Pervaporative Dehydration of Dimethyl Formamide (DMF) by

7210

Ind. Eng. Chem. Res. 2006, 45, 7210-7218

SEPARATIONS Pervaporative Dehydration of Dimethyl Formamide (DMF) by Crosslinked Copolymer Membranes Sagar Ray and Samit K. Ray* Department of Polymer Science and Technology, UniVersity of Calcutta, 92, A.P.C. Road, Kolkata 700009, India

Three different copolymers of acrylamide (AM) with increasing amounts of 2-hydroxyethyl methacrylate (HEMA), i.e., PAMHEMA-1, PAMHEMA-2, and PAMHEMA-3, were synthesized and the crosslinked (gelled) copolymer membranes that were made from these sol copolymer solutions (uncrosslinked) were used for pervaporative dehydration of dimethyl formamide (DMF) over the concentration range of 0-13.07 wt % water in feed. These hydrophilic gel copolymer membranes were found to be highly water-selective in both sorption and diffusion through the membranes. The water flux was found to decrease with an increasing degree of crosslinking from PAMHEMA-1 to PAMHEMA-3 membranes. Among the three membranes, the PAMHEMA-3 membrane showed the highest separation factor (464.3) and reasonable flux (23.91 g/(m2 h) for 0.5 wt % of water in the feed) for water, whereas the maximum water flux with high separation factor was exhibited by the PAMHEMA-l membrane (47.45 g/(m2 h) and 263.8 for 0.5 wt % of water in the feed) at 30 °C. When the pervaporation (PV) experiment was performed at 50 °C, the flux was observed to increase manifold with little change in the separation factor (51.03 g/(m2 h) and 369.6 for 0.5 wt % of water in the feed for the PAMHEMA-3 membrane). 1. Introduction Pervaporation (PV) is one of the most emerging separation processes for chemical industries where separation occurs through the preferential sorption and diffusion of one component. This separation process has already been commercialized for the dehydration of organics and the removal of traces of organics from the feed stream.1 Most of the PV membranes are prepared from various polymers. The choice of polymer for making PV membrane depends on the polarity of the component to be separated, with respect to the polymer. For pervaporative dehydration, a hydrophilic membrane is used, because water is to be preferentially permeated for this process. However, dehydration of an organic becomes difficult when it is also polar, like water. In this case, the stability of the membrane becomes questionable, because a high concentration of the polar organic (as used for any pervaporative dehydration) may dissolve or disintegrate the membrane polymer. Recently, extensive research2-10 has been carried out for the pervaporative separation of harsh or corrosive liquids. Various blends, composites,2-4 and copolymers6-9 have been tried for dehydration of these corrosive liquids with limited success. The use of these polymeric or ceramic membranes10,11 for dehydration of highly concentrated acetic acid, pyridine, 1,4-dioxane, tetrahydrofuran (THF), dimethyl formamide (DMF), etc. has been reported to give low flux and selectivity. Most of the acrylonitrile copolymer membranes used for the pervaporative dehydration of polar as well as corrosive organics are prepared * To whom correspondence should be addressed. Tel.: 91 3325673736. Fax: 91 3323508386. E-mail: [email protected], samitcu2@ yahoo.co.in.

by dissolving the polymer in a highly polar solvent such as DMF, followed by solution casting on a suitable surface.6,12,13 As most of the polymers are soluble in DMF, it is really challenging to find a suitable polymeric membrane for the dehydration of DMF. DMF is miscible in all proportions with water. Being highly hygroscopic, it readily absorbs moisture from the air, which makes it more corrosive.14 However, most of the applications of DMF, such as use as a solvent for polymers or as an extractant, demands its high purity; i.e., it should contain PAMHEMA-2 > PAMHEMA-3 Figure 3a also shows that the difference in the amount of sorption by the copolymers also increases as the concentration of water in the feed increases. In fact, as mentioned previously, with increasing amounts of HEMA from copolymer-1 to copolymer-3, the degree of crosslinking increases, resulting in less sorption of the feed mixtures. At higher feed concentration of water, the lesser-crosslinked polymer (e.g., PAMHEMA-1) is plasticized much more than the highly crosslinked polymer (e.g., PAMHEMA-3), which results in a large difference in the total sorption. The effect of temperature on the sorption of the membranes is also linear, as shown in Figure 4b for all three membranes at a 0.5 wt % concentration of water in the feed. It is observed from the figure that the amount of sorption also increases with temperature linearly for these membranes. The increased sorption of the membranes may be attributed to the increased solubility of both the permeants in the membranes at higher temperatures. 3.3.2. Sorption Selectivity. Figure 5 shows the variation of water content and sorption selectivity for water of all the three

PAMHEMA membranes with feed concentration of water. From this figure, it is observed that water sorption of the membranes follow a polynomial trend like total sorption while sorption selectivity for water decreases exponentially with feed concentration of water. It is also observed from the figure that as the degree of crosslinking from PAMHEMA-1 to PAMHEMA-3, the sorption selectivity for water increases. Higher crosslinking shows preference for sorption of smaller water molecules. 3.4. Permeation Studies. 3.4.1. Effect of Feed Concentration and Temperature on Dehydration. Figure 6a shows the variation (in wt %) of water in the permeate against the weight percentatge of water in the feed for dehydration of DMF by PAMHEMA membranes. These McCabe-Thiele-type xy diagrams suggest that the PAMHEMA membranes dehydrate DMF appreciably, up to 0-13.07 wt % water in feed without any pervaporative azeotrope and their dehydration characteristics show the following trend:

PAMHEMA-3 > PAMHEMA-2 > PAMHEMA-1 The water selectivity of the copolymer membranes is dependent on its degree of crosslinking, which decreases in the same order. The higher degree of crosslinking in PAMHEMA-3 gives networks with a stronger retractive force and a lower degree of swelling.24 Restricted permeation through the highly crosslinked structure gives maximum dehydration for the PAMHEMA-3 membrane.

Ind. Eng. Chem. Res., Vol. 45, No. 21, 2006 7215

Figure 4. (a) Sorption isotherm of copolymer membranes at 30 °C. (b) Variation of the total sorption of PAMHEMA membranes for 0.52 wt % water in the feed at different temperatures.

Figure 5. Variation of water content in the sorbed membrane and the separation factor for PAMHEMA membranes at 30 °C.

It is evident from Figure 6b that the dehydration characteristics of the PAMHEMA-3 membrane decrease as the temperature increases. Similar types of dehydration characteristics were also obtained with the other two copolymer membranes. 3.4.2. Effect of Feed Concentration on Flux and the Separation Factor. Figure 7a shows the effect of feed

Figure 6. a. Variation of the permeate concentration of water with its feed concentration (a) at 20 °C and (b) for PAMHEMA-3 at different temperatures.

concentration of water on its flux and separation factor for the three PAMHEMA copolymer membranes. This figure shows that for these copolymer membranes as the water concentration in the feed increases, the water flux increases at the cost of the separation factor. The decrease in the separation factor with increase in the water concentration in the feed for the binary mixture may be attributed to the plasticization of the hydrophilic membranes at high water content in the feed. Figure 6a also shows that for the same feed concentration, the water flux increases from the PAMHEMA-3 to the PAMHEMA-1 copolymer, whereas the reverse trend is observed for its separation factor. 3.4.3. Effect of Feed Temperature on Flux and the Separation Factor. It is evident from Figure 7b that the water flux increases as the temperature increases at the cost of the separation factor for the PAMHEMA-3 membrane. Similar results were also obtained for the other two PAMHEMA membranes. This may be due to the increase in both the total sorption and diffusion coefficient of the permeants with increases in temperature. Again, the thermal motion of the polymer chains increases as the temperature increases, which may facilitate easy permeation of the sorbed molecules through the membrane. 3.4.4. Effect of Feed Concentration on Activation Energies. Sorption and diffusion are activated processes and the temperature effect can be described by an Arrhenius-type relationship. The activation energy of sorption (∆HS) and that of permeation (∆EP) for the three copolymers were obtained from sorption

7216

Ind. Eng. Chem. Res., Vol. 45, No. 21, 2006

Figure 8. Variation of the activation energy of PAMHEMA membranes with the feed concentration of water.

coefficient can be expressed as25

Di ) Di0 exp(Wim + βWjm)

(4)

Dj ) Dj0 exp(Wjm + γWim)

(5)

Substituting eq 4 into eq 2 results in

dWim dl

Ji ) -FmDi0 exp(Wim + βWjm)

(6)

Integrating eq 7 over the membrane thickness results in the total flux of the ith component through the membrane: Figure 7. (a) Variation of flux and the separation factor of water with its feed concentration at 30 °C. (b) Variation of water flux and the separation factor with its feed concentration for the PAMHEMA-3 membrane at different temperatures.

and flux data, using the Arrhenius equation. The activation energy values, as calculated, were comparable to those reported in the literature for similar types of copolymer membranes.6,13 Figure 8 shows that the activation energies for sorption and permeation each decrease linearly as the feed concentration of water increases, because of plasticization of the hydrophilic membranes by water. It is also observed from the figure that, for the same feed concentration, activation energies increases from PAMHEMA-1 to PAMHEMA-3, because of increased crosslinking. 3.4.5. Model Calculation of the Theoretical Flux and Diffusion Coefficient. In a binary system with a nonporous dense membrane, pervaporative flux for the ith component can be described by Fick’s first law, as

Ji ) -

(

)

FmDi dWim 1 - Wim dl

(2)

For very small Wim, the above equation reduces to

dWim Ji ) -FmDi dl

(3)

However, the diffusion coefficient is dependent on the concentration of the permeating components in the membrane and their mutual coupling effect. For components i and j, the diffusion

∫0LJi dl ) -FmDi0∫WW

imp

imf

exp(Wim + βWjm) dWim

(7)

For the present system, both water (e.g., the ith component) and DMF (e.g., the jth component), being highly polar, will plasticize the membrane. Ignoring the very low concentration of the permeating component on the downstream side (because of the very low pressure on this side), eq 7 reduces to

Ji )

Di0Fm [exp(Wim + βWjm) - 1] L

(8)

Similarly, for the jth component (DMF),

Jj )

Dj0Fm [exp(Wjm + γWim) - 1] L

(9)

The density and thickness of the membranes were determined experimentally. The parameters Wim and Wjm were obtained from the sorption experiments, and the parameters Ji and Jj were obtained from permeation data. Using a linear regression of these permeation and sorption data and comparing these regressed equations with eqs 8 and 9, the diffusion coefficients at infinite dilution were calculated. The values of the plasticization coefficients β and γ were adjusted to make the predicted flux as close as possible to the experimental flux, maintaining a regression coefficient close to unity. The values of diffusion coefficients at infinite dilutions for both water and DMF along with the values of β and γ are given in Table 3. The parity plot of the experimental and theoretical flux of water for all three membranes at 30 °C is shown in Figure 9. The validity of the

Ind. Eng. Chem. Res., Vol. 45, No. 21, 2006 7217

Figure 9. Parity plot of experimental and predicted water flux for PAMHEMA membranes at 30 °C. Table 3. Diffusion Coefficients of Water (D0,water) and DMF (D0,DMF) through All the Membranes at Infinite Dilution membrane name

D0,water (× 1011 m2/s)

γ

D0,DMF (× 1013 m2/s)

β

PAMHEMA-1 PAMHEMA-2 PAMHEMA-3

Feed Temperature ) 30 °C 1.7094 0.72 1.90476 0.9091 0.72 1.17647 0.64516 0.72 0.571429

0.02 0.02 0.02



0.05 0.05 0.05



0.09 0.09 0.09

generalized model26 for the present system is quite satisfactory, as is evident from the closeness of the theoretical and experimental flux data shown in Figure 9. Similar types of parity plots were also obtained at higher temperatures. 3.4.6. Effect of Feed Concentration on Permeation Ratio. The permeation ratio gives a quantitative idea about the effect of one component on the permeation rate of the other component. Huang and Lin27 defined this permeation ratio (θ) as a measure of the deviation of the actual permeation rate (Jexpt) from the ideal rate (J0), to explain interactions between the membrane polymer and the permeants. Thus, the permeation ratio of the ith component is

θi ) -

Ji expt at x conc Ji0expt at x conc

(10)

water on the permeation rate of DMF. This figure also shows that, for the same feed concentration, the coupling effect decreases from PAMHEMA-1 to PAMHEMA-3. The increasing order of crosslinking from copolymer-1 to copolymer-3 membranes causes this decreasing order of coupling effect, because of restricted permeation. 4. Conclusions Copolymerization of acrylamide (AM) and 2-hydroxyethyl methacrylate (HEMA) with varied comonomer compositions by solution polymerization yielded three different copolymer sols. These sols, when cast onto glass plates, produced insoluble gel membranes on heating. These membranes were used for the dehydration of dimethyl formamide (DMF) by pervaporation. These membranes were found to be quite stable when in contact with highly corrosive DMF and showed very high water selectivity with reasonable flux. The copolymer membranes were observed to show both sorption and diffusion selectivity for water. The degree of crosslinking of the membranes was observed to increase as the HEMA content in the membrane increased, from copolymer-1 to copolymer-3. Water selectivity of the membranes was observed to increase as the HEMA content or crosslinking in the copolymers increased with a reduction in its flux. Acknowledgment The authors are grateful to the University Grant Commission (UGC) and the Council for Scientific and Industrial Research (CSIR), Government of India, for sponsoring this work.

where 0 Ji0expt at x conc ) Jpure i × xi

Figure 10. Variation of the permeation ratio with the feed concentration of water at 30 °C.

(11)

Equation 11 clearly shows that a θi value of greater than unity amounts to a positive coupling effect of the other component (j) on the permeation rate of the ith component. The variation in the permeation factor of DMF, with respect to the feed concentration of water, is given in Figure 10 for the three copolymer membranes. The highly hydrophilic membranes swelled too much to sustain pure water permeability and hence, the permeation factor for water could not be measured. From this figure, it is observed that the permeation factor of DMF exceeds its ideal value for even a very low concentration of water in the feed, which signifies a positive coupling effect of

Nomenclature Di ) diffusion coefficient of water (m2/s) Dio ) diffusion coefficient of water at infinite dilution (m2/s) Dj ) diffusion coefficient of dimethyl formamide (DMF) (m2/ s) Djo ) diffusion coefficient of DMF at infinite dilution (m2/s) i ) ith component (water) j ) jth component (DMF) J ) total flux ((kg/m2 s)) Ji ) water flux (kg/(m2 s)) Jj ) DMF flux (kg/(m2 s)) L ) thickness of membrane (m)

7218

Ind. Eng. Chem. Res., Vol. 45, No. 21, 2006

Wim ) membrane phase water mass fraction Wjm ) membrane phase DMF mass fraction ∆ED ) activation energy for diffusion ∆Ep ) activation energy for permeation ∆Hs ) activation energy for sorption R ) separation factor β ) coupling parameter of DMF γ ) coupling parameter of water F ) membrane density (kg/m3) Literature Cited (1) Baker, R. W. Membrane Technology and Applications; Wiley: West Sussex, England, 2004. (2) Devi, D. A.; Smitha, B.; Sridhar, S.; Aminabhavi, T. M. Pervaporation separation of dimethylformamide/water mixtures through poly(vinyl alcohol)/poly(acrylic acid) blend membranes. Sep. Purif. Technol. 2006, 51, 104-111. (3) Rao, P. S.; Smitha, B.; Sridhar, S.; Krishnaiah, A. Preparation and performance of poly(vinyl alcohol)/polyethyleneimine blend membranes for the dehydration of 1,4-dioxane by pervaporation: Comparison with glutaraldehyde cross-linked membranes. Sep. Purif. Technol. 2006, 48, 244. (4) Bhat, S. D.; Aminabhavi, T. M. Novel sodium alginate composite membranes incorporated with SBA-15 molecular sieves for the pervaporation dehydration of aqueous mixtures of 2-propanol and 1,4-dioxane at 30 °C. Microporous Mesoporous Mater. 2006, 91, 206. (5) Li, G.; Kikuchi, E.; Matsukata, M. Separation of water-acetic acid mixtures by pervaporation using a thin mordenite membrane. Sep. Purif. Technol. 2003, 32, 199. (6) Ray, S. K.; Sawant, S. B.; Joshi, J. B.; Pangarkar, V. G. Dehydration of acetic acid by Pervaporation. J. Membr. Sci. 1998, 138, 1. (7) Lee, Y. M.; Oh, B. K. Pervaporation of water-acetic acid mixture through poly(4-vinylpyridine-co-acrylonitrile) membrane. J. Membr. Sci. 1993, 85, 13. (8) Park, C. H.; Nam, S. Y.; Lee, Y. M.; Kujawski, W. Pervaporation of pyridine-water mixture through poly(acrylonitrile-co-monoacryloxyethyl phosphate) membrane. J. Membr. Sci. 2000, 164, 121. (9) Lee, Y. M.; Oh, B. K. Dehydration of water-pyridine mixture through poly(acrylonitrile-co-acryclic acid) membrane by Pervaporation. J. Membr. Sci. 1995, 98, 183. (10) ten Elshof, J. E.; Abadal, C. R.; Sekuli, J.; Chowdhury, S. R.; Blank, D. H. A. Transport mechanisms of water and organic solvents through microporous silica in the pervaporation of binary liquids. Microporous Mesoporous Mater. 2003, 65, 197. (11) Cuperus, F. P.; van Gemert, R. W. Dehydration using ceramic silica pervaporation membranessthe influence of hydrodynamic conditions. Sep. Purif. Technol. 2002, 27, 225. (12) Ray, S.; Ray, S. K. Effect of copolymer type and composition on separation characteristics of pervaporation membranessA case study with separation of acetone-water mixtures. J. Membr. Sci. 2006, 270, 73. (13) Ray, S. K.; Sawant, S. B.; Joshi, J. B.; Pangarkar, V. G. Development of new synthetic membranes for separation of benzene-

cyclohexane mixture by pervaporationsA solubility parameter approach. Ind. Eng. Chem. Res. 1997, 36, 5265. (14) Bipp, H.; Kieczka, H. Formamides. In Ullmann’s Encyclopedia of Industrial Chemistry, Volume A12; Elvers, B., Hawkins, S., Russey, W., Eds.; VCH Verlagsgesellschaft mbH, Weinheim, Germany, 1995; pp 1-12. (15) Hicke, H. G.; Lehmann, I.; Malsch, G.; Ulbricht, M.; Becker, M. Preparation and characterization of a novel solvent-resistant and autoclavable polymer membrane. J. Membr. Sci. 2002, 198, 187. (16) Peters, T. A.; Fontalvo, J.; Vorstman, M. A. G.; Benes, N. E.; van Dam, R. A.; Vroon, Z. A. E. P.; van Soest-Vercammen, E. L. J.; Keurentjes, J. T. F. Hollow fibre Microporous silica membranes for gas separation and pervaporation: Synthesis, performance and stability. J. Membr. Sci. 2005, 248, 73. (17) Schulz, R. C. Acrylamide Polymers. In Encyclopedia of Polymer Science and Engineering, Vol. 1; Mark, H. F., Ed.; Wiley: New York, 1985; pp 169-211. (18) Bikales, N. M. Preparation of Acrylamide Polymers. In Water Soluble Polymers; Bikales, N. M., Ed.; Plenum Press: New York, 1973; pp 213-225. (19) McCormick, C. L.; Bock, J. Water Soluble Polymers. In Encyclopedia of Polymer Science and Engineering; Mark, H. F., Ed.; Wiley: New York, 1989; Vol. 17, pp 730-784. (20) Silverstein, R. M.; Webster, F. X. Infrared Spectrometry, Spectrometric Identification of Organic Compounds, Sixth Edition; Wiley: New York, 1996. (21) Edwards, S. F. Polymer networks. In Structural and Mechanical Properties of Polymers; Chompaff, A. J., Newman, S., Eds.; Plenum Press: New York, 1971; pp 453-462. (22) Nielsen, L. E. Cross-Linking-Effect on Physical Properties of Polymers. In ReViews in Macromolecular Chemistry; Butler, J. B., O’Driscoll, K. F., Eds.; Marcel Dekker: New York, 1970; pp 69-103. (23) Roger, C. E. Permeation of gases and vapors in polymers. In Polymer Permeability; Comyn, J., Ed.; Elsevier Applied Science: New York, 1985; p 32. (24) Buchholz, F. L.; Burgert, J. H. Synthesis and Applications of Superabsorbent Polymers. In Industrial Water Soluble Polymers; Finch, C. A., Ed.; Royal Society of Chemistry: Cambridge, U.K., 1996; pp 92-105. (25) Brun, R. C.; Larchet, C.; Melet, R.; Bulvestre, G. Modeling of the Pervaporation of binary mixtures through moderately swelling. nonreacting membranes. J. Membr. Sci. 1985, 23, 257. (26) Rautenbach, R. A.; Herion, C.; Franke, M.; Asfour, A.-F. A.; Bemquerer-Costa, A.; Bo, E. Investigation of mass transport in asymmetric Pervaporation membranes. J. Membr. Sci. 1988, 36, 445. (27) Huang, R. Y. M.; Lin, V. J. C. Separation of liquid mixtures by using polymer membranes. I. Permeation of binary organic liquid mixtures through polyethylene. J. Appl. Polym. Sci. 1968, 12, 2615.

ReceiVed for reView April 6, 2006 ReVised manuscript receiVed July 20, 2006 Accepted July 27, 2006 IE060431B

Pervaporative Dehydration of Dimethyl Formamide (DMF) by

Recommend Documents