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Effect of heat stable salt bis-(2-hydroxyethyl) methylammonium formate on CO2 absorption in aqueous methyldiethanolamine solution
Accepted Manuscript Effect of heat stable salt bis-(2-hydroxyethyl) methylammonium formate on CO2 absorption in aqueous methyldiethanolamine solution
Shaghayegh Alborzi, Farzaneh Feyzi PII: DOI: Reference:
S0167-7322(18)36657-1 https://doi.org/10.1016/j.molliq.2019.02.089 MOLLIQ 10490
To appear in:
Journal of Molecular Liquids
Received date: Revised date: Accepted date:
22 December 2018 16 February 2019 19 February 2019
Please cite this article as: S. Alborzi and F. Feyzi, Effect of heat stable salt bis(2-hydroxyethyl) methylammonium formate on CO2 absorption in aqueous methyldiethanolamine solution, Journal of Molecular Liquids, https://doi.org/10.1016/ j.molliq.2019.02.089
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ACCEPTED MANUSCRIPT Effect of heat stable salt bis-(2-hydroxyethyl) methylammonium formate on CO2 absorption in aqueous methyldiethanolamine solution
Shaghayegh Alborzi, Farzaneh Feyzi
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Thermodynamics Research Laboratory, School of Chemical Engineering,
CR
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Iran University of Science and Technology, Tehran 16846-13114, Iran
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Abstract
In this work, the solubility of CO2 in aqueous methyldiethanolamine (MDEA) + bis-(2-
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hydroxyethyl) methylammonium formate (BHEMAF) system was measured using the static method at two temperatures (298.15 and 313.15 K) in the pressure range of 100-2100 kPa. Two
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sets of experiments were performed. In the first set, the weight percentage of BHEMAF, MDEA and H2O in H2O+MDEA+BHEMAF solutions were 7.5-10 wt.%, 15-39.38 wt.% and 53.12-75
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wt.%, respectively. In the second set of experiments concentration of BHEMAF was set as 0.3 and 0.6 M in the aqueous solutions of 25 wt.% MDEA. Experimental data showed that the solubility
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of CO2 in the H2O+MDEA+BHEMAF solutions decreases with increasing BHEMAF concentration. The Deshmukh-Mather thermodynamic model was then used to calculate the
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solubility of CO2 in H2O+MDEA+BHEMAF solutions. The binary interaction parameters of the model were adjusted by regression to experimental data. The modeling results showed that the
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average absolute relative percent deviation for all the data points was 7.87%.
Keywords: CO2 capture, Heat stable salt, Deshmukh-Mather model
Corresponding author email: [email protected] 1
ACCEPTED MANUSCRIPT
1. Introduction Over the past decades, the concentration of greenhouse gases in atmosphere is substantially increased [1]. Reports suggest that CO2 concentration, as the main greenhouse gas, will reach the range of 660-25507 ppm by 2030. Increased CO2 has numerous destructive effects on the
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environment including sea level rise and global warming [2]. Carbon capture and storage (CCS)
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is an efficient technology for reducing CO2 emissions [3]. One of the most important methods for
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CO2 capture is the absorption of CO2 in aqueous amine solutions. Among the alkanolamines the one that is most widely used for separation of acid gases is Methyldiethanolamine (MDEA) [4-
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10]. MDEA is a tertiary amine which in comparison to primary and secondary amines has a lower heat of reaction and vapor pressure. This is the reason why MDEA loss through vaporization is much lower. Also, due to the low heat of reaction with acidic gases, the energy consumed for
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solvent regeneration is very low [4]. Amines are degraded during the absorption process by two major methods of oxidation and thermal degradation and products such as carboxylic acids and
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heat stable salts are formed. Heat stable salts cause some costly operational problems including
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corrosion and foaming [11, 12]. The heat stable salt bis-(2-hydroxyethyl) methylammonium formate (BHEMAF) is one of the major products of MDEA degradation. It is an ionic liquid
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formed from the reaction of MDEA with formic acid which in turn is the product of MDEA oxidation and thermal degradation [13-16]. Chemical structure of BHEMAF is shown in Fig. 1.
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The reason for our choice was high concentrations of formate and high formation of this heat stable salt. Properties such as density and molecular weight of BHEMAF were also measured in this
AC
work.
So far, some research has been done on CO2 absorption with an amine and various ionic liquid mixtures [17-19]. Also, since viscosity in an important property in process design, effect of the presence of ionic liquids on viscosity of amine solutions are investigated and discussed [20-22]. However, the solubility of CO2 in aqueous MDEA including the ionic liquid formed during gas sweeting process has not yet been studied. Solubility of CO2 in pure BHEMAF is investigated by Huang et al. [23], however, to the best of our knowledge; CO2 solubility data for aqueous BHEMAF+MDEA solutions is not available. This issue is the subject of this study. For this
2
ACCEPTED MANUSCRIPT purpose, the solubility of CO2 in the H2O+MDEA+BHEMAF solutions was measured by static
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method. Then, thermodynamic modeling was carried out using the Deshmukh-Mather model.
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Fig. 1. Chemical structure of BHEMAF.
2. Experimental
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2.1. Material
Sources and purity of materials used in this work are presented in Table 1. The deionized water
M
was used to prepare the solutions.
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Table 1. Materials used in this work. Mass fraction Purity >99% >99% >99%
Source Hamta gas Institute of Chemistry and Chemical Engineering of Iran Merck
CE
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Material CO2 BHEMAF MDEA
2.2. Density measurement
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The density of BHEMAF was measured with density meter (model Anton paar DMA-5000) at three temperatures of 313.15, 323.15 and 333.15 K with accuracy 0.01 K. The results are presented in Table 2.
Table 2. Density of BHEMAF. T /K
ρ/ (gr .cm-3)
specific gravitya
313.15
1.18417
1.19344
323.15
1.17757
1.19184
3
ACCEPTED MANUSCRIPT 333.15 a
1.17081
Specific gravity =
1.19083
HHEMEF . Standard uncertainty of density is u (ρ)=0.000007 gr.cm-3 water
2.3. Apparatus and equipment The apparatus used in this work for measuring solubility of CO2 is schematically shown in Fig. 2.
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A cylindrical stainless steel autoclave with an internal diameter of 3 cm, a height of 6 cm and a
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volume of 42 cm3 is the main part of the apparatus. A magnetic stirrer was used to mix the liquid and vapor phases in the autoclave. A circulating water bath (model LAUDA ALPHA RA8) with
CR
temperature accuracy of 0.1 K was used to set the autoclave temperature. The temperature in the double-wall autoclave was controlled by the flow of water inside the wall. A stainless steel gas
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container is also installed. The temperature of the gas container was measured by a thermometer (Chromel- Alumel Type K) with accuracy of 0.1 K. Two pressure sensors (model BD Sensor)
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with accuracy of 1 kPa were used to measure the gas pressure inside the autoclave and gas container. The pressure and temperature transmitters were connected to a personal computer. A
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number of valves were used to control the gas flow and a vacuum pump was installed to evacuate
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the apparatus before each run. The apparatus is the same as used in [24].
4
ACCEPTED MANUSCRIPT Fig 2. Schematic diagram of the experimental apparatus. (V1-V8) valves; (PS1 and PS2) pressure sensors; (TS)
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thermometer.
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As seen in Fig. 2, the experimental apparatus consists of three sections A, B and C which their volumes were obtained by volumetric method. These sections are connected with 0.25-inch
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stainless steel pipes.
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2.4. Experimental procedure
Initially, all the three sections A, B and C were evacuated. Then, a certain amount of solution was
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injected into the autoclave and the autoclave temperature was set by the water bath. After this step, CO2 gas was injected from the gas cylinder into sections A and B. Then, valve (V5) between
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sections B and C was opened to inject CO2 into the autoclave. The number of moles of injected
initial Pcofinal V A+B Pco2 2 initial initial final final R T co2 Z T co2 Z
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(1)
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n coinjected 2
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CO2 into the autoclave was calculated by Eq. (1).
In Eq. (1), V, T and P are the volume, temperature and pressure of sections (A + B), respectively,
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and n is the number of moles of CO2. Superscripts initial and final refer to before and after CO2 injection into the autoclave. Z is the compressibility factor and was calculated by Peng-Robinson (PR) equation of state (EOS) [25] and R in the gas constant. By injection of CO2 into the autoclave, the pressure initially increases and then decreases due to absorption of CO2 in the solution. This decreasing trend continues until the system reaches equilibrium and pressure remains constant. In our experiments, according to the amount of CO2 injected and the MDEA concentration in the solution, the system took 8 to 16 h to reach equilibrium. Eq. (2) was used to calculate the partial pressure of CO2. 5
ACCEPTED MANUSCRIPT
sat PCo2 Ptotal Psolution
T
(2)
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sat In the above equation PCo2 , Ptotal and Psolution are the pressure of CO2 in the autoclave, total pressure
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in sections (A+B) and the saturation pressure of the solution at the measured temperature, v ) and the gas volume in the respectively. The moles of CO2 remained in the gas phase (nCO 2
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v PCo2V Co 2
Z PCo2 ,T RT
M
n
v Co2
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autoclave were obtained from Eqs. (3) and (4), respectively.
(4)
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v V CO V t V solution 2
(3)
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v In Eq. (4), V CO , Vsolution and Vt are the gas volume in the autoclave, the volume of solution in the 2
autoclave and the total volume of autoclave, respectively. The number of moles of absorbed CO2
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l in the solution (nCO 2 ) is then obtained from the Eq. (5). v ncol 2 ncoinjected nco 2 2
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(5)
3. Thermodynamic framework In the process of CO2 absorption in H2O+MDEA+BHEMAF solution, gas and liquid phases are present. First, the CO2 molecules are physically absorbed and transferred in the liquid phase, and then, CO2 reacts in the solution (chemical absorption). The chemical reactions cause the concentration of components to change in the liquid phase. These changes continue until the system becomes stable and gas-liquid phase and reaction equilibrium is established. 6
ACCEPTED MANUSCRIPT 3.1. Chemical equilibrium reactions CO2 is reacted with the H2O+MDEA+BHEMAF solution according to Eqs. (6) to (10) and species
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H+, OH-, HCO3-, CO32-, and MDEAH+ are formed.
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Dissociation of water:
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K1 OH- +H+ H2O
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Dissociation of CO2: K2 HCO3- +H+ H2O+CO2
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Dissociation of bicarbonate ion:
MDEA+H+ MDEAH+
(8)
(9)
PT
K4
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Dissociation of protonated MDEA:
(7)
M
K3 CO32- +H+ HCO3-
(6)
The primary and secondary amines react with CO2 and form carbamate. However, this is not the
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case with tertiary amines. Since MDEA is a tertiary amine, carbamate is not formed in the H2O+MDEA+BHEMAF solution. BHEMAF is among the ionic liquids that absorb gases
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physically and hence does not react with CO2. According to thermodynamics the general form of the equilibrium reaction constant is defined by Eq. (10).
K T ai ij x i i ij
(10)
where ai , x i and i are the activity, the molar fraction and the activity coefficient of component i and ij is the stoichiometric coefficient of component i in reaction j. The reaction equilibrium constant is a function of temperature as Eq. (11). 7
ACCEPTED MANUSCRIPT ln K A
B C ln T DT T
T /K
(11)
IP
T
The coefficients of Eq. (11) for reactions (6) to (9) are presented in Table 3.
Kj
A
B
C
D
K1
132.899
-13445.9
-22.4773
0
K2
231.465
-12092.1
-36.7816
0
K3
216.049
-12431.7
-35.4819
K4
-77.262
-1116.5
10.06
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Table 3. The coefficients of reaction equilibrium constants of Eq. (11). T/K
Ref [26]
273.15-498.15
[26]
0
273.15-498.15
[26]
0
273.15-423.15
[27]
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273.15-498.15
In addition to Eqs. (6) to (11), two mass balance and one electroneutrality balance equations should
M
be considered in the liquid phase to completely describe the governing equations.
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MDEA mole balance:
MDEA MDEAH+ MDEA
(13)
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CO2 HCO3- CO32-
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CO2 mole balance:
(12)
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Electroneutrality balance:
MDEAH H+ OH- HCO3- 2 CO32- HCOO-
(14)
Eqs. (6) to (14) make a nonlinear system of equations that must be solved simultaneously to obtain the concentration of all the species in the liquid phase.
3.2. Phase equilibrium
8
ACCEPTED MANUSCRIPT One of the conditions of thermodynamic equilibrium is the equality of chemical potentials or fugacities of each component in all the phases at equilibrium.
igas iliquid f igas f iliquid
T
(15)
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i and fi are the chemical potential and the fugacity of component i in solution. Due to the low
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vapor pressure of MDEA its presence in the vapor phase is neglected. According to this assumption sat the vapor pressure of water has been considered as the vapor pressure of the solution ( Psolution in
US
Eq. (2)) at each temperature. Also, BHEMAF and all the ionic components are considered nonvolatile. With these assumptions, the phase equilibrium equations are given only for water and
method is applied in this work.
2
2
H O y H O P x H O H O 2
2
2
sat H 2O
P
(16)
V coL P PHsatO 2 exp 2 RT
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2
sat H 2O
M
2
V co P PHsatO 2 H co2 exp 2 RT
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co y co P x co
* co2
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CO2 which are present in both phases. The following equilibrium equations show that the -
(17)
where , y and x are the fugacity coefficient and molar fraction in the gas and liquid phases,
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respectively. In Eqs. (16) and (17), fugacity coefficients in the vapor phase are obtained by the PR
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EOS [25]. H CO2 is the Henry’s law constant for CO2 solubility in mixed solvent which is calculated from Eq. (18).
H CO2 x j H CO2 , j
(18)
j
H CO2 , j / Pa ACO2 , j
BCO2 , j T
CCO2 , j ln T DCO2 , jT
T /K
(19)
where H CO2 , j is the Henry’s law constant for CO2 in pure solvent j. The coefficients of Eq. (19) are taken from Zhang and Chen [28].
9
ACCEPTED MANUSCRIPT Table 4. Coefficients of Henry’s law constants in Eq. (19) [28]. j
ACO2 , j
BCO2 , j
CCO2 , j
DCO2 , j
H2O
91.344
-5876
-8.589
-0.012
MDEA
-19.893
-1072.7
0
0
In Eq. (16) VCO is the partial molar volume of CO2 at infinite dilution in water which is correlated 2
T
L
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by Garcia [29] in Eq. (20). V H2O in Eq. (17) is the molar volume of water and is obtained from Eqs.
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(21) and (22).
VCO / cm3.mol1 37.51 9.585 102 T 8.74 104 T 2 5.044 107 T 3 2
999.83512 r T r T 1
2
2
r3 T 3 r4 T 4 r5 T
1 r6 T
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w / kg.m
-3
Mw W 103
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V HL2O / cm3 .mol1
T / C
5
(20) (21)
T /K
(22)
M
where Mw and W are the molecular weight and density of water. Density of water is obtained
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from Eq. (22) [30] and its coefficients are presented in Table 5.
1
ri
16.9451000
2
3
4
5
6
0.000799
0.000046
0.0000001
2.8100000
2.6900000
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i
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Table 5. Constants of water density correlation defined by Eq. (22) [30].
3.3. Thermodynamic modeling The Deshmukh and Mather [31] method is based on the excess Gibbs free energy. In this model, the extended Debye-Huckel theory proposed by Guggenheim and Stokes [32] and Scatchard [33] has been applied to calculate the activity coefficients.
ln i
Az i2 I 2 ij m j I B I J
(23)
10
ACCEPTED MANUSCRIPT
In the first term in the right hand side of Eq. (23) zi is the electric charge of ion i and I is the ionic strength obtained by Eq. (24), A is a function of temperature introduced by Eq. (25) and B is a constant which is equal 1.2 [34]. The second term in the right hand side of this equation accounts for short range interactions in which mj is the molality of component j and ij is the interaction
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parameter between the two components i and j in the liquid phase. In the Deshmukh and Mather
1 m j z j 2 J 2
(24) T / C
(25)
M
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A 1.11 1.335 103T 1.164 105T
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I
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model [31], activity of water is assumed to be equal to its mole fraction.
3.4. Data regression
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To calculate the activity coefficients from equation (23), the interaction parameters ij should be
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adjusted to the experimental data. Since there are 9 species present in the H2O+MDEA+BHEMAF liquid solution, 9 9 adjustable interaction parameters exist. The possible interactions are
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presented in Table 6. The number of these parameters is reduced by considering some assumptions as: 1) the interaction parameters are symmetric ( ij ji ) , 2) the interactions of water with other
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species are ignored, 3) self-interactions of all species are equal to zero, and 4) the interactions of species such as OH- and CO32- with low concentrations in the solution are ignored. Sensitivity analysis was also applied to ignore the parameters with little impact on the results. By applying these assumptions the number of adjustable parameters are reduced to 7, which are MDEA-CO2,
MDEAH+-CO2, MDEAH+- HCO3 , MDEAH+-MDEA, BHEMAF- HCO3 , BHEMAF-MDEA, BHEMAF-MDEAH+.
11
ACCEPTED MANUSCRIPT
CO2
H+
OH-
HCO-3
CO32
MDEA
MDEAH+
BHEMAF
H 2O
0
_
_
_
_
_
_
_
_
CO2
*
0
_
_
_
_
_
_
_
H+
*
*
0
_
_
_
_
_
_
OH-
*
*
*
0
_
_
_
_
_
HCO3
*
*
*
*
0
_
_
_
_
CO32
*
*
*
*
*
0
_
_
_
MDEA
*
?
*
*
*
*
0
_
_
MDEAH+
*
?
*
*
?
*
?
0
_
BHEMAF
*
*
*
*
?
*
?
?
0
CR
US
AN
IP
H2O
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Components
M
T
Table 6. The possible interaction parameter of Deshmukh-Mather model in H2O+MDEA+BHEMAF solution.
(–) interactions that have been ignored due to symmetry, (0) self-interaction and (*) other assumptions. (?) interactions
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which have been considered in modeling.
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To estimate the interaction parameters BUBL P calculation was performed in which the following objective function (OF) was minimized. All the data measured in this work were used to adjust the
OF
1 N
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parameters.
PCO ,Exp PCO ,Calc 2 2 PCO2 ,Exp i 1 N
100
(26)
N is the number of data points and subscript Exp and Calc denote the experimental and calculated values. The interaction parameters are considered as a function of temperature according to equation (27). Initially, binary parameters were adjusted at each temperature and then the coefficients of the
12
ACCEPTED MANUSCRIPT following equation were obtained. These coefficients are presented in Table 7 and the calculation algorithm is shown in the Fig. 3.
ij bij aijT
IP
T
(27)
phase.
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Table 7. Adjustable interaction parameter values in Deshmukh-Mather model for H2O+MDEA+BHEMAF liquid
a ij / kg.( mol.K)-1
MDEA-CO2
-0.0126×10-3
MDEAH+-CO2
-0.0581×10-3
0.0184
-3
0.0014
-HCO3-
-0.0042×10
0.0046
-3
0.0784
AN
MDEAH
+ +
MDEAH -MDEA
-0. 2497×10
BHEMAF-HCO3-
-0.1180×10-3
-0.0652×10
AC
CE
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BHEMAF-MDEAH
-0. 1916×10
+
M
BHEMAF-MDEA
13
b ij /kg .mol-1
US
Components
-3
-3
0.0371 0.0563 0.0195
IP
T
ACCEPTED MANUSCRIPT
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start
Calculation of equilibrium constants by Eq. (11)
AN
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Assume =1 Calculate xi and mi solving nonlinear system of Eqs. (10) and (12) to (14)
M
Calculation of activity coefficients by Eq. (23)
abs(x i ,new x i ,old )
No
yes
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PT
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Calculate mi and xi resolving the nonlinear system of Eqs. (10) and (12) to (14).
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Obtaining PCO2 by bubble P calculations and Eqs. (16) and (17) Minimize object function Eq. (26) with optimization of ij
No
Accept ij Yes Print ij and PCO2
Finish
14
ACCEPTED MANUSCRIPT Fig. 3. Flowchart of calculations.
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4. Results and discussions
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To ensure the accuracy of our data the solubility of CO2 in 25.73wt.% MDEA aqueous solution
CR
was measured at 298.15 K and compared with the reported data by Sidi-boumedine et al. [17]. The solubility results and the Absolute Relative Percent Deviation (ARD%) between the measured data
US
in this work and the published data are presented in Table 8. The Average Absolute Relative Percent Deviation (AAD%) calculated by Eq. (28) is 3.12.
(28)
M
1 N Exp-Calc 100 N i 1 Exp
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AAD%
(27)
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Exp-Calc ARD% 100 Exp
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Table 8. Comparison of the loading values for CO2 in 25.73 wt.% MDEA aqueous solution measured in this work and measured by Sidi-boumedine et al. [17].
a
a
(this work)
(literature)
265
0.875
0.893
2.38
765
0.994
1.017
2.54
1119
1.03
1.069
3.52
1385
1.044
1.079
3.36
1559
1.085
1.094
3.77
AAD%
3.12
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PCO2 /kPa
AC
T/K=313.15
a
ARD%
=mol CO2/mol MDEA. Standard uncertainty of temperature is u(T)= 0.1 K and standard uncertainty of pressure is u(P) =1 kPa.
To investigate effect of the presence of heat stable salt on solubility of CO2 two different sets of experiments were carried out. In the first set, weight percentage of water in solutions was kept 15
ACCEPTED MANUSCRIPT constant while the weight percentage of MDEA was decreased with increasing the amount of BHEMAF formed in a way that the total weight percentage of BHEMAF and MDEA remained constant. In the second set of experiments, the amount of MDEA and water were kept constant while BHEMAF was added to the solutions. These two sets of experiments were designed only to investigate the influence of the presence of the heat stable salt on the CO 2 solubility. Hence, to make better and clearer comparison two different definitions for loading introduced by Eqs. (29)
IP CR
nMDEA
(29)
l nCO 2
nMDEA nBHEMAF
(30)
M
AN
l nCO 2
US
T
and (30) were used.
4.1. CO2 solubility in the H2O+MDEA+BHEMAF in the first set of experiments
ED
CO2 solubility in H2O+MDEA+BHEMAF solutions was measured at two temperatures of 298.15 and 313.15 K in the pressure range of 100-2100 kPa. The measured data are presented in Table 9.
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These data and the data of Sidi-boumedine et al. [17] at 298.15 and 313.15 K are presented in Figs. 3-5. As it is observed, the solubility of CO2 decreases by increasing BHEMAF concentration. By
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comparing Figs. 3 and 4, it can be seen that at both temperatures of 298.15 and 313.15 the presence of BHEMAF have similar effect on CO2 absorption. In Figs. 3b, 4b and 5b in which loading is
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defined by Eq. (29), the decrease of CO2 solubility due to the presence of the heat stable salt is more evident.
16
ACCEPTED MANUSCRIPT
Table 9. CO2 partial pressure as a function of CO2 loading in H2O+MDEA+BHEMAF solutions at two
Pexp/kPa
± the uncertainties
H2O+MDEA+BHEMAF
± the uncertainties
173
0.8660.037
0.587
593
0.9830.033
0.666
845
1.0200.033
0.691
960
1.0320.033
0.699
1244
1.0720.033
0.726
1365
1.0860.034
0.735
1540
1.1050.034
0.748
75 wt.%+15 wt.%+10 wt.% 1.0450.032
0.799
1173
1.0740.030
0.822
1330
1.1110.030
0.850
1597
1.1290.030
0.864
1710
1.1370.030
0.870
M
AN
878
US
75wt.%+17.5 wt.%+7.5 wt.%
CR
T/K=298.15
T/K=313.15
ED
75wt.%+17.5 wt.%+7.5 wt.%
75 wt.%+15 wt.%+10 wt.%
0.7460.032
0.570
192
0.7490.036
0.511
340
0.8890.028
0.679
545
0.8980.033
0.616
0.9560.028
0.730
886
0.9450.033
0.652
1.0210.029
0.779
1240
0.9640.033
0.670
CE
1115
PT
137
620
1394
1.0460.029
0.799
1484
0.9860.034
0.687
1804
1.0820.030
0.826
1654
0.9910.034
0.693
2137
1.0100.035
0.717
AC
T/K=313.15
Pexp/kPa
IP
H2O+MDEA+BHEMAF
T
temperatures. Data measured in this work.
53.12 wt.%+39.38 wt.%+7.5 wt.%
53.12wt.%+36.88 wt.%+10 wt.%
126
0.5310.012
0.466
141
0.5640.015
0.470
160
0.6270.012
0.550
339
0.8160.014
0.681
272
0.7570.012
0.665
507
0.8750.013
0.730
640
0.8990.013
0.790
810
0.9250.013
0.771
1225
0.9600.013
0.850
1077
0.9470.013
0.790
1517
0.9940.013
0.873
1440
0.9690.014
0.809
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0.9880.014
b
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a
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Standard uncertainty of temperature is u(T)= 0.1 K and standard uncertainty of pressure is u(P) =1 kPa.
a
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Fig 3a-3b. CO2 partial pressure as a function of CO2 loading in H2O+MDEA+BHEMAF solutions at T= 298.15 K.
b
18
0.824
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b
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a
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Fig 4a-4b. CO2 partial pressure as a function of CO2 loading in H2O+MDEA+BHEMAF solutions at T= 313.15 K.
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Fig 5a-5b. CO2 partial pressure as a function of CO2 loading in H2O+MDEA+BHEMAF solutions at T= 313.15 K.
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4.2. CO2 solubility in the H2O+MDEA+BHEMAF in the second set of experiments
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In this set of experiments CO2 solubility in H2O+MDEA+BHEMAF solutions was measured at two temperatures of 298.15 and 313.15 K in the pressure range of 100-1800 kPa. Since in the second set of experiments, the amounts on MDEA, namely the denominator of Eq. (30), is constant the loading defined by this equation can present enough information on addition of BHEMAF. The obtained data are presented in Table 10. The comparison between the data obtained in this work with the data of solubility of CO2 in 25 wt.% aqueous solution of MDEA obtained by Sidiboumedine et al. [17] at 298.15 and 313.15 K, is shown in Figs. 6 and 7, respectively. As it is clearly seen, the solubility of CO2 in the solution is reduced by increasing BHEMAF concentration.
19
ACCEPTED MANUSCRIPT
Table 10. CO2 partial pressure as a function of CO2 loading in H2O+MDEA+BHEMAF solutions at two
Pexp/kPa
± the uncertainties
Pexp/kPa
± the uncertainties
91
0.6210.022
396
0.8670.022
1022
0.9460.022
1344
0.9770.022
1638
0.9980.023
0.8060.025
122
0.6650.026
0.9920.022
291
0.8470.022
H2O+MDEA+BHEMAF
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H2O+MDEA+BHEMAF
T
temperatures. Data measured in this work.
T/K =298.15
75 wt.%+25 wt.%+0.6 M
578
1.0040.022
920
1.0320.021
1189
1.0570.021
1358
1.0700.021
1610
1.0900.022
1771
1.0930.022
174
M 75 wt.%+25 wt.%+0.6 M
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645
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T/K=313.15 75 wt.%+25 wt.% +0.3 M
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0.7900.025
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110
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75 wt.%+25 wt.%+0.3 M
1.0220.021
517
0.9180.021
1126
1.0370.021
958
0.9690.022
1468
1.0600.022
1403
1.0080.022
1741
1.0730.022
1750
1.0270.023
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890
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Standard uncertainty of temperature is u(T)= 0.1 K and standard uncertainty of pressure is u(P) =1 kPa.
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Fig. 6. CO2 partial pressure as a function of CO2 loading for H2O+MDEA+BHEMAF solutions at T= 298.15 K.
Fig. 7. CO2 partial pressure as a function of CO2 loading in H2O+MDEA+BHEMAF solutions at T= 313.15 K.
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4.3. Modeling results By adjusting the binary interaction parameters, CO2 partial pressure was calculated. These data cover two temperatures of 298.15, 313.15 K and a pressure range of 100-2100 kPa. The modeling results are presented in Fig. 8. As it is observed, the results of modeling are in good agreement with experimental data in a wide range of pressure. ARD% between the experimental data and
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the model results of CO2 partial pressure are presented in Table 11. The AAD% of calculations
H2O +MDEA + BHEMAF ■ T=313.15 75wt.% +17.5wt.% +0.3 M ■ T=313.15 75wt.% +15wt.% +0.6 M ● T=298.15 75wt.% +17.5wt.% +0.3 M ● T=298.15 75wt.% +17.5wt.% +0.6 M ▲ T=298.15 75wt.% +17.5wt.% +7.5wt.% ▲ T=298.15 75wt.% +15wt.% +10wt.% ▼ T=313.15 75wt.% +17.5wt.% +7.5wt.% ▼ T=313.15 75wt.% +15wt.% +10wt.% * T=313.15 53.12wt.% +39.38wt.% +7.5wt.% * T=313.15 53.12wt.% +36.88wt.% +10wt.%
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M
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using the Deshmuch-Mather model for the total data points measured in this work is 7.87.
Fig. 8. Ratio of the experimental to calculated CO2 partial pressure as a function of CO2 loading (mole CO2/mole MDEA). Data points are obtained in this work.
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ACCEPTED MANUSCRIPT Table 11. Comparison between experimental data and calculated partial pressure of CO2 in H2O+MDEA+BHEMAF
298.15
75wt.%+17.5 wt.%+7.5 wt.%
Pcalc/kPa
ARD%
878
800
8.78
1173
1022
12.84
1330
1285
3.38
1597
1404
1710
1454
75 wt.%+15 wt.%+10 wt.% 173
0.00
587
0.86
845
866
2.59
960
963
0.37
1244
1280
2.91
1365
1383
1.36
1540
1524
1.03
137
110
19.83
340
237
30.01
620
418
32.46
1115
1014
9.05
1394
1393
0.00
1804
1667
7.56
192
192
0.34
545
545
7.10
886
919
3.76
1240
1161
6.33
1484
1484
4.83
1654
1575
4.72
AN M PT
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75wt.%+17.5 wt.%+7.5 wt.%
CE AC 313.15
14.95
173
593
313.15
12.03
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298.15
Pexp/kPa
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H2O+MDEA+BHEMAF
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T/K
T
solutions at two temperatures.
75 wt.%+15 wt.%+10 wt.%
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ACCEPTED MANUSCRIPT 11.12
126
94
24.66
160
160
0.00
272
256
5.60
640
577
9.82
1225
1124
8.23
1517
1517
53.12wt.%+36.88wt.%+10 wt.% 141
11.34
348
2.75
507
526
3.84
810
829
2.37
1077
1077
0.00
1440
1399
2.80
1769
1768
0.00
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PT
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75 wt.%+25 wt.%+0.3 M
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313.15
298.15
0.00
125
339
313.15
T
53.12wt.%+39.38wt.%+7.5wt.%
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313.15
1899
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313.15
2137
174
121
30.1
645
463
28.11
890
816
8.22
1126
1085
3.6
1468
1497
1.99
1741
1731
0.51
122
116
4.56
291
282
2.92
517
527
2.11
958
1133
18.28
1403
1685
20.1
1750
2089
19.37
75 wt.%+25 wt.%+0.6 M
75 wt.%+25 wt.%+0.3 M
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578
584
1.17
920
920
0.07
1189
1226
3.13
1358
1384
1.96
1610
1566
2.86
1771
1636
7.59
91
72
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20.87
366
7.40
1022
976
4.46
1344
1451
8.00
1638
1853
13.18
AAD%
7.87
M
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396
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75 wt.%+25 wt.%+0.6 M
T
110
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298.15
110
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5. Conclusion
In this research work, the solubility of CO2 in H2O+MDEA+BHEMAF solutions was measured
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using the static method at two temperatures of 298.15 and 313.15 K in the pressure range of 1002100 kPa. Experimental data showed that CO2 solubility in the solution decreases by increasing
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the concentration of BHEMAF. The Deshmukh-Mather thermodynamic model was used for modeling CO2 solubility in H2O+MDEA+BHEMAF solutions. The binary interaction parameters
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of the Deshmukh-Mather model was obtained with the regression on all the data points measured in this work. The AAD% of calculations for the total data points is 7.87.
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ACCEPTED MANUSCRIPT References: [1] S.D. Kenarsari, D. Yang, G. Jiang, S. Zhang, J. Wang, A.G. Russell, Q. Wei, M. Fan, Review of recent advances in carbon dioxide separation and capture, Rsc Advances. 45 (3) (2013) 2273973. [2] W.L. Theo, J.S. Lim, H. Hashim, A.A. Mustaffa, W.S. Ho. Review of pre-combustion capture
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methyldiethanolamine
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[26] M.L. Posey, G.T. Rochelle, A thermodynamic model of Methyldiethanolamine− CO2− H2S− Water, Industrial & engineering chemistry research. 36 (9) (1997) 3944-3953.
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[28] Y. Zhang, C.C. Chen, Thermodynamic modeling for CO2 absorption in aqueous MDEA solution with electrolyte NRTL model, Industrial & Engineering Chemistry Research, 50(1)
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[29] J.E. Garcia, Density of aqueous solutions of CO2, (2001) [30] G. S. Kell, Density, thermal expansivity, and compressibility of liquid water from 0. deg. to 150. deg.. Correlations and tables for atmospheric pressure and saturation reviewed and expressed on 1968 temperature scale, Journal of Chemical and Engineering Data. 20 (1) (1975) 97-105. [31] R. Deshmukh, A. Mather, A mathematical model for equilibrium solubility of hydrogen sulfide and carbon dioxide in aqueous alkanolamine solutions, Chemical Engineering Science. 36(2) (1981) 355-362. 28
ACCEPTED MANUSCRIPT [32] E.A. Guggenheim, R.H. Stokes, Activity coefficients of 2: 1 and 1: 2 electrolytes in aqueous solution from isopiestic data, Transactions of the Faraday Society. 54 (1958) 1646-1649. [33] G. Scatchard, Concentrated Solutions of Strong Electrolytes. Chemical Reviews, 19(3) (1936) 309-327. [34] A. Benamor, M.K. Aroua, Modeling of CO2 solubility and carbamate concentration in DEA,
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Graphical Abstract
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CO2 partial pressure as a function of CO2 loading in H2O+MDEA+BHEMAF solutions at T= 298.15 K.
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ACCEPTED MANUSCRIPT Research Highlights
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Heat stable salts are produced due to amine degradation during CO2 absorption. Solubility of CO2 in H2O+MDEA+BHEMAF solutions was measured. It is observed that CO2 solubility decreases with increasing BHEMAF concentration. Deshmukh-Mather thermodynamic model was used to obtain the solubility of CO2.
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Shaghayegh Alborzi, Farzaneh Feyzi PII: DOI: Reference:
S0167-7322(18)36657-1 https://doi.org/10.1016/j.molliq.2019.02.089 MOLLIQ 10490
To appear in:
Journal of Molecular Liquids
Received date: Revised date: Accepted date:
22 December 2018 16 February 2019 19 February 2019
Please cite this article as: S. Alborzi and F. Feyzi, Effect of heat stable salt bis(2-hydroxyethyl) methylammonium formate on CO2 absorption in aqueous methyldiethanolamine solution, Journal of Molecular Liquids, https://doi.org/10.1016/ j.molliq.2019.02.089
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ACCEPTED MANUSCRIPT Effect of heat stable salt bis-(2-hydroxyethyl) methylammonium formate on CO2 absorption in aqueous methyldiethanolamine solution
Shaghayegh Alborzi, Farzaneh Feyzi
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Thermodynamics Research Laboratory, School of Chemical Engineering,
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Iran University of Science and Technology, Tehran 16846-13114, Iran
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Abstract
In this work, the solubility of CO2 in aqueous methyldiethanolamine (MDEA) + bis-(2-
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hydroxyethyl) methylammonium formate (BHEMAF) system was measured using the static method at two temperatures (298.15 and 313.15 K) in the pressure range of 100-2100 kPa. Two
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sets of experiments were performed. In the first set, the weight percentage of BHEMAF, MDEA and H2O in H2O+MDEA+BHEMAF solutions were 7.5-10 wt.%, 15-39.38 wt.% and 53.12-75
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wt.%, respectively. In the second set of experiments concentration of BHEMAF was set as 0.3 and 0.6 M in the aqueous solutions of 25 wt.% MDEA. Experimental data showed that the solubility
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of CO2 in the H2O+MDEA+BHEMAF solutions decreases with increasing BHEMAF concentration. The Deshmukh-Mather thermodynamic model was then used to calculate the
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solubility of CO2 in H2O+MDEA+BHEMAF solutions. The binary interaction parameters of the model were adjusted by regression to experimental data. The modeling results showed that the
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average absolute relative percent deviation for all the data points was 7.87%.
Keywords: CO2 capture, Heat stable salt, Deshmukh-Mather model
Corresponding author email: [email protected] 1
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1. Introduction Over the past decades, the concentration of greenhouse gases in atmosphere is substantially increased [1]. Reports suggest that CO2 concentration, as the main greenhouse gas, will reach the range of 660-25507 ppm by 2030. Increased CO2 has numerous destructive effects on the
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environment including sea level rise and global warming [2]. Carbon capture and storage (CCS)
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is an efficient technology for reducing CO2 emissions [3]. One of the most important methods for
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CO2 capture is the absorption of CO2 in aqueous amine solutions. Among the alkanolamines the one that is most widely used for separation of acid gases is Methyldiethanolamine (MDEA) [4-
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10]. MDEA is a tertiary amine which in comparison to primary and secondary amines has a lower heat of reaction and vapor pressure. This is the reason why MDEA loss through vaporization is much lower. Also, due to the low heat of reaction with acidic gases, the energy consumed for
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solvent regeneration is very low [4]. Amines are degraded during the absorption process by two major methods of oxidation and thermal degradation and products such as carboxylic acids and
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heat stable salts are formed. Heat stable salts cause some costly operational problems including
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corrosion and foaming [11, 12]. The heat stable salt bis-(2-hydroxyethyl) methylammonium formate (BHEMAF) is one of the major products of MDEA degradation. It is an ionic liquid
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formed from the reaction of MDEA with formic acid which in turn is the product of MDEA oxidation and thermal degradation [13-16]. Chemical structure of BHEMAF is shown in Fig. 1.
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The reason for our choice was high concentrations of formate and high formation of this heat stable salt. Properties such as density and molecular weight of BHEMAF were also measured in this
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work.
So far, some research has been done on CO2 absorption with an amine and various ionic liquid mixtures [17-19]. Also, since viscosity in an important property in process design, effect of the presence of ionic liquids on viscosity of amine solutions are investigated and discussed [20-22]. However, the solubility of CO2 in aqueous MDEA including the ionic liquid formed during gas sweeting process has not yet been studied. Solubility of CO2 in pure BHEMAF is investigated by Huang et al. [23], however, to the best of our knowledge; CO2 solubility data for aqueous BHEMAF+MDEA solutions is not available. This issue is the subject of this study. For this
2
ACCEPTED MANUSCRIPT purpose, the solubility of CO2 in the H2O+MDEA+BHEMAF solutions was measured by static
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method. Then, thermodynamic modeling was carried out using the Deshmukh-Mather model.
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Fig. 1. Chemical structure of BHEMAF.
2. Experimental
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2.1. Material
Sources and purity of materials used in this work are presented in Table 1. The deionized water
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was used to prepare the solutions.
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Table 1. Materials used in this work. Mass fraction Purity >99% >99% >99%
Source Hamta gas Institute of Chemistry and Chemical Engineering of Iran Merck
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Material CO2 BHEMAF MDEA
2.2. Density measurement
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The density of BHEMAF was measured with density meter (model Anton paar DMA-5000) at three temperatures of 313.15, 323.15 and 333.15 K with accuracy 0.01 K. The results are presented in Table 2.
Table 2. Density of BHEMAF. T /K
ρ/ (gr .cm-3)
specific gravitya
313.15
1.18417
1.19344
323.15
1.17757
1.19184
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ACCEPTED MANUSCRIPT 333.15 a
1.17081
Specific gravity =
1.19083
HHEMEF . Standard uncertainty of density is u (ρ)=0.000007 gr.cm-3 water
2.3. Apparatus and equipment The apparatus used in this work for measuring solubility of CO2 is schematically shown in Fig. 2.
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A cylindrical stainless steel autoclave with an internal diameter of 3 cm, a height of 6 cm and a
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volume of 42 cm3 is the main part of the apparatus. A magnetic stirrer was used to mix the liquid and vapor phases in the autoclave. A circulating water bath (model LAUDA ALPHA RA8) with
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temperature accuracy of 0.1 K was used to set the autoclave temperature. The temperature in the double-wall autoclave was controlled by the flow of water inside the wall. A stainless steel gas
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container is also installed. The temperature of the gas container was measured by a thermometer (Chromel- Alumel Type K) with accuracy of 0.1 K. Two pressure sensors (model BD Sensor)
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with accuracy of 1 kPa were used to measure the gas pressure inside the autoclave and gas container. The pressure and temperature transmitters were connected to a personal computer. A
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number of valves were used to control the gas flow and a vacuum pump was installed to evacuate
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the apparatus before each run. The apparatus is the same as used in [24].
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ACCEPTED MANUSCRIPT Fig 2. Schematic diagram of the experimental apparatus. (V1-V8) valves; (PS1 and PS2) pressure sensors; (TS)
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thermometer.
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As seen in Fig. 2, the experimental apparatus consists of three sections A, B and C which their volumes were obtained by volumetric method. These sections are connected with 0.25-inch
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stainless steel pipes.
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2.4. Experimental procedure
Initially, all the three sections A, B and C were evacuated. Then, a certain amount of solution was
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injected into the autoclave and the autoclave temperature was set by the water bath. After this step, CO2 gas was injected from the gas cylinder into sections A and B. Then, valve (V5) between
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sections B and C was opened to inject CO2 into the autoclave. The number of moles of injected
initial Pcofinal V A+B Pco2 2 initial initial final final R T co2 Z T co2 Z
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(1)
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n coinjected 2
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CO2 into the autoclave was calculated by Eq. (1).
In Eq. (1), V, T and P are the volume, temperature and pressure of sections (A + B), respectively,
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and n is the number of moles of CO2. Superscripts initial and final refer to before and after CO2 injection into the autoclave. Z is the compressibility factor and was calculated by Peng-Robinson (PR) equation of state (EOS) [25] and R in the gas constant. By injection of CO2 into the autoclave, the pressure initially increases and then decreases due to absorption of CO2 in the solution. This decreasing trend continues until the system reaches equilibrium and pressure remains constant. In our experiments, according to the amount of CO2 injected and the MDEA concentration in the solution, the system took 8 to 16 h to reach equilibrium. Eq. (2) was used to calculate the partial pressure of CO2. 5
ACCEPTED MANUSCRIPT
sat PCo2 Ptotal Psolution
T
(2)
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sat In the above equation PCo2 , Ptotal and Psolution are the pressure of CO2 in the autoclave, total pressure
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in sections (A+B) and the saturation pressure of the solution at the measured temperature, v ) and the gas volume in the respectively. The moles of CO2 remained in the gas phase (nCO 2
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v PCo2V Co 2
Z PCo2 ,T RT
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n
v Co2
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autoclave were obtained from Eqs. (3) and (4), respectively.
(4)
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v V CO V t V solution 2
(3)
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v In Eq. (4), V CO , Vsolution and Vt are the gas volume in the autoclave, the volume of solution in the 2
autoclave and the total volume of autoclave, respectively. The number of moles of absorbed CO2
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l in the solution (nCO 2 ) is then obtained from the Eq. (5). v ncol 2 ncoinjected nco 2 2
AC
(5)
3. Thermodynamic framework In the process of CO2 absorption in H2O+MDEA+BHEMAF solution, gas and liquid phases are present. First, the CO2 molecules are physically absorbed and transferred in the liquid phase, and then, CO2 reacts in the solution (chemical absorption). The chemical reactions cause the concentration of components to change in the liquid phase. These changes continue until the system becomes stable and gas-liquid phase and reaction equilibrium is established. 6
ACCEPTED MANUSCRIPT 3.1. Chemical equilibrium reactions CO2 is reacted with the H2O+MDEA+BHEMAF solution according to Eqs. (6) to (10) and species
T
H+, OH-, HCO3-, CO32-, and MDEAH+ are formed.
IP
Dissociation of water:
CR
K1 OH- +H+ H2O
US
Dissociation of CO2: K2 HCO3- +H+ H2O+CO2
AN
Dissociation of bicarbonate ion:
MDEA+H+ MDEAH+
(8)
(9)
PT
K4
ED
Dissociation of protonated MDEA:
(7)
M
K3 CO32- +H+ HCO3-
(6)
The primary and secondary amines react with CO2 and form carbamate. However, this is not the
CE
case with tertiary amines. Since MDEA is a tertiary amine, carbamate is not formed in the H2O+MDEA+BHEMAF solution. BHEMAF is among the ionic liquids that absorb gases
AC
physically and hence does not react with CO2. According to thermodynamics the general form of the equilibrium reaction constant is defined by Eq. (10).
K T ai ij x i i ij
(10)
where ai , x i and i are the activity, the molar fraction and the activity coefficient of component i and ij is the stoichiometric coefficient of component i in reaction j. The reaction equilibrium constant is a function of temperature as Eq. (11). 7
ACCEPTED MANUSCRIPT ln K A
B C ln T DT T
T /K
(11)
IP
T
The coefficients of Eq. (11) for reactions (6) to (9) are presented in Table 3.
Kj
A
B
C
D
K1
132.899
-13445.9
-22.4773
0
K2
231.465
-12092.1
-36.7816
0
K3
216.049
-12431.7
-35.4819
K4
-77.262
-1116.5
10.06
CR
Table 3. The coefficients of reaction equilibrium constants of Eq. (11). T/K
Ref [26]
273.15-498.15
[26]
0
273.15-498.15
[26]
0
273.15-423.15
[27]
AN
US
273.15-498.15
In addition to Eqs. (6) to (11), two mass balance and one electroneutrality balance equations should
M
be considered in the liquid phase to completely describe the governing equations.
ED
MDEA mole balance:
MDEA MDEAH+ MDEA
(13)
CE
CO2 HCO3- CO32-
PT
CO2 mole balance:
(12)
AC
Electroneutrality balance:
MDEAH H+ OH- HCO3- 2 CO32- HCOO-
(14)
Eqs. (6) to (14) make a nonlinear system of equations that must be solved simultaneously to obtain the concentration of all the species in the liquid phase.
3.2. Phase equilibrium
8
ACCEPTED MANUSCRIPT One of the conditions of thermodynamic equilibrium is the equality of chemical potentials or fugacities of each component in all the phases at equilibrium.
igas iliquid f igas f iliquid
T
(15)
IP
i and fi are the chemical potential and the fugacity of component i in solution. Due to the low
CR
vapor pressure of MDEA its presence in the vapor phase is neglected. According to this assumption sat the vapor pressure of water has been considered as the vapor pressure of the solution ( Psolution in
US
Eq. (2)) at each temperature. Also, BHEMAF and all the ionic components are considered nonvolatile. With these assumptions, the phase equilibrium equations are given only for water and
method is applied in this work.
2
2
H O y H O P x H O H O 2
2
2
sat H 2O
P
(16)
V coL P PHsatO 2 exp 2 RT
PT
2
sat H 2O
M
2
V co P PHsatO 2 H co2 exp 2 RT
ED
co y co P x co
* co2
AN
CO2 which are present in both phases. The following equilibrium equations show that the -
(17)
where , y and x are the fugacity coefficient and molar fraction in the gas and liquid phases,
CE
respectively. In Eqs. (16) and (17), fugacity coefficients in the vapor phase are obtained by the PR
AC
EOS [25]. H CO2 is the Henry’s law constant for CO2 solubility in mixed solvent which is calculated from Eq. (18).
H CO2 x j H CO2 , j
(18)
j
H CO2 , j / Pa ACO2 , j
BCO2 , j T
CCO2 , j ln T DCO2 , jT
T /K
(19)
where H CO2 , j is the Henry’s law constant for CO2 in pure solvent j. The coefficients of Eq. (19) are taken from Zhang and Chen [28].
9
ACCEPTED MANUSCRIPT Table 4. Coefficients of Henry’s law constants in Eq. (19) [28]. j
ACO2 , j
BCO2 , j
CCO2 , j
DCO2 , j
H2O
91.344
-5876
-8.589
-0.012
MDEA
-19.893
-1072.7
0
0
In Eq. (16) VCO is the partial molar volume of CO2 at infinite dilution in water which is correlated 2
T
L
IP
by Garcia [29] in Eq. (20). V H2O in Eq. (17) is the molar volume of water and is obtained from Eqs.
CR
(21) and (22).
VCO / cm3.mol1 37.51 9.585 102 T 8.74 104 T 2 5.044 107 T 3 2
999.83512 r T r T 1
2
2
r3 T 3 r4 T 4 r5 T
1 r6 T
AN
w / kg.m
-3
Mw W 103
US
V HL2O / cm3 .mol1
T / C
5
(20) (21)
T /K
(22)
M
where Mw and W are the molecular weight and density of water. Density of water is obtained
ED
from Eq. (22) [30] and its coefficients are presented in Table 5.
1
ri
16.9451000
2
3
4
5
6
0.000799
0.000046
0.0000001
2.8100000
2.6900000
AC
CE
i
PT
Table 5. Constants of water density correlation defined by Eq. (22) [30].
3.3. Thermodynamic modeling The Deshmukh and Mather [31] method is based on the excess Gibbs free energy. In this model, the extended Debye-Huckel theory proposed by Guggenheim and Stokes [32] and Scatchard [33] has been applied to calculate the activity coefficients.
ln i
Az i2 I 2 ij m j I B I J
(23)
10
ACCEPTED MANUSCRIPT
In the first term in the right hand side of Eq. (23) zi is the electric charge of ion i and I is the ionic strength obtained by Eq. (24), A is a function of temperature introduced by Eq. (25) and B is a constant which is equal 1.2 [34]. The second term in the right hand side of this equation accounts for short range interactions in which mj is the molality of component j and ij is the interaction
T
parameter between the two components i and j in the liquid phase. In the Deshmukh and Mather
1 m j z j 2 J 2
(24) T / C
(25)
M
AN
A 1.11 1.335 103T 1.164 105T
US
I
CR
IP
model [31], activity of water is assumed to be equal to its mole fraction.
3.4. Data regression
ED
To calculate the activity coefficients from equation (23), the interaction parameters ij should be
PT
adjusted to the experimental data. Since there are 9 species present in the H2O+MDEA+BHEMAF liquid solution, 9 9 adjustable interaction parameters exist. The possible interactions are
CE
presented in Table 6. The number of these parameters is reduced by considering some assumptions as: 1) the interaction parameters are symmetric ( ij ji ) , 2) the interactions of water with other
AC
species are ignored, 3) self-interactions of all species are equal to zero, and 4) the interactions of species such as OH- and CO32- with low concentrations in the solution are ignored. Sensitivity analysis was also applied to ignore the parameters with little impact on the results. By applying these assumptions the number of adjustable parameters are reduced to 7, which are MDEA-CO2,
MDEAH+-CO2, MDEAH+- HCO3 , MDEAH+-MDEA, BHEMAF- HCO3 , BHEMAF-MDEA, BHEMAF-MDEAH+.
11
ACCEPTED MANUSCRIPT
CO2
H+
OH-
HCO-3
CO32
MDEA
MDEAH+
BHEMAF
H 2O
0
_
_
_
_
_
_
_
_
CO2
*
0
_
_
_
_
_
_
_
H+
*
*
0
_
_
_
_
_
_
OH-
*
*
*
0
_
_
_
_
_
HCO3
*
*
*
*
0
_
_
_
_
CO32
*
*
*
*
*
0
_
_
_
MDEA
*
?
*
*
*
*
0
_
_
MDEAH+
*
?
*
*
?
*
?
0
_
BHEMAF
*
*
*
*
?
*
?
?
0
CR
US
AN
IP
H2O
ED
Components
M
T
Table 6. The possible interaction parameter of Deshmukh-Mather model in H2O+MDEA+BHEMAF solution.
(–) interactions that have been ignored due to symmetry, (0) self-interaction and (*) other assumptions. (?) interactions
PT
which have been considered in modeling.
CE
To estimate the interaction parameters BUBL P calculation was performed in which the following objective function (OF) was minimized. All the data measured in this work were used to adjust the
OF
1 N
AC
parameters.
PCO ,Exp PCO ,Calc 2 2 PCO2 ,Exp i 1 N
100
(26)
N is the number of data points and subscript Exp and Calc denote the experimental and calculated values. The interaction parameters are considered as a function of temperature according to equation (27). Initially, binary parameters were adjusted at each temperature and then the coefficients of the
12
ACCEPTED MANUSCRIPT following equation were obtained. These coefficients are presented in Table 7 and the calculation algorithm is shown in the Fig. 3.
ij bij aijT
IP
T
(27)
phase.
CR
Table 7. Adjustable interaction parameter values in Deshmukh-Mather model for H2O+MDEA+BHEMAF liquid
a ij / kg.( mol.K)-1
MDEA-CO2
-0.0126×10-3
MDEAH+-CO2
-0.0581×10-3
0.0184
-3
0.0014
-HCO3-
-0.0042×10
0.0046
-3
0.0784
AN
MDEAH
+ +
MDEAH -MDEA
-0. 2497×10
BHEMAF-HCO3-
-0.1180×10-3
-0.0652×10
AC
CE
PT
ED
BHEMAF-MDEAH
-0. 1916×10
+
M
BHEMAF-MDEA
13
b ij /kg .mol-1
US
Components
-3
-3
0.0371 0.0563 0.0195
IP
T
ACCEPTED MANUSCRIPT
CR
start
Calculation of equilibrium constants by Eq. (11)
AN
US
Assume =1 Calculate xi and mi solving nonlinear system of Eqs. (10) and (12) to (14)
M
Calculation of activity coefficients by Eq. (23)
abs(x i ,new x i ,old )
No
yes
CE
PT
ED
Calculate mi and xi resolving the nonlinear system of Eqs. (10) and (12) to (14).
AC
Obtaining PCO2 by bubble P calculations and Eqs. (16) and (17) Minimize object function Eq. (26) with optimization of ij
No
Accept ij Yes Print ij and PCO2
Finish
14
ACCEPTED MANUSCRIPT Fig. 3. Flowchart of calculations.
T
4. Results and discussions
IP
To ensure the accuracy of our data the solubility of CO2 in 25.73wt.% MDEA aqueous solution
CR
was measured at 298.15 K and compared with the reported data by Sidi-boumedine et al. [17]. The solubility results and the Absolute Relative Percent Deviation (ARD%) between the measured data
US
in this work and the published data are presented in Table 8. The Average Absolute Relative Percent Deviation (AAD%) calculated by Eq. (28) is 3.12.
(28)
M
1 N Exp-Calc 100 N i 1 Exp
ED
AAD%
(27)
AN
Exp-Calc ARD% 100 Exp
PT
Table 8. Comparison of the loading values for CO2 in 25.73 wt.% MDEA aqueous solution measured in this work and measured by Sidi-boumedine et al. [17].
a
a
(this work)
(literature)
265
0.875
0.893
2.38
765
0.994
1.017
2.54
1119
1.03
1.069
3.52
1385
1.044
1.079
3.36
1559
1.085
1.094
3.77
AAD%
3.12
CE
PCO2 /kPa
AC
T/K=313.15
a
ARD%
=mol CO2/mol MDEA. Standard uncertainty of temperature is u(T)= 0.1 K and standard uncertainty of pressure is u(P) =1 kPa.
To investigate effect of the presence of heat stable salt on solubility of CO2 two different sets of experiments were carried out. In the first set, weight percentage of water in solutions was kept 15
ACCEPTED MANUSCRIPT constant while the weight percentage of MDEA was decreased with increasing the amount of BHEMAF formed in a way that the total weight percentage of BHEMAF and MDEA remained constant. In the second set of experiments, the amount of MDEA and water were kept constant while BHEMAF was added to the solutions. These two sets of experiments were designed only to investigate the influence of the presence of the heat stable salt on the CO 2 solubility. Hence, to make better and clearer comparison two different definitions for loading introduced by Eqs. (29)
IP CR
nMDEA
(29)
l nCO 2
nMDEA nBHEMAF
(30)
M
AN
l nCO 2
US
T
and (30) were used.
4.1. CO2 solubility in the H2O+MDEA+BHEMAF in the first set of experiments
ED
CO2 solubility in H2O+MDEA+BHEMAF solutions was measured at two temperatures of 298.15 and 313.15 K in the pressure range of 100-2100 kPa. The measured data are presented in Table 9.
PT
These data and the data of Sidi-boumedine et al. [17] at 298.15 and 313.15 K are presented in Figs. 3-5. As it is observed, the solubility of CO2 decreases by increasing BHEMAF concentration. By
CE
comparing Figs. 3 and 4, it can be seen that at both temperatures of 298.15 and 313.15 the presence of BHEMAF have similar effect on CO2 absorption. In Figs. 3b, 4b and 5b in which loading is
AC
defined by Eq. (29), the decrease of CO2 solubility due to the presence of the heat stable salt is more evident.
16
ACCEPTED MANUSCRIPT
Table 9. CO2 partial pressure as a function of CO2 loading in H2O+MDEA+BHEMAF solutions at two
Pexp/kPa
± the uncertainties
H2O+MDEA+BHEMAF
± the uncertainties
173
0.8660.037
0.587
593
0.9830.033
0.666
845
1.0200.033
0.691
960
1.0320.033
0.699
1244
1.0720.033
0.726
1365
1.0860.034
0.735
1540
1.1050.034
0.748
75 wt.%+15 wt.%+10 wt.% 1.0450.032
0.799
1173
1.0740.030
0.822
1330
1.1110.030
0.850
1597
1.1290.030
0.864
1710
1.1370.030
0.870
M
AN
878
US
75wt.%+17.5 wt.%+7.5 wt.%
CR
T/K=298.15
T/K=313.15
ED
75wt.%+17.5 wt.%+7.5 wt.%
75 wt.%+15 wt.%+10 wt.%
0.7460.032
0.570
192
0.7490.036
0.511
340
0.8890.028
0.679
545
0.8980.033
0.616
0.9560.028
0.730
886
0.9450.033
0.652
1.0210.029
0.779
1240
0.9640.033
0.670
CE
1115
PT
137
620
1394
1.0460.029
0.799
1484
0.9860.034
0.687
1804
1.0820.030
0.826
1654
0.9910.034
0.693
2137
1.0100.035
0.717
AC
T/K=313.15
Pexp/kPa
IP
H2O+MDEA+BHEMAF
T
temperatures. Data measured in this work.
53.12 wt.%+39.38 wt.%+7.5 wt.%
53.12wt.%+36.88 wt.%+10 wt.%
126
0.5310.012
0.466
141
0.5640.015
0.470
160
0.6270.012
0.550
339
0.8160.014
0.681
272
0.7570.012
0.665
507
0.8750.013
0.730
640
0.8990.013
0.790
810
0.9250.013
0.771
1225
0.9600.013
0.850
1077
0.9470.013
0.790
1517
0.9940.013
0.873
1440
0.9690.014
0.809
17
ACCEPTED MANUSCRIPT 1769
0.9880.014
b
ED
M
AN
US
a
CR
IP
T
Standard uncertainty of temperature is u(T)= 0.1 K and standard uncertainty of pressure is u(P) =1 kPa.
a
AC
CE
PT
Fig 3a-3b. CO2 partial pressure as a function of CO2 loading in H2O+MDEA+BHEMAF solutions at T= 298.15 K.
b
18
0.824
ACCEPTED MANUSCRIPT
b
ED
M
AN
US
a
CR
IP
T
Fig 4a-4b. CO2 partial pressure as a function of CO2 loading in H2O+MDEA+BHEMAF solutions at T= 313.15 K.
PT
Fig 5a-5b. CO2 partial pressure as a function of CO2 loading in H2O+MDEA+BHEMAF solutions at T= 313.15 K.
CE
4.2. CO2 solubility in the H2O+MDEA+BHEMAF in the second set of experiments
AC
In this set of experiments CO2 solubility in H2O+MDEA+BHEMAF solutions was measured at two temperatures of 298.15 and 313.15 K in the pressure range of 100-1800 kPa. Since in the second set of experiments, the amounts on MDEA, namely the denominator of Eq. (30), is constant the loading defined by this equation can present enough information on addition of BHEMAF. The obtained data are presented in Table 10. The comparison between the data obtained in this work with the data of solubility of CO2 in 25 wt.% aqueous solution of MDEA obtained by Sidiboumedine et al. [17] at 298.15 and 313.15 K, is shown in Figs. 6 and 7, respectively. As it is clearly seen, the solubility of CO2 in the solution is reduced by increasing BHEMAF concentration.
19
ACCEPTED MANUSCRIPT
Table 10. CO2 partial pressure as a function of CO2 loading in H2O+MDEA+BHEMAF solutions at two
Pexp/kPa
± the uncertainties
Pexp/kPa
± the uncertainties
91
0.6210.022
396
0.8670.022
1022
0.9460.022
1344
0.9770.022
1638
0.9980.023
0.8060.025
122
0.6650.026
0.9920.022
291
0.8470.022
H2O+MDEA+BHEMAF
IP
H2O+MDEA+BHEMAF
T
temperatures. Data measured in this work.
T/K =298.15
75 wt.%+25 wt.%+0.6 M
578
1.0040.022
920
1.0320.021
1189
1.0570.021
1358
1.0700.021
1610
1.0900.022
1771
1.0930.022
174
M 75 wt.%+25 wt.%+0.6 M
PT
645
ED
T/K=313.15 75 wt.%+25 wt.% +0.3 M
US
0.7900.025
AN
110
CR
75 wt.%+25 wt.%+0.3 M
1.0220.021
517
0.9180.021
1126
1.0370.021
958
0.9690.022
1468
1.0600.022
1403
1.0080.022
1741
1.0730.022
1750
1.0270.023
CE
890
AC
Standard uncertainty of temperature is u(T)= 0.1 K and standard uncertainty of pressure is u(P) =1 kPa.
20
US
CR
IP
T
ACCEPTED MANUSCRIPT
AC
CE
PT
ED
M
AN
Fig. 6. CO2 partial pressure as a function of CO2 loading for H2O+MDEA+BHEMAF solutions at T= 298.15 K.
Fig. 7. CO2 partial pressure as a function of CO2 loading in H2O+MDEA+BHEMAF solutions at T= 313.15 K.
21
ACCEPTED MANUSCRIPT
4.3. Modeling results By adjusting the binary interaction parameters, CO2 partial pressure was calculated. These data cover two temperatures of 298.15, 313.15 K and a pressure range of 100-2100 kPa. The modeling results are presented in Fig. 8. As it is observed, the results of modeling are in good agreement with experimental data in a wide range of pressure. ARD% between the experimental data and
IP
T
the model results of CO2 partial pressure are presented in Table 11. The AAD% of calculations
H2O +MDEA + BHEMAF ■ T=313.15 75wt.% +17.5wt.% +0.3 M ■ T=313.15 75wt.% +15wt.% +0.6 M ● T=298.15 75wt.% +17.5wt.% +0.3 M ● T=298.15 75wt.% +17.5wt.% +0.6 M ▲ T=298.15 75wt.% +17.5wt.% +7.5wt.% ▲ T=298.15 75wt.% +15wt.% +10wt.% ▼ T=313.15 75wt.% +17.5wt.% +7.5wt.% ▼ T=313.15 75wt.% +15wt.% +10wt.% * T=313.15 53.12wt.% +39.38wt.% +7.5wt.% * T=313.15 53.12wt.% +36.88wt.% +10wt.%
AC
CE
PT
ED
M
AN
US
CR
using the Deshmuch-Mather model for the total data points measured in this work is 7.87.
Fig. 8. Ratio of the experimental to calculated CO2 partial pressure as a function of CO2 loading (mole CO2/mole MDEA). Data points are obtained in this work.
22
ACCEPTED MANUSCRIPT Table 11. Comparison between experimental data and calculated partial pressure of CO2 in H2O+MDEA+BHEMAF
298.15
75wt.%+17.5 wt.%+7.5 wt.%
Pcalc/kPa
ARD%
878
800
8.78
1173
1022
12.84
1330
1285
3.38
1597
1404
1710
1454
75 wt.%+15 wt.%+10 wt.% 173
0.00
587
0.86
845
866
2.59
960
963
0.37
1244
1280
2.91
1365
1383
1.36
1540
1524
1.03
137
110
19.83
340
237
30.01
620
418
32.46
1115
1014
9.05
1394
1393
0.00
1804
1667
7.56
192
192
0.34
545
545
7.10
886
919
3.76
1240
1161
6.33
1484
1484
4.83
1654
1575
4.72
AN M PT
ED
75wt.%+17.5 wt.%+7.5 wt.%
CE AC 313.15
14.95
173
593
313.15
12.03
US
298.15
Pexp/kPa
IP
H2O+MDEA+BHEMAF
CR
T/K
T
solutions at two temperatures.
75 wt.%+15 wt.%+10 wt.%
23
ACCEPTED MANUSCRIPT 11.12
126
94
24.66
160
160
0.00
272
256
5.60
640
577
9.82
1225
1124
8.23
1517
1517
53.12wt.%+36.88wt.%+10 wt.% 141
11.34
348
2.75
507
526
3.84
810
829
2.37
1077
1077
0.00
1440
1399
2.80
1769
1768
0.00
US AN M
PT
ED
75 wt.%+25 wt.%+0.3 M
CE
AC
313.15
298.15
0.00
125
339
313.15
T
53.12wt.%+39.38wt.%+7.5wt.%
IP
313.15
1899
CR
313.15
2137
174
121
30.1
645
463
28.11
890
816
8.22
1126
1085
3.6
1468
1497
1.99
1741
1731
0.51
122
116
4.56
291
282
2.92
517
527
2.11
958
1133
18.28
1403
1685
20.1
1750
2089
19.37
75 wt.%+25 wt.%+0.6 M
75 wt.%+25 wt.%+0.3 M
24
ACCEPTED MANUSCRIPT 0.00
578
584
1.17
920
920
0.07
1189
1226
3.13
1358
1384
1.96
1610
1566
2.86
1771
1636
7.59
91
72
CR
20.87
366
7.40
1022
976
4.46
1344
1451
8.00
1638
1853
13.18
AAD%
7.87
M
AN
396
IP
75 wt.%+25 wt.%+0.6 M
T
110
US
298.15
110
ED
5. Conclusion
In this research work, the solubility of CO2 in H2O+MDEA+BHEMAF solutions was measured
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using the static method at two temperatures of 298.15 and 313.15 K in the pressure range of 1002100 kPa. Experimental data showed that CO2 solubility in the solution decreases by increasing
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the concentration of BHEMAF. The Deshmukh-Mather thermodynamic model was used for modeling CO2 solubility in H2O+MDEA+BHEMAF solutions. The binary interaction parameters
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of the Deshmukh-Mather model was obtained with the regression on all the data points measured in this work. The AAD% of calculations for the total data points is 7.87.
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Graphical Abstract
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CO2 partial pressure as a function of CO2 loading in H2O+MDEA+BHEMAF solutions at T= 298.15 K.
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ACCEPTED MANUSCRIPT Research Highlights
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Heat stable salts are produced due to amine degradation during CO2 absorption. Solubility of CO2 in H2O+MDEA+BHEMAF solutions was measured. It is observed that CO2 solubility decreases with increasing BHEMAF concentration. Deshmukh-Mather thermodynamic model was used to obtain the solubility of CO2.
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