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
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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
15
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
18
CO2 under a specific range of CO2 partial pressure.
19
Keywords:
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Carbon dioxide; Piperazine; Diethanolamine; vapor-liquid equilibrium; Electrolyte NRTL model
21
*
M AN U
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AC C
22
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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
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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
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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
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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
75
cyclic diamine, which has fast rate of reaction with CO2, higher CO2 capture capacity, and
76
resistance to degradation [6-10].
77
Literature reports about the effect of addition of PZ on the rate of CO2 absorption when blended
78
with amines like AMP, MDEA, TIPA, MEA, AEP, DAB etc. [11-15]. However, a limited
79
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
81
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
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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
87
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
89
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,
91
the volatilities of amines are undertaken into account. A thermodynamic framework is used to
92
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
94
solutions is collected over a CO2 partial pressure of 0.01-100 kPa, at 40-80 ◦C.
M AN U
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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
112
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
117
the aforementioned setup. For a typical run, an aliquot sample of 150 ml of the amine solution was put into the
119
reactor for ten minutes at a predefined temperature. Gas mixture of N2 and CO2, were mixed in a
120
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
123
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
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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
129
NaOH for 3 hours at 70 °C and atmospheric pressure. White, fine crystals of BaCO3 were then
130
formed and were allowed to settle. Later, which were separated from the clear liquid by
131
filtration. In order to remove the traces of NaOH from the crystals, they were washed with
132
distilled water. The crystals in water were then titrated with 1.0 N HCl using a PC controlled
133
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
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138
α=
(1)
139 Where,
α : CO2 loading in moles of CO2 per mole alkalinity
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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
152
system to describe the equilibrium behavior of the solution.
153
3.1 Chemical and phase equilibrium
154
EP
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AC C
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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:
ACCEPTED MANUSCRIPT
%$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:
192
[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 ][
196
Total mole fraction:
197
∑Sab a; xS = 1.0
(13)
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195
(14)
198 CO2 mole balance:
200
* * ( * αcd [total amine]YZm = [CO ][ + [HCO* ' ][ + [CO' ][ + [PZCOO ][ + [H PZCOO ][ +
201
2[PZ(COO* ) ][ + [DEACOO* ][
(15)
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SC
199
203
Where αcd is CO2 loading (mol CO2 /mol alkalinity). Using equation (16), the concentration of
204
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]
ACCEPTED MANUSCRIPT
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].
[@
n
=
[@,
n
+
[@,W
n
AC C
240
EP
239
%
+
[@,Z
(20)
n
%
241
,HN
242
Debye–Hückel parameter Aϕ and ionic strength of solvent Ix, are given by equations (22) and
243
(23), respectively.
244
n
A =
= − ∑m x m
