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Trends in design and preparation of polymeric membranes for pervaporation D. Roizard *, E. Favre CNRS Laboratory (LRGP - Nancy University), ENSIC, 1 rue Grandville, 54000 Nancy - France Abstract: Polymers are the favorite material for membrane applications because they have tunable properties, both chemical and physical ones, which allow the design of selective barriers, and also because they are prompt to give rise to active layers of low thicknesses under various forms, i.e. either as flat sheets, rolled sheets for spiral modules or even as hollow fibers; in addition, most of the time, polymers are also cheap materials that is always a good point for ulterior industrial developments. But in any cases, polymer membranes remain sensitive materials and of course fragile ones, that constitutes their Achilles’ heel. The particular key feature which mainly contributes to the strong potential of polymers is the large variety of chemical functions which can be found and modified when going from one polymer family to another one. Mastering both the structure and the functionality of polymers are certainly the clue to the design and to the preparation of advanced membranes such as mixed matrix or nanoporous membranes. Indeed, one of the fundamental aspects of the pervaporation transport is the strength of the physico-chemical interactions between the membrane and a given penetrant which can lead to the selectivity of the transport. Indeed, whereas in gas permeation the transfer is dominated by the diffusion step, the reverse situation is usually occurring with pervaporation where the absorption step is the decisive one. Unfortunately there is no universal membrane able to handle with success various separation problems, as far as molecular separations are targeted. Indeed the choice of a membrane is always closely related to the nature of the mixtures to be fractionized, i.e. water or organic removal, and even to the precise mixture composition; in the worst case, the use of an inappropriate membrane can even lead to its damage. Inorganic membranes are actually rarely used in pervaporation compared to organic ones; but it is certain that some specific needs will push the development of inorganic membranes in the near future thanks to the big improvements have been achieved in the last ten years in the controlled synthesis of inorganic structures having very narrow pores able for instance to promote a very efficient dehydration of alcohol mixtures. This paper intends to review the main routes which have been investigated for membrane design since the early time of pervaporation, and to analyze the current trends of polymeric membrane design and preparation published in the open literature. Taking into account the up-to-date knowledge in polymer science, some promising synthesis to new permselective pervaporation membranes are discussed as innovative routes.
INTRODUCTION If natural membranes have been dominating the life on the Earth since the origin of life, the use, control and understanding of natural and artificial membranes by mankind has really started only in the last century; thus one can imagine there is still a long way to go with membrane science in the field of separations, both from fundamental and applications points of view. Indeed as any new technologies, membrane separations have first to demonstrate their intrinsic interests and advantages versus other well established methods; then, even when competitive features are clear, competing with industrial running separation processes is the second key step and often the more difficult one to pass. The only exception is when membranes are able to bring on the market a
new technology having no competitor, as it has been the case with artificial kidney in the 1970’s [1,2]; this early world wide membrane application is still at the pole position of the membrane market as noted by Strathmann in 2000 with a 1.5109 € market sale and a growth of 8%/year [3]. As one can expect, heavy industries like petrochemical refineries and chemical plants are most of the time reluctant to carry out basic process modifications; however some real opportunities arise each time that a new process has to be evaluated in replacement of an old one, either because the productivity must be improved or because the availability of the traditional feedstock is decreasing or changing. Note that for a number of industrial key raw materials, it is actually the case and we can see for instance the rapid incoming of the new bio-industries, while the economical prospects
of the traditional chemical technologies look to turn particularly poor. In addition, energy price and fossil resources availability are today’s pushing industries to reconsider the costs and efficiency of their separation tools, because the separation steps are generally very energy consuming. This peculiar situation constitutes for emerging technologies and, in particular, for membranes processes like gas separation, pervaporation and vapor permeation, another opportunity to enter the industrial game because they can provide an easy way to cut with the energy demand in comparison to thermal separation processes as stressed by Koros [4] and Vane [5].
An emerging technology for difficult liquid separations since the 1980s In the chemical industries, a well-known example of the strong potential of pervaporation (PV) is the breaking of azeotropic mixtures (i.e. EtOH/H2O, MEK/EtOH, AcOEt/H2O, Bz/Cyclohexane,…) [6-7] because high energy penalties are paid by conventional processes. Indeed no simple solution exists and the membranes methods, i.e. pervaporation or the closely related vapor permeation, have proven to be able to solve the problem with competing costs versus vacuum distillation, extractive distillation or other azeotrope breaker processes (Fig.1). Thus performances of pervaporation for dewatering led to the study and development of many processes now installed in various industries [3-8-9]. Liquid Retentate: A + B as impurity
Azeotrope feed: A+B liquid mixture
Low Pressure
Permeate vapour: B + A as minor species
Fig.(1): General scheme of PV unit. Using a PVA water selective composite membrane and EtOH/H2O mixture 95/5 wt%, retentate composition can reach EtOH 99.99% according to the membrane area used. Besides azeotropic mixtures, the separation by distillation of components of close boiling points (i.e. Toluene (110°) / Methyl cyclohexane (101°)) leads to the same problem of high energy demand even if no azeotrope got formed. It is known that industrial distillations consume more than 40% of the total energy used per year in the chemical processing and the interest of a hybrid process coupling distillation and pervaporation to reduce the energy demand was already suggested in Europe in 1985 [10] and then in USA in 1991 [11]. For this type of organic-organic mixtures, the challenge to find an efficient membrane is much more difficult than for the azeotropic case where very often the involved components have
distinct physical and chemical properties. Hence there is a huge potential market with this type of separations and the design of high-performance membrane materials still remains a priority for the relevant applications [12-13].
Particular interests of pervaporation Pervaporation can be also an interesting method of purification for thermal sensitive chemicals such as unstable molecules, which are tricky to distillate safely (e.g. liquid propellant [14-15]); it is the same aim with natural products (e.g. flavors), which can be difficult to purify by thermal technologies without loosing their functionality [16]. Indeed, during the last 15 years, a lot of work have been dedicated to the purification of high added value compounds used in the food industries like aromas by membrane recovery [17-18-19]; this is due to the fact that membrane processes can be operated under mild conditions and that they can allow the concentration or the extraction aromas from complex diluted media [20-21-22]. Moreover, being a physical based method, membrane processes fulfill the international food legislation which prohibits the use of chemical methods. Experiments carried out with hollow fibre modules and industrial multicomponent feed have shown the interest of the method with feed containing aroma in the ppm range [23]. Another field of research where pervaporation has revealed to be attractive is bioprocesses [5] and early papers in this area are now more than 20years old [2425]. It is interesting to note that the initial aims were to improve fermentation processes by the selective removal of volatile products acting as inhibitors [26], using either batch [27] or continuous fermentors [2829], and to get alcohol free beverages [30-31]. Nowadays the combination of pervaporation with bioreactors is foreseen to give rise to innovative results in biotechnology [32] and applications to biofuels production [33] and to green chemistry [34] are also considered. Hence intensification of bioprocesses via pervaporation integration is currently investigated [35]. Apart the industrial applications as briefly reported above, pervaporation has also been identified as a very relevant tool in analysis due to the simplicity of the method, the ease of command and automation and the miniaturization ability; indeed pervaporation can be used as a pre-concentration step to improve the sensibility of numerous methods [36-37]. For these reasons pervaporation has been coupled to a lot of conventional analytic methods such as gas chromatography [38], electrophoresis [39], gas spectrometry [40] and to numerous specific detectors (MS, ICP, …) [41] or sensors, for monitoring bioreactors [42] or for the creation of tentative “electronic nose” [43]. A remarkable amount of information has been published on pervaporation and this paper does not pretend to be exhaustive, even a specific full book will
not be able to do it. Reviewing the main results of the open literature, i.e. going from aqueous mixtures to organic ones, this paper intends to underline the principles which can be helpful to an investigator to set up a research strategy to design an efficient membrane for a given separation case. Hence, the aim is to propose a roadmap to creativity which will be, in all cases, decisive to solve a difficult separation problem. Taking into account the up-to-date knowledge on polymer science, some promising routes to prepare new permselective PV membranes are presented and discussed in this review as perspectives. The reader is asked to consult carefully the given references and to learn more on pervaporation through specific textbooks which have been published since 1982 such as Mudler [44], Huang [45], or Neel [46] for fundamental aspects, and Rautenbach [47], Baker [48] or Nunes [49] for membrane applications.
must be evacuated continuously to reach the steady state of the separation and a constant mass transfer regime. The phase change between the two sides of the membrane is a characteristic feature of pervaporation transfer; in all other related membrane liquid separations (dialysis, RO, organic NF), liquids permeate without any phase change. The driving force is the activity gradient of each species which can be maintained between both sides of the membrane, by continuously removing the evolving vapors either with a vacuum pump or with an inert sweeping gas. The most important PV feature to be recalled is that the composition of the liquid feed mixture in equilibrium with the membrane is always different from the upstream side composition of the species into the membrane; this difference of composition is due to the mixture partitioning induced by the membrane and explains why pervaporation can basically act as an azeotrope breaker (Fig.3).
1. WHAT ARE THE PRINCIPLES AND ORIGINALITIES OF PERVAPORATION? The interest of pervaporation (PV) is to induce the separation of miscible liquid mixtures by the selective transfer of one mixture component through a membrane. At the downstream side of the membrane, the penetrant molecules freely evolved as vapor phase, as long as their vapor pressure is maintained well below their saturating vapor pressure (Fig.2). Retentate outlet
Upstream side
Fig.(3). Compared separations achieved by pervaporation and distillation (LVE) processes. Case of EtOH / ETBE mixtures. The upper dashed line indicates a strong EtOH enrichment.
Downstream side
Permeate outlet: Gas phase
Feed inlet: Liquid mixture
Fig.(2). Overview of pervaporation transfer of a binary liquid mixture. The dashed circle component is the slower component to cross the membrane barrier; at the downstream side the appearing vapor phase, mainly constituted by the faster component,
PV membrane characteristics In the early period of pervaporation, only natural polymers were used as dense films, e.g. natural rubber, cellophane; then, starting in the 1960s, it was the turn to many synthetic polymeric films to be tested i.e. polyethylene, polyimides, polyamides, polyelectrolytes, polydimethylsiloxane (PDMS). From this time, it was established that PV separation could occur only through pore-free layers, i.e. polymeric layers having no continuous physical pores linking the membrane upstream side to the downstream one. This view remained valid until the emergence of inorganic multilayer membranes developed in the mid 1990s. Indeed it was shown by Kita that it was possible to prepare membranes having a nanoporous top layer
formed of NaA zeolite, and that such membranes were able to permeate selectively water from water/organic mixtures [50]; then amorphous silica inorganic membranes were also successfully developed [51]. Hence if fifteen years ago it was the rule to state in the introduction part of the pervaporation textbook that pervaporation membranes are characterized by a top non porous polymer layer, this statement is not true anymore today as it has now been established from the PV results obtained with many nanoporous inorganic membranes, as reviewed by Sommer et al. [52]. So up to date, three types of membranes are known to exhibit pervaporation properties: - membranes having a polymeric dense top layer; at lab scale, self standing polymeric films can be used but for industrial applications only composite or asymmetric membranes are used (Fig.4-5); the selective layer is usually prepared on a mechanical highly porous support; - inorganic multilayer membranes having nanoporous top layer (4 to 7.4nm) due to zeolites or amorphous silica (Fig.6-7), and - membranes having a mixed organicinorganic dense top layer, also called mixed matrix membranes (MMM) [53]. Since the last ten years, the design and preparation of these two last membrane categories have been one of the most active research areas in membrane design.
8kV x1,100 10µm
Fig.(4): Composite polyimide membrane: a dense layer is formed on the top of a porous support.
X 3,500
5µm
Fig.(5): Asymmetric polyimide membrane prepared by wet phase inversion with a top dense layer.
Fig.(6): MFI-zeolite membrane prepared by hydrothermal synthesis. The densely polycrystalline zeolite is 10-30µm thick. Scale 1-1000. (Sommer [52])
Fig.(7): Amorphous silica composite membrane prepared on ceramic support made of α- and γalumina. Pervap® SMS. Scale 1-500. (Sommer [52]). For a binary mixture (A, B) where B is the preferentially pervaporated species, the performances of the membrane are usually described according the following criteria: a) Selectivity factor, α: Y .(1 − X ) (dimensionless number, 1
