WO1991001796A1

WO1991001796A1 - Orbital/oscillation filtration system with no rotary seal

Info

Publication number: WO1991001796A1
Application number: PCT/US1990/004206
Authority: WO
Inventors: Yao-Tzu Li
Original assignee: Y.T. Li Engineering, Inc.
Priority date: 1989-08-01
Filing date: 1990-07-26
Publication date: 1991-02-21

Abstract

A filtration system for a fluid containing suspended particles mounts a filter (3) within a closed container (2) having fluid inlet (7) and outlets (8, 9) for filtered fluid and retentate fluid. The inlet and outlets communicate with the interior of the container but do not rotate with respect to the container. An orbital and/or oscillatory drive (1) moves the container in either a continuous wobbling movement or in an oscillatory movement. In one form a mechanical element (4) pivots around the inlet side of the filter. In another form a rotating cylinder (17) with an eccentric center of gravity revolves or oscillates with respect to a surrounding non-porous cylinder (18) to create Taylor vortices in the fluid. In another, direct-drive form, a conduit (92) to the interior of a closed filter element (3), or a surrounding sleeve (77) which supports the container (2) is formed of an elastically resilient material.

Description

ORBITAL/OSCILLATION FILTRATION SYSTEM WITH NO ROTARY SEAL

Background Of The Invention:^'

This invention relates in general to filtration systems and in particular to a filtration system useful in the down-stream processing of biologically manufactured products.

In certain biotech applications a product such as a protein or enzyme is produced by cells in a bio-reactor. The cells are immersed in a processing fluid and the product is released into the fluid. While the use of actual naturally occurring or biologically engineered cells to produce a specific product is a well known commercial technique, difficulties in separating the product from the process fluid and undesired material carried by the fluid "downstream", of the bio-reactor has been a significant limitation on the utilization of this approach manufacturing, as compared to artificial, . chemical production of the product.

In these biotech applications, it is necessary to have a downstream processing which reliably filters the process medium, but it must do so at commercially acceptable rates, without damage to the product (which may be cells or molecules which can be damaged by mechanical forces or high shear forces in the process fluid), and without risk of biological contamination, as by the entry of bacteria from the external environment into the processing system, including the downstream filter.

One desirable feature of a filter is to minimize the piling up of he particles in front of the filter, a phenomenon known as polarization, which seriously diminish the flux rate of the filtration process. One conventional method to alleviate the polarization is to use a cross flow arrangement as is commonly used in spiral wound or hollow fiber types. A more direct scheme is to use a propeller-shaped stirrer placed in the vicinity of the filter surface to produce a turbulence to clear up the polarized filter surface. Another scheme involves a cylindrical-shaped filter rotating inside a fixed mating cylinder where a set of ring-shaped vortices, known as Taylor vortices, will be induced in the gap between the filter and the stationary cylinder to provide the mechanism of generating the stirring motion of the fluid to clear up the polarization.

Direct drive rotary filters, utilizing Taylor vortices for anti-polarization effect, are marketed by the Membrex company in the United States and by Sulzer Bros. Ltd. in Switzerland. The Membrex system involves the use-of an inward flowing rotary filter cartridge driven to rotate by a magnetic drive coupling to avoid one rotary seal. However, the flow passage of the filtrant from the inside volume of container still must be contained by a rotary seal to have a total sealed system.

A rotary filter can sustain much higher flow rate per unit area then the cross flow types. However, in either the propeller-stirrer types, or the rotating cylinder types of filters, a rotary motion is involved with the need of some form of rotary seal which is objectional for biotech applications.

Another drawback of rotary agitated filtration equipment is the progressive reduction of the surface/volume ratio, as the size of the equipment is increased.

It is therefore a principal object of this invention to provide a filtration system, particularly one for downstream processing in biotech applications, that resists polarization to provide continuous filtration at a high efficiency.

Another principal object is to provide a filtration system with the foregoing advantages that produces a low level of shear and mechanical action on cells and other particles carried in the fluid.

A further principal object is to provide a filtration system with the foregoing advantages that uses no rotary seals in the container.

Still another object is to provide a system with the foregoing advantages which may be scaled up in size without sacrificing a favorable surface/volume ratio.

Another object is to provide a filtration system with all of the foregoing advantages which is reliable, has a comparatively simple construction, and has a favorable cost of manufacture. Summary Of The Invention

A system for filtering suspended particles from a fluid, particularly a system for downstream processing of the process fluid of a bio-reactor in the production of biological products, has a filter mounted within a closed container having a fluid inlet and fluid outlets for the retentate and the filtrate, including the product. The inlet and outlet(s) are secured to the container in a non-rotating manner. The filter is typically a cylindrical membrane or mesh.

A key aspect of this invention is an arrangement to depolarize the inlet side of the filter without having a drive shaft that penetrates the container and requires a rotary seal, which is highly effective, and which does not damage cells and/or the product due to mechanical action or fluid shear forces.

One class of embodiments of the present invention uses dynamically coupled filters driven by an orbital/oscillatory motion applied to its container so that flexible tubings may be used to conduct* the processing fluids in and out of the system without the need of any rotary seal.

In certain embodiments the motion of an orbital drive swings a blade, whip rod, the filter itself, or an eccentrically weighted cylinder mounted within the filter to produce a turbulence at the inlet side of the filter or to produce Taylor vortices. These revolving elements are mounted within the container at their top and bottom for a free pivotal rotation in response to the orbital motion. In various of these forms, the container may have multiple filters and associated depolarizing members, such as surrounding tubes, blades, whip rods, etc., mounted within one container.

. In other embodiments the filter or the container is suspended by an elastic conduit which exerts a spring force in a torsional mode. A drive rotates the filter or the container in an oscillatory manner with the oscillation resisted by the torsional spring force of the coupling. The mutual oscillatory motion produces Taylor vortices in the fluid disposed between the filter and the container. In one form, the filtered fluid outlet from the filter is in part the elastic conduit and a magnetic drive oscillates the filter. In another form the conduit supports the container and a direct mechanical drive is coupled to the elastic ^"conduit at a point over the container.

The invention will be more fully understood from the following detailed description of the preferred embodiments which should be read in light 'of the accompanying drawings:

Brief Description Of The Drawings

Figure 1 is a simplified view in perspective, with portions broken away, of an orbital drive filter according to the present invention with revolving blade;

Figure 2 is a view in horizontal section of the rotating blade filter shown in Fig. 1;

Figure 3 is a view in horizontal section of an orbital drive rotating cylinder filter according to the present^'invention;

Figure 4 is a simplified perspective view of the filter system of the general type shown in Fig. 3 showing the formation of Taylor vortices;

Figure 5 is a graph of the flux for various types of filters as function of time;

Figure 6 is a simplified view in horizontal section of an oscillating type filter according to the present invention;

Figure 7 is a graph showing the acceleration, velocity and centrifugal force of the oscillating type filter shown in Fig. 6 as a function ^* of time;

Figure 8 is a simplified view in vertical section along the line 8-8 in Fig. 9 of a multi-tube orbital drive filter according to the present invention;

Figure 9 is a simplified view in horizontal section along the line 9-9 in Fig. 8;

Figure 10 is a simplified view inside elevation of an orbital drive platform according to the present invention;

Figure 11 is a simplified view in vertical section of a multi-tube oscillatory filter;

Figure 12 is a simplified view in vertical section of a filter system according to the present invention utilizing an orbital whip rod;

Figure 13 is a simplified view in vertical section of a filter system according to the present inyention with an orbiting filter cartridge;

Figure 14 is a view in vertical section of a direct drive oscillatory filter with magnetic coupling according to the present invention, and

Figure 15 is a view in vertical section of a. direct drive oscillatory filter with flexible seal.

Detailed Description Of The Preferred Embodiments

Figure 1 illustrates a simple orbital drive filter system according to the present invention with a revolving blade 4 inside a cylindrical filter 3 to perform the anti-polarization effect on the inlet (interior as shown) side of the filter 3. The filter is preferably formed of a membrane material such as the uniform pore size material sold by Millipore Corp. of Bedford, Massachusetts. In this diagram a conventional laboratory orbital shaker 1 provides the orbital drive. Upon, this shaker a filter container 2 is mounted with stand-offs 10. The filter 3, which is cylindrical, is mounted inside the container 2. Chamber 5 defines the space inside the filter and an annular chamber 6 defines the space outside the filter. Fluid to be filtered is fed through inlet port 7 into chamber 5 and exits through outlet port 8 as the retentate (or "filtrant"). A pressure gradiant (e.g. 10 psi) is maintained between chamber 5. and chamber 6 to drive the filtrate passing through the filter into chamber 6 where it exits through^" outlet port 9. Particles are intercepted by the filter and pile up behind the surface of the filter to block the flow of the filtrate producing the so-called polarization effect. The flow stream of the retentate that passes from the inlet 7 and out through outlet 8 can help to sweep away some of the particles that piled behind the filter. This is known as the cross flow anti-polarization effect and is used in each embodiment of the invention in conjunction with other depolarizing techniques. This cross flow is not very effective because the action needed to sweep the particles behind the filter screen is generated indirectly. The accumulation of filtered particles in the outlet chamber 5 at the interior of the filter is removed through retentate outlet 8 in the bottom wall of the container 2. The size of the outlet 8 and/or valving 8 ' on the port can control the withdrawal of the retentate carrying off a particle build up to maintain a desired fluid pressure across the filter.

The rotary blade 4 is hinged to revolve freely about the axis 11-11' . The blade 4 is made of material heavier than the density of the fluid so that under the influence of the orbital motion it will push the fluid inside the chamber 5 to revolve as shown by arrow 12 at the orbiting speed 13 of the shaker 1. The blade 4 is preferably formed of a rigid structural material that is compatible with the fluids being processed. Stainless steel is suitable. A narrow gap is maintained between the outer edge of blade 4 and the inside surface of the filter to avoid direct rubbing, but the turbulence created at the gap is quite effective to sweep away the particles piling up due to filtration. Fig. 2 is a cross section view of Fig. 1 showing the relationship of the container 2, the filter 3 and the wiper blade 4. Of course, in applications where mechanical action on the fluid and particles carried in the fluid is not a consideration, it may be possible to have the blade contact the filter.

Fig. 3 shows an orbital drive rotating cylinder filter system .of the present invention. Like parts in the various figures are denoted with the same reference number. In comparison with Fig. 2 the only change is the replacing of the wiper blade 4 by a rotating cylinder 14 which is also pivotally mounted to rotate freely about the center axis 11-11'. The mass center 15 of cylinder 14 is displaced away from axis 11-11' by a radius r so that in response to the orbital motion 13 the cylinder will revolve as indicated by arrow 12. Sector 16 represents a heavy bar that may be used to shift the mass center 15 away from the axis 11-11'.

Fig. 4 illustrates the effect of a revolving cylinder 17 inside a stationary cylinder 18 upon the fluid that fills the gap between the two cylinders. The gap is generally uniform. Arrow 20 represents the tangential velocity of the fluid near the surface of cylinder 17 which revolves with a speed indicated by the arrow 19. Corresponding to the tangential velocity 20 there exists a centrifugal force^" 21 which tends to move the fluid outwardly as shown by the interface between vortices 23 and 24. As the fluid thus moves outwardly to reach the stationary cylinder 18, its tangential velocity will slow down with the consequence of a lowering of the centrifugal force as represented by arrow 22. .Thus the outwardly moving fluid will have a higher centrifugal force than the inwardly moving fluid and thereby creates a condition for sustaining a sequence of ring-shaped vortices, known as Taylor vortices. These vortices are more effective for the depolarization of filters than the cross flow scheme.

Taylor vortices can be generated only when the inside cylinder is rotating to create the centrifugal force gradiant. However, function of either the inside cylinder 17 or the outside cylinder 18 may be performed by the filter 3. In the arrangement of Fig. 3 the filter 3 as shown corresponds to the stationary cylinder 18 of Fig. 4, while cylinder 14 has a non-porous wall and corresponds to the rotating cylinder 17. In this arrangement the container 2 collects the retentate. On the other hand, cylinder 14 of Fig. 3 may be the filter with the position of the filter 3 taken up by a non-porous cylinder and with the elimination of cylinder 2. However, the arrangement of Fig. 3 corresponds directly to Figs. 1 and 2 so that chamber 5 and chamber 6 and their corresponding ports 7, 8 and 9 are the same in Figs. 1-3. From these ports flexible hose may be used to transport fluid with no need of rotary seal. On the other hand, if cylinder 14 is used as the filter, one or two rotary seals would be needed even with an orbital drive.

Fig. 5 illustrates typical performances of three kinds of filters. Curve 26 shows that the flux (flow rate per unit area per unit pressure gradiant) of a simple filter drops from the initial value of Uo down to the zero base line very quickly. Curve 27 represents the situation with cross flow while curve 28 represents that of a rotary filter of the present invention.

Fig. 6 illustrates an oscillatory cylindrical filter system where the cylinder 17 oscillates about pivot 11 with a double amplitude of angle θ inside a stationary cylinder 18. On the surface of cylinder 17 every point would experience a tangential velocity vector 30, a centrifugal force vector 31 and a tangential acceleration vector 32. Fig. 7 illustrates the time function of the magnitude of these three vectors. Since the centrifugal force is proportional to the square of the tangential velocity, it remains positive at all times. For this reason there exists the necessary condition of generating vortices as shown in Fig. 4. In addition, the tangential acceleration is an effective means of shaking loose those particles polarizing the filter surface.

Under a test program with a 5 cm O^'.D. cylindrical inward flowing membrane filter subjecting to both a constant speed mode (about 1,000 cycles per minute) and a 120° oscillation mode and tested with 1% baker's yeast solution under 10 psi pressure gradient, it was found that operation in the oscillatory mode outperfoπris the constant speed mode by about 30% over a range of frequencies and similar speeds. At a 120° swing, the average tangential velocity is only 2/3 that of the constant speed of the same frequency and the average centrifugal force is even less, so that the improved anti-polarization effect as observed appears to be due to the angular acceleration effect.

Fig. 8 illustrates a multi-tube orbital drive filter with its horizontal cross-sectional view shown in Fig. 9. Filter tubes 3 are outward flowing type with their outside wall reinforced with grid structure 38 inside the container 35. The grid structure can be an open lattice of extruded plastic cemented to the filter where the materials chosen are compatible with the fluid being processed. The two ends of the filter tubes are connected to chambers 36 and 37 to. receive process fluid from an inlet port 40 and to discharge the retentate through an outlet 41. The filtrate collected from the outside surfaces of tubes 3 passes through the grid 38 and is discharged through port 42.

Inside each tube 3 there is a cylinder 14 with an Qff-axis mass center so that it will be driven by an orbital-type drive to rotate with respect to axis 11-11' and to generate Taylor vortices in the annular gap between cylinder 14 and filter cylinder 3 to achieve the desired anti-polarization effect.

A full-sized system of the type shown in Fig. 8 can weigh thousands of pounds and with a life, load varying according to the amount of fluid it carries. It* is important to dynamically balance a system of this kind that is in orbital motion. Even for a small laboratory-sized filter system, the setup shown in Fig. 1 by mounting the filter on top of a laboratory orbital shake table, operational stability cannot be • assured with the system running at the effective speed. Therefore, the proven orbital drive system of the type described and illustrated in applicant's U.S. Patent 4,762, 592, is preferred. The disclosure of this patent is incorporated herein by reference. A system of this type is, however, also shown in Fig. 10.

The filter container 35 of Figure 8 is shown in Fig. 10 as being supported by a number of struts 52 through elastic couplers 53 and 54 upon the base plate 50. The elastic support keeps the system standing upright with a certain degree of freedom for lateral movement, ^'such as to maintain a lateral mode of natural frequency around one Hertz. The supports allow a wobbling orbital motion of the container 35, but resist a revolving of the system.

A counterweight system with its mass center 55 placed in the same plane as the mass center 101 of the filter system 35, is driven to rotate at a radius R from the mass center 101. In Fig. 10 this mass center 55 of the counterweight system is illustrated by the use of two counterweights 55',55' one above and one below the filter system 35. Of course, a variety of other arrangements can be used to counterbalance and support the system given the structural features and performance criteria delineated above. As the counterweight 55 is driven to revolve with respect to mass center 101, they will automatically seek a common axis 100-100' to balance each other. This axis 100-100' may go through a spiral pattern as the system gets started. As soon as a stabilized speed is reached this common axis 100-100' will move to the system center line 102-102', while the system 35 will orbit a a radius R' . Radius R' automatically adjusts itself in proportion to the ratio of the mass of the counterweight system 55 and the mass of the system 35.

In principle, system 35 also carries many other revolving bodies such as the filter cylinders 14 of Fig. 8 and each cylinder revolves under the influence of the orbital motion of system 35 against certain viscous drag. All of these dynamic loadings will affect the relative location and phase angles between the mass centers of the counterweight and the main system 35.

The orbital filter of Figs. 8 and 10 may be driven to move in an orbital/oscillatory mode by driving the counterweights 55 in an oscillatory swing. Alternatively, the orbital filter system 35 may be driven to move in an oscillatory arc with respect to a fixed frame work. Detail dynamic tuning of this type of system to operate at or near a natural frequency is preferred. Those skilled in the art will readily calculate or empirically determine the desired values for variables such as the length of the struts, the stiffness of the supports and the mass of the container. Fig. 11 shows another orbital/oscillatory filter system according to the present invention with the filtrate passing inward through the surface of each filter tube and then upwardly to flow through a flexible hose 61 into chamber 60. Fluid pressure produces this flow. The process fluid will enter the system through port 40 and the filtrate exits through port 42. The lower ends of each filter tube may be secured by a pivot or by a torsional spring 62 to achieve some tuning effect. Note that the container of Fig. 11 must be driven to oscillate through an angle θ, and that this is the appropriate manner to use the inward flowing filter 17 without a rotary seal,

The system shown in Fig. 11 involves oscillating filter 17 inside non-porous tubes 18 which may be packed closely inside the shell 35. This is different from the construction of Fig. 8 where a grid structure is used to separate the filter tubes.

Fig. 12 shows another form of orbital filter system according to the present invention where a whip rod 65 is driven to revolve and is guided by two cables 66 and 67 so that rod 65 revolves, but does not rotate in contrast to the blade 4 of Figs. 1 and 2. The lengths of the cables 66 and 67 allows the whip rod to move around the inside surface of the filter tube 3 in a closely spaced relationship to avoid wear and excessive shear forces on the material being processed.

Fig. 13 shows^' yet another orbital filter system with a filter cartridge 3' coupled to a flexible tube 70. The cartridge is loaded by a dead .weight 72 to provide the needed offset of the mass center in a submerged system in order to respond to the orbital drive. Guide, rings 71 made of a low friction material such as that sold under the trade designation Teflon are mounted at the exterior of the filter cartridge 3' at its upper and lower ends to maintain the desired gap between the filter cartridge and the surrounding tube.

Fig. 14 shows a direct drive oscillatory filter system where a flexible tube 92 supports the filter 3" and forms parts of the outlet conduit 9 for the filtrate. The filter 3" has a cylindrical membrane, mesh or other porous material forming its side walls, but is enclosed by non-porous end walls 3a and 3b. A magnetic coupling 93 and 94 drives the filter 3 in an angular oscillatory mode 76 to produce oscillatory Taylor Vortices in the chamber 5 to depolarize the inward flow filter. Chamber 79 secured to bottom end wall 3a is shown as a void space^' to minimize the volume of the chamber of the filter. The tube 92 is preferable a length of rubber hose with a sufficient torsional spring constant to develope a natural resonant oscillation of the filter at the desired speed and amplitude to produce a good depolarization. The tube is suspended from a cap 90 mounted on a rigid sleeve 91 extending from the upper wall of the container 2. The cap 90 has a central opening that provides an outlet 9 for the filtrate. The retentate exits outlet 8 at the bottom wall of the container. The lower end of the tube 92 is open to an outlet opening in the upper end wall 3b.

Fig. 15 is a further improvement of Fig. 14 to drive the filter 3 in oscillatory mode 76 directly by a crank shaft 75 through hollow shaft 78 inside a flexible hose 77, also preferably of rubber.

These and other variations and modifications will occur to those skilled in the art from the foregoing detailed description and the accompanying drawings.

What is claimed is:

Claims

Claims :

1. A filtration system for a fluid to be filtered containing suspended particles, comprising a container having a fluid inlet, an outlet for retentate, and an outlet for filtered fluid, said inlet and said outlets being secured in fluid communication with the interior of said container without rotary seals, and said fluid having a higher pressure at said inlet than at said filtered fluid outlet, at least one filter mounted within said container having an inlet surface in fluid communication with said inlet and a second surface in fluid communication with said filtered fluid outlet, and means for depolarizing said inlet surface without a rotary seal on said container.

2. The filtration system of claim 1 wherein said depolarizing means comprises an orbital drive to move said container, said filter and the fluid within said container in a continuous revolving movement.

3. The filtration system of claim 1 wherein said depolarizing means comprises an orbital drive to move said container, said filter and the fluid within said container in an oscillatory movement through an angle θ.

4. The filtration system according to claim 2 further comprising a member pivotally mounted within said cylinder to revolve within said filter along said inlet surface in response to said orbital drive to sweep away the particles that accumulate on said inlet surface during operation of said system.

5. The filtration system according to claim 4 wherein said member is a rigid blade mounted for free rotation within said container and having a density greater than that of said fluid.

6. The filtration system according to claim

4 wherein said member is a whip rod.

7. The filtration system according to claim

5 wherein said whip rod is flexibly mounted at its upper and lower ends to said container and spaced from said inlet surface.

8. The filtration system according to claims 2 or 3 further comprising a non-porous cylinder mounted within said container in a closely spaced relationship with filter, and wherein one of said non-porous cylinder and said filter is weighted to revolve about the other to produce Taylor vortices at said inlet surface.

9. The filtration system according to claim 8 wherein said cylinder has a mass secured to it and offset from the axis of revolution of said cylinder. -20-

10. The filtration system of claim 8 wherein said filter is closed at its ends, except for. a flexible coupling to said inlet and wherein said filter is weighted to revolve with respect to said cylinder.

11. The filtration system according to claim

I wherein said depolarizing means comprises mounting said container and said filter in a closely spaced relationship, conduit means for supporting one of said container and said filter to allow an elastic torsional twisting, and means for driving said one member so elastically supported to produce Taylor vortices in said fluid in the region between said filter and said container.

12. The filtration system according to claim

II wherein said driving produces a continuous rotation.

13. The filtration system according to claim 11 wherein said driving is oscillatory.

14. The filtration system according to claim 13 wherein said conduit means forms a portion of said outlet for filtered fluid from the interior of said filter.

15. The filtration system according to claim 13 wherein said conduit means surrounds said filtered fluid outlet and supports said container.

16. The filtration system of claim 13 wherein said filter receives the fluid from said inlet at its exterior surface and has means at its interior for creating a void volume.

17. The filtration system of claims 2 or 3 comprising multiple filters mounted within said container each in fluid communication with said inlet and said outlets.

18. The filtration system according to claim 16 further comprising a plurality of tubes mounted within said container.