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Composite membranes prepared by concentrated emulsion polymerization and their use for pervaporation separation of water-acetic acid mixtures
Journal of Membrane Science, 66 (1992) 205-210
205
Elsevier Science Publishers B.V., Amsterdam
Composite membranes prepared by concentrated emulsion polymerization and their use for pervaporation separation of water-acetic acid mixtures E. Ruckenstein
and H.H. Chen
Department of Chemical Engineering, State University of New York at Buffalo, Buffalo, NY 14260 (USA)
(Received June 24,199l; accepted in revised form September 30,199l)
Abstract A composite membrane prepared by the concentrated emulsion polymerization method, containing poly (sodium acrylate ) as the dispersed phase and poly (divinyl benzene ) as the continuous phase, was investigated regarding its swelling as well as the pervaporation separation of water-acetic acid mixtures. The experiments were carried out in order to be compared with the pervaporation separation of waterethanol mixtures performed with the same membrane [J. Appl. Polym. Sci., 42 (1991) 2429 1. The swelling of this membrane was found to be higher for acetic acid solutions than for ethanol solutions at the same mixture concentration. The permeation rates through this membrane for the water-acetic acid mixtures were in the range of 0.41-2.36 kg/ m2-hr, and the permselectivity varied between 3.4 and 11.6. Compared with the separation of water-ethanol mixtures, the present experiments show much higher permeation rates and lower permselectivities. Keywords: concentrated emulsion polymerization; pervaporation; composite membranes; acrylate)-poly (divinyl benzene); separation of water-acetic acid mixtures
Introduction Previously, we prepared a new kind of composite membrane [l] via the concentrated emulsion polymerization method [ 21. The membrane was used for separating water from water-ethanol mixtures by pervaporation. A concentrated emulsion has a high volume fraction of dispersed phase (usually 80-95 vol. % ), and the appearance of a semi-solid gel. A large volume fraction of an aqueous sodium acrylate Correspondence to: E. Ruckenstein, Department of Chemical Engineering, State University of New York at Buffalo, Buffalo, NY 14260 (USA).
0376-7388/92/$05.00
poly (sodium
solution was dispersed in a small amount of divinyl benzene, each of the two phases containing a suitable initiator and the continuous phase containing also a surfactant. The concentrated emulsion thus obtained was sandwiched between two glass plates and subjected to polymerization at 45 oC, and subsequently to drying. In this composite membrane, poly (divinyl benzene) is in the form of a network of thin films which separate polyhedral cells of poly (sodium acrylate) when the volume fraction of the dispersed phase is sufficiently large. This membrane exhibited high permselectivity to water from a water-ethanol mixture. The permeation rate of a water-ethanol mixture was in the range
0 1992 Elsevier Science Publishers B.V. All rights reserved.
E. RUCKENSTEIN AND H.H. CHEN
of 96-560 g/m2-hr, and the permselectivity varied between 32 and 235 in favor of water. In the present paper experimental results will be presented regarding the pervaporation separation of the water-acetic acid mixtures using the same membrane, namely the poly (sodium acrylate )-poly (divinyl benzene) composite membrane. This is of interest for various reasons: Firstly, acetic acid ranks among the top 20 organic intermediates in the chemical industry [ 31. In addition the separation of acetic acid from water by distillation is an energy-expensive process [ 31. It is thus desirable to develop a new separation process for saving energy; the pervaporation membrane separation technique is a potential candidate for this purpose. Secondly, the pervaporation separation of water-ethanol and water-acetic acid mixtures by the same membrane can yield some useful comparisons. One might be tempted to expect that the results would be similar, because the hydrophilicities of ethanol and acetic acid are comparable. Our experiments reveal, however, different results. The two monomers, sodium acrylate and divinyl benzene, have been selected for the preparation of the membranes because water is much more soluble in poly(sodium acrylate) than acetic acid, and neither water nor acetic acid are soluble in poly (divinyl benzene). Consequently, the dispersed phase can dissolve a large amount of liquid much richer in water than the feed, while the continuous phase being insoluble in both components maintains the integrity of the membrane.
Preparation of the concentrated emulsion A small amount of divinyl benzene (Aldrich) containing sorbitane monooleate (Fluka) as surfactant and AIBN (Alfa) as initiator was placed in a round bottom flask (100 ml capacity) equipped with a mechanical stirrer and an addition funnel. An aqueous solution of sodium acrylate containing potassium persulfate (Aldrich) as initiator was introduced in the addition funnel. The solution of sodium acrylate was prepared by titrating the acrylic acid with a solution of sodium hydroxide to a pH slightly above 7 to ensure the stability of the concentrated emulsion which is prepared in the next step [ 11. The pH of the sodium acrylate solution should not be much higher than 7 in order to avoid the decomposition of potassium persulfate in the alkaline environment [ 41. The concentrated emulsions were prepared at room temperature by the dropwise addition of the sodium acrylate solution to the stirred mixture in the flask. The addition process lasted ca. 25 min. The amounts employed in various membranes (Ml, M2, and M3) are listed in Table 1. TABLE 1 Amounts of components used in the preparation of the concentrated emulsions (in g )
Dispersed phase aqueous solution of sodium acrylate acrylic acid content in aqueous solution initiator (K&O,)
Experimental The materials used as well as the experimental procedures were described previously [ 11. Since there are slight differences in the experimental conditions, some details are included in the following.
Continuous phase divinyl benzene initiator (AIBN)
Surfactant sorb&an monooleate
Ml
M2
M3
30
30
30 (water)
8
8
0
1.7 x 10e4 g/g of acrylic acid for Ml and M2
2 4 4 2.0 X low4 g/g of monomer for all emulsions
1
1
I
COMPOSITE MEMBRANES PREPARED BY CONCENTRATED EMULSION POLYMERIZATION
Preparation of the membrane The concentrated emulsion was transferred into a syringe of 10 ml capacity. Different centrifugation conditions, dependent upon the viscosity of the concentrated emulsion, were employed to eliminate the minute air bubbles introduced in the concentrated emulsion during preparation (M3 and M2: 2000 rpm, 5 min; Ml: 2500 rpm, 10 min). Two glass plates (10 cm x 15 cm) were dried in the oven before use. A small amount of glycerol was placed as a lubricant on the surface of the glass plates. The centrifuged concentrated emulsions were injected carefully on one of the glass plates and squeezed slowly with the other plate to avoid trapping air bubbles. For polymerization to occur, the glass plates sandwiching the concentrated emulsion were placed in a temperaturecontrolled oven at 45°C for 24 hr. After preparation, the membrane was dried at 45 ‘C in a circulating oven for 24 hr. The thickness of the membrane was approx. 0.3 mm. The membrane, which was brittle at room temperature and in dry air, became softer and elastic in a high-humidity environment. The swelling test The change in weight of the membrane produced by swelling was determined as follows: membrane strips of ca. 0.5 g were dried again in a circulating oven at room temperature (23 ‘C ) and the weight of the dried membrane was determined. Then, the membrane strips were placed in flasks filled with different wateracetic acid and water-ethanol mixtures, at room temperature. After different periods of time, the weight was determined. The swelling ratio of the membrane is defined as: S =
w,-w, w,
where W, and W, represent dry and swollen membrane,
the weights of the respectively.
207
Pervaporation experiments Pervaporation studies of water-acetic acid mixtures were carried out by employing the apparatus used in a previous paper [ 51. The pervaporation cell has a capacity of 250 ml, and the membrane area in contact with the feed solution was about 9 cm2. In the upstream compartment, the liquid mixture was under stirring, while the downstream compartment was subjected to a vacuum of 3 20.5 Torr. The membranes, swollen for 60, 30, and 15 min in 75,50, and 25 wt.% acetic acid solutions, were placed in the pervaporation cell. The times of swelling have been chosen shorter for those membranes for which S was higher, and longer for those for which S was smaller. The permeated vapor was collected in a liquid nitrogen cold trap only after running the apparatus for 5 hr at room temperature. This blank run was carried out to ensure the complete removal of the water-acetic mixture accumulated in the lower part of the membrane during its initial swelling, which otherwise will be evacuated to the cold trap. Experiments were conducted at room temperature (ca. 23 ’ C ) and between 30 and 50°C in order to investigate the temperature dependence. The products were analyzed with a gaschromatograph (Perkin-Elmer, Sigma 2000)) equipped with a Porapak QS column (Alltech) heated at 175°C. The permselectivity is defined by the relation: X,/X* (y=yw/yAp where Xw and XA represent the weight fractions of water and acetic acid in the permeate and Yw and YA those in the feed, respectively. Results and discussions The swelling test Figure 1 compares the swelling ratio of membrane M2 as a function of time in two different compositions of water-acetic acid and water-
208
E. RUCKENSTEIN AND H.H. CHEN
80 i
0
25s
ethanol
.
50%
ethanol
/
001, 00
10
I,
,
I,,
20
LO
30
T’me
50
I,
60
(hour)
Fig. 1. Swelling ratio of membrane M2 in two different concentrations of water-acetic acid and water-ethanol mixtures as a function of time at room temperature. TABLE 2 The swelling of membranes Ml, M2, and M3 in various pure solvents” Membr. Ethanol Acetic Acetone Pyridine Piperidine acid Ml M2 M3
Nb N 3.3
0.38 0.33 4.9
N N 4.6
N N 6.5
N N 5.6
“The time of swelling was 24 hr. bThe swelling ratio S is negligible; S is below 0.03.
ethanol mixtures. It is obvious that membrane M2 has a much higher swelling in water-acetic acid mixtures than in water-ethanol mixtures. This result is somewhat unexpected because the swelling of this membrane is inhibited by low pH [ 11. In order to clarify whether the higher swelling in the water-acetic acid solution is due to the dispersed or the continuous phase of the membrane, a porous membrane M3 free of the dispersed component, poly (sodium a&ate), as well as two other membranes Ml and M2 containing the dispersed phase were soaked in various pure solvents (all completely miscible with water). Table 2 indicates that both mem-
branes Ml and M2 swell to some extent in pure acetic acid but exhibit negligible swelling in the other solvents employed. The somewhat higher swelling of membranes Ml and M2 in acetic acid occurs probably because both the acetic acid and the dispersed phase have carboxilic groups. Due to its pores, the porous membrane M3 incorporates any of the solvents. Since the continuous phase is crosslinked, there are no major differences regarding the extent of incorporation. Nevertheless, membrane M3 has a somewhat higher swelling in acetic acid than in ethanol and even higher ones in pyridine and piperidine. The swelling is larger in acetic acid than in ethanol, probably because acetic acid modifies to some extent the elastic characteristics of the continuous phase. Pyridine leads to the highest swelling because both pyridine and the continuous phase posses aromatic rings, and this may affect in a somewhat stronger manner the elastic behavior of the continuous phase. Pervaporation experiments The pervaporation process consists of the dissolution of the components in the dispersed phase, their molecular diffusion through the network of films of the continuous phase, and finally, the evaporation of the components. Since the dispersed phase becomes ionized in water, the dissolution in the dispersed phase is selective in favor of water. As a result, the swollen membrane contains a larger proportion of water than the feed. The continuous phase being hydrophobic and extensively crosslinked does not swell itself appreciably in either of the two hydrophilic components. Its role is to maintain the integrity of the membrane. The components present in the swollen membrane diffuse through the films of the continuous phase and evaporate at the lower surface of the membrane. Because of the higher swelling of the membrane in the water-acetic acid mixture than in the water-ethanol mixture, a higher permeation rate is expected in the pervapora-
209
COMPOSITE MEMBRANES PREPARED BY CONCENTRATED EMULSION POLYMERIZATION
tion of a water-acetic acid mixture. Indeed Table 3 shows that the permeation rates through membranes Ml and M2 are much higher than those for the water-ethanol mixtures [ 11. As expected, the higher permeation rate is accompanied by a lower permselectivity. Although the present membranes are slightly thicker than those employed in the separation of water-ethanol mixtures, the comparison remains meaningful because a thicker membrane can only decrease the permeation rate. The permeation rates in Table 3 decrease with increasing acetic acid concentration in the feed and, surprisingly, with increasing poly (sodium acrylate) fraction in the membrane. The results are consistent with those obtained for the water-ethanol mixture [ 11. A possible explanation of the above surprising observation is as follows. For sufficiently large volume fractions of dispersed phase (ca. 0.95)) the gel has a relatively uniform structure consisting of polyhedral cells separated by thin films of continuous phase. For volume fractions of the order of 0.85 or smaller, the structure is less uniform and contains a size distribution of droplets of dispersed phase. Perhaps this non-uniformity is responsible for the higher permeability at the lower volume fraction of the dispersed phase. The temperature dependence of pervaporation for membrane M2 in the range of 30 to 50” C is presented in Table 4. Naturally, the permeation rate increases with the temperature of the feed. The permselectivity becomes TABLE 3 Permeation rate and permselectivity of the membrane Membr.
Ml M2
Permeation rate (kg/m’-hr) Acetic acid (wt.% )
Permselectivity,
(Y
Acetic acid (wt.% )
25
50
75
25
50
75
2.10 2.36
1.32 1.58
0.41 0.67
6.3 3.4
10.5 6.7
11.6 6.8
TABLE 4 Temperature-dependences of permeation rate and permselectivity for membrane M2 Temp.
30°C 40°C 50°C
Permselectivity,
cx
Permeation rate (kg/m’-hr) Acetic acid (wt.% )
Acetic acid (wt.% )
25
50
75
25
50
75
2.75 3.66 5.10
2.11 2.90 4.43
1.10 1.74 2.48
3.0 2.7 2.0
5.3 3.3 2.8
6.0 5.2 4.3
201
1.5
-I
AcetIc (WB) 25
10 i
Fig. 2. Arrhenius plots of the permeation rate through membrane M2 at various concentrations of acetic acid.
smaller when the temperature increases. An Arrhenius relationship can be established between the permeation rate and temperature (Fig. 2), and the activation energies are 5.41, 7.31, and 9.12 kcal/mole for the 25, 50 and 75 wt.% acetic acid concentrations, respectively. One may note that the membranes prepared via the concentrated emulsion polymerization are normally thicker than those prepared via casting from solution [ 61. The former membranes exhibit, however, for water-acetic acid mixtures, higher permeation rates and somewhat lower permselectivities than the latter ones.
E. RUCKENSTEIN AND H.H. CHEN
210
Conclusion A composite membrane prepared by the concentrated emulsion polymerization method, containing poly (sodium acrylate) as the dispersed phase and poly (divinyl benzene) as the continuous phase, was employed to carry out swelling tests as well as the pervaporation separation of water from water-acetic acid mixtures. The membrane was found to have a higher swelling in acetic acid solutions than in ethanol solutions of the same concentration. The permeation rate through the membrane is in the range of 0.41 to 2.36 kg/m2-hr, and is dependent on the acetic acid concentration in the mixture and on the composition of the membrane. The permselectivity is between 3.4 and 11.6 depending on the compositions of the mixture and membrane. The permeation rate of the water-acetic acid mixture through this membrane is higher than that of the waterethanol mixture. The permselectivity is, however, lower.
References E. Ruckenstein and H.H. Chen, Preparation of a waterpermselective composite membrane by the concentrated emulsion method: its swelling and permselectivity characteristics, J. Appl. Polym. Sci., 42 (1991) 2429. (a) E. Ruckenstein and J.S. Park, Hydrophilic-hydrophobic polymer composites, J. Polym. Sci., Polym. Lett. (Part C), 26 (1988) 528. (b) E. Ruckenstein, Emulsion pathways to composite polymeric membranes for separation processes, Colloid Polym. Sci., 267 (1989) 792. R.Y .M. Huang, A. Moreira, R. Notarfonzo and Y .F. Xu, Pervaporation separation of acetic acid-water mixtures using modified membranes. Part I. Blended polyacrylic acid (PAA)-Nylon 6 membranes, J. Appl. Polym. Sci., 35 (1988) 1191. Encyclopedia of Chemical Technology, 3rd edn., Vol. 17, John Wiley & Sons Inc., New York, NY, 1982, p. 1. J.S. Park and E. Ruckenstein, Selective permeation through hydrophobic-hydrophilic membranes, J. Appl. Polym. Sci., 38 (1989) 453. R.Y.M. Huang and C.K. Yeom, Pervaporation separation of aqueous mixtures using crosslinked polyvinyl alcohol membranes. Part III. Permeation of acetic acidwater mixtures, J. Membrane Sci., 58 (1991) 33.
205
Elsevier Science Publishers B.V., Amsterdam
Composite membranes prepared by concentrated emulsion polymerization and their use for pervaporation separation of water-acetic acid mixtures E. Ruckenstein
and H.H. Chen
Department of Chemical Engineering, State University of New York at Buffalo, Buffalo, NY 14260 (USA)
(Received June 24,199l; accepted in revised form September 30,199l)
Abstract A composite membrane prepared by the concentrated emulsion polymerization method, containing poly (sodium acrylate ) as the dispersed phase and poly (divinyl benzene ) as the continuous phase, was investigated regarding its swelling as well as the pervaporation separation of water-acetic acid mixtures. The experiments were carried out in order to be compared with the pervaporation separation of waterethanol mixtures performed with the same membrane [J. Appl. Polym. Sci., 42 (1991) 2429 1. The swelling of this membrane was found to be higher for acetic acid solutions than for ethanol solutions at the same mixture concentration. The permeation rates through this membrane for the water-acetic acid mixtures were in the range of 0.41-2.36 kg/ m2-hr, and the permselectivity varied between 3.4 and 11.6. Compared with the separation of water-ethanol mixtures, the present experiments show much higher permeation rates and lower permselectivities. Keywords: concentrated emulsion polymerization; pervaporation; composite membranes; acrylate)-poly (divinyl benzene); separation of water-acetic acid mixtures
Introduction Previously, we prepared a new kind of composite membrane [l] via the concentrated emulsion polymerization method [ 21. The membrane was used for separating water from water-ethanol mixtures by pervaporation. A concentrated emulsion has a high volume fraction of dispersed phase (usually 80-95 vol. % ), and the appearance of a semi-solid gel. A large volume fraction of an aqueous sodium acrylate Correspondence to: E. Ruckenstein, Department of Chemical Engineering, State University of New York at Buffalo, Buffalo, NY 14260 (USA).
0376-7388/92/$05.00
poly (sodium
solution was dispersed in a small amount of divinyl benzene, each of the two phases containing a suitable initiator and the continuous phase containing also a surfactant. The concentrated emulsion thus obtained was sandwiched between two glass plates and subjected to polymerization at 45 oC, and subsequently to drying. In this composite membrane, poly (divinyl benzene) is in the form of a network of thin films which separate polyhedral cells of poly (sodium acrylate) when the volume fraction of the dispersed phase is sufficiently large. This membrane exhibited high permselectivity to water from a water-ethanol mixture. The permeation rate of a water-ethanol mixture was in the range
0 1992 Elsevier Science Publishers B.V. All rights reserved.
E. RUCKENSTEIN AND H.H. CHEN
of 96-560 g/m2-hr, and the permselectivity varied between 32 and 235 in favor of water. In the present paper experimental results will be presented regarding the pervaporation separation of the water-acetic acid mixtures using the same membrane, namely the poly (sodium acrylate )-poly (divinyl benzene) composite membrane. This is of interest for various reasons: Firstly, acetic acid ranks among the top 20 organic intermediates in the chemical industry [ 31. In addition the separation of acetic acid from water by distillation is an energy-expensive process [ 31. It is thus desirable to develop a new separation process for saving energy; the pervaporation membrane separation technique is a potential candidate for this purpose. Secondly, the pervaporation separation of water-ethanol and water-acetic acid mixtures by the same membrane can yield some useful comparisons. One might be tempted to expect that the results would be similar, because the hydrophilicities of ethanol and acetic acid are comparable. Our experiments reveal, however, different results. The two monomers, sodium acrylate and divinyl benzene, have been selected for the preparation of the membranes because water is much more soluble in poly(sodium acrylate) than acetic acid, and neither water nor acetic acid are soluble in poly (divinyl benzene). Consequently, the dispersed phase can dissolve a large amount of liquid much richer in water than the feed, while the continuous phase being insoluble in both components maintains the integrity of the membrane.
Preparation of the concentrated emulsion A small amount of divinyl benzene (Aldrich) containing sorbitane monooleate (Fluka) as surfactant and AIBN (Alfa) as initiator was placed in a round bottom flask (100 ml capacity) equipped with a mechanical stirrer and an addition funnel. An aqueous solution of sodium acrylate containing potassium persulfate (Aldrich) as initiator was introduced in the addition funnel. The solution of sodium acrylate was prepared by titrating the acrylic acid with a solution of sodium hydroxide to a pH slightly above 7 to ensure the stability of the concentrated emulsion which is prepared in the next step [ 11. The pH of the sodium acrylate solution should not be much higher than 7 in order to avoid the decomposition of potassium persulfate in the alkaline environment [ 41. The concentrated emulsions were prepared at room temperature by the dropwise addition of the sodium acrylate solution to the stirred mixture in the flask. The addition process lasted ca. 25 min. The amounts employed in various membranes (Ml, M2, and M3) are listed in Table 1. TABLE 1 Amounts of components used in the preparation of the concentrated emulsions (in g )
Dispersed phase aqueous solution of sodium acrylate acrylic acid content in aqueous solution initiator (K&O,)
Experimental The materials used as well as the experimental procedures were described previously [ 11. Since there are slight differences in the experimental conditions, some details are included in the following.
Continuous phase divinyl benzene initiator (AIBN)
Surfactant sorb&an monooleate
Ml
M2
M3
30
30
30 (water)
8
8
0
1.7 x 10e4 g/g of acrylic acid for Ml and M2
2 4 4 2.0 X low4 g/g of monomer for all emulsions
1
1
I
COMPOSITE MEMBRANES PREPARED BY CONCENTRATED EMULSION POLYMERIZATION
Preparation of the membrane The concentrated emulsion was transferred into a syringe of 10 ml capacity. Different centrifugation conditions, dependent upon the viscosity of the concentrated emulsion, were employed to eliminate the minute air bubbles introduced in the concentrated emulsion during preparation (M3 and M2: 2000 rpm, 5 min; Ml: 2500 rpm, 10 min). Two glass plates (10 cm x 15 cm) were dried in the oven before use. A small amount of glycerol was placed as a lubricant on the surface of the glass plates. The centrifuged concentrated emulsions were injected carefully on one of the glass plates and squeezed slowly with the other plate to avoid trapping air bubbles. For polymerization to occur, the glass plates sandwiching the concentrated emulsion were placed in a temperaturecontrolled oven at 45°C for 24 hr. After preparation, the membrane was dried at 45 ‘C in a circulating oven for 24 hr. The thickness of the membrane was approx. 0.3 mm. The membrane, which was brittle at room temperature and in dry air, became softer and elastic in a high-humidity environment. The swelling test The change in weight of the membrane produced by swelling was determined as follows: membrane strips of ca. 0.5 g were dried again in a circulating oven at room temperature (23 ‘C ) and the weight of the dried membrane was determined. Then, the membrane strips were placed in flasks filled with different wateracetic acid and water-ethanol mixtures, at room temperature. After different periods of time, the weight was determined. The swelling ratio of the membrane is defined as: S =
w,-w, w,
where W, and W, represent dry and swollen membrane,
the weights of the respectively.
207
Pervaporation experiments Pervaporation studies of water-acetic acid mixtures were carried out by employing the apparatus used in a previous paper [ 51. The pervaporation cell has a capacity of 250 ml, and the membrane area in contact with the feed solution was about 9 cm2. In the upstream compartment, the liquid mixture was under stirring, while the downstream compartment was subjected to a vacuum of 3 20.5 Torr. The membranes, swollen for 60, 30, and 15 min in 75,50, and 25 wt.% acetic acid solutions, were placed in the pervaporation cell. The times of swelling have been chosen shorter for those membranes for which S was higher, and longer for those for which S was smaller. The permeated vapor was collected in a liquid nitrogen cold trap only after running the apparatus for 5 hr at room temperature. This blank run was carried out to ensure the complete removal of the water-acetic mixture accumulated in the lower part of the membrane during its initial swelling, which otherwise will be evacuated to the cold trap. Experiments were conducted at room temperature (ca. 23 ’ C ) and between 30 and 50°C in order to investigate the temperature dependence. The products were analyzed with a gaschromatograph (Perkin-Elmer, Sigma 2000)) equipped with a Porapak QS column (Alltech) heated at 175°C. The permselectivity is defined by the relation: X,/X* (y=yw/yAp where Xw and XA represent the weight fractions of water and acetic acid in the permeate and Yw and YA those in the feed, respectively. Results and discussions The swelling test Figure 1 compares the swelling ratio of membrane M2 as a function of time in two different compositions of water-acetic acid and water-
208
E. RUCKENSTEIN AND H.H. CHEN
80 i
0
25s
ethanol
.
50%
ethanol
/
001, 00
10
I,
,
I,,
20
LO
30
T’me
50
I,
60
(hour)
Fig. 1. Swelling ratio of membrane M2 in two different concentrations of water-acetic acid and water-ethanol mixtures as a function of time at room temperature. TABLE 2 The swelling of membranes Ml, M2, and M3 in various pure solvents” Membr. Ethanol Acetic Acetone Pyridine Piperidine acid Ml M2 M3
Nb N 3.3
0.38 0.33 4.9
N N 4.6
N N 6.5
N N 5.6
“The time of swelling was 24 hr. bThe swelling ratio S is negligible; S is below 0.03.
ethanol mixtures. It is obvious that membrane M2 has a much higher swelling in water-acetic acid mixtures than in water-ethanol mixtures. This result is somewhat unexpected because the swelling of this membrane is inhibited by low pH [ 11. In order to clarify whether the higher swelling in the water-acetic acid solution is due to the dispersed or the continuous phase of the membrane, a porous membrane M3 free of the dispersed component, poly (sodium a&ate), as well as two other membranes Ml and M2 containing the dispersed phase were soaked in various pure solvents (all completely miscible with water). Table 2 indicates that both mem-
branes Ml and M2 swell to some extent in pure acetic acid but exhibit negligible swelling in the other solvents employed. The somewhat higher swelling of membranes Ml and M2 in acetic acid occurs probably because both the acetic acid and the dispersed phase have carboxilic groups. Due to its pores, the porous membrane M3 incorporates any of the solvents. Since the continuous phase is crosslinked, there are no major differences regarding the extent of incorporation. Nevertheless, membrane M3 has a somewhat higher swelling in acetic acid than in ethanol and even higher ones in pyridine and piperidine. The swelling is larger in acetic acid than in ethanol, probably because acetic acid modifies to some extent the elastic characteristics of the continuous phase. Pyridine leads to the highest swelling because both pyridine and the continuous phase posses aromatic rings, and this may affect in a somewhat stronger manner the elastic behavior of the continuous phase. Pervaporation experiments The pervaporation process consists of the dissolution of the components in the dispersed phase, their molecular diffusion through the network of films of the continuous phase, and finally, the evaporation of the components. Since the dispersed phase becomes ionized in water, the dissolution in the dispersed phase is selective in favor of water. As a result, the swollen membrane contains a larger proportion of water than the feed. The continuous phase being hydrophobic and extensively crosslinked does not swell itself appreciably in either of the two hydrophilic components. Its role is to maintain the integrity of the membrane. The components present in the swollen membrane diffuse through the films of the continuous phase and evaporate at the lower surface of the membrane. Because of the higher swelling of the membrane in the water-acetic acid mixture than in the water-ethanol mixture, a higher permeation rate is expected in the pervapora-
209
COMPOSITE MEMBRANES PREPARED BY CONCENTRATED EMULSION POLYMERIZATION
tion of a water-acetic acid mixture. Indeed Table 3 shows that the permeation rates through membranes Ml and M2 are much higher than those for the water-ethanol mixtures [ 11. As expected, the higher permeation rate is accompanied by a lower permselectivity. Although the present membranes are slightly thicker than those employed in the separation of water-ethanol mixtures, the comparison remains meaningful because a thicker membrane can only decrease the permeation rate. The permeation rates in Table 3 decrease with increasing acetic acid concentration in the feed and, surprisingly, with increasing poly (sodium acrylate) fraction in the membrane. The results are consistent with those obtained for the water-ethanol mixture [ 11. A possible explanation of the above surprising observation is as follows. For sufficiently large volume fractions of dispersed phase (ca. 0.95)) the gel has a relatively uniform structure consisting of polyhedral cells separated by thin films of continuous phase. For volume fractions of the order of 0.85 or smaller, the structure is less uniform and contains a size distribution of droplets of dispersed phase. Perhaps this non-uniformity is responsible for the higher permeability at the lower volume fraction of the dispersed phase. The temperature dependence of pervaporation for membrane M2 in the range of 30 to 50” C is presented in Table 4. Naturally, the permeation rate increases with the temperature of the feed. The permselectivity becomes TABLE 3 Permeation rate and permselectivity of the membrane Membr.
Ml M2
Permeation rate (kg/m’-hr) Acetic acid (wt.% )
Permselectivity,
(Y
Acetic acid (wt.% )
25
50
75
25
50
75
2.10 2.36
1.32 1.58
0.41 0.67
6.3 3.4
10.5 6.7
11.6 6.8
TABLE 4 Temperature-dependences of permeation rate and permselectivity for membrane M2 Temp.
30°C 40°C 50°C
Permselectivity,
cx
Permeation rate (kg/m’-hr) Acetic acid (wt.% )
Acetic acid (wt.% )
25
50
75
25
50
75
2.75 3.66 5.10
2.11 2.90 4.43
1.10 1.74 2.48
3.0 2.7 2.0
5.3 3.3 2.8
6.0 5.2 4.3
201
1.5
-I
AcetIc (WB) 25
10 i
Fig. 2. Arrhenius plots of the permeation rate through membrane M2 at various concentrations of acetic acid.
smaller when the temperature increases. An Arrhenius relationship can be established between the permeation rate and temperature (Fig. 2), and the activation energies are 5.41, 7.31, and 9.12 kcal/mole for the 25, 50 and 75 wt.% acetic acid concentrations, respectively. One may note that the membranes prepared via the concentrated emulsion polymerization are normally thicker than those prepared via casting from solution [ 61. The former membranes exhibit, however, for water-acetic acid mixtures, higher permeation rates and somewhat lower permselectivities than the latter ones.
E. RUCKENSTEIN AND H.H. CHEN
210
Conclusion A composite membrane prepared by the concentrated emulsion polymerization method, containing poly (sodium acrylate) as the dispersed phase and poly (divinyl benzene) as the continuous phase, was employed to carry out swelling tests as well as the pervaporation separation of water from water-acetic acid mixtures. The membrane was found to have a higher swelling in acetic acid solutions than in ethanol solutions of the same concentration. The permeation rate through the membrane is in the range of 0.41 to 2.36 kg/m2-hr, and is dependent on the acetic acid concentration in the mixture and on the composition of the membrane. The permselectivity is between 3.4 and 11.6 depending on the compositions of the mixture and membrane. The permeation rate of the water-acetic acid mixture through this membrane is higher than that of the waterethanol mixture. The permselectivity is, however, lower.
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