Accepted Manuscript PDMS coating of used TFC-RO membranes for O2/N2 and CO2/N2 gas separation applications Mohammad Reza Moradi, Mahdi Pourafshari Chenar, Seyed Hossein Noie, Mehrdad Hesampour, Mika Mänttäri PII:
S0142-9418(17)30801-2
DOI:
10.1016/j.polymertesting.2017.07.024
Reference:
POTE 5104
To appear in:
Polymer Testing
Received Date: 16 June 2017 Revised Date:
0142-9418 0142-9418
Accepted Date: 24 July 2017
Please cite this article as: M.R. Moradi, M. Pourafshari Chenar, S.H. Noie, M. Hesampour, M. Mänttäri, PDMS coating of used TFC-RO membranes for O2/N2 and CO2/N2 gas separation applications, Polymer Testing (2017), doi: 10.1016/j.polymertesting.2017.07.024. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
PDMS coating of used TFC-RO membranes for O2/N2 and CO2/N2 gas separation applications Mohammad Reza Moradia,b, Mahdi Pourafshari Chenara,b,1, Seyed Hossein Noiea, Mehrdad Hesampourc, Mika Mänttärid a
c
Kemira Oyj, R&D Center, Espoo, Finland
Membrane Technology Research Group , LUT School of Engineering Science, Lappeenranta University of Technology, Lappeenranta 53851, Finland
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d
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Chemical Engineering Department, Faculty of Engineering, Ferdowsi University of Mashhad, Mashhad, Iran b Research Center of Membrane Processes and Membrane, Faculty of Engineering, Ferdowsi University of Mashhad, Mashhad, Iran
Abstract
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In this study, a new application for used reverse osmosis (RO) membranes as gas separation membranes was studied. In this regard, firstly, three pretreatment procedures were used to remove the foulants from the surface of used membrane and then they were coated with polydimethylsiloxane (PDMS). The results indicated that PDMS-coated used RO membranes were capable of separating O2/N2 and CO2/N2. The maximum O2/N2 and CO2/N2 selectivities of coated membranes were 5.9 and 32.5, respectively. The O2/N2 and CO2/N2 selectivities of PDMS membrane were reported in the range of 2.1-2.2 and 11-12, respectively. Finally, an economical assessment was carried out to compare prepared PDMS coated-RO membranes with commercial PPO membrane. Obtained results showed that coated membranes are less expensive than PPO membrane in CO2/N2 gas separation. The outcome of the research was a simple method for converting used RO membranes to cost effective gas separation membranes.
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Keywords: PDMS-coating, Used TFC-RO membrane, Gas separation, Economical assessment.
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Corresponding author. Tel.: +98 51 38805024; fax: +98 51 38816840. E-mail address:
[email protected] (M. Pourafshari Chenar)
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ACCEPTED MANUSCRIPT 1. Introduction Due to increased demand of clean water, the number of reverse osmosis (RO) desalination plants has been grown significantly during the last two decades. Currently, there are 14000 RO plants around the world. The capacity of these units now has also surpassed 500,000 m3/d in several countries [1]. The average lifetime of RO membranes is about 3 to 5 years that can
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be prolonged up to 7 years by using of proper pretreatment and periodic cleaning methods [2, 3]. Disposal of these used RO membranes generates big amount of solid waste which can be an issue for desalination plants and surrounding environment. Most often, these membrane elements are incinerated or discarded to the municipal landfills. It had been estimated that the
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total mass of disposed modules would be 12,000 tones (per year) by 2015 [1]. This large volume indicates that the problem of disposed membranes should be given more attention. Based on the concepts inherent in the hierarchy of waste management, direct reuse of the
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product is generally recommended over recycling, and the last option is disposal as waste. This principle is also applicable in the case of RO modules [4].
Reusing old (used) membranes refers to using them for the same application or new applications with little or no treatment, like lower grade wastewater treatment, seawater pretreatment or demineralization of brackish waters. In contrast, recycling is defined as
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breaking down the modules and using their components for another applications e.g. use of membrane and permeate spacer as geotextile [2, 5, 6]. As noted above, given that reusing membrane modules is preferred, several studies have been addressed reuse of membrane modules [2, 7-13]. Rodriguez and colleagues investigated the
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conversion of used RO membranes to microfiltration (MF) and ultrafiltration (UF) membranes. They removed the polyamide active layer by strong chemical oxidants, such as sodium hypochlorite (NaOCl), hydrogen peroxide (H2O2), potassium permanganate (KMnO4)
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and also by oxidants at the presence of sodium dodecyl sulphate (SDS) [7, 8]. The polyamide active layer was removed by recirculation contact method with different concentrations of above mentioned agents for 1 to 2 hours. Their results showed that KMnO4 was the most effective oxidative agent at about 1000 mg/L dosage. The NaCl rejection of degraded membrane declined to 2%. In another study, Lawler et al. [2] used sodium hydroxide (NaOH), KMnO4 and NaOCl to remove the polyamide dense layer of used RO membranes. The best results were observed using NaOCl. Membranes treated with at least 300,000 ppm.h of NaOCl showed water permeance of 175 ± 4 L.m-2.h-1.bar-1 and salt rejection lower than 4%. By studying the fouling
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ACCEPTED MANUSCRIPT behavior and properties of the resulting UF membranes, they concluded that the degraded membranes had better performance than commercial UF membranes in some cases. Molina et al. [13] also used NaOCl to convert end-of-life RO membranes into nanofiltration (NF) and UF membranes. Their results showed that exposure level of 30,000 ppm.h removes completely the polyamide layer.
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In this work, unlike above mentioned studies, the possibility of using such membranes for separation of gases (O2/N2 and CO2/N2) was investigated. To the best of authors' knowledge, there are limited studies on this topic. In our previous study [14] we successfully developed a method
to
convert
new
RO
membrane
into
the
gas
separation
membranes.
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Polydimethylsiloxane (PDMS) was used as primarily coating material. The selectivities of PDMS coated RO membranes for O2/N2 and CO2/N2 were in the range 1.6-4.5 and 6.7-22.5, respectively, which is comparable with reported selectivity values of 2.1-2.2 for O2/N2 and 11-
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12 for CO2/N2 [15-17].
In this study, the similar approach was applied for used RO membranes. The results were benchmarked against available data for PDMS membrane. The effect of different washing procedures on separation performance of uncoated TFC-RO membranes was also investigated by measuring pure water permeation and salt rejection using 2000 ppm NaCl solution. Finally,
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an economical assessment was carried out to compare prepared PDMS coated RO membranes with commercial PPO membrane.
3. Material and methods
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3.1. Materials
The end-of-life TFC-RO membrane with trademark of FILMTEC from Dow Chemical
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Company was used in this study. This membrane was composed of three layers, top (selective layer), support layer and polyester layer and its characteristics are summarized in Table 1. Table 1 The characteristics of studied used TFC-RO membrane. Model Thickness (µm) Conditions of use TW30-1812-50 135 Tap water treatment
Hydrochloric acid (HCl) and NaOCl were purchased from Merck Co., Germany. HCl was used for membrane cleaning and removing foulants from the membrane surface. NaOCl was used for cleaning and removal of polyamide active layer of used RO membranes. PDMS for coating TFC membranes was obtained from Wacker-Chemie GmbH. It was prepared by thoroughly mixing of two parts, ELASTOSIL LR 3003/40 A and B, in 1:1 weight ratio. 3
ACCEPTED MANUSCRIPT Hexane (as PDMS solvent) was purchased from Merck Co., Germany. N2 and O2 gases with purity of 99.9% were supplied by Khorakian Oxygen Gas Co., Mashhad, Iran. CO2 gas with purity of 99.9% was purchased from Technical Gas Services, Inc.
3.2. Pretreatment of used membranes before coating with PDMS
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Three different approaches were selected for pretreatment of the membranes before coating with PDMS.
In the first procedure, samples of membranes were only rinsed with deionized water and then coated with PDMS.
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In the second procedure, membrane rinsed with deionized water, followed by 24 h soaking in HCl (pH: 1.2). The procedure removed fouling layer from membrane surface. In the third procedure, active layer of membranes was removed by rinsing the membrane
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surface with water, followed by soaking in NaOCl solution at atmospheric pressure. The membranes were rinsed with deionized water at the end of pretreatment. The NaOCl exposure intensity was in the range of 10,000 to 240,000 ppm.h (Table 2). To achieve a desired exposure intensity, the concentration of the NaOCl was kept constant at 10,000 ppm (1 wt%) and the exposure time was varied.
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Table 2
The NaOCl exposure intensities in the third procedure. Exposure intensity (ppm.h) 10,000
TW2 TW3
20,000 50,000
TW4 TW5
140,000 240,000
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TW1
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Sample Code
3.3. Salt rejection test
The influence of pretreatment (washing) procedure on the membrane structure and performance was evaluated by filtering aqueous NaCl solution (2000 ppm). The feed-side pressure was 7 bar and the permeate side pressure was atmospheric. Details of the test were described in previous study [18]. Permeate flux and salt rejection were reported after 2 hours of NaCl filtration. Permeate flux, J (L/m2.h or LMH) and salt rejection, R (%), were defined as:
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V S ×t
(1)
C R = 1 − p ×100 C f
(2)
where V (L) is the total volume of permeate during the sampling time interval t (h) at steady
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state and S (m2) is the effective membrane area in the module. Cp (mg/L) and Cf (mg/L) are the NaCl concentration in the permeate and feed solutions, respectively. NaCl concentration was determined using electrical conductivity meter of Extech EC-400 (USA). 3.4. Composite membranes preparation and gas permeation test
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Following the salt rejection test, the membranes were washed with deionized water and dried. After drying, they were dipped and immersed in the 20 wt% PDMS solution for 8 min. The
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membranes were subsequently withdrawn from the coating solution and left in the air to drain off the excess solution. For evaporating the remaining solvent, the coated membranes were maintained at room temperature for 75 min and then cured at temperature of 90 oC for 210 min to complete PDMS crosslinking. After coating, their gas permeation performance was studied using constant volume-variable pressure setup. The details and methods of working with gas permeation setup were described in the previous study [14].
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Two significant characteristics in the gas permeation test are the gas permeance, P/l (GPU, 1 GPU = 10-6 cm3(STP)/cm2.s.cmHg) and the ideal membrane selectivity, αi,j, that described in
P Vl dpl = l ph ART dt
(P / l )i (P / l ) j
(4)
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αi, j =
(3)
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below equations:
where P (barrer) is the gas permeability (1 barrer = 10-10 cm3(STP).cm/cm2.s.cmHg), l (µm) is the effective thickness of the separation layer, Vl (cm3) is the downstream volume (lowpressure side), ph (cmHg) is the upstream absolute pressure (high-pressure side), A (cm2) is the effective membrane area, the gas constant R is 0.278 cmHg.cm3/(cm3(STP).K), T (K) is absolute temperature and dpl/dt (cmHg/s) is the steady-state rate of pressure rise in the downstream volume at fixed upstream pressure. The subscripts i and j represent the more and less permeable gases, respectively.
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ACCEPTED MANUSCRIPT 3.5. Membrane characterization 3.5.1. Fourier transform infrared spectroscopy The PDMS layer was characterized by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR, Thermo Nicolet Avatar 370) in the wave number range of 4000 to 650 cm-1. The average penetration depth of the infrared beam was 2 µm for the ZnSe crystal with a 45o
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angle of incidence. 3.5.2. Scanning electron microscopy (SEM)
The prepared membranes were examined with a scanning electron microscope (LEO 1450 VP). For preparation of cross section images, the samples were dipped in an ethanol bath for filling
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all the membrane pores with alcohol, and afterwards they were immersed in liquid nitrogen bath to freeze ethanol. Then, frozen segments of the membranes were broken. The samples were gold and palladium sputtered for producing electric conductivity. The micrographs were
3.6. Membrane process modeling
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obtained at an accelerating potential of 20 kV.
Most of the current gas separation systems operate under cross-flow conditions. According to experimental and theoretical studies conducted by Pan and Habgood in the field of cross-flow pattern for asymmetric membranes, the usefulness of this model for modeling of these
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membranes has been confirmed [19, 20]. Since membranes prepared in this study are asymmetric, cross-flow pattern was used for membrane modeling. The required membrane area was also calculated using an approximate method with reasonable accuracy after membrane process modeling [17]. In this method, if the mole fraction of more permeable gas
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in feed (xf) and retentate (xr) differ quite considerably (xr/xf < 0.5), the system must be divided
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into several stages with xr/xf = 0.5, because otherwise the error will become too large [17]. 3.7. Power consumption
Required energy for compressor is calculated by below equation with assumption of isentropic process.
γ q1 p1 HP = γ −1 η
γ −1 γ p 2 − 1 p1
(5)
where HP is the compressor power (W), γ is the isentropic exponent expressed as the ratio of the specific heat at constant pressure (cp) to specific heat at constant volume (cv), q1 and p1 are the intake flow rate (m3/h) and presuure (Pa), respectively. η is the compressor efficiency and p2 is the final delivery pressure [21]. 6
ACCEPTED MANUSCRIPT 3.8. Economical assessment There are several cost elements which should be taken into account when an economical assessment is performed such as module cost which depends on the type of module design (spiral wound or hollow fiber). In this study cost of prepared membrane was determined based on spiral wound module and for commercial gas separation membrane, hollow fiber module was considered.
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The economic parameters and other assumptions used to calculate the gas processing cost (GPC) and assess the economics of the membrane separation process are summarized in Table 3.
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$108/m2 $11003.061 × HP0.82 MC + CC 1.12 × FC 0.20 × BPC BPC + PC 0.10 × VOM (see below) TFI + SC
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Parameter Total plant investment (TPI) Total membrane module cost (Hollow fiber) (MC) Installed compressor cost (CC) Fixed cost (FC) Base plant cost (BPC) Project contingency (PC) Total facilities investment (TFI) Start-up cost (SC) TPI Annual variable operating and maintenance cost (VOM) Contract and material maintenance cost (CMC) Local taxes and insurance (LTI) Direct labor cost (DL) Labor overhead cost (LOC) Membrane replacement cost (MRC) Utility cost (UC) VOM Gas processing cost (GPC) Annual capital related cost (CRC) GPC Other assumptions Membrane life Payout period to calculate CRC Operation time to calculate DL Compressor efficiency (η) On-stream factor (OSF) Stage-cut equivalent (SCE)
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Table 3 Economic parameters and assumptions for calculation gas processing cost [21].
a
0.05 × TFI 0.015 × TFI $15/h 1.15 × DL $54/m2 $0.07/kW.h CMC + LTI + DL + LOC + MRC + UC 0.2 × TPI (CRC + VOM)/[365 × OSF × (1-SCE)× qa] 4 years 5-year 8 h/day per 25 MMSCFD of feed 0.8 96%
q = Retentate flow rate (m3/h) for O2/N2 separation; q = CO2 mass flow rate (kg/h) in permeate stream for
CO2/N2 separation
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ACCEPTED MANUSCRIPT Table 4 summarizes the cost elements used for calculating the cost of PDMS coated RO membrane.
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Table 4 The materials cost and oven power consumption for preparing PDMS/TFC-RO membrane. Value Reference PDMS $8.9/kg [22] n-Hexane $0.6/kg [22] Virgin TW30-1812-50 membrane module $10.93/unit [22] Active membrane area of TW30-1812-50 module 0.32 m2 [23] Virgin TW30-1812-50 membrane module $34.16/m2 Electrical load of Oven (Memmert, Model UNB 200) 1100 W -
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To determine the cost of TFC-RO membrane, the required surface area should be calculated. This area is equivalent to the area of the gas separation membrane whose value is determined by membrane modeling. The required mass of PDMS and n-hexane can also be determined
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with having the thickness of PDMS layer. At the end, after calculating the power consumption of the oven, the cost of prepared PDMS/TFC-RO membrane was calculated.
4. Results and discussion
4.1. Synthesis and crosslinking of PDMS/TFC-RO membrane
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ATR-FTIR spectra of two PDMS coated RO membranes are presented in Fig. 1. This figure confirms that the crosslinking of PDMS was carried out. The absorption peaks at 2961 and 1257 cm-1 are attributed to C-H bond (methyl group) and Si-CH3 bond, respectively. The Si-
Fig. 1.
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O-Si stretching multi-component peaks for PDMS are observed at 1100-1000 cm-1 [40].
4.2. First pretreatment procedure, rinsing with deionized water In this procedure, membrane module was opened and several samples were selected from its different layers. These samples were rinsed with deionized water and to investigate the effect of this pretreatment procedure, water flux was measured. After more than two hours of filtration, the permeate was negligible. Afterwards, they were coated with PDMS under conditions that were mentioned in experimental section. The gas permeation results of coated membranes that were partly reported in our earlier study [14] are presented in Table 5. Since the surface of used membranes were not uniform, there 8
ACCEPTED MANUSCRIPT are significant variations of permeances and selectivities between membrane samples. Compared to reference PDMS membrane (Table 5), selectivities of PDMS coated used RO membranes are higher.
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Table 5 N2 permeance and O2/N2 and CO2/N2 selectivities of used TW membranes after rinsing with deionized water and coating with PDMS. N2 permeance αO2/N2 αCO2/N2 Sample No. (GPU) (-) (-) PDMS 0.91 2.13 12.81 1 0.14 4.69 29.75 2 0.18 4.05 25.57 3 0.12 4.28 26.65 4 0.08 3.36 18.41 5 0.09 3.82 18.92 6 0.10 2.98 18.36 7 0.12 3.26 19.37 8 0.17 3.15 20.13 Average of PDMS 0.13 3.70 22.69 coated membranes
By comparing the CO2/N2 separation results of used membranes with virgin membranes [14], it is quite clear that the used membranes had better performance. The minimum value of αCO2/N2 in used membranes (18.36) is almost equal to the maximum value of αCO2/N2 in virgin
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membranes (18.91) and most N2 permeance values in used membranes are greater than virgin membranes. This means that used RO membranes have good potential for use as gas separation membrane.
4.3. Second pretreatment procedure, cleaning with HCl
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The used membranes rinsed with deionized water followed by soaking in HCl. Similar to first pretreatment the permeate flux was negligible after two hours of filteration. This could be due
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to dense fouling layer on membrane which prevent free passage of water through the membrane. In SEM image (Fig. 2 a) taken after acid cleaning indicates some foulant still left on surface. Difference can be seen compare to SEM image from the surface of virgin TW membrane (Fig. 2b).
Fig. 2.
After coating of these membranes with PDMS, the gas separation performance of coated membranes were determined. The results are presented in Table 6. Comparing these results with Table 5 indicates that average of N2 permeance for the first pretreatment procedure is higher than second procedure. However, selectivity values in Table 6 are averagely higher 9
ACCEPTED MANUSCRIPT than Table 5. This could be attributed to the intrusion of fouling layer into the support layer in the second procedure. Table 6 N2 permeance and O2/N2 and CO2/N2 selectivities of used TW membranes after washing with water and cleaning with HCl (pH=1.2) and coating with PDMS.
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1 2 3 4 5 Average
N2 permeance αO2/N2 αCO2/N2 (GPU) (-) (-) 0.09 3.70 22.08 0.17 3.78 21.26 0.06 4.10 26.51 0.11 4.52 32.48 0.11 4.40 28.21 0.11 4.10 26.11
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Sample No.
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4.4. Third pretreatment procedure, exposure to NaOCl solution
TW membranes were further treated with third pretreatment procedure (rinsing with deionized water followed by exposure to NaOCl solution). Membrane flux and salt rejection were measured at different NaOCl exposure times. As expected, by increasing NaOCl exposure intensity (ppm.h) and consequently increase in degradation level of polyamide, water flux and salt rejection of the membranes increased and decreased, respectively. The results are shown
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in Fig. 3 indicates that after 150,000 ppm.h, the changes in salt rejection and water flux became plateau. This is because of total removal of polyamide active layer at higher NaOCl exposure intensity.
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Fig. 3.
SEM image from the surface of used membrane after exposure to NaOCl solution (Fig. 4)
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shows less contaminants and foulants compared to the second pretreatment procedure (Fig. 2a). It seems that foulants as well as polyamide layer were removed effectively by NaOCl. The removal of active layer by NaOCl has reported in other references [2, 7, 8, 13]. Fig. 4.
Cross section SEM images of two PDMS coated RO membranes with different NaOCl exposure intensities are also provided and shown in Fig. 5. Fig. 5.
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ACCEPTED MANUSCRIPT The thickness of PDMS in PDMS/TW1 and PDMS/TW4 is 11.85 and 11.17 µm, respectively. Some of PDMS may penetrate into support layer. Energy-dispersive X-ray spectroscopy (EDS) from cross section of membranes (Fig. 6) indicates presence of elemental silicon (Si) from PDMS. The elements oxygen (O) and carbon (C) exist in both PDMS and support layer (PSf). Sulfur (S) is one of the elements of PSf polymer. The other two elements are related to
Fig. 6.
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the gold (Au) and palladium (Pd) sputtered coating.
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The gas permeation results of used TW membranes after pretreatment and coating with PDMS are listed in Table 7 and presented in Figs. 7 and 8.
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Table 7 Water flux and salt rejection of used TW membranes after exposure to NaOCl solution along with N2 permeance and O2/N2 and CO2/N2 selectivities after coating with PDMS. Exposure intensity N2 permeance αO2/N2 αCO2/N2 Sample No. (ppm.h) (GPU) (-) (-) 10,000 0.12 5.92 29.81 PDMS/TW1 20,000 0.24 3.43 17.15 PDMS/TW2 50,000 3.40 2.25 12.53 PDMS/TW3 140,000 6.52 2.33 11.81 PDMS/TW4 240,000 10.00 2.16 11.93 PDMS/TW5 Fig. 7. Fig. 8.
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Reducing the values of O2/N2 and CO2/N2 selectivities with increasing NaOCl exposure intensity implies that polyamide layer has an important effect on gas separation. With
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degradation of this layer, selectivities of coated membranes are similar to PDMS selectivity presented in the literature (2.1-2.2 and 11-12 for O2/N2 and CO2/N2, respectively [15-17]) and measured with reference PDMS membrane in this study (Table 5)
4.5. Gas processing cost Two case studies of (1) production of high purity N2 from air and (2) separation of CO2 from flue gas were investigated. According to procedure of membrane process modeling that was presented in reference [17], the calculations should be based on more permeable gas, therefore permeances of O2 and CO2 were used instead of N2 for PDMS coated membranes (Table 8). The upstream and downstream pressures of both two case studies were 5 and 1 bar, respectively. 11
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Table 8 The average values of permeances and selectivities of virgin and used TFC-RO membranes after coating with PDMS. Case study 2 Case study 1 Type of TFC-RO membrane α αCO2/N2 No Symbol (P/l)O2 (P/l)CO2 O2/N2 coated with PDMS (GPU) (GPU) (-) (-) 1 Virgin membrane (TW) PDMS/TW 0.33 3.18 1.85 17.80 2 First pretreatment (TW-D) PDMS/TW-D 0.47 3.70 2.81 22.15 3 Second pretreatment (TW-A) PDMS/TW-A 0.44 4.10 2.79 26.11 4 TW1 PDMS/TW1 0.73 5.92 3.70 29.81 5 TW2 PDMS/TW2 0.81 3.43 4.06 17.15 6 TW3 PDMS/TW3 7.66 2.25 59.59 17.53 7 TW4 PDMS/TW4 15.20 2.33 76.95 11.81 8 TW5 PDMS/TW5 21.65 2.16 119.31 11.93
Prepared PDMS/TFC-RO membranes were benchmarked against commercial PPO membrane.
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Gas separation performances values of commercial PPO membrane are summarized in the Table 9 [24], Upstream and downstream pressures were 5.1 and 1 bar, respectively.
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Table 9 The permeances and selectivities values of commercial PPO membrane [24]. Case study 2 Case study 1 αCO2/N2 (P/l)O2 αO2/N2 (P/l)CO2 (GPU) (GPU) (-) (-) 40 4 195 19.5
4.5.1. Case study 1: Production of high purity N2 from air The calculations of the required membrane area and compressor power consumption was
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done based on the production of high purity nitrogen stream from air. The parameters that are independent of the type of membrane are presented in Table 10.
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Table 10 The parameters used to calculate the required membrane area. Parameter Unit value Mole fraction of nitrogen in feed flow (xf) 0.79 Mole fraction of nitrogen in retentate flow (xr) 0.95 Retentate flow rate (qr) m3/h 10
Since the ratio of oxygen mole fraction in the feed to retentate stream is smaller than 0.5 (xr/xf = 0.238), calculations must be performed in two stages [17]: First stage: conversion of feed with oxygen mole fraction of 21% to 10% (xr/xf = 0.467 ≈ 0.5) Second stage: conversion of feed with oxygen mole fraction of 10% to 5% (xr/xf = 0.5)
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ACCEPTED MANUSCRIPT According to Fig. 9, the recycle stream is used to reduce the membrane area of the first stage and therefore the entire membrane area. Using this stream reduces the required flow rate of feed and thereby reduces the compressor power consumption. Fig. 9.
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The results of membrane process modeling of prepared PDMS/TFC-RO membranes are summarized in the Table 11. The compressor power consumption with efficiency of 80% was calculated from eq. 5. The isentropic exponent (γ) of air is equal to 1.4.
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1 2 3 4 5 6 7 8
Type of qf A1 A2 A Power membrane (m3/h) (m2) (m2) (m2) (kW) PDMS/TW 33 16675 7981 24656 3.8 PDMS/TW-D 27 9933 5181 15114 3.3 PDMS/TW-A 25 9812 5358 15170 3.0 PDMS/TW1 20 5105 3155 8260 2.6 PDMS/TW2 30 6204 3110 9314 3.5 PDMS/TW3 82 1704 528 2232 7.8 PDMS/TW4 70 741 248 989 6.9 PDMS/TW5 101 742 206 947 9.4
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Table 11 The required feed flow rate, membrane area and compressor power consumption for nitrogen production from air.
According to Fig. 5, the thickness of PDMS on surface of TFC-RO membrane is almost 12 µm. However, taking into account the PDMS penetration into PSf (support) layer, it is assumed that the thickness of PDMS is approximately 15 µm. Sensitivity analysis indicated that the
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impact of PDMS thickness on GPC is low.
The mass of PDMS layer on membrane calculated from PDMS thickness, and PDMS density
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(1.13 kg/l). The mass of solvent (n-hexane) calculated from PDMS concentration (20 wt%) in solvent (Table 12).
Table 12 The required mass of PDMS and n-Hexane. Type of PDMS mass n-Hexane mass No membrane (kg) (kg) 1 PDMS/TW 418 1672 2 PDMS/TW-D 256 1025 3 PDMS/TW-A 257 1029 4 PDMS/TW1 140 560 5 PDMS/TW2 158 631 6 PDMS/TW3 38 151 7 PDMS/TW4 17 67 8 PDMS/TW5 16 64
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ACCEPTED MANUSCRIPT The similar calculations as PDMS/TFC-RO membranes were done for commercial PPO membrane. The results are presented in Table 13.
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Table 13 The required feed flow rate, membrane area and power consumption for nitrogen production from air. qf A2 A1 A P (m3/h) (m2) (m2) (m2) (kW) 25 57 105 162 3.1
The GPC values for PDMS/TFC-RO and PPO membranes were calculated based on data presented in Tables 3 and 4.
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According to Table 14, commercial PPO membrane is less expensive than PDMS/TFC-RO membranes. Table 14 shows that the cost of prepared PDMS/TFC-RO membrane can
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significantly be reduced by using used membrane instead of virgin membrane.
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Table 14 Economic comparison of PDMS/TFC-RO and commercial PPO membranes. Type of MC CC CRC VOM GPC No membrane ($) ($) ($) ($) ($/(m3/h) retentate) 1 PDMS/TW 847242 32574 239742 162388 110.2 2 PDMS/TW-D 3173 28963 9754 55822 18.0 3 PDMS/TW-A 3184 27291 9306 55726 17.8 4 PDMS/TW1 1860 23744 7516 31686 10.7 5 PDMS/TW2 2062 30551 9495 36424 12.6 6 PDMS/TW3 705 59517 16540 17625 9.4 7 PDMS/TW4 467 53584 14775 12309 7.4 8 PDMS/TW5 459 69118 19004 15074 9.3 9 PPO 17509 27742 12324 8044 5.6
4.5.2. Case study 2: Separation of CO2 from flue gas Flue gas from factories and power plants often contain compounds such as N2, CO2, H2O and
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O2 and traces of CO, NOx and SO2. Information and assumption for calculating membrane area and compressors power consumption are presented in Table 15. Since the separation percentage of CO2 is 90%, so the flow rate of CO2 in the retentate flow is 1 m3/h. Table 15 The parameters used to calculate the required membrane area. Parameter Unit Value Mole fraction of CO2 in feed flow (xf) 0.1 Feed flow rate (qf) m3/h 100 CO2 separation percentage % 90
Similar procedure with case study 1 was applied except that the feed flow rate is known. It should be noted that the trial and error method was used to determine the retentate flow rate. 14
ACCEPTED MANUSCRIPT The GPC values for PDMS/TFC-RO and PPO membranes were calculated using assumptions presented in the Tables 3 and 4. According to the Table 16, PDMS/TW4, PDMS/TW5 and PDMS/TW3 membranes are less expensive than commercial PPO membrane. PDMS/TW membrane that was prepared by coating PDMS on virgin TFC-RO membrane is not cost effective due to virgin membrane cost.
No
MC ($)
CRC ($)
VOM ($)
144.2 62.7 71.7 61.3 37.4 7.0 6.0 5.6 7.5
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PDMS/TW 1186527 348956 581547 PDMS/TW-D 5458 27399 377258 PDMS/TW-A 6262 28750 433993 PDMS/TW1 5329 27183 368187 PDMS/TW2 3193 23594 217436 PDMS/TW3 480 19036 25984 PDMS/TW4 393 18890 19867 PDMS/TW5 353 18822 17016 PPO 36507 28791 19594
GPC ($/(kg/h) CO2)
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Type of membrane
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Table 16 Economic comparison of PDMS/TFC-RO and commercial PPO membranes.
In addition to lower cost of these membranes than commercial PPO membranes, it should be noted that the use of these membranes will solve the accumulation and environmental pollution
4.5.3. Sensivity analysis
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problems of used TFC-RO membranes.
As shown in Fig. 10, the thickness of PDMS has negligible effects on GPC of three
Fig. 10.
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5. Conclusions
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PDMS/TFC-RO membranes that are more cost effective.
A new application for used TFC-RO membranes were developed. In this application the membranes were coated with PDMS and used as gas separation membrane. Three different pretreatment procedures including rinsing with deionized water, rinsing with deionized water followed by cleaning with HCl, and rinsing with deionized water and exposing the samples to NaOCl solution were studied. Membranes pretreated with deionized water and HCl had negligible water flux. The flux increased after membrane pretreatment with NaOCl. With increasing NaOCl exposure intensity, water flux and salt rejection of membranes were increased and decreased, respectively due to an increase in degradation level of polyamide. This increasing NaOCl 15
ACCEPTED MANUSCRIPT exposure intensity decreased also the O2/N2 and CO2/N2 selectivities implying that polyamide layer has an important effect on gas separation. When polyamide layer was degraded, selectivities of coated membranes were similar to reference PDMS membrane and selectivities presented in the literature. At the best the achieved selectivity values were 2.5 fold compared to reference PDMS or literature values.
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Finally, an economical assessment was carried out to determine which pretreatment procedure is more suitable. Three PDMS coated membranes that were treated with at least 50,000 ppm.h of NaOCl are less expensive than PPO membrane in CO2/N2 gas separation.
The importance of the present work is generation of added value for used membranes. With
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described procedures used TFC-RO membranes will be converted into gas separation membranes which has higher added value compared to used membrane (given that the
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provision of used membranes has no cost). The above procedure can be an alternative for a sustainable way of using used membranes. Nomenclature
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Water permeate flux (L/m2.h or LMH) Salt rejection (%) Total volume of permeate (L) Sampling time interval (h) Effective membrane area in module of salt rejection test (m2) NaCl concentration in the permeate (mg/L) NaCl concentration in the feed solution (mg/L) Gas permeance (GPU) Gas permeability (barrer) Effective thickness of the separation layer (µm) Downstream volume (cm3) Downstream absolute pressure (cmHg) Upstream absolute pressure (cmHg) Effective membrane area in module of gas permeation test (cm2) Gas constant (cmHg.cm3/(cm3(STP).K)) Absolute temperature (K) Mole fraction of more permeable gas in feed Mole fraction of more permeable gas in retentate Feed flow rate Retentate flow rate Intake flow rate (m3/h) Intake presuure (Pa) Final delivery pressure (Pa) Compressor power (W)
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J R V t S Cp Cf P/l P l Vd pl ph A R T xf xr qf qr q1 p1 p2 HP
Greek letters αi,j αO2/N2 αCO2/N2 γ η
Ideal membrane selectivity of component i over component j O2/N2 selectivity CO2/N2 selectivity Isentropic exponent (cp/cv) Compressor efficiency
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ACCEPTED MANUSCRIPT References [1] W. Lawler, Z. Bradford-Hartke, M.J. Cran, M. Duke, G. Leslie, B.P. Ladewig, P. LeClech, Towards new opportunities for reuse, recycling and disposal of used reverse osmosis membranes, Desalination, 299 (2012) 103-112.
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[2] W. Lawler, T. Wijaya, A. Antony, G. Leslie, P. Le-Clech, Reuse of reverse osmosis desalination membranes, IDA World Congress, Perth Convention and Exhibition Centre Perth, Western Australia, 2011. [3] Huntington-Beach-Facility, Desalination Worldwide: Worldwide Seawater Desalination Capabilities, Available: http://hbfreshwater.com/desalination-101/desalination-worldwide, 2010.
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[4] C. Prince, M. Cran, P. Le-Clech, K. Uwe-Hoehn, M. Duke, Reuse and recycling of used desalination membranes, Proceedings of OzWater'11, Adelaide, 2011.
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[5] E.O. Mohamedou, D.P. Suarez, F. Vince, P. Jaouen, M. Pontie, New lives for old reverse osmosis (RO) membranes, Desalination, 253 (2010) 62-70. [6] M. Pontié, Old RO membranes: solutions for reuse, Desalination and Water Treatment, 53 (2015) 1492-1498. [7] J.J. Rodríguez, V. Jiménez, O. Trujillo, J. Veza, Reuse of reverse osmosis membranes in advanced wastewater treatment, Desalination, 150 (2002) 219-225.
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[8] J.M. Veza, J.J. Rodriguez-Gonzalez, Second use for old reverse osmosis membranes: wastewater treatment, Desalination, 157 (2003) 65-72. [9] H.D. Raval, V.R. Chauhan, A.H. Raval, S. Mishra, Rejuvenation of discarded RO membrane for new applications, Desalination and Water Treatment, 48 (2012) 349-359.
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[10] A. Ambrosi, I.C. Tessaro, Study on potassium permanganate chemical treatment of discarded reverse osmosis membranes aiming their reuse, Separation Science and Technology, 48 (2013) 1537-1543.
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[11] W. Lawler, A. Antony, M. Cran, M. Duke, G. Leslie, P. Le-Clech, Production and characterisation of UF membranes by chemical conversion of used RO membranes, Journal of Membrane Science, 447 (2013) 203-211. [12] R. García-Pacheco, J. Landaburu-Aguirre, S. Molina, L. Rodríguez-Sáez, S.B. Teli, E. García-Calvo, Transformation of end-of-life RO membranes into NF and UF membranes: Evaluation of membrane performance, Journal of Membrane Science, 495 (2015) 305-315. [13] S. Molina, R. García-Pacheco, L. Rodríguez-Sáez, E. García-Calvo, E. Campos-Pozuelo, D. Zarzo Martínez, J. González de la Campa, J. de Abajo González, Transformation of endof-life RO membranes into recycled NF and UF membranes, surface characterization, Proceedings of the IDAWC'15, San Diego, 2015. [14] M.R. Moradi, M. Pourafshari Chenar, S.H. Noie, Using PDMS coated TFC-RO membranes for CO2/N2 gas separation: Experimental study, modeling and optimization, Polymer Testing, 56 (2016) 287-298. 17
ACCEPTED MANUSCRIPT [15] C. Yeom, S. Lee, J. Lee, Study of transport of pure and mixed CO2/N2 gases through polymeric membranes, Journal of Applied Polymer Science, 78 (2000) 179-189. [16] I. De Bo, H. Van Langenhove, P. Pruuost, J. De Neve, J. Pieters, I.F. Vankelecom, E. Dick, Investigation of the permeability and selectivity of gases and volatile organic compounds for polydimethylsiloxane membranes, Journal of Membrane Science, 215 (2003) 303-319.
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[17] M. Mulder, Basic principles of membrane technology, Springer Science & Business Media, 2012.
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[18] H. Azizi Namaghi, A. Haghighi Asl, M. Pourafshari Chenar, Identification and optimization of key parameters in preparation of thin film composite membrane for water desalination using multi-step statistical method, Journal of Industrial and Engineering Chemistry, 31 (2015) 61-73. [19] C.Y. Pan, H. Habgood, Gas separation by permeation part I. Calculation methods and parametric analysis, The Canadian Journal of Chemical Engineering, 56 (1978) 197-209.
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[20] C.Y. Pan, H. Habgood, Gas separation by permeation Part II: Effect of permeate pressure drop and choice of permeate pressure, The Canadian Journal of Chemical Engineering, 56 (1978) 210-217. [21] J. Hao, P. Rice, S. Stern, Upgrading low-quality natural gas with H2S-and CO2-selective polymer membranes: Part II. Process design, economics, and sensitivity study of membrane stages with recycle streams, Journal of Membrane Science, 320 (2008) 108-122.
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[22] Anonymous, India's import and export data, Available: www.zauba.com, 2016. [23] Anonymous, DOW FILMTEC RO Membrane, Available: www.watereco.com/products_ view.asp?id=110, 2016.
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[24] M. Pourafshari Chenar, M. Soltanieh, T. Matsuura, A. Tabe-Mohammadi, C. Feng, Gas permeation properties of commercial polyphenylene oxide and Cardo-type polyimide hollow fiber membranes, Separation and Purification Technology, 51 (2006) 359-366.
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Fig. 1. ATR-FTIR spectra of two used RO membranes after contact with NaOCl solution and coating with PDMS.
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Fig. 2. The surface SEM image of (a) used TW membranes after rinsing with water and cleaning with HCl (pH=1.2) and (b) virgin TW membrane.
Fig. 3. Water flux and salt rejection of used TW membranes after exposure to NaOCl solution.
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Fig. 4. The surface SEM image of the used TW membranes after contact with NaOCl solution (Exposure intensity = 50,000 ppm.h).
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Fig. 5. Cross section SEM images of two used membranes after contact with NaOCl solution and coating with PDMS, (a) PDMS/TW1 and (b) PDMS/TW4.
Fig. 6. EDS spectra of cross section of two used membranes after exposing to NaOCl solution and coating with PDMS, (a) PDMS/TW1 and (b) PDMS/TW2.
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Fig. 7. N2 permeances and O2/N2 selectivities of used TW membranes after exposure to NaOCl solution and coating with PDMS.
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Fig. 8. N2 permeances and CO2/N2 selectivities of used TW membranes after exposure to NaOCl solution and coating with PDMS.
Fig. 9. Two-stage design for production of high purity N2 from air with recycle stream.
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Fig. 10. The effect of PDMS thickness on GPC of three PDMS/TFC-RO membranes, (a) case study 1, (b) case study 2, ●: PDMS/TW3, ♦: PDMS/TW4,▲: PDMS/TW5.
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