CA1053417A - Separation process - Google Patents

Abstract

Abstract of the Disclosure The flux rate of ultrafiltration membranes is markedly improved by the presence of inert, water-insoluble, impervious particles, particularly of metals and their alloys, distributed wholly within the membrane. The particles are particularly effective in the region adjacent the interface of the membrane with a support surface and may be distributed previously in the casting dope, or applied in a thin coating to the surface on which the membrane is subsequently prepared by casting from the dope. The membrane is in flat, tubular or filamentary form.

Description

cA.153 1053~17 This invention relates to membrane filtration processes and to improved semi-permeable membranes for use in such proces~es.
Semi-permeable membranes used in membrane filtration processes enable the separation to be effected of material down to molecular dimensions, usually from aqueous systems. According to the selectivity of the membranes, otherwise expressed as the rejection character-istics, they ~ind widespread application for example in desalinating brine, purifying effluent and concentrating milk protein, particularly in whey.
In hyper~iltration processes in which small solute molecules of molecular weight le~s than about 100 can be separated, membranes of fine pore size are employed in conjunction with filtration pressures of 1,000 psi or more which are necessary to overcome the considerable osmotic pressure generated by the small molecules. The selective rejection of much larger molecules, eg proteins, of molecular weight generally over 1,000, is effected on the other hand by membranes of more open pore structure, in ultrafiltration processes in which osmotic pressure is negligible and in which therefore substantially lower filtration pressures are adequate, generally about 100 psi or even less.
The present invention provides a semi-permeable, ultrafiltration membrane, suitable for use in ultra-filtration processes, in which a minor amount of an inert, impervious, water-insoluble, preferably inorganic, solid material in the form of finely-divided non-colloidal particles is dispersed wholly within the membranes so as to cA.153 swell or otherwise change the structure of the membrane, thereby increasing its flux rate.
It has been found that the membranes of the invention can ~xhibit up to 2-3 times the flux, at a given pressure and temperature of otherwise identical membranes without the added particles. On the other hand, the rejection characteristics of the membranes towards protein and other large organic molecules which can normally be separated by ultrafiltration remain substantially unaffected. The flux rate is the flow rate that can be treated by unit area of membrane, and i9 commonly expressed in gallons per 24 hours per ft2, either US or Imperial gallons.
The exact mechani~m by which the inorganic material improves the membranes is not known. The filtration of proteins in milk or other aqueous systems is adversely affected by the build-up of proteins on the surface of the membrane, forming a second "filter" having a poor flux rate.
Without wishing to diminish the scope of the invention described by any expression as to its mechanism, it is believed that filtration through the membranes of this invention is improved by charged groups carried by the particles, effective amounts of which thus repel milk proteins from the membrane surface. This leads to higher permeation rates of water and dissolved salts through the membrane, by lining the surface of passages through the membrane skin to give these areas a negative charge and thereby allowing effusion of neutrally-charged molecules through the membrane while rejecting charged molecules such as p~otein. It should be emphasised that the membranes of the present invention being of open pore structure, cA.153 lQ534~7 exercise no selective filtration action on aqueous solutions of small solutes, eg brine solutions 9 capable of exerting a strong osmotic pressure and are thus distinguished from reverse osmosis membranes which do so.
Their application in ultrafiltration processes lies in their selective rejection of comparatively large molecules, for example, proteins. The limits of their effectiveness for this purpose, that i9, the minimum size of molecules which they are capable of rejecting, depends largely upon the effective pore size of the membrane and hence upon the conditions and materials of its preparation but also upon the conditions under which it is used, particularly the operating pressure, increased pressure often effectively decreasing pore size. This i9 particularly observed where, as in milk and whey concentration using ultrafiltration methods, a wide spectrum of solute molecular sizes is present, providing a build-up of the bigger rejected solute molecules on the membrane in a layer which itself exercises a filtration action in the smaller molecules to which the membrane itself is non-rejecting. Thus, lactose solutions may be found to be filtered unchanged through a membrane which will however at least partially reject the lactose in milk or whey in the presence of the protein molecules, and the degree of rejection may then be enhanced with increased pressure above that customarily adopted for ultrafiltration.
Semi-permeable membranes are generally cast from a solution, usually referred to as dope, of a film-forming polymer in an organic solvent, the membranes used in hyper-filtration processes being cast from volatile solvents, for example acetone. The ultrafiltration membranes of the _ ~ _ cA. 153 ~0534~7 present invention, however, are conveniently prepared from dope comprising non-volatile organic solvents having a boiling point substantially in excess of 100C. Suitable solvents include dimethyl formamide, dimethyl sulphoxide and triethyl phosphate. It is surprising that membranes cast from volatile solvents show no improvement when particles are incorporated but on the contrary often exhibit flaws and are then wholly unsuitable for use. An important feature of the present invention is in the preparation of cellulose acetate ultrafiltration membranes, particularly from solutions in dimethyl formamide. These membranes can be used at elevated temperatures up to approximately 80C, enabling ultrafiltration processes to be carried out at temperatures at which, for example, milk or whey may be pasteurised.
The concentration of polymer in the dope is not critical. Solutions from about 5% to 50% and above may be used if desired, up to the limits of solubility of the polymer. Preferably, however, a solution of 10-30% by weight concentration is used. Very dilute solutions tend to form very fragile membranes, while those prep~red from very concentrated solutions may be tough but are often slow in use.
The invention also provides a method of preparing improved ultrafiltration membranes in which a solution of film-forming polymeric material, for example a cellulose ether or ester, is dissolved in a non-volatile solvent and an inert, finely-divided inorganic water-insoluble material is added having a mean specific surface area of preferably 30 at least 50 m2/g, and distributed throughout the solution, cA.153 a film is cast and the solvent is leached by contact with a miscible solvent in which the polymer is insoluble.
The particles should be small compared to the molecular size of the membrane material. Bigger particles 5 exceeding the cellular dimensions tend to form gaps in the membrane. Only a comparatively small concentration is needed to provide effective cover for all the membrane interfaces with the liquid to be filtered. The particles should preferably constitute at least 1% of the membrane casting solution, preferably 1-4% for carbon particles and from 4-25% is particularly preferred for metal particles.
These amounts are expressed in the specification by weight, as grammes per cc of solution, 1% therefore representing 1 gramme as additive per 100 cc of solution. Greater 15 amounts, up to 50%~ may enhance the membrane flux still further, but some loss may then occur of selectivity, to - give a lower rejection factor towards molecules of specified size. The concentration at which this occurs is dependent upon the nature of the casting dope, including the size and nature of the active material. With these greater quantities the membranes may then become selective only towards the bigger molecules such as bacteria, while passing even milk protein, or defects in the membrane may develop.
However, as much as 50% may be acceptable of some material, 25 eg metal particles, without loss of milk protein rejection.
In general also a greater change is effected using dimethyl sulphoxide than dimethyl formamide as the casting solution.
Suitable material to be added to the membrane in the form of particles in accordance with the invention include lamp black, carbon black and soot. Other inorganic cA.153 1053~7 materials which may be used include iron and ferrous alloys including steel, metals generally if these are stable, both elements and their alloys, particularly nickel, cobalt, aluminium and their oxides, silica, silicon, sulphur and alumina. The materials should be substantially insoluble in water and the casting solvent, and exert no hydrolytic, catalytic, oxidative, reductive or other chemical change likely to lead to deterioration of the casting solvent or membrane material. They should be impermeable and should not penetrate the membrane when this is formed. The particles ~hould not form suspensions in water.
A wide range o~ particle sizes may be adopted, but the best size range may vary from one material to another.
Thus, for carbon particles a range of 10-30 millimicrons is preferred, whereas for silica and metal particles the individual particle size should preferably be within the range 1-5 micron. Particles of metals, for example stainless steel, exhibit a tendency to aggregation and may be used in aggregated form, up to 200 microns in size, or even more, and selected ranges of aggregates may show improved behaviour compared with the rest, according to the nature of the dope solvent and the concentration of the added particles. While the coarser fractions of aggregated metal particles may exercise a greater effect, they may alternatively lead to membrane defect.
The particles themselves exhibit no permeability and when slurried in water they should give a pH of 3-7.
The membranes of the invention may be prepared from a variety of polymers. These are preferably cellulose-based, preferably lower esters or ethers, eg acetate, cA.153 1053~17 propïonate and butyrate, or methyl, ethyl or propylcellulose. Other polymers which may be used to prepare the membrane include poly-ion polymers, prepared by reaction of poly-anions with poly-cations, polyvinyl chloride, polyacrylonitrile, poly-olefins and polyacrylic esters, particularly of lower alcohols. Apart from the addition of the inorganic particles, the membranes of the invention may be prepared by methods which are conventional for the preparation of ultrafiltration membranes. Thus, the casting solution after the addition and distribution therein of the inorganic particles, is cast as film in flat, tubular or other convenient form, for example as fibres, preferably at room temperature, but if desired at other temperatures~ and i9 preferably contacted less than a minute afterwards, in a leaching bath, for example water, where the solvent diffuses out through the membrane into the leaching bath while the water from the bath passes through the membrane.
The membrane may also be cast directly onto a porous backing providing adequate mechanical support for the membrane. In any case, preferably the membrane when completed is between 5 and 25 mils in thickness, ie 0.012-0.0625 cms, but membrane thicknesses up to 1 mm or even more may be suitable. Thicker membranes are more robust, but show a corresponding decrease in flux rate. As in conventional procedure in the preparation of semi-permeable membranes, the thickness of the membrane may be controlled by the method of applying the dope to the support on which the film is prepared, and its concentration.

cA.153 ~053417 In contrast to membranes cast from volatile solvents, which form an active layer at the air interface that perl'orms the selective filtration function and must be exposed to the solution side of the filtration system for best effect,membranes cast from non-volatile solvents form a corresponding skin serving the same purpose at the interface with the surface on which the film is cast, and this must be exposed with the skin on the filtrate side, remote from the solution undergoing filtration, to exhibit a high flux while being selective to larger molecules in an ultrafiltration capacity. In the preparation of a membrane according to the invention it is found that the greatest flux improvement effected by a given quantity of additive particlesoccur~ when they settle in the membrane casting, concentrating near the interface with the support material in the active layer and thus providing an ani~trnpic, ie asymmetric distribution. To this end, aggregated particles are preferred which settle rapidly in the dope. Sufficient time should be permitted for this to occur, but the membrane should in any event be leached to remove solvent, within five minutes of completing the casting. ~n asymmetric distribution may however be encouraged in internal membranes supported on tubes, by rotating these to apply centrifugal force to the particles in the casting. In use, the prepared membrane is mounted in a suitable test cell or similar arrangement providing adequate mechanical support for the membrane and the milk or other liquid system to be filtered is supplied under pressure to the contact surface of the membrane. The liquid is usually circulated continuously until the degree of concentration required is g cA.153 ~053~17 obtaincd.
In the following Examples a series of membranes was prepared using a variety of particulate additive materials, all of which exhibited a specific surface area o~ at least 502m /gm and a pH of about 5. This was measured by immersing an electrode oi a pH meter in the supernatant liquor obtained by slurrying about 5~ of the material under test in water.
The stainless steel particles used were of 316L
stainless steel, containing 14~ nickel, 17~ chro~ium and

2 . 5% molybdenum. They were nominally 5 microns diameter but aggregated. In Examples 5 and 6 the particles were sieve-graded and the fractions obtained were used in separate tests to demonstrate the effect of the extent of 15 aggregation between the particles upon the flux rate.
About three-quarters of the aggregate was of mesh sieve size 60-90 microns.
In each case the same grade of secondary cellulose acetate was used to prepare the membrane, which was cast at about 15C.
~LE 1 ' A semi-permeable membrane was prepared from a casting solution having the following composition:-20 gms cellulose acetate 100 mls dimethyl formamide 4 gms carbon black, particle size 14 millimicrons, pH 5Ø
The carbon was added to a solution of the cellulose acetate in the dimethyl formamide, giving a viscous liquid which was cast into a membrane by spreading the solution with a doctor blade onto a plate of optical glass permitting I ~

cA.153 lOS3q~17 a blade opening 0.2 cms. After 1 minute, the plate was immersed in water to provide a membrane 0.018 cms in thickness .
A similar membrane, prepared without the introduction of carbon black particles was also prepared. The two membranes were then compared by filtering skim milk having a pH of 6.8, at a working pressure of 200 psig at 50C.
The results of the comparison are set out in Table 1, from which it will be apparent that a marked improvement in flux 10 rate and selectivity results from the presence of carbon black in the membrane. `
TABLE I
Example Control Temp. C 15 50 15 50 Flux rate usgfd* 10.5 13.5 4.9 5.8 Lactose in permeate wt % 2.5 1.9 3.7 3.3 * US gallons per ft2 of membrane per 24-hour day.
From this data the rejection characteristics of the membrane according to the invention, with respect to lactose, was calculated as 10/, wbile that for the control was 8 ~ .

Membranes were prepared as described in Example 1, except that instead of carbon black, particles of micronised stainless steel were used having a particle size range of 1-5 microns. The thickness of the resulting membranes was 8 mils in each case, ie 0.020 cms.
The results of test runs carried out at 16C and 200 psig, on skim milk with a pH 6.8, are given in Table II.

cA.153 1~5341'7 TABLE II

Wt /0 Flux Rate Wt % lactose Steel usgfd in permeate 0 5.46 ~ 3.3 12 9.1 ~ 3.0 18 9.8 ~ 2.8 24 10.5 2.2 *
A In this Example a micronised silica (Gasil 200) of mean particle size 5 microns was incorporated in a series of membranes otherwise prepared as described in Example 1. The membranes were tested at various temperatures as previously described, and the results appear in Table III.
TABLE III
oh Silica Operatin~ Temp.C Flux (usgfd) 0 16 5.46 0 50 5.95 6 16 9.1 12 16 9.8 12 50 12.6 18 16 11.~
24 16 10.5 The effect of changing the solvent on the properties of the membranes according to the invention was examined in this Example. Membranes were prepared otherwise as described in Example 1, from dimethyl formamide and dimethyl sulphoxide, using as the particulate matter Supercarbovar carbon, of particle size 14 millimicrons and pH 5. The membrane A~rK

cA. 153 1053~7 thickness in each case was 8 mils, and the membranes were tested for the concentration of skim milk, against similarly prepared control membranes containing no inert particles, at room temperature (20C) and 200 psig, with the results reported in Table IV.
TABLE IV

Solvent Dimethyl formamide Dimethyl sulphoxide I .
Film thickness 8 15 8 15 I_ l_ _ _ . _ .
Wt % carbon 02 ¦ 40 2 4 0 2 4 0 2 4 I_ l _ _ _ . _ Flux usgfd ~ 9.1¦10.5 _ _ ~ 9.5 11.9 14.7 7.7 _ 11.8 Cellulose acetate dopes were prepared containing 20 grammes of cellulose acetate powder, 100 mls of dimethyl formamide as solvent and powdered steel. A control dope of the same proportions of cellulose acetate and solvent was also prepared. The cellulose acetate was of grade E 3983, as supplied by Eastman Kodak Ltd, containing 39.8% acetyl groups and a viscosity No. 3. The dopes containing powdered steel were vigorously shaken to give a uniform mixture before being cast.
Casting was carried out in tubular modules mounted vertically, each consisting of a fibreglass porous support tube 4 ft long and 4 inch in internal diameter, with walls about ~
inch thick. A plug of the dope was drawn through the tube in each case, at about 2 feet per minute from the bottom of the tube to the top at about 15C on a stainless steel, conical casting bob about 3 inches in length, by means of which the plug of dope was pushed up the tube to apply a uniform layer of the dope about 1 ml thick.

1053~7 The tubes containing the bigger aggregates in the range examined were rotated by hand before admitting water into the tube, to centrifuge the particles toward the membrane-support tube interface.
Finally the tubes were emptied and mounted in a tube separation unit for testing, which was carried out as follows:-Pasteurised milk of zero fat content and 3-3.5 wt %
protein was circulated throught the unit at 50C and at measured pressures and circulation rates. The protein rejection of each membrane was determined by examining the filtrate, using a Pro-milk Analyser (Ross Electric Co.), by a dye-binding detection method using an amido-black dye.
From an examination of the results, it appeared that a substantial increase in flux rate was provided by the membranes according to the invention, from as little as 8 wt % (grammes per cc of solvent) being sufficient to double the flux rate compared with the control membrane. Greater quantities provided a corresponding increase in flux rate, up to 50O/o which was the maximum concentration measured. On the other hand, the complete protein rejection of the membranes was sustained for all particle si~es, from 50-200 microns, except at greater concentrations than 25% with the smallest particles. Further particulars appear in Tables V and VI, obtained at an operating pressure of 60 psig, an operating temperature of 50C and a milk circulation throughput of 700 gals per hour. In regard to Table V, protein rejection failure was observed at other pressures and flow rates using more than 25% of the steel particles.

Further tests esta~lish 3 that the flux remained substantially higher for all the membranes made in accordance with the invention by comparison with the control membrane, whatever the extent to which the milk was concentrated, the flux falling progressively at similar rates with increase in the extent to which the milk was concentrated, both for the control and for the membranes according to the invention. A
comparison with commercially-available tubular modules showed that these exh~bited similar flux rates at the same operational pressure, as the control membranes prepared in the Example. Details of these further tests are given in Table VII.
TABLE V

Concentration of Flux Rate Rejection to steel of particle sizeIG/ft2 daY protein 15120-150 micron %
%
0 15 lO0 lO0 lS 45 lO0 . lO0 42 lO0 200 nil 300 nil 450 nil cA. 153 1053'~7 TABLE VI
( 100% protein rejection throughout) Particle size Flux microns IG/ft2 d (20 g/100 ml solvent) ' ay TABLE VII

10Circulation Rate De~rl ,e of Concn. (X-fold) Gals/hr. 1 2 l 3 4 60 psi~

100 psi~

(16)(10) (6) (4 5) In Table VII the flux obtained from the control tubes, supporting membranes free from these steel particles, is given in brackets. It will be observed that the flux obtained using tubes according to the invention is substantially higher, even at the 60 psig operating pressure, than that obtained with the control at the higher operating pressure of 100 psig for the same circulation rate and at all degrees of concentration at which the measurements were made, the flux falling at substantially the same rate in all cases as the degree of concentration is increased.

cA.153 1053~17 In this Example a comparison is made of the efiect of diiierent solvents on the rejection/flux rates of membranes prepared in accordance with the invention. A similar method was used for preparing tube-supported membranes as described in Example ~, with 45~ in each case oi similar steel particles. One pair oi tubes containing unfractionated aggregates and made irom dope with dimethyl sulphoxide (DMSO) and dimethyl iormamide (DMF) as solvent. Both gave a three-fold improvement in ilux rate over corresponding control tubeswith no additive, in the concentration oi skim milk, protein rejection remaining at 100~. ~he control made using DMSO
had a substantially higher ilux rate itseli than that made using DMF, lS When these comparative tests were repeated, using however the 60-90 micron sieve fraction oi the steel aggregate, a complete 1099 of selectivity ior milk protein was observed with the DMSO tube, which however showed lOOyo rejection of milk bacteria with no ilaws, enabling the milk to be cold-sterilised by iiltration through the membrane without change in composition.
In the case oi the DMF membrane, protein re~ection remained at 100~, but the flux rate increased iurther, to a similar rate to that obtained using the whole aggregate with ~5 DMSO.
In comparable tests in which in all respects the particulars oi Example 1 were iollowed to obtain closely similar membranes with the exception that the dope solvent was acetone, the membrane was found to be wholly unacceptable when particles oi any oi the additive material~ were present.

~ cA.153 ~053'~17 `

This Example illustrates an alternative method o~ preparing the membranes of the invention, in which instead of incorporating the particulate matter in the dope before the film is cast on the support surface, it is previously spread on the latter and the film is cast over it. The particles should not of course embed into the support layer, so that the dope is free to percolate and thus substantially wholly incorporate the particles.
A slurry of the particulate matter, comprising the aggregated stainless steel particles described in the preceding Examples in a 20~ gms/cc. concentration in acetone, was painted on a glass plate and the solvent evaporated, leaving a deposit of the particles on the plate oi about 20 mgms per cm2.
A cellulose acetate film was formed on the plate as described in Example 1, over the coating of steel particles.
On testing the film as described in Example 1 a three-fold improvement in flux rate was observed over a control membrane prepared under similar conditions but with no particles, in the concentration of skim milk at a circulation of 6 gallons per hour and a pressure of 50 psig, through a membrane 112 inches in diameter.
A comparable improvement was obtained using similar particles to those described in Example 1. It was also found that the amount of the particles deposited could be varied at least between 5 and 500 mgms. per cm2 and could be applied directly to porous support means for the film, on which the dope was cast in situ, either in plate or tubul~r form.

Claims (24)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A semi-permeable ultrafiltration membrane having a pore size incapable of rejecting solutes exerting substantial osmotic pressure, the membrane comprising an organic polymeric film cast from an organic film-forming polymer dissolved in a non-volatile solvent the removal of the solvent being effected entirely by leaching using a liquid in contact with the film in which the solvent is miscible and in which the polymer is insoluble and from 2 to 50 wt % finely-divided inert, impervious, water-insoluble particulate matter distributed wholly within the film to enhance the flux of the membrane.

2. A membrane according to claim 1 in which the particulate matter is distributed anisotropically through the membrane.

3. A membrane according to Claim 1 or 2 in which the particulate matter comprises a minor amount of carbon of particle size range 10-30 millimicrons.

4. A membrane according to Claim 1, 2 or 3 in which the particulate matter comprises a minor amount of metallic particles from 1-5 microns in size.

5. A membrane according to Claim 4 in which the metallic particles are aggregated.

6. A membrane according to claim 4 in which the particles are selected from the group consisting of iron, cobalt, nickel, molybdenum, chromium and their inert oxides and alloys.

7. A membrane according to Claim 6 in which the particles comprise stainless steel.

8. A membrane according to Claim l, 2 or 3 in which the particles comprise silica or sulphur.

9. A membrane according to Claim 1 in which the membrane thickness is between 5 and 25 mils.

10. A membrane according to claim 1 in which the membrane polymer comprises a lower cellulose ester or ether.

11. A membrane according to Claim 10 in which the membrane polymer comprises secondary cellulose acetate.

12. A process of preparing an ultrafiltration membrane in which a film is cast from a casting dope comprising a solution in a non-volatile organic solvent of a film-forming organic poly-mer and the solvent is removed solely by leaching from the film using a liquid in contact with the film, in which the solvent is miscible and which the the polymer is insoluble to form the membrane, and wherein at least 1% by weight of the dope of a finely-divided, inert, impervious, water-insoluble particulate matter having a mean specific area of at least 50 m2/g is incor-porated wholly within the dope to improve the flux rate of the membrane.

13. Process according to Claim 12 in which the partic-ulate matter is present in a minor amount with respect to the solvent.

14. Process according to Claim 12 or 13 in which the particulate matter comprises carbon in an amount from 1-4 grammes per 100 cc of the solvent.

15. Process according to Claim 12 or 13 in which the particulate matter comprises metallic particles in an amount from 4-25 grammes per 100 cc of the solvent.

16. Process according to Claim 12 or 13 in which the polymer solution contains from 10-30% by weight of the polymer.

17. Process according to Claim 12 or 13 in which the solvent is dimethyl sulphoxide or dimethyl formamide.

18. Process according to Claim 12 or 13 in which the polymer comprises a lower cellulose ester or ether.

19. Process according to Claim 12 or 13 in which the polymers comprises secondary cellulose acetate.

20. Process according to Claim 12 in which the particu-late matter is incorporated into the dope when the film is cast.

21. Process according to Claim 20 in which the particu-late matter is spread on a support surface in a volatile solvent, the solvent is evaporated and the film is cast over the particu-late matter.

22. Process according to Claim 21 in which the slurry solvent is acetone.

23. Process according to claim 22 in which the particu-late matter is applied on the support surface in an amount from 5-500 mgms. per cm2.

24. Process according to claim 12 in which the membrane is cast in situ on a porous support therefor.