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Sorption and pervaporation of benzene-cyclohexane mixtures through composite membranes prepared via concentrated emulsion polymerization
ournall of EMBRANE SCIENCE ELSEVIER
Journal of Membrane Science 99 ( 1995 ) 273-284
Sorption and pervaporation of benzene-cyclohexane mixtures through composite membranes prepared via concentrated emulsion polymerization Fuming Sun, Eli Ruckenstein* Department of Chemical Engineering, State University of New York at Buffalo, Box 60, Buffalo, NY 14260-4200, USA Received 12 July 1994; accepted in revised form 3 October 1994
Abstract Composite membranes have been prepared starting from concentrated emulsions of styrene in aqueous solutions of acrylic acid as precursors. Each of the phases of the emulsion contained a suitable initiator and the continuous phase contained a dispersant (sodium dodecylsulfate). The polymerization of the precursor generated a composite consisting of particles of dispersed phase imbedded in the continuous phase. To obtain the membranes, the polymer powders generated via grinding were subjected to hot pressing. The mechanical properties of the membranes have been improved by dissolving either butyl acrylate or styrene-butadiene-styrene three block copolymer in styrene. The membranes have been used to separate benzene from benzene-cyclohexane mixtures. The components of the membranes have been selected because polystyrene swells well in benzene and much less in cyclohexane, while the poly(acrylic acid) does not swell well in either of the components and can, therefore, maintain the integrity of the membrane. The mechanical and sorption behaviors of the membranes as well as their pervaporation capabilities were investigated. Keywords: Pervaporation; Benzene-cyclohexane mixture; Composite membranes; Concentrated emulsion polymerization
1. Introduction Pervaporation differs from all the other membrane processes, except membrane distillation, because of the change of the permeate from liquid to vapor. The permeation process consists of selective sorption of the components of the liquid into the membrane, their transport by molecular diffusion through it, and evaporation at the membrane surface since the partial pressures on the permeate side are lower than the vapor pressures of the components at the same side [ 1-3 ]. By utilizing suitable membranes, selected components * Corresponding author. 0376-7388/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI0376-7388(94)00254-1
of a liquid mixture can be efficiently removed or enriched and direct or indirect energy saving achieved. Thus, the development of membrane materials plays an important role in the pervaporation research [ 4 ]. Various hydrophilic membranes have been suggested for the dehydration of organic solvents which have positive azeotropic mixtures with water [5-9]. The finding of organophilic materials which could be employed for the recovery of alcohol or other organic substances from aqueous systems has been also attempted [ 10-14]. Investigations have been also carried out to evaluate the performance of pervaporation in the separation of isomers and close-boiling point mixtures [ 15-21 ].
274
F. Sun, E. Ruckenstein / Journal of Membrane Science 99 (1995) 273-284
A novel method to prepare composite membranes from emulsions was developed in our laboratory [22]. The dispersed phase of the emulsion was selected to yield a polymer which swells well in those components for which the membrane should be selective, while the continuous phase was chosen to yield a polymer which does not swell in any of the components of the mixture. Conventional emulsions or microemulsions could be employed to generate such composites. However, the concentrated emulsions, in which the volume fraction of the dispersed phase is very large (larger than 0.74, which represents the most compact arrangement of spheres of equal radii), are most suitable precursors for such membranes, because the thickness of the continuous phase films separating the polyhedral cells of the dispersed phase of the emulsion is small. The membranes have been prepared by polymerizing the emulsion. In a previous paper [ 16], such a membrane was prepared from a concentrated emulsion whose dispersed phase was styrene and the continuous phase was an aqueous solution of acrylamide. Acrylamide alone could not be used because it is not hydrophilic enough to ensure the stability during polymerization at 50°C for 24 h of the concentrated emulsion. The membrane was prepared by spreading the gel-like concentrated emulsion between two parallel plates and subjecting the system to polymerization. While the selectivity for toluene from toluene-cyclohexane mixtures and the permeability of the membrane were good, the mechanical properties of the membranes have not been comTable 1 Amounts of components involved in the preparation of the concentrated emulsionsa Membrane number
M-0 M-1 M-2 M-3 M-4 M-5 M-6
Composition of membranes
pletely satisfactory. In the present paper new membranes were prepared, which had better mechanical properties, and mixtures containing the close boiling point components benzene and cyclohexane subjected to pervaporation. Since the membranes have dispersed phases based on styrene and continuous phases based on acrylic acid, they are expected to be permeable to the aromatic component, which is compatible with styrene, and impermeable to the aliphatic component, which is incompatible with styrene. The thin films of poly(acrylic acid), which are not compatible with either benzene or cyclohexane, ensure the integrity of the membranes.
2. Experimental 2.1. Materials Styrene (ST, ACS reagent, Aldrich), butyl acrylate (BA, ACS reagent, Aldrich) and acrylic acid (AA, ACS reagent, Aldrich) were purified by distillation under reduced pressure; the initiators azobisisobutyronitrile (AIBN, ACS reagent, Kodak) and ammonium persulfate [ (NH4)2S208, ACS reagent, Aldrich] were purified by recrystallization from methanol and water, respectively. Styrene-butadiene-styrene three block copolymer (SBS, Aldrich) containing 28 wt% styrene, the dispersant sodium dodecylsulfate (SDS, ACS reagent, Fluka), benzene (HPLC grade, Aldrich), cyclohexane (HPLC grade, Aldrich) and methanol (MeOH, 99.9%, Aldrich) were used as received. Water was distilled and deionized. 2.2. Preparation of the concentrated emulsion
Disperse phase
Continuous phase
ST 20.0 g ST 20.0 g ST 20.0 g ST/SBS (95/5 wt/wt) 20.0 g ST/BA (95/5 wt/wt) 20.0 g ST/BA (90/10 wt/wt) 20.0g ST/BA (80/20 wt/wt) 20.0 g
AA AA AA AA AA AA
0.4 g 1.0 g 1.0 g 1.0 g 1.0g 1.0 g
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 (NH4)zS208 (0.001 g / g AA), the surfactant was SDS ( 1.050 g).
A small amount of an aqueous solution of acrylic acid containing sodium dodecylsulfate (SDS) and the initiator ammonium persulfate was placed in a 250 ml three-neck flask equipped with a mechanical stirrer. Styrene, a mixture of styrene and butyl acrylate, or a solution of SBS in styrene, containing the initiator AIBN, was injected into the mixture under stirring (600-650 rpm). Then the flask was heated at 50°C in an oil bath under stirring; and a nitrogen gas purging for 1/2 to 2 h, depending upon the viscosity of the resulting gel, was employed. (The heating of the flask
F. Sun, E. Ruckenstein / Journal of Membrane Science 99 (1995) 273-284
A
275
G
E
Figure 1-b
Figure 1-a
Fig. 1. The pervaporation apparatus: (A) permeation cell, (B) membrane, (C) sintered glass support, (D) entering mixture, (E) band heater, (F) mechanical stirrer, (G) switch valve, (H) vent to atmosphere, (l) collection trap, (J) safety trap, (K) Pirani gauge, (L) vacuum pump.
i
i
//
40.0
a
i
2.3. Preparation of the membranes
i
d
The obtained polymer composites were washed with methanol in an extractor for 24 h, dried over night at the ambient temperature, and then further dried at 50°C in a vacuum oven. After grinding, the fine composite powders were pressed at 150°C. The membranes were further dried at 50°C in a vacuum oven for 2 days before use. The thickness of the membrane was in the range of 104 to 137/zm.
o
/
30.0
,m 20.0
°
t
2.4. Measurement of stress-strain behavior aM-1
10,0
b M-2
e M-5 f M-5
0.0
0.0
2.0
4.0 6.0 Strain ~ (%)
8.0
10.0
Fig. 2. Stress-strain behavior of membranes.
was necessary, because without this heating treatment, the stability of the gel could not be ensured during the subsequent polymerization at 50°C for 60 h). The slightly polymerized emulsion was then transferred to a tube of 20 ml capacity, which was sealed with a septum. To avoid the presence of oxygen, nitrogen gas was passed through the tube for 20 min. Polymerization was conducted in a temperature-controlled water bath (50°C) for 60 h. The amounts of components involved in the preparation are listed in Table 1.
The stress-strain behavior of the composite membranes was measured using an Instron Testing Instrument (model 1000), the extension rate being 10 ram/ min. Table 2 The stress-strain behavior of membranesa Membrane
E (MPa)
o-y (MPa)
ob (MPa)
~b (%)
M-I M-2 M-3 M-4 M-5 M-6
1999.00 1334.67 532.14 921.90 744.00 573.38
39.98 40.02 37.25 38.72 33.48 31.56
39.98 40.02 37.25 38.72 33.48 31.56
2.50 3.00 7.00 4.20 4.50 5.50
"E, o'y, o-b and % stand for Young's modulus, yield stress, breaking stress and breaking strain, respectively.
F. Sun, E. Ruckenstein / Journal of Membrane Science 99 (1995) 273-284
276
8.0
i
i
i
i
c
6.0
a
d
,
0
4.0
Cap Carbowax (30 m × 0.32 mm i.d. × 0.25/xm) capillary column (Alltech) and with a flame ionization detector; helium was used as the carrier. The column, injector and detector temperatures were 50, 260 and 300°C, respectively. Two quantities can be used to characterize the equilibrium sorption properties: ( 1) the equilibrium swelling ratio S¢, and (2) the equilibrium sorption selectivity a s, defined by
w~-wd Wd
Se
E= "~ 2.0
aM-i
~.
b M-2 c M-3 d M~
Iii
e M-5 f M-6 i
I
I
I
0.0 ).=0 20.0 40.0 60.0 80.0 100.0 Weight fraction of benzene in the mixture (%)
Fig. 3. Equilibrium sorption as a function of benzene concentration in the mixture at 20°C in various membranes: ( V ) M-I; ( A ) M-2; ( 0 ) M-3; (1-1) M-4; (<)) M-5; ( * ) M-6.
as = Cb/ C~
x./xc where Wd and Wc denote the weights of the dry and equilibrium swollen membrane strips, respectively, Ch and Cc represent the weight fractions of benzene and cyclohexane at equilibrium in the membrane and Xb and X~ those in the feed, respectively.
2.5. Sorption kinetic experiments Strips of dry membranes (about 120/xm thick and 100 mg weight) were immersed in various benzenecyclohexane mixtures at ambient temperature and rapidly removed from the mixtures at various times between 16 to 6400 min, wiped off with tissue paper to remove the surface adherent liquid and weighed on a Mettler semimicrobalance. The swelling ratio is defined as
S
w~-w~ %
where Wo and Ws denote the weights of the dry and swollen samples, respectively.
8.0
et~
"~ 6.0 E
o
4.0
E 2.0
2.6. Equilibrium membrane sorption experiments
g
UJ
After the sorptions reached equilibrium, the membranes were removed from the mixtures, the liquid on their surface wiped off with tissue paper, weighed as quickly as possible, and then placed into a dry flask connected to a cold trap and a vacuum pump. The collected liquids were analyzed with a Hewlett Packard 5890A gas chromatograph equipped with an Econo-
o.o~
20.0 40.0 60.0 Weight fraction of benzene in the mixture (%)
Fig. 4. Equilibrium total and individual sorptions for membrane Ml as a function of benzene concentration in the mixture at 20°C: ( O ) total sorption; (/X) benzene sorption; (V) cyclohexane sorption.
F. Sun, E. Ruckenstein / Journal of Membrane Science 99 (1995) 273-284
2.7. Pervaporation
8.0
-~ 6.0 E
cn
._o
4.0
==
E ._~
.~ 2.0 I.IJ
0.0
277
20.0 40.0 60.0 80.0 1( Weight fraction of benzene in the mixture (%)
Fig. 5. Equilibrium total and individual sorptions for membrane M2 a,s a function of benzene concentration in the mixture at 20°C: (O) total sorption; ( A ) benzene sorption; ( V ) cyclohexane sorption.
Fig. 1 presents the pervaporation apparatus. The membrane with an effective surface area of 9.6 cm 2 was inserted in the pervaporation cell and supported on a sintered glass disk. About 250 ml of feed mixture was introduced in the upstream compartment, and the downstream pressure (3 + 1 torr) was maintained with a vacuum pump. A band heater mounted on the cell and a mechanical stirrer were used to control the feed temperature which was varied between 20 and 50°C. The pervaporation vapor was condensed in a cold trap of liquid nitrogen after 2 h of running the apparatus at the operating temperature. The amount of permeate collected during a run was kept small compared with the amount of feed, hence the composition of the feed can be assumed to remain constant throughout a run. The trap filled with permeate was warmed up to the ambient temperature, the permeate removed, weighed, and analyzed with the gas chromatograph. The membrane performance was measured in terms of two quantities: ( 1 ) the separation factor a p, and (2) the permeation rate (P), expressed in g / m 2 h, or the normalized flux (J), expressed in g / m h, defined by
8.0
8.0
~ ,
i
I
i
!
|
I
6.0 E /
/
/
/
/
/
/
6.0 E
d o
~ 4.0
.£
.~_
==
ig 2.0
"~ 2.0
4.0
E
g U.I
0.0
) 20.0 40.0 60.0 80.0 Weight fraction of benzene in the mixture (%)
Fig. 6. Equilibrium total and individual sorptions for membrane M3 as a function of benzene concentration in the mixture at 20°C: (O) total sorption; ( A ) benzene sorption; (~7) cyclohexane sorption.
0.0~
. . . . ' 20.0 40,0 60.0 80.0 10 .0 Weight fraction of benzene in the mixture (%
Fig. 7. Equilibrium total and individual sorptions for membrane M4 as a function of benzene concentration in the mixture at 20°C: ( O ) total sorption; ( A ) benzene sorption; (V) cyclohexane sorption.
278
F. Sun, E. Ruckenstein / Journal of Membrane Science 99 (1995) 273-284
ozP =
YblY~ XhlX~
W A.t J=
8.0
== c~
~
6.0
W.d A.t co
where Yb and Y~represent the weight fractions of benzene and cyclohexane in the permeate and Xb and Xc those in the feed, respectively, and W, A, t and d represent the weight of the permeate (g), the effective membrane area (m2), the operating time (h) and the membrane thickness (m), respectively. The pervaporation experiments were carried out between 20 and 50°C.
o
4.o
N 2.0 W
0.0
3. Results and discussion
3.1. Mechanical properties The hydrophobic-hydrophilic composite membranes prepared in a previous paper[ 16] by the concentrated emulsion polymerization exhibited high efficiency in the separation of aromatic-aliphatic mixtures. In that paper, the membrane was prepared by spreading the gel-like concentrated emulsion of styrene dispersed in an aqueous solution of acrylamide between two parallel plates and subjecting the system to polymerization. While the selectivity for toluene from the toluene-cyclohexane mixture and the permeability of the membrane were good, the mechanical properties of the membranes have not been completely satisfactory, since they could not be used repeatedly. The poor mechanical properties were caused by the presence of the surfactant and by the brittleness of the polystyrenepolyacrylamide composite. In the present paper, the membranes were prepared by hot-pressing fine powders of composites from which most of the surfactant was extracted by washing with methanol; the mechanical properties were somewhat improved by this heating treatment. The addition of butyl acrylate or SBS to styrene made the membranes less brittle. This is because the copolymer formed in the first case has side chains which impede the compact packing of the polymer molecules, thus generating some flexibility. Sim-
) 20.0 40.0 60.0 80.0 Weight fraction of benzene in the mixture (%)
Fig. 8. Equilibrium total and individual sorptions for membrane M5 as a function of benzene concentration in the mixture at 20°C: (O) total sorption; ( A ) benzene sorption; (V) cyclohexane sorption.
ilarly, the molecules of SBS do not allow the compact packing of the polymer molecules. Fig. 2 and Table 2 present the stress-strain behavior of the membranes. 3.2. Sorption capacity In pervaporation, the driving force for transport is the concentration difference across the membrane. The transport process consists of three steps: (i) sorption into the membrane at the upstream side, (ii) diffusion through the membrane, and (iii) desorption into a vapor phase at the down stream side. The solubility (sorption) is a thermodynamic and the diffusivity a kinetic property. Benzene is a good solvent for polystyrene (PS), cyclohexane is a nonsolvent for PS, while both benzene and cyclohexane are nonsolvents for poly (acrylic acid) (PAA). The sorptions of these compounds in the PS/ PAA based polymer composites, prepared via the concentrated emulsion polymerization, are therefore expected to differ considerably. Fig. 3 shows the equi-
279
F. Sun, E. Ruckenstein / Journal of Membrane Science 99 (1995) 273-284
8.0 / 8.0
i
i
i
'
'
'
i
6.0 R
i
[]
6.0
ft, 4.0 J-//
~
2.0
0.0
0 Square Rootof Time (rainI/2)
2010
8;.o
'
Weight fraction of benzene in the mixture (%) Fig. 9. Equilibrium total and individual sorptions for membrane M6 as a function of benzene concentration in the mixture at 20°C: ( 0 ) total sorption; (/~ ) benzene sorption; (~7) cyclohexane sorption.
Fig. 11. Dependence of the swelling ratio (S) on the square root of absorption time in different benzene/cyclohexane mixtures for membrane M-l: (~7) cyclohexane; (A) 10 wt% benzene; ((3) 30 wt% benzene; (D) 50 wt% benzene; (O) 70 wt% benzene.
8.0 l 2.5
b~
M-2 M-3 M-4 M-5 M-6
~ 2.0
6.0 ~-
~
>
'
'
/
=M-1 b c d • f
'
X
>(
4.0
c o
n
o 1.5 c f
2
"
0
~
8
0.0 1.0
.
.
.
.
O.O 20.0 40.0 60.0 80.0 100.O Weight fraction of benzene in the mixture (%)
Fig. 10. Dependence of sorption selectivity on the benzene concentration in various membranes: (V) M-I; (A) M-2; (O) M-3; (Fq) M-4; (O) M-5; ( * ) M-6
O.O SquareRootof Time(rain1/2)
Fig. 12. Dependence of the sorption ratio (S) on the square root of absorption time in different benzene/cyclohexane mixtures for membrane M-2: (~7) cyclohexane; (A) 10 wt% benzene; (©) 30 wt% benzene; ([]) 50 wt% benzene; ( ~ ) 70 wt% benzene; ( * ) 90 wt% benzene; ( X ) benzene.
280
F. Sun, E. Ruckenstein / Journal of Membrane Science 99 (1995) 273-284
8.0
i
6.0
~
i
i
/
4.0
¢.0
2.0
0.01m
03
v*
I
J
t
20.0 40.0 60.0 Square Root of Time (min 1/2)
80.0
Fig. 13. Dependence of the swelling ratio (S) on the square root of absorption time in different benzene-cyclohexane mixtures for membrane M-3: ( ~ ) cyclohexane; (A) 10 wt% benzene; (O) 30 wt% benzene; ([]) 50 wt% benzene; ( ~ ) 70 wt% benzene; ( * ) 90 wt% benzene.
librium sorptions for various membranes. The membranes have similar sorption characteristics, the equilibrium sorption increasing monotonically with increasing benzene concentration. However, when the latter concentration becomes sufficiently high, some membranes partially dissolve. Comparing the behavior of the membranes, one can see that the sorption increases with decreasing PAA content, because of the increase of the hydrophobicity in the composite (membranes M-1 and M-2). When the membrane contains only PS (M-0), it dissolves in benzene and partially in the binary mixtures, even at low benzene concentrations. The swelling ratio is also affected by the composition of the dispersed phase of the membrane. From the sorptions of membranes M-2, M-3 and M-4 it is clear that, at a given PAA content, the membrane which contains SBS in the dispersed phase has the highest sorption over the entire concentration range. This is due to the higher solubility of cyclohexane in SBS compared to PS and poly(butyl acrylate) (PBA). In the membranes which contain BA and ST in the dispersed phase, the swelling ratio increases with increasing PBA content. It should be noted that the membranes
with high swelling ratios partially dissolve in benzenecyclohexane mixtures with sufficiently high benzene concentrations. Figs. 4-9 present the equilibrium total and individual sorptions in different membranes, respectively, while Fig. 10 provides the equilibrium sorption selectivities of the membranes as a function of the feed concentration. For all the membranes prepared, the sorption selectivity is greater than unity, indicating that benzene is preferentially absorbed, but decreases monotonically with increasing benzene concentration. The effect of the membrane composition on the sorption selectivity is in the reverse direction to that on the swelling ratio. In order to examine the effect of the feed concentration and polymer composition on the diffusion inside the membrane, the swelling kinetics was investigated. Fig. 11-16 present the dependence of the swelling ratio on the square root of absorption time for various membranes. They show that with increasing benzene concentration, the time needed to reach equilibrium sorption decreases for all the membranes. At a given benzene concentration, the time needed to reach equilibrium sorption decreases with decreasing AA content 8.0
i;
i
J
i
I
6.0
~
><
o
~.4.0
f 2.0
O.
60.0 Square Root of Time (rain 1/2)
80.0
Fig. 14. Dependence of the swelling ratio (S) on the square root of absorption time in different benzene/cyclohexane mixtures for membrane M-4. (~7) cyclohexane; (A) 10 wt% benzene; (O) 30 wt% benzene; (El) 50 wt% benzene; (©) 70 wt% benzene; ( * ) 90 wt% benzene; ( × ) benzene.
F. Sun, E. Ruckenstein / Journal of Membrane Science 99 (1995) 273-284
Square Root of Time (rnin1/2) Fig. 15. Dependence of the swelling ratio (S) on the square root of absorption time in different benzene-cyclohexane mixtures for membrane M-5: ( V ) cyclobexane ( ~ ) 10 wt% benzene; (©) 30 wt% benzene; (r-I) 50 wt% benzene; (©) 70 wt% benzene; ( * ) 90 wt% benzene.
in the continuous phase (M-1 and M-2) (Figs. 11 and 12) and increasing BA content in the dispersed phase (M-2, M-4, M-5, and M-6) (Figs. 12, 14-16). Among the membranes with the same weight contents in the continuous and dispersed phase, but with different additives in the dispersed phase, the one which contains SBS needs a shorter time to reach equilibrium (M-3 and M-4) (Figs. 13 and 14). As already noted in a previous section, the inclusion of BA or SBS in the disperse phase improves the mechanical properties of the membranes (Fig. 2).
3.3. Pervaporation experiments From the above sorption characteristics of the hydrophobic-hydrophilic composite membranes, one can conclude that these membranes have a higher affinity for benzene than for cyclohexane, and hence that they are expected to extract benzene from the benzenecyclohexane mixtures. Figs. 17a and b present the pervaporation performances of the prepared membranes, and indeed, all the membranes are permselective toward benzene over the
281
entire range of feed concentrations. It was found that the permeation rate or normalized flux increases tremendously with increasing benzene concentration in the feed, whereas the separation factor decreases. In agreement with the sorption experiments, the pervaporation performance is affected by the membrane composition, the normalized flux increasing with a decrease of the PAA content in the continuous phase and increase of BA content in the dispersed phase of the membrane. For the membranes with the same dispersed and continuous phase wt. contents, but different additives, the one which contains SBS has the higher flux; the effect on the separation factor is in the reverse direction. Table 3 lists the pervaporation characteristics of a benzene-cyclohexane mixture with 50 wt% benzene at 20°C, through various membranes. M-2 has the largest separation factor (aP=9.57), whereas M-6 has the largest permeation rate (P = 794 g/m2h). It is interesting to note that the permeation rates of the present composite membranes are much higher than those of poly(vinyl fluoride) membranes [19], and that the separation factors are higher than those of polyethylene 6.0
i
~
i
6.0
O
.4.0
2.0
0.0.
20.0 40.0 60.0 Square Root of Time (rain1/2)
80.q
Fig. 16. Dependence of the swelling ratio (S) on the square root of absorption time in different benzene-cyclohexane mixtures for membrane M-6; (~7) cyclohexane; (A) 10 wt% benzene; (O) 30 wt% benzene; (IS]) 50 wt% benzene; (O) 70 wt% benzene; ( * ) 90 wt% benzene.
282
F. Sun, E. Ruckenstein / Journal of Membrane Science 99 (1995) 273-284 100.0 a
G)
80.0
but the permeation rates of the latter membranes are more than one order of magnitude higher. Table 4 lists the pervaporation performances of some membranes.
//S
,"
c
3.4. Effect of temperature on pervaporation Q. t-
.=_
60.0
E G)
~6
40.0
tO
"3 j-
t-
20.0
}'! /
,"
b M-2
//
0.~.'~
-
dM-4
20.0 40.0 60.0 80.0 100.0 Weight fraction of benzene in the mixture (%)
16.0 aM-] bM-2
cM-3 12.0
~
d M-4 eM-5 f M-6
Table 3 Characteristics of the different membranes in the pervaporation of benzene-cyclohexane mixture containing 50% benzene at 20°C
f
/"
/
/
/
E
~
e 8.0
0 z
Fig. 18 presents the effect of the operating temperature on the pervaporation characteristics of some membranes for a benzene-cyclohexane mixture containing 10 wt% benzene. It shows that the normalized flux increases and the separation factor decreases with increasing temperature. This is because an increase of temperature increases the polymer chain mobility and the free volume of the membranes, and hence increases the diffusivity; besides, increasing the temperature, the interactions between the polymer and the components of the liquid become weaker, consequently, the separation factor lower.
b
Membrane d (p.m)
Permeate composition (wt% benzene)
ap
P (g/m2h)
10 2 X J (g/mh)
M-1 M-2 M-3 M-4 M-5 M-6
82.66 90.54 71.16 85.55 75.35 63.10
4.77 9.57 2.47 5.92 3.06 1.71
513 465 535 476 481 794
7.04 4.84 7.06 5.57 6.36 10.72
137 104 132 117 132 135
4.0 Table 4 Comparison of pervaporation performance of benzene-cyclohexane mixture containing 50% benzene for some membrane materials
(b) O~
O.
i
i
i
20.0 40.0 60.0 80.0 100.0 Weight fraction of benzene in the mixture (%)
Membrane
d T ctv (/zm) (°C)
P (g/m2h)
Reference
40 12.5 80.2 ~ 520 465
18 18 19 20 this work
Fig. l 7. Dependence of pervaporation characteristics on the benzene concentration for various membranes at 20°C: ( V ) M-l; ( A ) M-2; (©) M-3; (IS]) M-4; ( ~ ) M-5; ( * ) M-6; (a) weight fraction of benzene in the permeate vs. weight fraction of benzene in the feed; (b) normalized flux.
P/A-50 a 20 30 P/A-30 ~ 20 44 Polyvinylidene fluoridec 25 56 Polyethylene 25 25 M-2 100 20
or polypropylene membranes [20]. Compared to the membranes based on polymeric alloy of polyphosphonates and acetyl cellulose [ 18 ], the separation factor of our composite membranes are somewhat lower,
" P / A - 5 0 represents a polyphosphonate-acetyl cellulose alloy which contains 50 wt% polyphosphonate. b P/A-30 represents a polyphosphonate-acetyl cellulose alloy which contains 30 wt% polyphosphonate. c The membrane was modified with 23% 3-methylsulfolene.
13.3 14.0 5.00 1.632 9.57
F. Sun, E. Ruckenstein / Journal of Membrane Science 99 (1995) 273-284
30.0
~- 20.0
g~ 10.0
(a) l
0.0 0.0
20.0
I
I
30.0 40.0 Temperature (°C)
I
50.0
60.0
3.0
283
method exhibit selective absorption and permeation toward benzene in the benzene-cyclohexane mixtures. Benzene is sorbed in the membrane to a much higher extent than cyclohexane. The sorption increases with increasing benzene concentration in the mixture; for a given benzene concentration, the sorption increases with decreasing PAA content in the continuous phase and increasing butyl acrylate content in the dispersed phase. For membranes with the same dispersed and continuous phase contents, but different additives in the dispersed phase, the one which contains SBS has the higher sorption. The permeation rate or normalized flux increases tremendously and the separation factor decreases with increasing benzene concentration in the feed; the effect of the membrane composition is in the same direction as on the sorption characteristics. The flux increases and the separation factor decreases as the operating temperature is increased. Compared to other types of membranes, large separation factors and particularly high fluxes have been achieved with the hydrophobic-hydrophilic composite membranes. The prepared composite membranes exhibit good mechanical properties and chemical resistance.
d~
E
References
"~ 2.0 0
X
E 1.0 0 Z
(b) J
O.q 0.0
20.0
J
I
30.0 40.0 Temperature (°C)
I
50.0
60.0
Fig. 18. Dependence of pervaporation characteristics of 10 wt% benzene feed on temperature for different membranes: (V) M- l ; (A) M-2; (I-q) M-4; (a) separation factor; (b) normalized flux.
4. Conclusions Polystyrene (PS) and poly(acrylic acid) (PAA) based hydrophobic-hydrophilic composite membranes p r e p a r e d via the c o n c e n t r a t e d e m u l s i o n p o l y m e r i z a t i o n
l l] R. Rautenbach and R. Albrecht, in Membrane Processes, Wiley, New York, 1989, Chap. 12. 12] M.H.V. Mulder, A.C.M. Franken and C.A. Smolders, On the mechanism of separation of ethanol-water mixtures by pervaporation. I. Calculations of concentration profiles, J. Membrane Sci., 17 (1984) 289. 131 M.H.V. Mulder, A.C.M. Franken and C.A. Smolders, On the mechanism of separation of ethanol-water mixtures by pervaporation. 11. Experimental concentration profiles, J. Membrane Sci., 23 (1985) 41. 141 S.M. Zhang, A. Basile and E. Drioli, A Study on polyetheretherketone pervaporation membranes, lnt'l Symp. Membranes Membrane Processes, Hangzhou, China, 1994, p. 205. I51 S.M. Dinh, B. Bemer, Y.M. Sun and P.I. Lee, Sorption and transport of ethanol and water in poly(ethylene-co-vinyl acetate) membranes, J. Membrane Sci., 69 (1992) 223. [6l R.Y.M. Huang and J.W. Rhim, Separation characteristics of pervaporation membrane separation processes using modified poly(vinyl alcohol) membranes, Polym. International, 30 (1993) 123. [7] R.Y.M. Huang and J.W. Rhim, Modification of polY(Vinyl alcohol) using maleic acid and its application to the separation of acetic acid-water mixtures by the pervaporation technique, Polym. International, 30 (1993) 129.
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[ 8 ] H.L. Fleming, Consider membrane pervaporation, Chem. Eng. Prog., 88 (1992) 46. [9] E. Ruckenstein and F.M. Sun, Anomalous sorption and pervaporation of aqueous organic mixtures by poly(vinyl acetal) membranes, J. Membrane Sci., 95 (1994) 207. [ 10] F.M. Sun and E. Rucken~tein, Membranes of block copolymerpoly (divinylbenzene) blends for the pervaporation of alcohol / water mixtures, J. Membrane Sci., 90 (1994) 275. [ 11 ] J. Bai, A.E. Fouda, T. Matsuura and J.D. Hazlett, A study on the pervaporation and performance of polydimethylsiloxanecoated polyetherimide membranes in pervaporation, J. Appl. Polym. Sci., 48 (1993) 999. 112] H.H. Nijhuis, M.H.V. Mulder and C.A. Smolders, Selection of elastomeric membranes for the removal of volatile organics from water, J. Appl. Polym. Sci., 47 (1993) 2227. [ 13] M.E. Hollein, M. Hammond and C.S. Slater, Concentration of dilute acetone water solutions using pervaporation, Sep. Sci. Technol., 28 (1993) 1043. 114] C. Dotremont, S. Goethaert and C. Vandecasteele, Pervaporation behavior of chlorinated hydrocarbons through organophilic membranes, Desalination, 91 ( 1993 ) 177. I151 M.H.V. Mulder, F. Kruitz and C.A. Smolders, Separation of isomeric xylenes by pervaporation through cellulose ester membranes, J. Membrane Sci., 11 (1982) 349.
[ 16] J.S. Park and E. Ruckenstein, Selective permeation through hydrophobic-hydrophilic membranes, J. Appl. Polym. Sci., 38 (1989) 453. [ 17] C.H. Lee, Separation of liquids through polymer membranes. Benzene and cyclohexane system, Sep. Sci. Technol., 16 (1981) 25. [18] 1. Cabasso, J.G. Grodzinski and D. Vofsi, A study of permeation of organic solvents through polymeric membranes based on polymeric alloys of polyphosphonates and acetyl cellulose. 11. Separation of benzene, cyclohexane and cyclohexene, J. Appl. Polym. Sci., 18 (1974) 2137. [ 19] F.P. McCandless, Separation of aromatics and naphthenes by permeation through modified vinylidene fluoride films, Ind. Eng. Chem., Process Des. Dev., 12 (1973) 354. [20] R.Y.M. Huang and V.J.C. Lin, Separation of liquid mixtures by using polymer membranes. I. Permeation of binary organic liquid mixtures through polyethylene, J. Appl. Polym. Sci., 12 (1968) 2615. [21] M. Fels and R.Y.M. Huang, Theoretical interpretation of the effect of mixture composition on separation of liquids in polymers, Macromol. Sci., Sect. B, 5 ( 1971 ) 89. [22] E. Ruckenstein, Emulsion pathway to composite polymeric membranes, Colloid Polym. Sci., 267 (1989) 792.
Journal of Membrane Science 99 ( 1995 ) 273-284
Sorption and pervaporation of benzene-cyclohexane mixtures through composite membranes prepared via concentrated emulsion polymerization Fuming Sun, Eli Ruckenstein* Department of Chemical Engineering, State University of New York at Buffalo, Box 60, Buffalo, NY 14260-4200, USA Received 12 July 1994; accepted in revised form 3 October 1994
Abstract Composite membranes have been prepared starting from concentrated emulsions of styrene in aqueous solutions of acrylic acid as precursors. Each of the phases of the emulsion contained a suitable initiator and the continuous phase contained a dispersant (sodium dodecylsulfate). The polymerization of the precursor generated a composite consisting of particles of dispersed phase imbedded in the continuous phase. To obtain the membranes, the polymer powders generated via grinding were subjected to hot pressing. The mechanical properties of the membranes have been improved by dissolving either butyl acrylate or styrene-butadiene-styrene three block copolymer in styrene. The membranes have been used to separate benzene from benzene-cyclohexane mixtures. The components of the membranes have been selected because polystyrene swells well in benzene and much less in cyclohexane, while the poly(acrylic acid) does not swell well in either of the components and can, therefore, maintain the integrity of the membrane. The mechanical and sorption behaviors of the membranes as well as their pervaporation capabilities were investigated. Keywords: Pervaporation; Benzene-cyclohexane mixture; Composite membranes; Concentrated emulsion polymerization
1. Introduction Pervaporation differs from all the other membrane processes, except membrane distillation, because of the change of the permeate from liquid to vapor. The permeation process consists of selective sorption of the components of the liquid into the membrane, their transport by molecular diffusion through it, and evaporation at the membrane surface since the partial pressures on the permeate side are lower than the vapor pressures of the components at the same side [ 1-3 ]. By utilizing suitable membranes, selected components * Corresponding author. 0376-7388/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI0376-7388(94)00254-1
of a liquid mixture can be efficiently removed or enriched and direct or indirect energy saving achieved. Thus, the development of membrane materials plays an important role in the pervaporation research [ 4 ]. Various hydrophilic membranes have been suggested for the dehydration of organic solvents which have positive azeotropic mixtures with water [5-9]. The finding of organophilic materials which could be employed for the recovery of alcohol or other organic substances from aqueous systems has been also attempted [ 10-14]. Investigations have been also carried out to evaluate the performance of pervaporation in the separation of isomers and close-boiling point mixtures [ 15-21 ].
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F. Sun, E. Ruckenstein / Journal of Membrane Science 99 (1995) 273-284
A novel method to prepare composite membranes from emulsions was developed in our laboratory [22]. The dispersed phase of the emulsion was selected to yield a polymer which swells well in those components for which the membrane should be selective, while the continuous phase was chosen to yield a polymer which does not swell in any of the components of the mixture. Conventional emulsions or microemulsions could be employed to generate such composites. However, the concentrated emulsions, in which the volume fraction of the dispersed phase is very large (larger than 0.74, which represents the most compact arrangement of spheres of equal radii), are most suitable precursors for such membranes, because the thickness of the continuous phase films separating the polyhedral cells of the dispersed phase of the emulsion is small. The membranes have been prepared by polymerizing the emulsion. In a previous paper [ 16], such a membrane was prepared from a concentrated emulsion whose dispersed phase was styrene and the continuous phase was an aqueous solution of acrylamide. Acrylamide alone could not be used because it is not hydrophilic enough to ensure the stability during polymerization at 50°C for 24 h of the concentrated emulsion. The membrane was prepared by spreading the gel-like concentrated emulsion between two parallel plates and subjecting the system to polymerization. While the selectivity for toluene from toluene-cyclohexane mixtures and the permeability of the membrane were good, the mechanical properties of the membranes have not been comTable 1 Amounts of components involved in the preparation of the concentrated emulsionsa Membrane number
M-0 M-1 M-2 M-3 M-4 M-5 M-6
Composition of membranes
pletely satisfactory. In the present paper new membranes were prepared, which had better mechanical properties, and mixtures containing the close boiling point components benzene and cyclohexane subjected to pervaporation. Since the membranes have dispersed phases based on styrene and continuous phases based on acrylic acid, they are expected to be permeable to the aromatic component, which is compatible with styrene, and impermeable to the aliphatic component, which is incompatible with styrene. The thin films of poly(acrylic acid), which are not compatible with either benzene or cyclohexane, ensure the integrity of the membranes.
2. Experimental 2.1. Materials Styrene (ST, ACS reagent, Aldrich), butyl acrylate (BA, ACS reagent, Aldrich) and acrylic acid (AA, ACS reagent, Aldrich) were purified by distillation under reduced pressure; the initiators azobisisobutyronitrile (AIBN, ACS reagent, Kodak) and ammonium persulfate [ (NH4)2S208, ACS reagent, Aldrich] were purified by recrystallization from methanol and water, respectively. Styrene-butadiene-styrene three block copolymer (SBS, Aldrich) containing 28 wt% styrene, the dispersant sodium dodecylsulfate (SDS, ACS reagent, Fluka), benzene (HPLC grade, Aldrich), cyclohexane (HPLC grade, Aldrich) and methanol (MeOH, 99.9%, Aldrich) were used as received. Water was distilled and deionized. 2.2. Preparation of the concentrated emulsion
Disperse phase
Continuous phase
ST 20.0 g ST 20.0 g ST 20.0 g ST/SBS (95/5 wt/wt) 20.0 g ST/BA (95/5 wt/wt) 20.0 g ST/BA (90/10 wt/wt) 20.0g ST/BA (80/20 wt/wt) 20.0 g
AA AA AA AA AA AA
0.4 g 1.0 g 1.0 g 1.0 g 1.0g 1.0 g
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 (NH4)zS208 (0.001 g / g AA), the surfactant was SDS ( 1.050 g).
A small amount of an aqueous solution of acrylic acid containing sodium dodecylsulfate (SDS) and the initiator ammonium persulfate was placed in a 250 ml three-neck flask equipped with a mechanical stirrer. Styrene, a mixture of styrene and butyl acrylate, or a solution of SBS in styrene, containing the initiator AIBN, was injected into the mixture under stirring (600-650 rpm). Then the flask was heated at 50°C in an oil bath under stirring; and a nitrogen gas purging for 1/2 to 2 h, depending upon the viscosity of the resulting gel, was employed. (The heating of the flask
F. Sun, E. Ruckenstein / Journal of Membrane Science 99 (1995) 273-284
A
275
G
E
Figure 1-b
Figure 1-a
Fig. 1. The pervaporation apparatus: (A) permeation cell, (B) membrane, (C) sintered glass support, (D) entering mixture, (E) band heater, (F) mechanical stirrer, (G) switch valve, (H) vent to atmosphere, (l) collection trap, (J) safety trap, (K) Pirani gauge, (L) vacuum pump.
i
i
//
40.0
a
i
2.3. Preparation of the membranes
i
d
The obtained polymer composites were washed with methanol in an extractor for 24 h, dried over night at the ambient temperature, and then further dried at 50°C in a vacuum oven. After grinding, the fine composite powders were pressed at 150°C. The membranes were further dried at 50°C in a vacuum oven for 2 days before use. The thickness of the membrane was in the range of 104 to 137/zm.
o
/
30.0
,m 20.0
°
t
2.4. Measurement of stress-strain behavior aM-1
10,0
b M-2
e M-5 f M-5
0.0
0.0
2.0
4.0 6.0 Strain ~ (%)
8.0
10.0
Fig. 2. Stress-strain behavior of membranes.
was necessary, because without this heating treatment, the stability of the gel could not be ensured during the subsequent polymerization at 50°C for 60 h). The slightly polymerized emulsion was then transferred to a tube of 20 ml capacity, which was sealed with a septum. To avoid the presence of oxygen, nitrogen gas was passed through the tube for 20 min. Polymerization was conducted in a temperature-controlled water bath (50°C) for 60 h. The amounts of components involved in the preparation are listed in Table 1.
The stress-strain behavior of the composite membranes was measured using an Instron Testing Instrument (model 1000), the extension rate being 10 ram/ min. Table 2 The stress-strain behavior of membranesa Membrane
E (MPa)
o-y (MPa)
ob (MPa)
~b (%)
M-I M-2 M-3 M-4 M-5 M-6
1999.00 1334.67 532.14 921.90 744.00 573.38
39.98 40.02 37.25 38.72 33.48 31.56
39.98 40.02 37.25 38.72 33.48 31.56
2.50 3.00 7.00 4.20 4.50 5.50
"E, o'y, o-b and % stand for Young's modulus, yield stress, breaking stress and breaking strain, respectively.
F. Sun, E. Ruckenstein / Journal of Membrane Science 99 (1995) 273-284
276
8.0
i
i
i
i
c
6.0
a
d
,
0
4.0
Cap Carbowax (30 m × 0.32 mm i.d. × 0.25/xm) capillary column (Alltech) and with a flame ionization detector; helium was used as the carrier. The column, injector and detector temperatures were 50, 260 and 300°C, respectively. Two quantities can be used to characterize the equilibrium sorption properties: ( 1) the equilibrium swelling ratio S¢, and (2) the equilibrium sorption selectivity a s, defined by
w~-wd Wd
Se
E= "~ 2.0
aM-i
~.
b M-2 c M-3 d M~
Iii
e M-5 f M-6 i
I
I
I
0.0 ).=0 20.0 40.0 60.0 80.0 100.0 Weight fraction of benzene in the mixture (%)
Fig. 3. Equilibrium sorption as a function of benzene concentration in the mixture at 20°C in various membranes: ( V ) M-I; ( A ) M-2; ( 0 ) M-3; (1-1) M-4; (<)) M-5; ( * ) M-6.
as = Cb/ C~
x./xc where Wd and Wc denote the weights of the dry and equilibrium swollen membrane strips, respectively, Ch and Cc represent the weight fractions of benzene and cyclohexane at equilibrium in the membrane and Xb and X~ those in the feed, respectively.
2.5. Sorption kinetic experiments Strips of dry membranes (about 120/xm thick and 100 mg weight) were immersed in various benzenecyclohexane mixtures at ambient temperature and rapidly removed from the mixtures at various times between 16 to 6400 min, wiped off with tissue paper to remove the surface adherent liquid and weighed on a Mettler semimicrobalance. The swelling ratio is defined as
S
w~-w~ %
where Wo and Ws denote the weights of the dry and swollen samples, respectively.
8.0
et~
"~ 6.0 E
o
4.0
E 2.0
2.6. Equilibrium membrane sorption experiments
g
UJ
After the sorptions reached equilibrium, the membranes were removed from the mixtures, the liquid on their surface wiped off with tissue paper, weighed as quickly as possible, and then placed into a dry flask connected to a cold trap and a vacuum pump. The collected liquids were analyzed with a Hewlett Packard 5890A gas chromatograph equipped with an Econo-
o.o~
20.0 40.0 60.0 Weight fraction of benzene in the mixture (%)
Fig. 4. Equilibrium total and individual sorptions for membrane Ml as a function of benzene concentration in the mixture at 20°C: ( O ) total sorption; (/X) benzene sorption; (V) cyclohexane sorption.
F. Sun, E. Ruckenstein / Journal of Membrane Science 99 (1995) 273-284
2.7. Pervaporation
8.0
-~ 6.0 E
cn
._o
4.0
==
E ._~
.~ 2.0 I.IJ
0.0
277
20.0 40.0 60.0 80.0 1( Weight fraction of benzene in the mixture (%)
Fig. 5. Equilibrium total and individual sorptions for membrane M2 a,s a function of benzene concentration in the mixture at 20°C: (O) total sorption; ( A ) benzene sorption; ( V ) cyclohexane sorption.
Fig. 1 presents the pervaporation apparatus. The membrane with an effective surface area of 9.6 cm 2 was inserted in the pervaporation cell and supported on a sintered glass disk. About 250 ml of feed mixture was introduced in the upstream compartment, and the downstream pressure (3 + 1 torr) was maintained with a vacuum pump. A band heater mounted on the cell and a mechanical stirrer were used to control the feed temperature which was varied between 20 and 50°C. The pervaporation vapor was condensed in a cold trap of liquid nitrogen after 2 h of running the apparatus at the operating temperature. The amount of permeate collected during a run was kept small compared with the amount of feed, hence the composition of the feed can be assumed to remain constant throughout a run. The trap filled with permeate was warmed up to the ambient temperature, the permeate removed, weighed, and analyzed with the gas chromatograph. The membrane performance was measured in terms of two quantities: ( 1 ) the separation factor a p, and (2) the permeation rate (P), expressed in g / m 2 h, or the normalized flux (J), expressed in g / m h, defined by
8.0
8.0
~ ,
i
I
i
!
|
I
6.0 E /
/
/
/
/
/
/
6.0 E
d o
~ 4.0
.£
.~_
==
ig 2.0
"~ 2.0
4.0
E
g U.I
0.0
) 20.0 40.0 60.0 80.0 Weight fraction of benzene in the mixture (%)
Fig. 6. Equilibrium total and individual sorptions for membrane M3 as a function of benzene concentration in the mixture at 20°C: (O) total sorption; ( A ) benzene sorption; (~7) cyclohexane sorption.
0.0~
. . . . ' 20.0 40,0 60.0 80.0 10 .0 Weight fraction of benzene in the mixture (%
Fig. 7. Equilibrium total and individual sorptions for membrane M4 as a function of benzene concentration in the mixture at 20°C: ( O ) total sorption; ( A ) benzene sorption; (V) cyclohexane sorption.
278
F. Sun, E. Ruckenstein / Journal of Membrane Science 99 (1995) 273-284
ozP =
YblY~ XhlX~
W A.t J=
8.0
== c~
~
6.0
W.d A.t co
where Yb and Y~represent the weight fractions of benzene and cyclohexane in the permeate and Xb and Xc those in the feed, respectively, and W, A, t and d represent the weight of the permeate (g), the effective membrane area (m2), the operating time (h) and the membrane thickness (m), respectively. The pervaporation experiments were carried out between 20 and 50°C.
o
4.o
N 2.0 W
0.0
3. Results and discussion
3.1. Mechanical properties The hydrophobic-hydrophilic composite membranes prepared in a previous paper[ 16] by the concentrated emulsion polymerization exhibited high efficiency in the separation of aromatic-aliphatic mixtures. In that paper, the membrane was prepared by spreading the gel-like concentrated emulsion of styrene dispersed in an aqueous solution of acrylamide between two parallel plates and subjecting the system to polymerization. While the selectivity for toluene from the toluene-cyclohexane mixture and the permeability of the membrane were good, the mechanical properties of the membranes have not been completely satisfactory, since they could not be used repeatedly. The poor mechanical properties were caused by the presence of the surfactant and by the brittleness of the polystyrenepolyacrylamide composite. In the present paper, the membranes were prepared by hot-pressing fine powders of composites from which most of the surfactant was extracted by washing with methanol; the mechanical properties were somewhat improved by this heating treatment. The addition of butyl acrylate or SBS to styrene made the membranes less brittle. This is because the copolymer formed in the first case has side chains which impede the compact packing of the polymer molecules, thus generating some flexibility. Sim-
) 20.0 40.0 60.0 80.0 Weight fraction of benzene in the mixture (%)
Fig. 8. Equilibrium total and individual sorptions for membrane M5 as a function of benzene concentration in the mixture at 20°C: (O) total sorption; ( A ) benzene sorption; (V) cyclohexane sorption.
ilarly, the molecules of SBS do not allow the compact packing of the polymer molecules. Fig. 2 and Table 2 present the stress-strain behavior of the membranes. 3.2. Sorption capacity In pervaporation, the driving force for transport is the concentration difference across the membrane. The transport process consists of three steps: (i) sorption into the membrane at the upstream side, (ii) diffusion through the membrane, and (iii) desorption into a vapor phase at the down stream side. The solubility (sorption) is a thermodynamic and the diffusivity a kinetic property. Benzene is a good solvent for polystyrene (PS), cyclohexane is a nonsolvent for PS, while both benzene and cyclohexane are nonsolvents for poly (acrylic acid) (PAA). The sorptions of these compounds in the PS/ PAA based polymer composites, prepared via the concentrated emulsion polymerization, are therefore expected to differ considerably. Fig. 3 shows the equi-
279
F. Sun, E. Ruckenstein / Journal of Membrane Science 99 (1995) 273-284
8.0 / 8.0
i
i
i
'
'
'
i
6.0 R
i
[]
6.0
ft, 4.0 J-//
~
2.0
0.0
0 Square Rootof Time (rainI/2)
2010
8;.o
'
Weight fraction of benzene in the mixture (%) Fig. 9. Equilibrium total and individual sorptions for membrane M6 as a function of benzene concentration in the mixture at 20°C: ( 0 ) total sorption; (/~ ) benzene sorption; (~7) cyclohexane sorption.
Fig. 11. Dependence of the swelling ratio (S) on the square root of absorption time in different benzene/cyclohexane mixtures for membrane M-l: (~7) cyclohexane; (A) 10 wt% benzene; ((3) 30 wt% benzene; (D) 50 wt% benzene; (O) 70 wt% benzene.
8.0 l 2.5
b~
M-2 M-3 M-4 M-5 M-6
~ 2.0
6.0 ~-
~
>
'
'
/
=M-1 b c d • f
'
X
>(
4.0
c o
n
o 1.5 c f
2
"
0
~
8
0.0 1.0
.
.
.
.
O.O 20.0 40.0 60.0 80.0 100.O Weight fraction of benzene in the mixture (%)
Fig. 10. Dependence of sorption selectivity on the benzene concentration in various membranes: (V) M-I; (A) M-2; (O) M-3; (Fq) M-4; (O) M-5; ( * ) M-6
O.O SquareRootof Time(rain1/2)
Fig. 12. Dependence of the sorption ratio (S) on the square root of absorption time in different benzene/cyclohexane mixtures for membrane M-2: (~7) cyclohexane; (A) 10 wt% benzene; (©) 30 wt% benzene; ([]) 50 wt% benzene; ( ~ ) 70 wt% benzene; ( * ) 90 wt% benzene; ( X ) benzene.
280
F. Sun, E. Ruckenstein / Journal of Membrane Science 99 (1995) 273-284
8.0
i
6.0
~
i
i
/
4.0
¢.0
2.0
0.01m
03
v*
I
J
t
20.0 40.0 60.0 Square Root of Time (min 1/2)
80.0
Fig. 13. Dependence of the swelling ratio (S) on the square root of absorption time in different benzene-cyclohexane mixtures for membrane M-3: ( ~ ) cyclohexane; (A) 10 wt% benzene; (O) 30 wt% benzene; ([]) 50 wt% benzene; ( ~ ) 70 wt% benzene; ( * ) 90 wt% benzene.
librium sorptions for various membranes. The membranes have similar sorption characteristics, the equilibrium sorption increasing monotonically with increasing benzene concentration. However, when the latter concentration becomes sufficiently high, some membranes partially dissolve. Comparing the behavior of the membranes, one can see that the sorption increases with decreasing PAA content, because of the increase of the hydrophobicity in the composite (membranes M-1 and M-2). When the membrane contains only PS (M-0), it dissolves in benzene and partially in the binary mixtures, even at low benzene concentrations. The swelling ratio is also affected by the composition of the dispersed phase of the membrane. From the sorptions of membranes M-2, M-3 and M-4 it is clear that, at a given PAA content, the membrane which contains SBS in the dispersed phase has the highest sorption over the entire concentration range. This is due to the higher solubility of cyclohexane in SBS compared to PS and poly(butyl acrylate) (PBA). In the membranes which contain BA and ST in the dispersed phase, the swelling ratio increases with increasing PBA content. It should be noted that the membranes
with high swelling ratios partially dissolve in benzenecyclohexane mixtures with sufficiently high benzene concentrations. Figs. 4-9 present the equilibrium total and individual sorptions in different membranes, respectively, while Fig. 10 provides the equilibrium sorption selectivities of the membranes as a function of the feed concentration. For all the membranes prepared, the sorption selectivity is greater than unity, indicating that benzene is preferentially absorbed, but decreases monotonically with increasing benzene concentration. The effect of the membrane composition on the sorption selectivity is in the reverse direction to that on the swelling ratio. In order to examine the effect of the feed concentration and polymer composition on the diffusion inside the membrane, the swelling kinetics was investigated. Fig. 11-16 present the dependence of the swelling ratio on the square root of absorption time for various membranes. They show that with increasing benzene concentration, the time needed to reach equilibrium sorption decreases for all the membranes. At a given benzene concentration, the time needed to reach equilibrium sorption decreases with decreasing AA content 8.0
i;
i
J
i
I
6.0
~
><
o
~.4.0
f 2.0
O.
60.0 Square Root of Time (rain 1/2)
80.0
Fig. 14. Dependence of the swelling ratio (S) on the square root of absorption time in different benzene/cyclohexane mixtures for membrane M-4. (~7) cyclohexane; (A) 10 wt% benzene; (O) 30 wt% benzene; (El) 50 wt% benzene; (©) 70 wt% benzene; ( * ) 90 wt% benzene; ( × ) benzene.
F. Sun, E. Ruckenstein / Journal of Membrane Science 99 (1995) 273-284
Square Root of Time (rnin1/2) Fig. 15. Dependence of the swelling ratio (S) on the square root of absorption time in different benzene-cyclohexane mixtures for membrane M-5: ( V ) cyclobexane ( ~ ) 10 wt% benzene; (©) 30 wt% benzene; (r-I) 50 wt% benzene; (©) 70 wt% benzene; ( * ) 90 wt% benzene.
in the continuous phase (M-1 and M-2) (Figs. 11 and 12) and increasing BA content in the dispersed phase (M-2, M-4, M-5, and M-6) (Figs. 12, 14-16). Among the membranes with the same weight contents in the continuous and dispersed phase, but with different additives in the dispersed phase, the one which contains SBS needs a shorter time to reach equilibrium (M-3 and M-4) (Figs. 13 and 14). As already noted in a previous section, the inclusion of BA or SBS in the disperse phase improves the mechanical properties of the membranes (Fig. 2).
3.3. Pervaporation experiments From the above sorption characteristics of the hydrophobic-hydrophilic composite membranes, one can conclude that these membranes have a higher affinity for benzene than for cyclohexane, and hence that they are expected to extract benzene from the benzenecyclohexane mixtures. Figs. 17a and b present the pervaporation performances of the prepared membranes, and indeed, all the membranes are permselective toward benzene over the
281
entire range of feed concentrations. It was found that the permeation rate or normalized flux increases tremendously with increasing benzene concentration in the feed, whereas the separation factor decreases. In agreement with the sorption experiments, the pervaporation performance is affected by the membrane composition, the normalized flux increasing with a decrease of the PAA content in the continuous phase and increase of BA content in the dispersed phase of the membrane. For the membranes with the same dispersed and continuous phase wt. contents, but different additives, the one which contains SBS has the higher flux; the effect on the separation factor is in the reverse direction. Table 3 lists the pervaporation characteristics of a benzene-cyclohexane mixture with 50 wt% benzene at 20°C, through various membranes. M-2 has the largest separation factor (aP=9.57), whereas M-6 has the largest permeation rate (P = 794 g/m2h). It is interesting to note that the permeation rates of the present composite membranes are much higher than those of poly(vinyl fluoride) membranes [19], and that the separation factors are higher than those of polyethylene 6.0
i
~
i
6.0
O
.4.0
2.0
0.0.
20.0 40.0 60.0 Square Root of Time (rain1/2)
80.q
Fig. 16. Dependence of the swelling ratio (S) on the square root of absorption time in different benzene-cyclohexane mixtures for membrane M-6; (~7) cyclohexane; (A) 10 wt% benzene; (O) 30 wt% benzene; (IS]) 50 wt% benzene; (O) 70 wt% benzene; ( * ) 90 wt% benzene.
282
F. Sun, E. Ruckenstein / Journal of Membrane Science 99 (1995) 273-284 100.0 a
G)
80.0
but the permeation rates of the latter membranes are more than one order of magnitude higher. Table 4 lists the pervaporation performances of some membranes.
//S
,"
c
3.4. Effect of temperature on pervaporation Q. t-
.=_
60.0
E G)
~6
40.0
tO
"3 j-
t-
20.0
}'! /
,"
b M-2
//
0.~.'~
-
dM-4
20.0 40.0 60.0 80.0 100.0 Weight fraction of benzene in the mixture (%)
16.0 aM-] bM-2
cM-3 12.0
~
d M-4 eM-5 f M-6
Table 3 Characteristics of the different membranes in the pervaporation of benzene-cyclohexane mixture containing 50% benzene at 20°C
f
/"
/
/
/
E
~
e 8.0
0 z
Fig. 18 presents the effect of the operating temperature on the pervaporation characteristics of some membranes for a benzene-cyclohexane mixture containing 10 wt% benzene. It shows that the normalized flux increases and the separation factor decreases with increasing temperature. This is because an increase of temperature increases the polymer chain mobility and the free volume of the membranes, and hence increases the diffusivity; besides, increasing the temperature, the interactions between the polymer and the components of the liquid become weaker, consequently, the separation factor lower.
b
Membrane d (p.m)
Permeate composition (wt% benzene)
ap
P (g/m2h)
10 2 X J (g/mh)
M-1 M-2 M-3 M-4 M-5 M-6
82.66 90.54 71.16 85.55 75.35 63.10
4.77 9.57 2.47 5.92 3.06 1.71
513 465 535 476 481 794
7.04 4.84 7.06 5.57 6.36 10.72
137 104 132 117 132 135
4.0 Table 4 Comparison of pervaporation performance of benzene-cyclohexane mixture containing 50% benzene for some membrane materials
(b) O~
O.
i
i
i
20.0 40.0 60.0 80.0 100.0 Weight fraction of benzene in the mixture (%)
Membrane
d T ctv (/zm) (°C)
P (g/m2h)
Reference
40 12.5 80.2 ~ 520 465
18 18 19 20 this work
Fig. l 7. Dependence of pervaporation characteristics on the benzene concentration for various membranes at 20°C: ( V ) M-l; ( A ) M-2; (©) M-3; (IS]) M-4; ( ~ ) M-5; ( * ) M-6; (a) weight fraction of benzene in the permeate vs. weight fraction of benzene in the feed; (b) normalized flux.
P/A-50 a 20 30 P/A-30 ~ 20 44 Polyvinylidene fluoridec 25 56 Polyethylene 25 25 M-2 100 20
or polypropylene membranes [20]. Compared to the membranes based on polymeric alloy of polyphosphonates and acetyl cellulose [ 18 ], the separation factor of our composite membranes are somewhat lower,
" P / A - 5 0 represents a polyphosphonate-acetyl cellulose alloy which contains 50 wt% polyphosphonate. b P/A-30 represents a polyphosphonate-acetyl cellulose alloy which contains 30 wt% polyphosphonate. c The membrane was modified with 23% 3-methylsulfolene.
13.3 14.0 5.00 1.632 9.57
F. Sun, E. Ruckenstein / Journal of Membrane Science 99 (1995) 273-284
30.0
~- 20.0
g~ 10.0
(a) l
0.0 0.0
20.0
I
I
30.0 40.0 Temperature (°C)
I
50.0
60.0
3.0
283
method exhibit selective absorption and permeation toward benzene in the benzene-cyclohexane mixtures. Benzene is sorbed in the membrane to a much higher extent than cyclohexane. The sorption increases with increasing benzene concentration in the mixture; for a given benzene concentration, the sorption increases with decreasing PAA content in the continuous phase and increasing butyl acrylate content in the dispersed phase. For membranes with the same dispersed and continuous phase contents, but different additives in the dispersed phase, the one which contains SBS has the higher sorption. The permeation rate or normalized flux increases tremendously and the separation factor decreases with increasing benzene concentration in the feed; the effect of the membrane composition is in the same direction as on the sorption characteristics. The flux increases and the separation factor decreases as the operating temperature is increased. Compared to other types of membranes, large separation factors and particularly high fluxes have been achieved with the hydrophobic-hydrophilic composite membranes. The prepared composite membranes exhibit good mechanical properties and chemical resistance.
d~
E
References
"~ 2.0 0
X
E 1.0 0 Z
(b) J
O.q 0.0
20.0
J
I
30.0 40.0 Temperature (°C)
I
50.0
60.0
Fig. 18. Dependence of pervaporation characteristics of 10 wt% benzene feed on temperature for different membranes: (V) M- l ; (A) M-2; (I-q) M-4; (a) separation factor; (b) normalized flux.
4. Conclusions Polystyrene (PS) and poly(acrylic acid) (PAA) based hydrophobic-hydrophilic composite membranes p r e p a r e d via the c o n c e n t r a t e d e m u l s i o n p o l y m e r i z a t i o n
l l] R. Rautenbach and R. Albrecht, in Membrane Processes, Wiley, New York, 1989, Chap. 12. 12] M.H.V. Mulder, A.C.M. Franken and C.A. Smolders, On the mechanism of separation of ethanol-water mixtures by pervaporation. I. Calculations of concentration profiles, J. Membrane Sci., 17 (1984) 289. 131 M.H.V. Mulder, A.C.M. Franken and C.A. Smolders, On the mechanism of separation of ethanol-water mixtures by pervaporation. 11. Experimental concentration profiles, J. Membrane Sci., 23 (1985) 41. 141 S.M. Zhang, A. Basile and E. Drioli, A Study on polyetheretherketone pervaporation membranes, lnt'l Symp. Membranes Membrane Processes, Hangzhou, China, 1994, p. 205. I51 S.M. Dinh, B. Bemer, Y.M. Sun and P.I. Lee, Sorption and transport of ethanol and water in poly(ethylene-co-vinyl acetate) membranes, J. Membrane Sci., 69 (1992) 223. [6l R.Y.M. Huang and J.W. Rhim, Separation characteristics of pervaporation membrane separation processes using modified poly(vinyl alcohol) membranes, Polym. International, 30 (1993) 123. [7] R.Y.M. Huang and J.W. Rhim, Modification of polY(Vinyl alcohol) using maleic acid and its application to the separation of acetic acid-water mixtures by the pervaporation technique, Polym. International, 30 (1993) 129.
284
F. Sun, E. Ruckenstein / Journal of Membrane Science 99 (1995) 273-284
[ 8 ] H.L. Fleming, Consider membrane pervaporation, Chem. Eng. Prog., 88 (1992) 46. [9] E. Ruckenstein and F.M. Sun, Anomalous sorption and pervaporation of aqueous organic mixtures by poly(vinyl acetal) membranes, J. Membrane Sci., 95 (1994) 207. [ 10] F.M. Sun and E. Rucken~tein, Membranes of block copolymerpoly (divinylbenzene) blends for the pervaporation of alcohol / water mixtures, J. Membrane Sci., 90 (1994) 275. [ 11 ] J. Bai, A.E. Fouda, T. Matsuura and J.D. Hazlett, A study on the pervaporation and performance of polydimethylsiloxanecoated polyetherimide membranes in pervaporation, J. Appl. Polym. Sci., 48 (1993) 999. 112] H.H. Nijhuis, M.H.V. Mulder and C.A. Smolders, Selection of elastomeric membranes for the removal of volatile organics from water, J. Appl. Polym. Sci., 47 (1993) 2227. [ 13] M.E. Hollein, M. Hammond and C.S. Slater, Concentration of dilute acetone water solutions using pervaporation, Sep. Sci. Technol., 28 (1993) 1043. 114] C. Dotremont, S. Goethaert and C. Vandecasteele, Pervaporation behavior of chlorinated hydrocarbons through organophilic membranes, Desalination, 91 ( 1993 ) 177. I151 M.H.V. Mulder, F. Kruitz and C.A. Smolders, Separation of isomeric xylenes by pervaporation through cellulose ester membranes, J. Membrane Sci., 11 (1982) 349.
[ 16] J.S. Park and E. Ruckenstein, Selective permeation through hydrophobic-hydrophilic membranes, J. Appl. Polym. Sci., 38 (1989) 453. [ 17] C.H. Lee, Separation of liquids through polymer membranes. Benzene and cyclohexane system, Sep. Sci. Technol., 16 (1981) 25. [18] 1. Cabasso, J.G. Grodzinski and D. Vofsi, A study of permeation of organic solvents through polymeric membranes based on polymeric alloys of polyphosphonates and acetyl cellulose. 11. Separation of benzene, cyclohexane and cyclohexene, J. Appl. Polym. Sci., 18 (1974) 2137. [ 19] F.P. McCandless, Separation of aromatics and naphthenes by permeation through modified vinylidene fluoride films, Ind. Eng. Chem., Process Des. Dev., 12 (1973) 354. [20] R.Y.M. Huang and V.J.C. Lin, Separation of liquid mixtures by using polymer membranes. I. Permeation of binary organic liquid mixtures through polyethylene, J. Appl. Polym. Sci., 12 (1968) 2615. [21] M. Fels and R.Y.M. Huang, Theoretical interpretation of the effect of mixture composition on separation of liquids in polymers, Macromol. Sci., Sect. B, 5 ( 1971 ) 89. [22] E. Ruckenstein, Emulsion pathway to composite polymeric membranes, Colloid Polym. Sci., 267 (1989) 792.