CKSAR=3 mol/L;

406

(b) CKSAR=2 mol/L)

407

Based on the distribution of CO2 concentration in the droplet, the kL can be

408

calculated by Eq. (21) to (23). Combining the kG result calculated by Eq. (24), the KGa

409

can be finally determined. A comparison of KGa between the experimental data and the

410

calculated values is plotted in Figure 9. It can be found that this model offers relatively

411

accurate predictions on average KGa in RPB, with a deviation within ±25% compared to

412

the experimental values. 8 7

Experimental KGa

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

Energy & Fuels

+25%

6 5 4

-25%

3 2 1 0

0

1

413 414

2

3 4 5 Predicted KGa

6

7

8

Figure 9. Comparison of predicted and experimental KGa

415

When KGa is known, the CO2 concentration at outlet of RPB can be obtained

416

through a reverse calculation of Eq.(15). The calculated results are plotted in Figure 10. It

417

can be seen that the predicted values agreed well with experimental data under the

418

conditions of high liquid flow rate and rotation speed. This is probably because the

419

assumption of all liquid elements existing in the form of spherical liquid droplets is only

420

valid when rotation speed and liquid flow rate is relatively high, which is also pointed out

421

by Yi et al.42 and Gao et al.43 23


Energy & Fuels

0.6

1.0 (a). 3 mol/L KSAR N =1000 r/min

0.4 0.2 0.0

(b).

0.6

2 mol/L KSAR N =1000 r/min

0.4 0.2 0.0

(c).

0.8 CO2 concentration at outlet (%)

CO2 concentration at outlet (%)

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 24 of 31

3 mol/L KSAR L = 20 L/h

0.6 0.4 0.2 0.0 0.8

(d).

0.6

2 mol/L KSAR L = 20 L/h

0.4 0.2

10

20

30

40

600

900

1200

1500

 r/min

L (L/h) Experimental Values

422

0.0

Predicted Values

423

Figure 10. Predicated values of CO2 concentration at outlet in RPB by the Model

424

(Condition: T=313 K, G=3.5 m3/h, yin=4%; (a) CKSAR=3 mol/L, N=1000 r/min; (b)

425

CKSAR=2 mol/L, N=1000 r/min; (c) CKSAR=3 mol/L, L=20 L/h; (d) CKSAR=2 mol/L, L=20

426

L/h)

427 428

6 CONCLUSION

429

In this work, rotating packed bed (RPB) reactor was employed to enhance the CO2

430

capture in KSAR solution from simulated flue gas of natural gas power plants with low

431

CO2 content. The effects of various operation conditions on KGa and CO2 capture

432

efficiency were explored. Results indicate that higher rotational speed and temperature

433

favor CO2 absorption in RPB, while higher gas-liquid ratio and CO2 loading in lean

434

solution are unfavorable to CO2 capture. Changing CO2 concentration in the range of

435

2%~6% has limited effect on capture performance.

436

Comparison results show that RPB with smaller packing volume can obtain higher 24


Page 25 of 31 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

Energy & Fuels

437

CO2 capture performance under similar operation conditions as compared to θ-ring

438

packed column. This means RPB can be used for larger flue gas treatment with a smaller

439

device size due to its intensification on mass transfer process.

440

A mathematical model was developed to describe the mass transfer process in RPB.

441

Calculated values of KGa well agreed with experimental data with a deviation within ±25%

442

and the tendency of the CO2 concentration at outlet of RPB can be well predicted under

443

high liquid flow rate and high rotation speed.

444

This work show that the method, combining both advantages of RPB and KSAR,

445

exhibits a good potential for CO2 capture from natural gas power plant with high flue gas

446

temperature and low CO2 content.

447 448

ACKNOWLEDGEMENTS

449

This work was financially supported by National Key R&D Program of China (No.

450

2017YFB0603300) and the National Natural Science Foundation of China (Nos.

451

21725601).

452 453

NOMENCLATURE

454

a

effective gas-liquid contact area, m2/m3

455

at

surface area of packing, m2/m3

456

ac

centrifugal acceleration, m/s2

457

CCO2

concentration of CO2 in a spheral droplet, kmol/m3

458

CCO2,e

equilibrium concentration of CO2 in a spheral droplet, kmol/m3

459

CCO2,0

concentration of CO2 at the gas–liquid interface, kmol/m3 25


Energy & Fuels 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

460

d

diameter of liquid droplet, m

461

dp

spherical equivalent diameter of packing, m

462

DG

diffusivity of CO2 in the reactive gas phase, m2/s

463

G

gas flow rate, m3/h

464

GI

465

He

Henry constant, Pa/m3/mol

466

I

ionic strength

467

k-1

reverse reaction rate constant, m3/kmol/s

468

k2

forward reaction rate constant, m3/kmol/s

469

kB

reaction rate constant by a base, m6/kmol2/s

470

kov

observed reaction rate constant, 1/s

471

kion

ionic strength correction factor

472

Z kKSAR

 k2 kKSAR / k1 , m6/kmol2/s

473

kHZ2O

 k2 kH2O / k1 , m6/kmol2/s

474

kL

liquid-side mass transfer coefficient, m/s

475

kG

gas-side mass transfer coefficient, mol/Pa/m2/s

476

K Ga

overall volumetric mass-transfer coefficient, kmol/m3/h/ kPa

477

L

liquid flow rate, L/h

478

N

rotating speed, r/min

479

NCO2

absorption rate of CO2 per unit volume, mol/m3/s

480

P

total pressure, Pa

481

QG

superficial mass velocity of gas phase, kg/s/m2

482

r

geometric average radius of the packing, m

inert gas molar flow rate, kmol/L

26


Page 26 of 31

Page 27 of 31 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

Energy & Fuels

483

rin

inner radius of the packing, m

484

rout

outer radius of the packing, m

485

R

radial coordinate of a spheral droplet, m

486

t

mean lifetime of the liquid droplet, s

487

T

temperature, K

488

u

average radial flow rate of the liquid flow, m/s

489

yCO2

mole fraction of CO2 in the gas phase

490

yCO2*

equilibrium mole fraction of CO2 in the gas phase

491

yin

inlet mole fraction of CO2

492

yout

outlet mole fraction of CO2

493

z

ion charge

494

Z

axial length of the packing, m

495

Greek symbols

496

ω

angular speed (=2πN/60), rad/min

497

η

CO2 capture efficiency

498

ρL

density of ionic liquid, kg/m3

499

υG

kinematic viscosity of gas, m2/s

500

σ

surface tension of ionic liquid, N/m

501

Dimensionless quantities

502

ReG

gas Reynolds number, QG/atυG

503

ScG

gas Schmidt number, υG/DG

504 505

REFERENCES

27


Energy & Fuels 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

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[2] Xu, Y.; Ramanathan, V. Proc Natl Acad Sci: USA. 2017, 114, 10315-10323.

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[3] Haszeldine, R. S. Science 2009, 325, 1647-1652.

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