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Purification and dehydration of methylal by pervaporation
Journal of Membrane Science 217 (2003) 159–171
Purification and dehydration of methylal by pervaporation E. Carretier a , Ph. Moulin a,∗ , M. Beaujean b , F. Charbit a a
Laboratoire en Procédés Propres et Environnement (LPPE, EA 884), ENSSPICAM, Université de St Jérˆome, Avenue Escadrille Normandie Niemen, 13397 Marseille Cedex 20, France b Lambiotte & Cie S.A., 18 av. des Aubépines, B-1180 Brussels, Belgium Received 30 July 2002; received in revised form 5 March 2003; accepted 6 March 2003
Abstract Pervaporation studied for the removal of the VOC traces from effluents is now used to dehydrate organic solutions and this paper is focused on an acetal called methylal. Different membranes and different operating conditions were investigated: pure quality or anhydrous qualities were obtained from technical or intermediate qualities. However, due to degradations and weak permeate flows, it is impossible to consider an industrial development using organic membranes. A ceramic membrane yields high permeate fluxes without degradation. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Pervaporation; Methylal; Dehydration; Purification; Industrial solution
1. Introduction For historical reasons, Lambiotte & Cie S.A. [1] are located in Brussels, Belgium, and the plant, sawmill and laboratories are in Marbehan, Province of Luxemburg, Belgium. In 1850, the development of the company began with forestry, sawmill and commercialisation of railway sleepers. Around 1890, Lambiotte & Cie S.A. used sawmill waste for the carbonisation of wood and in 1901, for the first time used carbonisation derivatives, gas and tars for diverse chemical activities (first formaldehyde unit in 1905). Until 1965, this society had developed field by perfecting the vertical continuous carbonisation retorts, and had also grown geographically in 1939 when several plants were started in France in spite of difficult times during the two World Wars. In 1970, Lambiotte ∗ Corresponding author. Tel.: +33-4-42-90-85-05; fax: +33-4-91-02-77-76. E-mail address: [email protected] (Ph. Moulin).
& Cie S.A. had developed its own production process for formaldehyde and methylal. Since 1992, the company is ISO 9002 certified. Continued research on new products has made it possible to elaborate other acetals and to invest in new acetal production units. The methylal (Table 1), also called dimethoxymethane, is a colourless, highly volatile solvent with a low boiling point, low viscosity and an excellent dissolving power. It has a good toxicological profile and it is biodegradable. On the other hand, it is very flammable and thus requires suitable conditions of storage. Thanks to an exceptional solvent power, its amphiphilic character (methylal is both hydrophilic and lipophilic), an extremely low viscosity, a low surface tension, a particularly high evaporation rate, methylal is useful in several fields: aerosols for cosmetic and technical applications, paints and varnishes, paint strippers, cleaning and degreasing solvents, pharmaceuticals, synthesis, polymers, resins, adhesives, extraction, fuel additive, insecticides, etc. Thus, Lambiotte & Cie S.A. proposes various methylal grades
0376-7388/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0376-7388(03)00125-X
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E. Carretier et al. / Journal of Membrane Science 217 (2003) 159–171 Table 1 Methylal physical properties
Nomenclature J P T wi
permeate flux (mol s−1 m−2 ) pressure (Pa) temperature (◦ C) weight fraction of i component
Greek letter β selectivity Subscript col column det detector do downstream inj injector p permeate r retentate
Name
including pure and anhydrous grades (Fig. 1) products for the pharmaceutical use, the cosmetic and is looking for further purification processes. A purification treatment is consequently required in order to adjust different quantities of methanol and variable water quantities. One synthesis way is the condensation of formaldehyde with methanol in the presence of an acid catalyst: HCHO + 2CH3 OH CH2 (OCH3 )2 + H2 O
Formula Molecular weight Surface tension (N cm−1 (dyn cm−1 )) Vapour pressure (Pa) Viscosity (cps) Density Boiling point CAS registry number
Methylal; dimethoxymethane; dimethyl formal; formal; formaldehyde dimethyl acetal; methyl formal C 3 H8 O 2 76.1 21.12 × 10−5 (21.12) (20 ◦ C) 44.103 (20 ◦ C) 0.325 0.8593 42.3 ◦ C 109-87-5
The aim of this paper is to study the purification of the methylal by a membrane process. The process considered is pervaporation, which makes it possible to eliminate organic compounds from aqueous solutions or, on the contrary, water from organic mixture
Fig. 1. Various methylal grades proposed by Lambiotte & Cie S.A.
E. Carretier et al. / Journal of Membrane Science 217 (2003) 159–171
[2,3]. In recent years, pervaporation has emerged as an economical and easy to operate replacement of traditional processes for dehydration of organics [4–7], removal of organics from water [8–10] and separation of organic mixtures [11,12]. In this work, depending on the initial solution, we have used pervaporation membrane process to remove organic compounds or dehydrate organic solutions. The removal of methanol and/or water contained in the methylal is carried out. We have investigated several pervaporation membranes (material, thickness, chemistry, geometry), with different operating conditions and for various methylal grades. We have studied the performances in terms of permeate flux and membrane selectivities but also the possibilities of an industrialisation of this process.
161
• membrane 2256 to remove the methanol, • membrane 2200 to dehydrate organic solutions, and a mineral membrane from Isotronics Company, referenced membrane SS304. Generally, performances of pervaporation membranes are highly influenced by changes in operating conditions such as concentration and temperature. Since they are resistant and chemically stable, ceramic membranes could represent a major improvement. The interest in using such membranes in purification increases. Fortunately, ceramic membranes with narrow pore size distributions are now commercially available [14]. In addition, the permeate flux obtained is nicely high. 2.3. Experimental plant Experimental plant and procedure are usual and already described [13].
2. Experimental 2.1. Solutions
2.4. Analysis Characteristics of the industrial solutions provided by the Lambiotte & Cie S.A. are given in Fig. 1 and Table 1. Starting from grade A (technical grade), the aim is to obtain grades C or D (methanol removal or both water and methanol removal). Starting from grade B (intermediate stage of the production), the goal is grade C or D (water removal). Compositions of these grades (C and D) are given as standards to compare the different products finally obtained.
During the experiment, both permeate and retentate samples were collected and analysed by a gas chromatograph (VARIAN 3350, Varian chromatography system, California, USA) with a Porapak Q column (Q80/100, Varian SA, California, USA) equipped with a catharometer detector (W2X, Varian chromatography system, California, USA). The chromatography parameters are: column: Porapak Q80/100; gas: helium; pressure: 3.2 bars; Tinj : 200 ◦ C; Tdet : 200 ◦ C.
2.2. Membranes Three organic membranes provided by Sulzer Company were tested (Table 2): • membrane 1060 previously used for the purification of VOCs [13],
Due to the low boiling point of the methylal (42.3 ◦ C), the initial temperature of the column is
Table 2 Membrane characteristics Membrane
Geometry
Thickness
Area (cm2 )
Materiala
Interests
Sulzer 1060 Sulzer 2256 Sulzer 2200 Isotronics SS304
Plane Plane Plane Tube
20 m 20 m 20 m 1.5 mm
55 55 55 70
PVA/PAN PVA/PAN PVA/PAN Zeolite
Removal of VOCs Removal of methanol Dehydration Mineral, dehydration
a
PVA: polyvinylalcohol, PAN: polyacrilonitrile.
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low and increases as 5 ◦ C min−1 from 100 to 150 ◦ C. The three components are consequently separated: water (3.6 min), methanol (6.4 min) and the methylal (18.4 min). This analysis procedure was validated by Lambiotte & Cie S.A.
Table 3 Selectivity of the membrane 1060 (grade A, T = 40 ◦ C, Pdo = 4.2 mbar) Compounds
100β
β i /β water
Water Methanol Methylal
81.5 79.9 102.4
1.00 0.98 1.26
3. Results and discussion 3.1. Organic membrane 3.1.1. Membrane 1060 The membrane selectivity β i for i compound is dep fined as the ratio of i composition in the permeate (wi ) r on i composition in the retentate (wi ) as: p
βi =
wi wir
Membrane 1060 was tested in order to remove methanol from the grade A solution (technical methylal) in which methanol concentration is somewhat high (up to 6%). Table 3 presents results obtained for grade A. This table suggests that the separation aimed cannot succeed using the membrane tested since the different selectivities are close to each other. On the Fig. 2, the initial solution corresponds to initial grade A, the final retentate corresponds to the solution obtained at the end of the experiment. Between these two samples, compositions are presented of perme-
Table 4 Selectivity of the membrane 2256 (grade A, T = 40 ◦ C, Pdo = 0.4 mbar) Compounds
100β
β i /β water
Water Methanol Methylal
2500 700 50
1.00 0.28 0.02
ates collected after different time laps. Fig. 2 shows that different compositions (initial, permeates, retentate) are similar (0.4% of water, 6.3% of methanol and 93.3% of methylal). There is no separation and no further study was developed using this membrane. 3.1.2. Membrane 2256 According to Sulzer Company, this membrane is designed for methanol removal, the initial feed solution is grade A (>5% of methanol). However, the membrane selectivity measured (Table 4) reveals a
Fig. 2. Variation of weight fractions in the feed and permeate streams vs. time (membrane 1060, grade A, T = 40 ◦ C, Pdo = 4.2 mbar).
E. Carretier et al. / Journal of Membrane Science 217 (2003) 159–171
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Fig. 3. Variation of weight fractions in the feed and permeate streams vs. time (membrane 2256, grade A, T = 40 ◦ C, Pdo = 0.4 mbar).
strong affinity for water. Fig. 3 shows that some methanol and water pass through membrane 2256 whereas methylal is more retained. Except the first permeate, which is collected before the stationary non-equilibrium state is achieved, all permeate compositions are similar. Methanol and water are removed and represented approximately 40 and 8% of the permeate flux. At the end of the purification, dehydration is achieved (no water in the final solution) and methanol composition is close to 4%. This product is not exactly corresponding to grade C (<3.5%). The permeate flux (Fig. 4) is close to 1 kg h−1 m−2 and slightly decreases versus time. The influence of the temperature is weak: the selectivity is constant whatever the temperature from 20 to 40 ◦ C, whereas the permeate flux increases very slightly with the temperature. The influence of the pressure is much stronger and Fig. 5 shows that the results are different. Permeate composition reveals no membrane selectivity at an industrial downstream pressure (100 mbar). However, at reduced pressure (3 mbar), the permeate composition shows that more methanol and water are removed and the results are similar to previous ones (Fig. 3). Analogous conclusions are deduced from permeate flux, that is, 0.3 kg h−1 m−2 at 100 mbar and increases to 1 kg h−1 m−2 at lower pressure, 3 mbar.
This can be discussed in the light of thermodynamics. Fig. 6 shows the variations of the vapour pressure versus temperature calculated by means of ProPhy-Plus software for the mixture of 53% methylal, 40% methanol and 7% water which is the average composition of the feed considered. It appears that condensation might occur at 100 mbar within the 15–25 ◦ C range. This is consistent with our experiments. Our lab-scale set-up ensures that the upstream side of the membrane is maintained at 20 ◦ C but temperature decreases in the downstream compartment to the cold traps. A strict control of downstream temperature is an important operating parameter of the process. At 100 mbar, the condensation occurs on the downstream side of the membrane which reduces desorption and permeate flux. Due to the respective vapour pressure values, water and methanol are more sensitive than methylal to the pressure variation. In addition, water and methanol compositions are very low at the beginning, so we understand better the inversion of the membrane selectivity observed. This suggests that water removal from a grade B solution could be undertaken using a 2256 membrane, though this membrane is not specific. Two experiments at 20 and 40 ◦ C were carried out and the results at 40 ◦ C are presented in Fig. 7. The initial feed
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Fig. 4. Permeate flux vs. time (membrane 2256, grade A, T = 40 ◦ C, Pdo = 0.4 mbar).
Fig. 5. Variation of weight fractions in the feed and permeate streams vs. time (membrane 2256, grade A, T = 20 ◦ C, Pdo = 100 and 3 mbar).
E. Carretier et al. / Journal of Membrane Science 217 (2003) 159–171
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Fig. 6. Liquid and vapour pressure vs. temperature (ProPhy-Plus software, permeate composition: wmethylal = 0.53, wmethanol = 0.40, wwater = 0.07).
composition is 1.4% water, 0.2% methanol and 98.4% methylal and the final purified methylal obtained is 0.5% water and 99.5% methylal. The purification from grade B to grade C is demonstrated and our pur-
pose is achieved. However, permeate flux is very weak with an average value close to 100 g h−1 m−2 . As above-mentioned, the temperature has low influence on the membrane selectivity and the permeate flux
Fig. 7. Variation of weight fractions in the feed and permeate streams vs. time (membrane 2256, grade B, T = 40 ◦ C, Pdo = 6.2 mbar).
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E. Carretier et al. / Journal of Membrane Science 217 (2003) 159–171
Fig. 8. Variation of weight fractions in the feed and permeate streams vs. time (membrane 2256, grade B, T = 20 ◦ C, Pdo = 100 and 5.6 mbar).
slightly decreases with temperature. At typical industrial downstream pressure (100 mbar), the membrane is no more selective. The selectivity modification is well highlighted in Fig. 8 when the pressure downstream is changed. The average value of permeate flux is 40 g h−1 m−2 and increases to 100 g h−1 m−2 at 5.6 mbar. The purification from grade A to grade C by methanol removal does not seem to be possible us-
ing membrane 2256 from Sulzer Company. However, dehydration from grade B to grade C is much easier though the permeate flux is weak even at laboratory operating conditions (Pdo , 10 mbar). Moreover, the membrane is degraded with time: the active layer is destroyed. For these main reasons (weak flux and membrane degradation), the industrial methylal purification with this membrane seems impossible by pervaporation.
Fig. 9. Variation of weight fractions in the feed and permeate streams vs. time (membrane 2256, grade B, T = 40 ◦ C, Pdo = 6 mbar).
E. Carretier et al. / Journal of Membrane Science 217 (2003) 159–171
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Fig. 10. Variation of weight fractions in the feed and permeate streams vs. time (membrane SS304, grade B, T = 50 ◦ C, Pdo = 10 mbar).
Fig. 11. Permeate flux vs. water weight fractions in the feed (membrane SS304, grade B, T = 50 ◦ C, Pdo = 10 mbar).
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Fig. 12. Chromatographs of the anhydrous grade: (a) obtained by pervaporation; (b) sample of Lambiotte & Cie S.A. (grade D).
E. Carretier et al. / Journal of Membrane Science 217 (2003) 159–171
Fig. 13. Chromatographs of the pure grade: (a) obtained by pervaporation; (b) sample of Lambiotte & Cie S.A. (grade C).
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3.1.3. Membrane 2200 Such membranes are planned for dehydration and were tested with a grade B solution in order to remove water. Fig. 9 shows that the permeate only contains water and thus allows the dehydration aimed. Thus, it should be possible to obtain grade C or D from grade B. Unfortunately, the same conclusions as before for the other membrane can be made: very weak permeate flux (lower than 50 g h−1 m−2 ) and degradation of the membrane by the feed solution (removal of the active layer by sheets). Thus, these results obtained with the membranes from Sulzer Company demonstrate that the constraints of Lambiotte & Cie S.A. are fulfilled in the case of dehydration of grade B to grade C or D. Unfortunately, low permeate flux and possible degradation of the membrane again do not allow scaling up. So, we considered the dehydration from grade B to grade C or D using a mineral tubular membrane which, according to published results [5], is efficient. 3.1.4. Membrane Isotronics SS304 This membrane is very hydrophilic and therefore allows methylal dehydration. The interest in using this membrane is to obtain a pure grade C from grade B. Table 5 is consistent with the manufacturer data: the membrane does not retain water. The selectivity observed is in agreement with the results published for a zeolite membrane [16]. The permeate only contains water and at the end the purified methylal obtained is grade C or D (Fig. 10). This kind of membrane resists pressure drop and thus it is possible to work with 10 mbar as an industrial downstream pressure as recommended by the membrane supplier. For such industrial pressures used, permeate flux are significant and depend on the water composition in the feed solution (Fig. 11). These results are in agreement with previous published results [15]. Moreover, no degradation of the membranes is observed after several experiments. Experiments were carried out at different temperatures. The operating temperature chosen was 50 ◦ C given Table 5 Selectivity of the membrane SS304 (grade B, T = 20 ◦ C, Pdo = 4.2 mbar) Compounds
100β
β i /β water
Water Methylal
11079.3 0.8
1.00 0.00
that temperature increase does not improve the results significantly, but increases evaporation of methylal, energy consumption, in addition with the risks due to this flammable product. Chromatograms presented in Figs. 12 and 13 show that the dehydration is aimed. 4. Conclusion The aim of the purification of the methylal by pervaporation for Lambiotte & Cie S.A. is the removal of water and/or methanol from a water/methanol/methylal mixture. In short, it can be concluded that the initial study turned to the dehydration of a water/methylal mixtures. The main conclusions are the following: • It is possible, using selective membranes from Sulzer Company, to remove either water or methanol. This purification was only carried out at the laboratory scale and scaling up was no longer considered due to the permeate fluxes obtained: they are so low that industrial plants would require very large membrane areas. In addition, membranes are feed sensitive after some experiments; the active layer was damaged. • Dehydration of methylal is also possible with membrane Isotronics SS304. We can finally obtain a grade C or D starting from an intermediate solution grade B. The difference between these two final grades obtained only comes from the membrane area required. Membrane Isotronics SS304 does not undergo physical degradations meanwhile permeate flux obtained are significant whence industrial scale-up is possible. • The operating conditions of the experiments considered in this paper are those used as industrial conditions and we could estimate the membrane area required (from grade B to grade C) for an industrial purification: less than 10 m2 would be sufficient and correspond of approximately to 305 000. This first estimation could be refined using a larger membrane than that tested. References [1] http://www.lambiotte.com/. [2] J. Néel, Pervaporation, Génie des procédés de l’école de Nancy, Technique et documentation, 1997.
E. Carretier et al. / Journal of Membrane Science 217 (2003) 159–171 [3] R. Clement, La pervaporation: application au traitement des effluents aqueux, Trib. Eau 603–605 (2000) 108–135. [4] J.M. Neto, M.N. Pinho, Mass transfer modelling for solvent dehydration by pervaporation, Sep. Purif. Tech. 18 (2001) 151–161. [5] A.W. Verkerk, P. van Male, M.A.G. Vorstman, J.T.F. Keurentjes, Properties of high ceramic pervaporation membranes for dehydration of alcohol/water mixtures, Sep. Purif. Tech. 22–23 (2001) 689–695. [6] K.S. Sportsman, J.D. Way, W.J. Chen, G.P. Pez, D.V. Laciak, The dehydration of nitric acid using pervaporation and a nafion perfluorosulfonate/perfluorocarboxylate bilayer membrane, J. Membr. Sci., in press. [7] R. Rautenbach, M. Franke, S.T. Klatt, Dehydration of organic mixtures by pervaporation, J. Membr. Sci. 61 (1991) 31– 48. [8] S. Schnabel, P. Moulin, Q.T. Nguyen, D. Roizard, P. Aptel, Removal of volatile organic components (VOC) from water by pervaporation with dense silicone hollow fibres. Separation improvement by secondary flows, J. Membr. Sci. 142 (1998) 129–141. [9] C.K. Yeom, H.K. Kim, J.W. Rhim, Removal of trace VOCs from water through PDMS membranes and analysis of their permeation behaviors, J. Appl. Polym. Sci. 73 (1999) 601– 611. [10] R.W. Baker, J.G. Wijmans, A.L. Athayde, R. Daniels, J.H. Ly, M. Le, The effect of concentration polarization on the
[11]
[12]
[13]
[14] [15]
[16]
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separation of volatile organic compounds from water by pervaporation, J. Membr. Sci. 137 (1997) 159–172. I. AbouNemeh, A. Das, A. Saraf, K.K. Sikdar, A composite hollow fiber membrane-based pervaporation process for separation of VOCs from aqueous surfactant solutions, J. Membr. Sci. 158 (1999) 187–209. T. Allouane, P. Moulin, F. Charbit, Modélisation par la thermodynamique des processus irréversibles du transfert de matière en pervaporation de mélanges complexes, 8ème Congrès SFGP, Nancy, France, 17–19 Octobre 2001, in: Récent Progrès en Génie des Procédés, Edition Technique et Documentation, Lavoisier, vol. 15, 2001, pp. 99–106. P. Moulin, T. Allouane, L. Latapie, C. Raufast, F. Charbit, Valorisation of an industrial fuel by pervaporation, J. Membr. Sci. 197 (2002) 103–115. F.M. Velterop, Book of Abstract, No. 2 Euromembrane 99, Leuven, 19–22 September 1999, p. 118. R. Jiraratananon, A. Chanachai, R.Y.M. Huang, D. Uttapap, Pervaporation dehydration of ethanol–water mixtures with chitosan/hydroxyethlcellulose (CS/HEC) composite membranes. Effect of operating conditions, J. Membr. Sci. 195 (2002) 143–151. T. Gallego-Lizon, E. Edwards, G. Lobiundo, L.F. dos Santos, Dehydration of water/t-butanol mixtures by pervaporation: comparative study of commercially available polymeric, microporous silica and zeolite membranes, J. Membr. Sci. 197 (2002) 309–319.
Purification and dehydration of methylal by pervaporation E. Carretier a , Ph. Moulin a,∗ , M. Beaujean b , F. Charbit a a
Laboratoire en Procédés Propres et Environnement (LPPE, EA 884), ENSSPICAM, Université de St Jérˆome, Avenue Escadrille Normandie Niemen, 13397 Marseille Cedex 20, France b Lambiotte & Cie S.A., 18 av. des Aubépines, B-1180 Brussels, Belgium Received 30 July 2002; received in revised form 5 March 2003; accepted 6 March 2003
Abstract Pervaporation studied for the removal of the VOC traces from effluents is now used to dehydrate organic solutions and this paper is focused on an acetal called methylal. Different membranes and different operating conditions were investigated: pure quality or anhydrous qualities were obtained from technical or intermediate qualities. However, due to degradations and weak permeate flows, it is impossible to consider an industrial development using organic membranes. A ceramic membrane yields high permeate fluxes without degradation. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Pervaporation; Methylal; Dehydration; Purification; Industrial solution
1. Introduction For historical reasons, Lambiotte & Cie S.A. [1] are located in Brussels, Belgium, and the plant, sawmill and laboratories are in Marbehan, Province of Luxemburg, Belgium. In 1850, the development of the company began with forestry, sawmill and commercialisation of railway sleepers. Around 1890, Lambiotte & Cie S.A. used sawmill waste for the carbonisation of wood and in 1901, for the first time used carbonisation derivatives, gas and tars for diverse chemical activities (first formaldehyde unit in 1905). Until 1965, this society had developed field by perfecting the vertical continuous carbonisation retorts, and had also grown geographically in 1939 when several plants were started in France in spite of difficult times during the two World Wars. In 1970, Lambiotte ∗ Corresponding author. Tel.: +33-4-42-90-85-05; fax: +33-4-91-02-77-76. E-mail address: [email protected] (Ph. Moulin).
& Cie S.A. had developed its own production process for formaldehyde and methylal. Since 1992, the company is ISO 9002 certified. Continued research on new products has made it possible to elaborate other acetals and to invest in new acetal production units. The methylal (Table 1), also called dimethoxymethane, is a colourless, highly volatile solvent with a low boiling point, low viscosity and an excellent dissolving power. It has a good toxicological profile and it is biodegradable. On the other hand, it is very flammable and thus requires suitable conditions of storage. Thanks to an exceptional solvent power, its amphiphilic character (methylal is both hydrophilic and lipophilic), an extremely low viscosity, a low surface tension, a particularly high evaporation rate, methylal is useful in several fields: aerosols for cosmetic and technical applications, paints and varnishes, paint strippers, cleaning and degreasing solvents, pharmaceuticals, synthesis, polymers, resins, adhesives, extraction, fuel additive, insecticides, etc. Thus, Lambiotte & Cie S.A. proposes various methylal grades
0376-7388/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0376-7388(03)00125-X
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E. Carretier et al. / Journal of Membrane Science 217 (2003) 159–171 Table 1 Methylal physical properties
Nomenclature J P T wi
permeate flux (mol s−1 m−2 ) pressure (Pa) temperature (◦ C) weight fraction of i component
Greek letter β selectivity Subscript col column det detector do downstream inj injector p permeate r retentate
Name
including pure and anhydrous grades (Fig. 1) products for the pharmaceutical use, the cosmetic and is looking for further purification processes. A purification treatment is consequently required in order to adjust different quantities of methanol and variable water quantities. One synthesis way is the condensation of formaldehyde with methanol in the presence of an acid catalyst: HCHO + 2CH3 OH CH2 (OCH3 )2 + H2 O
Formula Molecular weight Surface tension (N cm−1 (dyn cm−1 )) Vapour pressure (Pa) Viscosity (cps) Density Boiling point CAS registry number
Methylal; dimethoxymethane; dimethyl formal; formal; formaldehyde dimethyl acetal; methyl formal C 3 H8 O 2 76.1 21.12 × 10−5 (21.12) (20 ◦ C) 44.103 (20 ◦ C) 0.325 0.8593 42.3 ◦ C 109-87-5
The aim of this paper is to study the purification of the methylal by a membrane process. The process considered is pervaporation, which makes it possible to eliminate organic compounds from aqueous solutions or, on the contrary, water from organic mixture
Fig. 1. Various methylal grades proposed by Lambiotte & Cie S.A.
E. Carretier et al. / Journal of Membrane Science 217 (2003) 159–171
[2,3]. In recent years, pervaporation has emerged as an economical and easy to operate replacement of traditional processes for dehydration of organics [4–7], removal of organics from water [8–10] and separation of organic mixtures [11,12]. In this work, depending on the initial solution, we have used pervaporation membrane process to remove organic compounds or dehydrate organic solutions. The removal of methanol and/or water contained in the methylal is carried out. We have investigated several pervaporation membranes (material, thickness, chemistry, geometry), with different operating conditions and for various methylal grades. We have studied the performances in terms of permeate flux and membrane selectivities but also the possibilities of an industrialisation of this process.
161
• membrane 2256 to remove the methanol, • membrane 2200 to dehydrate organic solutions, and a mineral membrane from Isotronics Company, referenced membrane SS304. Generally, performances of pervaporation membranes are highly influenced by changes in operating conditions such as concentration and temperature. Since they are resistant and chemically stable, ceramic membranes could represent a major improvement. The interest in using such membranes in purification increases. Fortunately, ceramic membranes with narrow pore size distributions are now commercially available [14]. In addition, the permeate flux obtained is nicely high. 2.3. Experimental plant Experimental plant and procedure are usual and already described [13].
2. Experimental 2.1. Solutions
2.4. Analysis Characteristics of the industrial solutions provided by the Lambiotte & Cie S.A. are given in Fig. 1 and Table 1. Starting from grade A (technical grade), the aim is to obtain grades C or D (methanol removal or both water and methanol removal). Starting from grade B (intermediate stage of the production), the goal is grade C or D (water removal). Compositions of these grades (C and D) are given as standards to compare the different products finally obtained.
During the experiment, both permeate and retentate samples were collected and analysed by a gas chromatograph (VARIAN 3350, Varian chromatography system, California, USA) with a Porapak Q column (Q80/100, Varian SA, California, USA) equipped with a catharometer detector (W2X, Varian chromatography system, California, USA). The chromatography parameters are: column: Porapak Q80/100; gas: helium; pressure: 3.2 bars; Tinj : 200 ◦ C; Tdet : 200 ◦ C.
2.2. Membranes Three organic membranes provided by Sulzer Company were tested (Table 2): • membrane 1060 previously used for the purification of VOCs [13],
Due to the low boiling point of the methylal (42.3 ◦ C), the initial temperature of the column is
Table 2 Membrane characteristics Membrane
Geometry
Thickness
Area (cm2 )
Materiala
Interests
Sulzer 1060 Sulzer 2256 Sulzer 2200 Isotronics SS304
Plane Plane Plane Tube
20 m 20 m 20 m 1.5 mm
55 55 55 70
PVA/PAN PVA/PAN PVA/PAN Zeolite
Removal of VOCs Removal of methanol Dehydration Mineral, dehydration
a
PVA: polyvinylalcohol, PAN: polyacrilonitrile.
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low and increases as 5 ◦ C min−1 from 100 to 150 ◦ C. The three components are consequently separated: water (3.6 min), methanol (6.4 min) and the methylal (18.4 min). This analysis procedure was validated by Lambiotte & Cie S.A.
Table 3 Selectivity of the membrane 1060 (grade A, T = 40 ◦ C, Pdo = 4.2 mbar) Compounds
100β
β i /β water
Water Methanol Methylal
81.5 79.9 102.4
1.00 0.98 1.26
3. Results and discussion 3.1. Organic membrane 3.1.1. Membrane 1060 The membrane selectivity β i for i compound is dep fined as the ratio of i composition in the permeate (wi ) r on i composition in the retentate (wi ) as: p
βi =
wi wir
Membrane 1060 was tested in order to remove methanol from the grade A solution (technical methylal) in which methanol concentration is somewhat high (up to 6%). Table 3 presents results obtained for grade A. This table suggests that the separation aimed cannot succeed using the membrane tested since the different selectivities are close to each other. On the Fig. 2, the initial solution corresponds to initial grade A, the final retentate corresponds to the solution obtained at the end of the experiment. Between these two samples, compositions are presented of perme-
Table 4 Selectivity of the membrane 2256 (grade A, T = 40 ◦ C, Pdo = 0.4 mbar) Compounds
100β
β i /β water
Water Methanol Methylal
2500 700 50
1.00 0.28 0.02
ates collected after different time laps. Fig. 2 shows that different compositions (initial, permeates, retentate) are similar (0.4% of water, 6.3% of methanol and 93.3% of methylal). There is no separation and no further study was developed using this membrane. 3.1.2. Membrane 2256 According to Sulzer Company, this membrane is designed for methanol removal, the initial feed solution is grade A (>5% of methanol). However, the membrane selectivity measured (Table 4) reveals a
Fig. 2. Variation of weight fractions in the feed and permeate streams vs. time (membrane 1060, grade A, T = 40 ◦ C, Pdo = 4.2 mbar).
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Fig. 3. Variation of weight fractions in the feed and permeate streams vs. time (membrane 2256, grade A, T = 40 ◦ C, Pdo = 0.4 mbar).
strong affinity for water. Fig. 3 shows that some methanol and water pass through membrane 2256 whereas methylal is more retained. Except the first permeate, which is collected before the stationary non-equilibrium state is achieved, all permeate compositions are similar. Methanol and water are removed and represented approximately 40 and 8% of the permeate flux. At the end of the purification, dehydration is achieved (no water in the final solution) and methanol composition is close to 4%. This product is not exactly corresponding to grade C (<3.5%). The permeate flux (Fig. 4) is close to 1 kg h−1 m−2 and slightly decreases versus time. The influence of the temperature is weak: the selectivity is constant whatever the temperature from 20 to 40 ◦ C, whereas the permeate flux increases very slightly with the temperature. The influence of the pressure is much stronger and Fig. 5 shows that the results are different. Permeate composition reveals no membrane selectivity at an industrial downstream pressure (100 mbar). However, at reduced pressure (3 mbar), the permeate composition shows that more methanol and water are removed and the results are similar to previous ones (Fig. 3). Analogous conclusions are deduced from permeate flux, that is, 0.3 kg h−1 m−2 at 100 mbar and increases to 1 kg h−1 m−2 at lower pressure, 3 mbar.
This can be discussed in the light of thermodynamics. Fig. 6 shows the variations of the vapour pressure versus temperature calculated by means of ProPhy-Plus software for the mixture of 53% methylal, 40% methanol and 7% water which is the average composition of the feed considered. It appears that condensation might occur at 100 mbar within the 15–25 ◦ C range. This is consistent with our experiments. Our lab-scale set-up ensures that the upstream side of the membrane is maintained at 20 ◦ C but temperature decreases in the downstream compartment to the cold traps. A strict control of downstream temperature is an important operating parameter of the process. At 100 mbar, the condensation occurs on the downstream side of the membrane which reduces desorption and permeate flux. Due to the respective vapour pressure values, water and methanol are more sensitive than methylal to the pressure variation. In addition, water and methanol compositions are very low at the beginning, so we understand better the inversion of the membrane selectivity observed. This suggests that water removal from a grade B solution could be undertaken using a 2256 membrane, though this membrane is not specific. Two experiments at 20 and 40 ◦ C were carried out and the results at 40 ◦ C are presented in Fig. 7. The initial feed
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Fig. 4. Permeate flux vs. time (membrane 2256, grade A, T = 40 ◦ C, Pdo = 0.4 mbar).
Fig. 5. Variation of weight fractions in the feed and permeate streams vs. time (membrane 2256, grade A, T = 20 ◦ C, Pdo = 100 and 3 mbar).
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165
Fig. 6. Liquid and vapour pressure vs. temperature (ProPhy-Plus software, permeate composition: wmethylal = 0.53, wmethanol = 0.40, wwater = 0.07).
composition is 1.4% water, 0.2% methanol and 98.4% methylal and the final purified methylal obtained is 0.5% water and 99.5% methylal. The purification from grade B to grade C is demonstrated and our pur-
pose is achieved. However, permeate flux is very weak with an average value close to 100 g h−1 m−2 . As above-mentioned, the temperature has low influence on the membrane selectivity and the permeate flux
Fig. 7. Variation of weight fractions in the feed and permeate streams vs. time (membrane 2256, grade B, T = 40 ◦ C, Pdo = 6.2 mbar).
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Fig. 8. Variation of weight fractions in the feed and permeate streams vs. time (membrane 2256, grade B, T = 20 ◦ C, Pdo = 100 and 5.6 mbar).
slightly decreases with temperature. At typical industrial downstream pressure (100 mbar), the membrane is no more selective. The selectivity modification is well highlighted in Fig. 8 when the pressure downstream is changed. The average value of permeate flux is 40 g h−1 m−2 and increases to 100 g h−1 m−2 at 5.6 mbar. The purification from grade A to grade C by methanol removal does not seem to be possible us-
ing membrane 2256 from Sulzer Company. However, dehydration from grade B to grade C is much easier though the permeate flux is weak even at laboratory operating conditions (Pdo , 10 mbar). Moreover, the membrane is degraded with time: the active layer is destroyed. For these main reasons (weak flux and membrane degradation), the industrial methylal purification with this membrane seems impossible by pervaporation.
Fig. 9. Variation of weight fractions in the feed and permeate streams vs. time (membrane 2256, grade B, T = 40 ◦ C, Pdo = 6 mbar).
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Fig. 10. Variation of weight fractions in the feed and permeate streams vs. time (membrane SS304, grade B, T = 50 ◦ C, Pdo = 10 mbar).
Fig. 11. Permeate flux vs. water weight fractions in the feed (membrane SS304, grade B, T = 50 ◦ C, Pdo = 10 mbar).
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Fig. 12. Chromatographs of the anhydrous grade: (a) obtained by pervaporation; (b) sample of Lambiotte & Cie S.A. (grade D).
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Fig. 13. Chromatographs of the pure grade: (a) obtained by pervaporation; (b) sample of Lambiotte & Cie S.A. (grade C).
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3.1.3. Membrane 2200 Such membranes are planned for dehydration and were tested with a grade B solution in order to remove water. Fig. 9 shows that the permeate only contains water and thus allows the dehydration aimed. Thus, it should be possible to obtain grade C or D from grade B. Unfortunately, the same conclusions as before for the other membrane can be made: very weak permeate flux (lower than 50 g h−1 m−2 ) and degradation of the membrane by the feed solution (removal of the active layer by sheets). Thus, these results obtained with the membranes from Sulzer Company demonstrate that the constraints of Lambiotte & Cie S.A. are fulfilled in the case of dehydration of grade B to grade C or D. Unfortunately, low permeate flux and possible degradation of the membrane again do not allow scaling up. So, we considered the dehydration from grade B to grade C or D using a mineral tubular membrane which, according to published results [5], is efficient. 3.1.4. Membrane Isotronics SS304 This membrane is very hydrophilic and therefore allows methylal dehydration. The interest in using this membrane is to obtain a pure grade C from grade B. Table 5 is consistent with the manufacturer data: the membrane does not retain water. The selectivity observed is in agreement with the results published for a zeolite membrane [16]. The permeate only contains water and at the end the purified methylal obtained is grade C or D (Fig. 10). This kind of membrane resists pressure drop and thus it is possible to work with 10 mbar as an industrial downstream pressure as recommended by the membrane supplier. For such industrial pressures used, permeate flux are significant and depend on the water composition in the feed solution (Fig. 11). These results are in agreement with previous published results [15]. Moreover, no degradation of the membranes is observed after several experiments. Experiments were carried out at different temperatures. The operating temperature chosen was 50 ◦ C given Table 5 Selectivity of the membrane SS304 (grade B, T = 20 ◦ C, Pdo = 4.2 mbar) Compounds
100β
β i /β water
Water Methylal
11079.3 0.8
1.00 0.00
that temperature increase does not improve the results significantly, but increases evaporation of methylal, energy consumption, in addition with the risks due to this flammable product. Chromatograms presented in Figs. 12 and 13 show that the dehydration is aimed. 4. Conclusion The aim of the purification of the methylal by pervaporation for Lambiotte & Cie S.A. is the removal of water and/or methanol from a water/methanol/methylal mixture. In short, it can be concluded that the initial study turned to the dehydration of a water/methylal mixtures. The main conclusions are the following: • It is possible, using selective membranes from Sulzer Company, to remove either water or methanol. This purification was only carried out at the laboratory scale and scaling up was no longer considered due to the permeate fluxes obtained: they are so low that industrial plants would require very large membrane areas. In addition, membranes are feed sensitive after some experiments; the active layer was damaged. • Dehydration of methylal is also possible with membrane Isotronics SS304. We can finally obtain a grade C or D starting from an intermediate solution grade B. The difference between these two final grades obtained only comes from the membrane area required. Membrane Isotronics SS304 does not undergo physical degradations meanwhile permeate flux obtained are significant whence industrial scale-up is possible. • The operating conditions of the experiments considered in this paper are those used as industrial conditions and we could estimate the membrane area required (from grade B to grade C) for an industrial purification: less than 10 m2 would be sufficient and correspond of approximately to 305 000. This first estimation could be refined using a larger membrane than that tested. References [1] http://www.lambiotte.com/. [2] J. Néel, Pervaporation, Génie des procédés de l’école de Nancy, Technique et documentation, 1997.
E. Carretier et al. / Journal of Membrane Science 217 (2003) 159–171 [3] R. Clement, La pervaporation: application au traitement des effluents aqueux, Trib. Eau 603–605 (2000) 108–135. [4] J.M. Neto, M.N. Pinho, Mass transfer modelling for solvent dehydration by pervaporation, Sep. Purif. Tech. 18 (2001) 151–161. [5] A.W. Verkerk, P. van Male, M.A.G. Vorstman, J.T.F. Keurentjes, Properties of high ceramic pervaporation membranes for dehydration of alcohol/water mixtures, Sep. Purif. Tech. 22–23 (2001) 689–695. [6] K.S. Sportsman, J.D. Way, W.J. Chen, G.P. Pez, D.V. Laciak, The dehydration of nitric acid using pervaporation and a nafion perfluorosulfonate/perfluorocarboxylate bilayer membrane, J. Membr. Sci., in press. [7] R. Rautenbach, M. Franke, S.T. Klatt, Dehydration of organic mixtures by pervaporation, J. Membr. Sci. 61 (1991) 31– 48. [8] S. Schnabel, P. Moulin, Q.T. Nguyen, D. Roizard, P. Aptel, Removal of volatile organic components (VOC) from water by pervaporation with dense silicone hollow fibres. Separation improvement by secondary flows, J. Membr. Sci. 142 (1998) 129–141. [9] C.K. Yeom, H.K. Kim, J.W. Rhim, Removal of trace VOCs from water through PDMS membranes and analysis of their permeation behaviors, J. Appl. Polym. Sci. 73 (1999) 601– 611. [10] R.W. Baker, J.G. Wijmans, A.L. Athayde, R. Daniels, J.H. Ly, M. Le, The effect of concentration polarization on the
[11]
[12]
[13]
[14] [15]
[16]
171
separation of volatile organic compounds from water by pervaporation, J. Membr. Sci. 137 (1997) 159–172. I. AbouNemeh, A. Das, A. Saraf, K.K. Sikdar, A composite hollow fiber membrane-based pervaporation process for separation of VOCs from aqueous surfactant solutions, J. Membr. Sci. 158 (1999) 187–209. T. Allouane, P. Moulin, F. Charbit, Modélisation par la thermodynamique des processus irréversibles du transfert de matière en pervaporation de mélanges complexes, 8ème Congrès SFGP, Nancy, France, 17–19 Octobre 2001, in: Récent Progrès en Génie des Procédés, Edition Technique et Documentation, Lavoisier, vol. 15, 2001, pp. 99–106. P. Moulin, T. Allouane, L. Latapie, C. Raufast, F. Charbit, Valorisation of an industrial fuel by pervaporation, J. Membr. Sci. 197 (2002) 103–115. F.M. Velterop, Book of Abstract, No. 2 Euromembrane 99, Leuven, 19–22 September 1999, p. 118. R. Jiraratananon, A. Chanachai, R.Y.M. Huang, D. Uttapap, Pervaporation dehydration of ethanol–water mixtures with chitosan/hydroxyethlcellulose (CS/HEC) composite membranes. Effect of operating conditions, J. Membr. Sci. 195 (2002) 143–151. T. Gallego-Lizon, E. Edwards, G. Lobiundo, L.F. dos Santos, Dehydration of water/t-butanol mixtures by pervaporation: comparative study of commercially available polymeric, microporous silica and zeolite membranes, J. Membr. Sci. 197 (2002) 309–319.