DK156813B - A NISOTROP SYNTHETIC MEMBRANE WITH MULTILAYER STRUCTURE - Google Patents

Description

DK 156813 B

The present invention relates to novel highly permeable synthetic polymer membranes which can be used in membrane separation of compounds in such processes as ultrafiltration, dialysis, electrodialysis and reverse osmosis.

5 In such processes semipermeable membranes are used which distinguish between soluble molecules and solvent molecules on the basis of differences in molecular size, shape, chemical structure or electrical charge.

This type of membrane can also advantageously be used in gas permeation 10 and gas diffusion.

Ultrafiltration (UF) is the term used to denote the separation of relatively high molecular weight solutes and colloidly dispersed substances from their solvents. The osmotic pressure of the solute is usually insignificant does not play an important role in the separation process.

Reverse osmosis (RO) is the term commonly used to denote the separation of low molecular weight solutes from their solvent. In this case, the driving pressure for effective separation must exceed the osmotic pressure of the solution.

Both at UF and RO a solution under pressure flows across the surface of a supported membrane, and under the influence of the applied pressure gradient across the membrane, solvent and certain solutes pass through the membrane and are collected as permeate. The solvent and solute retained by the membrane is referred to as the retentate.

By a suitable membrane selection, it is possible to concentrate, purify and fractionally separate virtually any solution by simple physical path, the only energy requirement being the compression energy of the supplied liquid. Such a technique is particularly well-suited for the production of thermally or chemically unstable products, in which 30 conventional separation processes, e.g. evaporation, selective

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2 extraction and selective precipitation, often leading to product loss or destruction.

Membranes commonly used for UF are so-called anisotropic membranes, originally developed by S. Loeb and S. Sourirajan 5 at the University of California, Los Angeles, in the late 1950s.

These membranes are made or "stumped" by a solution of a polymer in a solvent (e.g., cellulose acetate dissolved in an acetonic medium). A thin layer of the solution is spread on a suitable surface, e.g. a glass plate, and the solvent is allowed to evaporate to such an extent that a semi-solid matrix is formed with a surface "skin" which results from the surface layer drying faster than the layers below. The membrane is then immersed in another solvent, usually a water-based one, to precipitate the remaining polymer rapidly; the rapid precipitation or coagulation of the polymer forms a sponge-like backing on the membrane.

The resulting membrane is an extremely thin layer or extremely thin film of polymer with a very fine pore structure (thickness of less than 5 µ), which is supported by a much thicker layer of strong porous material (thickness of over 100 µ). In such membranes, only the surface layer 20 or the film is active at UF. Because the flow rate through such membranes is low, in ultrafiltration processes using such membranes, relatively large amounts of energy are consumed and are time consuming and require relatively high capital investment in plants or equipment to ensure that the use of the membrane is economical or practical, expressed in the flow rates obtainable with such membranes.

Recently, different types of membranes have been developed using polyelectrolyte, polysulfone and polycarbonate, especially by such companies as Amicon Corporation and Dorr-Oliver (U.S.A.), 30 Satorius and Gelman (West Germany) and DDS (Denmark). These membranes have either a skin layer as described above or a regular fungal structure. Further, General Electric Company has developed a technique for producing extremely thin membranes with howls,

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3 formed by nuclear bombardment, in which case the membranes have a structure similar to that of a mono-sieve. Between 1965 and 1970, Du Pont (U.S.A.) and OPI (France) developed polyamide membranes, either in flat or hollow filter form. Due to limited permeability and "skinned" properties, these membranes cannot be considered to be different from classical membranes in terms of practical application.

In all previously known membranes, only the surface of the membrane is active, and it is only the properties of the active side that are in contact with the liquid that give the membrane its exclusion property.

Due to the structure of the prior art membranes, ultrafiltration throughput is in most cases limited by the formation of a gel layer without the membrane, which gel layer is formed by the substances which are stopped by the membrane. The permeability and exclusion characteristics of the gel layer determine the performance of the membrane.

For this reason, the flow of permeate through the membrane even at a low pressure becomes independent of the pressure drop through the membrane.

In addition, the permeation permeation is highly dependent on the wall transition velocity, and high throughput can only be achieved by an expensive pumping equipment capable of establishing sufficient velocity of fluid in contact with the membrane to minimize the effect of the gel layer. Furthermore, the flow rate decreases rapidly as the concentration builds up. Because of this effect, UF is neither attractive nor practical for removing solvent from highly concentrated solutions.

Another consequence of the structure of the classical, previously known membranes is the extremely low flow rate in low molecular weight limit membranes. This factor, the water permeability, is very often the limiting general use of 30 membranes.

Furthermore, in the manufacture of known types of membranes, there is a need for very strict quality control with regard to the structure of the membrane surface, which entails high manufacturing costs.

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The present invention relates to an anisotropic multilayer synthetic membrane, which membrane is characterized in that each layer is active in the separation process and acts as a molecular sieve having an accurate molecular weight exclusion limit.

According to the present invention, there is provided a novel type of multi-layered anisotropic synthetic membranes, preferably of 4-12 layers, each layer serving as a molecular sieve having an accurate molecular weight exclusion limit. The accompanying drawing schematically shows a 7-layer membrane, denoted from I to VII. Between two arbitrary layers of the membrane, parallel to the surface of the membrane, is enclosed an alveolar structure of alveoli with exact dimensions. From top to bottom of the membrane, each alveolar oil varies in dimension with respect to the immediately preceding and / or subsequent alveoli expressed as a geometric progression, i.e., the average volume of the alveoli between the first and second membrane layers is a fixed ratio of the average volume of the alveoli between the second and third membrane layers, etc., expressed as a geometric progression of degree rx (see below).

When the degree of geometric progression is above 1 (i.e., rx> 1), the membrane is called a divergent membrane, and when the degree is below 1 (i.e., rx <1), the membrane is called a convergent membrane. The geometric progression can be expressed by the following equation:

Vol [n, n + l] rx = - 25 Vol [nl, n] where n-1, n and n + 1 are the consecutive numbers of adjacent layers of the membrane in the direction from top to bottom of the membrane, and Vol [n, n + l] is the average volume of the alveoli between layers n and layers n + 1; Similarly, Vol [n-l, n] is the average volume of alveoli between layers n-1 and layer n.

The size of this average volume can be measured by measuring the water content of each layer. The water content (WC) in layer n is expressed as:

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5 weight of wet layer - weight of tort layer wcn = _ weight of wet layer. wcn + l 5 rx __ _ WCn convergence in membrane C = _ rx 10 wcn WCn + l

If C is less than 1, the membrane is called divergent and if C is greater than 1, the membrane is called convergent.

For membranes of the present invention, the logarithm of the molecular weight exclusion boundary in the various layers varies with geometrical progression: log of molecular weight exclusion boundary + in ΡχΡχ 20 log of molecular weight boundary end

The molecular convergence of the membrane is defined as 7 = _ Ρχ 25 If y is greater than 1, the membrane is called molecular convergence.

If 7 is less than 1, the membrane is called molecularly divergent.

This gives 4 types of membrane, namely:

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6 Convergent-Convergent C> 1 7> 1 Convergent-Divergent C> 1 7 <1 Divergent-Convergent C <1 7> 1 Divergent-Divergent C <1 7 <1 5 The membranes of the present invention are highly permeable to prior art. known anisotropic membranes, which is mainly due to the intercellular or interalveolar structure.

The structure is made up of polymer macromolecules which form an amorphous "huiler" or "channel" polymer. These ducts are strongly twisted and are slightly and elastically deformable under pressure; or they are intact during coagulation by assembling the macromolecules and leaching the chemical by-products from the coagulation. The flow areas of the "holes" or "ducts" are s tort, and the retention characteristics of the membranes stem primarily from the obstruction of 15 than the total exclusion of the stream of retained substances.

Thus, the theory and performance of the membranes of the present invention is quite different from the prior art anisotropic membranes. By appropriately designing the size of the "channels" or "holes", membranes having different flow and exclusion characteristics can be produced.

The anisotropic synthetic membranes of the invention with graduated porosity and consisting of a mixture formed by depolymerization of a polymeric material are characterized in that it is formed by dissolving a polymeric material in acid solution, maturing the solution of the polymeric material for a fixed period of time. for obtaining the desired degree of depolymerization, stripping a thin layer of the solution onto an inert carrier, coagulating the thin layer by immersing in a bath containing a compound capable of diluting the acid concentration, and curing and removing it formed membrane from the carrier.

Known membranes are prepared from a specific polymer to produce a membrane with known, predetermined characteristics. According to the present invention, it is possible to vary the molecular weight exclusion limit point and the solvent agent.

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7, regulating the depolymerization or repolymerization of the polymeric material by varying the maturation time of a given polymer's "dope".

With any type of polymeric material it is possible to form 5 membranes with a predetermined molecular weight exclusion limit point (MWCO) and to produce a whole series of membranes between 2 exclusion viewpoints, ie. from the case where the coated film for the coagulation (dope) is not at all matured, to the second extreme with multiple days of maturation and maximum depolymerization compatible with the stretch or strength of the membrane.

The more the mixture to be coated matures, the greater the permeability of the membrane. The lower the polymer concentration, the higher the membrane permeability. For a given polymer, the following applies: 15 The more "doping" matures, the greater the MWCO value. The lower the polymer concentration, the higher the MWCO value. Increase in the polymerization level of the starting polymer or increase in the degree of crystallinity of the polymer leads to a lower MWCO value.

The multilayer structure of the membranes of the present invention 20 is advantageous in that it provides a multilayer effect expressed as separation wherein a single layer defect is corrected by the following layer. For this reason, with this type of membrane, strict quality control is not necessary during manufacture. The cost of manufacturing membranes according to the present invention has therefore been greatly reduced in relation to the cost of manufacturing the prior art membranes.

Another advantage of the membranes of the present invention is the extremely high water content which can be as high as 98% by weight, which leads to very high water permeability in the membrane.

Another advantage of the membranes of the present invention is the fact that a convergent-convergent membrane is converted into a divergent-divergent membrane when its direction of application is reversed. Similarly, a convergent-divergent membrane can be transformed into one

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8 divergent-convergent membrane. This means that it is only necessary to prepare two types of membranes, a convergent-convergent membrane and a divergent-convergent membrane, where the orientation of the membrane is determined by its intended use.

5 All convergent-divergent membranes and divergent-divergent membranes act as "skin" membranes for forming a gel layer, which means that the gel layer is formed on the outside of the membrane. In this case, the classical theories apply to membranes expressed as the various equations which give the flow rate 10 in relation to pressure, temperature, transition rate and concentration.

In this case, the advantage of the membranes according to the invention is the high water permeability and the low cost.

In the case of both convergent-convergent and divergent-convergent membranes by ultrafiltration, a gel layer of 15 molecular weight substances is built between the first-layer molecular weight exclusion limit and the last-layer molecular weight exclusion limit. In this case, the membrane acts as a mechanical support for the gel layer and the apparent molecular weight exclusion limit for the membrane is the molecular gel exclusion limit of the inner layer. This means that in a given solution there is the possibility of selecting one of the components or a mixture of components to form this gel layer. Furthermore, it is advantageously possible to select the components which will cause the maximum permeability of the minimum molecular weight exclusion limit. An outer gel layer built on the outside of the membrane 25 does not adhere to this inner gel layer and is removed more effectively, e.g. by laminar or turbulent flow. The permeability of the system starts mostly independent of concentration. This can be expressed as an internal dynamic membrane effect with the advantage that the compression of the dynamic membrane is avoided due to the internal support provided by the membrane, especially in the case of pulsating current.

Membranes according to the present invention are obtained by controlled, unidirectional coagulation of the polymeric material from solution by coating on a suitable inert surface.

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For the preparation of the membrane, it is necessary to make a "dope" by opening the poles. This so-called dissolution effect is obtained by cutting off the hydrogen bonds that bind the molecular chain in the polymer together. The solvent 5 used for this effect may itself exert a depolymerizing effect on the polymer to effect the depolymerization to a level controlled for this purpose; if not, a chemical additive must be used to obtain the desired result. To control the rate of dissolution and depolymerization rate, a limited amount of tensioactive agent is added and the temperature is adjusted. The uniformity of the reaction is controlled by stirring. The level of depolymerization is obtained after a period of time called the maturation time. The appropriate level of depolymerization is determined by viscosity measurement in the "dope". The "dope" is then stacked out on a glass plate or other suitable inert, non-porous support, by the conventional technique with a knife to form a layer of controlled thickness. The "dope" is then coagulated by immediate immersion in a coagulation bath containing any chemical product capable of diluting the solvent and curing the depolymerization compound used. The number of layers that the membrane consists of 20 depends on the film thickness, coagulation and can easily range from 4 to 40, although it is unusual to produce more than 10-12 layers.

For a given concentration of polymer in the "dope", an equilibrium exists between the "dope" volume and the volume of the farthest layer.

This concentration is called the neutral concentration and the alveoli 25 in the membrane are of constant starch. The membrane is then called a parallel membrane with convergence C = 1. For any concentration of polymer in the "dope" above the neutral concentration, the membrane obtained is a divergent membrane, which can be seen by the upper surface of the membrane being clear and the lower surface. is food. For any concentration of polymer below the neutral concentration, the membrane obtained is a convergent membrane where the outer surface is matte and the lower surface is clear. In this normal procedure, the exclusion point of the farthest layer is generally above the exclusion point of the bottom layer. By this method, it is possible to obtain convergent-convergent membranes or divergent-convergent membranes, which may also be used as herein, respectively.

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10 divergent-divergent and convergent-divergent membranes by reversing the application orientation of the membrane.

One can, for example. for example, a polyamide "dope" consisting of polyamide-6.6 in a mixture of 50 ml of ION HCl, 25 ml of H 2 O and 5 ml of CH 3 CH 2 OH.

In this mixture, a neutral concentration (C = 1) is obtained by the addition of 27.5 g of polyamide-6.6, and such a "dope" mixture will give a parallel membrane. A divergent membrane can be obtained by the addition of 40 g of polyamide-6.6, or a convergent membrane can be obtained by the addition of 17.5 g of polyamide-6.6.

10 As noted above, the coagulation of the coated film of polymeric material on the inert surface is subjected to controlled, unidirectional coagulation from the averaged surface of the coated film and through to the inert surface. The farthest layer is directly coagulated, but the second and descending layers are coagulated via coagulation 15 of the immediately preceding layer. This results in different coagulation conditions, expressed as concentration, which cause the formation of different layers of alveolar cells, the alveoli of each layer having dimensions other than the adjacent alveolar layer on each side.

20 Some of the factors affecting coagulation in the material film are: pH, temperature and redox potential.

An example is a membrane based on a polyamide-6 which has a degree of polymerization of approx. 120. The polymer is dissolved in an acid (HCl, HNO3 or formic acid) and optionally an alcohol (e.g., methyl or ethyl alcohol or glycol), a blood gore and an inert 9 site. The variation in the molecular weight or molecular weight exclusion point of the polymeric material in the resulting membrane is determined by the type of acid used. For molecular weights between approx. 300 and 2000 can be 'used formic acid; for molecular weights between approx. 2000 and 80,000 can be used nitric acid; and for molecular weights between ca. 80,000 and 800,000 can be used hydrochloric acid. To form the membrane, the solution is applied to a suitable even inert surface, e.g. a glass plate, in a thickness of approx. 100 μ, after which it is coagulated by setting

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u The pH value of a process soin includes interface contact with a basic solution such as ammonia. The coagulation in the solution covering the smooth surface takes place in steps. The different coagulation rates lead to a multilayer membrane comprising between 3 and approx. 15 layers. With respect to the microstructure of the coagulated polymer, the diameter of each channel is in the order of 100 °, but it may vary between approx. 20Â and approx. 1000Â, depending on the rate of coagulation and the salt concentration. The size or diameter of the channels can be adjusted or determined in advance by a number of independent parameters, including the nature of the acid used in the initial solution of the polymeric material, the concentration of the polymeric material in the solution, the pH value determined by the base concentration, temperature and amount. of added salts. If neither the solution of polymeric material nor the coagulation base contains added site, there is an average coagulation rate which results in channels of medium size in the resulting membrane. If only the inert site is added to the solution of the polymeric material and not to the coagulation base, a higher current of salt desorption is obtained, which leads to larger diameter 20 channels. On the other hand, if only a seat is added to the coagulation bath, a very low coagulation rate is obtained, which results in less duct diameter in the membrane. The degree of coagulation or rate can be controlled or modified by adding to the coagulation base an inert site of the same ion system as (or having 25 common ions with) the acid used to dissolve the polymeric material. For example, the acid used is hydrochloric acid, the inert added site may be sodium chloride. Substitution of the sodium ion with a larger ion also affects the coagulation rate.

Below is an example of forming a multilayer membrane 30 in accordance with the present invention.

Example.

Mix 100 ml of 10N HCl with 50 ml of water. Then add 10 ml of ethyl alcohol to the mixture. Meanwhile, 80 g of polyamide-6 is washed in finely divided form (20 denier, shark stretching ratio, 3 filaments, light yarn) to remove surface oil, terres and weighed. The washed yarn dissolves

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12 then into the pre-prepared mixture of HCl, water and alcohol over approx. 20 minutes while maintaining the temperature below 25 ° C. This is an exothermic reaction and overheating can cause an increase in the polymer's depolymerization level. The solution is then degassed and allowed to mature at 20 ° G for 1 day. The matured solution is then laid out as a film for approx. 100 µ in thickness on a clean glass plate. The coated plate is then gently placed in a water bath to coagulate the polymeric material film onto the glass. The coagulation reaction is completed in approx. 2 minutes and the membrane flows 10 free of the glass plate. The membrane is removed from the coagulation bath, washed in warm water (90 ° C for 30 seconds) to remove monomer and salts, e.g. C1 ions, and to cure the membrane, and then absorb Λ and terres. This membrane has a water permeability of 200 liters / nry hour.

15 Using different polymers, different coagulation techniques and different inserted ions, it is possible to obtain membranes with different properties.

A cross-section of the membrane structure of the present invention is schematically illustrated in the figure. This structure has been confirmed both by transmission and by scanning electron microscope. Cell or alveolar walls are of the order of 1-2 µ in thickness, the shape of the cells or alveoli varying in direction away from the conductive surface, ie. that the size of the cells in each layer rises from the top or the conductive surface to the bottom or the bottom surface of the membrane. Electron micrographs show that the ratio of the smallest to the largest half-axis in the cells (counted as elongated spheroids) varies in an approximate geometric progression depending on the distance from the membrane surface. The connecting channels between the cells in adjacent layers are not shown.

These channels are perpendicular to the membrane surface, and to pass from one cell to the next and then through the membrane, a molecule or ion follows a path from a cell in the conductive surface layer to neighboring cells in the next layer with negligible lateral transition between cells.

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The separation of compounds by the membranes of the present invention results from single phenomena which depend on which separation is carried out, i.e. dialysis, ultra-filtration or reverse osmosis.

In dialysis, the membranes of the present invention can be used as a sieve, even for very small molecules which, due to their random zig-zag movement in their solvent, have a mean path length whose amplitude is greater than the diameter of the molecular channels between the cells. in the membrane.

For example, For example, a UF membrane according to the present invention with a molecular weight exclusion limit of 500,000 can have molecular channels having a diameter of 100Â. A small molecule of molecular weight 200 and an average diameter of 7Â can have an average path length of 300Â and is therefore severely prevented from passing through the molecular channels. Thus, the same membrane used for ultrafiltration can separate or concentrate molecules. of molecular weight 500,000 is also used in dialysis to purify molecules of molecular weight 100-200.

As indicated above, the membranes of the present invention are useful for membrane separation of compounds by such processes as dialysis, electrodialysis, gas permeation, gas diffusion, ultrafiltration and reverse osmosis. A discussion of these various processes and specific applications of the membranes is given below.

Dialysis.

In dialysis processes, the membrane is used as a partition between two chambers where one chamber is filled with solvent and solute and the other chamber is filled with solvent only. The exchange between the two chambers through the membrane is driven by the concentration gradient which is between the two membrane sides. The separation 30 of various reads through the membrane is determined by the size of the molecule and the mean path length.

14

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Used soin dialysis membrane and to separate Cu * -1 "and Co2 ions from a mixture of CuCl2 and COCl2, it is possible to increase the Co2 concentration by 130% in one step. Such a concentration increase can be achieved by a membrane according to the present invention is a strong indication that each layer of the membrane is active in the concentration process, thereby providing a multi-stage concentration effect at one passage through the membrane.

If the membrane is used as a divergent membrane, the dilution level from the side with small alveoli to the side with large alveoli 10 is constant step by step, so that there is a multi-layer separation, with each layer effecting a maximum degree of separation.

Immobilized enzyme technology.

An interesting option given with polyamide membranes is the possibility of fixing enzymes in two different ways.

15 1. Fixation by adsorption of enzyme bound to a membrane of blood in the membrane. Water as blood gores in the membranes contributes to the adsorption of any hydrated enzyme on the membrane.

2. Fixation by hydrogen bonding to the oxygen of the -C-N group i

0 H

The polyamide. The amino group in the enzyme forms the basis for an enzyme-polyamide-hydrogen bond. This leads to the possibility of fixing a large amount of enzyme, e.g. it is possible to fix 35% by weight of α-amylase on the polyamide 6,6 membrane. The use of this membrane in ultrafiltration allows for enzymatic conversion of a compound 25 during its passage through the membrane, e.g. removal of lactose and galactose from milk during ultrafiltration.

Electrodialysis.

It is possible to modify the polarity of the membrane and obtain anionic, neutral or cationic membranes. Eg. a membrane formed from a "dope" containing HCl has a strong electrovalent fixation of chloride within the polymer and has the same properties as a 15 dope.

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highly electronegative membrane. By chemically curing this membrane, e.g. by treatment with 5N CH 3 COOH for 1 minute and washing with water, it is possible to replace the chlorine position with dissolved water during the polymer hydrolysis reaction. In this case, the membrane is converted into a neutral membrane. In the same way, it is possible to obtain electropositive charged membranes by chemically curing the membrane with strong hydroxides.

The advantage of the membrane according to the present invention for electrodialysis is mainly due to the extremely high water content of the membrane, which results in high permeability and low potential loss.

Gaspermeation.

For this use, the membrane is used to induce separation between various liquid components or separation between a liquid-gas two-phase system. 1) The membrane can be used for separating n-propane and isopropane. The liquid mixture under pressure is gasified through the membrane by constant supply of heat on the other side of the membrane. The rate of transfer of the isopropane through the membrane is higher than that of the n-propane and the liquid phase is enriched with n-propane.

Ex. 2) For the recovery of liquid fuel from wet natural gas, some membrane types (e.g. highly hydrated polymer) allow the gas fraction to pass through the membrane, whereas the liquid fraction is retained in the form of droplets which cannot enter the membrane due to its water repellent property.

25 Gas diffusion.

The membrane in ter form can be calendered to form a diffusion barrier with extremely thin pore structure on e.g. above 15Â which gives good selectivity for low molecular weight gas.

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Isotopseparering.

Membranes of the present invention can be used to separate or concentrate compounds of different molecular weights, including compounds of heavy metal isotopes, e.g. isotopes 5 of actinide elements and in particular isotopes of uranium.

In isotope separation methods, the operation depends on either small physical (ie mass) differences or on the small chemical differences that exist between isotopes. The separation processes usually depend on a very large number of steps, each providing only a small 10 isotope enrichment.

Membranes of the type described herein provide the basis for a new enrichment technique for separating and enriching heavy metal isotopes, especially uranium isotopes. Unlike existing processes, it is carried out in liquid phase rather than in vapor phase. A solution 15 containing isotopes of uranium is subjected to a mild centrifugal force field, while at the same time being found to flow radially inward through a membrane of the type described herein, whose channels serve to partially separate and immobilize the heavier isotope. By largely preventing the molecular back diffusion, which limits the efficiency of conventional centrifuges, a significant separation can be achieved at each contact step. Furthermore, since the membrane is relatively permeable in comparison to the porous membrane used in the gas diffusion process, the throughput of enriched material leaving the contact step is quite high despite being a liquid being treated.

Ultrafiltration.

Some specific uses of membranes according to the present invention include: 1) Treatment of effluent or effluent.

Treatment of wastewater or effluent to reduce effluent volume so that water free of most of the contaminant

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The 17 minants can be recycled and products from the retentate can be recovered. In conventional wastewater treatment, a combination of large precipitation tanks, bacterial cultures and sludge thickeners is used to remove impurities from wastewater and concentrate the solid residue.

While the primary treatment for the recovery of precipitated solids is still required, the present secondary treatment can be replaced by ultrafiltration using membranes according to the present invention. Thus, filtered solids from the effluent stream can be concentrated, facilitating recirculation or removal thereof.

2) Tanning fluff treatment.

Treatment of tannery effluent to minimize tanning odor and effluent gene by dissipating such effluent in public sewage systems.

15 3) Recycling of surfactants.

Recovery of surfactants and other similar macromolecular agglomerates from aqueous effluent, so that the surfactants can be recovered and the water recycled.

4) Recycling of waste from food industry.

20 Recovery of waste from the food industry in the manufacture and refinement of waste liquids from the food industry and by-products containing large amounts of food, but at concentrations too low to make recycling economically practical. However, if the liquids are simply discarded, they can cause serious pollution problems. Examples here include dairy whey and wastewater effluents from the extraction of protein from soybean meal and from wet milling of cereals. By using membranes according to the present invention, it is possible to concentrate and recover valuable products from such effluents.

Other uses include the use of membranes according to the present invention

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The invention relates to biofiltration and sterilization of beverages, including OI, wine and non-alcoholic beverages.

5) Production or recovery of animal blood protein.

Production or recovery of animal blood protein from slaughterhouses.

The whole blood can be readily fractionated using membranes of the present invention to isolate the plasma. The plasma can again be concentrated for drying by further ultrafiltration technique using a different type of membrane according to the present invention.

10 6) Purification and sterilization of water.

For this application, membranes with relatively large channel diameters are used, and the dimensions of the selected channel vary from one membrane type to another (from 0.2 to 1.5 µ). Bacteria and viruses are stopped because of their size or the size of the solid particles to which they may be attached. The membrane acts as a biological screen and can produce a throughput several times worse than that available with known membranes.

In fact, in any chemical process comprising the following, membranes of the present invention can be effectively used: a) removing and recovering small amounts of dissolved or colloid dispersed substance from solution; b) concentration of solutions or dispersions of valuable products which are thermally or chemically unstable or volatile; and c) separating and purifying macromolecular or colloidal solutes from solutions containing micromolecular impurities.

Although the present invention is described above with reference to preferred embodiments, specific examples and drawings, it is to be understood that various variations, modifications

Claims (2)

An anisotropic multilayer synthetic membrane, characterized in that each layer is active in the separation process and acts as a molecular sieve with an accurate molecular weight exclusion limit. An anisotropic graduated porosity synthetic membrane consisting of a mixture formed by depolymerization of a polymeric material, characterized in that it is formed by dissolving a polymeric material in acid solution, maturing the dissolution of the polymeric material for a fixed period of time to obtain the desired degree of depolymerization, irradiation of a thin layer of the solution on an inert carrier, coagulation of the thin layer by immersion in a bath. 15 containing a compound which can dilute the concentration of the acid, and curing and removing the membrane thus formed from the support.