Dehydration of tetrahydrofuran by pervaporation using a composite membrane

Journal of Membrane Science 268 (2006) 13–19

Dehydration of tetrahydrofuran by pervaporation using a composite membrane Peter D. Chapman a , Xiaoyao Tan a , Andrew G. Livingston a , K. Li a,∗ , Teresa Oliveira b a

Department of Chemical Engineering, Imperial College London, University of London, South Kensington, London SW7 2AZ, UK b GlaxoSmithKline, Stevenage SG1 2NY, UK Received 8 March 2005; received in revised form 13 May 2005; accepted 5 June 2005 Available online 19 July 2005

Abstract Tetrahydrofuran (THF) is a strong aprotic solvent, commonly used in the pharmaceuticals industry due to its broad solvency for both polar and non-polar compounds. THF and water form a homogeneous azeotrope at 5.3 wt.% water thus simple distillation is not feasible to dehydrate THF below this concentration. Pervaporation offers a solution since it is not governed by vapour–liquid equilibria. However many polymer-based pervaporation membranes are cast utilizing THF as the casting solvent and so these membranes have a tendency to swell excessively in its presence. This results in poor separation performance and poor long-term stability and thus renders these membranes unsuitable for THF dehydration. In this study, a new membrane available from CM Celfa, CMC-VP-31 has been tested for the dehydration of THF. The membrane shows excellent performance when dehydrating THF with a flux of over 4 kg m−2 h−1 when dehydrating THF containing 10 wt.% water at 55 ◦ C dropping to 0.12 kg m−2 h−1 at a water content of 0.3 wt.%. The permeances of water and THF in the membrane were calculated to be 11.76 × 10−6 and 7.36 × 10−8 mol m−2 s−1 Pa−1 , respectively, at 25 ◦ C and found to decrease in the membrane with increasing temperature to values of 6.71 × 10−6 and 1.63 × 10−8 mol m−2 s−1 Pa−1 at 55 ◦ C. The flux and separation factor were both found to increase with an increase in temperature thus favouring the operation of CMC-VP-31 at high temperatures to optimize separation performance. © 2005 Elsevier B.V. All rights reserved. Keywords: Pervaporation; Solvent resistant; Tetrahydrofuran; Dehydration; CMC-VP-31

1. Introduction Pervaporation has long been recognized as an economically effective way to separate azeotropic mixtures and has been increasingly been turned to for problems of this nature since its first commercial application in an alcohol dehydration plant installed by Gesellchaft fur Trenntechnik mbH (GFT) of Germany in 1982. Much of the research on pervaporation has been focused on alcohol dehydration [1–3], particularly ethanol [4–8]. However there are many applications where it would be desirable to dehydrate other common organics such as acetic acid, acetone and tetrahydrofuran.

∗

Corresponding author. Tel.: +44 207 5945676; fax: +44 207 5945629. E-mail address: [email protected] (K. Li).

0376-7388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2005.06.003

Tetrahydrofuran (THF) is frequently utilized as a solvent in many pharmaceutical synthetic procedures because of its broad solvency for polar and non-polar compounds. THF is particularly capable of dissolving many ionic species and organometallics which are commonly used in specialty syntheses. In addition, THF’s high volatility and very high purity facilitate solvent removal and recovery without leaving residues in the desired product. THF is a relatively expensive solvent and thus being able to recover used solvent by dehydration can offer significant savings whilst also being environmentally beneficial. Since THF forms an azeotrope with water at 94.7 wt.% [9] this prevents the use of simple distillation. Adding an entrainer to the mixture in order to break the azeotrope results in an impure THF product containing some of the entrainer rendering it unsuitable for many applications where pure THF is required. Badische

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Anilin- & Soda-Fabrik AG (BASF) details a system for recovering THF using multi-stage distillation requiring the first stage, azeotropic distillation to be followed by dehydration with solid or aqueous sodium or potassium hydroxide and a final distillation stage to obtain pure THF. The use of pervaporation could cut this lengthy, complicated and costly process down to just two stages and also reduce impurities present in the product. A common method used for producing membranes is phase inversion. In this process a suitable solvent is required in which it is possible to dissolve the polymeric material to form a casting solution. Strong aprotic solvents such as THF are often used as the casting solvent, however when a membrane fabricated from polymeric material soluble in these types of solvents is used in contact with them, the solvent can swell the membrane excessively or re-dissolve the material, rendering the membrane unsuitable for applications involving these solvents. Some success has been had with producing a THF stable polymeric membrane, previously Kurkuri et al. [10] used sodium alginate-based membranes in order to separate THF–water mixtures. They grafted the sodium alginate with polyacrylamide and compared the performance of membranes fabricated from these grafted polymers by solvent evaporation with those produced from sodium alginate. The membranes produced from the grafted sodium alginate were dense films and thus showed poor fluxes of approximately 0.1 kg m−2 h−1 , slightly greater than that of the pure sodium alginate membrane and separation factors ranged between 216 and 591. They did not comment specifically on the stability of sodium alginate in THF. Poly(vinyl alcohol)/poly(vinyl pyrrolidone) (PVA/PVP) blends of different compositions were used by Lu et al. [11] and crosslinked with UV at elevated temperature. They found that permeation flux increased with increasing PVP content without a loss of selectivity and that a PVP content of 80 wt.% was optimal for the process as this gave a high flux of around 0.33 kg m−2 h−1 when dehydrating a 95 wt.% THF solution at 40 ◦ C but was far less fragile than a pure PVP membrane. Membranes fabricated from inorganic material however do not swell in the way polymeric membranes do and so membrane separations involving these solvents can be performed using ceramic or zeolite membranes. Urtiaga et al. [12] looked at dehydrating THF using a zeolite-based membrane from the SMART chemical company (This company has since ceased trading). They found that the separation factor was as high as 20,000, increasing with decreasing water content in the feed down to 0.15 wt.% water at which point the permeate becomes enriched in THF. The flux was also reasonable, above 0.3 kg m−2 h−1 at all water concentrations and as high as 1 kg m−2 h−1 at the relatively low chosen operating temperature of 45 ◦ C. Li et al. [13] looked at four types of zeolite membranes, A, Al-ZSM-5, Y and mordenite. They showed that the Y-type membrane had the highest flux and selectivity of 2.4 kg m−2 h−1 and 290, respectively, at 60 ◦ C. Ortiz et al. [14] looked at the dehydration of THF with two different types of commercial membranes, the poly-

meric CMC-CF-23 (CM Celfa) and an inorganic NaA Zeolite membrane from the SMART chemical company as tested by Ultiaga et al. above. They showed that both membranes appeared short term stable in the solvents and were successful at dehydrating THF. Performance of the polymeric membrane compared well with that of the zeolite membrane however dehydration to a much lower water level was shown to be feasible with the zeolite membrane. So far there has been limited success in producing a solvent resistant polymeric membrane suitable for the dehydration of strong aprotic solvents such as THF, acetone, dimethylformamide (DMF) and n-methyl pyrrolidone. A new composite membrane has recently become available from a Swiss company CM Celfa (a subsidiary of Folex Imaging—www.cm-celfa.ch) and is considered to be suitable for the dehydration of THF, acetone, pyridine, acetonitrile, ethyl acetate and ethoxyethanol as well as ethanol and isopropanol. The composite membrane consists of a thin active layer of approximately 1–2 ␮m with a sponge like gutter layer on a non-woven support. Due to the novel nature of the membrane, details of the components used were not available. The objective of this study was to investigate the performance of CMC-VP-31 whilst dehydrating THF and to determine how the factors of feed temperature and flow rate affect the separation performance of the membrane. The experimental data was then used to evaluate the permeance of water and THF in the membrane.

2. Experimental 2.1. Materials Tetrahydrofuran (GPR, >99%, BP 66 ◦ C) from the British Drug Houses company (BDH) was mixed with deionized water to produce test solutions of varying concentrations for experiments into the dehydration of THF. THF is totally miscible with water and forms an azeotrope at 5.3 wt.% water. Karl Fisher reagents Hydranal-Coulomat A and HydranalCoulomat CG from Riedel de Ha¨en were used for coulometric Karl Fischer titration. 2.2. Pervaporation experiments The experimental setup is shown in Fig. 1. Half a liter of feed solution is prepared, typically containing ≈10 wt.% H2 O and poured into a 500 ml volumetric flask which acts as the feed tank. This volumetric flask is then placed in a water bath to regulate the temperature between 25 and 90 ◦ C whilst a gear pump (Cole palmer Gear Pump Drive, EW07001-40 Drive Head) circulates the feed solution into the test module (which is also immersed in the water bath) at a desired flow rate, typically 1.8 l min−1 , and then returns the retentate to the volumetric flask. The vacuum on the permeate side is monitored on a digital rough vacuum gauge (Vacuubrand DVR2) and maintained by a two stage rotary vane vacuum

P.D. Chapman et al. / Journal of Membrane Science 268 (2006) 13–19

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Organic content in the permeate samples was analyzed by total organic carbon (TOC) using a Shimadzu TOC-5050. Permeate samples were diluted down using deionized water so the solvent content of the samples to be analyzed was below 1000 ppm. Three injections were made for each sample and analysis of the variance showed that the coefficient of variation was less than 3.5% for samples containing low solvent contents and much below this for samples containing higher solvent concentrations, within the limits of detection.

3. Theory

Fig. 1. Experimental setup for membrane pervaporation.

pump (Varian DS202) connected to the module via a cold trap system. The vacuum level was maintained below 30 mbar for all experiments. The permeate vapour is collected in one of the two cold traps in the system for a given length of time after which the second cold trap is brought into operation and the first one isolated, as the permeate sample is removed for analysis. Liquid nitrogen is used to provide the required cooling duty in the two cold traps. The membrane cell is sealed using silicone rubber orings and the membrane supported on top of a stainless steel mesh. The cell houses membrane discs of 75 mm diameter and when sealed with the o-ring has an active area of 41.3 cm2 . The sheets of Celfa CMC-VP-31 were kindly donated by CM Celfa. Experiments were typically carried out for 8–10 h with permeate samples being collected every 30 or 60 min, depending on the permeation rate, and a retentate sample being collected simultaneously for each permeate sample. An additional experiment was also performed using a large feed volume of 5 l contained in a 5 l round bottomed flask as a feed tank. This large feed volume ensured that the feed concentration entering the cell remained close to constant with time. During experimentation, the system temperature was increased in steps to allow the pseudo steady state flux and selectivity to be evaluated at various temperatures and thus allow the permeances of both THF and water in the membrane at these temperatures to be calculated. 2.3. Sample analysis The water content in the retentate was measured using a coulometric Mettler Toledo DL37 Karl Fisher titrator. For each retentate sample, three injections of a 5 ␮l sample were made and the average water content of the three samples taken as the retentate concentration. The largest deviation between the injections and the average value is less than 2%.

Celfa CMC-VP-31 is a non-porous membrane which allows water to preferentially permeate compared to THF. Since it is a non-porous membrane, it is assumed that transport across the membrane occurs by solution diffusion. Assuming that: (1) the membrane process (dissolution and diffusion) is the controlling step; (2) negligible pressure drop of the feed and permeate streams occurs along the flow path; and (3) perfect mixing is present in both compartments of the membrane cell, the mass conservation equations for water and THF in the membrane cell are, thus, given by UF xF,w = UR xR,w + Jw Am

(1a)

UF (1 − xF,w ) = UR (1 − xR,w ) + Jt Am

(1b)

where UF and xF,w are the feed flow rate (mol s−1 ) and the molar fraction of water in the feed, respectively; UR and xR,w the flow rate of retentate and the molar fraction of water in the retentate solution; Am the effective membrane area for permeation (m2 ); Jw and Jt are, respectively, the permeation fluxes of water and THF, which are given by Jw = Pw (psw γw xR,w − pp yP,w )

(2a)

Jt = Pt (pst γt (1 − xR,w ) − pp (1 − yP,w ))

(2b)

where Pw and Pt are the permeances of water and THF in the membrane, respectively (mol m−2 s−1 Pa−1 ); psw and pst the saturated pressure of water and THF (Pa); γ w and γ t the activity coefficients of water and THF; pp the permeate pressure (Pa); and yP,w is the molar fraction of water in the permeate gas that may be implicitly given by yP,w =

Jw Jw + J t

(3)

The activity coefficients of water and THF in the binary solution, γ w and γ t may be determined by the Wilson equation: ln γw = −ln(xR,w + (1 − xR,w )Λ12 ) + (1 − xR,w )
 Λ12 Λ21 × − xR,w + (1 − xR,w )Λ12 1 − xR,w + xR,w Λ21 (4a)

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ln γt = −ln(1 − xR,w + xR,w Λ21 ) − xR,w 
Λ12 Λ21 × − xR,w + (1 − xR,w )Λ12 1 − xR,w + xR,w Λ21 (4b) where Λ12 and Λ21 are the Wilson parameters for the THF–water binary solution, the values of which depend on the solution’s temperature. The variation of these activity coefficients was calculated over all compositions using Aspen properties 12.1 for a binary system of THF and water and used for subsequent calculations. The separation factor, α is defined as yP,w /(1 − yP,w ) α= xR,w /(1 − xR,w )

(5)

When the downstream pressure approaches to zero, the separation factor is equal to the selectivity of the membrane for water and THF [15], namely, Sw/t =

Pw = α(pv =0) Pt

(6)

In the feed tank, the material balance equations for water and THF are given by d(NF xF,w ) = −Jw Am dt

(7a)

d[NF (1 − xF,w )] = −Jt Am dt

(7b)

where NF is the amount of mixture in the feed tank (mol). Mass balances were performed around the system for each experiment to ensure that calculated values for fluxes and sample concentrations were in line with the quantities of solvent and water initially added to the system.

4. Results and discussion 4.1. Kinetic study of the permeation of water and THF in the membrane The experiment using a large feed volume of 5 l was performed to study how the permeance of water and THF in the CMC-VP-31 membrane varied with temperature. The feed flow rate was 1.8 l min−1 and the starting water concentration 10.66 wt.%. Due to the large volume of the feed tank used, the feed concentration remained virtually unchanged

Fig. 2. Plot of permeance against the reciprocal of temperature.

as the water permeation proceeds. The permeances of water and THF, Pw and Pt in the membrane could therefore be calculated with Eqs. (1)–(4) based on the obtained permeation fluxes, which are summarized in Table 1. The results indicate that the permeances of both water and THF decrease as temperature increases. Based on the solution–diffusion model, vapour permeance in a rubbery membrane is related to the solubility and diffusivity of the vapour in the membrane, and may be expressed as
 DS Ea P= = P0 exp − (8) δ RT where D is the diffusion coefficient (m2 s−1 ); S the gas solubility in the membrane (mol m−3 Pa−1 ); δ the membrane thickness (m); Ea the activation energy for permeation, Ea = E + H in which E and H are the activation energy for diffusion and the heat of solution of the gas, respectively (J mol−1 ). The permeances of water and THF in the membrane are plotted against the reciprocal of temperature in Fig. 2. Since the diffusivity always increases with temperature, the positive values of the lines suggest that the permeation of water and THF in the CMC-VP-31 membrane is dominated by the solubility rather than the molecular diffusion. Increasing temperature leading to a decrease in permeation indicates that the permeation is dominated by solubility as opposed to diffusion where increasing temperature would lead to an increase in permeation rate. Fig. 3 shows the membrane’s selectivity for water and THF as a function of temperature. It can be seen that the selectivity increases with temperature, indicating that the increase in the operating temperature favours the permeation of water than that of THF. With the intercept and

Table 1 Permeances of water and THF in the CMC-VP-31 membrane Temperature (◦ C)

Permeance of water, Pw (×l06 mol m−2 s−1 Pa−1 )

Permeance of THF, Pt (×108 mol m−2 s−1 Pa−1 )

Selectivity, Sw/t = Pw /Pt

25 40 45 50 55

11.76 9.41 9.36 7.47 6.71

7.36 4.92 3. 12 2.51 1.63

159.79 191. 11 300.24 297. 35 410.99

P.D. Chapman et al. / Journal of Membrane Science 268 (2006) 13–19

Fig. 3. Relationship between membrane selectivity and temperature.

the slop of the lines, the relationships between permeances and temperature are thus given as
 14707 −8 Pw = 3.288 × 10 exp (9a) RT
 39608 Pt = 9.585 × 10−15 exp (9b) RT It should be noted that the above equations could only predict the permeation values of water and THF within the examined temperature and concentration ranges.

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Fig. 5. Total flux and water flux across membrane vs. water concentration in the retentate for the dehydration of THF at 25, 40 and 55 ◦ C, vacuum 2–17 mbar, Q = 1.8 l min−1 .

gests that other resistances mainly of the liquid boundary layer have made negative contribution to the permeation of water in the membrane. It further confirms that the feed flow rate of 1.8 l min−1 set in the above kinetic experimental studies is reasonable because the liquid mass transfer resistance had been eliminated. 4.3. Effect of temperature on the dehydration of THF

By varying the feed flow rate, it is possible to identify whether hydrodynamic effects within the cell would affect the membrane performance. At low flow rates, a larger boundary layer is likely to develop close to the membrane surface. Experiments were performed with flow rates of 1.8, 1.0 and 0.5 l min−1 under otherwise identical operating conditions (55 ◦ C) and the effect of this operation on the total flux across the membrane can be seen in Fig. 4. The water fluxes through the membrane do not vary significantly when the feed flow rate changes from 1 to 1.8 l min−1 , thus showing that operation above 1 l min−1 is enough to minimize any hydrodynamic effects on membrane performance. When operating at 0.5 l min−1 however the flux is clearly reduced. This sug-

The membrane samples from Celfa, CMC-VP-31 were then tested at various operating temperatures, a comparison of the results for these experiments can be seen in Fig. 5. It can clearly be seen from the figure that increasing feed temperature increases the overall flux however it should also be noted that with increase in temperature, the total flux and water flux become closer indicating an improvement of selectivity with increase in operating temperature. Fig. 6 shows the variation of the water content remaining in the retentate with time for these same experiments dehydrating THF at various operating temperatures, where the feed flow rates for all runs were kept constant, i.e., 1.8 l min−1 . It can be seen that the final content of water in the THF solution may be experimentally reduced to just below 0.25 wt.%, although the theoretical results indicate that the water may be removed completely by pervaporation if the downstream pressure is low enough. The experimental data obtained when dehydrating solutions initially containing high water

Fig. 4. Effect of feed flow rate on permeation flux.

Fig. 6. Effect of temperature on the performance of CMC-VP-31.

4.2. Effect of feed flow rate on the dehydration of THF

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concentrations are in good agreement with the predicted values using the kinetic parameters obtained above while some deviation (lower than the experimental data) occurs when using these values to model the dehydration of solutions initially containing low concentrations of water. This is possibly because the permeance values are dependent on the concentration of the solution. Further, although the permeances of both water and THF in the membrane are reduced as temperature increases, as presented above, the saturate pressures of the mixture solution increases with temperature, which means the driving force for permeation is increased. As a result, the water in THF can be more easily removed at higher temperature than at lower temperatures. In another words, the operation of CMC-VP-31 at higher temperatures is favourable since this results in an increase in both flux and selectivity. 4.4. Effect of water content on the dehydration of THF As described above, the permeances of both water and THF in the CMC-VP-31 membrane may be lowered as the water content in the THF solution decreases. Fig. 7 compares the experimental data for which the initial concentration of water in the feed solution is only 1.8% and the theoretical results using the obtained permeance values. Obviously, the theoretical values are lower than the experimental data, indicating that the actual permeances are lower than the values calculated with Eq. (9). This may be due to the way that the membrane initially is conditioned when in contact with a high water content feed which does not occur when commencing operation at a lower water content. To better understand the relationship between permeance and water concentration however, it would be necessary to perform additional experiments and this is a possible area which it would be interesting to study in more detail in the future. Restrictions in operating the pervaporation rig prevented long-term operational testing of the membrane. Immersion testing (azeotropic mixture of THF and water, 55 ◦ C, 2 months) did not appear to adversely affect the membranes structure but the long-term operational stability of the membrane must still be tested.

Fig. 7. Performance of the membrane for dehydration of THF at low feed concentration.

5. Conclusions The dehydration of THF and other strong aprotic solvents is complicated by the separation problems caused by azeotropy coupled with the poor availability of membranes suitable for use in these solvents. From the results of this study, it is clearly evident that in short term performance, Celfa CMC-VP-31 offers good performance whilst dehydrating THF and therefore overcomes a gap in the market where previously there was no commercially available polymeric membrane suitable for this separation. The permeances of water and THF in the membrane decrease with increasing temperature with the permeance of THF falling more quickly than that of water from 25 to 55 ◦ C. Both flux and separation factor can be seen to increase with temperature and therefore the operation of this membrane at higher temperatures is favourable. This improvement of performance with temperature would make it interesting to test this membrane under vapour permeation since its operation may well be further improved whilst operating with a vapour feed.

Acknowledgements A CASE studentship provided by EPSRC and GSK to one of the authors, Peter Chapman, is gratefully acknowledged.

Nomenclature Am D E Ea H Jw , Jt NF pp psi Pi Q R S Sw/t t T UF UR xF,w xR,w yP,w

effective membrane area for permeation (m2 ) molecular diffusion coefficient (m2 s−1 ) activation energy for diffusion (J mol−1 ) activation energy for permeation, Ea = E + H (J mol−1 ) heat of solution of species (J mol−1 ) permeation flux of water and THF through the membrane (mol m−2 s−1 ) amount of mixture in feed tank (mol) permeate pressure (Pa) saturated pressure of species i (Pa) permeance of species i in the membrane (mol m−2 Pa−1 s−1 ) feed flow rate (l min−1 ) ideal gas constant, R = 8.314 J mol−1 K−1 solubility in the membrane (mol m−3 Pa−1 ) selectivity of the membrane, Sw/t = Pw /Pt time variable (s) operation temperature (K) feed flow rate (mol s−1 ) retentate flow rate (mol s−1 ) molar fraction of water in the feed molar fraction of water in the retentate molar fraction of water in the permeate gas

P.D. Chapman et al. / Journal of Membrane Science 268 (2006) 13–19

Greek letters α separation factor γ w , γ t activity coefficient of water and THF δ membrane thickness (m) Λ12 , Λ21 Wilson parameters for the THF–water binary solution Subscripts F feed entering cell P permeate leaving cell R retentate leaving cell t tetrahydrofuran (THF) w water References [1] A. Yamasaki, T. Iwatsubo, T. Masuoka, K. Mizoguchi, Pervaporation of ethanol/water through a poly(vinyl alcohol)/cyclodextrin (PVA/CD) membrane, J. Membrane Sci. 89 (1994) 111–117. [2] I. Cabasso, Z.-Z. Liu, The permselectivity of ion-exchange membranes for non-electrolyte liquid mixtures. 1. Separation of alcohol/water mixtures with Nafion hollow fibers, J. Membrane Sci. 24 (1985) 101–119. [3] W.-H. Chan, C.-F. Ng, S.-Y. Lam-Leung, X. He, O.-C. Cheung, Water–alcohol separation by pervaporation through poly(amidesulfonamides) (PASAs) membranes, J. Appl. Polym. Sci. 65 (6) (1997) 1113–1119. [4] M.H.V. Mulder, C.A. Smolders, On the mechanism of separation of ethanol/water mixtures by pervaporation. 1. Calculations of concentration profiles, J. Membrane Sci. 17 (1984) 289–307.

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