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MDEA nanofluid
Accepted Manuscript Title: CO2 Absorption Enhancement in Graphene-Oxide/MDEA Nanofluid Authors: Vahid Irani, Amin Maleki, Ahmad Tavasoli PII: DOI: Reference:
S2213-3437(18)30706-1 https://doi.org/10.1016/j.jece.2018.11.027 JECE 2782
To appear in: Received date: Revised date: Accepted date:
3 September 2018 3 November 2018 13 November 2018
Please cite this article as: Irani V, Maleki A, Tavasoli A, CO2 Absorption Enhancement in Graphene-Oxide/MDEA Nanofluid, Journal of Environmental Chemical Engineering (2018), https://doi.org/10.1016/j.jece.2018.11.027 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.
CO2 Absorption Enhancement in Graphene-Oxide/MDEA Nanofluid Vahid Irani, Amin Maleki, Ahmad Tavasoli* School of Chemistry, College of Science, University of Tehran, Tehran 11155-4563, Iran
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* Corresponding author: E-mail: tavasoli.a@ ut.ac.ir; Tel: +982161113643
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Graphical abstract
Highlights
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GO was synthesized by modified hummers method.
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Solubility of carbon dioxide was investigated in a solution an MDEA solution by addition of GO. Results showed 9.1% enhancement for addition of 0.1 mass % GO to the 40 mass % MDEA. These experiments for addition of 0.2 mass % GO presented a capacity absorption promotion up to 10.4% in different temperatures.
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Further concentration of graphene oxide into the amine solution showed no significant enhancement in absorption capacity
Abstract Graphene-Oxide (GO)/MDEA nanofluid was prepared for CO2 absorption
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enhancement in gas sweetening process. GO was synthesized using a modified Hammer method and characterized by XRD, BET, SEM and IR spectroscopy to
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determine the structure. The XRD analysis showed a strong peak at 2θ=10.3°. GO was dispersed in MDEA by ultrasonic treatment and zeta potential analysis
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was employed to examine the stability of the nanofluid. The measured zeta
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potential (35 mv) confirmed the excellent stability in the solution. GO/MDEA
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nanofluid showed high absorption capacities toward CO2 due to the high surface
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area and existence of OH groups on the GO surface and enhancement in mass
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transfer coefficient. When 0.1 wt.% GO was introduced to the 40 wt.% MDEA,
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the absorption capacity of the solution was promoted up to 9.1%. This enhancement for 0.2 wt. % slightly more while the solutions with higher GO
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content showed no significant enhancement in absorption capacity. This enhancement has a reverse and direct relationship with increasing temperature
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and pressure, respectively. Keywords: CO2; Nanofluid; Graphene oxide; Solubility; MDEA
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1. Introduction Global warming is because of increasing greenhouse gases such as carbon dioxide (CO2), which is arisen from fossil fuel combustion [1,2]. However, these fuels are important as an essential energy source and it is necessary to develop some
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methods for reducing CO2 emissions [3,4]. CO2 capture has attracted a global attention because of different adverse effects of CO2 emissions. Researchers have
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focused on reducing the greenhouse gas emissions using different procedures and techniques including chemical and physical absorption, membranes, adsorption
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with solid state materials and cryogenic separation processes [5]. One of these
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methods is CO2 absorption in the gas sweetening process as a preferred method
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for reducing CO2 emissions. Gas sweetening is commonly referred to as an
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important industrial process for removing of acid gases such as CO2 and hydrogen
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sulfide (H2S) from gas stream [6–9]. Based on the origin of the natural gas, there
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are different levels amounts of CO2 and H2S impurities in the natural gas [10]. Several CO2 capture technologies such as chemical absorption, physical
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absorption, and membrane have been studied [11,12]. Alkanoamine-based solvents are widely used for the removal of acidic gases. The most common
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alkanoamines used in the CO2 absorption process are monoethanolamine (MEA), diethanolamine (DEA), diglycolamine (DGA), methyldiethanolamine (MDEA) and Triethanolamine (TEA) [13–15]. The advantage of MDEA over other amines is improving properties against degradation and corrosion than other
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alkanoamine. MDEA is a tertiary amine with a stable structure and has no carbamate formation in the CO2 absorption process. However, these processes are highly energy intensive and corrosive and making it necessary to develop new studies based on the solid adsorbents with a high
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performance and low costs [16]. Besides, these processes are potentially releasing
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carcinogenic compounds[17,18]. Recently, many efforts have been focused on
developing new techniques or processes for CO2 capture, especially sorption using solid state materials [19–21]. For this purpose, a variety of solid materials
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have been considered such as zeolites [22,23], clays [24], MOFs [25,26], COFs
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[27] and carbon-based materials [5,28,29]. Carbon-based materials have been
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studied for promising gas sorption, storage, and separation because of the availability, robust pore structure, thermal and chemical stability, high surface
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area and comfortable synthesis in industrial scale [30]. Some carbonaceous
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materials such as carbons and carbon molecular sieves are extremely used as a solid adsorbent and catalyst in the gas related processes, because of their
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availability, high surface area and thermal and chemical stability [31–33]. Graphene has a higher capacity for adsorption of acid gases due to its very high
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specific surface area and ability to control the chemical structure [34,35]. Graphene and reduced graphene oxide (GO) have attracted significant interest in different areas such as gas adsorption due to intrinsic properties including large theoretical surface area and chemical stability [36]. The physical adsorption is
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considered a new approach [37–39]. Graphite oxide is also a carbon material with high stability at room temperature. The structure of graphite oxide is not defined precisely, but it has been used in many fields because of its unique and unusual chemical properties. Graphene can be doped and functionalized with amine
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functional groups to enhance the adsorption of acidic gases. The GO structure contains oxygen-rich functional groups including hydroxide and epoxide groups
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on the basal plane and carbonyl and carboxyl groups on edges [40]. Therefore, the GO has a hydrophilic nature and it is soluble in water and several solvents.
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GO with its largely expanded and tuneable layer structure provides a platform for
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engineering a wide range of chemical reactions [41,42]. Therefore, it is possible
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to simultaneously use the properties of alkanoamine solutions and GO by addition
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of GO into the alkanoamine solution and examination the absorption.
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In this work, for the first time, the solubility of CO2 in a 40 wt.% MDEA solution
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in the presence of 0.1 and 0.2 wt.% GO was evaluated at different temperatures (303.15, 313.15, 323.15 and 333.15 K). GO/MDEA nanofluid was prepared for
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CO2 absorption enhancement in gas sweetening process. GO was synthesized using a modified Hammer method and characterized by XRD, BET and IR
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spectroscopy to determine the structure. GO was dispersed in MDEA by ultrasonic treatment. 2. Materials and methods 2.1. Materials 6
(CO2) was obtained from Roham gas company with a minimum grade of 99.5%. All the materials used in this study were reagent grade without further purification. Deionized water used for a solution was degassed in an ultrasonic bath (FUNGILAB, model UA10MFD) at 353.15 K and wave frequency of 50 kHz for an hour. The specifications of the chemical materials used in this study
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are summarized in Table 1. **Table 1**
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2.2. Preparation of GO
GO was synthesized according to a modified Hummers method [43]. 7 g graphite
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powder, 3 g sodium nitrate, and 100 ml concentrated sulphuric acid and 12 ml
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phosphoric acid was added to a beaker and stirred for 10 min in an ice bath. 15 g
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KMnO4 was slowly added to the solution under stirring conditions so that the
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temperature of the mixture remained below 5 °C. The mixture was heated to
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maintain a temperature within the range of 35-40 °C. After half an hour, as the mixture was thickened, 235 ml deionized water was added to the reaction mixture
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and stirred for 15 min. An additional 800 ml of water was added followed by the
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slow addition of 50 ml H2O2 (30%). The mixture was then centrifuged and washed with deionized water and 5% HCl to remove any residue from the
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suspension. Finally, the suspension was dried in an oven at 60 °C to obtain a powder. 2.3. Solution preparation
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Firstly, a mass fraction of 40% MDEA and 60% water were prepared. Then, the desired mass fraction of GO including 0.1 wt.% or 0.2 wt.% were added to this solution using an analytical balance with the accuracy of 0.1 mg. GO was dispersed and stabilized into the base fluid (MDEA 40 wt.%) with ultrasonication
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process. With the application of sonication process, the use of a surfactant is not needed to obtain a desired dispersion in the solution. Since nanofluids are
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suspensions, their stability is one of the most important roles that they play. Various procedures can be used for stability examination, such as zeta potential,
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UV, etc. Zeta potential is a fundamental verification for the stability of
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nanofluids. GO should be stable in the solution and this stability was verified by
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zeta potential analysis. Small levels of zeta potential indicate that the particles
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tend to settle but high levels of zeta potential to confirm the stability of the
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solution. So, high zeta potential more than 25 mv (positive or negative) shows that the solution will be stable. Fig.1 illustrates the zeta potential diagram for this
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solution (40 wt.% MDEA+ 60 wt.% water with the addition of 0.1 wt.% GO).
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The measured zeta potential for this solution was 35 mv that confirms the colloidal dispersion of the solution.
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**Fig. 1**
2. Apparatus and Procedure The details of experimental measurement of gas solubility in amine solution can be found elsewhere [7,9,44]. However, the equilibrium cell is a double-well 8
autoclave reactor which was connected to a recirculation bath with a temperature stability within ±0.02 K. Temperature was measured using a digital thermometer with 0.01 K resolution with a sensor inserted into the cell via thermowell. The equilibrium cell pressure was measured using a calibrated Baroli type BD
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SENSOR gauge with an accuracy of 0.1% of full scale. The range of the transmitter sensor for equilibrium cell is (0-4) MPa. Gas container pressure is
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measured by a Baroli type BD SENSOR pressure gauge in the range of (0-6) MPa which is accurate to within 0.1% of full scale. Fig. 2 presents the schematic design
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3.1 Operation of the apparatus
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**Fig. 2**
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of the system.
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Initially, a vacuum was applied to the equilibrium cell using a vacuum pump. Then, a known amount of amine solution that was modified by the addition of the
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GO was injected into the equilibrium cell so that equilibrium cell pressure
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maintains vacuum property. The temperature was then adjusted to the desired value; when the equilibrium state was obtained, the pressure that is showed by the pressure sensor is the vapor pressure of the solution. The volume of the gas
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container was well known and the amount of CO2 injected into the equilibrium cell was calculated with a procedure presented by Park and Sandal [45]. The accurate PVT data was obtained from the National Institute of Standards and
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Technology that provides data for all fluids including pure CO2. So, the total mole of gas injected into the equilibrium cell can be calculated by equation 1. nCO2
VC P1 P2 ( ) RTa Z1 Z 2
(1)
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where VC presents the volume of the CO2 container, P1, P2 are the initial and final partial pressures of the CO2 container before and after the gas injection into the
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equilibrium cell respectively, Ta is the ambient temperature, Z1, Z2 denotes the compressibility factors relating to P1, P2 respectively. Then, the equilibrium
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pressure of the equilibrium cell (PCO2) is calculated by
(2)
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PCO2 PT PV
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where PT and PV present the absolute pressure and vapor pressure of the solution
V g PCO2 Z CO2 RT
2
(3)
PT
g nCO 2
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g respectively. The remaining CO2 gas in the gas phase nCO is represented as
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where Vg is equilibrium cell volume minus the solution volume used in the equilibrium cell that represents the gas-phase volume and Z CO is the 2
compressibility factor of CO2 at PCO and T. The amount of CO2 available in the
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2
liquid phase is presented as l g nCO nCO2 nCO 2 2
(4)
Finally, the molality of the CO2 available in the liquid phase is determined with
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mCO2
l nCO 2
(5)
wsol
where wsol (kg) denotes the mass of solvent. The volume of the gas phase (V g), is the difference between the equilibrium cell volume and the volume of the l uncharged solvent. The uncertainty corresponding to nCO is related to the
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2
uncertainties of the thermometer, pressure sensors and balance that are
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determined by suppliers.
All volumes are determined by pressure swing experiments (PSE). In this method
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pressure drop is measured simultaneously with the opening the valve between
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unknown and reference volume using Charles-Gay-lussac law so that the volume
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of the gas container and equilibrium cell are equal to 309.42±1.7 and 49.50±1.8 cm3, respectively. It should be noted that the partial pressure of MDEA in the
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vapor phase was neglected [46]. The vapor pressure can be calculated using
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different equations of state [47,48]. 1. Results and discussion Validation experiments
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1.1.
In order to verify the accuracy of the method and set-up, several validation tests
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were performed with CO2 and the solubility results were compared with the data available in the literature. The validation experiments are carried out with 30 and 50 wt.% aqueous MDEA solution at different temperatures. Figs. 3 and 4 showed that the results have a good agreement with the other data.
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**Fig. 3** **Fig. 4**
XRD analysis
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1.2.
X-ray diffraction (XRD) analysis is used to illustrate the average crystalline
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properties of the GO sheet. As shown in Fig. 5 the strong peak at 2θ=10.3° is in a good agreement with the literature [40,49].
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FTIR spectra analysis
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1.3.
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**Fig. 5**
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This analysis is employed to evaluate the functional groups and the structure of materials. As shown in Fig. 6, GO sheet shows apparent adsorption bands for
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functional groups including hydroxyl –OH (3187 cm-1), carboxyl C=O (1733 cm), aromatic C=C (1622 cm-1), alkoxy C-O (1047 cm-1) and epoxy C-O (1220 cm-
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). Oxygen-containing functional groups such as C=O and C-O confirm that
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graphite is oxidized into GO that is consistent with the literature [40,50–52].
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These results confirm the successful synthesis of GO.
1.4.
**Fig. 6**
BET analysis
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The surface area of GO powders was 293 m2/g measured by Brunauer–Emmett– Teller (BET). GO was expanded in water in an ultrasonic water bath to ensure the stability of GO. Fig. 7 shows the N2 adsorption/desorption isotherms of GO.
1.5.
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**Fig. 7** SEM morphology
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Fig. 8 presents the SEM images of GO. As observed, GO consists of randomly aggregated thin sheets that are forming a porous and disordered network. GO
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layer thickness is 29.3 to 35.16 nm. Elemental composition of GO was measured
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by Energy-dispersive X-ray spectroscopy(EDS) analysis and results showed that
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the sample contains C, O, and S.
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**Fig. 8**
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2. Experimental solubility tests
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In Tables 2, 3 and 4, the experimental data of CO2 solubility for 40 wt.% MDEA and (40 wt.% MDEA +0.1 wt.% and 0.2 wt.%) GO are reported respectively. The
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results indicate that CO2 solubility of the solution was increased in the presence of the GO. When 0.1 wt.% GO was added to the solution, solubility
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measurements show that the increase of CO2 sorption is in average more than 9.1%. These measurements for 0.2 wt.% GO show an average increase in solubility up to 10.4%. Further concentration of nanofluid was employed and showed no significant enhancement in CO2 absorption capacity. These results
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confirm that the addition of nanoparticle into the MDEA solution will increase the solubility of CO2 effectively. CO2 solubility increased with increasing the partial pressure and decreased with increasing temperature.
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**Table 2** **Table 3**
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**Table 4**
Fig. 9 and 10 show the solubility of CO2 at different temperatures (303.15,
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313.15, 323.15 and 333.15 K); and different partial pressure ranged from 100 to
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2100 kPa. The data were firstly obtained for 40 wt.% aqueous MDEA solution
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without addition of nanoparticles. Then, CO2 absorption of MDEA solution was obtained in the presence of different addition concentrations of GO. Each graph
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presents the solubility at a particular temperature. The experimental data are reported in terms of partial pressure of CO2 versus the molality. Increasing the
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temperature decreased the solubility results, while increasing the partial pressure improved the solubility. When 0.1 wt.% GO was introduced to the solution, the
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average absorption capacity was promoted up to 9.1%, while this enhancement rate for the solution in the presence of 0.2 wt.% was slightly more at any temperature. Higher GO content showed no significant enhancement in
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absorption capacity. All the solutions were significantly under effect of temperature and partial pressure.
**Fig. 9** 14
**Fig. 10** As shown in Fig. 9 and 10, each graph presents the solubility of CO2 at a particular temperature. Increasing the temperature decreased the solubility results, while increasing the partial pressure improved the solubility in all solutions. When 0.1
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wt.% GO was introduced to the solution, the average absorption capacity was
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slightly less than that of the solution in the presence of 0.2 wt.% at any
temperature. These results verify the positive effect of the addition of GO into the MDEA solution. The absorption capacity was increased with increasing the
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partial pressure to the extent that a linear trend was observed in the results.
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Fig. 10 presents the comparison of CO2 solubility at different temperatures. As
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shown, temperature has a negative effect on solubility. When temperature was
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increased, we observed a decrease in absorption capacity for all solutions. The trend in each graph is pretty much the same after increasing the temperature.
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There are different factors affecting the CO2 absorption rate and capacities such
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as improvement in mass transfer coefficient in the presence of nanostructures, additional reactions of CO2 in the presence of nanoparticles and zwitterion
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mechanism. This increase could be due to the high surface area of the nanostructure and the presence of the –OH and epoxy groups but based on previous works mass transfer coefficient plays a critical role in this process. There are several mechanisms that can be considered for the improvement in the mass transfer coefficient. First, the hydrodynamic effect, in which nanoparticles collide 15
with the gas-liquid interface and make the interface thinner. So, the diffusion coefficient between the two phases will be increased in the presence of nanostructures and CO2 absorption will be subsequently improved [53]. This phenomenon has been depicted in Fig. 11.
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**Fig. 11**
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Second, bubble breaking effects in the presence of nanoparticles lead to the
smaller bubbles which are the desired phenomenon for increasing the mass
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transfer [54]. Fig. 12 shows the bubble breaking effect after the addition of
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nanoparticles to a solution. However, there are different studies focused on the
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nanofluids and their unique features.
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**Fig. 12**
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Esmaeili Faraji et al. [55] observed that nanofluids enhance mass transfer relative
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to the base fluid up to 40% when just 0.02 wt.% nanoparticle dosage was employed. Komati and Suresh [56] investigated the absorption of CO2/MDEA
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solution by using nanofluid in a wetted column, their experimental results showed that mass transfer coefficient enhanced up to 92.8% for a magnetite dosage about
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0.39 vol.%. Lee et al. [56] studied methanol-based fluids containing Al2O3 and SiO2 nanoparticles at different concentrations to remove CO2. They observed that the maximum CO2 absorption enhancements (compared to base fluid) were 4.5% at 0.01 vol.% of Al2O3 at 20°C and 5.6% at 0.01 vol.% of SiO2 at -20 °C,
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respectively. Park et al. [57] also studied the effect of SiO2 nanoparticles on the absorption rate of CO2 in several aqueous solutions and found that the absorption rate decreased with increasing the concentration of nanoparticles because of elasticity of the solution.
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Pineda et al. [55] carried out the CO2 absorption in a tray column absorber with
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a methanol-based nanofluid by using Al2O3 and SiO2 nanoparticles. They found that the maximum enhancement in absorption rates (relative to base fluid) was
9.4 and 9.7% for Al2O3 and SiO2 nanoparticles, respectively. Taheri et al.[6]
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showed an enhancement in mass transfer for Al2O3-DEA nanofluid compared to
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the base fluid up to 14% in 0.1 wt.%. nanoparticle concentration. They observed
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that the CO2 absorption rate enhanced up to 40% at 0.05 wt.% of SiO2/DEA nanofluid. Nanoparticles have a high surface area, leading to an improvement in
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mass transfer.
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Generally, the reaction between MDEA and CO2 is slow. The absorption of CO2
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is directly affected by changes in pressure and temperature. In fact, the increase in pressure and temperature have a positive and negative effect on the CO 2
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absorption, respectively. In an aqueous solution, CO2 reacts with water and hydroxide according to the following reactions: H3O HCO3 CO2 2 H 2O
(6)
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HCO3 CO2 OH
(7)
CO2 reacts indirectly with MDEA in the presence of water to form bicarbonate and protonated MDEA. Three different mechanisms have been proposed for this reaction. The first mechanism suggests that MDEA neutralizes the species formed
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by the CO2-water reactions (reactions 6 and 7). The second mechanism (reaction
H
O KMDEA-CO
H
O +
2
C
+
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+
R1R2R3N
H +
-
HCO3
(8)
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R1R2R3N:
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8) assumes that MDEA associates with water before reacting with CO2:
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O
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The third mechanism (reaction 9) follows the zwitterion mechanism. The
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zwitterion mechanism is generally accepted as the mechanism behind the reaction of primary and secondary amines with CO2. The reaction involves the formation
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of a zwitterion in the presence of GO, which is followed by the base-catalyzed
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deprotonation of the zwitterion to form carbamate [58,59]:
C
O
kI
+
k -1 I
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RR'R''N + O
O RR'R''N
C
-
(9) O
The zwitterion formed immediately reacts with water and hydroxide to produce respectively bicarbonate and protonated MDEA and bicarbonate and MDEA according to the following reaction: 18
O RR'R''N + O
C
O
kI
+
RR'R''N
k -1 I
-
(10)
C O
Therefore, the presence of hydroxide groups on GO will increase the rate of (CO2)
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with MDEA. Amine molecules are inserted to the intergallery space of GO due to the interaction between amines and oxygen-containing groups, for example,
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hydrogen-bonding interactions, protonation of amine by acidic sites on the GO layers and chemical grafting of the amine to the GO surface through nucleophilic
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reactions on the epoxy groups [60]. Primary and secondary amines react with
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(CO2) and generate stable carbamate [61]. The GO structure is full of oxygen-
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rich functional groups such as hydroxide, epoxide, carbonyl, and carboxyl
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groups. Thus GO provides a wide range of reaction sites and functional groups
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for gas sorption. Different oxy-functional groups provide interlayer spaces for gas
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sorption. Surface area is also activated by functional groups and the ability of functional groups for (CO2) adsorption is in the following order: -OH˃ -COOH˃
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-NH2˃ -CH3 [62].
We used the error propagation theory to determine the uncertainties of results
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after calculation [63]. The uncertainty of some variables was calculated as
q [(
q 2 q )] ... [( ) du ]2 r u
(11)
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This value is mainly dependent upon some variables including r, …, u which have changed in a random and independent manner. These uncertainties are related to the instruments used in this work. Some of errors in the pressure sensors (equal to ±0.003 MPa), temperature measurement instrument (±0.01 K), and balance
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accuracy for solvent added into the equilibrium cell (±0.0001 g) lead to an uncertainty of data obtained by tests employed on the equilibrium cell. Using the
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equation 11, the uncertainties of mCO2 was calculated as given in final data in Tables 2, 3 and.
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3. Conclusion
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In this study, the effect of GO addition on the solubility of CO2 in MDEA aqueous
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solution was studied. GO was synthesized using a modified hummer method and characterized by XRD, BET and IR spectroscopy to determine the structure. The
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XRD analysis revealed a strong peak at 2θ=10.3° which confirmed the structure.
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GO sheet showed apparent adsorption bands for some functional groups such as hydroxyl –OH (3187 cm-1), carboxyl C=O (1733 cm-1). The results showed that
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GO has a positive effect on the solubility of CO2 in MDEA solution because of high surface area and existence of OH groups on the surface of the GO and
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enhancement in mass transfer coefficient. Experimental measurements were carried out at temperatures of 303.15, 313.15, 323.15 and 333.15 K. Results showed that increasing the temperature has a negative effect on the absorption rate, while an increase in pressure improved the absorption capacity. In the
20
presence of 0.1 wt.% GO in 40 wt.% MDEA, an increase of 9.1% was observed in the absorption of CO2 at all temperatures while in the presence of 0.2 wt.% GO in 40 wt.% MDEA, capacity absorption was promoted up to 10.4%. Further addition of GO showed no significant change in absorption capacity. The exact
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mechanism of this enhancement is unknown so far, but different factors can be considered for this phenomenon such as a zwitterion mechanism, hydrodynamic
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effect, and bubble breaking effects.
S. Shirazian, M. Pishnamazi, M. Rezakazemi, A. Nouri, M. Jafari, S. Noroozi, A.
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[1]
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Figure captions Fig. 1. Zeta potential of the prepared solution (40 wt.% MDEA + 60 wt.% water + 0.1 wt.% GO). Fig. 2. Schematic design of the system used for experiments.
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Fig. 3. Validation experiments for solubility of CO2 in 30 wt.% MDEA at 313.15 K.
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Fig. 4. Validation experiments for solubility of CO2 in 50 wt.% MDEA at 328.15 and 343.15 K.
U
Fig. 5. X-ray diffraction (XRD) pattern of GO
N
Fig. 6. Infrared (IR) spectrum of GO
A
Fig.7. N2 adsorption/desorption on GO
M
Fig.8. Scanning electron microscope (SEM) images of GO
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Fig. 9. Comparison of CO2 solubility in 40 wt.% MDEA and 40 wt.% MDEA with the presence of 0.1 wt.% GO and 0.2 wt.% GO at a temperature of (A)
PT
303.15, (B) 313.015, (C) 323.15 and (D) 333.15 K. Fig. 10. Comparison of CO2 solubility at different temperatures (A) without
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addition of GO, (B) with addition of 0.1 wt.% GO and (C) with addition of 0.2 wt.% GO.
A
Fig. 11. Schematic description for hydrodynamic effect Fig. 12. Schematic description for bubble breaking effect
29
A ED
PT
CC E Fig. 1
30
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A
M
Figures
A ED
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U
N
A
M
Fig. 2
Fig. 3
31
A ED
PT
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SC R
U
N
A
M Fig. 4
32
A ED
PT
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Fig. 6
33
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SC R
U
N
A
M Fig. 5
A ED
PT
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SC R
U
N
A
M Fig.7
Fig.8
34
35
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PT
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U
N
A
M
36
A ED
PT
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U
N
A
M
37
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PT
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SC R
U
N
A
M
A ED
PT
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SC R
U
N
A
M Fig. 9
38
39
A ED
PT
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U
N
A
M
40
A ED
PT
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U
N
A
M
A ED
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Fig. 11
41
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Fig. 12
42
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A
M
Tables Table 1 Specifications and sources of materials used in this study Cas number
Purity
CO2
124-38-9
99.5 % Roham gas company
Methyldiethanolamine
CH3N(CH2CH2OH)2
105-59-9
˃ 99 %
Hydrochloric acid
HCl
7647-01-0
37 %
Potassium permanganate
KMnO4
7722-64-7
˃ 99 %
Hydrogen peroxide
H2O2
7722-84-1 7782-42-5
Sulfuric acid
H2SO4
7631-99-4
A
NaNO3
7664-93-9
Sigma-Aldrich Merck
Sigma-Aldrich
30 %
Sigma-Aldrich
˃99.5 %
Merck
˃99 %
Sigma Aldrich
˃99 %
Sigma Aldrich
ED
M
Sodium nitrate
N
Graphite oxide
source
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Carbon dioxide
registery
SC R
Molecular formula
U
Chemical name
A
CC E
PT
Table 2 Experimental equilibrium partial pressure of CO2 in 40 mass % MDEA at different temperatures mCO2 (moleCO2/ kgsolvent)±δmCO2
T (K)±0.01
PCO2 (kPa)± 1
303.15
111.82
2.750± 0.18
303.15
359.42
3.089± 0.19
303.15
623.01
3.387±0.18
303.15
862.62
3.563±0.20
303.15
1110.22
3.699±0.20
303.15
1341.85
3.766±0.19
303.15
1541.53
3.821±0.20
43
3.834±0.19
303.15
2092.65
3.902±0.18
313.15
126.62
2.559±0.20
313.15
311.13
2.862±0.20
313.15
518.61
3.139±0.19
313.15
748.96
3.324±0.18
313.15
979.18
3.430±0.19
313.15
1278.73
3.522±0.19
313.15
1523.91
3.588±0.20
313.15
1853.78
313.15
2099.24
323.15
127.62
323.15
300.30
323.15
533.03
323.15
765.76
3.057±0.18
983.48
3.188±0.19
323.15
1343.84
3.346±0.20
323.15
1591.59
3.438±0.19
323.15
1876.88
3.490±0.19
323.15
2049.55
3.503±0.20
333.15
105.42
1.750±0.18
333.15
293.67
2.184±0.19
333.15
474.39
2.5130±0.19
333.15
692.77
2.763±0.18
333.15
858.43
2.894±0.20
333.15
1091.87
3.026±0.19
PT CC E A
3.667±0.19
U
3.707±0.20
M
A
N
2.165±0.18
ED
323.15
333.15
2.532±0.18 2.874±0.20
1295.18 3.092±0.18
44
IP T
1733.23
SC R
303.15
333.15
1566.27 3.157±0.19
333.15
1897.59 3.171±0.19
Table 3
135.78
3.052±0.18
303.15
303.51
3.362±0.18
303.15
559.10
3.672±0.18
303.15
902.55
3.956±0.19
303.15
1158.15
4.103±0.18
303.15
1373.80
4.148±0.18
303.15
1589.46
4.213±0.19
303.15
1685.30
N
303.15
2036.74
4.269±0.18
313.15
149.71
2.878±0.20
265.02
3.080±0.19
313.15
549.35
3.502±0.19
313.15
725.92
3.634±0.19
313.15
940.83
3.764±0.19
313.15
1247.73
4.040±0.20
313.15
1470.23
4.157±0.19
313.15
1800.10
4.041±0.19
313.15
2037.87
4.069±0.19
323.15
112.613
2.356±0.19
323.15
247.74
2.660±0.18
323.15
472.97
3.080±0.19
323.15
713.21
3.326±0.20
323.15
968.47
3.499±0.19
A
ED
PT CC E A
4.226±0.19
45
IP T
303.15
313.15
mCO2 (moleCO2/ kgsolvent)±δmCO2
SC R
PCO2 (kPa)± 1
U
T (K)±0.01
M
Experimental equilibrium partial pressure of CO2 in 40 mass % MDEA + 0.1 mass % GO at different temperatures
1298.80
3.672±0.19
323.15
1561.56
3.788±0.19
323.15
1816.82
3.851±0.19
323.15
1997.00
3.862±0.20
333.15
135.54
1.985±0.20
333.15
316.26
2.449±0.19 512.04 2.826±0.19
333.15
753.01 3.105±0.19
333.15
978.91 3.276±0.20
333.15
1257.53 3.407±0.19
333.15
1498.49 3.451±0.19
333.15
1799.70 3.494±0.20
333.15
1965.36 3.495±0.19
A
N
U
SC R
333.15
IP T
323.15
Table 4
ED
T (K)±0.01
M
Experimental equilibrium partial pressure of CO2 in 40 mass % MDEA + 0.2 mass % GO at different temperatures mCO2 (moleCO2/ kgsolvent)±δmCO2
303.15
134.87
3.095±0.18
303.15
309.64
3.382±0.18
303.15
561.17
3.699±0.18
303.15
798.72
3.924±0.18
303.15
1110.22
4.104±0.18
303.15
1413.74
4.195±0.18
303.15
1621.41
4.255±0.19
303.15
1853.04
4.286±0.18
303.15
1988.82
4.297±0.19
313.15
165.09
2.928±0.20
313.15
349.60
3.264±0.19
313.15
587.74
3.558±0.19
PT CC E A
PCO2 (kPa)± 1
46
787.96
3.689±0.19
313.15
1017.55
3.820±0.19
313.15
1316.79
3.937±0.20
313.15
1593.01
4.001±0.19
313.15
1761.76
4.007±0.20
313.15
1984.17
4.008±0.20
323.15
150.15
2.417±0.19
323.15
270.27
2.738±0.19
323.15
570.57
3.231±0.20
323.15
825.82
3.433±0.20
323.15
1073.57
3.583±0.19
323.15
1373.87
3.728±0.20
323.15
1524.02
3.801±0.20
323.15
1779.28
3.803±0.19
323.15
2087.09
N
333.15
120.48
SC R
U
1.971±0.19 338.85 2.564±0.19 579.82 2.935±0.20 798.19 3.155±0.19
333.15
1039.16 3.334±0.19
333.15
1340.36 3.446±0.20
333.15
1536.14 3.398±0.19
333.15
1746.99 3.492±0.20
333.15
2018.07 3.493±0.20
PT
333.15
A
CC E
3.837±0.20
A
ED
333.15
M
333.15
IP T
313.15
47
S2213-3437(18)30706-1 https://doi.org/10.1016/j.jece.2018.11.027 JECE 2782
To appear in: Received date: Revised date: Accepted date:
3 September 2018 3 November 2018 13 November 2018
Please cite this article as: Irani V, Maleki A, Tavasoli A, CO2 Absorption Enhancement in Graphene-Oxide/MDEA Nanofluid, Journal of Environmental Chemical Engineering (2018), https://doi.org/10.1016/j.jece.2018.11.027 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.
CO2 Absorption Enhancement in Graphene-Oxide/MDEA Nanofluid Vahid Irani, Amin Maleki, Ahmad Tavasoli* School of Chemistry, College of Science, University of Tehran, Tehran 11155-4563, Iran
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* Corresponding author: E-mail: tavasoli.a@ ut.ac.ir; Tel: +982161113643
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Graphical abstract
Highlights
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GO was synthesized by modified hummers method.
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Solubility of carbon dioxide was investigated in a solution an MDEA solution by addition of GO. Results showed 9.1% enhancement for addition of 0.1 mass % GO to the 40 mass % MDEA. These experiments for addition of 0.2 mass % GO presented a capacity absorption promotion up to 10.4% in different temperatures.
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Further concentration of graphene oxide into the amine solution showed no significant enhancement in absorption capacity
Abstract Graphene-Oxide (GO)/MDEA nanofluid was prepared for CO2 absorption
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enhancement in gas sweetening process. GO was synthesized using a modified Hammer method and characterized by XRD, BET, SEM and IR spectroscopy to
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determine the structure. The XRD analysis showed a strong peak at 2θ=10.3°. GO was dispersed in MDEA by ultrasonic treatment and zeta potential analysis
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was employed to examine the stability of the nanofluid. The measured zeta
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potential (35 mv) confirmed the excellent stability in the solution. GO/MDEA
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nanofluid showed high absorption capacities toward CO2 due to the high surface
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area and existence of OH groups on the GO surface and enhancement in mass
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transfer coefficient. When 0.1 wt.% GO was introduced to the 40 wt.% MDEA,
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the absorption capacity of the solution was promoted up to 9.1%. This enhancement for 0.2 wt. % slightly more while the solutions with higher GO
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content showed no significant enhancement in absorption capacity. This enhancement has a reverse and direct relationship with increasing temperature
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and pressure, respectively. Keywords: CO2; Nanofluid; Graphene oxide; Solubility; MDEA
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1. Introduction Global warming is because of increasing greenhouse gases such as carbon dioxide (CO2), which is arisen from fossil fuel combustion [1,2]. However, these fuels are important as an essential energy source and it is necessary to develop some
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methods for reducing CO2 emissions [3,4]. CO2 capture has attracted a global attention because of different adverse effects of CO2 emissions. Researchers have
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focused on reducing the greenhouse gas emissions using different procedures and techniques including chemical and physical absorption, membranes, adsorption
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with solid state materials and cryogenic separation processes [5]. One of these
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methods is CO2 absorption in the gas sweetening process as a preferred method
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for reducing CO2 emissions. Gas sweetening is commonly referred to as an
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important industrial process for removing of acid gases such as CO2 and hydrogen
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sulfide (H2S) from gas stream [6–9]. Based on the origin of the natural gas, there
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are different levels amounts of CO2 and H2S impurities in the natural gas [10]. Several CO2 capture technologies such as chemical absorption, physical
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absorption, and membrane have been studied [11,12]. Alkanoamine-based solvents are widely used for the removal of acidic gases. The most common
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alkanoamines used in the CO2 absorption process are monoethanolamine (MEA), diethanolamine (DEA), diglycolamine (DGA), methyldiethanolamine (MDEA) and Triethanolamine (TEA) [13–15]. The advantage of MDEA over other amines is improving properties against degradation and corrosion than other
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alkanoamine. MDEA is a tertiary amine with a stable structure and has no carbamate formation in the CO2 absorption process. However, these processes are highly energy intensive and corrosive and making it necessary to develop new studies based on the solid adsorbents with a high
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performance and low costs [16]. Besides, these processes are potentially releasing
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carcinogenic compounds[17,18]. Recently, many efforts have been focused on
developing new techniques or processes for CO2 capture, especially sorption using solid state materials [19–21]. For this purpose, a variety of solid materials
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have been considered such as zeolites [22,23], clays [24], MOFs [25,26], COFs
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[27] and carbon-based materials [5,28,29]. Carbon-based materials have been
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studied for promising gas sorption, storage, and separation because of the availability, robust pore structure, thermal and chemical stability, high surface
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area and comfortable synthesis in industrial scale [30]. Some carbonaceous
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materials such as carbons and carbon molecular sieves are extremely used as a solid adsorbent and catalyst in the gas related processes, because of their
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availability, high surface area and thermal and chemical stability [31–33]. Graphene has a higher capacity for adsorption of acid gases due to its very high
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specific surface area and ability to control the chemical structure [34,35]. Graphene and reduced graphene oxide (GO) have attracted significant interest in different areas such as gas adsorption due to intrinsic properties including large theoretical surface area and chemical stability [36]. The physical adsorption is
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considered a new approach [37–39]. Graphite oxide is also a carbon material with high stability at room temperature. The structure of graphite oxide is not defined precisely, but it has been used in many fields because of its unique and unusual chemical properties. Graphene can be doped and functionalized with amine
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functional groups to enhance the adsorption of acidic gases. The GO structure contains oxygen-rich functional groups including hydroxide and epoxide groups
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on the basal plane and carbonyl and carboxyl groups on edges [40]. Therefore, the GO has a hydrophilic nature and it is soluble in water and several solvents.
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GO with its largely expanded and tuneable layer structure provides a platform for
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engineering a wide range of chemical reactions [41,42]. Therefore, it is possible
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to simultaneously use the properties of alkanoamine solutions and GO by addition
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of GO into the alkanoamine solution and examination the absorption.
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In this work, for the first time, the solubility of CO2 in a 40 wt.% MDEA solution
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in the presence of 0.1 and 0.2 wt.% GO was evaluated at different temperatures (303.15, 313.15, 323.15 and 333.15 K). GO/MDEA nanofluid was prepared for
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CO2 absorption enhancement in gas sweetening process. GO was synthesized using a modified Hammer method and characterized by XRD, BET and IR
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spectroscopy to determine the structure. GO was dispersed in MDEA by ultrasonic treatment. 2. Materials and methods 2.1. Materials 6
(CO2) was obtained from Roham gas company with a minimum grade of 99.5%. All the materials used in this study were reagent grade without further purification. Deionized water used for a solution was degassed in an ultrasonic bath (FUNGILAB, model UA10MFD) at 353.15 K and wave frequency of 50 kHz for an hour. The specifications of the chemical materials used in this study
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are summarized in Table 1. **Table 1**
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2.2. Preparation of GO
GO was synthesized according to a modified Hummers method [43]. 7 g graphite
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powder, 3 g sodium nitrate, and 100 ml concentrated sulphuric acid and 12 ml
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phosphoric acid was added to a beaker and stirred for 10 min in an ice bath. 15 g
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KMnO4 was slowly added to the solution under stirring conditions so that the
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temperature of the mixture remained below 5 °C. The mixture was heated to
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maintain a temperature within the range of 35-40 °C. After half an hour, as the mixture was thickened, 235 ml deionized water was added to the reaction mixture
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and stirred for 15 min. An additional 800 ml of water was added followed by the
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slow addition of 50 ml H2O2 (30%). The mixture was then centrifuged and washed with deionized water and 5% HCl to remove any residue from the
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suspension. Finally, the suspension was dried in an oven at 60 °C to obtain a powder. 2.3. Solution preparation
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Firstly, a mass fraction of 40% MDEA and 60% water were prepared. Then, the desired mass fraction of GO including 0.1 wt.% or 0.2 wt.% were added to this solution using an analytical balance with the accuracy of 0.1 mg. GO was dispersed and stabilized into the base fluid (MDEA 40 wt.%) with ultrasonication
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process. With the application of sonication process, the use of a surfactant is not needed to obtain a desired dispersion in the solution. Since nanofluids are
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suspensions, their stability is one of the most important roles that they play. Various procedures can be used for stability examination, such as zeta potential,
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UV, etc. Zeta potential is a fundamental verification for the stability of
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nanofluids. GO should be stable in the solution and this stability was verified by
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zeta potential analysis. Small levels of zeta potential indicate that the particles
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tend to settle but high levels of zeta potential to confirm the stability of the
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solution. So, high zeta potential more than 25 mv (positive or negative) shows that the solution will be stable. Fig.1 illustrates the zeta potential diagram for this
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solution (40 wt.% MDEA+ 60 wt.% water with the addition of 0.1 wt.% GO).
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The measured zeta potential for this solution was 35 mv that confirms the colloidal dispersion of the solution.
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**Fig. 1**
2. Apparatus and Procedure The details of experimental measurement of gas solubility in amine solution can be found elsewhere [7,9,44]. However, the equilibrium cell is a double-well 8
autoclave reactor which was connected to a recirculation bath with a temperature stability within ±0.02 K. Temperature was measured using a digital thermometer with 0.01 K resolution with a sensor inserted into the cell via thermowell. The equilibrium cell pressure was measured using a calibrated Baroli type BD
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SENSOR gauge with an accuracy of 0.1% of full scale. The range of the transmitter sensor for equilibrium cell is (0-4) MPa. Gas container pressure is
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measured by a Baroli type BD SENSOR pressure gauge in the range of (0-6) MPa which is accurate to within 0.1% of full scale. Fig. 2 presents the schematic design
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3.1 Operation of the apparatus
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**Fig. 2**
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of the system.
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Initially, a vacuum was applied to the equilibrium cell using a vacuum pump. Then, a known amount of amine solution that was modified by the addition of the
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GO was injected into the equilibrium cell so that equilibrium cell pressure
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maintains vacuum property. The temperature was then adjusted to the desired value; when the equilibrium state was obtained, the pressure that is showed by the pressure sensor is the vapor pressure of the solution. The volume of the gas
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container was well known and the amount of CO2 injected into the equilibrium cell was calculated with a procedure presented by Park and Sandal [45]. The accurate PVT data was obtained from the National Institute of Standards and
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Technology that provides data for all fluids including pure CO2. So, the total mole of gas injected into the equilibrium cell can be calculated by equation 1. nCO2
VC P1 P2 ( ) RTa Z1 Z 2
(1)
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where VC presents the volume of the CO2 container, P1, P2 are the initial and final partial pressures of the CO2 container before and after the gas injection into the
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equilibrium cell respectively, Ta is the ambient temperature, Z1, Z2 denotes the compressibility factors relating to P1, P2 respectively. Then, the equilibrium
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pressure of the equilibrium cell (PCO2) is calculated by
(2)
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PCO2 PT PV
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where PT and PV present the absolute pressure and vapor pressure of the solution
V g PCO2 Z CO2 RT
2
(3)
PT
g nCO 2
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g respectively. The remaining CO2 gas in the gas phase nCO is represented as
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where Vg is equilibrium cell volume minus the solution volume used in the equilibrium cell that represents the gas-phase volume and Z CO is the 2
compressibility factor of CO2 at PCO and T. The amount of CO2 available in the
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2
liquid phase is presented as l g nCO nCO2 nCO 2 2
(4)
Finally, the molality of the CO2 available in the liquid phase is determined with
10
mCO2
l nCO 2
(5)
wsol
where wsol (kg) denotes the mass of solvent. The volume of the gas phase (V g), is the difference between the equilibrium cell volume and the volume of the l uncharged solvent. The uncertainty corresponding to nCO is related to the
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2
uncertainties of the thermometer, pressure sensors and balance that are
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determined by suppliers.
All volumes are determined by pressure swing experiments (PSE). In this method
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pressure drop is measured simultaneously with the opening the valve between
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unknown and reference volume using Charles-Gay-lussac law so that the volume
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of the gas container and equilibrium cell are equal to 309.42±1.7 and 49.50±1.8 cm3, respectively. It should be noted that the partial pressure of MDEA in the
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vapor phase was neglected [46]. The vapor pressure can be calculated using
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different equations of state [47,48]. 1. Results and discussion Validation experiments
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1.1.
In order to verify the accuracy of the method and set-up, several validation tests
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were performed with CO2 and the solubility results were compared with the data available in the literature. The validation experiments are carried out with 30 and 50 wt.% aqueous MDEA solution at different temperatures. Figs. 3 and 4 showed that the results have a good agreement with the other data.
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**Fig. 3** **Fig. 4**
XRD analysis
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1.2.
X-ray diffraction (XRD) analysis is used to illustrate the average crystalline
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properties of the GO sheet. As shown in Fig. 5 the strong peak at 2θ=10.3° is in a good agreement with the literature [40,49].
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FTIR spectra analysis
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1.3.
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**Fig. 5**
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This analysis is employed to evaluate the functional groups and the structure of materials. As shown in Fig. 6, GO sheet shows apparent adsorption bands for
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functional groups including hydroxyl –OH (3187 cm-1), carboxyl C=O (1733 cm), aromatic C=C (1622 cm-1), alkoxy C-O (1047 cm-1) and epoxy C-O (1220 cm-
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). Oxygen-containing functional groups such as C=O and C-O confirm that
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1
graphite is oxidized into GO that is consistent with the literature [40,50–52].
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These results confirm the successful synthesis of GO.
1.4.
**Fig. 6**
BET analysis
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The surface area of GO powders was 293 m2/g measured by Brunauer–Emmett– Teller (BET). GO was expanded in water in an ultrasonic water bath to ensure the stability of GO. Fig. 7 shows the N2 adsorption/desorption isotherms of GO.
1.5.
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**Fig. 7** SEM morphology
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Fig. 8 presents the SEM images of GO. As observed, GO consists of randomly aggregated thin sheets that are forming a porous and disordered network. GO
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layer thickness is 29.3 to 35.16 nm. Elemental composition of GO was measured
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by Energy-dispersive X-ray spectroscopy(EDS) analysis and results showed that
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the sample contains C, O, and S.
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**Fig. 8**
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2. Experimental solubility tests
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In Tables 2, 3 and 4, the experimental data of CO2 solubility for 40 wt.% MDEA and (40 wt.% MDEA +0.1 wt.% and 0.2 wt.%) GO are reported respectively. The
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results indicate that CO2 solubility of the solution was increased in the presence of the GO. When 0.1 wt.% GO was added to the solution, solubility
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measurements show that the increase of CO2 sorption is in average more than 9.1%. These measurements for 0.2 wt.% GO show an average increase in solubility up to 10.4%. Further concentration of nanofluid was employed and showed no significant enhancement in CO2 absorption capacity. These results
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confirm that the addition of nanoparticle into the MDEA solution will increase the solubility of CO2 effectively. CO2 solubility increased with increasing the partial pressure and decreased with increasing temperature.
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**Table 2** **Table 3**
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**Table 4**
Fig. 9 and 10 show the solubility of CO2 at different temperatures (303.15,
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313.15, 323.15 and 333.15 K); and different partial pressure ranged from 100 to
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2100 kPa. The data were firstly obtained for 40 wt.% aqueous MDEA solution
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without addition of nanoparticles. Then, CO2 absorption of MDEA solution was obtained in the presence of different addition concentrations of GO. Each graph
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presents the solubility at a particular temperature. The experimental data are reported in terms of partial pressure of CO2 versus the molality. Increasing the
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temperature decreased the solubility results, while increasing the partial pressure improved the solubility. When 0.1 wt.% GO was introduced to the solution, the
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average absorption capacity was promoted up to 9.1%, while this enhancement rate for the solution in the presence of 0.2 wt.% was slightly more at any temperature. Higher GO content showed no significant enhancement in
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absorption capacity. All the solutions were significantly under effect of temperature and partial pressure.
**Fig. 9** 14
**Fig. 10** As shown in Fig. 9 and 10, each graph presents the solubility of CO2 at a particular temperature. Increasing the temperature decreased the solubility results, while increasing the partial pressure improved the solubility in all solutions. When 0.1
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wt.% GO was introduced to the solution, the average absorption capacity was
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slightly less than that of the solution in the presence of 0.2 wt.% at any
temperature. These results verify the positive effect of the addition of GO into the MDEA solution. The absorption capacity was increased with increasing the
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partial pressure to the extent that a linear trend was observed in the results.
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Fig. 10 presents the comparison of CO2 solubility at different temperatures. As
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shown, temperature has a negative effect on solubility. When temperature was
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increased, we observed a decrease in absorption capacity for all solutions. The trend in each graph is pretty much the same after increasing the temperature.
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There are different factors affecting the CO2 absorption rate and capacities such
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as improvement in mass transfer coefficient in the presence of nanostructures, additional reactions of CO2 in the presence of nanoparticles and zwitterion
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mechanism. This increase could be due to the high surface area of the nanostructure and the presence of the –OH and epoxy groups but based on previous works mass transfer coefficient plays a critical role in this process. There are several mechanisms that can be considered for the improvement in the mass transfer coefficient. First, the hydrodynamic effect, in which nanoparticles collide 15
with the gas-liquid interface and make the interface thinner. So, the diffusion coefficient between the two phases will be increased in the presence of nanostructures and CO2 absorption will be subsequently improved [53]. This phenomenon has been depicted in Fig. 11.
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**Fig. 11**
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Second, bubble breaking effects in the presence of nanoparticles lead to the
smaller bubbles which are the desired phenomenon for increasing the mass
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transfer [54]. Fig. 12 shows the bubble breaking effect after the addition of
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nanoparticles to a solution. However, there are different studies focused on the
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nanofluids and their unique features.
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**Fig. 12**
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Esmaeili Faraji et al. [55] observed that nanofluids enhance mass transfer relative
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to the base fluid up to 40% when just 0.02 wt.% nanoparticle dosage was employed. Komati and Suresh [56] investigated the absorption of CO2/MDEA
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solution by using nanofluid in a wetted column, their experimental results showed that mass transfer coefficient enhanced up to 92.8% for a magnetite dosage about
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0.39 vol.%. Lee et al. [56] studied methanol-based fluids containing Al2O3 and SiO2 nanoparticles at different concentrations to remove CO2. They observed that the maximum CO2 absorption enhancements (compared to base fluid) were 4.5% at 0.01 vol.% of Al2O3 at 20°C and 5.6% at 0.01 vol.% of SiO2 at -20 °C,
16
respectively. Park et al. [57] also studied the effect of SiO2 nanoparticles on the absorption rate of CO2 in several aqueous solutions and found that the absorption rate decreased with increasing the concentration of nanoparticles because of elasticity of the solution.
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Pineda et al. [55] carried out the CO2 absorption in a tray column absorber with
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a methanol-based nanofluid by using Al2O3 and SiO2 nanoparticles. They found that the maximum enhancement in absorption rates (relative to base fluid) was
9.4 and 9.7% for Al2O3 and SiO2 nanoparticles, respectively. Taheri et al.[6]
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showed an enhancement in mass transfer for Al2O3-DEA nanofluid compared to
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the base fluid up to 14% in 0.1 wt.%. nanoparticle concentration. They observed
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that the CO2 absorption rate enhanced up to 40% at 0.05 wt.% of SiO2/DEA nanofluid. Nanoparticles have a high surface area, leading to an improvement in
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mass transfer.
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Generally, the reaction between MDEA and CO2 is slow. The absorption of CO2
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is directly affected by changes in pressure and temperature. In fact, the increase in pressure and temperature have a positive and negative effect on the CO 2
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absorption, respectively. In an aqueous solution, CO2 reacts with water and hydroxide according to the following reactions: H3O HCO3 CO2 2 H 2O
(6)
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HCO3 CO2 OH
(7)
CO2 reacts indirectly with MDEA in the presence of water to form bicarbonate and protonated MDEA. Three different mechanisms have been proposed for this reaction. The first mechanism suggests that MDEA neutralizes the species formed
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by the CO2-water reactions (reactions 6 and 7). The second mechanism (reaction
H
O KMDEA-CO
H
O +
2
C
+
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+
R1R2R3N
H +
-
HCO3
(8)
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R1R2R3N:
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8) assumes that MDEA associates with water before reacting with CO2:
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O
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The third mechanism (reaction 9) follows the zwitterion mechanism. The
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zwitterion mechanism is generally accepted as the mechanism behind the reaction of primary and secondary amines with CO2. The reaction involves the formation
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of a zwitterion in the presence of GO, which is followed by the base-catalyzed
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deprotonation of the zwitterion to form carbamate [58,59]:
C
O
kI
+
k -1 I
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RR'R''N + O
O RR'R''N
C
-
(9) O
The zwitterion formed immediately reacts with water and hydroxide to produce respectively bicarbonate and protonated MDEA and bicarbonate and MDEA according to the following reaction: 18
O RR'R''N + O
C
O
kI
+
RR'R''N
k -1 I
-
(10)
C O
Therefore, the presence of hydroxide groups on GO will increase the rate of (CO2)
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with MDEA. Amine molecules are inserted to the intergallery space of GO due to the interaction between amines and oxygen-containing groups, for example,
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hydrogen-bonding interactions, protonation of amine by acidic sites on the GO layers and chemical grafting of the amine to the GO surface through nucleophilic
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reactions on the epoxy groups [60]. Primary and secondary amines react with
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(CO2) and generate stable carbamate [61]. The GO structure is full of oxygen-
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rich functional groups such as hydroxide, epoxide, carbonyl, and carboxyl
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groups. Thus GO provides a wide range of reaction sites and functional groups
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for gas sorption. Different oxy-functional groups provide interlayer spaces for gas
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sorption. Surface area is also activated by functional groups and the ability of functional groups for (CO2) adsorption is in the following order: -OH˃ -COOH˃
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-NH2˃ -CH3 [62].
We used the error propagation theory to determine the uncertainties of results
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after calculation [63]. The uncertainty of some variables was calculated as
q [(
q 2 q )] ... [( ) du ]2 r u
(11)
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This value is mainly dependent upon some variables including r, …, u which have changed in a random and independent manner. These uncertainties are related to the instruments used in this work. Some of errors in the pressure sensors (equal to ±0.003 MPa), temperature measurement instrument (±0.01 K), and balance
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accuracy for solvent added into the equilibrium cell (±0.0001 g) lead to an uncertainty of data obtained by tests employed on the equilibrium cell. Using the
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equation 11, the uncertainties of mCO2 was calculated as given in final data in Tables 2, 3 and.
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3. Conclusion
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In this study, the effect of GO addition on the solubility of CO2 in MDEA aqueous
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solution was studied. GO was synthesized using a modified hummer method and characterized by XRD, BET and IR spectroscopy to determine the structure. The
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XRD analysis revealed a strong peak at 2θ=10.3° which confirmed the structure.
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GO sheet showed apparent adsorption bands for some functional groups such as hydroxyl –OH (3187 cm-1), carboxyl C=O (1733 cm-1). The results showed that
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GO has a positive effect on the solubility of CO2 in MDEA solution because of high surface area and existence of OH groups on the surface of the GO and
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enhancement in mass transfer coefficient. Experimental measurements were carried out at temperatures of 303.15, 313.15, 323.15 and 333.15 K. Results showed that increasing the temperature has a negative effect on the absorption rate, while an increase in pressure improved the absorption capacity. In the
20
presence of 0.1 wt.% GO in 40 wt.% MDEA, an increase of 9.1% was observed in the absorption of CO2 at all temperatures while in the presence of 0.2 wt.% GO in 40 wt.% MDEA, capacity absorption was promoted up to 10.4%. Further addition of GO showed no significant change in absorption capacity. The exact
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mechanism of this enhancement is unknown so far, but different factors can be considered for this phenomenon such as a zwitterion mechanism, hydrodynamic
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effect, and bubble breaking effects.
S. Shirazian, M. Pishnamazi, M. Rezakazemi, A. Nouri, M. Jafari, S. Noroozi, A.
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[1]
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Figure captions Fig. 1. Zeta potential of the prepared solution (40 wt.% MDEA + 60 wt.% water + 0.1 wt.% GO). Fig. 2. Schematic design of the system used for experiments.
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Fig. 3. Validation experiments for solubility of CO2 in 30 wt.% MDEA at 313.15 K.
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Fig. 4. Validation experiments for solubility of CO2 in 50 wt.% MDEA at 328.15 and 343.15 K.
U
Fig. 5. X-ray diffraction (XRD) pattern of GO
N
Fig. 6. Infrared (IR) spectrum of GO
A
Fig.7. N2 adsorption/desorption on GO
M
Fig.8. Scanning electron microscope (SEM) images of GO
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Fig. 9. Comparison of CO2 solubility in 40 wt.% MDEA and 40 wt.% MDEA with the presence of 0.1 wt.% GO and 0.2 wt.% GO at a temperature of (A)
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303.15, (B) 313.015, (C) 323.15 and (D) 333.15 K. Fig. 10. Comparison of CO2 solubility at different temperatures (A) without
CC E
addition of GO, (B) with addition of 0.1 wt.% GO and (C) with addition of 0.2 wt.% GO.
A
Fig. 11. Schematic description for hydrodynamic effect Fig. 12. Schematic description for bubble breaking effect
29
A ED
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CC E Fig. 1
30
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A
M
Figures
A ED
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U
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A
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Fig. 2
Fig. 3
31
A ED
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SC R
U
N
A
M Fig. 4
32
A ED
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Fig. 6
33
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SC R
U
N
A
M Fig. 5
A ED
PT
CC E
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SC R
U
N
A
M Fig.7
Fig.8
34
35
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PT
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U
N
A
M
36
A ED
PT
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U
N
A
M
37
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PT
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SC R
U
N
A
M
A ED
PT
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SC R
U
N
A
M Fig. 9
38
39
A ED
PT
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U
N
A
M
40
A ED
PT
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SC R
U
N
A
M
A ED
PT
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M Fig. 10
Fig. 11
41
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Fig. 12
42
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A
M
Tables Table 1 Specifications and sources of materials used in this study Cas number
Purity
CO2
124-38-9
99.5 % Roham gas company
Methyldiethanolamine
CH3N(CH2CH2OH)2
105-59-9
˃ 99 %
Hydrochloric acid
HCl
7647-01-0
37 %
Potassium permanganate
KMnO4
7722-64-7
˃ 99 %
Hydrogen peroxide
H2O2
7722-84-1 7782-42-5
Sulfuric acid
H2SO4
7631-99-4
A
NaNO3
7664-93-9
Sigma-Aldrich Merck
Sigma-Aldrich
30 %
Sigma-Aldrich
˃99.5 %
Merck
˃99 %
Sigma Aldrich
˃99 %
Sigma Aldrich
ED
M
Sodium nitrate
N
Graphite oxide
source
IP T
Carbon dioxide
registery
SC R
Molecular formula
U
Chemical name
A
CC E
PT
Table 2 Experimental equilibrium partial pressure of CO2 in 40 mass % MDEA at different temperatures mCO2 (moleCO2/ kgsolvent)±δmCO2
T (K)±0.01
PCO2 (kPa)± 1
303.15
111.82
2.750± 0.18
303.15
359.42
3.089± 0.19
303.15
623.01
3.387±0.18
303.15
862.62
3.563±0.20
303.15
1110.22
3.699±0.20
303.15
1341.85
3.766±0.19
303.15
1541.53
3.821±0.20
43
3.834±0.19
303.15
2092.65
3.902±0.18
313.15
126.62
2.559±0.20
313.15
311.13
2.862±0.20
313.15
518.61
3.139±0.19
313.15
748.96
3.324±0.18
313.15
979.18
3.430±0.19
313.15
1278.73
3.522±0.19
313.15
1523.91
3.588±0.20
313.15
1853.78
313.15
2099.24
323.15
127.62
323.15
300.30
323.15
533.03
323.15
765.76
3.057±0.18
983.48
3.188±0.19
323.15
1343.84
3.346±0.20
323.15
1591.59
3.438±0.19
323.15
1876.88
3.490±0.19
323.15
2049.55
3.503±0.20
333.15
105.42
1.750±0.18
333.15
293.67
2.184±0.19
333.15
474.39
2.5130±0.19
333.15
692.77
2.763±0.18
333.15
858.43
2.894±0.20
333.15
1091.87
3.026±0.19
PT CC E A
3.667±0.19
U
3.707±0.20
M
A
N
2.165±0.18
ED
323.15
333.15
2.532±0.18 2.874±0.20
1295.18 3.092±0.18
44
IP T
1733.23
SC R
303.15
333.15
1566.27 3.157±0.19
333.15
1897.59 3.171±0.19
Table 3
135.78
3.052±0.18
303.15
303.51
3.362±0.18
303.15
559.10
3.672±0.18
303.15
902.55
3.956±0.19
303.15
1158.15
4.103±0.18
303.15
1373.80
4.148±0.18
303.15
1589.46
4.213±0.19
303.15
1685.30
N
303.15
2036.74
4.269±0.18
313.15
149.71
2.878±0.20
265.02
3.080±0.19
313.15
549.35
3.502±0.19
313.15
725.92
3.634±0.19
313.15
940.83
3.764±0.19
313.15
1247.73
4.040±0.20
313.15
1470.23
4.157±0.19
313.15
1800.10
4.041±0.19
313.15
2037.87
4.069±0.19
323.15
112.613
2.356±0.19
323.15
247.74
2.660±0.18
323.15
472.97
3.080±0.19
323.15
713.21
3.326±0.20
323.15
968.47
3.499±0.19
A
ED
PT CC E A
4.226±0.19
45
IP T
303.15
313.15
mCO2 (moleCO2/ kgsolvent)±δmCO2
SC R
PCO2 (kPa)± 1
U
T (K)±0.01
M
Experimental equilibrium partial pressure of CO2 in 40 mass % MDEA + 0.1 mass % GO at different temperatures
1298.80
3.672±0.19
323.15
1561.56
3.788±0.19
323.15
1816.82
3.851±0.19
323.15
1997.00
3.862±0.20
333.15
135.54
1.985±0.20
333.15
316.26
2.449±0.19 512.04 2.826±0.19
333.15
753.01 3.105±0.19
333.15
978.91 3.276±0.20
333.15
1257.53 3.407±0.19
333.15
1498.49 3.451±0.19
333.15
1799.70 3.494±0.20
333.15
1965.36 3.495±0.19
A
N
U
SC R
333.15
IP T
323.15
Table 4
ED
T (K)±0.01
M
Experimental equilibrium partial pressure of CO2 in 40 mass % MDEA + 0.2 mass % GO at different temperatures mCO2 (moleCO2/ kgsolvent)±δmCO2
303.15
134.87
3.095±0.18
303.15
309.64
3.382±0.18
303.15
561.17
3.699±0.18
303.15
798.72
3.924±0.18
303.15
1110.22
4.104±0.18
303.15
1413.74
4.195±0.18
303.15
1621.41
4.255±0.19
303.15
1853.04
4.286±0.18
303.15
1988.82
4.297±0.19
313.15
165.09
2.928±0.20
313.15
349.60
3.264±0.19
313.15
587.74
3.558±0.19
PT CC E A
PCO2 (kPa)± 1
46
787.96
3.689±0.19
313.15
1017.55
3.820±0.19
313.15
1316.79
3.937±0.20
313.15
1593.01
4.001±0.19
313.15
1761.76
4.007±0.20
313.15
1984.17
4.008±0.20
323.15
150.15
2.417±0.19
323.15
270.27
2.738±0.19
323.15
570.57
3.231±0.20
323.15
825.82
3.433±0.20
323.15
1073.57
3.583±0.19
323.15
1373.87
3.728±0.20
323.15
1524.02
3.801±0.20
323.15
1779.28
3.803±0.19
323.15
2087.09
N
333.15
120.48
SC R
U
1.971±0.19 338.85 2.564±0.19 579.82 2.935±0.20 798.19 3.155±0.19
333.15
1039.16 3.334±0.19
333.15
1340.36 3.446±0.20
333.15
1536.14 3.398±0.19
333.15
1746.99 3.492±0.20
333.15
2018.07 3.493±0.20
PT
333.15
A
CC E
3.837±0.20
A
ED
333.15
M
333.15
IP T
313.15
47