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Asymmetric Membranes : Preparation and Applications By H.-D. Saier and H.

Strathmann[*l

The separation of molecular mixtures by semipermeable membranes under the driving force of hydrostatic pressure has assumed major importance in recent years. Among other factors the development of membranes with asymmetric structure which, while having the same separating properties, afford a filtration output many times greater than that of the previously known symmetrical membranes, was decisive for this method. In the present progress report the structures of asymmetric membranes are discussed, as well as their preparation from various polymers and their application to the separation of molecular mixtures.

1. Introduction Selective mass transport through membranes has been the subject of numerous scientific investigations for more than a century. In this work attention was first focused on biologi-

[*I

Dr. H.-D. Saier [ +] and Dr. H. Strathmann Forschungsinstitut Berghof GmbH 74 Tubingen-Lustnau, Berghof (Germany) [+] To whom correspondence should be addressed.

452

cal membranes, the importance of which for metabolic processes in living organisms was recognized very early, but more recently synthetic membranes in particular and their application to separation problems in chemistry and industry have acquired economic significance. For example, membrane filtration is nowadays used to extract drinking water from the sea, for fractionating, concentrating, and purifying macromolecular solutions, for the treatment of industrial waste waters, and for the recovery of valuable constituents. Membrane filtraAngeir. Chem. internat. Edit. I Vol. 14 (197.5)

No. 7

tion has proved particularly useful in the treatment of thermally sensitive substances in biochemistry, pharmacology, and the food industry[’-”! Here and in many other problems of separation it is often greatly superior to the more conventional distillation, crystallizaition, extraction, and other processes, for it can often be carried out faster, more economically, and under milder conditions. In view of the wealth of possible applications of membrane filtration and their economy it is surprising that its industrial exploitation became possible only a few years ago, even though the principle of the process and its industrial potential have been known for almost a century from Pfeffer’s investigations’”’ and uun’t Hojf’s theoretical studied’ ‘ I on osmosis. However, the industrial application of the method was held back not least by the difficulty of developing membranes with sufficient filtration efficiency. Only the studies of Loeb and Sourirajan“ ”1, which led to the integral-asymmetric membranes, have made industrial scale membrane filtration feasible, and this stimulus was followed by some very hectic development, particularly in the desalination of seawater and brackish waters. Recent progress in polymer chemistry considerably extended the scope of membrane filtration. Asymmetric membranes consist, in principle, of a relatively thick, highly porous carrier and the actual membrane, the latter being only 0.1 to 0.5pm thick. They are characterized primarily by a greatly increased rate of filtration.

3. Phenomenological Description of Membranes and Membrane Transport Processes Since the membrane itself is decisive for the success of membrane filtration, this section will be devoted to a short discussion of membrane construction and of mass transport through membranes. Quite generally, a membrane is defined as a liquid or solid intermediate phase separating two homogeneous phases and offering different resistances to the passage of different chemical substances. The mass transport through this intermediate phase can be described by the following phenomenological relationship[’ 81:

“ J,

= k= 1

LikXk

i = 1 , 2 , 3 ,... n

(1)

~

2. The Principle of Membrane Filtration Membrane filtration is based fundamentally on a simple principle’‘’! A molecular mixture is brought by convection onto the surfaceofa membrane. While some of the components pass through the membrane under the influence of hydrostatic pressure, others are retained more or less completely. Viewed superficially, membrane filtration differs from conventional filtration only by the size of the particles to be separated. The real difference, however, is that a conventional filter acts asa sieve and separates a mixture of substances simply according to particle sizes, whereas in membrane filtration specific interactions between the mixture components and the membrane matrix are, or can also be, responsible for the separation. It is thus possible not only to carry out separations in the molecular range but also to separate substances having the same or nearly the same molecular dimensions. Terminologically, a distinction is made in membrane filtration between reverse osmosis and ~ltrafiltration[~. l7]. The process is called reverse osmosis when the osmotic pressure difference between the starting solution and the filtrate is not negligible compared with the hydrostatic pressure applied; this is the case when low-molecular compounds are to be separated from a solution, e. g . in the desalination of seawater. When, however, the aim is to separate macromolecular substances with molecular weights of about 10OO0, the osmotic pressure difference between the starting solution and the filtrate is negligibly small compared with the hydrostatic pressure applied, and the process is one of ultrafiltration. Although at first sight this differentiation may appear to be very arbitrary, it has a degree of justification in respect of the membranes used and the hydrostatic pressure required, and there are also genuine differences between the techniques used for reverse osmosis and ultrafiltration (see Section 7).

where J i is the material flow of the component i, Lik is a phenomenological coefficient, and X k is a generalized driving force provided by the chemical and electrochemical potential gradients of the component k in the membrane. The applicability of eq. (1) is restricted by boundary conditions set by the limited validity of linear laws in thermodynamics of irreversible processes ia discontinuous system^[^^-^^^. Although eq. ( 1 ) describes the transport process in membranes in such a way that it includes all the interactions between the particle flows as well as chemical reactions, its value for the description of membrane filtration processes is limited because the phenomenological coefficients are mathematically difficult to obtain and because the equation is strictly valid only in a state close to equilibrium. The generalized definition of a membrane is also of little value when membranes are to be developed for special purposes, because this phenomenological description, like eq. (I), does not take into account the membrane structure or the molecular interactions between the transported particles and the membrane matrix. For the development of membranes in practice it is therefore often more advantageous to work with membrane models.

4. Membrane Models There are two models for the description of filtration membranes, each representing an idealized limiting case: the “pore membrane” and “the solubility membrane”. The ideal pore membrane comes nearer in its structure to a conventional filter. Here mass transport takes place through well-defined pores and the selectivity of the membrane is determined by the pore diameter. Pore membranes can be used only for the separation of substances that differ in molecular dimensions. In contrast, the ideal solubility membrane is a homogeneous polymer layer through which all the components are transported independently of one another by molecular diffusion. The selectivity of the solubility membrane depends on differences between the diffusion coefficients and concentrations of individual components in the membrane matrix.

4.1. Model of a Pore Membrane The mass transport during filtration through an ideal pore membrane depends on volume flow through the pores and 453

can be described by the Hagen-Poiseuille equation. For a system consisting of a solvent and a dissolved component the volume flow density J , is given by the relation[231: J,=L,(A~-cJAR)

(2)

Here L, is the hydrodynamic permeability, A p the specified hydrostatic pressure, A n the difference between the osmotic pressures in the original solution and the filtrate, and CJ a so-called reflection coefficient[241that is a measure of the separating characteristics of the membrane. For a strictly semipermeable membrane C J =1 L 2 1 1 . The hydrostatic permeability L , is given by:

Here (0 is the porosity, r the mean pore radius, q the viscosity, i. a correction factor for pore length, and A x the thickness of the membrane. For dilute solutions and membranes with good selectivity the flow density JI of the solvent is equal, in the first approximation, to the volume flow density J,. For the dissolved component the flow density J , is given by the following

J,

=

J,c?

-

D?(dC'/d.x)

v,

For description of the filtration flow density of the dissolved component J , in a dilute solution the pressure term R A p can become negligible in comparison with the concentration term RTAc,/c,L2h1; we then have to a first approximation:

(4)

Here cF and D? are, respectively, the concentration and the diffusion coefficient of the dissolved component in the membrane, and dcy/d.x is the concentration gradient of the dissolved component in the membrane. Eq. (4) consists of two additive terms: the first describes convective transport of the dissolved component with the volume flow, and the second diffusion of this component in the volume flow. The second term is negligible in membranes with high filtration efficiency[22! The two partial flows of solvent and solute yield a further parameter that can in general be used to characterize a membrane. This is the membrane's retention capacity R for the dissolved component 5l :

Putting J,= J , d we obtain:

Here cz and c{ are the concentrations of the dissolved component in the original solution (0) and the filtrate (f). For a strictly semipermeable membrane J , and thus cfJ,=O and R = 1, whereas for a membrane with no separating capacitv R =O.

4.2. Model of a Solubility Membrane In an ideal solubility membrane the transport of the components depends on molecular diffusion in which coupling of the material flows is neglected. The flow density of any one component through the membrane can be described by the following equationI2.

454

Here, J , is the mass flow of a component i through the membrane, DY the diffusion coefficient in the membrane, k , the distribution coefficient between the membrane and the adjoining solution, c, the concentration in the adjoining solution, and A c , the concentration difference of component i between the two sides of the membrane. is the partial molar volume of component i. A p I S a specified pressure difference, R is the gas constant, T is the absolute temperature, and Ax is the membrane thickness. If the filtration flow density is to be derived for a relatively dilute solution, the concentration term RTAc,/c, in eq. ( 7 ) can be expressed as P A R . The term An is the osmotic pressure difference between the original solution and the filtrate. The filtration flow density for the solvent J , is given by eq. (8):

5. Production of Asymmetric Membranes Equations (2)-(4), and (8), (9) illustrate the difference between a pore membrane and a solubility membrane. While a pore membrane merely differentiates between the particle geometries, i. e. acts only as a molecular sieve. the selectivity of a solubility membrane depends mainly on the distribution coefficients of the components between the membrane and the external phases. Because of this, a pore membrane can separate only materials that differ considerably in molecular size. while a solubility membrane can also separate materials having the same-or nearly the same-molecular size if their solubilities in the membrane phase are sufficiently different. A further difference between solubility and pore membranes lies in their different filtration flow densities. Owing to the diffusion process the filtration flow density of a solubility membrane is 1-2 orders of magnitude lower than that of a pore membrane under the same driving force. In both cases, however, as follows from the transport equations, the filtration flow density is inversely proportional to the membrane thickness. Nevertheless, for economic reasons the filtration flow density should be as large as possible, i. e. the membrane thickness should be minimized. In the present state of the art it is not possible to produce perfect, self-supporting films thinner than about 10 pm on a large scale, whereas to achieve economically justifiable filtration flow densities the membranes must not beappreciably more than 0.1 to0.5 pm thick. These difficultiesare overcome by the use of asymmetric membranes. Figure 1 shows side-by-side scanning electron micrographs of an asymmetric and a symmetrical membrane. The total thickness of the asymmetric membrane amounts to 0.1 to 0.5mm. It is constructed of a relatively thick, very porous support carrying an extremely thin skin measuring 0.1 to 0.5pm. This skin is the actual semipermeable membrane, while the coarse, highly porous support has no selective properties and does

5.2 Production of Composite Asymmetric Membranes

Fig. 3. Scanning electron micrographs of sections through asymmetric membranes with a foam structure (left) and finger structure (right).

allowing high filtration flow densities even at low hydrostatic pressures; these have no ability to retain salts. At pressures above 10 bar the structure collapses and the membranes lose their good filtration properties; such structures are therefore usually reserved for ultrafiltration, i. e. for the separation of macromolecular substances at low hydrostatic pressures.

5%

15 %

10%

18 %

Comparatively recently, a “composite” method of construction was devised for the preparation of asymmetric memb r a n e ~ [ ~ ’ -that ~ ~ had ] extremely high filtration flow densities and could be used especiallyfor reverse osmosis. For composite membranes a highly porous, readily permeable, and mechanically very strong support is prepared, on which is placed a homogeneous separating layer 200 to ca. IOOOA thick. For this purpose a very dilute polymer solution is poured-once or several times-onto the support layer and the solvent is evaporated. To prevent the polymer solution from penetrating the support, the latter is usually protected by an inert intermediate layer which is removed in subsequent washing processes. Present-day composite membranes consist of two materials, since the support must not be attacked by the solvent of the separating layer added. Cellulose acetate, cellulose nitrate mixed polymers, and polysulfones are the main materials used for the highly porous carriers. The pore diameter of the carrier material must be smaller than the thickness of the active film, in order to prevent rupture of the separating layer. Any polymer can be considered for the homogeneous separating layer, provided that the solvent used does not also attack or dissolve the material of the support. The range of materials that can be used is limited by the conditions that can be withstood by both, but this disadvantage is compensated by the very high filtration flow densities. Other methods of preparing asymmetric membranes are still in the experimental stage, for example of polymers on a liquid foundation which is then applied directly to the porous supporting structure, or coating the porous supports in a plasma stream of monomers which polymerize on the s ~ p p o r t [ ~ ~ - ~ ~ l . Very good filtration results for special applications have 561, been achieved with dynamically formed which are not true composite membranes although they behave as such. Almost any ultrafiltration membrane can be used for dynamically formed membranes if its pore diameter is not more than about 0.2pm. During the filtration a very small amount of macromolecular or colloidal substance is added to the initial solution, this additive being retained by the pores of the support and thereby forming a film that can separate even salt from water.

6. Choice of Polymers Suitable for Membrane Production

20 %

22 %

Fig. 4. Scanning electron micrographs of membrane section in dependence on polymer content of the initial solution (figures in wt-%).

These two membrane types are the limiting cases; in practice a continuous transition from one to the other can be achieved by variation of the preparation parameters, and thereby a continuous transition from dense desalting membranes to coarse-pored ultrafiltration membranes (Fig. 4). 456

Membrane materials are required to have high mechanical, chemical, and thermal stability as well as resistance to microbiological degradation. While in pore membranes the material responsible for the separation plays a subordinate role, in solubility membranes it exerts a decisive influence on the separating characteristics. Solubility membranes are always used when molecules of approximately equal size are to be separated, the basis for this being special interactions between the molecules and the membrane matrix. Transport ofwater and salt has been the subject of numerous studies,particularly through cellulose acetate but also through other polymers[**2 8 * 571. Water transport is associated with the number of polar groups present in the polymer matrix. Angew. Chem. internat. Edit.

1 Vol. 14 (1975) 1 No. 7

These polar groups serve as adsorption centers with which the penetrating water can interact by hydrogen bonding. According to this view the water molecules are transported through the homogeneous matrix by activated boundary layer diffusion along the polar groups. A prerequisite for this to happen is that the polar groups should be distributed in the polymer matrix as close together and as regularly as possible['" "I.

t L

AFig. 5. Relation between the salt flow IS), water flow (WI. salt retention capacity (R), and water absorption IA). schematic.

Exclusion of the salt depends on the inability of its unhydrated ions to form hydrogen bonds to any large extent with the polar groups. Hydrated alkali metal and halide ions are, however, too large to penetrate the polymer matrix. Polymers with desalination properties can usually absorb up to about 20 wt-'%, of water; at high water uptakes water clusters are increasingly formed in the homogeneous polymer, and the hydrated ions of the salt can penetrate, so that the retention capacity is affected. The distribution of water within the polymer matrix can be determined with the aid of a function devised by Zimm and Lundberg[601. Clusters can also be present at very small water uptakes-much below 20 wt-'%,-arising from theall-powerful water-water interactions; such a polymer would be unsuitable for desalination purposes. Figure 5 shows the relation between the salt flow, water flow, salt-retention capacity, and water abs~rption["~.

7. Problems of Membrane Filtration in Industrial Practice 7.1. Concentration Polarization

surface and, owing to the concentration gradient, diffuse back into the original solution: after some time a stationary state is produced, with a constant concentration profile: convective transport of the dissolved particles to the membrane surface is compensated by a diffusion current directed back into the original solution. Concentration polarization considerably impairs the economics of membrane filtration. With substances of low molecular weight, e.y. salts, the osmotic pressure of the original solution, which must be overcome by the hydrostatic pressure, increases with increasing concentration at the membrane surface. Moreover, since in general filtration membranes are not strictly semipermeable, and the amount of material passing through the membrane is also directly proportional to the concentration on the membrane surface, the quality of the filtrate is impaired by the concentration polarization. The separation capacity of the membrane is thus reduced. During filtration of macromolecular substances it is very often found that, as a result of concentration polarization, the solubility limit is exceeded and a covering film is formed on the membrane surface. Such films act as secondary membranes, which not only drastically reduce the filtration flow densities but can also completely alter the separation characteristics of the original membrane. Although concentration polarization cannot be wholly avoided, it can be kept in check by suitably arranged flow parallel to the membrane surface, as has been shown by penetrating investigations of transport processes at the laminar boundary of membrane surfaces.

7.2. Development of Membrane Filtration Systems In large industrial filtration systems the disadvantageous effects of concentration polarization must be kept as small as possible by suitable arrangements for the flow of the original solution and lay-out of the installation. In addition, the filtration unit should be distinguished by a low cost and long service life, and should not occupy much space. Three systems have passed into industrial use (Sections 7.2.1 to 7.2.3). 7.2.1. The Tube Bundle System The tube bundle system is represented schematically in Figure 6rh7]. The tube-shaped asymmetric polymer membranes are placed in porous tubes made of metal or plastic. T o maintain the concentration increase at a minimum level, the flow through the tubes must be turbulent, and the relatively large tube diameter (ca.2.5 cm) therefore requires a high pump output. The ratio of the installed membrane surface to the volume of the apparatus, i. e. the packing density, is low; the relatively large costs of sealing the ends, support of the

Even when an optimally suitable membrane is available for a given problem of material separation, its practical industrial application may run into difficulties seriously detracting from economics of the process. One of the most important problems in technical processes is concentration polarization[hl -661 . Th'IS appears because, on membrane filtration, a solution is brought to the surface of a semipermeable membrane by convecrion. While the solvent permeates the membrane under the driving force of hydrostatic pressure, the dissolved components are more or less completely retained; they concentrate in the boundary layer on the membrane

Fig. 6. Construction of the tube bundle module. schematic

457

membranes, and changing of the membranes are also unfavorable. However, the possibility of cleaning the membrane surfaces mechanically with small foam balls constitutes an advantage of this system. 7.2.2. The Roga Module The Roga module[681consists of two asymmetric membranes separated by an incompressiblebut porous intermediate layer. Flow is introduced through a lattice on each side of the membrane. Figure 7 gives a schematic representation of the construction. The whole arrangement is rolled up, made watertight, and placed in a pressure tube. This mode of construction gives a filtration module with favorable cost and compactness. However, here too problems arise with controlling the concentration polarization, particularly when colloidal or macromolecular material has to be separated. Fig. 8. Schematic representation of filtration through hollow fibers with an inner active layer.

\ \

direction o t t l o w

All hollow-fiber filtration systems give not only purified filtrates but also highly concentrated solutions. The latter can be further evaporated and finally dumped or, simply burnt, if their content of organic matter is high.

8. Present and Future Applications of Asymmetric Membranes

Fig. 7. Construction of the Roga module, schematic.

7.2.3. The Hollow Fiber System The membranes developed by D u P ~ n t [701~ ~are , in the form of hollow fibers of asymmetric structure; the actual separating layer is on the outside of the fibers. The fibers are bundled together, bent into a U-shape, inserted in a pressure tube, and sealed up. The packing density of such a module is extremely high. The fibers are selfsupporting, so that for work at pressures up to about 60 bar only a pressure-tight outer tube of steel, aluminum, or fiberglass-reinforced plastic is required. Filtration units can then be constructed from hollow fiber modules distinguished by very small bulk and favorable cost characteristics, which have proved valuable above all for desalination. They appear to be unsuitable for the separation of macromolecular substances because precipitates are formed on the membrane. The precipitates formed on the outside of the hollow fibers can only be partially removed by the insufficientflow running over the fiber surfaces, so that a very large part of the membrane surface becomes unavailable for filtration. have developed The A m i ~ o n [and ~ ~ Berghof ] a different hollow-fiber concept, in which the active separating layer is in the interior of the hollow fibers. This filtration from the inside to the outside, even with low flow rates in the laminar flow region, can reduce the concentration polarization to economically acceptable levels. Figure 8 is a schematic representation of the filtration process. This new hollow-fiber concept combines the advantages of the DuPont module and the tube bundle module. So far it has only been used for ultrafiltration, since the resistance of the fibers to pressure does not yet satisfy the demands of reverse osmosis. 458

The main applications of asymmetric membranes are in the field of desalination of sea water, brackish waters, and river waters. Desalination plants have been built especially in the USA, in Israel, and recently in the Federal Republic of Germany. The water obtained from these plants is sterile, desalted, and completely free from particles. The recent progress in polymer chemistry now makes it possible to purify the sometimes extremely corrosive industrial waste waters. Here membrane filtration has the edge over other methods of separation in that its mode of operation conserves energy. It has proved its value particularly in the treatment of waste water from dairy plants, from dipping tanks for electrophoretic coating in the automobile industry, and for the separation of oil emulsions. In this way valuable components such as proteins, varnishes, and oils can be recovered. Other fields of application are in the purification of the waste waters from breweries and from factories dealing with fish, starch, dyeing, and coffee. In the pharmaceutical and clinical industry asymmetric membranes are used principally to provide sterile liquids free from microparticles. It would appear advantageous also to use them as artificial kidneys; it might be possible to replace the slow and relatively unspecificdialysis by membrane filtration of blood that would be faster and give considerably sharper separation. The industrial application of membrane processes is certainly still in its infancy[73* 74]. The development of asymmetric membranes is being directed toward better selectivity and faster rates of filtration; above all, membranes with still greater chemical, mechanical, and thermal resistance are being sought. In the development of membranes and membrane systems Nature itself can be taken as a model. In Nature membranes are of decisive importance for life, as they fulfill a variety of functions in plant and animal metabolism. Natural membranes are extremely selective,have variable separating properAngew. Chem. internat. Edit.

Vol. 1 4 ( 1 9 7 5 ) 1 No. 7

ties. serve as information carriers, give extremely fast transport rates, and take active part in mass transport. A very large number of new uses will follow if membrane research can incorporate just a few of these properties in synthetic membranes. Received: December 2. 1974 [A 65 IE] German version: Angew. Chem. 87.476 (1975) Translated by Express Translation Service, London

[I]

[2] [3] [4] [5] [6] 171

[XI 191 [lo] [I I] [ 121

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12-11 1241 [2S] 1261 [27]

[ZX] 1291

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