A review of membrane selection for the dehydration of aqueous ethanol by pervaporation

A review of membrane selection for the dehydration of aqueous ethanol by pervaporation

Chemical Engineering and Processing 50 (2011) 227–235 Contents lists available at ScienceDirect Chemical Engineering and Processing: Process Intensi...

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Chemical Engineering and Processing 50 (2011) 227–235

Contents lists available at ScienceDirect

Chemical Engineering and Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep

Review

A review of membrane selection for the dehydration of aqueous ethanol by pervaporation Brian Bolto ∗ , Manh Hoang, Zongli Xie CSIRO Materials Science and Engineering, Private Bag 33, Clayton South MDC, Victoria 3169, Australia

a r t i c l e

i n f o

Article history: Received 12 July 2010 Received in revised form 19 December 2010 Accepted 11 January 2011 Available online 28 January 2011 Keywords: Pervaporation Polar polymers Polyanions Polycations Polysalts

a b s t r a c t Four broad types of membranes are categorised: organic polymers generally, crosslinked poly(vinyl alcohol), organic–inorganic hybrids and charged polymers. The best performers in terms of flux, which reaches a maximum of 5 kg/m2 h, are anionic or cationic polymers, including polysalts. Polyanion and polysalt membranes are superior. Two examples are thin layers of the active polysalt membrane on a supporting membrane. The best combination for flux and selectivity is a polyethyleneimine/poly (acrylic acid) polysalt deposited on a reverse osmosis membrane, at 4 kg/m2 h and 1075 respectively. It is noticeable that hybrid poly(vinyl alcohol)/inorganic membranes do not show enhanced fluxes. Very high separation factors were observed, covering a range of polymers, of neutral, anionic or cationic character. The top results (>10,000) were for charged membranes, either cationic or anionic, but not polysalts. The fluxes encountered here were miniscule, the best being caesium alginate at about 1 kg/m2 h. The ideal structure for high fluxes would appear to be one containing discrete domains of oppositely charged species of optimal size. Fresh approaches are being actively studied, such as layer-by-layer deposition of oppositely charged polyelectrolytes, with due attention to appropriate separation of the sites of opposite character. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.

Contents 1. 2.

3. 4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pervaporation membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Membranes based on organic polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Crosslinked PVA membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Hybrid PVA/inorganic polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Modified chitosan membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5. Other polymers containing charged groups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6. Membranes formed from polysalts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Best performing membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

227 228 228 228 229 230 231 232 233 234 234

1. Introduction Membranes can be used for the selective removal of water from aqueous organic mixtures. The knowledge gained from this area can influence the design of membranes for other water treatment processes. Pervaporation (PV), aimed at the separation of liquid mixtures, involves a membrane that is in contact with the feed solu-

∗ Corresponding author. Tel.: +61 3 9545 2037; fax: +61 3 9545 1128. E-mail address: [email protected] (B. Bolto).

tion on one side, while permeate is removed as a vapour from the other side [1]. Transport through the membrane is driven by the vapour pressure difference between the feed solution and the permeate vapour. The vapour pressure difference can be maintained by applying a vacuum on the permeate side, or by cooling the permeate vapour so that it condenses, thus creating a partial vacuum. Commercial systems for the dehydration of concentrated alcohol and other solutions have been developed since the 1980s, much of the push coming from interest in the production of pure ethanol as an alternative liquid fuel, where PV can be used to dehydrate

0255-2701/$ – see front matter. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cep.2011.01.003

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Table 1 PV dehydration of ethanol through various polymeric membranes [4,5]. Polymer

Feed, wt% water

Temp., ◦ C

Regenerated cellulose Cellulose acetate Teflon-g-polyvinylpyrrolidone Perfluorinated polymer on PAN support Nafion-H+ Polyacrylonitrile-polyvinylpyrrolidone Poly(maleimide-co-acrylonitrile) Poly(acrylic acid-co-acrylonitrile) Polystyrene Poly(vinyl chloride) Alginic acid

50 4 4 1.3 4 4 15 18 4 4 4 5

45 60 25 50 70 20 15 15 40 40 40 60

Chitosan Chitosan acetate salt Chitosan/glutaraldehyde PVA/25% TEOS, annealed at 160 ◦ C PVA/25% TEOS, annealed at 130 ◦ C

4 4 4 15 15

40 40 40 40 40

azeotropic ethanol/water mixtures [2]. The 200–500-fold separation achieved is due entirely to the selectivity of the membranes used, which are much more permeable to water than to ethanol. More than 100 plants for the dehydration of ethanol have been installed, the largest processing 5000 kg/h [1]. PV data can be expressed in a number of ways [1]. The bulk of the literature expresses them in terms of the total flux through the membrane and a separation factor. The flux of a component i through a PV membrane can be expressed in terms of the partial vapour pressures pio and pit on either side of the membrane by Ji =

PG i (pio − pit ) l

where Ji is the flux, l is the membrane thickness and PG i is the gas separation permeability coefficient. The separation factor ˛sep can be defined for a two-component liquid system as the ratio of the two components on the permeate side divided by the ratio of the two components on the feed side of the membrane. Thus ˛sep =

PH2 O /PEtOH

Separation factor 5.0 5.9 2.9 387 2.5 3.2 33 877 101 63 8.8 13 2208 2556 202 329 893

Flux, g/m2 h

Reference

2060 200 2200 1650 5000 2200 8 13 5 3 48 2800

[4,47] [4] [50] [5] [36] [51] [52] [53] [54] [55] [56] [37]

4 2 7 5 4

[16] [21] [21] [4] [4]

these membranes may be highly crosslinked. This survey of the different polymer types is somewhat crude as there are crucial variables that are not taken into account. Only one publication has been found where further variables are addressed, with the data being normalised by the authors to remove the effects of different membrane thicknesses and an applied pressure or vacuum [6]. Crosslinked membranes exhibited less swelling, had a higher separation factor, but lower water permeability, measured as g␮m/m2 h kPa. The results are included in Table 2 and will be discussed further under the heading of charged polymers. Of course the physical format of the membranes is crucial, but there are no data to tell whether any nanoporosity or the like is present in the membranes mentioned so far. It may be that the different heat treatment on crosslinking PVA results in enhanced crystallinity, with more pronounced channels on the molecular scale being formed between the crystalline regions, thus facilitating transport. In the choice of appropriate membranes, crosslinked PVA is pertinent because of its very hydrophilic nature [7]. A wide range of crosslinking agents has been employed in the fabrication of PVA membranes.

FH2 O /FEtOH

where PH2 O and PEtOH are the mass fractions of water and ethanol in the permeate, and FH2 O and FEtOH are the mass fractions of water and ethanol in the feed [3]. 2. Pervaporation membranes 2.1. Membranes based on organic polymers A general survey of the permeation characteristics of a variety of organic polymers in the treatment of aqueous ethanol solutions of low water content was published by an early reviewer in the field, and is given in Table 1 [4]. It is difficult to get a consistent picture, but it would seem that the non-polar polystyrene and poly(vinyl chloride) membranes are responsible for moderate separation factors and low fluxes. A recent example of a hydrophobic active layer of a commercial amorphous perfluorinated polymer on a polyacrylonitrile (PAN) supporting membrane gives, a moderate separation factor, but with a much higher flux [5]. However, the very hydrophilic poly(vinyl alcohol)/tetraethoxysilane (PVA/TEOS) and chitosan products also show low flux behaviour. The highest fluxes are those for the hydrophilic membranes based on cellulose and Nafion, and grafts of hydrophilic poly(vinyl pyrrolidone) on Teflon and polyacrylonitrile. The PVA/TEOS membranes are exceptions in that they are hydrophilic, but exhibit low fluxes. However,

2.2. Crosslinked PVA membranes An amic acid has been used as a crosslinker for PVA [8]. Imidisation at 150 ◦ C gave an improved membrane for the PV of aqueous ethanol when there was 12 wt% crosslinker present. More crosslinker showed the reverse effect because of the dispersion of unreacted crosslinker within the membrane. Separation factors ranged from 70 to 380 and permeation rates from 30 to 1600 g/m2 h at 30–75 ◦ C, depending on the operating temperature and feed mixture composition. At 45 ◦ C the separation factor was 100 and the permeation rate 200 g/m2 h when the feed contained 30 wt% water (Table 3). As the water concentration increased, the separation factor decreased and the permeation rate increased. This was explained by the plasticising effect of water, making the amorphous regions of the PVA membrane more swollen, as the polymer chains become more flexible. At 75 ◦ C the permeation rate was 500 g/m2 h, with the separation factor little changed. Crosslinking of PVA with maleic acid or citric acid has been used to prepare membranes for permeation studies of water and lower aliphatic alcohol mixtures [9]. Citric acid gave more selective behaviour than maleic acid, with the selectivity being highest for water–isopropanol and water–isobutanol systems. For the water–ethanol system the selectivity was 80 and the flux 495 g/m2 h for a 30% aqueous solution at 30 ◦ C.

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Table 2 Normalised separation data for PV dehydration of ethanol with modified PVA membranes [6]. Membrane synthesis

Feed, wt% water

Temp., ◦ C

Poly(ethylene glycol) blend and TEOS Poly(acrylic acid) copolymer and TEOS Sulphated zirconia crosslinker Acrylic acid graft ␥-aminopropyl-triethoxysilane Quaternised PVA uncrosslinked Quaternised PVA and glutaraldeyde

15 15

50 40

10 4.4 15 15 15

50 60 50 50 50

*

Water permeability, g␮m/m2 h kPa 0.17 18 5.6 3.0 3.0 13 8.0

Permselectivity

Reference

218 17* 250 80 3.5 63 58 75

[17] [16] [7] [8] [3] [6] [6]

No poly(ethylene glycol).

Table 3 PV dehydration of ethanol as a function of PVA crosslinking agent. Crosslinker

Feed, wt% water

None Amic acid to imide

4.6 30 30

Citric acid PAA PVA/GA containing PAA/EG IPNs Fumaric acid

Glutaraldehyde

Temp., ◦ C

Flux, g/m2 h

Separation factor

Reference

75 45 75

721 100 95

247 200 500

[32] [8] [8]

30 50 5 20 20 20

30 75 50 100 80 60

80 60 50 211 411 779

495 2800 260 1511 771 217

[9] [11] [12] [13] [13] [13]

10 20 10 20

30 30 40 30

180 150 104 –

50 130 280 93

[14] [15] [15] [34]

PVA (MW 50 kDa) has been crosslinked with poly(acrylic acid) or PAA (MW 2 kDa) by heat treatment at 150 ◦ C for 1 h [10]. The products have been used for the PV of methanol/methyl tert-butyl ether mixtures, and later ethanol–water mixtures [11]. For 50 wt% water ethanol solutions the separation factor was 60 at 75 ◦ C and the flux 2800 g/m2 h. Interpenetrating networks (IPNs) of PVA crosslinked with glutaraldehyde (GA) containing PAA crosslinked with ethylene glycol (EG) have similarly low permeation rates to PVA/GA membranes, with semi-IPNs giving the better results [12]. Fumaric acid, trans-HO2 C CH CH CO2 H, has been employed as the crosslinking agent at 0.05 mole per mole of PVA in multilayer membranes formed on a polyacrylonitrile support membrane [13]. In the PV of 20 wt% water in ethanol, the separation factors varied from 779 to 211 over the temperature range 60–100 ◦ C, and fluxes from 217 to 1511 g/m2 h. The dehydration of ethanol and other alcohol/water mixtures has been explored with PVA membranes crosslinked with GA [14]. At 30 ◦ C the flux was 50 g/m2 h for ethanol of 10 wt% water content, and 130 g/m2 h when the water content was 20 wt%. The corresponding separation factors were 180 and 150. The flux increased with increasing water content, while the reverse was the case with the separation factor. The flux also increased with the feed temperature, but the separation factor was lowered. From the summary of the results obtained for variously crosslinked PVA membranes given in Table 3 it can be seen that the highest fluxes are obtained when the crosslinking agent is a carboxylic acid, which could arise from the ionic component in the polymer network. The charge is dependent on the pH of the system, so it would be illuminating to see how performance varies with pH. There is no comparable performance with alginic acid though (Table 1). Likewise with the ethylenediamine salt of Nafion, or chitosan, except for the GA crosslinked material, the latter having a high flux. Similar GA crosslinked membranes have been reported and their swelling properties explored [15]. The more crosslinked, less swollen membranes had lower fluxes but higher selectivities in the

PV of 10 wt% water in ethanol at 40 ◦ C, the fluxes ranging from 249 to 313 g/m2 h and the separation factors from 69 to 108. The details are given in Table 4. The more crosslinked membranes are less hydrophilic as OH groups are consumed in the crosslinking reaction, and the membranes are more rigid, making for greater obstruction to diffusion within. 2.3. Hybrid PVA/inorganic polymers Membranes made from PVA crosslinked with 25 wt% of TEOS have been prepared for the PV of aqueous ethanol, with the aim of minimising the swelling of the PVA [4]. Annealing of the membranes under nitrogen at temperatures of 100, 130 and 160 ◦ C was needed to complete the condensation reaction that introduced bridging and crosslinking, when higher selectivity resulted. As shown in Table 5, the best result, for a 15 wt% water mix in ethanol at 40 ◦ C, was a separation factor of 893 and a flux of 40 g/m2 h when the annealing temperature was 130 ◦ C, versus 329 and 50 g/m2 h respectively when annealing was at 160 ◦ C. It was postulated that the crosslinking reaction took place in the non-crystalline parts of the PVA membrane, forming denser non-crystalline regions. Annealing also improved the selectivity of similar membranes made from poly(vinyl alcohol-co-acrylic acid) [16]. In one example the separation factor was 250 for a 15 wt% aqueous ethanol at 40 ◦ C, and a flux of 18 g/m2 h. Blends of PVA and poly(ethylene glycol) or PEG have been crosslinked with TEOS and annealed at 130 ◦ C to Table 4 Influence of degree of GA crosslinking of PVA on PV of 10% aqueous ethanol at 40 ◦ C [15]. Swelling degree, %

Flux, g/m2 h

Water selectivity

28.4 27.9 27.1 26.5 24.8

313 302 289 279 249

69 84 93 107 108

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Table 5 PV dehydration of ethanol using PVA/inorganic hybrid membranes. Crosslinker ◦

TEOS (160 C) TEOS (130 ◦ C) PEG blend and TEOS No PEG Poly(acrylic acid) copolymer and TEOS ␥-aminopropyl-triethoxysilane Sulphated zirconia

Feed, wt% water

Temp., ◦ C

Separation factor

Flux, g/m2 h

Reference

15 15 15 15 15 5 5 10 20 30

40 40 50 50 40 50 50 50 50 50

329 893 300 160 250 537 263 142 86 61

50 40 46 500 18 36 10 105 183 1036

[4] [4] [17] [17] [16] [3] [18] [18] [18] [18]

produce a membrane with a separation factor of 300 for a 15 wt% water mix in ethanol at 50 ◦ C [17]. The presence of PEG decreased the flux from 500 to 46 g/m2 h, but almost doubled the separation factor (Table 5). When ␥-aminopropyltriethoxysilane was used for crosslinking PVA, the hydrophilicity of the membranes increased when the silane content was <5%, and the permeation increased remarkably while the selectivity increased at the same time, thus breaking the trade-off between the two [3]. With a feed containing 5 wt% water the separation factor was 537 and the flux 36 g/m2 h at 50 ◦ C. PVA has been crosslinked with the solid acid of sulphated zirconia by an acid-catalysed reaction which affected the degree of swelling and the crosslinking density of the membrane [18]. For the same duty, both flux and selectivity were lower than for the silane products (Table 5). The flux increased with feed water content, while the selectivity decreased. Overall, by comparison with the results for organically crosslinked PVA membranes (Table 4), the data in Table 5 show that there is no advantage in introducing inorganic particulates into PVA. The story is quite different for totally inorganic membranes such as those based on zeolites, which have been reviewed recently [19]. They are beyond the scope of the present paper. An LTA zeolite membrane can give extremely high fluxes of 12, 15, 18 and 22 kg/m2 h at 100,110, 120 and 130 ◦ C respectively in dehydrating a 93 wt% ethanol mixture. 2.4. Modified chitosan membranes Another highly hydroxylated polymer, chitosan, or poly(2amino-2-deoxy-d-glucose), has been used to make PV membranes. It has a cationic charge when the amino group is protonated. At neutral pH levels it is slightly charged with 17% of the amino groups being protonated [20]. A chitosan acetate membrane has more than 10 times the separation factor of chitosan itself, as does a GA crosslinked chitosan [21], as detailed in Table 6. Very low flux levels were obtained with all these membranes for feeds of low water contents.

Chemical modification of chitosan has been carried out by carboxylation, sulphonation or phosphorylation to produce PV membranes [22]. The phosphorylated products [23] gave the highest flux in dehydrating ethanol, with a selectivity of 541 and a flux of 180 g/m2 h for mixtures containing 10 wt% water at 70 ◦ C. Other variants such a sulphonation or carboxymethylation had much higher selectivities, but lower fluxes. It was proposed that phosphorylation formed phosphates at the C-6 position in the glucose units, reducing the chain relaxation markedly because of the bulky side chain and crosslinking. A less packed structure would make the membrane more relaxed and mobile, allowing passage of ethanol and resulting in a lower selectivity. Blends of chitosan with hydroxyethylcellulose that have been crosslinked with a urea–formaldehyde–sulphuric acid mixture produce PV membranes with the very high selectivity of 10,491 when the hydroxyethylcellulose content is 50 wt% [24]. The addition of the blending agent increases the hydrophilicity of the membranes, and since the hydroxyethyl group is larger than the amino group in chitosan, there is an increased flexibility and openness in the polymer network. The fluxes measured were many times those of the unmodified chitosan membranes. Likewise, in the preparation of PV membranes for ethanol dehydration, chitosan has been crosslinked with 3aminopropyltriethoxysilane to form chitosan–silica hybrid membranes [25]. The hydrophilicity of the membranes increased with increasing amounts of the crosslinker, reaching a maximum when the content was 10 wt%. The flux and separation factor increased as a result, compared to the unmodified chitosan membrane. The crosslinking limited the swelling of the polymer. It was claimed that there were strong hydrogen bonds as well as covalent bonds formed, along with a lowering of crystallinity. Thus the FTIR spectrum showed an intense absorption band at ∼1072 cm−1 , indicating that esterification took place between the OH groups in the polymer and the silanol groups in the crosslinker, to form C O Si bonds. If the second ethoxy group reacts with the OH in another chain, bridges can occur. The crosslinking makes the polymer chains in the amorphous regions more compact, resulting

Table 6 PV dehydration of ethanol using chitosan-based membranes. Chitosan product GA crosslinked Uncrosslinked Acetate salt Phosphorylated Acetate salt Sulphonated & GA Carboxymethylated Carboxyethylated Cyanoethylated Hydroxyethylcellulose 50% blend 10% 3-aminopropyl-triethoxysilane Uncrosslinked

Temp., ◦ C

Separation factor

Flux, g/m2 h

Reference

4

40

10

70

10 15 15

60 50 50

2208 202 2556 541 242 1560 1294 301 52 10,491 597 200

4 7 2 180 142 52 36 30 80 112 887 275

[21] [21] [21] [22,23] [22,23] [22,23] [22,23] [22,23] [22,23] [24] [25] [25]

Feed, wt% water

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Table 7 PV dehydration of aqueous ethanol with chitosan membranes in various forms [26]. Chitosan format

Feed, wt% water

Temp., ◦ C

Sepn. Factor

Flux, g/m2 h

99% deacetylated 99% deacetylated 73% deacetylated Crosslinked with GA 98% deacetylated-HOAc 98% deacetylated-HCl 98% deacetylated-H2 SO4 98% deacetylated-non-ionised

5 4.4 8 10–50 10 10 10 10

70 40 35 40 60 60 60 60

5.4 17 1240 >2000 123 593 1040 36

840 65 6 – 400 210 160 350

Separation factor

Flux, g/m2 h

Table 8 PV dehydration of 20% aqueous ethanol at 70 ◦ C using graft polymer membranes having different charges [27]. Host polymer

Grafted polymer

Graft site charge

Polyvinylidene fluoride Polyvinylidene fluoride Polyvinyl fluoride Polyvinyl fluoride Polyacrylonitrile Polyacrylonitrile Polyvinyl fluoride Polyvinylidene fluoride Polyvinylidene fluoride Polyvinylidene fluoride Polyvinyl fluoride

4-vinylpyridine N-vinyl-imidazole N-vinylmethyl- acetamide N-vinyl-pyrrolidone Acrylic acid K+ acrylate K+ acrylate Quaternised 4-vinylpyridine Quaternised N-vinylimidazole 4-vinylpyridine/BrCH2 COOH Vinylimidazole/BrCH2 COOH

Neutral Neutral Neutral Neutral Neutral Anionic Anionic Cationic Cationic Zwitterionic Zwitterionic

in less space for species to permeate through the membrane and making the resistance to flow higher for the larger species. Research into various forms of chitosan has shown that a lighter level of deacetylation of the original chitin is beneficial as regards the separation factor, as is crosslinking of chitosan with GA, or having the amino groups in the salt form when the anion involved is divalent, as shown in Table 7 [26]. However, the fluxes obtained were all low. 2.5. Other polymers containing charged groups The use of hydrophilic membranes for the dehydration of organic solvents in general by PV has been the subject of comprehensive reviews [27,28]. The emphasis is on selectivity, postulated to be determined by selective sorption and selective diffusion. Selective sorption is governed by the presence in the membrane of active centres such as charged sites which are capable of specific interaction with water, while selective diffusion is governed by the rigidity and regularity of the polymer structure and the nature of the polymer interspace, exemplified by the degree of swelling and the frequency of the crosslinks. The results for a series of membranes made by grafting neutral or charged polymers onto supporting membranes are given in Table 8. A neutral amide graft and an anionic acrylate graft produced the highest moderate fluxes. Charged membranes formed by crosslinking PVA with 7 wt% of a sulphosuccinic acid yields membranes that in the separation of 10/90 wt% water/ethanol at 60 ◦ C have a separation factor of 240 and a flux of 180 g/m2 h [29]. At 70 ◦ C the results are 175 and 300 g/m2 h respectively. When the lithium salt of the crosslinker was prepared, it showed lower results of 44 and 59 g/m2 h, but at 50 ◦ C and with 5 wt% crosslinker, as shown in Table 9 [30]. Quaternised poly(acrylonitrile-co-3-N ,N -dimethylaminopropylacrylamide) has been used as a PV membrane for dehydrating ethanol [31]. The selectivity factor increased with decreasing levels of water in the feed solution, reaching over 15,000 for the fully quaternised polymer at 30 ◦ C when the water content was 10 wt%. However, the flux was then only 10 g/m2 h. Cationic and anionic PVA membranes have been made by reacting PVA with a catalyst and 3-chloro-2hydroxypropyltrimethylammonium chloride or phosphoric

9 10 4 7 10 500 156 175 61 76 63

200 1600 2900 1000 1800 3000 4700 400 1800 200 1250

acid [32]. Performance depended on the degree of substitution (DS), the separation factor increasing with an increase in the DS for the cationic membrane, to 1540 for the higher DS, but decreasing in the case of the anionic membrane. For the cationic membrane both selectivity and flux were improved by a higher DS, which is unusual. The highest flux was still not impressive though, and levels were higher for the anionic than the cationic membrane. Cationic and anionic PVA membranes have been made also via copolymers with acrylamidopropyltrimethylammonium chloride or with itaconic acid [15]. The degree of swelling decreased from neutral to cationic to anionic PVA with values of 26.5%, 15.7% and 14.0% respectively. Here the anionic PVA gave better performance than the cationic derivative, in contrast with the other examples in Table 9. The anionic membrane had 2.5 times the separation factor of the neutral PVA, at 837 versus 335 for one duty. The flux was correspondingly lower (86 g/m2 h versus 189 g/m2 h). The plasma modification of GA crosslinked PVA membranes by grafting acrylic acid onto them gave membranes which had enhanced hydrophilicity [33]. The fluxes ranged from 122 to 144 g/m2 h for 96 wt% ethanol at 25–60 ◦ C, and the separation factors from 23 to 8. The respective results for unmodified membranes were 75–120 g/m2 h and 19–10. Other work on PVA crosslinked with GA reports fluxes of up to 93 g/m2 h for the PV of ethanol containing 20 wt% water at 30 ◦ C [34]. Incorporating sericin, the gelatinous protein that binds the fibres in silk, into PVA that is crosslinked with dimethylolurea gives a membrane that is only moderately selective to water [35]. The crosslinker reacts with the OH groups of the polymers leaving the amino groups in sericin free to interact with water. As shown in Table 2, a quaternised PVA, made by grafting 2,3-epoxypropyltrimethylammmonium chloride onto the polymer and then crosslinking with GA has been tested [6]. The quaternary ammonium groups reduced the crystallinity of the polymer and enhanced the hydrophilicity and water permselectivity. The results are still modest: a membrane with 4% quaternisation had a water permselectivity of 58 and a water permeability of 13 g␮m/m2 h kPa for 15% aqueous ethanol at 50 ◦ C. The data here have been normalised by the authors to remove the effects of different membrane thickness and applied pressure or vacuum. Crosslinked membranes exhibited less swelling, and had a higher

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Table 9 PV dehydration of ethanol with membranes containing charged groups. Polymer and charged species

Feed, wt% water

Temp., ◦ C

Separation factor

Flux, g/m2 h

Reference

PVA/7 wt% sulphosuccinic acid

10 10

70 60

175 240

300 180

[29] [29]

PVA/5 wt% Li+ sulphosuccinate Quaternised poly(acrylo-nitrile-co-3-N ,N dimethyl-aminopropyl acrylamide) Cationic PVA*

10 10

50 30

44 >15,000

59 10

[30] [31]

4.6 4.6

75 75

424a 1540b

216 249

[32] [32]

Anionic PVA*

4.6 4.6

75 75

398c 72d

632 407

[32] [32]

Unmodified PVA Cationic PVA/GA† Anionic PVA/GA† PVA/GA† PVA/GA acrylic acid Plasma graft Unmodified PVA

4.6 4 4 4 4 4 4 4

75 40 40 40 40 60 40 60

721 709(216) 837(228) 335(107) 14 8.4 15 10

247 89(123) 86(119) 189(279) 135 144 91 120

[32] [15] [15] [15] [33] [33] [33] [33]

15 15

800 700

7 10

[27] [27]

50

90

70

[35]

PVA/9% acrylic acid graft PVA/sericin blend * †

10 20 8.5

DS: a, 2.9%; b, 5.2%; c, 2.3%; d, 5.0%. The data in parentheses are for a down stream pressure of 0.4 kPa; the rest are for a down stream pressure of 1.4 kPa.

water permselectivity of 75, but a lower water permeability of 8.0 g␮m/m2 h kPa. Nafion sulphonic acid membranes have been tested for the influence of the counterion on PV performance [36]. The data (Table 10) show that the acid form has a much greater flux than the various salt forms, no doubt because of the less space occupied by the proton versus the cations studied. Alginic acid has been tested for PV behaviour [37]. It was found that the selectivity factor increased enormously when the acid was neutralised with alkali, from 19 to over 10,000, while the flux decreased markedly, with the acid form again exhibiting by far the highest flux. The counter ion influenced the flux in the order Na+ < K+ < Rb+ < Cs+ < H+ , which was ascribed to a change in the conformation of the alginate molecule so that its mobility was increased, as was the mobility of the ion hydration shell and the crystallinity of the membrane. The conformational change was verified by 13 C NMR. A blend of sodium alginate with PVA crosslinked with 1.5 wt% maleic acid had a lower separation factor than the membrane without the PVA [38]. Ion exchange membranes in the form of styrene-based quaternary ammonium and sulphonated polyethylene containing different counter ions have been tested, to give the results shown in Table 10 [2]. The sulphonic acid membrane had a much higher flux than the sodium salt, by a factor of several hundred, somewhat similar to the Nafion and alginic acid experiences. The selectivity was correspondingly lower. Diffusion of liquid water through the continuous pathway of water shells around charged groups is claimed to be faster than diffusion through clusters of free water in the membrane [39]. Membranes have been made by modifying poly(vinyl chloride) or PVC membranes by reacting them with sodium N-methyldithiocarbamate or sodium N-methyl-Ncarboxydithiocarbamate, and then with metal ions such as Li+ , Ca++ and Cu++ [40]. The Cu++ complex showed an extremely high selectivity of >5000 in the dehydration of ethanol at high water contents (80 wt%). Various salt forms of sulphonated polyamide membranes have been tested [41]. There was little variation in selectivity, but the flux decreased in the order PEI+ > K+ > NEt4 + > Cs+ > Na+ , where PEI is polyethyleneimine, which had by far the greatest influence

on selectivity (Table 10). Mixing two polymers of opposite charge like this forms a polysalt. 2.6. Membranes formed from polysalts It has been speculated that polysalts, formed from anionic and cationic polyelectrolytes, would be appropriate for obtaining both highly permeable and highly selective membranes [27]. Earlier work on PV with membranes formed from sodium polyacrylate and various cationic polyelectrolytes [42,43] is summarised in Table 11. Having a quaternary ammonium structure in the backbone of the cationic polymer appears to be advantageous as regards selectivity, but less so for the epichlorohydrin/dimethylamine (ECH/DMA) polymer, where the presence of an OH group may be a handicap. The flux levels are again not impressive. Several membranes based on polysalts have been explored, as detailed in Table 12. The highest flux results were from structures based on cellulose sulphate as the polyanion [44], plus one based on sodium carboxymethyl cellulose, or Na+ CMC, and chitosan [45] or poly(Nethyl-4-vinylpyridinium bromide) [46]. Other physical structures affecting membrane behaviour have been examined as ways to enhance the flux. Thus a porous polysulphone membrane has been coated with a polysalt formed from chitosan and poly(acrylic acid), and the composite membrane tested for PV of aqueous ethanol [47]. For dehydration of 5 wt% aqueous ethanol at 30 ◦ C, when compared with the dense homogeneous membrane that is unsupported, the composite version had a flux of 132 g/m2 h versus 33 g/m2 h and a separation factor of 1008 versus 2216 (Table 12). The composite membrane was only superior for solutions of high ethanol content. For all membranes both parameters increased with heating for feeds of high water content (>30 wt%), which is unusual. This was ascribed to the strong sorption contribution to the activation energy for PV with these membranes. For optimal performance the ratio of membrane thickness to the permeation characteristics of the dense layer and the porous supporting membrane had to be appropriate. One way of achieving this is to use a layer-by-layer deposition method, which allows one to choose which thickness of active membrane skin is the best.

B. Bolto et al. / Chemical Engineering and Processing 50 (2011) 227–235

233

Table 10 Effect of counter ion on PV dehydration of ethanol with membranes containing charged groups. Feed, wt% water

Temp., ◦ C

Separation factor

Flux, g/m2 h

Reference

Nafion-H Nafion-Na+ Nafion-K+ Nafion-N(CH3 )3 + Alginic acid Li+ alginate Na+ alginate K+ alginate Rb+ alginate Cs+ alginate Na+ alginate-PVA blend Anion exchanger-SCN− Sulphonated PE-H+ Sulphonated PE-Na+ PVC/dithiocarbamate-Cu++

4 4 4 4 10 10 10 10 10 10 10 15 16 16 40 80

70 70 70 70 60 60 60 60 60 60 45 60 26 26 25 25

2.5 5.0 9.8 10.5 19 11,400 11,600 11,600 11,600 10,000 380 38 2.6 671 63 >5000

5000 500 200 340 1300 100 210 550 750 1030 380 470 1360 80 20 103

[36] [36] [36] [36] [37] [37] [37] [37] [37] [37] [38] [2] [2] [2] [40] [40]

Polyamide sulphonate-Na+ Polyamide sulphonate-K+ Polyamide sulphonate-Cs+ Polyamide sulphonate-NEt4 + Polyamide sulphonate-PEI+

20 20 20 20 20

60 60 60 60 60

1.3 2.1 1.4 1.5 11

160 230 11 18 14

[41] [41] [41] [41] [41]

Polymer and charged species +

Table 11 PV dehydration of 5% aqueous ethanol at 70 ◦ C with membranes based on polysalts where the polyanion is sodium polyacrylate [42,43]. Polycation

Carbon backbone

Polyethyleneimine Polyallylamine Ethylene/phenylene dimethylammonium Propylenedimethyl-ammonium ECH/DMA polymer Trimethylammoniumethyl methacrylate

–CH2 CH2 – –CH2 CH2 – Ditto & –CH2 C6 H4 CH2 – –(CH2 )3 – –CH2 CH(OH)CH2 – –CH2 C(CH3 )–

Charged group

Charged site

+

–NH2 – –CH2 NH3 + –N+ (CH3 )2 – –N+ (CH3 )2 – –N+ (CH3 )2 – –N+ (CH3 )3

Separation factor

Backbone Pendant Backbone Backbone Backbone Pendant

220 750 1710 1940 830 380

Flux, g/m2 h 830 510 790 820 340 220

Table 12 PV dehydration of aqueous ethanol with membranes based on various polysalts. Polyanion

Polycation

Feed, wt% water

Temp., ◦ C

Separation factor

Poly(acrylonitrile-co-acrylic acid) Cellulose-SO3 − Na+ Cellulose-SO3 − Na+ Cellulose-SO3 − Na+ Cellulose-SO3 − Na+

10 16 16 16 16

– 50 50 50 50

5000 295 140 123 123

400 1900 3200 4900 2700

[44] [45] [45] [45] [45]

Aromatic polyamide sulphonate Poly(acrylic acid) On polysulphone No supporting

Poly(acrylonitrile-co-vinyl pyridine) Polyethyleneimine PolyDADMAC, linear Same, but branched Poly-N,N-dimethyl-3,5dimethylenepiperidine chloride Polyethyleneimine Chitosan Supporting membrane Membrane

20

60

15

300

[41]

5 5

30 30

1008 2216

132 33

[47] [47]

Na+ polystyrene sulphonate Na+ CMC Na+ CMC Anionic PVA, DS 2.3% DS 5.0%

Polyallylamine.HCl Chitosan N-ethyl-4-vinyl-pyridinium bromide Cationic PVA, DS 2.9% DS 5.2%

6.2 10 10

70 70 70

70 1062 782

230 1140 1320

[57] [45] [46]

4.6 4.6

75 75

2250 1910

378 284

[32] [32]

3. Best performing membranes The best performers in terms of flux, which at a maximum of 5 kg/m2 h never achieve high values, are charged polymers of one type or another, including polysalts. Table 13 shows that anionic and polysalt membranes are superior. For anionic polymers, the proton form has a significantly higher flux than the metal or quaternary ammonium salt versions, owing to the greater free space within the polymer network. Two of the polysalt examples involve thin layers of the active membrane on an RO or UF supporting membrane. They also exhibit the highest separation factors. The best combination for flux and selectivity is PEI/PAA on an RO membrane, at 4 kg/m2 h and 1075 respectively. It is interesting to note that

Flux, g/m2 h

Reference

hybrid PVA/inorganic membranes do not show enhanced fluxes, the best being PVA/sulphated zirconia, at just over 1 kg/m2 h, so there is no great advantage in incorporating inorganic fillers within the membrane. External factors enhancing the flux include heating, and an increased water content, which follows from greater swelling and a low level of crosslinking. However, crosslinking can occur while maintaining the swollen state. It is very difficult to compare different membranes in terms of fluxes and separation factors as these parameters are not only a function of the intrinsic properties of the membranes themselves, but rely also on the operating conditions such as temperature permeate pressure and the feed concentration [48,49]. So for membranes tested under different conditions, caution needs to be applied in attempting comparisons.

234

B. Bolto et al. / Chemical Engineering and Processing 50 (2011) 227–235

Table 13 Highest fluxes for PV dehydration of aqueous ethanol. Membrane polymer +

Nafion-H Cellulose-SO3 − Na+ and polyDADMAC, branched K+ acrylate graft on poly(vinyl fluoride) PEI/PAA on RO membrane Cellulose-SO3 − Na+ and polyDADMAC, linear K+ acrylate graft on PAN PolyDADMAC/Na+ CMC on UF membrane *

Mem. type

Feed, wt% water

Temp., ◦ C

Flux, g/m2 h

Anionic Polysalt Anionic Polysalt Polysalt Anionic Polysalt

4 16 20 10* 16 20 10*

70 50 70 70 50 70 75

5000 4900 4700 4050 3200 3000 3000

Separation factor 2.5 125 156 1075 140 500 960

Reference [36] [44] [27] [58] [44] [27] [59]

Aqueous isopropanol, not ethanol.

Table 14 Highest separation factors for PV dehydration of aqueous ethanol. Membrane polymer

Mem. type

Feed, wt% water

Temp., ◦ C

Separation factor

Flux, g/m2 h

Reference

Quaternised poly(acrylo-nitrile-co-3-N ,N dimethyl-aminopropyl acrylamide) Metal alginate salts (Na+ , K+ , Rb+ , Li+ or Cs+ ) Chitosan/50% blend with hydroxyethylcellulose PVC/dithiocarbamate-Cu++ Chitosan acetate PVA crosslinked with GA

Cationic amide

10

30

>15,000

10

[31]

Anionic Cationic Anionic Cationic Neutral

10 10 80 4 10–50

60 60 25 40 40

11,600–10,000 10,491 >5000 2556 >2000

100–1030 112 103 2 –

[37] [24] [40] [57] [26]

A range of very high separation factors is shown in Table 14 for the top selective performers. A variety of polymer types are included, of neutral, anionic or cationic character. The top results (>10,000) are for charged membranes again, either cationic or anionic, but not polysalts. The fluxes encountered here are miniscule, the best being caesium alginate at about 1 kg/m2 h. External factors that improve the flux, such as heating and increased water content, have detrimental effects on the separation factor. 4. Conclusions The ideal structure for good fluxes would appear to be one containing discrete domains of oppositely charged species of optimal size, such that internal neutralisation is minimised. Possible solutions could be a tightly crosslinked non-polar host polymer containing charged nanoparticles, or a crosslinked block copolymer that has dual substitution. The influence of a number of parameters needs to be determined, such as the minimum desirable size of the charge domain, the distance between oppositely charged sites, and the role of counter ions. Fresh approaches are needed; one such could be layer-by-layer deposition of oppositely charged polyelectrolytes, with due attention to appropriate separation of the sites of opposite character. Imperfect matching of the oppositely charged centres would create an open structure. A greater understanding of the free volume within charge mosaic membranes, as provided for example by positron annihilation lifetime spectroscopy, is needed. Better insight into the detailed crosslinked structure of the membranes could be provided by using a synchrotron light source. As well as the water content of the aqueous ethanol feed, temperature is a crucial parameter, the benefit of which is attributed to the increase in diffusivity and reduction in flow viscosity that occurs on heating. Mechanical energy in the form of extra applied pressure or vacuum can likewise be called upon to enhance the flux. The thickness of the membrane is another vital factor: the active layer should be as thin as possible. The inherent permeability of the membrane polymer is similarly important. The main parameters for better water flux are therefore • • • •

Higher feed water temperature Increased pressure/vacuum Thinner membranes Improved membrane permeability

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