Analysis of Sorption and Permeation of AcetonitrileâWater Mixtures

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Separations

Analysis of sorption and permeation of acetonitrile-water mixtures through nano clay filled copolymer membranes Swastika Choudhury, and Samit Kumar Ray Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05882 • Publication Date (Web): 21 Feb 2019 Downloaded from http://pubs.acs.org on February 21, 2019

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Industrial & Engineering Chemistry Research

Analysis of sorption and permeation of acetonitrile-water mixtures through nano clay filled copolymer membranes Swastika Choudhury, Samit Kumar Ray* Department of Polymer Science and Technology, University of Calcutta, 92, A.P.C Road, Kolkata-700009 Abstract Several hydrophilic membranes were prepared by solvent casting from filled copolymers. These filled copolymers were synthesized by free radical polymerization of acrylonitrile (AN) and N-vinylpyrrolidone (NVP) monomers in different comonomer ratios (AN: NVP) in the presence of a nano sized clay. The membrane polymers were analyzed by NMR, FTIR, XRD, DTA-TGA, SEM, mechanical properties and DMA for characterization of its structural configuration, functionalities, copolymer composition, distribution of clay in the copolymer and the strength and stability of the membrane under dynamic and thermal stress. The sorption and pervaporation of acetonitrile-water mixtures through these membranes were studied at varied copolymer compositions, mass% of clay, feed concentrations of water and feed temperatures. Sorption data were analyzed by binary and ternary solvent/membrane interaction parameters while the diffusion coefficient of solvents through the membranes was determined by a solution-diffusion model. The synthesis and process variables were optimized by a central composite design (CCD) of the response surface methodology

(RSM) with flux and separation factor as output responses. The membrane prepared with the AN: NVP mole ratio of 12:1 and 1 mass% nano clay showed the optimized flux and separation factor of 160 g/m2h and 196, respectively for 90 mass% acetonitrile in feed which was dehydrated to 95.6 mass % after pervaporation. The long- term stability of the membrane was also evaluated. Comparison of tensile properties of unused and used membranes confirmed mechanical stability of the membrane while FTIR analysis revealed that there was no change in structure or composition of the membrane after prolonged use. Key words: synthesis & characterization; filled membrane; interaction parameter & plasticization coefficient; pervaporation; diffusion coefficient; RSM Corresponding author (email: [email protected], fax: +91 33 351 9755).

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1. Introduction The conventional distillation used for solvent dehydration is an energy intensive equilibrium-controlled process which becomes more complicated and expensive for close boiling or azeotropic liquid mixtures. In contrast, solvent dehydration by pervaporation is not equilibrium controlled and this low energy and environment friendly separation process may be carried out at ambient temperature requiring potentially much lower operating cost than distillation or solvent extraction. Solvent dehydration plants based on pervaporation still lacks suitable membrane showing high permeability and selectivity along with mechanical and chemical stability 1. Membranes used in pervaporation are prepared from inorganic ceramic materials by sol-gel techniques 2,3,4,5 or organic polymer materials by solution casting6. Inorganic membranes are highly selective because of the presence of micro pores in its structures and. also blessed with excellent mechanical stability and chemical resistance. However, it is difficult to make membrane module with high surface area from inorganic or ceramic material because of its structural rigidity. In contrast, membranes based on organic polymers are more flexible and demand much less processing cost for fabrication. However, selectivity of hydrophilic polymer membranes is reduced significantly because of its extensive swelling in water. Further, mechanical stability or chemical resistance of polymer membranes is also poor in comparison to inorganic membranes. Thus, the mixed matrix membranes were developed where inorganic adsorptive fillers such as zeolite, bentonite, carbon black, carbon nano fibers 36, etc., were dispersed in a polymer matrix. Accordingly, flexibility of an organic polymer is combined with the high selectivity and stability of inorganic filler in a mixed matrix membrane. The inorganic filler present in the polymer contributes to enhanced selectivity and stability of the filled membrane. In fact, the porous morphology and the presence of shape and size selective micropores (3-10oA) of a nano filler enhances both permeability and selectivity of the resulting organic-inorganic hybrid type membranes 7. Thus, the flux-selectivity trade off relationship often encountered in an organic polymer membrane is eliminated in a mixed matrix membrane. However, the elimination of the ‘trade off’ limitation of flux and selectivity of these membranes strongly depends on the polymer-filler compatibility. The poorer the filler-polymer compatibility, the higher will be the interfacial stresses and subsequent cracks and micro voids in the membrane affecting its selectivity. The compatibility 2 ACS Paragon Plus Environment

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issue is further aggravated for nano sized filler because of its susceptibility to easy agglomeration even at a low dose (mass% of total polymer mass). The problem of polymer-filler compatibility is eliminated by choosing suitable polymer and filler and ensuring their proper mixing. In situ mixing of clay in the growing polymer networks during its synthesis from monomers results in much better filler-polymer mixing than conventional mixing where the filler materials are physically mixed with the polymer by dispersing in a liquid medium such as water. Thus, in the present wok several copolymers (CP) filled in situ with different amounts of nano clay were prepared from copolymer of acrylonitrile (AN) and N-vinylpyrrolidone (NVP) and nano sized sodium montmorillonite (NaMMT) clay. Both polyacrylonitrle (PAN) and polyvinyl pyrrolidone (PVP) are amorphous polymers. Hence, the copolymer of AN and NVP will also be amorphous. In the copolymer, the AN moiety will give mechanical stability while NVP moiety will provide hydrophilicity since PAN is resistant to water while PVP is soluble in water. The clay will contribute to both permeability and selectivity. The membranes prepared from these filled copolymers were studied for sorption and pervaporation of acetonitrile-water binary mixtures. Acetonitrile is a water-soluble organic intermediate used in various chemical processes such as separating C4 hydrocarbons in petrochemical industries and also as column solvent for HPLC 8. Acetonitrilewater mixture containing 82.2 mass% acetonitrile forms an azeotrope at atmospheric pressure. Thus, acetonitrile is industrially dehydrated by a pressure swing or azeotropic distillation which is expensive with many limitations 9. As an alternative, a hybrid separation process of solvent extraction and batch distillation

10

or

pervaporation-distillation has also been studied. Pervaporative dehydration of acetonitrile have been reported with few hydrophilic membranes such as membranes prepared from nano sized iron oxide filled polyvinyl alcohol (PVA)

11,

zeolite filled blend IPN of sodium alginate-PVA and polyaniline

acrylonitrile and itaconic acid

13.

12

and also copolymer of

Thus, in the present work acetonitrile-water mixtures were dehydrated by

pervaporation using the filled copolymer membranes.


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2. Experimental 2.1. Materials High purity analytical grade acetonitrile solvent, laboratory reagent grade acrylonitrile and N’vinyl pyrrolidone (NVP) monomers, sodium lauryl sulfate emulsifier and potassium peroxo disulfate initiator were purchased from M/s. E. Merck (India) Ltd, Mumbai. The nano sized clay (sodium montmorillonite, NaMMT), polymer grade containing 98% montmorillonite, particle size 30-90 nm, mineral’s thickness 1nm, cation exchange capacity 120 meq/100 g) obtained from Amrfeo Pte. Ltd., Kolkata, was used after drying in a hot air oven. 2.2. Methods 2.2.1. Synthesis of the filled copolymer The clay filled copolymer poly (AN-co-NVP) was prepared by emulsion polymerization in a glass reactor as shown in scheme 1. The emulsifier (0.4 g/l), clay (0.5, 1, 1.5 and 2 mass% of total monomer mass), monomer (AN: NVP molar ratio of 10:1, 15:1, 20:1 and 25:1, respectively) and initiator (1 mass% of total monomer mass) were added to 50 ml of water in the reactor and the free radical polymerization was carried out for around 3 h at 70oC followed by breaking the emulsion in excess water in the presence of trace amount of sodium chloride salt. The polymers were washed repeatedly with methanol, ethyl acetate and finally by distilled water. Unfilled copolymer was also synthesized in a similar way without adding any clay. The unfilled copolymer prepared with AN:NVP molar ratio of 10:1, 15:1, 20:1 and 25: 1 were noted as CP10, CP15, CP20 and CP25, respectively. The filled copolymers prepared with 0.5, 1, 1.5 and 2 mass% clay in CP15 were termed as F0.5, F1, F1.5 and F2, respectively. 2.2.2. Membrane casting The polymer as prepared was dried in a vacuum oven. Membrane was prepared from the polymer solution (~2% w/v) in N’N-di-methyl formamide (DMF) by solvent casting on a smooth surface of a clean glass plate. After casting, the membrane was dried overnight at ambient temperature and then in a hot air oven at around 80 oC for 3 h.


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2.2.3. Membrane characterization The unfilled and filled membranes were characterized by Fourier transform Infrared spectroscopy (FTIR, Perkin Elmer, model-Spectrum-2, Singapore), 13C nuclear magnetic resonance (NMR, JEOL, ECX400, 400 MHz NMR spectrometer), scanning electron microscopy (SEM, Model no. S3400N, VP SEM, Type-II, made by Hitachi, Japan) and wide-angle x-ray diffraction (XRD, model X'Pert PRO, PANalytical B.V., The Netherlands). For these characterizations a thin (~ 10 micron) membrane sample was used. For XRD, Nifiltered Cu K radiation (= 1.5418 Å) was used and the scanning rate was 2 deg/s (2/s). For SEM the gold coated membrane sample was used at an accelerating voltage of 2-5 kV and a high magnification of 10000x. Thermal properties of the membrane samples were evaluated by differential thermal analysis and thermogravimetric analysis (DTA-TGA, Perkin Elmer, Singapore). For DTA-TGA, the membrane sample was heated from 30 to 600 oC at a heating rate of 10 oC per minute. The mechanical stability of the membrane under static load was determined by its tensile strength (TS) and elongation at fracture (EAF). The experiment was carried out as per the ASTM D 882-97 in a Lloyd-Tensile tester (Lloyd instruments, England). Similarly, the stability of the membrane under dynamic load in a temperature range of 40-100 oC were carried out to find its storage (G’) and loss (G”) modulus and also tan δ in a dynamic mechanical analyzer (DMA 7, Perkin Elmer, Singapore).


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Scheme 1 Synthesis , casting and use of the membrane in pervaporation. 2.2.4. Sorption & Pervaporation experiments For sorption experiment thick membrane samples (~0.1 g) were swollen in acetonitrile-water mixtures in a stoppered conical flask at constant temperature till sorption equilibrium was attained. The swollen samples were then carefully taken out from the flask and the superfluous solvents adhered on the membrane surfaces were wiped out with a tissue paper. The total sorption mass (ST, g/g dry membrane) was determined from the ACS Paragon Plus Environment

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difference of mass of the dry and swollen sample. Partial solvent sorption was determined by taking the swollen membrane sample in a conical flask which was connected to a vacuum pump. The solvent mixture vaporized from the membrane at low pressure was condensed in a glass trap immersed in liquid nitrogen. The amount of water (solvent 1) or acetonitrile (solvent 2) in the membrane after sorption was obtained by analyzing the condensed solvent mixture in a Gas Chromatograph (GC, Shimadju, Japan, Column Stabilwax-DAGC, EB624). The permeation of solvents through the membranes was studied by pervaporation experiment in a stirred membrane cell in a batch mode as shown in scheme 1. The area of the membrane in contact with the solvent mixture was 19.6 cm2 while the total feed volume was 150 cm3. The lower half of the membrane cell was equilibrated for 3h for the first experiment and 1 h for the subsequent experiments. The sorption and pervaporation experiments were carried out at a fixed feed temperature of 30 oC, 40 oC, 50 oC and 60 oC which were maintained by circulating water of constant temperature from a water bath to a jacket surrounding the pervaporation cell. These permeation experiments were also carried out at a fixed downstream pressure of 1.5kPa maintained by a vacuum pump connected to the pervaporation cell. The solvent mixtures vaporized from the downstream side of the membrane were condensed in a glass traps immersed in liquid nitrogen and analyzed with the same GC used for sorption for determining the partial solvent mass of the permeate. Each experiment of sorption and pervaporation was repeated thrice and the results were reproducible with the inherent errors of around

±3%. The sorption selectivity,

α s 14, the total solvent flux (JT) and separation factor for water ( α PV )

were determined using the sorption and permeation data from the following Eq. 1, 2 and 3, respectively. u1 u αs  2 φ1 φ2

(1) ; J T 

m At

y1 y (2); α PV = 2 x1 x2

(3)

Here ui and φ i is the volume fraction of solvent i in the ternary water/ acetonitrile /membrane and the binary acetonitrile /water in feed, respectively, ‘m’ is the total mass of water and acetonitrile mixtures coming out of the membrane of cross-sectional area ‘A’ at time ‘t’ while ‘yi’ and ‘xi’ are the mass fraction of solvent ‘i’ in the permeate and feed, respectively. Partial molar flux (Jim) of the solvent ‘i’ was determined by multiplying the ACS Paragon Plus Environment

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total flux (J) with the mass fraction of the solvent ‘i’ in the permeate and dividing the same with molecular weight (18 and 41 for water and acetonitrile, respectively) of the solvent. The vapor pressure and thickness normalized partial flux or permeability of solvent i (Pi) was obtained by multiplying its molar flux (Jmi) with the thickness of the membrane (L) and dividing the same by the difference of vapor pressure of the solvent on the feed and permeate side 15 as Pi 

J im L (x f i γ i p sat fi  y pi p p )

(4)

Here x fi and y pi are the mole fraction of solvent i on the feed and permeate side, respectively, while γ i is the activity coefficient of solvent i. The activity coefficient of water and acetonitrile were obtained by using sat Willson parameter 16. Similarly, p f1 is the saturated vapor pressure obtained by using the Antoine’s equation.

Ignoring very low permeate pressure (pp), the above Eq.4 reduces to Pi 

J im L x f i γ i p sat fi



J im L fi

(4a)

where fi is the fugacity of the solvent i. The intrinsic membrane selectivity, i.e., the selectivity due to membrane only was obtained as α mem = f1 f α evap = 2 x1 x2

P1 P2

(4b). Further, selectivity due to only solvent evaporation

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is defined as

(4c). Accordingly, α PV =α mem .α evap (4d)

3. Results and Discussion 3.1. Membrane Characterization The FTIR and NMR analysis of the membrane samples as shown in Fig.1a and 1b, respectively, were carried out to characterize the formation of the copolymer and also its structure. The unfilled CP15 shows an absorption peak at 2240cm-1 corresponding to -CN functionality of its AN moiety, at 2937 cm-1 corresponding to –CH2 stretching and also at 1670 cm-1due to carbonyl of amide of its pyrrolidone moiety 18. The ratio of the peaks due to –CN at 2240 cm-1 and carbonyl of pyrrolidone at 1670 cm-1 are 2.65/4.05 (= 0.655), 2.75/3.91 ACS Paragon Plus Environment

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(=0.701)


and 2.98/3.85 (=0.774) for the CP10, CP15 and CP20 copolymer, respectively which signifies an

increase in the AN content in the copolymer (CP) from CP10 to CP20 though the exact copolymer composition could not be determined because of the unavailability of standard copolymer sample with known composition. In Fig.1a the absorption peaks of virgin clay at 3622 cm-1 due to O-H stretching of its silanol (Si-OH) group 19 is observed at 3617 cm-1, 3611 cm-1, 3617 cm-1 and 3617 cm-1 in the filled copolymer F0.5, F1, F1.5 and F2, respectively. Similarly, the absorption peaks of clay at 1062 cm-1 and 527 cm-1 due to Si-O-Si groups of its tetrahedral sheet 19 is also observed at 1035 cm-1, 1035 cm-1, 1066 cm-1, 1065 cm-1 and 527 cm-1, 537 cm-1, 534 cm-1 and 533 cm-1 in the filled copolymers in the same order. The presence and marginal shifting of the characteristic absorption peaks of clay in the filled copolymers ensure its successful entrapment in the polymer matrix. The formation of the copolymer of AN and NVP was also evident from NMR analysis as shown in Fig.1b. The homo polyacrylonitrile (PAN) has been reported 20 to show well resolved triplet in the –CN region at 118.8-120.10 ppm. However, the resolution of the –CN splitting in the copolymer of acrylonitrile becomes broad due to the overlapping of the compositional sequence of the comonomers units 20. Thus, the signal due to the resonance of the carbon atom on nitrile (-CN) functionality is observed at 121.499 ppm in the present copolymer. Similarly, the signal due to the resonance of the carbon atom on methylene (-CH2) of the homo polyacrylonitrile and homo polyvinyl pyrrolidone ring at 33.3 ppm 21 and 30.3 ppm 22, respectively merges to a strong single peak at 31.208 ppm in the copolymer. The signal due to the resonance of carbonyl carbon of pyrrolidone is observed as a small peak at 175.91 ppm in the copolymer as also reported elsewhere

22.

These

results ensure the formation of the copolymer. The thermal characteristic of the unfilled CP15 and the filled copolymer F1 is shown in Fig.1c in terms of TGA and DTG (inset of Fig.1c). The marginal weight loss (~2-4%) of the unfilled and filled copolymer in the initial stage is due to the loss of absorbed water which is more significant for the inorganic clay. The onset of the major degradation temperature is observed as an endothermic DTG peak at around 300oC and 400oC for CP15 and F1, respectively. It is also observed that though the degradation profile of the CP15 and F1 are similar, the filled copolymer F1 shows much higher thermal resistance over the temperature range. This may be because of the presence of the inorganic clay in the copolymer matrix. The amount of residue left at 600oC was 80.80%, 41.90% and 42.25% for clay, CP15 and 9 ACS Paragon Plus Environment


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F1, respectively which shows an actual ((42.25-41.90)/(80.8-42.25)) x 100 or 0.89% of clay incorporation in the filled copolymer F123. This is very close to the amount (1%) mixed experimentally with the filled copolymer (F1). The XRD analysis of the CP15, F1 and the clay is shown in Fig.1d. The amorphous copolymer (CP15) does not show any characteristic crystalline XRD peak. The inorganic clay shows crystalline peaks at 2θ of 3.7o and 10.2o corresponding to the reflection at its 001 plane and another peak at 2θ of 17.5o due to its 002 plane 24. The 3.7 o and 10.2 o peaks of clay broadens and shifts to 5.9 o and 9.8 o, respectively in F1 while other peaks of clay are not observed in F1 which may be due to exfoliation and breaking of the crystalline structure of the clay in the copolymer matrix. From Fig.1e it is observed that the unfilled CP 25 copolymers shows a brittle failure (no bending of the curve because of failure before yield point) with a tensile strength (TS) of only 15 MPa and a low elongation at fracture ( EAF) of only 0.03%. As the AN content reduces or NVP content increases in the CP, TS increases and the polymer becomes more flexible as required for making a good membrane. This is evident from the results of TS and EAF of the CP15 copolymer which shows a ductile failure (bending of the curve with failure above yield point) with TS and EAF of 35 MPa and 0.06%, respectively. The membrane made from CP15 will be hard as well as flexible. From Fig.1e it is also observed that in the presence of clay the TS increases significantly though only the filled copolymer F1 shows ductile failure with TS and EAF of 60 MPa and 0.11%, respectively. In fact, the NVP monomer with its 5 membered rigid pyrrolidone substituent increases the stiffness of the CP. Similarly, the presence of clay also improves the tensile properties of the filled copolymers by acting as physical crosslinks. The variation of storage modulus (G’) and loss modulus (G”) of the CP15 and F1 at different temperatures and at a constant frequency of 1 Hz is shown in Fig.1f while the variation of the ratio of loss to storage modulus (G”/G’ = Tan δ) or Tan δ with temperature is shown in the inset of Fig.1f. It is evident from the figure that the storage modulus (G’) of F1 is higher than the G’ of CP15 over the temperature range of 40-100oC. The Tan δ value is also observed to be much less than unity over the range of temperatures for both CP15 and F1 polymers signifying elastic nature of the membrane. As the temperature rises, the movements of polymer molecules increase resulting in decrease in both storage and loss modulus. Accordingly, there is marginal variation of Tan δ over the temperature range as observed in the inset of Fig. Thus, the membranes made from these unfilled or filled copolymer will retain its dimensional stability under 10 ACS Paragon Plus Environment

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dynamic stress and over a wide range (40-100oC) of temperature. The distribution of clay in the copolymer membrane is shown in terms of the surface morphology of the filled copolymer membranes, i.e., F0.5, F1, F1.5 and F2. The clay particles are observed to be uniformly distributed without any crack or agglomeration. However, a significant amount of agglomeration of clay particles is observed with non-uniform distribution in the membrane containing 2 mass % of clay signifying the poor compatibility of this nano sized clay with the

0 .4 0 .3 0 .2 0 .1 0 .0 0 .5 0 .4 0 .3 0 .2 0 .1 0 .0 .5 - 0 .1 0 .4 0 .3 0 .2 0 .1 0 .0 .5 - 0 .1 0 .4 0 .3 0 .2 0 .1 0 .0 0- 0 .0.1 8 0 .0 6 0 .0 4 0 .0 2 0 .0 0 0 .5 0 .4 0 .3 0 .2 0 .1 0 .0 - 0 .1

3617 3468

1a

2243

1672 1065 533 1389

3617

31.208

1659 1065 534 1389 2243

3469

F 1 .5

3611

1659 1066 1389 2243

537

1035

527

3468

F1

3617

2243

1659

1389

3468

121.499

F 0 .5

3622 3439

1062

3464

2243

527

175.14

C la y

1666 1389

CP15

4000 3500 3000 2500 2000 1500 1000 W a v e n u m b e r [c m -1 ]

1c

In te n s ity

600

0 .3 0 0 .1 5 0 .0 0 -0 .1 5 -0 .3 0 -0 .4 5

500 400

150

100

200

300 ToC 450 300

400

T em p eratu re , T 0 C

600 500

300

75 100 125 150 175 200

140 120 100 80 60 40 20

C lay

0 10 20 30 40 50 60 70 80

200

0 600

50

F1

100

0

25

ppm

1d 700

0

0

500

CP15 F1 c la y

d w /d T

100 90 80 70 60 50 40 30 20 10 0

1b F2

In te n s ity

Transm ittance (% )

copolymer at this concentration.

W t% (w )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CP15

2

0 10 20 30 40 50 60 70 80 90 2


11

1e

60 50 40 30 20 10 0 0.000

2 .0 x 1 0

G 'C P 1 5 G "C P 1 5 G "F 1 G 'F 1

7

1 .5 x 1 0

7

1 .0 x 1 0

7

5 .0 x 1 0

6

1f

0 .4 0 .2 0 .0 -0 .2

C P15 F1

Tan 

0.5F 1F 1.5 2 C P 25 C P 15

G ' o r G ", P a

70

S tre s s , M P a

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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40

60

80

T ,o C A

0 .0 0.025

0.050

0.075

S train, %

0.100

0.125

40

50

60 70 80 T e m p e ra tu re ,T o C

90

100

1 .6 x 1 0

8

1 .4 x 1 0

8

1 .2 x 1 0

8

1 .0 x 1 0

8

8 .0 x 1 0

7

6 .0 x 1 0

7

4 .0 x 1 0

7

2 .0 x 1 0

7

G ' of F1, Pa


100

Fig.1 Characterization of the membrane polymers-a) FTIR, b) NMR, c) DTA-TGA, d) XRD, e) Mechanical properties, f) DMA.

2a (0.5F)

2c (1.5F)

2b(1F)

2d (2.5F)

Figure 2a-d SEM of filled CP ACS Paragon Plus Environment

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3.2. Sorption & Pervaporation Effect of NVP molar% and clay content The effect of molar% of NVP of the copolymer on sorption and pervaporation for the azeotropic feed concentration (82.5 mass% acetonitrile) is shown in Fig.3ai-aiv. It is observed that with an increase in molar% of NVP, the partial solvent sorption (Fig.3ai), flux (3bii) and permeability increases though partial sorption, flux and permeability of water is observed to be much higher than the same of acetonitrile for all copolymers. This may be because of much higher polarity index (water 10.2, acetonitrile 5.8 25) and smaller kinetic diameter of water molecules(water 2.64Å and acetonitrile 3.64Å26) for which it shows preferential sorption and diffusion through the membrane. Accordingly, as the molar% of NVP in the copolymer increases from 3.85% (CP25 membrane) to 9.09% (CP10 membrane), the partial sorption, flux and permeability of water (Sw, Jw and Pw, respectively) increases from 0.423 to 0.6614mg/g (56.2% increase), 120 to 160 g/m2h (33.3% increase) and 50.4 to 65.8 Barrer (30.5% increase), respectively while the same of acetonitrile, i.e., Sace, Jace and Pace increases from a very low value of 0.020 to 0.047 mg/g (135% increase), 4.2 to 8.51 g/m2h (103% increase) and 0.478 to 0.969 Barrer (103% increase), respectively. Because of higher rate of increase of Sace, Jace and Pace, sorption selectivity (SS), separation factor (SF) and intrinsic membrane selectivity (MS) of water decreases with the increase in molar% of NVP as observed in Fig.3ai, ii and iv, respectively. From Fig.3aiii it is observed that the azeotropic feed containing 17.5 mass% water is concentrated much above 90 mass% after sorption and pervaporation. It is observed that with an increase in the NVP molar% from 3.85 to 9.09, the membrane/permeate mass% of water decreases marginally from 95.1/96.5 to 92.3/94.6. In fact, hydrophilicity of the membrane increases with an increase in the molar% of NVP since NVP contains pyrrolidone functionality which shows strong hydrogen bonding interaction with water (the homo polyvinylpyrrolidone is also soluble in water). However, the vinyl pyrrolidone repeating unit of the copolymer containing pendant 5member-pyrolidone group also increase the separation between the polymer chains27and allow sorption and permeation of bigger acetonitrile molecules through the membrane at higher NVP content. From Fig.3ai-iv it is clear that the copolymer containing 6.25 molar% NVP (CP15 membrane) shows the best results for sorption, flux and permeability. This CP15 copolymer was further filled with 0.5, 1.0, 1.5 and 2 mass% clay and the ACS Paragon Plus Environment

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effect of clay on sorption and pervaporation for these filled membranes, i.e., F0.5, F1, F1.5 and F2 is observed to be similar as shown in Fig.3bi-iv. Initially in the presence of clay, sorption and permeation of water increases but above around 1 mass% of clay in the membrane, the sorption, flux or solvent permeability decreases. Clay contributes to increased sorption and permeation by selective adsorption through electrostatic interaction of its ionic groups with solvent molecules. However, this nano sized clay agglomerate at higher mass% of loading and cause microphase separation

28

in the membrane which allows increased sorption and

permeation of acetonitrile above 1 mass% clay in the membrane resulting in decrease in sorption selectivity, separation factor and membrane selectivity. Similar trend of results was reported for pervaporative dehydration of azeotropic ethanol-water mixture with bentonite filled PVA29 and SDS clay filled polyamide

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5

6

7

8

0 .0 4

9 10

0 .5 0

0 .0 3

ST SW S a c e 0 .0 2

0 .4 5 0 .4 0 3

4

5

6

7

8

9

10

SF

160 150

3

4

5

6

7

8

N V P m o la r% in c o p o ly m e r

8

9 10

JW J ace

110

P w , B arrer

, m ass% pw or C mw

60 50

C pw

8 7

120 3

4

5

6

7

8


110 100 90 80 70 60 50

10

3

4

5

6

7

4 10

0 .9 8

9 10

0 .8

60

0 .7

55

45

5

1 .0 MS

Pw P ace

50

9

9

6

3 a iv

MS

65

A z e o tro p ic fe e d (8 2 .5 m a s s % w a te r)

6

7

70

C mw

5

JT

90

70

4

130

3 a iii

80

3

140


100

SF

2

4

9

J ace , g /m h

3

3 a ii

3

4

5 6 7 8 N V P m o la r% in c o p o ly m e r


9

P ace , B arrer

SS

130 120 110 100 90 80 70 60

170

0 .0 5

2

0 .5 5

3ai

J T o r J w , g /m h

0 .6 0

80 75 70 65 60 55 50

S ace , m g /g m em b ran e

S T o r S w , m g /g m em b ran e

0 .6 5

ss

nanocomposite membranes.

C

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 40

0 .6 0 .5 0 .4 10

14

2 .0

2 .5

0 .0 4 5

0 .6 5 0 .6 0

0 .0 4 0

0 .5 5 0 .5 0

0 .0 3 5

0 .4 5 0 .4 0 0 .0

0 .5

1 .0

1 .5

2 .0

0 .0 3 0 2 .5

180 170

8 6

150 140

JT

4

130

JW J ace

2

120 0 .0

0 .5

80 75

90

1 .0

1 .5

2 .0

2 .5

C la y m a s s % in c o p o ly m e r

MS

3 b iii

180 160 140 120 100 80 60 40 0 .0

3 b iv

MS

0 .5

1 .0

1 .5

2 .0

1 .5

2 .5

1 .2

70 80

C mw

70

C pw

A z e o tro p ic fe e d (8 2 .5 m a s s % w a te r)

60 50 0 .0

0 .5

1 .0

1 .5

2 .0


2 .5

P w , B arrer

, m ass% pw or C mw

10

160


100

12

65

0 .9

60 55

Pw

50

P ace

45 0 .0

2

1 .5

3 b ii

0 .5 1 .0 1 .5 2 .0 N V P m o la r% in c o p o ly m e r

J ace , g /m h

1 .0

200 175 150 125 100 SF 75 50 0 .0 0 .5 1 .0 1 .5 2 .0 2 .5

P ace , B arrer

0 .5

SF

ss

0 .7 0

SS

190

0 .0 5 0

2

S T o r S w , m g /g m em b ran e

0 .7 5

ST SW S ace

3bi

J T o r J w , g /m h

100 90 80 70 60 50 40 0 .0

0 .8 0

C

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


S ace , m g /g m em b ran e

Page 15 of 40

0 .6

0 .3 2 .5

Fig.3 Effect of a) molar% of NVP and b) mass% of clay on i) sorption & sorption selectivity (SS), ii) Flux& separation factor (SF), iii) membrane& permeate concentration (Cmw & Cpw, mass%) and iv) permeability (P) & membrane selectivity (MS). Effect of feed concentration The effect of feed water concentration (Cw) on sorption and pervaporation of acetonitrile-water binary mixture for filled F1 membrane is shown in Fig.4a-d. Similar results with the unfilled CP15 membrane is also shown in the same figures for comparison. From these figures it is evident that for the same Cw, the FI membrane shows higher sorption, flux, permeability, sorption selectivity, separation factor and intrinsic membrane selectivity than the unfilled CP15 membrane. The total sorption of solvent mixture (ST) increases with an increase in Cw while the sorption selectivity (SS) decreases at higher Cw as observed in Fig.4a. In fact, the filled F1 membrane show a dual mode of Type V30 sorption where the continuous and bulk membranepolymer phase shows Henry’s type sorption while the dispersed clay particles within the membrane shows ACS Paragon Plus Environment

15


Langmuir type sorption. Similar to sorption, in Fig.4b the total flux (JT) is observed to increase with Cw while separation factor (SF) decreases at higher Cw. The increase in solvent sorption or flux with concentration (Cw) may be ascribed to the increase in the plasticization and swelling of the hydrophilic membranes at higher Cw. However, the polymer chains also loosen at higher Cw allowing increased permeation of acetonitrile molecules. Thus, the sorption selectivity or separation factor of water decreases at higher Cw. In Fig.4c the membranes phase (Cmw) or permeate (Cpw) concentration of water after pervaporation (PV) is observed to be much higher than its vapor-liquid equilibrium (VLE) composition. Thus, around 6 mass% water in the feed is concentrated to only 12 mass % after distillation (VLE) while the filled copolymer or F1 membrane concentrates it up to 90.5% after sorption and 92.7% after pervaporation. The vapor pressure and thickness normalized flux or solvent permeability shows an opposite trend of flux and sorption as observed in Fig.4d. The partial solvent permeability is observed to decrease with an increase in the Cw. However, the water permeability (Pwater) is much higher than the acetonitrile permeability (Pace). Thus, the intrinsic membrane selectivity (MS) increases with an increase in the Cw as shown in the inset of Fig.4d. The decrease in the permeability with an increase in the Cw may be ascribed to an increase in the vapor pressure with Cw which more than offset the increase in flux with Cw. From the inset of Fig.4c it is observed that the increase in fugacity is marginal for Cw >12 mass%. Thus, above this feed concentration (Cw) the variation of solvent permeability with Cw is also not significant as

4a

ST C P15

200

ST F1

4b

240 180

180 150 SS C P15 120 SS F1 90

J T , g /m 2 h

210

SS

1 .0 0 .9 0 .8 0 .7 0 .6 0 .5 0 .4 0 .3 0 .2 0 .1 0 .0

JT C P 15 JT F 1

160 140

SF CP15 SF F1

120

60 0

3

6

9 12 15 18 C w (m a s s % )

21

24

100

0

3


6

9 12 15 18 C w (m a s s % )

21

24

300 270 240 210 180 150 120 90 60

SF

observed in Fig.4d.

S T , m g /g m em b ran e

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 40

16

100 F u g a c ity , k P a

80 60 40

21 18 15 12 9 6 3 0

A c e to n itrile

C mw

200

C P 1 5 C pw

175

F1

150

VLE

4d

125

3

6

M S C P15 M S F1

W a te r

0 3 6 9 1 21 51 82 12 42 7

4 Pw CP15 Pw F1

75 50

9 12 15 18 21 24 27 30 33 C , (m a s s % ) w

0

5

0 3 6 9 1 21 51 82 12 42 7

P ace C P 1 5

25 0

300 250 200 150 100 50

100

20 0

C pw

P w , B arrer

F1

P ace F 1

0

3

6

C

9 w

3 2

P ace , B arrer

C P15 C mw

4c C m w o r C p w , m ass%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


MS

Page 17 of 40

1 0 12 15 18 21 24 27 , (m a s s % )

Figure 4. Effect of feed concentration of water (Cw) on a) sorption (ST) & sorption selectivity (SS), b) Flux (JT) & separation factor (SF), c) permeability(P) & membrane selectivity (MS), d) membrane (Cmw)& permeate (Cpw) concentration. Effect of temperature The sorption and pervaporation experiments with the F1 membrane was also carried out at 30 oC, 40 oC, 50 oC and 60oC temperatures for different feed water concentrations, i.e., 1.26, 6.27, 12.38, 18.33 and 24.13 mass% water in feed noted as C1.26, C6.27, C12.38, C18.33 and C24.13, respectively as shown in Fig.5a-f. From these figures it is evident that sorption, flux and permeation are activated processes. Partial solvent sorption and flux was found to increase linearly with temperature (not shown) indicating its endothermic nature. At higher temperature there is an increase in transient free volume in the membrane polymer because of increased segmental motion of the polymer chains. Thus, the ‘endothermic’ movement of solvent molecules from bulk feed to membrane surface (desorption) and increase in free volume in membrane during sorption of solvents more than offset the ‘exothermic’ sorption of solvent molecules into the bulk of the membrane to make the overall sorption process ‘endothermic’31. The low heat of dissipation (low exotherm) during sorption into the bulk of the membrane owes to large difference in the solubility parameter (in MPa0.5 unit) values between solvents (water-48 MPa0.5, acetonitrile-24.4 MPa0.5) and the polymer membrane (PAN-26.2 MPa0.5, PVP-23.9 MPa0.5)31. The molar heat of sorption (∆Hs) was determined from the slope of the linear Arrhenius type plot of ln (molar sorption) against inverse of absolute temperature (1/T) as shown in Fig.5a. Similarly, at higher temperature, apart from the increase in transient free volume of the membrane matrix, vapor pressure of the solvents also increases as observed in the inset of Fig.5c. Thus, flux increases at higher temperature. The ACS Paragon Plus Environment

17


apparent activation energy of permeation (Eap) consisting of activation energy for permeation (Ep) and molar heat of vaporization (∆Hv; Eap = Ep +∆Hv) was obtained from the slope of the ln (molar solvent flux) versus 1/T plot as shown in Fig.5b. The effect of temperature on solvent permeability is shown in Fig. 5c. It is observed that permeability shows an opposite trend of sorption and flux, i.e., partial solvent permeability decreases almost exponentially with feed temperature. This is because of the increase in vapor pressure of both water and acetonitrile at higher temperature. As solvent fugacity increases at higher feed concentration, the decrease in permeability with temperature is more significant. The activation energy for permeation (Ep) is obtained from a similar plot of ln (P) versus 1/T as shown in Fig.5d. Activation energy for diffusion (Ed) is obtained as Ep = Ed + ∆Hs. The effect of feed concentration on these activation energies are shown in Fig.5e. It is observed that with an increase in feed water concentration activation energy for permeation and diffusion of water increases as reported elsewhere 32. Vapor-liquid equilibrium selectivity (αevap ) and membrane selectivity (αms) The effect of vapor-liquid equilibrium (VLE)-based water selectivity (αevap) of the pervaporation process on separation factor (αPV) at different feed temperatures is shown in Fig.5f. For low feed concentration αPV increases with the increase in αevap. However, with the increase in feed water concentration (>~20 wt% water) the trendlines becomes gradually parallel to the αPV axis (y) indicating marginal effect of αevap on αPV or the temperature on αevap. The decrease of αPV at higher temperature is more significant at higher feed water

w a te r

5a

-4 .0 -4 .5 -5 .0 -5 .5

a c e to n itrile

-6 .0 -6 .5 -7 .0 -7 .5

3 .0 0 3 .0 5 3 .1 0 3 .1 5 3 .2 0 3 .2 5 3 .3 0 1 /T x 1 0 0 0 , K -1

C 1 .2 6 w a te r C 6 .2 7 w a te r C 1 2 .3 8 w a te r C 1 8 .3 3 w a te r C 2 0 .6 7 w a te r C 2 4 .1 3 w a te r C 1 .2 6 a c e to C 6 .2 7 a c e to C 1 2 .3 8 a c e to C 1 8 .3 3 a c e to C 2 0 .6 7 a c e to C 2 4 .1 3 a c e to

5b

ln (m o la r flu x )

-3 .0 -3 .5

ln (s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 40

0 .0 -0 .3 -0 .6 -0 .9 -1 .2 -1 .5 -1 .8 -2 .1 -2 .4 -2 .7 -3 .0 -3 .3 -3 .6

w a te r C 1 .2 6 w C 6 .2 7 w C 1 2 .3 8 w C 1 8 .3 3 w C 2 4 .1 3 w C 1 .2 6 a c e to C 6 .2 7 a c e to C 1 2 .3 8 a c e to C 1 8 .3 3 a c e to C 2 4 .1 3 a c e to

a c e to n itrile 3 .0 0

3 .0 5


3 .1 0 3 .1 5 3 .2 0 1 /T x 1 0 0 0 , K -1

3 .2 5

3 .3 0

18

C 2 4 .1 3 a c e C 1 8 .3 3 a c e C 1 2 .3 8 a c e C 6 .2 7 a c e C 1 .2 6 a c e

kP a

P w , m 3 .m /m 2 s k P a

20 18 16 14 12 10 8 6 4 2 0

60 50 40 30 20 10 0

C 1 .2 6 w C 6 .2 7 w C 1 2 .3 8 w C 1 8 .3 3 w C 2 4 .1 3 w A c e to n itrile w a te r

30

36

42

48

54

0 .3 0

-2 0

0 .2 5

-2 1

0 .2 0

-2 2

P a c e , m 3 .m /m 2 s k P a

5c

0 .1 5

60

A

-2 6

0 .0 0

Epw Edw

-4 0 .2 -4 0 .4

0

-4 0 .6

-5

-4 0 .8

-1 0

-4 1 .0 -4 1 .2

-1 5

-4 1 .4

-2 0

-4 1 .6

-2 5 0

5

10 15 C w (m a ss% )

20

25

-4 1 .8

C 1 .2 6 C 6 .2 7 C 1 2 .3 8 C 1 8 .3 3 C 2 4 .1 3

360 320 280 240

PV

5



E pace E dace

5e

3 .0 0 3 .0 5 3 .1 0 3 .1 5 3 .2 0 3 .2 5 3 .3 0 1 /T x 1 0 0 0 , K -1

60

E p , E d , k J/m o l, a c e to n itrile

10

42 48 54 T e m p e ra tu re , o C

-2 3

-2 5

0 .0 5 36

5d

C 1 .2 6 w C 6 .2 7 w C 1 2 .3 8 w C 1 8 .3 3 w C 2 4 .1 3 w C 1 .2 6 a c e C 6 .2 7 a c e C 1 2 .3 8 a c e C 1 8 .3 3 a c e C 2 4 .1 3 a c e

-2 4

0 .1 0

30

E p , E d , k J/m o l, w a te r

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ln P

Page 19 of 40

5f

200 160 120 80 40 1

2

3

4

5

6

evap

Figure 5. Arrhenius plot for a) sorption b) flux, c) permeability & temperature, d) Arrhenius plot for permeability, e) activation energy & feed conc. f) αevap & αPV concentration. In fact, the less selective and least sorbing solvent, i.e., acetonitrile undergoes the largest influence of temperature at higher feed water concentration 31 resulting in significant decrease in αPV 3.3. Modeling of sorption and pervaporation Sorption Flory-Huggins (F-H) Thermodynamics The initial Flory-Huggins (F-H) sorption model assumed constant thermodynamic F-H interaction parameter ( χ ) over the entire feed activity range. However, the variation of solvent-solvent interaction parameter ( χ ), 12 solvent 1-membrane interaction parameter ( χ

1m

) and solvent 2-membrane interaction parameter ( χ

2m

) with

feed concentration should be considered for better prediction of equilibrium sorption of polar solvents by a polymer membrane as in the present case. The thermodynamic F-H interaction parameter χ between 12 ACS Paragon Plus Environment

19


Page 20 of 40

solvents, i.e., acetonitrile and water in the feed33 and membrane at different feed concentrations were determined using the following Eq.5. χ

12



1 x v 1 2

  x  x   x ln 1   x ln 2   (x 1 ln  1  x 2 ln  2 ) 2 v   1  v1    2

(5)

Here x1 and v1 is the mole fraction and volume fraction, respectively, of solvent 1. The activity coefficient of the solvents (  ) was determined by using the Willson equation16. The Wilson parameter for water and acetonitrile was obtained from its vapour-liquid equilibrium data 16 as 0.23965 and 1.67589, respectively. The  between membrane-solvent for single solvent (i) and binary solvent mixtures were determined by using the following Eq.6 and 7, respectively 34,35.

lnγ im  φ m  χ im φ 2m  (1  φ i )  χ im (1  φ i ) 2

(6)

χ 1m  χ 1m (u 2  0)  pu 2  q[φ m  φ m (u 2  0)]

(7a)

χ 2m  χ 2m (u 1  0)  ru 1  s[φ m  φ m (u 1  0)]

(7b)

 χ (u2  0) and similarly, when u1  0 , χ χ (u  0) For the limiting case when u2  0 , χ 1m 1m 2m 2m 1 Here φ Also,

m

is volume fraction of membrane polymer.

χ 1m χ 1m  p , (8a); q u 2 φ m

(8b);

χ 2m χ 2m  r (8c) and s u 1 φ m

(8d)

The volume fraction of solvent φ i in binary solvent-membrane or u1 in ternary solvent1 /solvent2/membrane is obtained from the sorption mass (mi) of the solvent by the membrane and the density of the solvent (  i ) and membrane (  p ). From the experimental sorption values of binary water-acetonitrile mixture in the membrane30, the values of p, q , r and s were obtained by using Eq-8 a, b, c and d, respectively with a statistical regression 30,31

and these values were

0.423, 3.76, 0.392 and 2.89, respectively. Similar values were reported for

separation of methanol from its mixture with ethylene glycol34 and recovery of ethanol from ethanol-water mixture using PDMS rubber membrane 35. Substituting these values and the values of  between membrane and single solvent in Eq.7a and 7b, the following Eq.9a and 9b are obtained. ACS Paragon Plus Environment

20

Page 21 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


χ 1m  0.312  0.423u 2  3.76(φ m  0.428)

9a

χ 2m  0.389  0.392u 1  2.89(φ m  0.604)

9b

These interaction parameters and experimental sorption data were used to calculate the membrane phase concentration of solvent 1 and solvent 2 in terms of activity coefficient (  m )using the following Eq.10 and Eq.11, respectively 36. lnγ 1m  φ

lnγ 2m  φ

m

m

χ

χ

V V χ 12 φ 2  (1  1 )φ  χ φ 2  (χ  χ  1 χ )φ φ  u u φ 1m m 2 12 2 12 1m 2m 2 m 1 2 2 V V u 2 2 2

V χ V V V V 12 φ 2  (1  2 )φ  2 χ φ 2  2 (χ  χ  1 χ )φ φ  2 u 12 φ 2m m 1 12 1 12 1m 2m 1 m 2 V V V V V u 1 1 1 2 1 2

(10)

(11)

Analysis of sorption data with Flory Huggins thermodynamics In general, the lower the value of interaction parameter, (  ) between two solvents or membrane polymer and solvent, the higher will be the solvent-solvent or solvent-membrane interaction. Accordingly, sorption or sorption selectivity will increase with the decrease in  value because of increased interaction. From Fig.6a it is observed that  between water-acetonitrile decreases with the increase in the feed water concentration which justifies increased sorption at higher feed water concentration. It is also observed from Fig.6a that for the same feed concentration  between the two solvents in the membrane is lower than the  in feed which signifies sorption by the membrane after its immersion in the solvent mixture. From Fig.6b it is observed that both water-membrane  and acetonitrile -membrane  decreases with feed concentration while for the same feed concentration water-membrane  is lower than the acetonitrile-membrane  . This result confirms increase in sorption and sorption selectivity for water with feed concentration. The higher sorption and sorption selectivity for F1 membrane may be ascribed to the lower  of water- F1 membrane than waterCP15 membrane as observed in Fig.6b. The calculated water activity in membrane (Caltd. amw) based on the Flory-Huggins thermodynamics (Eq.10 and 11) is shown in Fig.6c as a parity plot against experimental water activity (Expt. amw) in the CP15 and F1 membrane. It is observed that in spite of considering variable interaction


21


Page 22 of 40

parameters between membrane and solvent at different feed concentrations, the calculated values are not in close agreement with the experimental values. Engaged species induced clustering (ENSIC) model In fact, F-H thermodynamics is not suitable for polar solvents like the present water and acetonitrile solvents because of strong solvent-solvent interaction and solvent clustering 37. Thus, the experimental sorption data were also fitted to the following ‘Engaged species induced clustering (ENSIC) model Eq.12

Here the constant ks describe the elementary affinity between non- polymeric species, i.e., the solvent water and acetonitrile and kp is the same between polymer segment and solvent, viz., water-membrane and acetonitrilemembrane. The values of these affinity parameters were obtained by plotting experimental volume fraction of solvent in the membrane (  m ) against feed activity (afw) and doing exponential regression as per the Eq.12 as shown in Fig.6d. For sorption, the solvent-membrane interaction should be more than the solvent-solvent interaction, i.e., kp should be less than ks38. Accordingly, kp was 0.5054 for CP15 and 0.1032 for F1 membrane while ks was 3.51 for CP15 and 0.896 for F1 membrane. These values also indicate preferential water sorption by both CP15 and F1 membrane and better sorption selectivity for F1 membrane. The values of r2 were 0.9434 for CP15 and 0.9909 for F1 membrane indicating good fitting. The calculated membrane phase volume fraction of water was also close to y = x line as shown in Fig.6e which signifies good fitting of the ENSIC model for predicting sorption of water by the present hydrophilic membrane.


22

Page 23 of 40

F e e d IP C P 1 5 IP s o l F 1 IP s o l

C P 1 5 W a te r F 1 W a te r C P 1 5 A c e to n itrile F 1 A c e to n itrile

m

 m caltd



a fw

IP ,   

6a

ip

6b 0 .9 2 .1 0 .8 2 .4 1 .8 0 .7 2 .2 F e e d a c tiv ity 0 .6 1 .5 0 .5 2 .0 1 .2 0 .4 1 .8 0 .9 0 .3 1 .6 0 .2 0 .6 0 .1 1 .4 0 3 6 9 12 15 18 21 24 27 0 3 6 9 12 15 18 21 24 27 Cw Cw 0 .5 0 6d 6c 6e 0 .5 0 .4 5 1 .0 0 0 .4 0 F 1 , r 2 = 0 .9 9 0 9 0 .4 0 .9 5 0 .3 5 0 .3 0 0 .3 0 .9 0 0 .2 5 0 .2 C P15cal 0 .2 0 0 .8 5 F1cal 0 .1 5 0 .1 C P 1 5 c a l 0 .1 0 0 .8 0 C P 1 5 , r 2 = 0 .9 4 3 4 F1cal 0 .0 5 0 .0 0 .7 5 0 .2 0 .3 0 .4 0 .5 0 .6 0 .7 0 .8 0 .1 0 .2 0 .3 0 .4 0 .5 0 .9 2 0 .9 4 0 .9 6 0 .9 8 a  m expt fw E x p t. a m w 2 .6

C altd . a m w

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. Effect of feed conc. of water (Cw) on a) interaction parameter ( χ12 ) and activity of solvents, b) solvent-polymer IP ( χ1p ), c) Calculated versus experimental water sorption data with F-H model, d) and e) ENSIC model Pervaporation Based on the transfer solution volume fraction model (TSVF) 39, integrating the membrane phase volume fraction(φ) from the upstream (u) to the downstream side of the membrane and ignoring very low concentration on the downstream side of the membrane due to low pressure, the ratio of partial solvent flux of water (J1) to acetonitrile (J2) through the membrane may be expressed as

J 1 D10 V2 dφ1 D10 V2 φ1u   J 2 D 20 V1 dφ 2 D 20 V1 dφ 2u

(13)

Here Ji, Di0, Vi and φi are the partial molar flux, diffusion coefficient at infinite dilution, molar volume and membrane phase volume fraction of solvent i. Plotting of molar flux ratio of component 1 (water) and 2


23


Page 24 of 40

(acetonitrile) against ratio of membrane phase upstream concentration of component 1 and 2 gave the following regressed Eq.14a for CP15 and Eq.14b for F1 membrane as also shown in Fig. 7a. φ J1  3.921 1u J2 φ 2u

(14a),

 J1  7.050 1u 2u J2

(14b)

The ratio of the molar volume of acetonitrile (V2) and water (V1) is 2.889. Comparing Eq.14a and Eq.14b with Eq.15 and putting the values of ratio of molar volume of solvents, the values of

D1o was obtained as 1.357 for D 2o

CP15 and 2.329 for F1 membrane. Further, ignoring the very low concentration of solvent 1 at downstream side ( 1d  0 ), the partial flux of solvent-1 (water) may be expressed as 39 J1 =

D10 exp(u )  1 V1L 1  2u

(15)

iu

Here, ‘u’ denotes the total volume fraction of two solvents in the upstream side of the membrane, i.e.,

u  1u  2u , L is membrane thickness and  is plasticization coefficient of solvent 1. Plotting of partial molar

7a

200 F1

 2 J1  7.050 1u r = 0.9553 J2 2u

150

CP15 F1

J1 J 2 100

50

J1

CP15 J 2

0

0

3

6

9

 3.9211

1u 2 2u r = 0.9681

12 15 18 21 24 27 1u 2u


24

Page 25 of 40

7b

CP15 J

1

 10.027

exp(u )  1

 1  2u iu

10

J1, mol/m2h

r2 = 0.9300

CP15 F1

8 6

F1

J1  3.777

4

exp(u )  1 1

2 u iu

r2 = 0.9392

2 0 0.0

0.4

0.8

1.2

1.6

7C

0.0030

2.0

exp(u )  1 1

Calculated water Flux, mol/m2s

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.4

2.8

2 u iu

CP15 F1

0.0025 0.0020 0.0015 0.0010 0.0005 0.0000 0.0012 0.0015 0.0018 0.0021 0.0024 0.0027 Experimental water Flux, mol/m2s

Figure 7. Fitting of flux data to a) Eq. 16, b) Eq.17 and c) Experimental versus calculated flux for water

flux of solvent -1 (J1) against

membrane.

J1 =10.027

exp(u )  1

 1  2u iu

exp(1.5u )  1

 1  2u iu

yielded the following two regressed Eq.15a for CP15 and 15b for F1

, r2 = 0.930 (15a) J1 =3.777

exp(2.77u )  1


 1  2u iu

, r2 = 0.939

(15b)

25


Page 26 of 40

The fitting is shown in Fig.7b. From the above Eq.15, the plasticization coefficient is obtained as 1.5 for CP15 and 2.77 for F1 membrane. The diffusion coefficient of water (Dio) and acetonitrile (Djo) at infinite dilution, was calculated as 2.53 x 10-6 m2/s and 1.86 x 10-6 m2/s, respectively for CP15 and 1.52 x 10-6 m2/s and 0.65 x 10-6 m2/s, for F1, respectively. The small water molecules diffuse faster than the big acetonitrile molecules under the same concentration gradients resulting in higher diffusion coefficient for water. The diffusivity of solvent through a polymer matrix depends on its structure, i.e., the presence of polar groups, unsaturation, crosslinking, grafting or % crystallinity strongly influence the diffusivity of solvent through a membrane40. The present unfilled CP15 copolymer contains pendant nitrile and a 5 membered pyrrolidone ring in its structure which also increases separation between polymer chains of the membrane and hence the diffusivity. However, the presence of clay decreases the chain separation and hence diffusion coefficient of water. Similar order of diffusion coefficients for solvents through polymers were also reported elsewhere

41, 42.

The experimental water flux is

plotted against calculated water flux for these two membranes as shown in Fig.7c. It is evident that the calculated flux is close to y = x line signifying close fitting. 3.4. Optimization of synthesis and process parameters The i) copolymer composition in terms of NVP molar%, ii) mass% of clay and iii) feed concentration strongly influence the flux and separation factor of the membranes and hence these three (3) variables noted as x1, x2 and x3, respectively were considered for a central composite design (CCD) with two (2) different levels43 for 2n+2n+nc = 23+2.3+6 = 20 number of different compositions as shown below in Table 2a. The coded values were -1, +1 for the 8 (=23) factorial points, -1.68, 0, + 1.68 for the 2.3 =6 axial points and 0 for the 6 center points. The origin of 8 factorial and 6 replicate compositions were at the center while the origin of the 6 axial points was at a distance α (α = 2 n/4 = 2.3/4 = 1.68), i.e. 1.68 from the center. Multiple regressions with the experimental data of the three variables yielded two regressed equations-one for flux and another for separation factor as two different ‘response’. The intercept and the coefficients of linear, quadratic and interactive variables for flux and separation factor as response is shown in Table 2bi and ii, respectively.


26

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The values of mean square (MS) are obtained by dividing the sum of square with degree of freedom while ‘F’ value is obtained by dividing MS with error. Hence, the values of F should be high. It is observed from Table 2a and b that the regression shows a R2 value >0.9 indicating more that 90% of experimental data fitting the model. The ‘p” value is observed to be less than 0.05 for quadratic effect of clay mass% on flux (Table 2a) and separation factor (Table 2b) indicating its ‘significant’ effect on flux and separation factor. Similarly, the feed concentration is observed to have ‘significant’ effect on separation factor as evident from its ‘p’ value in both linear and quadratic form. The positive and negative coefficient of these three variables signify increase or decrease, respectively in ‘response’, i.e., the flux and separation factor. The interactive effect of the parameters, viz., copolymer composition (in terms of NVP molar%), clay% and feed concentration on flux and separation factor is shown below in Fig.8a i-iii for flux and Fig.8a iv-vi for separation factor. In these figures the simultaneous effect of two variables on flux and separation factor are shown with the third variable keeping at its central (0) level. From these figures it is evident that feed concentration and NVP molar% (Fig.8ai) increase flux to a greater extent than feed concentration and clay mass% (Fig.8aii). However, for constant feed concentration (central level), the increase of flux by increasing NVP molar% is more than offset by the decrease in flux at higher clay mass% (Fig.8aiii). Similarly, the reduction in separation factor with the increase in feed water concentration and NVP molar% (Fig.8aiv) and feed water concentration and clay mass% (Fig.8av) is also evident from the negative coefficient of these respective variables while with the increase in clay mass%, initially separation factor increases followed by the decrease in separation factor above 1 mass% clay though the effect of NVP molar % on separation factor is not significant (Fig.8avi). The residual plot of experimental versus predicted (based on this model) flux and separation factor are shown in Fig.8bi and ii respectively. It is observed that the predicted values closely fit the experimental values though flux data shows a better fitting than separation factor as also evident from their respective statistical parameters shown in Table 2a and b, respectively.


27


Page 28 of 40

Table 1a: Composition (coded values) of membranes based on central composite design and flux and sep. factor for pervaporative dehydration of acetonitrile Factor (input variables) Response variables x1 x2 x3 Response 1 Response 2 Membrane VP wt% Clay wt% Feed conc. Flux (g/m2h) Separation Factor 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

-1 -1 -1 -1 1 1 1 1 -1.68 -1.68 0 0 0 0 0 0 0 0 0 0

-1 -1 1 1 -1 -1 1 1 0 0 --1.68 -1.68 0 0 0 0 0 0 0 0

-1 1 -1 1 -1 1 -1 1 0 0 0 0 --1.68 -1.68 0 0 0 0 0 0

132.57

155.13

166.32

90.66

134.43

149.44

172.44

79.43

139.67

148.32

173.23

85.45

140.56

139.67

180.66

72.34

153.12 176.33

205.34 165.16

143.82

79.11

152.34 133.54

56.69 307.40

179.43

98.27

167.23

198.46

168.17

196.12

165.73

198.89

167.89

196.16

166.44

198.11

167.11

198.37

Table 1b: Coded and experimental values of synthesis parameters for different levels Parameters

Levels Lowest (-1.68) 3.85 (25:1)

(i)VP molar % (x1) (ii) Clay mass% (x2)

(iii) Feed conc. (mass% water) (x3)

Low (-1) 4.76 (20:1)

Center (0) 6.25 (15:1)

High (+1) 7.69 (12:1)

0

0.5

1.0

1.5

1.26

6.27

14.3

20.67

Highest (+1.68) 9.09 (10:1) 2.0 24.0

Table 2: Analysis of variances (ANOVA) for a) Flux and b) separation factor a) Flux (R2 = 0.9782, Adj.R2 = 0.9541, Std. error = 3.47, Model F= 40.45, p = 0) Source Coefficients Sum of squares DF Mean Square F-value p-value Intercept (a0) x1 x2

63.63 9.66 42.39

28.73 39.75

3 1 1

28.74 39.75


2.37 3.28

0.158* 0.103*

28

Page 29 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

x3 Two way x1 x2 x1 x3 X2 x3 Three way x1 x2 x3 Quadratic x12 X22 X32 Total *significant


3.57 -0.670 -0.018 0.037 0.054 -0.486 -19.624 -0.0572

51.15 15.72 0.428 0.060 0.007 0.649 0.649 664.71 19.41 623.77 101.97

5003.29

1 3 1 1 1 1 1 3 1 1 1

51.15

4.23

0.070*

0.428 0.0601 0.007

0.035 0.005 0.0006

0.855 0.945 0.980

0.649

0.0537

0.822

19.41 623.77 101.97

1.604 51.56 8.43

0.23 0.004* 0.388

19 53

b) Separation factor (R2 = 0.9555, Adj.R2 = 0.9060, Std. error = 16.22, Model F= 19.30, p = 0) Source Coefficients Sum of squares DF Mean Square F-value p-value Intercept (a0) x1 x2 x3 Two way x1 x2

202.38 -5.3624 -5.1409 -33.123

401.259 420.034 13,984.7

1.5241 1.5954 53.117

0.248 0.238 0.000*

17.038

3 1 1 1 3

-0.6102

2.9794

1

2.9794

0.0113

0.918

x1 x3 X2 x3 Three way x1 x2 x3 Quadratic x12 X22 X32 Total *significant

0.3104 -1.285

0.0029 0.0505

0.958 0.827

0.1445

0.0005

0.982

-8.315 -35.32 -32.57

1,231.92 32,327.4 12,306.1

1 1 1 1 3 1 1 1

0.7754 13.303

0.1344

0.7754 13.303 0.1445 0.1445

1,231.92 32,327.4 12,306.1

4.6791 122.787 46.741

0.059 0.000* 0.000*

401.259 420.034 13,984.7

19 53


29

Flux (g/m2h)

ii

195 170

6.3 8.1 9.9 11.7 13.5 15.3 17.1 18.9 20.7

145

7.7 7.0 6.2 5.5 VP 4.8 molar%

Separation Factor For water

220

Feed conc.(wt% water) iv

205 180 155 130 105 80 6.3 9.9 13.5 17.1 Feed conc. (wt% water) 20.7

Page 30 of 40

170 165 160 155 150

1.4 1.1 0.8 0.5 4.8 5.5 6.2 7.0 7.7 Clay% VP molar%

iii

1.5 1.3 1.0 0.8 0.5 Clay%

Separation Factor for water

20.7

17.1

13.5

1.4 1.1 0.8 0.5 Clay%

Feed conc. (wt% water)

i

120

9.9

171 159 147 135 6.3

20.7

17.1

13.5

9.9

6.3

7.4 6.5 5.6 VP 4.8 molar % Feed conc. (wt% water)

Flux (g/m2h)

180 165 150 135

Separation Factor For water

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Flux (g/m2h)


200 180 160 140 4.8 5.5 6.2 7.0 7.7 VP molar%

v

1.4 1.1 0.8 0.5Clay%

vi

Figure 8a Interactive Effect of feed conc. of water, molar% of VP in the copolymer and wt% of clay on flux (iiii) and separation factor for water (iv-vi).

i

ii

Figure 8b. Residual plot for i) Flux and ii) Separation Factor (S.F.) ACS Paragon Plus Environment

30

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3.5. Stability of the membranes The good mechanical stability of the membrane is evident from its high tensile strength and EAF. Further, the high storage modulus and low Tan δ under dynamic stress over the wide temperature range (40100oC) indicated its stability over long use under the pressure differential of pervaporation. The low Tan δ (

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