AU777755B2 - Annular hollow fibre membrane and process for preparation thereof - Google Patents

Description

Regulation 3.2

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT

(ORIGINAL)

9 9 U.

Name of Applicant: Actual Inventor: 9 .9 9 NATIONAL UNIVERSITY OF SINGAPORE LI Kang WANG Dong Liang TEO Wah Koon DAVIES COLLISON CAVE, Patent Attorneys, 1 Little Collins Street, Melbourne, Victoria 3000.

"ANNULAR HOLLOW FIBRE MEMBRANE AND PROCESS FOR PREPARATION THEREOF" Address for Service: Invention Title: Details of Associated Provisional Application No: PQ1380/99 filed on 2 July, 1999 The following statement is a full description of this invention, including the best method of performing it known to us: P:\0PERAXD\2309037.CAP JUNE 30. -2- ANNULAR HOLLOW FIBRE MEMBRANE AND PROCESS FOR PREPARATION THEREOF The present invention relates to annular hollow fibre membranes and a process for the preparation thereof. The hollow fibre membranes include two distinct layers, which may be completely or partially detached from each other. The annular hollow fibre membranes described in this invention may advantageously be used for further development of membrane bioreactors.

Vasudevan et al. [Vasudevan et al., Simultaneous bioreaction and separation by an 10 immobilized yeast membrane reactor, Separation Science and Technology, 22 (1987) 1651; Yong et al., Study of transport phenomena in an immobilized yeast membrane bioreactor, Ind.

Eng. Chem. Res., 28 (1989) 231] studied the alcoholic fermentation using a "sandwich" :i membrane bioreactor. The reactor contains a biocatalyst, which is sandwiched between an ultrafiltration membrane and a reverse osmosis membrane. The ultrafiltration membrane provides free passage for all nutrients to the cell mass, while the reverse osmosis membrane is used to immobilise the cells as well as separates the products from the reaction mixture.

Since the feed was forced through under pressure, it overcomes the diffusional resistance usually present in the conventional configurations. In addition to the advantages mentioned above, the substrate uptake becomes more efficient and cell layer thickness can be minimised to prevent diffusion gradients in the cell mass due to substrate depletion. The disadvantage of the bioreactor is that the surface area of the membrane is not high enough resulting in a bulky volume of the reactor even for a small quantity of the alcoholic production.

An alternative method to the one mentioned above is to fabricate the membrane module by inserting one hollow fibre within another and to grow the cells in the annulus between the two membranes. However, the level of difficulties for making such a membrane module will be greatly increased by placing one hollow fibre within another even for a very short length. In order to fabricate such hollow fibre membrane reactors, one may propose to use annular hollow fibre membranes. However, to date, no such hollow fibre membranes are available commercially. The present invention advantageously provides various annular hollow fibre membranes for bioreactor application.

According to one aspect of the invention there is provided an annular hollow fibre P:NOPER\AXD\2309037.CAP JUNE 30, -3membrane comprising an inner layer and an outer layer having a discontinuity therebetween, the inner layer having an integral skin on an inner surface thereof and the outer layer having an integral skin on an outer surface thereof, wherein each of the inner and outer layers includes a plurality of finger-like microvoids and/or is of a sponge-like structure.

According to another aspect of the invention there is provided a method for producing an annular hollow fibre membrane comprising an inner layer and an outer layer having a discontinuity therebetween, the method comprising the steps of: preparing first and second polymer dopes for forming said inner and outer layers respectively; S 10 (ii) extruding the first and second polymer dopes respectively through inner and outer orifices of a spinneret while simultaneously injecting an internal coagulant through a central tube of the spinneret to form a nascent hollow fibre; (iii) passing the nascent hollow fibre through an air gap; and (iv) solidifying the nascent hollow fibre in an external coagulant to form the hollow fibre membrane.

Annular hollow fibre membranes can be prepared by the Lob-Sourirajan phase inversion method with a triple-orifice spinneret. The triple-orifice spinneret is constructed with two annular-jets and a central tube. In the spinning process, two polymer dopes are prepared; the first is extruded from the outer annular-jet, while the second is extruded from 20 the inner annular-jet. The internal coagulant is, as described above, simultaneously injected into the central tube. During the spinning process, the first polymer dope is in contact with the internal coagulant, while the second polymer dope is first exposed to air and then to the external coagulant. Due to the difference in shrinkage between the two layers caused by the coagulation process, an annulus may be formed between the two layers of the hollow fibre.

The formed annular hollow fibres exhibit complex structures. More particularly, both the inner layer and the outer layer display an asymmetric structure and they, respectively, have a skin integrally located in the inner surface of the inner layer and the external surface of the outer layer. Between these two skins are the inner and outer layers which respectively include finger-like microvoids and/or sponge-like structures with a clear discontinuity therebetween. The discontinuity may be a clear transition between the inner and outer layers.

However, the discontinuity is preferably an annular gap between the inner and outer layers, P:\OPER\AXD\309037.CAP JUNE 30, 2000 -4the layers being partially or completely detached in the presence of the gap. Preferably the annular gap has a width of from about 10,tm to about 100im.

The finger-like microvoids are preferably long, regular, closely packed and completely open ended cavities. These may be beneficial in immobilization for biocatalysis to improve the capacity of the bioreactor. Open cavities would also improve free convective transport of spores into the finger-like microvoids. This would advantageously simplify the inoculation procedure and the fungus could establish itself within the microvoids of the membrane, thus creating a thick fungal layer. Preferably the finger-like microvoids open towards the discontinuity between the inner and outer layers.

10 The annular hollow fibre membranes are preferably produced by a dry/wet phase inversion process. Two polymer dopes need to be formulated for making the annular hollow fibre. The polymer dopes each comprise a polymeric material, a solvent for the polymeric material and an additive. The concentration and nature of the constituents are factors which are advantageously taken into consideration when formulating the polymer dopes. In 15 particular, the polymer dopes are preferably formulated such that the first polymer dope has a coagulation rate which is greater than that of the second polymer dope.

As for the spinning process, solution thickness is reduced with the exchange of solvent and coagulant. Also, high polymer concentration may result in less shrinkage during the coagulation because a high polymer concentration relates to high polymer content per unit 20 volume. Therefore, it is generally conceivable that the annular width of the membrane reduced with increasing polymer concentration. On the other hand, membranes prepared from low polymer concentration generally include relatively large pores. The preferred polymer concentration for providing desirable annular width and pore size is in the range from 10wt% to 40wt% of the respective polymer dopes. More preferably, the polymer concentration is in the range of from 15wt% to Preferably, the polymer of each of the polymer dopes is selected from the group consisting of polysulfone, polyethersulfone, cellulose polymers (particularly cellulose diacetates, cellulose triacetate and their blends, and cellulose butyrate), aromatic polyamides, polyimides, polyacrylonitrile, polyvinylidene fluoride and polyetherimide. These polymers exhibited good physicochemical and membrane forming properties.

The solvents used in the polymer dopes are also important, as the rate of coagulation P:\OPER\AXD\2309037CAP JUNE 30.2000 in the precipitation process is different for different solvents. If the two coagulation rates for the inner and outer dopes are widely separated in magnitude, the formation of the annular gap may be expected. In making asymmetric membranes from the above mentioned polymers, any organic polar solvent may be used provided that it is capable of dissolving the polymer.

The preferred solvents are N,N,-disubstituted amides solvents, particularly, N-methyl-2pyrrolidone, dimethylacetamide, dimethylformamide, and acetone.

The additives used in the membrane forming system are selected to obtain the desired membrane structure and pore size. For making the membranes in different applications, the additives used are different. The additives should be soluble in and compatible with the o solution of polymer and solvent. They should also be soluble in the coagulation medium.

There are a large number of nonsolvents which meet these requirements. Water miscible polymers (particularly polyvinylidene pyrrolidone and polyethylene glycol) may be used as a pore former to increase the viscosity of the polymer dopes and increase pore size. The disadvantage of these polymeric additives is that they are not easily washed out from the membranes. Small molecular nonsolvents and inorganic salts are also important additives.

In this invention, small molecular non-solvent additives including water and ethanol may be employed.

Proper choice of the additive concentration is also a consideration which should be taken into account when formulating the polymer dopes. The additive tolerance for different 20 polymers and solvents is different. The mixture of solvent and nonsolvent with a given ratio may be a good solvent for one polymer, but may be a poor solvent or even act as a swelling agent for another [Wang et al., Phase separation phenomena of polysulfone/solvent/organic nonsolvent and polyethersulfone/solvent/organic nonsolvent systems, J. Appl. Polym. Sci., (1993) 1693]. The successful preparation of annular hollow fibres may further be attributed to the solvent mixture with proper ratio. Proper choice of the solvent mixtures for both the polymer dopes is favorable to form the annular hollow fibres.

Nonsolvents for the polymers may be used as the internal and/or external coagulant.

From an economic or environmental point of view, tap water is preferred, particularly as the external coagulant. A mixture of aqueous solutions containing various compounds may also be used as the internal coagulant because only a small amount of internal coagulant is introduced in the hollow fibre spinning. In spinning annular hollow fibres, the internal P:\OPER\AXDUl09037.CAP JUNE 30. 2000 -6coagulant is responsible for the formation of the inner layer, while the external coagulant is responsible for the outer layer. Therefore, based on general formation mechanisms of conventional hollow fibres, while the inner surface of the inner layer and the external surface of the outer layer may have skins, the surfaces facing the discontinuity between the inner and outer layers may have no skins.

The accompanying figures are scanning electron microscopic photographs of crosssections of asymmetric annular hollow fibres according to embodiments of the invention.

FIGURE 1 shows the cross-sectional structure of annular PEI/PSf hollow fibre obtained in Example 1.

10 FIGURE 2 shows the cross-sectional structures of annular PESf/PSf hollow fibre oooo obtained in Example 2. fibre wall; inner edge of outer layer.

FIGURE 3 shows the cross-sectional structures of annular PSf/PESf hollow fibre "!"obtained in Example 3. fibre wall; inner edge of outer layer.

FIGURE 4 shows the cross-sectional structures of annular PSf/ PESf hollow fibres obtained in Example 4. concentric bores; non-concentric outer orifice FIGURE 5 shows the cross-sectional structures of annular CAB/PEI hollow fibre obtained in Example 5. fibre wall; outer edge of inner layer; (c) S .inner edge of outer layer 20 FIGURE 6 shows the cross-sectional structure of annular CAB/PEI hollow fibre obtained in Example 6.

The annular hollow fibres were spun using a triple orifice spinneret in laboratory-scale hollow fibre spinning equipment. Two preformulated polymer dopes were poured into the two solution tanks, respectively. During the membrane spinning, the dopes were fed under nitrogen pressure and/or the gear pump through the spinneret. The polymer dope 1 was fed to the inner orifice and the polymer dope 2 was fed to the outer orifice, while the internal coagulant was introduced into the inner central tube of the spinneret. The extrusion rates of both the spinning dopes were properly controlled so as to control both wall thicknesses of the fibres and also annular width. Tap water at room temperature (25 1 0 C) was used as both the internal and the external coagulants. The nascent fibre was passed through an air-gap and then into the coagulation bath at ambient conditions (25 1C and RH 60-65%). Different P:\OPER\AXD\309037.CAP JUNE 30. 2000 -7spinning conditions were selected according to the specific polymer dopes. The spinneret was arranged such that the nascent fibre was extruded vertically downwards into the coagulation bath. After the coagulation, the fibre was guided through the wash bath, and then coiled into the storage tank. The hollow fibre was kept in a storage tank for at least three days to stabilise the structure.

The invention will now be described in more detail with reference to the accompanying Examples which are provided for illustration only and which should not be construed as limiting on the invention in any way.

EXAMPLE 1 S* 10 Polyetherimide (PEI)-polysulfone (PSf) bilayer hollow fibre membrane was prepared.

The PEI dope in the outer orifice contained a composition of 25% PEI, 72.34% NMP and 2.66% H 2 0, while the composition of the PSf dope in the inner orifice was 28% PSf, 67.78% NMP and 4.22% water. The air-gap was controlled at 5cm. Figure 1 shows a SEM graph of the hollow fibre membrane prepared from the PEI and PSf dopes. As can be seen in Figure 1, no clear annulus can be observed between the two layers although the boundary of the two layers is clearly shown. In the inner layer, finger-like voids beneath of the inner skin layer are open to the outer layer. The outer PEI layer exhibits a sponge-like structure. It seems that the PEI layer was shrunk more than the PSf layer resulting in no annulus being formed in the fibre.

20 EXAMPLE2 Polyethersulfone (PESf)-polysulfone (PSf) annular hollow fibre was prepared from an outer dope containing 27.4% PESf, 64.8% NMP and 7.8% H20 and an inner dope containing PSf and 70% DMAC using an air-gap of 5cm. The SEM graph of the cross-section structure is shown in Figure 2. The annular width of the hollow fibre formed is about 20 /zm.

In the inner layer, finger-like voids are formed beneath the skin layer and are open towards the annular gap. The outer layer, in general, exhibits a sponge-like structure. Further investigation of the outer layer reveals that a more porous structure is observed near the annulus. As shown in Figure 2b, there is no clear skin layer formed at the annular gap.

EXAMPLE 3 Polysulfone (PSf)-polyethersulfone (PSEf) annular hollow fibre was prepared from an outer PSf dope (30% PSf and 70% DMAC) and an inner PESf dope (30% PESf, 48.71% P:\OPERXAXD\2309037.CAP JUNE 30. 2000 -8- NMP and 21.38% ethanol) using an air-gap of 5cm. It can be seen in Figure 3a that long finger-like voids were still present in the inner layer and were open to the annular gap. The annular width is not uniform. Figure 3a further reveals that stringy structures were formed between the internal opposing surfaces of the inner and outer layers. These stringy structures appear to be tangled with each other within the annular gap. Figure 3b shows the outer layer structure near the annulus at high magnification (5,000). As can be seen, only porous structure is observed on the surface facing to the annulus.

EXAMPLE 4 Polysulfone (PSf)-polyethersulfone (PES) annular hollow fibre was prepared using 10 low polymer concentrations. Figure 4 shows cross-sectional structures of the annular hollow fibre prepared from an outer PSf dope (20% PSf, 71.02 DMAC and 8.98% EtOH) and i an inner PESf dope (20% PESf, 71.45% NMP and 8.55% H 2 0) using an air-gap of Both polymer dopes had very close phase separation points. As can be seen, a very clear annulus was formed and the cross-sectional structures of both the inner layer and outer layer are similar to those prepared using higher polymer concentrations (Figures 2 and It was also observed that the formation of a uniform annular gap was mostly affected by the spinneret. A slight non-concentricity of the inner tube, inner orifice or outer orifice of the spinneret resulted in the non-concentric annular hollow fibres as shown in Figure 4b. In some cases, the inner fibre attached to the lumen of the outer fibre. Due to the non-uniform 20 distribution of the outer polymer dope, the thinner part coagulates faster than the thicker part resulting in non-uniform shrinkage, forming the non-concentric annular hollow fibres shown in Figure 4b.

EXAMPLE Cellulose acetate butyrate (CAB) may also be used as the polymer for fabrication of the annular hollow fibre membranes. Figure 5 shows cross-sectional structures of annular hollow fibres prepared from an outer dope containing 12% CAB 80% DMAC and 8% H 2 0, and an inner dope containing 16% PEI, 80.05% NMP and 3.95% H 2 0 using an air-gap of As can be seen, the outer layer exhibits a sponge-like structure, while the inner PEI layer contains finger-like voids in the middle area. The two layers of the membrane are detached and a clear and wide annular gap has been formed in this CAB-PEI annular hollow fibre. However, the inner layer is not centered and is partially attached to the inner wall of P:\OPER\AXD\2309037.CAP JUNE 30, 2000 -9the outer layer. Further examination of the annular gap at high magnification reveals that non-skin and porous structures were formed at both edges of the inner (Figure 5b) and outer (Figure 5c) layers.

EXAMPLE 6 Figure 6 illustrates the cross-sectional structure of a CAB-PEI annular hollow fibre prepared from a higher PEI concentration and a slow extrusion rate. The polymer solution used in the outer layer contained 11% CAB, 77.8% DMAC and 11.1% EtOH, while the polymer solution used for the inner layer contained much higher PEI concentration PEI, 59.3wt% NMP and 10.7% MeOH). It can be seen that a much wider annulus was l0 formed. Again, the two layers of the membranes are completely detached from each other.

It is understandable that the inner and outer layers are detached completely, as these two polymers are very compatible. It thus follows that the annular gap of the annular hollow ee fibres can be easily formed if incompatible polymers are employed. On the other hand, the coagulation rate of the inner PEI solution is much faster than that of the outer CAB solution.

This is also beneficial to form completely detached annular hollow fibres. However, such an annular hollow fibre will never be a concentric one since there is no binding between the two layers and inevitably, the inner layer will fall on to the outer layer as shown in Figure 6.

The reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that that prior art forms part of the common 20 general knowledge in Australia.

Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is to be understood that the invention includes all such variations and modifications which fall within its spirit and scope. The invention also includes all the steps, features, compositions and compounds referred to or indicated in this specification, individually or collectively, and any and all combinations of any two or more of said steps or features.

Claims (18)

1. An annular hollow fibre membrane comprising an inner layer and an outer layer having a discontinuity therebetween, said inner layer having an integral skin on an inner surface thereof and said outer layer having an integral skin on an outer surface thereof, wherein each of said inner and outer layers includes a plurality of finger-like microvoids and/or is of a sponge-like structure.

2. An annular hollow fibre membrane according to claim 1, wherein said discontinuity is an annular gap between said inner and outer layers.

3. An annular hollow fibre membrane according to claim 2, wherein said annular gap has a width of from about 10gm to about 100mm.

4. An annular hollow fibre membrane according to claim 1, wherein said inner and/or outer layers include finger-like microvoids which open towards said discontinuity between the inner and outer layers.

A method of producing an annular hollow fibre membrane comprising an inner layer and an outer layer having a discontinuity therebetween, said method comprising the steps of: preparing first and second polymer dopes for forming said inner and outer layers respectively; (ii) extruding said first and second polymer dopes respectively through inner and outer orifices of a spinneret while simultaneously injecting an internal coagulant through a central tube of said spinneret to form a nascent hollow fibre; (iii) passing said nascent hollow fibre through an air gap; and (iv) solidifying said nascent hollow fibre in an external coagulant to form said hollow fibre membrane.

6. A method according to claim 5, wherein said first and second polymer dopes each comprise a polymeric material, a solvent for said polymeric material and an additive, and wherein said first polymer dope has coagulation rate which is greater than that of said second P:\OPER\AXD\2309037.CAP JUNE 30. 11 polymer dope.

7. A method according to claim 6, wherein said polymeric materials are selected from the group consisting of polysulfone, polyethersulfone, cellulose polymers (particularly cellulose diacetates, cellulose triacetate and their blends, and cellulose butyrate), aromatic polyamides, polyimides, polyacrylonitrile, polyvinylidene fluoride and polyetherimide.

8. A method according to claim 6 or 7, wherein said solvents are selected from the group consisting of N,N,-disubstituted amides solvents, particularly, N-methyl-2-pyrrolidone, dimethylacetamide, dimethylformamide, and acetone.

9 A method according to any one of claims 6 to 8, wherein said additives are selected from the group consisting of water-miscible polymers, particularly polyvinylene pyrrolidone and polyethylene glycol, small molecular non-solvents, particularly C 1 -C 3 aliphatic alcohols and acids, water and mixtures of water and the aliphatic alcohol and acids, and inorganic salts.

10. A method according to any one of claims 6 to 9, wherein said first and second polymer dopes have polymer concentrations in the range from 15wt% to

11. A method according to any one of claims 5 to 10, wherein said internal and external coagulants are water and/or aqueous solution containing organic liquid or inorganic salts.

12. A method according to any one of claims 5 to 11, wherein said first and second polymer dopes are spun into air and then into the external coagulant.

13. A method according to any one of claims 5 to 11, wherein the first and second polymer dopes are extruded from the spinneret at a controlled extrusion rate so as to adjust the thickness of the inner and outer layers and the annular width of the annular hollow fibre membrane. P:\OPER\AXD\Z309037.CAP -JUNE 30. 2000 -12-

14. A method according to any one of claims 5 to 13, further comprising desolvating and washing the formed hollow fibre membranes.

A method according to claim 14, wherein the desolvation and washing of the hollow fibre membranes takes place in water bath at room temperature.

16. A method according to any one of claims 5 to 15, wherein said air-gap is from about 2 to about 15cm in width. *se

17. An annular hollow fibre membrane substantially as hereinbefore described with reference to the drawings and/or Examples.

18. A method of producing an annular hollow fibre membrane substantially as hereinbefore described with reference to the drawings and/or Examples. DATED this 30th day of JUNE, 2000 NATIONAL UNIVERSITY OF SINGAPORE by DAVIES COLLISON CAVE Patent Attorneys for the Applicant