Ind. Eng. Chem. Res. 1995,34, 3581-3589
3581
Concentrated Emulsion Pathway to Novel Composite Polymeric Membranes and Their Use in Pervaporation Eli Ruckenstein* and Fuming Sun Department of Chemical Engineering, State University of New York at Buffalo,Amherst, New York 14260 Novel composite polymeric membranes have been prepared, using concentrated emulsions as precursors, and employed in the pervaporation of various liquid mixtures. In a concentrated emulsion, which has the appearance of a gel, the volume fraction of the dispersed phase is larger than 0.74 (which represents the most compact arrangement of spheres of equal radii) and can be as large as 0.99. The concentrated emulsions were prepared at room temperature by dispersing a hydrophobic (hydrophilic) monomer(s) into a small amount of a hydrophilic (hydrophobic) monomer( s) containing a suitable dispersant. In order to improve the stability of the concentrated emulsion, the hydrophilicity a n d o r the hydrophobicity of the phases involved must be increased by replacing them with their solutions in water and/or in a hydrocarbon, respectively. Another possibility of improving the stability is to increase the viscosity of the phases, by partial polymerization of one or both phases before preparing the concentrated emulsion. The emulsion gel was subsequently transformed into a polymer composite by polymerizing both phases. The dispersed phase should be selected to yield a hydrophobic (hydrophilic) polymer which is compatible with the components selected for separation and incompatible with the other components, while the continuous phase should be selected to yield a hydrophilic (hydrophobic) polymer which is incompatible with all of the components of the mixture, and thus it can ensure the integrity of the membrane. As examples, several composite polymeric membranes were designed, prepared, and employed in the separation by pervaporation of water-ethanol, aromatics-paraffinics, and aromatics-alcohol mixtures.
Introduction Pervaporation is becoming recognized as an energyefficient alternative to distillation and other separation methods of liquid mixtures (Nee1et al., 1985; Fleming, 19921, especially in cases in which the traditional separation techniques are not efficient, such as the separation of azeotropic mixtures (Yoshikawa et al., 1986;Zhu et al., 1989;Jansen et al,, 19921, close-boilingpoint components (Huang and Lin, 1968; McCandless, 1973; Cabasso et al., 19741, isomeric components (Mulder et al., 1982),and recovery or removal of trace organic substances from aqueous solutions (Bai et al., 1993; Nijhuis et al., 1993). In pervaporation, the feed of liquid mixtures contacts the upstream side of the membrane, and the driving force in the membrane is achieved by lowering the activity of the permeating components on the permeate side by applying vacuum on the downstream side. The permeation process consists of selective dissolution (sorption)of the components of the liquid mixture into the membrane, their transport by diffusion through the membrane, and evaporation on the membrane downstream side (Fleming, 1992). The separation is achieved because the membrane has the ability to transport one component more readily than the other one. In earlier studies, cellulose and its derivatives were used as membrane materials (Aptel et al., 1976; Nagy et al., 1980) in the pervaporation of water-ethanol azeotropic mixtures; more recently extensive studies have been focused on radiation- or plasma-grafted composite membranes (Hirotsu, 1987; Hirotsu and Nakajima, 1988; Hegazy et al., 1989)and polymer blend membranes (Xu and Hung, 1988; Zhao and Huang, 1988); interpenetrating polymer networks (Lee et al., 1990), zeolite-polymer membranes (Goldman et al.,
* Author to whom correspondence should be addressed.
1989), polyelectrolyte membranes (Reineke et al., 19871, and complex polyion hollow fiber membranes (Maeda et al., 1991) were also reported. A new method for the preparation of composite polymeric membranes was developed recently in our laboratory (Ruckenstein and Park, 1988; Ruckenstein, 1989). The composite membranes were prepared starting from concentrated emulsions as precursors. In a concentrated emulsion, which has the appearance of a gel, the volume fraction of the dispersed phase is larger than 0.74 (which represents the volume fraction of the most compact arrangement of spheres of equal radii) and can be as large as 0.99 (Princen et al., 1980; Ruckenstein et al., 1989). In this type of emulsion and for large volume fractions, the dispersed phase is composed of polyhedral cells separated by thin films of a continuous phase. The selection of the components of the concentrated emulsion is based on the consideration that the dispersed phase should yield a hydrophobic (hydrophilic) polymer which is compatible with the components selected for separation and incompatible with the other ones, while the continuous phase should yield a hydrophilic (hydrophobic) polymer which is incompatible with all of the components of the mixture and thus ensures the integrity of the membrane. Conventional emulsions or microemulsions could also be employed to generate such composites. The concentrated emulsions are, however, most suitable precursors, because to ensure maximum permeability, the volume fraction of the dispersed phase should be as large as possible and the thickness of the continuous phase films as small as possible. It is important t o emphasize that the polymerized concentrated emulsion maintains the structure of the concentrated emulsion precursor. Let us assume that we want to separate water from water-ethanol mixtures. Because polyacrylamide (PAAM) or polfisodium acrylate) (PSA)are compatible with water but incompatible with ethanol, whereas the
Q888-5885/95I2634-3581$09.QQlQ 0 1995 American Chemical Society
3582 Ind. Eng. Chem. Res., Vol. 34, No. 10, 1995 Table 1. Solubility Parameters of Some Components and Polymers solubility solubility components components and parameter 6 and parameter 6 (cal/~m~)~'~ polymers ( c a l / ~ m ~ ) ~ / ~ polymers 23.4 butyl acrylate 8.8 water 9.5-14.7 ethanol 12.7 acrylamide benzene 9.2 acrylic acid 12.0 8.9 polystyrene 8.5-10.3 toluene cyclohexane 8.2 poly(buty1 acrylate) 7.4-12.1 styrene 9.3
cross-linked polystyrene (PS)is incompatible with both water and ethanol, the membrane prepared from a concentrated emulsion precursor containing acrylamide (AAM) or sodium acrylate (SA) in the dispersed phase and styrene together with a cross-linking agent in the continuous phase will lead to a membrane permselective toward water from water-ethanol mixtures. In earlier studies (Ruckenstein and Park, 1990; Ruckenstein and Chen, 1991), such composite membranes were prepared by sandwiching emulsion gels (containing either an aqueous AAM or aqueous SA solution as dispersed phase and divinylbenzene (DVB)or styrene with a certain amount of DVB as continuous phase) between two glass plates and polymerizing the system by heating at about 50 "C. The resulting membranes exhibited high permselectivity to water from water-ethanol mixtures, but their mechanical properties were not satisfactory. To improve the mechanical properties of the composite membranes, the preparation procedure was changed in two ways (Ruckenstein and Sun, 1993): (i)butyl acrylate (BA) was introduced in addition to styrene in the continuous phase and (ii) the me,mbrane was prepared by the polymerization of the concentrated emulsion followed by its grinding into fine powders and finally by the hot pressing of the powders into a membrane. To improve the permeation rate, a porogen was introduced in the continuous phase of the emulsion precursor (Sun and Ruckenstein, 1993). Now assume that we want to separate aromatics from paraffnics or alcohols. One can observe that PAAM or PSA is sufficiently hydrophilic t o be incompatible with the above components; on the other hand, PS due to its benzenic ring is expected to be compatible with aromatics but not with paraffinics or alcohols. Therefore the membranes were prepared from concentrated emulsions in which styrene constituted the dispersed phase and an aqueous solution of AAM or SA the continuous phase. Consequently, the design of a membrane for a given mixture can be made on the basis of very simple chemical considerations involving the vague but fruitful concepts of compatibility and incompatibility. In principle, the solubility parameters [Table 1(Brandrup and Immergut, 1989)l may constitute a more quantitative characterization of the compatibility. If the solubility parameters are closer, the compatibility is higher. Indeed, the compatibility between styrene [6 = 9.3, where 6 ( ~ a l / c m ~ )denotes l'~ the solubility parameter] and benzene (6 = 9.2) or toluene (6= 8.9) is higher than that between styrene and cyclohexane (6 = 8.2). However, for PS the values provided by the literature are 8.5-10.3 and on the basis of these values it is no longer possible t o conclude which species is more compatible with PS. It should be also noted that the presence of another component may change the interactions between the component and polymer. Indeed, when the mixture is water-ethanol and the polymer is PAAM, the interactions between the hydrophobic moiety of ethanol and the hydrophobic moiety of PAAM lead to preferential
Table 2. Amounts of Components Used in the Preparation of the Polymer Composite continuous phase styrene (g) divinylbenzene (g) initiator (AIBN)
0.8 0.2
dispersed phase acrylamide (g) water (mL) initiator (K2S208)
4.0 28
surfactant (sorbitan monooleate)
0.8 4.0 4.0 0.2 1.0 1.0 2.0 10-4 gig monomers for all four systems
2.0 2.0 0 28 28 28 1.7 10-4 gig acrylamide for all four systems 1mL for all four systems
Table 3. Amounts of Components Used in the Preparation of the Concentrated Emulsion dispersed phase aqueous solution of sodium acrylate" (g) initiator (K2S208) continuous phase divinylbenzene (g) initiator (AIBN) surfactant (sorbitan monooleate) (g)
Mzi
M22
M23
30
30
30
1.7 x 10-4gig acrylic acid for all emulsions 2
4 6 2.0 x 10-4 gig of monomer for all emulsions 1 1 1
Containing 8 g of acrylic acid.
adsorption of alcohol and hydrophilize the membrane by exposing OH groups. As a result, the compatibility between water and the membrane is additionally increased. Further details regarding the preparation of the composite membranes and of their separation performances are described below.
Separation of Water from Water-Ethanol Mixtures Membrane Preparation by the Sandwiching Method (Ruckenstein and Park, 1990; Rucken. stein and Chen, 1991). A small amount of a mixture of styrene (St) and DVB, or only DVB, containing sorbitan monooleate as surfactant and azobisisobutyronitrile (AIBN) as initiator was placed in a three-neck flask. An aqueous solution of AAM or SA, which was prepared by titrating acrylic acid (& with I an) aqueous solution of NaOH to a pH slightly above 7, was introduced dropwise into the flask with mechanical stirring, and an aqueous solution of potassium persulfate was added as initiator. A concentrated emulsion having the appearance of a gel was thus generated. Tables 2 and 3 list the amounts used in the preparation of the gels. In order to prepare the membrane, the gel was placed between two glass plates and squeezed slowly to avoid trapping air bubbles. A small amount of glycerol was placed as a lubricant on the surface of the glass plates. Subsequently, the sandwiched gel was placed in an oven at 50 "C for 24 h. After polymerization, the membrane was dried. The thickness of the membrane was in the range 150-250 pm. Membrane Preparation by the Hot-Pressing Method (Ruckensteinand Sun,1993). The preparation process of the emulsion gel was as described above, but a partially polymerized mixture of StJBA instead of SuDVI3 or DVB was used as the continuous phase. It is important to emphasize that the initial partial polymerization was absolutely necessary, because only
Ind. Eng. Chem. Res., Vol. 34, No. 10, 1995 3583 Table 4. Amounts of Components Involved in the PreDaration of the Concentrated Emulsion@ ~
~~
~~
dispersed phase aqueous solution of sodium acrylate (SA) crosslinking agent (MBAAM) initiator (KzS208) continuous phase SVBA mixtures initiator (AIBN) surfactant (sorbitan monooleate)
~~
15.0 mL 0.042 glg SA 0.01 g/g SA 3.00 g 0.01 glg mixture 0.30 g/g mixture
a The amount of SA and the weight ratio of St and BA were varied as follows: Set A contains 3.00 g SA in aqueous solution. A-1, SVBA 30170; A-2, SVBA 50150; A-3, SVBA 70130; A-4, St. In Set B, the weight ratio of SVBA is 50150. B-1, 1.50 g SA, B-2, 2.00 g SA, B-3, 3.00 g SA, B-4, 3.75 g SA.
in this manner one could ensure the formation of a stable gel a t room temperature and its stability in the subsequent complete polymerization at about 50 "C (Ruckenstein and Sun, 1992). The generated gel was then transferred to a tube and sealed with a septum. The polymerization was conducted at 50 "C for 60 h. The amounts of components used in the preparation of the gel are listed in Table 4. The resulting polymer composites were washed with methanol in an extractor for 24 h and dried overnight in a vacuum oven. After grinding, the fine powders of the composites were hot pressed at 150 "C. The prepared membranes were further dried in vacuum oven for 2 days. The thickness of the membranes was in the range 150-250 pm. The swelling experiments showed that the swelling ratio, defined as s = (w, - w d ) / w d , where w d and w, denote the weights of the dry and the liquid-swollen membrane strips, increased rapidly for all membranes as the water concentration increased. The swelling ratio varied from 0 to 4.4 and 0 t o 55.72 with changes in the water concentration from 0 to 100 w t % for the membranes prepared from the gels of Table 2 and Table 4, respectively. The swelling ratio varied from 0.71 to 22.2 for water concentration changes from 25 t o 100 wt % for the membranes prepared from the gels of Table 3. The swelling ratio also increased with increasing amounts of hydrophilic polymers (AAM or SA) in the dispersed phase and decreasing SVBA weight ratios in the continuous phase. Permeation Experiments. A standard pervaporation apparatus was employed. The feed was charged in the upper compartment of the pervaporation cell with a capacity of about 250-300 mL. The membrane was inserted into the pervaporation cell being supported on a sintered glass disk. The membrane area in contact with the feed was 9.6 cm2. A constant downstream pressure (3 f 1 Torr) was maintained by a vacuum pump. The pervaporation vapor was condensed in a cold trap of liquid nitrogen after a few hours of running the apparatus at room temperature. In order to keep the feed composition constant, the amount of pervaporate collected during a run was kept small compared to the amount of the feed. The products were analyzed with a gas chromatograph (Varian 37001, equipped with a thermal conductivity detector and a column packed with Porapak T or Q (Alltech)heated a t 150 "C; helium was used as carrier. The separation factor or selectivity a is defined as a = (YA/YB)/(XPJXB), where YAand YBrepresent the weight fractions of liquid components A and B in the permeate and XA and XB those in the feed, respectively. Figures 1 and 2 show the dependence of the permeation rate and the separation factor on the membrane composition and ethanol concentration in the feed, respectively. The permeation rate decreased with de-
I-
I 10
30
20
40
50
70
60
Wt% of Ethanol in Feed (%)
Figure 1. Permeation rate through the membrane against ethanol concentration in the feed. The symbols 0 , A, and 0 correspond to membranes M11, Mlz, and M13 (of Table 21, respectively.
r-,
e' A'
/
/' /
' A
A '
A
20
30
40
50
60
70
SO
Wt% of Ethanol in Feed (70) Figure 2. Separation factor through the membrane against ethanol concentration in the feed. The symbols 0 , A, and 0 correspond to membranes M11, Ml2, and M13 (of Table 2), respectively.
creasing PAAM content in the dispersed phase and with increasing ethanol concentration in the feed through the membranes prepared from gels M11 and M1z of Table 2, whereas the permeation rate had a minimum through membrane M13. The separation factor increased with increasing ethanol concentration in the feed for membranes M11 and Mlz but decreased for membrane M13. The permeation rate was in the range 30-lo3 g/(m2h), and the separation factor ranged between 2 and 50. The pervaporation performances through membranes prepared from gels Mzi, where i = 1 , 2 , 3 (Table 3), were also determined. These membranes exhibited permselectivities toward water, the permeation rate decreasing and the separation factor increasing as the ethanol concentration increased in the feed. The temperature dependence of membrane M21 in the range 30-50 "C is presented in Table 5. Naturally, the permeation rate increased and the separation factor became slightly smaller when the temperature increased. An Arrhenius
3584 Ind. Eng. Chem. Res., Vol. 34,No. 10,1995 Table 5. Temperature Dependence of Permeation Rate and Permselectivity for Membrane M21 permeation rate (kg/(m2 h)) separation factor a ethanol (wt %) 30 "C 40 "C 50 "C
25 0.61 0.83 1.11
50 0.29 0.50 0.74
75 0.14 0.22 0.30
25 64 60 60
50 70 72 62
75 222 222 172
30.0
A-A
$
15.0
0 m
0 5
e
G,
00
Ni
EtOH R
-05
25
Ad
8
-10
d
c
.s a
-1
0.0 Strain E ( A M u )
50
5
Figure 4. Stress-strain curves of four membranes with systematically decreasing weight ratio of SUBA at fxed content of SA (set A of Table 4).
U
E
P)
-20
P)
3
75
5 -2J
30.0
-3 0
f
-3 5
1
31
3.2
I
3.3
I
3.4
3
lOOO/r (K') Figure 3. Arrhenius plots of the permeation rate through membrane Mzl (of Table 3).
relationship exists between the permeation rate and the temperature (Figure 31,with activation energies of 6.58, 7.91,and 8.14 kcallmol for the 25, 50, and 75 w t % ethanol concentrations, respectively. "he permeation rate through the membranes was in the range 95-560 g/(m2h), whereas the separation factor ranged between 32 and 235,depending upon the ethanol concentration in the feed and the compositions of the membranes. The above composite membranes exhibited high permselectivities toward water from water-ethanol mixtures, but their mechanical properties were not satisfactory. They could not be used repeatedly. The membranes prepared by hot pressing exhibited good mechanical properties because of the presence of BA in the continuous phase of the emulsion precursor (Ruckenstein and Sun, 1993). Figures 4 and 5 present the stress-strain curves for membranes Sets A and B (Table 41, respectively. The presence of BA in the continuous phase profoundly affected the mechanical properties of the composite membranes. In its absence, the membrane was brittle; it became more rubber like as the weight ratio of BA increased without becoming too large. This occurs because the copolymer formed between styrene and BA is less rigidly packed than PS and has therefore more flexibility. At a fixed weight ratio StAA, the change of the SA content in the dispersed phase affected the stress-strain behavior, its increase increasing the brittleness of the membrane. Table 6 lists the stress-strain data of the membranes. Because of their more suitable mechanical properties and swelling ratios, set B was selected to separate ethanol-water mixtures. The effect of the water concentration in the feed on the separation factor and permeation rate of the selected membranes is presented in Figures 6 and 7. The water permselectivity is highly influenced by the feed composition,the separation factor increasing and the permeation rate decreasing with decreasing water concentration in the feed from 50 t o
B
15.0
0
z2
0.0
0.40
(
(
10
Strain E (AWJ
Figure 5. Stress-strain curves of four membranes with systematically increasing content of SA at fxed weight ratio of StBA (set B of Table 4). Table 6. The Stress-Strain Properties of Membranes" membrane A-1 A-2b A-3 A-4 B-1 B-2 B-3 B-4
E (MPa) 16.69 196.53 543.05 1224.00 135.24 189.20 196.53 324.56
uY (MPa)
3.02 12.46 19.83 24.48 7.07 7.63 12.46 18.50
Ob
(MPa)
5.73 19.92 19.83 24.48 11.08 14.94 19.92 23.24
€b
(%I
66.33 15.13 3.67 2.00 31.80 20.83 15.13 8.66
a E , a? ut,, and Eb stand for Young's modulus, yield stress, stress, and strarn at break, respectively. A-2 and B-3 represent the same sample.
4.5 w t % (the latter concentration being that of the ethanol-water azeotrope),which is of practical interest. For the same composition of the ethanol-water mixture, the separation factor decreased and the permeation rate increased with increasing PSA content in the membrane. The increase in the permeation rate with increasing water concentration is a &sult of both higher water partial pressure, hence higher driving force, and the plasticizing action of water on the membrane (Nagy et al., 1980; Reineke et al., 1987). According to the solution-diffusion theory (Fleming, 19921,the permeation of liquids through polymer membranes involves a dissolution of the liquids in the upstream surface of the membranes and diffusion within them, the thermal
Ind. Eng. Chem. Res., Vol. 34, No. 10, 1995 3686 800.0
a v
% P
I 2
e
.s
400.0
J!
a
0.01 (
60.0
30.0 Wt% of Water in Feed (%)
Figure 6. Dependence of the separation factor (a)on the water concentration in the feed. 300.0
PSA. As a result, the permeation rate should increase and the water permselectivity decrease. The separation factor varied from about 23 at 50 w t % water concentration in the feed to greater than 700 at 4.5 wt %, whereas the permeation rate decreased from 246 to 13 g/(m2h), respectively. To increase the permeation rate, a porogen (hexane) which does not participate in the polymerization process is introduced in the continuous phase of the concentrated emulsion to generate pores in the thin films of the continuous phase (Sun and Ruckenstein, 1993).By increasing the amount of porogen (hexane),the porosity of the thin films of the continuous phase increases, and thus the liquid permeates through the films with less difficulty. As a result, the permeation rate increases, but the separation factor decreases. The porogen increases the permeation rate by a factor of about 2. For comparison purposes, typical results regarding the permselectivity through various synthetic-polymer membranes prepared by various researchers are listed in Table 7. The composite membranes prepared by us also exhibited long term stability,, since the permeation rate and separation factor almost did not change after 10 days of pervaporation. This can be attributed to the special structure of the composite membranes. In pervaporation, the aging of the PSA membranes is caused by the elution of the alkali ions out of the membrane with the generation of the acidic group COOH (Habert et al., 1980;Yoshikawa et al., 1986).In the composite membranes prepared by the concentrated emulsion pathway, the sodium ions are located in the cells of the dispersed phase, and their elution is prevented by the surrounding hydrophobic films.
Separation of Toluene from TolueneCyclohexane Mixtures (Park and Ruckenstein, 1989) 150.0
/
/B-2
/B-1
0.0 Wt% of Water in Feed (90) Figure 7. Dependence of the permeation rate on the water concentration in the feed.
motion of the polymer chains randomly producing “holes” through which the permeating molecules can diffuse. In the hydrophilic-hydrophobic composite membranes developed by us, the hydrophilic cells are separated by hydrophobic thin films. Water swells the cells easily, and the migration through the thin films (by diffusion and flow through its pores) is likely to be the rate-determining step. With increasing content of PSA, the relative content of the hydrophobic films surrounding the hydrophilic cells decreases, and the components of the liquid permeate through the films more easily. On the other hand, the hydrophilicity of the membranes increases with increasing content of
Membrane Preparation. A small amount of an aqueous solution of AAM containing sodium dodecylsulfate (SDS)was placed in a three-neck flask. Styrene containing AIBN was added dropwise with mechanical stirring to generate an emulsion gel. Table 8 lists some variations used in the preparation of the gels. The membrane was prepared by the sandwiching method described in a previous section. The thickness of the dried membrane was around 600pm. The swelling ratio was 14.86 in toluene and 1.03 in cyclohexane at 30 “C for the membrane prepared from gel PC1 of Table 8. Permeation Experiments. The pervaporation separation of a toluene-cyclohexane mixture through the composite membranes was carried out in the range 3070 “C in the pervaporation cell described in a previous section. The permeate was analyzed by HPLC with a p Bondapak C I S column; the eluent was a 50/50 vol % methanol-water mixture. It was found that the permeation rate increased with increasing temperature and with decreasing PAAM fraction in the composite and that it had an Arrhenius dependence on temperature. The permeation rate increased markedly with increasing toluene concentration in the feed and varied between lo3 and lo4 g/(m2 h) (Figure 8). These composite membranes exhibited permselectivity toward toluene. In contrast to the permeation rate, the separation factor decreased with increasing temperature but increased with increasing PAAM content in the membrane and with increasing toluene concentration in the feed. Figure 9 shows the dependence of the separation factor on temperature for
3586 Ind. Eng. Chem. Res., Vol. 34, No. 10,1995 Table 7. Comparison of EtOH/II20 Permselectivity through Polymeric Membranesa EtOH in feed membrane d b m ) (wt%) T("C) a P(kg/(m2h)) aP 30 99.7 84 1630 poly sulfonate 0.006 9.78 poly sulfonamide 30 96.0 84 450 0.016 7.20 25 95.0 40 8.99 0.172 poly(acrylamide-co-acrylonitrile)/ 1.55 cellulose acetate 81.5 40 poly(acry1ic acid-co-acrylonitrile) 14 -500 -0.035 -17.5 poly(acry1ic acid)/nylon 66 blend 42-81 95.0 75 10.1 0.858 8.67 poly(acry1ic acid)/nylon 6 blend 90.0 35 25-89 35 0.120 4.20 89.0 25 -25 2700 50b carboxymethylcellulose/ 135.0 poly(acry1ic acid, sodium salt) blend grafted acrylamide-co-acrylic acid, 88.5 40 29.7 0.140 4.16 sodium salt onto porous polypropylene grafted acrylic acid, sodium salt 90.0 40 52.1 0.130 6.77 onto porous polypropylene polyurethane/polystyrene (70/30) 90.0 30 4.3 0.001c interpenetrating polymer network (IPN) polyion complex containing 95.0 60 -5000 0.420 2100 poly(acry1ic acid) PSAP(StA3A)composite -170 95.5 20 568 0.073 41.46 PSAP(StA3A)composite with porogen -170 95.5 20 331 0.203 67.19 a
ref Jansen et al., 1992 Zhu et al., 1989 Yoshikawa et al., 1986 Zhao and Huang, 1988 Xu and Huang, 1988 Reineke et al., 1987 Hirotsu and Nakajima, 1988 Hirotsu, 1987 Lee et al., 1990 Maeda et al., 1991 Ruckenstein and Sun, 1993 Sun and Ruckenstein, 1993
The best performance measured by the product aP is given. The unit is g mil/(m2 h). The unit is g cm/(cm2h).
Table 8. Amounts of Components Used in the Preparation of the Polymer Composite PC1 dispersed phase styrene 27 g initiator (AIBN) 2.0 x gfg styrene continuous phase acrylamide 0.5 g water 4 mL initiator (K2S208) 1.7 x g/g acrylamide surfactant (SDS) 0.3 g PC2 dispersed phase styrene 27 g initiator (AIBN) 2.0 x g/g styrene continuous phase acrylamide 0.75 g water 4 mL initiator (K2SzOs) 1.7 x g/g acrylamide surfactant (SDS) 0.3 g ~
r
~
toluene-cyclohexane mixtures. The membranes prepared by the sandwiching method do not have, however, satisfactory mechanical properties.
L4
-
It IO 06
-
06
-
04 i
0
20
0
10
80
60
100
Wt% of Toluene in Feed (%)
Figure 8. Permeation rate through membrane PC1 (of Table 8) as a function of concentration of toluene.
Separation of Benzene from BenzeneCyclohexane Mixtures (Sun and Ruckenstein, 1995)
Membrane Preparation. A small amount of an aqueous solution of acrylic acid containing SDS and the initiator ammonium persulfate was placed in a threeneck flask equipped with a mechanical stirrer. Styrene, a mixture of styrene and BA, or a solution of styrene1 butadiene/styrene three-block copolymer (SBS) in styrene, containing the initiator AIBN, was injected into the flask under stirring to generate an emulsion gel. The amounts of components involved in the preparation are listed in Table 9. The membranes were obtained by the hot-pressing method described in a previous section. The thickness of the membrane was in the range 104-137 ,urn. The composite membranes exhibited selective sorption toward benzene. The swelling ratio increased with increasing benzene concentration in the mixture, with decreasing poly(acry1ic acid) (PAA) content in the continuous phase and with increasing BA content in the dispersed phase. Benzene swelled the membrane to a much higher extent than cyclohexane did.
4 1
I
I
1
I
I
I
I
Temperature (OC)
Figure 9. Separation factor for toluene from toluene-cyclohexane separation factors mixtures as a function of temperature: (0,O) obtained for a toluene-cyclohexane mixture (75125 by weight) through membranes PC1 and PC2 (of Table 8), respectively; (A) separation factor obtained for a toluene-cyclohexane mixture (50/ 50 by weight) through membrane PC1.
Permeation Experiments. The permeation of benzene-cyclohexane mixtures was carried out in the pervaporation cell described in a previous section. The
Ind. Eng. Chem. Res., Vol. 34, No. 10, 1995 3687 Table 9. Amounts of Components Involved in the Preparation of the Concentrated Emulsion@
100.0 r
r
composition of membranes membrane
disperse phase
continuous phase
M-0 M-1 M-2 M-3 M-4 M-5 M-6
20.0 g St 20.0 g St 20.0 g St 20.0 g SVSBS (95/5) 20.0 g StBA (95/5) 20.0 g St/BA (90/10) 20.0 g StA3A (80/20)
OgAA 0.4 g AA 1.0 g AA 1.0 g AA 1.0 g AA 1.0 g AA 1.0 g AA
80.0
60.0
a The continuous phase was an aqueous acrylic acid solution containing 5.25 mL of water; the initiator used for the disperse phase was AIBN (0.001 g/g monomer) and for the continuous phase was (N€&)zSzOs(0.001 g/gAA), and the surfactant was SDS (1.050
40.0
g).
permeate was analyzed with a gas chromatograph (Hewlett Packard 5890A), equipped with an Econo-Cap Carbowax capillary column (Alltech) and with a flame ionization detector; helium was used as carrier. It was found that all the membranes were permselective toward benzene over the entire range of feed concentrations. Figure 10 presents the pervaporation performance of the composite membranes for the separation of benzene from benzene-cyclohexane mixtures. The permeation rate or the normalized flux, which is the product of permeation rate and the membrane thickness, increased tremendously with increasing benzene concentration in the feed, whereas the separation factor decreased. In agreement with the sorption experiments, the pervaporation performance was affected by the membrane composition, the normalized flux increasing with a decrease in the PA4 content in the continuous phase and the BA content in the dispersed phase of the membrane. For membranes which differed only in the identity but not the amount of additive employed (SBS or BA), the one which contained SBS had the higher permeation rate; the effect on the separation factor was in the reverse direction. Table 10 lists the pervaporation performances of a benzenecyclohexane mixture with 50 w t % benzene a t 20 "C, through various membranes. It is interesting to note that the permeation rate of the present composite membranes are much higher than those of poly(viny1 fluoride) membranes (McCandless, 1973) and that the separation factors are higher than those of polyethylene or polypropylene membranes (Huang and Lin, 1968) which were used for the same mixture. Compared to the membranes based on a polymeric alloy of polyphosphonates and acetylcellulose (Cabasso et al., 19741, the separation factor of our composite membranes are somewhat lower but the permeation rates of our membranes are with more than one order of magnitude higher. Table 11 lists the pervaporation performances of some selected polymeric membranes.
Separation of Benzene from Benzene-Ethanol Mixtures (Ruckenstein and Sun, 1996) Membrane Preparation. The preparation of the emulsion gel was as described in previous sections. The amounts of components used in the preparation are listed in Table 12. The membrane was prepared using the hot-pressing method which was also described in a previous section. The thickness of the membrane was in the range 109-117 pm. The membranes which were modified by introducing SBS in the dispersed phase of the concentrated emulsion showed better mechanical properties than the other composite membranes. The
20.0
l
0.0 0.0
20.0
.
l
.
,
,
l
40.0 60.0 80.0 W/O of benzene in feed ( O h )
.
100.0
d M-4 e M-5
f M-6
12.0
3 E
9
I
J
P C Pd
I
WW. of benzene in feed (%) Figure 10. Dependence of pervaporation characteristics on the benzene concentration for various membranes (of Table 9) a t 20 "C: (v) M-1, (A)M-2, (0)M-3, (0)M-4, (0) M-5, and (*) M-6; (a, top) wt % of benzene in permeate vs wt % of benzene in feed and (b, bottom) normalized permeation rate (permeation rate times thickness of the membrane in meters).
Table 10. Performance of Different Membranes in the Pervaporation of Benzene-Cyclohexane Mixtures Containing SO w t % Benzene at 20 "C permeate d composition P lo2 x J membrane (um) (wt % benzene) a (g/(m2h)) (g/(m h)) 513 7.04 4.77 137 82.66 M-1 465 4.84 90.54 9.57 M-2 104 535 7.06 2.47 132 71.16 M-3 476 5.57 85.55 5.92 M-4 117 6.36 3.06 481 132 75.35 M-5 794 10.72 63.10 1.71 M-6 135
mechanical properties were also affected by the nature and amounts of the continuous phase monomers.
3588 Ind. Eng. Chem. Res., Vol. 34, No. 10, 1995 Table 11. Comparison of Pervaporation Performance of Benzene-Cyclohexane Mixture Containing 50 wt % Benzene for Some Membrane Materials
d membrane (um) P/A-50" 20 P/A-30b 20 pol(yviny1idene 25 fluorideP polyethylene 25 M-2 (Table 9) 104
T p (g/ ("C) a (m2h)) reference 40 Cabassoet al., 1974 30 13.3 12.5 44 14.0 82.2 McCandless, 1973 56 5.00 25 20
1.632 -520 9.57 465
Huang and Lin, 1968 Sun and Ruckenstein, 1995
a PIA-50 represents a polyphosphonate/acetylcellulose alloy which contains 50 wt % polyphosphonate. P/A-30 represents a polyphosphonate/acetylcellulose alloy which contains 30 wt % polyphosphonate. The membrane contains 23 wt % 3-methylsulfolene.
Table 12. Amounts of Components Used in the Preparation of the Concentrated Emulsion# composition of membranes membrane disperse phase continuous phase 20.0 g SVSBS (85/15 wVwt) A- 1 OgAA 20.0 g SVSBS (85/15 d w t ) 0.4 g AA A-2 1.0 g AA 20.0 g SVSBS (85115 wVwt) A-3 B-1 20.0 g SVSBS (90/10 wVwt) 0.4 g AA 0.4 g AA B-2 (=A-2) 20.0 g SVSBS (85115 wt/wt) 20.0 g St/SBS (80/20 wt'wt) B-3 0.4 g AA C-1 (=A-3) 20.0 g SVSBS (85115 wt/wt) 1.0 g AA 20.0 g St/SBS (85/15 wt'wt) c-2 1.0 g DMAA 20.0 g St/SBS (85/15 wVwt) c-3 1.0 g AAM 20.0 g SVSBS (85/15 wVwt) 1.0 g DMAAM c-4 c-5 20.0 g SVSBS (85/15 wVwt) 1.0 g SSSF 20.0 g SVSBS (85/15 wVwt) C-6 1.0 g AMPSA The continuous phase was water or an aqueous solution of hydrophilic monomer; the volume of the continuous phase was 5.25 mL; the initiator used for the dispersed phase was AIBN (0.001 g/g St); the initiator used for the continuous phase was ("&SZOS (0.001 g/g monomer); the surfactant was SDS (1.050 g). a
All membranes exhibit similar swellingbehaviors, the sorption increasing with increasing benzene concentration in the binary mixtures; when the benzene concentration reaches extremely high values, the membranes either partially dissolve or disintegrate. The sorption is also affected by the composition of the dispersed phase of the composite membranes. At a fixed PAA content in the continuous phase, the equilibrium swelling ratio increases monotonically with increasing SBS content in the dispersed phase; it should be, however, noted that when the SBS content becomes too large, the membrane disintegrates at a benzene concentration of 70 w t %. The sorption characteristics also depend on the hydrophilic polymer employed in the continuous phase. The sorption selectivities of all the generated membranes are greater than unity, indicating that benzene is preferentially absorbed, but decrease monotonically with increasing benzene concentration in the mixture. Permeation Experiments. The permeation of benzene-ethanol mixtures through the composite membranes was carried out in the pervaporation cell described in a previous section. The collected liquids were analyzed with a Hewlett Packard 5890A gas chromatograph equipped with a 6 ft x 118 in. stainless steel column packed with 0.2% Silar-1OC on Graphpac-GC 80/100 (Alltech) and with a flame ionization detector; helium was used as carrier. All the membranes were permselective toward benzene over the entire range of feed concentrations. The separation factor decreased with increasing benzene concentration in the feed, whereas the permeation rate or the normalized flux increased markedly. In agree-
Table 13. Characteristics of Different Membranes in Pervaporation at 20 "C of Benzene-Ethanol Mixtures Containing 50 wt % Benzene permeate d composition P 102x J membrane h m ) (wt % benzene) a (g/(m2h)) (g/(m h)) 87.56 7.04 4.23 A-2 114 371.05 4.33 89.40 8.43 397.25 A-3 109 2.97 87.8 7.20 253.85 B-1 117 4.23 87.56 7.04 371.05 B-2 114 6.66 79.11 3.79 584.21 B-3 114 89.40 4.33 8.43 397.25 c-1 109 87.49 6.42 6.99 563.16 c-2 114 2.12 90.24 9.25 181.20 c-4 117 88.67 2.83 C-6 112 7.83 252.68
ment with the sorption experiments, the pervaporation performance was affected by the membrane composition. The normalized flux increased with increasing SBS content in the dispersed phase, whereas the separation factor decreased. The separation factor also increased with increasing PAA content while the normalized flux remained almost the same. The pervaporation performance was also affected by the nature of the hydrophilic polymer in the continuous phase. The largest permeation rate was 1040 g/(m2h) and occurred in the feed containing 70 wt % benzene through membrane C-2 of Table 12. The highest separation factor (a= 25) took place for the feed containing 10 wt % benzene through membrane C-4 of Table 12. Table 13 lists the pervaporation performance through different membranes for 50 wt % benzene.
Conclusion Novel composites, namely the hydrophilidhydrophobic composites, were prepared via the concentrated emulsion pathway. These composites contained hydrophilic (hydrophobic) polymer or copolymer cells dispersed in a continuous hydrophobic (hydrophilic) polymer phase. The composite membranes were prepared by either sandwiching or hot-pressing methods. The dispersed phase was selected to yield a hydrophilic (hydrophobic) polymer which is compatible with the component being selected for separation and incompatible with the other compoiient, whereas the continuous phase was selected to yield a hydrophobic (hydrophilic) polymer which is incompatible with any of the components of the mixture and thus can maintain the integrity of the membrane. The composite membranes have been employed for the pervaporation separation of waterethanol and organic-organic liquid mixtures. The mechanical properties of the composite membranes were improved by introducing additional suitable compounds in one of the two phases of the concentrated emulsion and using the hot-pressing method in their preparation.
Nomenclature AA = acrylic acid AAM = acrylamide AIBN = azobisisobutyronitrile AMPSA = 2-acrylamido-2-methyl-1-propanesulfonic acid BA = butyl acrylate DMAA = dimethylacrylic acid DMAAM = dimethylacrylamide DVB = divinylbenzene MBAAM = NJV-methylenebisacrylamide PAA = poly(acry1ic acid) PAAM = polyacrylamide PS = polystyrene PSA = poly(sodium acrylate)
Ind. Eng. Chem. Res., Vol. 34, No. 10, 1995 3589
SA = sodium acrylate SDS = sodium dodecylsulfate SBS = styrene/butadiene/styrenethree-block copolymer SSSF = styrene sodium sulfonate St = styrene d = membrane thickness (ccm) J = Pd (g/(m h)) P = permeation rate (g/(m2h)) S = swelling ratio = (W, - w d ) / w d w d = weight of dry membrane (g) W,= weight of swollen membrane (g) XA = weight fraction of liquid component A in the feed XB = weight fraction of liquid component B in the feed YA= weight fraction of liquid componentA in the permeate YB= weight fraction of liquid component B in the permeate a = separation factor = (YA/YB)/(XA/XB) 6 = solubility parameter ( c a l / ~ m ~ ) ~ ’ ~
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Abstract published in Advance A C S Abstracts, September 1, 1995. @
