Study on CO2 Desorption Behavior of a PDMS-SiO2 Hybrid

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Study on CO2 Desorption Behavior of a PDMS-SiO2 Hybrid Membrane Applied in a Novel CO2 Capture Process Ebrahim Ataeivarjovi, Zhigang Tang, and Jian Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08630 • Publication Date (Web): 26 Jul 2018 Downloaded from http://pubs.acs.org on July 31, 2018

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ACS Applied Materials & Interfaces

Study on CO2 Desorption Behavior of a PDMS-SiO2 Hybrid Membrane Applied in a Novel CO2 Capture Process

Ebrahim Ataeivarjovi1, Zhigang Tang1*, Jian Chen1 1

State Key Laboratory of Chemical Engineering, Department of Chemical Engineering,

Tsinghua University, Beijing 100084, P.R. China

Keywords: CO2 capture; Desorption PDMS-SiO2 membrane; DMC; Energy consumption; Pervaporation; Wilson plot mass transfer

* Corresponding Author: Zhigang Tang, Email: [email protected]

Abstract In this present work, a novel approach has been proposed by which a solvent absorption and membrane desorption methods can significantly reduce the carbon dioxide (CO2) emission and energy consumption. In this method, the CO2 capture processing is based on the combination of membrane and physical solvent. Here, dimethylcarbonate (DMC) and polydimethylsiloxane (PDMS)-SiO2 nanocomposite were used as the physical solvent and desorption membrane, respectively. However, the main focus of this research was on the CO2 desorption behavior of PDMS-SiO2 hybrid membrane. To do so, the influence of the operating conditions and membrane properties on the pervaporation process to capture CO2 have been investigated. The PDMS-SiO2 hybrid membrane containing 10% wt. SiO2 was the most effective membrane. Results revealed that increase in CO2 concentration from 1.5 wt% to 3 wt% led to decrease the selectivity from 94 to 47 and increase in flux from 1.7 (kg/m2.h) to 5.38 (kg/m2.h). In addition, an increase in temperature increased the flux and reached the 1 ACS Paragon Plus Environment

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highest level (8.17 kg/m2.h) at 40 °C. However, the selectivity decreased to 36.13. It was found that the addition of SiO2 nanoparticles to the PDMS membrane not only enhanced the membrane performance but also decreased the energy consumption about 75% compared with gas stripping method and mass transfer about 49% compared with pure PDMS membrane. Finally, these results illustrated that such novel technique used for pervaporation separation process is a green and promising alternative to separate CO2 from the physical solvent.

1. Introduction Over last decades, the dramatic rising emissions of greenhouse gases has been enumerated of controversial global issues from which some other threatening problems have been caused, including the rise in global warming, subsequent death of the polar glaciers and affecting human societies. Carbon dioxide (CO2) component comprises approximately 80% of the total greenhouse gases. CO2 capture technique has been identified as an effective way to overcome the challenges attributed to the considerable emission of CO2 and as an opportunity for reutilization of carbon source.1,2 Over a few years ago, researchers have been trying to address the issue by presenting various methods to lessen the CO2 emission using the implement and development of the CO2 capture and storage (CCS) techniques, such as post-combustion, precombustion and oxy-fuel combustion.3 Among the indicated CCS approaches, precombustion is the most suitable option due to its high CO2 partial pressure, large trapping capacity, high separation driving force and low energy consumption.4 For pre-combustion CO2 capture, different capturing technologies can be applied, including hybrid applications, adsorption,5 absorption,6 and membranes.7 Among these capturing methods, absorption approach has grasped more attention because of its mature and successful utilization in industrial scale. However, the common absorption technique uses heating or gas stripping for

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solvent regeneration, which contributes a considerable energy consumption and requires some secondary separations. In conventional CO2 capture methods, i.e. solvent-based absorption technique, CO2 is absorbed in some solvents, and subsequently, the captured CO2 is released upon introducing gas stripping or heating (Figure 1a). The major problem of the conventional technology is high-energy consumption used for solvent recovery and secondary separation. Moreover, using membrane technologies can be considered as an ideal option for gas absorption,8,9,10,11 and gas separation.12,13,14 An ideal membrane for CO2 capture should have the following properties: high gas selectivity, and high thermal and mechanical stability. Regarding membrane absorption case, a membrane with a very large area is needed owing to the considerable amount of gaseous feedstock and risk of membrane poisoning, i.e. being polluted with the impurity of the gas mixture. Therefore, to overcome the limitations of CO2 capture methods, researchers try to use novel and effective methods. In view of the above situations, absorption method possesses a high selectivity and membranebased approach has a low energy consumption characteristics. Consequently, the innovative integration forms a solvent absorption-membrane desorption coupling the line technology shown in Figure 1. Therefore, to deal with the energy consumption issue, a novel process combining with solvent absorption and membrane desorption were presented in Figure 1b and Figure 2.



Figure 1. Schematic diagram of the conventional method (a) and novel method (b) used in the present study for CO2 capture In this method, CO2 is absorbed in some solvents, and then, CO2 is desorbed by pervaporation (PV) membrane from the rich liquid solvent (Figure 2). In this method solvent absorption and membrane desorption were used in CO2 capture under medium or high CO2 partial pressure, such as the integrated gasification combined cycle (IGCC).15 After absorption process in the absorption tower (No. 3, under pressure range 3-4 MPa), the pressure of the rich liquid solution would be decreased to 0.8 MPa and 0.2 MPa by the decompressions No. 6 and No. 8, respectively. On the other side, since DMC is a physical solvent, thereby, the solubility of CO2 in DMC has a direct relation with pressure (based on Henry’s law).16 Consequently, a considerable amount of CO2 would be desorbed in the decompressions (No. 6 and 8) due to the pressure drop. Eventually, the negligible CO2 remained would be desorbed in solvent by the membrane (No. 9) because of the most significant advantage of the membranes for desorption which is the ability to strip CO2 while in the solvent phase with a low energy consumption and without the vaporization of the solvent.


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Figure 2. DMC absorption-membrane desorption process for CO2 capture under medium or high CO2 partial pressures Compared with the case of gas separation using membrane, the advantages of desorption membranes are: • There is no necessity for a membrane with a very large area because a significant amount of CO2 is desorbed using the reduced pressure exerted by two initial decompressions prior to reaching PV membrane (Figure 2). In other words, after so-called pre-desorption stage, there would be only a few amount of remaining CO2 to be desorbed by PV membrane. It also lessens the risk of being polluted because of contacting with loaded absorbent instead of gas-mixture feedstock. • The difference of molecular weight between the absorbent and CO2 is large, so the selectivity of the membrane does not need to be too high • Absorption after decompression in the high-pressure combined with decompression and the CO2 flux dramatically reduced through the membrane, thus the membrane flux does not need to be too large



• Mixed gas including CO2 may have some impurities and if it contact directly with membrane leads to block the pores and decrease the efficiency of the membrane. However, in novel process (desorption method using membrane), contact between mixed gas and solvent results in absorption of CO2 in solvent. Then, the CO2 will desorbed from solvent in contact with desorption membrane. This leads to decrease the amount of impurities and thus improve performance of membrane for CO2 capturing. In this work, we have tried to improve the CO2 capture performance of desorption membrane method using dimethyl carbonate (DMC) as the physical solvent and a flat hybrid PV membrane prepared from polydimethylsiloxane (PDMS) and silicon dioxide nanoparticles (PDMS-SiO2). According to the results of some previous investigations,17,18 DMC is favored as an environmentally friendly, nontoxic, cost-effective, and highperformance CO2 absorbent. Furthermore, DMC has other significant properties such as high selectivity with high solubility of CO2, low boiling point and low desorption energy consumption. These breakthrough properties present DMC as a preferential alternate for CO2 absorption solvent.19 PDMS, as a linear silicone rubbery material, is one of the most effectively used membrane materials among commercially applied polymeric substances, which its effectiveness attributes to gas permeability, excellent mechanical properties, dielectric properties, high free volume, low temperature flexibility and low chemical reactivity.20,21 The PDMS material possesses a benchmark hydrophobic structure that is suitable for separation purposes. Recently, Xia

22

and Zhimin

23

have verified the feasibility of PDMS desorption membrane

with different cross-linking agents. In addition, they showed that membrane desorption process has an obvious energy saving effect compared with the conventional technologies. However, these polymeric membranes suffer from the restriction of the increase between selectivity and permeability.24 Different methods are used for improving polymeric


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membrane gas separation properties. One of them is the incorporation of nanoparticles with polymer matrix. The incorporation of nanoparticles with polymers, so-called mixed matrix membranes, is a type of membrane with modified properties. Different types of inorganic fillers have been applied for the modification of membranes performance, such as zeolitic imidazolate framework materials, carbon nanotubes, carbon nanotubes, mesoporous silica, titanium dioxide (TiO2),25,26 zeolite,27 and SiO2 nanoparticles.28 Compared with other nanoparticles, SiO2 is cheaper and can be easily found in the earth's crust. Moreover, due to natural advantages, nontoxicity, small particle size, narrow pore size distribution, large pore volume, high surface area, hydrophilic properties, containing large number of hydroxyl groups, SiO2 nanoparticle has high potentiality to be used in preparation of effective membranes.29,30 Some researchers have used hollow fiber membranes to desorb CO2 from aqueous solutions of organic amines or carbonates.31,32 However, those porous membrane materials are likely to cause the leakage of solvent DMC applied in the current study. Additionally, it seems difficult to ensure the axial uniformity of the film thickness by preparing a dense layer of PDMS-SiO2 on hollow fiber membrane surface. Considering those assumptions, the preparation of a flat membrane can easily ensure the film thickness and uniformity of the nano-material dispersion that is why a flat membrane has been used in this study. Moreover, in the future enlargement experiments, the preferences of other membrane types will be considered. Therefore, the preferences of other membrane modules will be explored in further scale-up researches. In this study, we used cellulose acetate (CA) ultrafiltration membrane as a substrate, tetraethoxysilane (TEOS) as a cross-linking agent, and nano-SiO2 particles as filler in PDMS to prepare asymmetric pervaporation membrane. In addition, the CO2 desorption and separation from DMC-CO2 mixture has been studied. To investigate the process precisely, the 7 ACS Paragon Plus Environment


effect of different parameters such as SiO2 concentration and operating conditions (feed temperature, permeate pressure, feed flow rate and overall mass transfer resistance) on PV process have been investigated. Eventually, a preliminary calculation and evaluation of desorption energy consumption were performed.

2. Experimental

2.1. Material Dibutyltin dilaurate (DBTDL), TEOS, DMC, and n-heptane were obtained from AladdinReagent Company, Shanghai, China. Fumed silica with specific surface areas of 200 m2/g was obtained from Meryer, Shanghai. Hydroxyl-terminated polydimethylsiloxane (H-PDMS) (molecular weight 10,000) was supplied from Shanghai Resin Factory Co., Ltd., China. CA support was purchased from the Membrane Science & Technology Research Center, Nanjing University of Technology. CO2 with a volume fraction of 0.9999 was supplied from Bei Wen Gas, Beijing.

2.2. Preparation of membranes

2.2.1. Preparation of PDMS membrane The preparation of the PDMS and PDMS-SiO2 membranes were accomplished through the following procedure. First, PDMS solution was prepared by 6 g hydroxyl-terminated polydimethylsiloxane, 14 mL n-heptane and 1.2 g TEOS through a continuous mixing in a vigorous magnetic stirring for 2 h. Then 0.48 g DBTDL was added, and the final obtained mixture was stirred for 5 min. Finally, PDMS composite membrane was obtained by casting the prepared solution on a CA support for 24 h at room temperature. Then, the final product was dried for 4 h at 80 °C. 8 ACS Paragon Plus Environment

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2.2.2. Preparation of PDMS-SiO2 nanocomposite membranes The preparation of PDMS-SiO2 nanocomposite membranes was similar to the pure PDMS membranes. To prepare PDMS-SiO2 nanocomposite membranes, 14 mL n-heptane, 1.2 gr TEOS and Nano- Fumed silica with different concentrations (1%, 5%, and 10%) were mixed with 6 gr PDMS, and then 0.48 gr of DBTDL as a catalyst was added. In order to reach a good dispersion of nanoparticles, a probe ultrasonication was employed for 2 h at ambient temperature.33 The prepared solution was cast on a CA sheet at room temperature and dried after one day at 80 °C for 4 h.

2.3. Characterization of prepared membranes The Fourier transform infrared (FTIR) spectra of the prepared membranes were collected using the attenuated total reflection (ATR) technique by a spectrophotometer (Bruker, Vertex 70v, Germany). To assess the cross-sectional and surface morphology of the composite PDMS membrane and nanocomposite PDMS-SiO2 membrane, a scanning electron microscopy (SEM) with a field emission SEM (ZEISS, ULTRA 55 FE-SEM, Germany) and an atomic force microscope (AFM) (Dimension Icon, Bruker, Germany) were used.

2.4. PV measurements Figure 3 shows the laboratory PV setup. As seen in Figure 3, the device has two parts for absorption and desorption. In the adsorption part, CO2 dissolved in a physical solvent, subsequently, the rich liquid solvent passed through the membrane, and consequently, CO2 separated from the solvent by the membrane. The experimental PV device for desorption membrane and regeneration includes a gas compartment, stirring vessel, plate membrane pool (23.75 cm2), gas–liquid separator, membrane separator, and lye tank. Temperature, pressure, and flow rate were controlled by electronic sensing devices. To explore parameters, the effects of CO2 concentration (1.5 wt% (mole fraction of 0.0278) to 3 wt% (mole fraction of 9 ACS Paragon Plus Environment


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0.0549)), flow rate (0.3, 0.6, 0.9, and 1.2 L/min) and temperature (25 °C to 40 °C) on the rich-liquid side were studied in the above mentioned ranges. The measured desorption time and membrane area were 1 h and 23.75 cm2, respectively. All the experiments in this study were conducted in triplicate to assure the accuracy of the results. CO2 loaded in DMC

1-CO2 cylinders 2-Gas chamber 3-Thermostatic bath

4-inlet tube 5-Dissolving tank 6-liquid pump

CO2 desorption by membrane

7-plate membrane pool 8-gas-liquid separator 9-electronic balance

10-lye tank 11-buffer tank 12-Vacuum pump

Figure 3. CO2 absorption in the rich liquid–membrane desorption test PV device Most important parameters in PV membrane are selectivity (α) and permeation flux (J). J =

Q A×t

(1)

Wherein, Q is the mass of permeate (g), A is the effective area of PV membrane (m2), and t is operating time (h). The mass of the permeate side of DMC was obtained using a weighing balance. Selectivity:

(2)

Where in, α is membrane selectivity. YA is the mass fraction of CO2 in permeate, whereas XA refers to the mass fraction of CO2 in rich solution.34 10 ACS Paragon Plus Environment

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2.5. Wilson-plot mass transfer Mass transfer can be figured out by experimental methods based on the so-called Wilson plot. 35

Wilson plot is widely applied to determine the membrane mass transfer for liquid-liquid

operations. However, in the present study, this method has been used to evaluate the mass transfer in a gas-liquid operation. In the Wilson plot method, the permeation and vaporization process is generally described by the solution-diffusion mechanism. The transfer of the components can be divided into the following three steps:35 1) From the liquid phase to the upstream surface of the membrane 2) Adsorption on the membrane surface and dissolution into the membrane 3) Desorption downstream of the membrane. The mass transfer equation of the pervaporation process is

−

,

(3)

Where ρ (g/m2·s) is the permeation flux, K (m/s) is the total mass transfer coefficient, ρ0 and ρ1 (g/m3) are the CO2 concentration in the liquid and gaseous phase, the partition coefficient of CO2 between liquid and membrane. Since the downstream pressure of the membrane is very low, as a result, ρ1 is approximately zero. Consequently, Equation 3 can be reduced to

.

(4)

Considering Equation 4, one can obtain the total mass transfer coefficient as / , where and are obtained from experimental results. Using the tandem resistance model and based on the following five assumptions for the pervaporation process: (1) the pervaporation process is a steady-state process, (2) the dissolution of the membrane surface component is in equilibrium, (3) the diffusion of the components in the membrane is onedimensional mass transfer process, (4) the internal temperature and pressure are constant, and (5) the downstream mass transfer resistance is negligible due to a very small concentration of 11 ACS Paragon Plus Environment


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CO2 in the downstream, we can figure out the resistance of the liquid film boundary layer and the resistance in the film as below:

+

,

(5)

Where, and stand for the liquid mass transfer coefficient and membrane mass transfer coefficient, respectively. In order to determine the liquid mass transfer within a controlled temperature condition, we have

ν ,

(6)

Where A and ν are the area of the membrane and feed flow rate (m/s), respectively. It should be noted that α is an empirical parameter. Finally, by introducing Equation 6 to Equation 5 one has

+

+

(7)

Where ν is the material flow rate, m/s. The mass transfer resistance of the liquid side is proportional to the liquid velocity as , where α is an empirical parameter and is the liquid velocity.36 A plot of 1/K versus

results in a straight line, which is known as Wilson plot. Then, the membrane mass transfer resistance can be calculated from the intercept of the plot. It is believed that this method is more accurate than the predictions achieved by the traditional correlations,37,38 therefore, this method was applied throughout the current study.


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3. Results and discussion

3.1. FTIR characterization of membranes The pure SiO2, pure PDMS and PDMS-SiO2 membranes were analyzed by ATR-IR spectrophotometry, and a membrane with pure PDMS was used for comparison (Figure 4). In Figure 4, several transmitted peaks for pure SiO2 are apparent. The absorption band at 794 cm-1, and 1099 cm-1 can be attributed to the symmetric stretching vibration, and asymmetric stretching vibration of the Si–O–Si bond, respectively. The band assigned to 950 cm-1 represents the bending vibration and stretching vibration of the Si–OH bond. The band around 1631 cm-1 can be ascribed to the stretching and bending vibration of free OH groups. A broad band around 3434 cm-1 is caused by the stretching vibration of hydroxyl. I addition for pure PDMS and PDMS-SiO2 membranes can be see, the bond at 3400 cm−1 corresponded to the stretching vibration of the O–H group. The absorption bands at 2972 cm−1 are attributed to the vibration mode of the C–H stretching. An absorption band characteristic of the deformation of the –CH3 radical appeared at 1434 cm−1, confirming previous groupings.39 The strong absorption band at 1261 cm−1 is attributed to the vibration mode of the –Si–C stretching. The absorption bands at 1026 cm−1 and 1087 cm-1 correspond to vibrations of the Si–O–Si bonds. The presence of the Si–OH bond confirms hydrogen bond formation between organic components and interconnected inorganic components.40 The PDMS materials with different loadings have similar peak shapes and no new absorption peaks, indicating that only a physical interaction occurs between SiO2 and PDMS; and no chemical cross-linking has occurred.41



Figure 4. FTIR-ATR spectra of SiO2, PDMS, PDMS-SiO2-1%, PDMS-SiO2-5%, and PDMSSiO2-10%

3.2. Morphological and topographical features of prepared membranes

3.2.1. SEM characterization of membranes The SEM images of the support (CA), PDMS and PDMS-SiO2 membranes are shown in Figure 5. As it is clear, in Figure 5 (a, b), the support (CA) has a porous structure and the fabricated PDMS membranes are dense with a quite smooth surface (see Figure 5 (c, d)). Compared with the PDMS membrane, the surfaces of the PDMS-SiO2 membranes are more rough and heterogeneous, so that these properties can be due to the presence of SiO2 nanoparticles on their surface. Figure 5 (e, f) shows that by introducing the low amount of SiO2 nanoparticles (1 wt.%) to the PDMS, the morphology and surface roughness had slightly changed. It could be due to the inappropriate distribution SiO2 nanoparticles. Moreover, the silica nanoparticles appeared to be embedded in the dense PDMS film. 14 ACS Paragon Plus Environment

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Noteworthy, by varying SiO2 concentration from 1 wt. % to 10 wt.%, the amount of the SiO2 nanoparticles increased on the membrane surface (see Figure 5e to 5j). As it can be observed in Figure 5 (i, j), SiO2 nanoparticles inside PDMS have more appropriate distribution and better compatibility between nanoparticle and polymer is observed. On the other side, the SEM images illustrate that, SiO2 nanoparticles well dispersed on the membrane surface. Therefore, it gives a uniform surface in SEM images. In addition, the thickness of the active layer of prepared membranes was measured by SEM, which is approximately 12 µm.



Figure 5. SEM images of the cellulose acetate (CA) support layer, PDMS, and PDMS-SiO2 membranes: (a) surface of CA support layer, (b) cross section of CA support layer, (c) 16 ACS Paragon Plus Environment

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surface of PDMS membrane, (d) cross section of PDMS membrane, (e) surface of 1% PDMS-SiO2 membrane, (f) cross section of 1% PDMS-SiO2 membrane, (g) surface of 5% PDMS-SiO2 membrane, (h) cross section of 5% PDMS-SiO2 membrane, (i) surface of 10% PDMS-SiO2 membrane, (j) cross section of 10% PDMS-SiO2 membrane

3.2.2. AFM characterization of membranes The surface roughness and morphology of PDMS and PDMS-SiO2 membranes with different percentages incorporation of SiO2 nanoparticles were characterized by AFM. As seen in Figure 6, by varying the particle loading from 1 wt% to 10 wt%, the roughness of PDMS, PDMS-SiO2-1 wt.%, PDMS-SiO2-5 wt.%, and PDMS-SiO2-10 wt.% were 5.49 nm, 15.5 nm, 57.8 nm and 136 nm, respectively. The reason for that behavior is that, when the surface roughness is increased, the surface content area of the membrane is also increased, so that the membrane permeate flux is improved. Results revealed that the incorporation of nanoparticles leads to an increase in the surface roughness and effective area in the membrane. Similar results have been reported by Zhang et al.42 in which the effect of incorporation of Material Institute Lavoisier-53 (MIL-53) to PDMS membrane has been studied.



Figure 6. AFM images of the PDMS membranes (a), PDMS-SiO2-1 wt.% (b), PDMS-SiO2-5 wt.% (c) and PDMS-SiO2-10 wt.% (d)

3.3. Effect of operating conditions Generally, operating conditions will significantly affect permeation flux, selectivity, and feed flow rate. To determine the membrane performance in PV process some important parameters should be tested, such as the temperature, concentration, and flow rate of 18 ACS Paragon Plus Environment

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feedstock. The effect of aforementioned three parameters on the performance of membranes have been examined in this work.

3.3.1. Effect of temperature According to previous studies, pervaporation process strongly depends on operating temperature.43 Figure 7 (a, b) shows the effect of the feed temperature (ranging from 25 °C to 40 °C) on the PV of the PDMS and PDMS-SiO2 membranes at a feed flow rate of 0.6 L/min. As seen in Figure 7, by the increase of the operating temperature from 25 °C to 40 °C, the flux increased from 5.38 kg/m2.h to 8.17 kg/m2.h, whereas selectivity decreased from 47.61 to 36.13. According to the literature, by the increase of the feed temperature, the driving force, resulting from the partial pressure difference across the membrane, rised considerably, which lead to an improvement in permeate flux.44,45 Results revealed that the prepared membranes reached the highest flux and selectivity of 8.17 kg/m2.h and 36.13 respectively at 40 ºC. On the other hand, the selectivity of membranes decreased with increment in temperature, which such phenomenon might be due to the penetration of the DMC particles along with CO2 through the membrane at high temperatures.37 According to the transport theory by the solution-diffusion, by the increase of the operating temperature, the permeation flux increased and usually selectivity decreased, which all are in agreement with results reported in the literature.44,46



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Figure 7. Effects of the feed temperature on the PV performance of the membranes The effect of the temperature may be described by an Arrhenius-type relationship:20 ln "# ln "

%$(8)

&'


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where, "# represents the kinetic constant, " refers to the pre-exponential factor (kg/m2.h), () corresponds to activation energy (kJ/mol), R is the gas constant, and T is temperature (K).47 () represents activation energy, which is calculated from the slope of LnJ versus 1/RT plot (Figure 8). () is an important parameter in separation processes, which provides some information about the process mechanism. Typically, a separation process with physical interactions is characterized by low values of () (80 kJ/mol).48,49 Using Equation 8, the activation energies of PDMS and PDMS-SiO2 were calculated. The PDMS membrane possessed an activation energy of 33.39 kJ/mol, which was higher than that of PDMS-SiO2 (22.09 kJ/mol). The higher activation energy indicates the more sensitive behavior of the PDMS membrane toward variation in temperature, which in turn means, the PDMS-SiO2 membranes can be more effective than PDMS. In general, the permselectivity increase with increasing concentration of nanoparticle (1 wt.% to 10 wt.%) in polymer matrix due to the hydroxyl groups on the surface of SiO2 nanoparticles.50 PDMS-SiO2 membrane show lower activation energies than PDMS membrane due to the existence of silica nanoparticles creates excess spaces in interface of polymer and silica nanoparticles, which results of increase in gas permeability. These results shown that, the main of the excessive spaces generated according to incorporation of inorganic silica nanoparticles.51 In a previous literature, a linearized Arrhenius-type equation has been used to evaluate the activation energy of CO2 adsorption on the surface of activated carbon.49 In another study on the desorption of CO2 on mesoporous silica materials, the activation energy of CO2 physical desorption has been calculated from the Arrhenius equation.52 According to the results, the general performance trend for polymeric membranes in PV demonstrated an increase in



permeation flux accompanied by a decline in selectivity when the operating temperature increased, which verified what had been reported previously.53

Figure 8. Arrhenius plots of the permeation flux for the separation of a DMC/CO2 mixture

3.3.2. Effects of CO2 concentration CO2 concentration is enumerated as another important operating parameter (a high vacuum condition) that is directly linked to a high energy cost.54 The CO2 concentration difference of components represents the driving force in PV, which is linked to the activity gradient of composites. CO2 partial pressure is directly correlated to the component activity at the downstream side of membranes, and selectivity affects PV characteristics. Zero Partial pressure resulted in maximum gradient, and for higher Partial pressure, PV characteristics were affected by partial pressure.55 In our study, generally, the flux declined as partial pressure was elevated, which was because of a decrease in driving force for mass transport. Furthermore, according to Henry's law, the amount of dissolved gas into the solvent at a constant temperature is directly correlated to the partial pressure of the gas. According to 22 ACS Paragon Plus Environment

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Xia,22 it has been reported that the solubility of CO2 in the DMC solvent increased with increasing partial pressure. The effects of the CO2 concentration (from 1.5% to 3%) on the selectivity and flux of the prepared membranes were investigated, and the results have been shown in Figure 9. Figure 9a indicates that the flux increased by increasing CO2 mass fraction. An increase in the CO2 concentration of the rich liquid intensified the vapor pressure; however, the pressure of the other side was almost constant due to vacuum. As a result, the driving force in the membrane increased. The PDMS membrane prepared by SiO2 nanoparticles showed a higher total flux compared with the PDMS membrane because the addition of the SiO2 nanoparticles may lead to an increase in free volume within the membrane and a decrease in mass transfer resistance. Consequently, the flux of the membrane increased. Figure 9b illustrates that the selectivity decreased with increasing CO2 mass fraction. This result may be due to the higher driving force at higher partial pressure.56 Diffusion through the membrane was a rate-determining step in reducing pressure in PV. In other words, at a higher partial pressure, diffusing molecules will experience a larger driving force, which will enhance desorption rate at the downstream side of the membrane. The driving force decreased by increasing pressure, leading to reduced desorption rates of sorbed molecules.57 Notably, pressure variations at lower ranges affected membrane performance more significantly. Lowering CO2 concentrations (especially below 2%) resulted in higher fluxes and decreased the membrane selectivity toward CO2. According to Equation 2, when the solvent (DMC) does not pass the membrane, YA equals to 1, and XA increases with increasing CO2 concentration. Thus, the selectivity declines when the CO2 concentration elevated.



Figure 9. Effect of the CO2 concentration on the PV performance of the prepared membranes


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3.3.3. Effect of feed flow rate Figure 10 shows the effect of the feed flow rate on the pervaporation performance of the PDMS and PDMS/SiO2 membranes. In this scenario, the feed flow rate varied from 0.3 to 1.2 L/min at a CO2 concentration of 3 wt.% and feed temperature of 25 °C. According to the literature, generally, an increase of the feed flow rate reduces the polarization effect of the concentration and increases the flux due to a reduction of the transport resistance in liquid boundary layer.58 Similarly, in our work, an increase in the feed flow rate prompted the improvement of the CO2 mass transfer rate. As discussed previously, obviously the CO2 permeation flux of the PDMS and PDMS-SiO2 membranes were increased by increasing the feed flow rate from 0.3 to 1.2 L/min. The permeation flux of the PDMS membrane was considerably enhanced with the addition of the SiO2 nanoparticles so that it was larger than the pure PDMS membrane. At the feed flow rate of 1.2 L/min, the CO2 permeation flux of the PDMS-SiO2 film was 8.68, which is 4.88 times larger than the permeation flux of the PDMS layer. This phenomenon can be ascribed to the modified structure of the PDMS membrane by incorporation of the SiO2 nanoparticles in the sake of increasing the O−H functional groups of the nanoparticles located on the membrane, which also improved the membrane performance for CO2 absorption. Moreover, in a constant feed liquid flow rate of 1.2 L/min, the CO2 permeation flux of the PDMS-SiO2 membrane increased 56% compared with the PDMS membrane. Therefore, the CO2 permeation flux of the PDMS membrane was lower than that of the PDMS-SiO2 membrane prepared in this study.



Figure 10. Effect of the feed flow rate on the PV performance of the prepared membranes

3.4. Analysis of the overall mass transfer According to the mass transfer equations of the pervaporation process, by neglecting gas phase mass transfer resistance in flat sheet membranes, due to a small CO2 concentration in the downstream, the membrane mass transfer can be determined using the Wilson plot shown in Figure 11. At a certain temperature and flow rate of the liquid, the permeation flux of CO2 under different liquid flow rate was determined, subsequently, the permeability flux (J) and the liquid flow rate (ρ0) were obtained, and finally, a straight line was obtained. From Equation 4, the slope for the total mass transfer coefficient K calculated under these operating conditions.59,30 To fit 1/K and υ-α, from Equation 7 it can be seen that α must meet the 1/K and υ-α linear relationship; as shown in Figure 11, using the intercept the resistance within film and mass transfer coefficient were determined.


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The best straight line fit to the data of 1/Kl versus 1/uα was obtained when α equaled to 0.99. Similarly, Nabian et al.60 and Yang et al.61 have reported an analogous relationship of 1/Kl with 1/u0.93 to evaluate the liquid stream in designing some hollow fiber contactors. They calculated the mass transfer resistance based on the intercept of the plot, which in turn shows the contribution of the overall mass transfer resistance. The overall mass transfer resistances of the prepared membranes (i.e. PDMS and PDMSSiO2) have been listed in Table 1. The detailed information and calculations of the mass transfer of the prepared membranes have been mentioned in the supporting information file, 1. Mass Transfer section (Table S1 to S20). Table 1. Overall mass tarnsfer resistance obtained by Wilson plot Membrane

Overall mass transfer resistance (s/m)

PDMS

12.17⨯106

PDMS-SiO2

6.27⨯106

As seen in Figure 11, the overall mass transfer resistance declined with an elevation in the feed flow rate, consequently, the contribution of the membrane resistance increased. This behavior can be attributed to the disturbance of the boundary layer in the liquid phase with an increase in the velocity. According to these data, the overall mass transfer resistance of the PDMS-SiO2 membrane reduced to approximately 49% compared with the PDMS membrane. With this respect, such trend confirms a better performance of the PDMS-SiO2 membrane in terms of effective mass transfer.



Figure 11. Wilson plot mass transfer of the PDMS and PDMS-SiO2 membranes

3.5. Comparison of the PV performance of the composite and nanocomposite membranes Table 2 presents the separation performance of the membranes prepared by PDMS. The PDMS-SiO2 nanocomposite membrane prepared in the present research performed better than the membranes without incorporated materials. Table 2 compares the separation performances of the PDMS and PDMS-SiO2 membranes with various membranes reported in the previous literature. The PDMS/SiO2 membrane exhibited a relatively high flux with comparable selectivity.


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Table 2. Performances of different membranes for CO2 separation from DMC–CO2 Membrane

SiO2 wt.%

Temperature Pressure (°C)

Cross

Flux

linking

(kg/m2.h)

Selectivity

Reference

agent PDMS

-

25

3%

TSD

0.9696

24.4

62

PDMS

-

25

1%

VTEO

5.144

36.5

63

PDMS

-

25

3%

TEOS

1.75

38.07

This study

PDMS/SiO2

1%

25

3%

TEOS

3.2

41.13

This study

PDMS/SiO2

5%

25

3%

TEOS

3.86

45.61

This study

PDMS/SiO2

10%

25

3%

TEOS

5.38

47.61

This study

3.6. Energy consumption of the absorption and desorption processes The cost of the separation process is a useful benchmark for the comparison of different mitigation strategies. The mitigation cost provides various important information for the comparison of different technologies, especially for the trading systems. For instance, the cost of a sequestration project can be directly compared to the cost of a renewable energy project or an energy efficiency project.64 In a previous paper, in order to verify the effect of the energy consumption of a desorption membrane, the energy consumption of three methods, including thermal desorption, gas stripping, and membrane desorption, have been studied.17 The energy consumptions of three different desorption methods, including membrane desorption,22,23 gas stripping,65 and thermal desorption,65 have been adopted from previous studies done using Aspen plus simulation (Figure 12). Then, their results were compared with the energy consumption of the membrane desorption used in the present study (see Table 3). To be specific, the highest capture cost of 153 RMB/t CO2 was observed for the gas stripping method, which could be due to the higher solvent loss in this method so that a 29 ACS Paragon Plus Environment


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secondary separation step is also required to separate CO2 and N2.65 However, compared with the gas stripping method, thermal desorption method revealed a slightly lower capture cost (109 RMB/t CO2), due to the elimination of the secondary separation step; whilst the energy consumption is still high because of the solvent loss as well as the consumption resulted from the heating and cooling systems.65 The detailed formula and calculations of the cost and energy consumption of the prepared membranes have been mentioned in the supporting information file, the section of 2. Calculation of Cost and Energy Consumption. Noteworthy, some of the constant values in the used formula have been adopted from the same processes studied by the research group using Aspen plus simulation.22,23 Table 3. Comparison of different desorption methods for CO2 capture Solvent

Evaporation

Total Energy

Total

Total

loss

heat of DMC

consumption

Cost

Cost

(RMB/t

(USD/t

CO2)

CO2)

Desorption Process

Ref. (kg DMC/t (kj/kg DMC)

(GJ/t CO2)

CO2) Gas stripping

14.84

*

0.1302

153

23.08

65

Heat desorption

6.11

*

1.722

109

16.4

65

PDMS membrane

7.52

0.7

0.897

59.8

8.98

22

PDMS membrane

4.216

0.39

0.587

45.5

6.83

23

PDMS membrane

3.99

0.37

0.513

37

5.56

This study

PDMS/SiO2 membrane

3.13

0.29

0.433

33.2

5

This study

* Note: should be consideration about evaporation heat of DMC in gas stripping and heat desorption. The use of the desorption membrane method greatly reduces the energy consumption of the novel CO2 capture process. Xia22 and Zhimin23 have reported that the capture cost of the desorption membrane process could be saved by 61% and 70%, respectively, compared with


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the gas stripping method. Noteworthy, in the current study, the cost saving for the composite membrane (PDMS) and nanocomposite membrane (PDMS-SiO2) are 75% and 78%, respectively. From these results, it can be inferred that the performance of the PDMS desorption membranes for the present study has demonstrated a slightly better function than the data reported by Zhimin23. In the present study, not only the capture cost reduced to 37 RMB/t CO2 but also a solvent loss of 3.99 kg DMC/t CO2 was observed for the PDMS desorption membrane, which is slightly better than the result reported by Zhimin.23 However, according to the results, an addition of the SiO2 nanoparticles led to the decrease of both solvent loss and capture cost to 3.13 kg DMC/t CO2 and 33.2 RMB/t CO2, respectively. Moreover, in this study, a higher separation factor was observed for the PDMS-SiO2 which is better than the results reported by Xia.22 and Zhimin23.

Figure 12. Comparison of the energy consumption of different desorption methods



4. Conclusion In this study, a combination of membrane and solvent processes has been explored as a novel method for CO2 capture from a liquid solution. The membranes made from the PDMS and SiO2 nanoparticles were fabricated on the surface of the CA substrate for the PV separation of CO2 from DMC solutions. The effects of various operating parameters, such as feed flow rate, pressure, and temperature, on CO2 capture were investigated. According to results, the addition of the SiO2 nanoparticles improved the performance of the membrane for CO2 separation. Generally, increase in operating parameters, i.e. feed flow arte, pressure, and temperature, resulted in increasing the flux and decreasing the selectivity. Moreover, the CO2 permeability increased with temperature, indicating the increased diffusivity of CO2 through the PDMS polymeric matrix. Importantly, the temperature trend in CO2 permeability is directly proportional to the trend observed for the gas permeability measurements. In addition, by the increase of the feed flow rate, the overall mass transfer resistance decreased to 49% compared with the PDMS membrane, while the permeation flux increased. This result agrees with the findings obtained from Wilson plot mass transfer method. The hybrid PDMS-SiO2 membrane with 10 wt.% SiO2 loading showed the high total flux of 5.38 kg/m2.h with comparable selectivity of 47.61 at 25 °C and 3% CO2 concentration. Results revealed that, by the incorporation of the SiO2 nanoparticles into the PDMS membrane structure, the capture cost of the separation process declined to 78% compared with the gas stripping method. The high performance of the PDMS-SiO2 nanocomposite membrane may be found as a feature potential in CO2 capture.

Supporting Information The detailed information and calculations of the mass transfer of the prepared membranes have been mentioned in the supplementary file, 1. Mass Transfer section (Table S1 to S20).


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In addition, the detailed formula and calculations of the cost and energy consumption of the prepared membranes have been mentioned in the supporting information file, the section of 2. Calculation of Cost and Energy Consumption.

Acknowledgement This study has been supported by the National Key Technology Support Program of China (No. 2015BAC04B00). We are grateful to the editors and anonymous reviewers for their thorough reviews and constructive comments.

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227x106mm (96 x 96 DPI)

ACS Paragon Plus Environment

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Study on CO2 Desorption Behavior of a PDMS-SiO2 Hybrid

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