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Accepted Manuscript Effect of piperazine on solubility of carbon dioxide using aqueous diethanolamnie Lubna Ghalib, Brahim Si Ali, Wan Mohd Ashri, Shaukat Mazari PII:

S0378-3812(15)30298-3

DOI:

10.1016/j.fluid.2015.12.056

Reference:

FLUID 10937

To appear in:

Fluid Phase Equilibria

Received Date: 7 July 2015 Revised Date:

11 December 2015

Accepted Date: 30 December 2015

Please cite this article as: L. Ghalib, B.S. Ali W.M. Ashri, S. Mazari, Effect of piperazine on solubility of carbon dioxide using aqueous diethanolamnie, Fluid Phase Equilibria (2016), doi: 10.1016/ j.fluid.2015.12.056. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Effect of piperazine on solubility of carbon dioxide using aqueous diethanolamnie

2

Lubna Ghalib1,2, Brahim Si Ali1,*, Wan Mohd Ashri1, Shaukat Mazari1 1

3 4 5

RI PT

1

Department of Chemical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia 2 Department of Material Engineering, University of Al-Mustansiriyah, 10052 Baghdad, Iraq

Abstract

Experimental results of vapor-liquid equilibrium of this study for CO2 capture in

7

solutions of activated diethanolamine (DEA) are presented at temperatures ranging from 313.15

8

to 353.15 K. Piperazine (PZ), which is used as an activator in this study, its concentration ranged

9

from 0.01 to 0.1 M. Total alkalinity of the solution was kept 2 M. The partial pressures of CO2

10

varied from 0.01 to 100 kPa. A thermodynamic model is developed to predict the vapor liquid

11

equilibrium of CO2 in aqueous mixtures of DEA/PZ. For CO2+H2O+DEA+PZ system, the e-

12

NRTL model is used to develop the VLE model, which defines equilibrium behavior of the

13

solution. Model is validated using data generated from this work as well as data available in

14

literature. Results of the current model are in an acceptable degree of agreement with the

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experimental data of CO2 solubility of this work as well as of those stated in literature. The

16

species concentration, activity coefficients, pH of the CO2 loaded solutions, and the model

17

predicts amine volatility. Addition of PZ to DEA, as an activator, has increased the solubility of

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CO2 under a specific range of CO2 partial pressure.

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Keywords:

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Carbon dioxide; Piperazine; Diethanolamine; vapor-liquid equilibrium; Electrolyte NRTL model

21

*

M AN U

TE D

EP

AC C

22

SC

6

To whom correspondence should be addressed. E-mail: [email protected]

24

D

dielectric constant

25

ds

density of mixed solvent (kg /m3)

26

e

electron charge (1.60219 ×10-19) (C)

27

Gex

excess Gibbs Energy

28



Henry’s coefficient of CO2 (kPa)

29

K

equilibrium constant

30

k

Boltzman constant (1.38065 ×10-23)(J/K)

31

M

molarity (kmol /m3)

32

NA

Avogadro’s number (6.02205 ×1023) (mol-1)

33

N

number of moles

34

Pt

total pressure (kPa)

35



saturation pressure (kPa)

36

PCO2

equilibrium CO2 partial pressure (kPa)

37

R

gas constant (8.314) (J /mol.K)

38

r

Born radius (m)

39

T

Solution temperature (K)

40



partial molar volume (m3/kmole)

41



42

y

43

zi

charge number of ion i

44



mole loading, (mole CO2 / mole alkalinity)

45



activity coefficient

SC

Nomenclature

AC C

EP

TE D

M AN U

23

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mole fraction in liquid phase mole fraction in vapour phase

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closest approach parameter

47

∅

vapor phase fugacity coefficient

48

Subscripts and superscripts

49



infinite dilution

50

ɑ ,  ́

anion

51

c ,  ́

cation

52

i,j,k

species index

53

m

molecule

54

s

solvent

55

w

water

SC M AN U

56 57

AC C

EP

TE D

58 59

RI PT

46

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1. Introduction Importance of removal of acid gases like carbon dioxide (CO2) and hydrogen sulfide

62

(H2S) using amines cannot be denied [1]. The maturity of amine solvents surrounds

63

monoethanolamine (MEA), methyldiethanolamine (MDEA) and diethanolamine (DEA) for acid

64

gas absorption. MEA solutions are known for their high reactivity with CO2, low hydrocarbons

65

solubility, low cost and ease in reclamation [2]. Stability of carbamate plays a vital role in CO2

66

solubility using amines. Due to the formation of stable carbamates, CO2 loading for primary and

67

secondary amines is about 0.5 mole of CO2 /mole of amine [3, 4]. Tertiary amines, like MDEA

68

has higher CO2 loading capacity (about 1.0 mole of CO2 /mole of amine) and lower enthalpy of

69

reaction. MDEA is also attractive due to lower regeneration energy. However, the slow rate of

70

reaction of MDEA with CO2 limits its usage.

M AN U

SC

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61

71

Solutions of two or more amines improve the CO2 absorption rate and may reduce the solvent

73

regeneration energy when blended together because of combined properties of their constituent

74

amines [5]. One of the frequently investigated solvent at present is piperazine. Piperazine is a

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cyclic diamine, which has fast rate of reaction with CO2, higher CO2 capture capacity, and

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resistance to degradation [6-10].

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Literature reports about the effect of addition of PZ on the rate of CO2 absorption when blended

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with amines like AMP, MDEA, TIPA, MEA, AEP, DAB etc. [11-15]. However, a limited

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literature is available in public domain on CO2 solubility in aqueous mixtures of DEA and PZ.

80

Mondal [16] investigated the solubility of CO2 in aqueous solutions of PZ and DEA. The molar

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ratio of PZ ranged from 0.01 to 0.2 in total amine concentration of 1-4 mol/l and CO2 partial

82

pressure was up to 20.265 kPa. However, Mondal did not take into account the presence of

AC C

EP

TE D

72

ACCEPTED MANUSCRIPT

83

carmabate species in the liquid phase, such as diethanolamine carbamate (DEACOO-),

84

piperazine dicarbamate PZ(COO-)2 and protonated piperazine carbamate (H+PZCOO-). Even

85

though, formation of carbamates species is reported for PZ and DEA in other studies [17-19]. In this work, effect of addition of PZ on solubility of CO2 using aqueous DEA is

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investigated. Concentration and temperature of solutions are varied. An additional, CO2

88

solubility data is also provided to extend the existing experimental database of the mixture. The

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data includes not only equilibrium of CO2 loading, but also corresponds information on the pH of

90

the loaded solvents. In order to estimate the loss of solvent during CO2 absorption and stripping,

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the volatilities of amines are undertaken into account. A thermodynamic framework is used to

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model amine solution VLE with minimal experimental effort to investigate the feasibility of the

93

electrolyte nonrandom two-liquid (e-NRTL) model. The data of CO2 loading in aqueous

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solutions is collected over a CO2 partial pressure of 0.01-100 kPa, at 40-80 ◦C.

M AN U

SC

RI PT

86

TE D

95 2. Experimental

97

2.1 Chemicals and sample preparation

98

Following chemicals were used in this study.

99

Table 1

List of chemicals used in this study.

AC C

100

EP

96

Chemical

Molecular weight

Purity (%)/Concentration (N)

Supplier

Piperazine flakes

86.14

99 %

Across

Diethanolamine

105.14

99.5 %

Merck Millipore

sodium hydroxide

40

1.0 N

J.T.Baker® Chemicals

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Hydrochloric Acid

37.5

Barium chloride 208.23 dihydrate

1.0 N

J.T.Baker® Chemicals

99%

Merck Millipore

RI PT

101

Required amounts of both amines were dissolved in degassed and deionized water in a

103

volumetric flask, and water was poured until the required volume was achieved. Amine

104

concentration was verified titrating a fixed amount of amine solution with 1 N HCl. Throughout

105

the investigation, oxygen free nitrogen and high purity CO2 (99.8%) were used as the reaction

106

gases.

107 108 109

Fig. 1. Schematic diagram of the experimental setup for CO2 absorption

110

2.2 Apparatus and experimental procedure

AC C

111

EP

TE D

M AN U

SC

102

The Fig. 1 is the schematic diagram of the experimental setup used for this study. The

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system has a double-jacketed reactor, stirred cell resistant glass reactor with a volume of 250 ml.

113

A pressure transducer, a thermocouple, a magnetic stirrer, and a pH meter were fitted with the

114

reactor, which were linked to a data acquisition system. A water saturator at a constant

ACCEPTED MANUSCRIPT

115

temperature using a water bath was connected with the system in order to keep the temperature

116

uniform. The reaction temperature was adjustable by changing the temperature of the bath using

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the aforementioned setup. For a typical run, an aliquot sample of 150 ml of the amine solution was put into the

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reactor for ten minutes at a predefined temperature. Gas mixture of N2 and CO2, were mixed in a

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desired proportion, and were fed into the reactor through a sparger using Brooks mass flow

121

controller. Prior feeding the reaction gas into the reactor, it was passed through a water saturator

122

placed in the water bath. This was to ensure that the reaction gas is saturated and is at the

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reaction temperature, in order to avoid any change in the concentration of the solution. Change in

124

pH of the solution during reaction was observed. On achieving the equilibrium observed through

125

pH, the flow of the gas was stopped.

126

M AN U

SC

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118

The CO2 loading (moles of CO2/mole alkalinity), was verified by titration. A 5 ml

128

(Vsample), of the loaded amine solution was reacted with excess volumes of 0.5 N BaCl2 and 0.5 N

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NaOH for 3 hours at 70 °C and atmospheric pressure. White, fine crystals of BaCO3 were then

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formed and were allowed to settle. Later, which were separated from the clear liquid by

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filtration. In order to remove the traces of NaOH from the crystals, they were washed with

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distilled water. The crystals in water were then titrated with 1.0 N HCl using a PC controlled

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metrohm 716 DMS titrino auto titrator. The set method for titration was DET (Dynamic

134

Equivalence point Titration) for the determination of the endpoint. Thus, the key for the

135

determination of concentration of solution was the volume of HCl consumed to neutralize the

136

crystals. Equation (1) was used to determine CO2 loading of carbonated solutions.

137

AC C

EP

TE D

127

ACCEPTED MANUSCRIPT

138

α=



(1)

 

139 Where,

α : CO2 loading in moles of CO2 per mole alkalinity

RI PT

140 141

VHCl : Volume of HCl in ml needed to neutralize the basic species

142

M: Alkalinity of amine solution in moles per liter

SC

143

To determine the concentration of amine solution at the end of each experiment, an aliquot

145

sample was taken and titrated with a solution of 1 N HCl. In, most of the cases variation between

146

initial and final amine concentration was less than 5%.

147

3. Thermodynamic modeling

Various thermodynamic models are available for the estimation of acid gas solubility in

149

different solvents. Using an appropriate model, interpolation and more importantly, extrapolation

150

of experimental data to other regions, where data are not available with a good degree of

151

accuracy. The e-NRTL model was selected to build the VLE model for CO2+H2O+DEA+PZ

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system to describe the equilibrium behavior of the solution.

153

3.1 Chemical and phase equilibrium

154

EP

TE D

148

AC C

M AN U

144

Dash et al. [20] and Austgen et al. [21] has discussed the chemical and phase equilibria

155

models development for the systems CO2+PZ+H2O and CO2+DEA+H2O respectively. In the

156

present study, in order to develop the chemical and phase equilibrium model for the system

157

CO2+DEA+PZ+H2O, following reactions in the aqueous phase were taken into account.

158 159

R1: Dissociation of water:

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%$2H O ↔ H' O( + OH * 160

R2: Formation of bicarbonate ions: $

161

R3: Dissociation of bicarbonate ions:

162

* ( HCO* ' + H O ↔ CO' + H' O

163

R4: Dissociation of protonated piperazine:

RI PT

CO + 2H O ↔ HCO' * + H' O(

SC

$,

$/

PZH ( + H O ↔ PZ + H' O( R5: Formation of piperazine carbamate:

165

PZ + CO + H O ↔ PZCOO* + H' O(

166

R6: Dissociation of zwitterion (protonated carbamate):

167

H ( PZCOO* + H O ↔ PZCOO* + H' O(

168

R7: Formation of piperazine dicarbamate:

169

PZCOO* + CO + H O ↔ PZ(COO*) + H' O(

170

R8: Dissociation of protonated diethanolamine:

$0

TE D

$1

$2

$8

M AN U

164

R9: Diethanolamine carbamate reversion to bicarbonate

AC C

171

EP

DEAH ( + H O ↔ DEA + H' O(

$9

DEACOO* + H O ↔ DEA + HCO' * 172 173

The equilibrium constant for the above equations are expressed as:

174 175

K; =

? @ >? ABγ>C @>C D , , B= > @ > D



(2)

ACCEPTED MANUSCRIPT

K =

177

K' =

C @>C A? @ >? A , , , ,

(3)

E=>C @>C F? @ >? A , , , ,

(4)



B=> @> DB= > @ > D

C @>C AB=> @ > D ,

,

RI PT

176

(=HI @HI )? @ >? A , , DB= @ ? ?  > HI HI  @  > D

178

K G = B=

179

KJ =

180

K K = B=

181

K L = (=

182

K M = B=

183

K Q = (=

(5)

(=HI>>C @HI>>C )? @ >? A , , (=HI @HI )B=> @> DB= > @ > D

SC

(6)

M AN U

(=HI>>C @HI>>C )? @ >? A , , ? HI>>C @? HI>>C DB= >@ > D

(7)

B=HI(>>C ) @HI(>>C) D? @ >? A , , HI>>C @HI>>C )B=> @> DB= > @ > D

(8)

(=NOP @NOP )? @ >? A , , DB= @ ? ?  > NOP NOP  @  > D

(9)

(=NOP @NOP )C @>C A ,

,

(10)

TE D

NOP>>C @NOP>>C )B= > @ > D

184

Where, xi, and γS are mole fraction and activity coefficients of the specie i respectively.

186

In addition to the above equations, the following set of conditions are also satisfied.

EP

185

187

DEA mole balance:

189

[DEA]VWSXSYZ = [DEA][ + [DEAH ( ][ + [DEACOO* ][

190

AC C

188

191

PZ mole balance:

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[PZ]SWSXSYZ = [PZ][ + [PZH ( ][ + [PZCOO*][ + [H ( PZCOO*][ + [PZ(COO*) ][

193

(11)

(12)

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194

Electroneutrality balance: [DEAH ( ][ + [PZH ( ][ + [H' O( ][ = * * * * * [HCO* ' ][ + [OH ][ + 2[CO' ][ + [PZCOO ][ + 2[PZ(COO ) ][ + [DEACOO ][

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Total mole fraction:

197

∑Sab a; xS = 1.0

(13)

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195

(14)

198 CO2 mole balance:

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* * ( * αcd [total amine]YZm = [CO ][ + [HCO* ' ][ + [CO' ][ + [PZCOO ][ + [H PZCOO ][ +

201

2[PZ(COO* ) ][ + [DEACOO* ][

(15)

M AN U

202

SC

199

203

Where αcd is CO2 loading (mol CO2 /mol alkalinity). Using equation (16), the concentration of

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CO2 in the liquid phase is calculated by using Henry’s law.

206

Pcd = Hcd [CO ][

207

TE D

205

(16)

Table 2 depicts the temperature dependence of the equilibrium constants based on the mole-

209

fraction and their sources using equation (17).

EP

208

AC C

210

c

211

ln K = C; +

212

Table 2

213

Coefficients of chemical equilibrium constants used in e-NRTL model.

K1

n

+ C' ln T + CG T

(17)

C1

C2

C3

132.9

-13445.9 -22.48

C4

T(K)

Reference

0.0

273-498

[22]

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231.47

-12092.1 -36.78

0.0

273-498

[22]

K3

216.05

-12431.7 -35.48

0.0

273-498

[22]

K4

-9.642

-5008.4

0.0

0.0

270-350

[23]

K5

466.5

1614.5

-97.54

0.2471

273-343

[20]

K6

6.82

-6066.9

-2.29

0.0036

273-343

[20]

K7

-11.56

1769.4

-1.47

0.0024

373-343

[20]

K8

-13.34

-4218.71 0.0

0.009872

313-353

[21]

K9

16.5027

-4068.76 -1.5027

0.0

313-353

[21]

SC

214

RI PT

K2

Water, PZ and DEA are assumed as solvents making a mixed solvent system. The

216

assumption for the standard state for each solvent is the pure liquid under defined system

217

conditions. However, for the ionic solutes and molecular solute (CO2) the assumed reference

218

state is the ideal solution.

M AN U

215

Table 3 Henry’s law constants a *p  q

170.72

b

c

d

Source

-8477.71

-21.96

0.005781

[24]

EP

220 221

TE D

219

222 223

AC C

ln  =  + r +  ln s + t s : where T is in K and H is in Pa

3.2 Vapor phase model

224

Under the established phase equilibria, the fugacity of each constituent in the liquid and

225

vapor phases are identical. Under equilibrium conditions, the CO2 molecules distribution take

ACCEPTED MANUSCRIPT

226

place between the liquid and vapor phase as per Eq. (18). However, water, PZ, and DEA

227

distribute themselves according to activity coefficient approach following the Eq. (19) [25].

228 ∗ ∅uv wuv  = uv  y exp E  uv uv

230

∅ w  =   ° ∅° exp E

} B~*~ ° D {|  

€r

F

RI PT

229

(18)

 B~*~° D

F

(19)

SC

€r

The coefficients for Henry’s constant for CO2 in water are taken from Austgen [24] and

232

are presented in Table 3 The Soave–Redlich-Kwong (SRK) equation of state is used to calculate

233

the vapor phase fugacity coefficients for equations (18) and (19) [26] and the liquid phase

234

activity coefficients were calculated using the e-NRTL model.

M AN U

231

235

3.3 Electrolyte NRTL activity coefficient model

TE D

236 237 238

The e-NRTL equation used in the present study to calculate the excess Gibbs energy is presented in equation (20) [21].

‚ [@

ƒn

=

‚ [@,…†‡

ƒn

+

‚ [@,ˆ‰ŠW

ƒn

AC C

240

EP

239

%

+

‚ [@,Z‹

(20)

ƒn

%

241

‚ Œ,HN

242

Debye–Hückel parameter Aϕ and ionic strength of solvent Ix, are given by equations (22) and

243

(23), respectively.

244

ƒn

A” =

= − ∑m x m