Dehydration of organics by pervaporation with polyelectrolyte complex membranes: some considerations concerning the separation mechanism

Dehydration of organics by pervaporation with polyelectrolyte complex membranes: some considerations concerning the separation mechanism

' ~ ELSEVIER Mjournal EMBRAN ofE SCIENCE Journal of Membrane Science 113 (1996) 3 1 4 1 Dehydration of organics by pervaporation with polyelectrolyt...

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' ~ ELSEVIER

Mjournal EMBRAN ofE SCIENCE Journal of Membrane Science 113 (1996) 3 1 4 1

Dehydration of organics by pervaporation with polyelectrolyte complex membranes: some considerations concerning the separation mechanism K. Richau *, H.-H. Schwarz, R. Apostel, D. Paul GKSS Forschungszentrum, Institut fiir Chemie, Kantstr. 55, D-14513 Teltow-Seehof Germany Accepted 7 August 1995

Abstract Polyelectrolyte complex (PELC) membranes were prepared by simultaneous interfacial reaction of aqueous solutions of two oppositely charged poly-ions, i.e. from cellulose sulfate and various polycations as well as a cationic surfactant. Pervaporation investigations proved that such membranes prepared with polycations may be successfully used for dehydration of various organic solvents. Measurements of swelling and pervaporation properties of model membranes confirm, that the anionic polysaccharide Na-cellulose sulfate is the only component responsible for good separation capability in dehydration of organics with PELC membranes. Water molecules we assume to be preferentially transferred from one hydrogen bonding site to another across the polysaccharide chains. We can conclude that not the sorption into the upstream surface, but the diffusion selectivity along a swelling gradient across the stabilized cellulose sulfate mainly governs the separation behaviour. Kevwords." Pervaporation; Dehydration of organics; Polyelectrolyte complex membranes; Separation mechanism

1. Introduction Dehydration of organic solvents is probably the best known and developed application of the pervaporation principle. A large amount of polymeric materials have been used in order to prepare pervaporation membranes for dehydration purposes, as e.g. cellulose, cellulose esters, polyvinylalcohol, polyhydroxymethylene and related copolymers, polyamides, aromatic polycondensates, synthetic ion-exchange membranes, ionexchange polysaccharides and polyelectrolyte complexes [ 1 ]. The presence of hydrophilic groups in the polymer membrane material is assumed to be very important to * Corresponding author. Tel.: ( + 3328) 46 490; FAX: ( + 3328) 46 452. 0376-7388/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSD10376-7388 ( 95 ) 0 0 2 1 2 - X

provide a sufficient dewatering effect. Two types of hydrophilic groups are to be distinguished: nonionic groups, as e.g. - O H , and ionic groups, e.g. R a N +, SO 3, - C O O - or - O S O 3 . From that point of view polysaccharides should be a very suitable class of polymers for the preparation of pervaporation membranes. Having a high content of hydroxyl groups, they can be easily modified into the form of polyelectrolytes, also possessing anionic groups. The only problem for charged polysaccharides is their water solubility, since polymer membranes for dehydration have to be insoluble in aqueous feed solutions. The usual way to transform these compounds into the water insoluble state is by the crosslinking reaction with small ions or poly-ions. Interaction between two oppositely charged macro-ions results in formation of

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K. Richau et al. / Journal of Membrane Science 113 (1996) 31-41

a polyelectrolyte complex (PELC). In 1961 Michaels [2] systematically studied the preparation and application of PELC membranes, mainly for ultrafiltration purposes. He obtained porous membranes by dissolution of PELC in a suitable solvent followed by usual precipitation techniques. An extensive review dealing with various types of polyelectrolyte membranes was published by Maeda and Kai [ 3 ]. We used another way for the PELC membrane preparation. The possibility of simultaneous interfacial reaction of aqueous solutions of two oppositely charged poly-ions was employed by Philipp et al. [4] in order to obtain spherical microcapsules. Flat membranes prepared according to this principle from cellulose sulfate and a polycation containing quaternary ammonium groups like poly(dimethyldiallyl ammonium chloride ), we described recently [ 5,6 ], are especially suited for water separation from water-alcohol mixtures by pervaporation with high separation efficiency and high permeation rates. The TEM micrograph of the cross-section of a freeze-dried PELC membrane from cellulose sulfate and poly (dimethyldiallyl ammonium chloride) shows a spongy-like porous internal structure embedded between outer dense surfaces. XPS measurements demonstrated identical chemical compositions in both near surface regions [5,8]. Concentration potential was found to be independent on orientation of PELC membrane surfaces towards surrounding aqueous KC1 solutions [7]. Contact angle and streaming potential investigations also indicate identical properties of both membrane surfaces [ 8 ]. Furthermore, under pervaporation as well as dialysis conditions the experimental results are independent on which membrane surface contacts the feed or concentrate. From all these findings we have to conclude that the PELC membrane formation procedure described leads to homogeneous symmetric samples. Results of electrochemical investigations show, that PELC membranes from cellulose sulfate and polycations exhibit a positive net charge due to protonation o f - N H 3 at low pH, corresponding to a nonstoichiometric nature of the PELC membranes investigated [ 79]. The aim of this study was to present pervaporation results of PELC membranes based on cellulose sulfate and prepared by interfacial reaction. A larger variety of counter-ions was included now in membrane for-

mation investigations, i.e. polycations with the ionogenic group in the main chain were compared with other polycations, bearing its ionogenic sites in different pendant groups, as well as with low molecular weight cations. All these samples were tested with respect to dehydration of other organics than ethanol also. Furthermore, at that point it was desired to obtain information about the possible range of applicability of the materials and membranes used. Therefore a deeper knowledge of the transport mechanism, even only at a qualitative level, was necessary. In order to verify at first the applicability of known transport models to the description of pervaporation behaviour of PELC membranes, swelling investigations and pervaporation experiments were carried out with model membranes also.

2.

Theory

Assuming homogeneous membranes between homogeneous mixtures, isothermal conditions and stationary fluxes, classical Nernst-Planck flux equations as well as thermodynamics of irreversible processes leads to li= Zk( Lik × Xk)

where I is the generalized flux density, X the conjugated driving force, Lik=f(c~ .... c~) are the coupling coefficients between the ith and kth species and Lii = f ( c l c k) the conductivities. By introduction of assumptions on the transport mechanism on the molecular level we have to consider three groups of models: 1. solution-diffusion models; 2. models based on the theory of free volume; 3. models according to the theory of reaction rates. ....

In the framework of the widely accepted solutiondiffusion model the following assumptions and approximations are used in the literature: 1. The pervaporation process is divided into subsequent steps: • sorption into the upstream surface of the membrane, • diffusion through the membrane and • desorption out of the downstream surface. It is assumed that desorption from the downstream sur-

K. Richau et al. /Journal of Membrane Science 113 (1996) 31-41

face is nonselective and does not limit the pervaporation process. 2. Partial pressure differences of the ith species between upstream and downstream surfaces of the membrane Op~ are approximated by concentration gradients ac, over the cross-section of the membrane ax. 3. Plastification coefficients, which are available from pervaporation experiments only, are introduced in order to take into account coupling of fluxes. 4. Permeability coefficients are introduced instead of conductivities L, according to

Pi =

DiS,

where D, is the diffusion coefficient and S~the solubility coefficient, with linear or exponential dependencies D,,S~ = f ( c , ( x ) ) on the location x within the membrane. With respect to the process design further assumptions are introduced: • The concentration polarization at the feed side is negligible due to high circulation velocity and sufficiently low fluxes. • No liquid permeate layer exists at the downstream surface of the membrane due to sufficiently low vapour pressure in the permeate compartment.

3. Experimental 3. I. Materials The polyanion used for complex formation was sodium cellulose sulfate (CS) made by sulfation of cellulose acetate [ 10] in all cases. Poly(ethyleneimine) (PEI) was chosen as a linear polycation with the ionogenic group in the main chain. It is a typical polyelectrolyte of the integral type [4]. The other polycations used, namely poly ( dimethyldiallyl ammonium chloride ) ( P D M D A A C ) and poly(N,N-dimethyi-3,5-dimethylenepiperidinium chloride) (PPIP), are also polyelectrolytes of the integral type, but their ionogenic sites are not located in the main chain. The preparation of P D M D A A C ( M ~ = 6 0 000) is described elsewhere [11]. PEI ( M ~ = 4 0 0 0 0 ) and PPIP ( M ~ = 3 5 0 0 0 ) used in this study were obtained from Aldrich ( 18, 1978 and 18, 177-3, p.a. grade). Representing the class of low molecular weight cations, the surfactant Quarto-

33

lan c~' (dodecylamidoethyldimethylbenzyl ammonium chloride, DDC) delivered by Hydrierwerk Rodleben, Germany, was also used. Casting solutions with a content of 20-30 wt% polycationic polyelectrolyte were obtained by dissolution in water or by dilution with 2 N HCI in the case of PEI, respectively. The DDC concentration in the solution obtained was about 50 wt%. All solutions were filtered and degassed prior to use.

3.2. Membr(mes An aqueous solution of CS (usually 2-6 wt%) was cast as a 0.2 mm thick film on a glass plate. On this film a layer of the aqueous counter-ion solution with a thickness of 0.2 mm was spread out immediately. The simultaneous interfacial reaction between the different poly-ions leads to formation of the PELC membrane. After a reaction time of 15 min the glass plate with adjacent unreacted polyelectrolytes and with the polyelectrolyte complex membrane in between was dipped into a water bath. The membrane was loosened from the support, intensively soaked and dried. All these steps were performed under ambient conditions. For investigations of single polyelectrolyte films the aqueous polyelectrolyte solution was cast on a support. The CS films could be spread out on a glass plate as well as on a porous polyvinylidenefluoride (PVDF) sublayer, the P D M D A A C film could be obtained only on a glass plate. These films could be loosened from the glass plate after drying under ambient conditions. For application purposes wet PELC membranes were transferred onto porous ultrafiltration membranes from polyacrylonitrile, polyvinylidenefluoride or polysulfone. After drying under ambient conditions easy to handle composite membranes were obtained. It was found that these supports do not influence pervaporation selectivity, but suppress flux as compared with nonwoven supports in some cases only.

3.3. Membra,e thickness Membrane thickness was measured by a differential transformer based distance sensor type AE2 DS delivered by Feinmel3zeugfabrik Suhl, Germany, in the dry state under ambient conditions. Reproducibility of the single measurement was better than + 0 . 4 /zm. In the tables the averages from

34

K. Richau et al. / Journal of Membrane Science 113 (1996) 31~t l

measurements of at least 30 locations on at least two samples are given, the variability (given as the standard deviation divided by the average value) is lower than + 20%. 3.4. Swelling ratio Air dried membrane strips were immersed in water or in a water-ethanol mixture respectively at 20°C. After 3 h, the adherent liquid was removed by centrifugation for 15 min at 1200g ( m / s 2) and the sample was weighed (W). Then the sample was dried at 105°C and weighed again (Wo). The swelling ratio SR ( % ) was obtained from the weights of a sample in the dry state and in the swollen state, according to S R = 1 0 0 × ( W - Wo) /Wo. In the tables the mean values from two samples (400 cm 2 each) are given, the deviations of which are less than ± 10%. 3.5. Pervaporation investigations Pervaporation experiments were carried out with a laboratory scale apparatus using a stainless steel measuring cell P 28 supplied by C E L F A AG, Switzerland, with an effective membrane area of 17 cm 2. PELC membrane samples were supported by a sheet of filter paper on top of a stainless steel sintered plate. Composites and supported single polyelectrolyte films were inserted directly on the sintered plate. If not stated otherwise, downstream pressure and feed temperature were kept constant below 1 mbar and at 50°C, respectively. The feed circulating velocity was enlarged up to 40 1/h in order to assure minimized influence of concentration polarization. Under these conditions, fluxes and water concentration in the permeate are found independent on circulation velocity. Furthermore, the flux for ethanol-water mixtures is much smaller than that for pure water (see Fig. 3 later). The permeate was collected in a trap cooled with liquid nitrogen. Fittings and valves between feed tank (volume 1 1), measuring cell and cooling traps were heated in order to avoid condensation. The flux was determined gravimetrically. Compositions of feed and permeate were determined by density measurements using the vibrating tube density meter D M A 48 delivered by A. Paar KG, Austria.

Feed solutions were prepared from bidistilled water and p.a. grade ethanol, n-propanol, isopropanol, acetone and N-methylmorpholine. Performance and separation efficiency of the membranes were estimated after the steady-state transport regime was established by measuring the mass flux J ( k g / h m 2) and the water concentration in the permeate Cwp ( w t % ) as well as in the feed cwf ( w t % ) , respectively. For various membranes J and Cwp were found to be independent of downstream pressure P~ow,, < 10 mbar. The separation factor ~ according to a = (Cwp/Corgp) / (CwdCorgO is given in some cases for the sake of comparison with data of commercial products, where Corgp and Corgf are the organics concentrations in the permeate and in the feed. Reproducibilities of concentration measurements are better than ___0.2 wt%. Reproducibilities of flux measurements are assumed to be much better than + 10% of the absolute value, but cannot be given exactly because of the instantaneous decrease of water concentration in the feed during a test loop. The tables show the mean values obtained from at least two samples, the variability amounts to + 1 % and + 10%, respectively.

4. Results and discussion 4.1. Dehydration performance o f P E L C membranes To demonstrate their excellent separation performance, results obtained with PELC membranes made from CS and different counter-ions are summarized in Table I. The fluxes are high, especially considering the unusually low process temperature of 50°C. Using different polycations, a broad range of fluxes from 0.65 Table 1 Performance of PELC membranes in pervaporation (feed: EtOH/ H20; Tf= 50°C; Pdow,< 1 mbar) PELC composition J (kg~/h m2) Cwf(wt%)

Cwp(wt%)

a

CS + PEI CS + PDMDAAC CS + PPIP CS + DDC

98.1 96.0 97.0 47.5

208 153 134 3

0.65 2.1 2.8 4.6

20.0 | 8.0 19.4 20.0

K. Richau et al. / Journal o[ Membrane Science 113 (1996) 31~:Jl Table 2 Dehydration of other organic solvents with PELC membranes from CS and P D M D A A C ( Tt = 50°C; pd,,~,, < I mbar) Solvent

J ( k g / h m e)

c,,t ( w t % )

L'~p(wt%)

~

Acetonc n-Propanol lsopropanol N-Methyhnorpholine

2.4 2.8 1.8 I. I

9.0 18.5 19.2 22.7

99.6 99.4 99.8 99.9

2518 730 2100 3402

up to 2.8 kg/h m 2 is covered at nearly the same high level of separation. PEI with the ionogenic group in the main chain yields the lowest flux, whereas PDMDAAC and PPIP having the ionogenic groups outside of the main chain were leading to higher fluxes. Contrarily using cationic surfactants instead, membranes with extraordinary large flux but vanishing selectivity for water are obtained. This difference is discussed in terms of different concentrations of cationic groups in PELC membranes elsewhere [ 12]. The performance of a PELC membrane made from CS and PDMDAAC in dehydration of different solvents is shown in Table 2. The fluxes between 1.1 and 2.8 kg/h m 2 are dependent on the chemical character of the organic solvent. At the same time the water content in the permeate reaches more than 99% in all cases, thus making possible very efficient separation not only for alcohol-water mixtures. Long-term experiments were performed in order to control membrane stability and to check changes in the membrane performance at varying feed concentrations and feed temperatures. For these experiments PELC membranes made from CS and PDMDAAC were used. Pervaporation conditions were altered by periodic changing of water content in the feed. The measurements were carried out at 50 and 80°C. The duration of the membrane test was 270 h. Data characterizing the dependence of flux and of water concentration in permeate on feed composition are summarized in Fig. 1 for the system isopropanol/ water. Performance data are nearly independent of time, giving evidence of the good long-term efficiency in dehydration. The flux is twice as high at 80°C as compared with 50°C (Fig. l a). Contrarily, the water content in the permeate and thus the separation factor is independent of feed temperature (Fig. lb). The PELC membrane possesses a very good pervaporation performance over a broad range of isopropanol con-

35

centration in the feed. Moreover, the steady state of membrane performance was achieved during a few minutes after changing the feed composition, indicating fast adaptation of the membrane on variation of the water content in the feed. According to Ndel [13] the flux decline observed for polyvinylalcohol membranes at low feed concentrations is caused by the semicrystalline character of the polymer in the non-swollen state. Further investigations will show whether this explanation is applicable to PELC membranes too. Analogous pervaporation investigations during 90 h for the system N-methylmorpholine/water show a similar behaviour ( Fig. 2a and b ). The flux decreases with decreasing feed water content. The separation factor is very high since the water content in the permeate is nearly 100%. These first long-term investigations indicate that PELC membranes made from CS and PDMDAAC are (a)

,4 3.5

-e-

Tf : 5 0 ~ C ' ~

-e-

Tf : 8 0 ° C ]

I

/

3

g

c~

/

E 2.5

/

2

~ 1.5 1 0.5

5

10

15

20

cwf [ wt % ]

100

(b)

©

©

99 C)

98 ~ 97 _

n

i O Tf = 50°C

95

i • Tf = 8 0 ' c

0

5

10

15

20

Cwf [ wt-% ]

Fig. I. Long-term pervaporation performance of PELC membranes made from CS and P D M D A A C (feed: isopropanol/water; Po,,w,, < I mbar): (a) flux J vs. water concentration in feed c~,: (b) water concentration in permeate Cwp vs. water concentration in feed c~.

K. Richau et al. / Journal of Membrane Science 113 (1996) 31-41

36 (a)

3 2.5

Tf = 50°C

-e-

Tf = 80°C

2

E

5

-..e-

i

1.5

/

/

A s Fig. 3 demonstrates, the flux for ethanol-water mixtures is much smaller than that for pure water under pervaporation conditions for a P E L C membrane also made from CS and P D M D A A C .

,e

//

1

0.5

0" 5

0,''/¢

4.2. Swelling behaviour and pervaporation properties o f PELC membranes

~

10

15

20

25

o

25

Cwf [ wt-% ]

(b)

100 99 98

j-

97 96

Tf = 50°C

• T t = 80°C

95

10

15 Cwf [ wt-% ]

20

Fig. 2. Long-term pervaporation performance of PELC membranes made from CS and P D M D A A C (feed: N-methylmorpholine/water; paow._< 1 mbar): (a) flux J vs. water concentration in feed cwj; (b) water concentration in permeate Cwo vs. water concentration in feed ewt.

60

~

feed::H20

. . . . .

/

50

.c 30 ~

20

In order to clarify the correlation between swelling and pervaporation properties, the dependence of swelling on the ethanol concentration in ethanol/water mixtures was investigated. Fig. 4 shows typical results obtained at room temperature for a PELC membrane made from CS and PDMDAAC. The swelling ratio, i.e. the amount of equilibration solution sorbed in the membrane, decreased with increasing ethanol concentration. Furthermore, PELC membranes prepared from casting solutions of different concentrations were examined in pure water as well as in an ethanol/water mixture containing 20 wt% water, which corresponds to the pervaporation feed composition usually used. In this way, both extreme values of the swelling state in the downstream surface as well as in the upstream surface of a perfectly dehydrating membrane were approximated. The results of these investigations are summarized in Table 3. First of all the thickness of the membrane increases with concentration of CS during polyelectrolyte complex formation. The influence of the addition of bivalent cations (Cu 2+) to the P D M D A A C solution and the influence of the P D M D A A C concentration in the casting solutions on resulting membrane thickness is not so pronounced. 300

10

, * .......

0 15

20

25

• ........ 30

,", . . . . . .

35 40 Tf [ °C ]

45

'* 50

.

250 55

Fig. 3. Flux of water and of ethanol/water ( 8 0 / 2 0 wt% ) feed mixture in dependence on feed temperature Tt of a PELC membranes made from CS and P D M D A A C (Pdow. < 1 mbar).

n,03

200 150 j

promising candidates for dehydration applications. The chemical stability against organic compounds was found to be satisfactory [ 14]. PELC membranes are destroyed by inorganic electrolytes due to suppression of coulomb interactions.

100 t 0

10

20

30

40

50

60

70

80

CEtOH [ Wt-% ]

Fig. 4. Swelling ratio of a PELC membrane made from CS and P D M D A A C vs. ethanol concentration CE,OHin the water/ethanol feed-like equilibration liquid at 20°C.

K. Richau et al. / Journal of Membrane Science 113 ( 19961 31-41

37

Table 3 Membranes thickness d. swelling ratio in water SRH.~o and in ethanol/water SR~,on/mo as well as results of pervaporation tests / teed: EtOH/ H:O = 8 0 / 2 0 wt%: Tf= 50°C; Pdown< 1 mbar) Composition of the casting solution (wt%)

d (/xm I

CS

PDMDAAC

3 4 5 6 4 4 4 4 4

26 26 26 26 15 10 2 6 + 0 . 1 N Cu 2+ 2 6 + 0 . 5 N Cu 2+ 2 6 + 1 N Cu e+

1.9 3.8 4.1 6.8 4. I 4.3 2.3 2.1 2.4

The swelling ratio of PELC membranes in the ethanol/water (feed-like) mixture is practically not influenced by the concentration of the poly-ions as well as by addition o f C u 2 + and it is nearly constant: The mean value of S R E t o H / H 2 0 a m o u n t s to 1 8 5 + 2 5 % . Consequently we have to assume that the upstream surfaces show the same swelling behaviour for all PELC membranes made from CS and PDMDAAC. Therefore the conditions for mass transfer, i.e. sorption into the upstream surface, should be very similar. On the other hand swelling in water is significantly higher than in the ethanol/water mixture. The amount of sorbed water depends strongly on the details of membrane formation conditions. An addition of Cu 2 + to the polycation casting solution has the most drastic consequences on the swelling ratio in water. Like that, []

/

2.5

~

].5



/

/ []

i

/

0, . 0

0.1

0.2

,

variation CS doubled membrane variation PDMDAAC addition of Cu2+

0.3 0.4 l l d [ I /.um]

0.5

Pervaporation performance

Swelling ratio

0.6

Fig. 5. Pervaporation flux J vs. reciprocal membrane thickness l / d for PELC membranes differing with respect to preparation conditions as well as for a single and a doubled PELC membrane made from CS and P D M D A A C (Tr = 50°C; PJow. < 1 mbar).

SRH2o ( c~ )

SR~,,~H mo (%)

Jlkg/hln'-~

c,,r ( w t % )

667 471 268 262 615 994 508 1074 1546

170 201

2.5 2. I

162

1.4

163 170 160 227 213 166

I. I 2. I 2. I 1.9 1.8 2.5

96.3 95.6 96.9 96.7 97.6 97.6 g6.6 98 97.7

the swelling ratio increases with reduced CS concentration during polyelectrolyte complex formation as well as with decreasing concentration of the polycation casting solution. In conclusion, the swelling ratio in the downstream surface of a dehydrating PELC membrane under equilibrium conditions is considerably higher and depends on membrane preparation conditions, in contrast to its upstream surface. Concerning the pervaporation properties, the water content in the permeate is nearly constant ( from 96 to 98%) for all membranes. Water uptake as well as feed uptake do not correlate with the separation capability. That means, the separation effect is independent of details of PELC formation. Having in mind the constancy of swelling ratio in the feed-like solution observed, the assumption raises that selectivity is governed by sorption into equally swollen upstream surfaces. On the other hand the flux depends on the concentration of CS in the casting solution during PELC membrane formation. Considering the rehttion between membrane thickness and CS content in the casting solution, flux and thickness data from Table 1 are completed by results of a single and a doubled PELC membrane made from CS and P D M D A A C and summarized in Fig. 5. There is a nearly linear correlation between flux and reciprocal membrane thickness with respect to variation of CS concentration and to the doubled membrane. This indicates that the water transport may be governed by a diffusion mechanism.

38

K. Richau et al. / Journal of Membrane Science 113 (1996) 31~t1

4.3. Pervaporation investigations with model membranes and sandwiches In order to clarify whether sorption or diffusion determine membranes selectivity, we were following this idea: Laminating a PELC membrane A with high EtOH retention on a PELC membrane B with low EtOH retention and investigating such sandwich A + B under pervaporation conditions should enable us to decide whether sorption selectivity or diffusion selectivity determine the separation behaviour of PELC membrane A: • if sorption into the upstream surface governs the separation capability of A, the sandwich in the case that its A-layer is facing the feed should exhibit the separation capability of A • if diffusion through the cross section of the membrane governs the separation capability of A, the sandwich in the case that its A-layer is facing the permeate compartment should reach the separation capability of A (its B-layer is facing the feed, respectively). Results of respective investigations of sandwiches as well as its single PELC membranes are given in Table 4. The model PELC membranes used were prepared from CS and PDMDAAC ( A 1, A2), CS and PEI (A3) as well as CS and DDC (B1, B2). It is observed, that the high water content in the permeate Cwpof PELC membrane A as well as nearly its flux J is reached by sandwiches A + B only in the case that PELC membrane B (poor EtOH retention, high flux) is directed towards the feed compartment (A facing the permeate compartment, respectively). We have to assume that good separation capability of PELC membrane A is determined not by sorption selectivity of their upstream surface, but mainly by diffusion selectivity within the cross section of the membrane, most probably within regions near the downstream surface.

4.4. Pervaporation investigations with single polyelectrolyte films As mentioned above, our way to transform the water soluble anionic polysaccharide CS into the water insoluble state, which is necessary for its use in dehydration applications, is the interfacial polyelectrolyte complex formation. In order to clarify whether the counter-ions

Table 4 Results of pervaporation tests with sandwiches as well as its single PELC membrane components (feed: E t O H / H 2 0 = 8 0 / 2 0 wt%" Tf = 50°C; Pdow. < 1 mbar) Membrane

Single A1 Single B 1 Sandwich (AI+BI) Sandwich (AI+B1) Single A2 Single A2 Single B2 Sandwich (A2+B2)

Single component which faces the feed

J (kg/h m 2)

Cwp (wt%)

B1

3.20 12.60 2.70

97.3 34. I 97.3

AI B1

1.10 2.40

66.4 97.1

B2

2.10 2.80 5.10 2.00

96.0 96.5 42.0 96.8

B2

0.46 0.78 0.89

81.5 97.6 99.2

A3

0.23

87.1

A2 Single A3 Sandwich (A3+B2)

used contribute separately to the PELC membranes pervaporation performance or whether they act as network stabilizers in the aqueous environment only, it was desired to determine permeability and separation capability of films of the individual components used. In the case of CS and PDMDAAC it was possible to prepare single films as described in the experimental section, but the water concentration in the feed had to be decreased down to 10 wt% in order to retain both films stable. The results of pervaporation measurements are summarized in Table 5. First of all it becomes evident that the PVDF support does not influence the pervaporation properties of the CS film essentially. Comparing the results for Cwf= 10 wt%, the CS film yields a four times lower flux at 150 times higher separation factor as compared with the PDMDAAC film. The PELC membrane made from CS and PDMDAAC yields a somewhat lower separation performance than the CS film at a higher flux. According to these results, cellulose sulfate is confirmed as the only component responsible for separation capability, PELC membrane formation using polycations is accompanied by increasing flux density as compared with the single CS film.

K. Richau et al. /Journal of Membrane Science 113 (1996) 31-41

39

Table 5 Results of pervaporation tests with single polyelectrolyte films and with P E L C m e m b r a n e made from these polyelectrolytes I leed: E t O H / H 2 0 - 8 0 / 2 0 w t % at T~-= 50°C; " E t O H / H z O = 9 0 / 1 0 w t % at Tr = 20°C; puown < 1 m b a r in all cases ) Membrane

J ( k g / h m e)

Cw, ( w t % )

{,,e ( wt~c ~

c~

CS film on P V D F CS film

0.6 0.92 0.75 0. I ~ 0.38 ~ 0.45 ~ 0.19 ~

18.0 18.9 18.7 10.0 10.0 10.0 I 0.0

99.3 99.3 99.5 99.7 68.5 63.8 98.6

646 609 865 299 I 20 16 624

P D M D A A C film CS + P D M D A A C P E L C m e m b r a n e on P V D F

This conclusion is in agreement with the results summarized in Table 1 and Table 3. As long as polycations are used, the separation capability changes only slightly with the kind of polycation (see Table 1 ) or the polycation casting solution composition (see Table 3), but the flux density can be changed over a range from 0.65 to 2.8 kg/h m e. Therefore we can conclude, that polycations act as stabilizers but additionally as the component of the PELC membrane which governs the flux density. They do not influence essentially the separation capability inherent to the polyanion CS, provided by its large number of hydrophilic groups. With respect to the polycations used in this study, the polycation with the ionogenic group in the main chain yields the most dense network (PEI), whereas the other polycations of the integral type results in membranes with higher fluxes, probably due to the larger volume available for the transport of water. To explain the very high flux and vanishing selectivity of the PELC membrane made with the cationic surfactant DDC instead of polycations (see Table 1 ), there are two possibilities: first, this membrane exhibits micropores or, second, the hydrophilic character of CS is fully suppressed by the surfactant. Comparison between results of concentration potential measurements and streaming potential investigations confirm the latter assumption [ 12]. Taking into account the results concerning the intrinsic separation capability of pure CS, composites were prepared, consisting of CS layers located between a porous ultrafiitration membrane as the support and PELC membranes made from CS and DDC as described above as the feed facing protective membrane. Dehydration investigations confirm our assumptions: independent of the poor separation capa-

bility of the teed facing PELC membrane made from CS and DDC such composite reaches the excellent separation capability of the single CS film and is useful at Cwf> 10 wt% too [ 15]. It means that a simple protection of an unmodified free swelling CS film against dissolution in the feed mixture provides a ,,ery selective dehydration membrane.

4.5. Remarks concerning the applicability of model calculations in the case of the PELC membranes investigated The aim of a comparison between theoretical predictions and experimental data is to obtain correlations between membrane properties (in terms of diffusion coefficients, etc.) and preparation conditions (choice of polycations, details of membrane formation, etc. ) in order to develop more efficient membranes. In principle, model calculations can yield the desired membrane parameters for pervaporation of pure liquids, provided that the simplifying assumptions mentioned above are fulfilled. For binary feed mixtures the analysis becomes more complex due to the coupling of the individual fluxes. In both cases a key point is the description of the concentration dependence of conductivities Li~ and coupling coefficients Lik, the concentration itself being dependent on the location in the membrane. The resulting equations can be solved exactly for special cases only, even for pure liquids parameters are accessible only by fitting theoretical results, based on the solutiondiffusion model, to pervaporation data [ 16 ]. Of course, the reliability of e.g. calculated diffusion coefficients. with respect to conclusions concerning the membrane preparation conditions, will suffer from an increasing number of adaptable parameters contained in the

40

K. Richau et al. / Journal of Membrane Science 113 (1996) 31-41

model. An extensive discussion of the related problems is given in [ 13]. For the investigated PELC membranes, we deduced from the swelling investigations discussed above, that 1. independent of membrane formation conditions, their properties should be nearly the same in its upstream surface regions under pervaporation conditions; 2. dependent on membrane formation conditions, the swelling ratio should increase towards the downstream surface of the dehydrating membrane under pervaporation conditions due to decreasing EtOH concentration. The concentration profile established by that gradient of swelling is superposed under pervaporation conditions by another gradient, which is induced by drying of the downstream layer of the membrane [ 13 ]. Therefore, we have to describe a non-homogeneous and unevenly swollen membrane. In this case it seems not to be allowed to assume a simple exponential correlation Di=f(ci(x)) [ 16]. Unfortunately, direct measurement of the concentration profile according to Matsuura's procedure [ 17] using a membrane stack is not applicable to PELC membranes due to mechanical reasons. Therefore it seems more appropriate to use models proposed recently by Kedem [18] and by Bode [19]. These models account for the coupling of fluxes, but do not require any assumptions on the concentration profile within the membrane. Our further work will focus on collecting the necessary data for such an analysis.

4.6. Preferential transport of water through PELC membranes Remembering the results mentioned above, especially 1. the pure water permeability being much higher than the permeability for ethanol containing feed-like mixtures; 2. the correlation between mass flux and reciprocal membrane thickness; 3. the fact that cellulose sulfate only is responsible for dehydration properties of PELC membranes; we can conclude that the polysaccharide component in the polyelectrolyte complex interacts with water more preferentially than with the organic component in the feed. The pervaporation experiments led us to the

assumption, that the high selectivity toward water could be attributed to a selective hydrogen-bonding interaction between water and oxygen atoms of the polysaccharide component in the polyelectrolyte complex. Already Reid [ 20] discussed a mechanism of water and ion flow across cellulosic membranes for reverse osmosis. According to his scheme the water molecules combine with one side of the membrane, they migrate across by transfering from one hydrogen bonding site to another and are finally discharged from the other side. Cellulose sulfate provides excellent conditions for hydrogen bonding because there are large numbers of oxygen atoms in each polymer unit. The adoption of this mechanism explains clearly the preferential transport of water molecules across the PELC membranes. As compared with water, the transport of for example alcohols is suppressed due to the larger size of alcohol molecules and their lower polarity [ 21 ].

5. Conclusions

( 1 ) Long-term pervaporation investigations proved that PELC membranes obtained by simultaneous interfacial reaction from Na-cellulose sulfate and different polycations may be successfully used for dehydration of various organic solvents. (2) Such membranes exhibit fast adaptation with respect to variation of the actual pervaporation conditions, especially with respect to the water content in the feed. (3) The anionic polysaccharide Na-cellulose sulfate is the only component responsible for good separation capability in dehydration of organics with PELC membranes. (4) The use of polycations during membrane formation leads to enhanced stability against water and can be applied for adaptation of flux density. (5) The mechanism of water transport in case of PELC membranes is assumed to be a preferential transport due to transference of water molecules from one hydrogen bonding site to another across the polysaccharide chains in the membrane. (6) The selectivity is governed not by sorption into the upstream surface of PELC membranes investigated, but by different diffusion coefficients of feed compo-

K. Richau et al. /Journal of Membrane Science 113 (1996) 3 l ~ l l

nents along a rather complex swelling gradient across the stabilized cellulose sulfate.

Acknowledgements The authors wish to thank Gisela Frigge for her excellent assistance.

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