Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D65/00—Accessories or auxiliary operations, in general, for separation processes or apparatus using semi-permeable membranes
  • B01D65/08—Prevention of membrane fouling or of concentration polarisation
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D63/00—Apparatus in general for separation processes using semi-permeable membranes
  • B01D63/16—Rotary, reciprocated or vibrated modules
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D2321/00—Details relating to membrane cleaning, regeneration, sterilization or to the prevention of fouling
  • B01D2321/16—Use of chemical agents
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D2321/00—Details relating to membrane cleaning, regeneration, sterilization or to the prevention of fouling
  • B01D2321/20—By influencing the flow
  • B01D2321/2008—By influencing the flow statically
  • B01D2321/2016—Static mixers; Turbulence generators

Definitions

• a feed fluid such as waste water can be separated into permeate, such as pure water which passes through a membrane, and concentrate which includes water with a high concentration of particles.
• permeate such as pure water which passes through a membrane
• concentrate which includes water with a high concentration of particles.
• separation can be accomplished by the use of a stack of membrane packs lying within a container. Fouling of the membrane packs, by the buildup of particles at the surface which block pores of the membranes, can be reduced by rapidly rotating the membrane packs, as described in U.S. Patent Number 4,025,425 by Croopnick. Fouling can be further reduced by placing stationary separator elements between pairs of membrane packs, to create turbulence in the gap between the rotating surface of the membrane pack and the stationary surface of the separating element. It is noted that when a membrane pack has large pores (many microns wide), it may be referred to as a filter pack, but applicant uses the term membrane pack herein for both.
• the turbulence-enhancing separator elements lying between membrane packs should be relatively thin to take up little space, but must not touch the rapidly rotating membrane packs or they will be destroyed . It would be desirable if the separator elements could be designed for maximum strength against deflection while minimizing conditions that could cause their deflection.
• a common filtration construction directs the feed fluid in series through the gaps. For example, if there is a stack of fifty membrane packs and corresponding stationary elements to produce one hundred gaps, the fluid may flow in a series serpentine path through the one hundred gaps.
• Such serial flow has the advantage that the feed fluid moves along a long path in contact with the surfaces of the membrane packs, to remove a considerable portion of the filtrate.
• such serial flow has a disadvantage that the feed fluid is not homogeneous, in that the concentration of particles in the feed fluid may increase by many times between the upstream and downstream ends of the feed fluid path. Also, there can be a large pressure drop along the long path, due to friction applied to the moving feed fluid, especially for more viscous liquids.
• a filtration system of the type wherein a stack of rotatable membrane packs is spaced by separator elements to leave gaps through which feed fluid moves, which produces enhanced filtration .
• a filtration system is operated so the feed fluid is flowed into a conduit that connects the radially outer edges of the gaps that separate the rotating membrane packs from the substantially stationary separator elements. Portions of the feed fluid flow largely radially inwardly along inward paths that lie adjacent to the stationary elements, and flow largely radially outwardly along outward paths that lie adjacent to the rotating membrane packs, to produce a largely circulating flow along each gap.
• Portions of the fluid that have moved radially inwardly and outwardly along a path pass into the feed conduit to move to another gap, while other portions move back into the same gap along the inward paths.
• the feed fluid moves along a long path in moving into and out of each of the gaps and flowing radially inwardly and outwardly along each gap, and yet a substantially homogenous feed fluid is maintained because fluid in each gap is constantly mixed with fluid from other gaps by way of the feed conduit.
• the stationary separator elements have apertures to leave spokes, which helps create shear and which equalizes pressure on opposite faces of the elements.
• An accumulator is preferably connected to the feed conduit, as at the feed inlet. Such accumulator assures that the feed fluid pressure can only gradually decrease, to prevent blowout of the membranes as the membrane packs stop rotating in the event of loss of feed fluid pressure.
• Fig. 1 is an isometric view of a rotary filtration apparatus constructed in accordance with one embodiment of the present invention.
• Fig. 2 is a sectional side view of the apparatus of Fig. 1.
• Fig. 3 is a view taken on the line 3 - 3 of Fig. 2.
• Fig. 4 is a sectional view of a portion of the apparatus of Fig. 2, taken on the line 4 - 4 of Fig. 3.
• Fig. 5 is an enlarged view of a portion of the apparatus of Fig. 4.
• Fig. 6 is an enlarged view of a portion of the apparatus of Fig. 5.
• Fig. 7 is a view taken on the line 7 - 7 of Fig. 3, but with the axial dimension exaggerated, and with pluses (+) indicating flow into the paper and circles indicating flow out of the paper.
• Fig. 8 is a partial sectional side view of a rotary filtration apparatus constructed in accordance with another embodiment of the invention.
• Fig. 1 illustrates a rotary filtration apparatus 10 which has a feed fluid inlet 12 for receiving feed fluid.
• the feed fluid generally includes a liquid, and particles in the liquid which are of micron size (average diameter less than ten microns) or submicron size.
• the apparatus separates the feed fluid into filtrate, or permeate which flows out of permeate outlets 14, 16, and concentrate which flows out of an outlet 20 (or out of an inlet 12 in the case of batch processing). Concentrate comprises liquid with a high concentration of particles, and is left after much of the permeate in the original feed fluid has been removed.
• the apparatus includes a rotor 22 which lies within a sealed container 24.
• a motor 26 is coupled to the rotor to rapidly rotate it about an axis 28.
• the rotor 22 includes a stack 30 of axially-spaced (in direction X that is parallel to the axis) membrane packs 32 that lie within the container.
• the apparatus also includes a stack 34 of plate-like separator elements 36.
• the separator elements 36 are stationary, and positioned so a separator element 36 lies between each pair of membrane packs 32. This leaves gaps 40 between each surface of the membrane pack and adjacent surfaces of separator elements.
• the gaps extend radially (parallel to radial directions Y), in that they have large radial dimensions Y and short axial dimensions X.
• FIG. 2 shows first and second membrane packs 32A, 32B and shows separator elements 36A and 36B and shows first and second gaps 40A, 40B on opposite sides of the first membrane pack 32A.
• Fig. 2 shows that the membrane packs 32 are mounted at their axial middles on a shaft 50 which is hollow to form a permeate conduit 52.
• the permeate conduit extends through the entire length of the shaft, to form the permeate outlets 14, 16 at the shaft opposite ends.
• the shaft is rotatably mounted on bearing 54, 56, 58, with a stand indicated at 59 to support the lower bearings, and with the upper bearing supported on the container 24. It is possible to rotatably support the rotor on a bearing assembly that includes a single bearing.
• the membrane packs 32 have radially inner and outer ends 60, 62.
• the inner ends 60 are mounted on the shaft, while the outer ends 62 are free and therefore unsupported.
• the separator elements 36 have radially inner and outer ends 64, 66.
• the outer ends 66 are mounted on a group of tie rods 70, and are spaced apart by spacers 72.
• the radially inner ends 64 of the separator element are free and therefore unsupported.
• each of the separator elements 36 has through apertures 131 - 138 that leave spokes 141 - 148.
• the apertures extend radially (away from axis 28) further than the peripheries 150 of the membrane packs. This leaves spaces 152 at the radially outer portions of the apertures, along which feed fluid can move.
• the radially outer ends 66 of the separator elements are radially spaced from the sidewalls 80 of the container 24. This leaves additional space 154 along which feed fluid can move.
• the spaces 152, 154 form a feed conduit 82, the conduit 82 being of largely toroidal shape (with the peripheries of the spacer element lying in it). As shown in Fig.
• the cross sectional area of the feed conduit 82 is much greater than that of any of the gaps 40.
• the feed fluid tends to be substantially homogenous throughout the filtration apparatus, in that there is a substantially uniform pressure of the feed fluid and a substantially uniform concentration of solids in the liquid of the feed fluid.
• the radially inner ends 60 of the membrane packs are spaced apart by seal spacers 84. Permeate flows radially inwardly along each membrane pack, and through holes 86 in the rotor shaft and along the permeate conduit 52 of the shaft.
• Fig. 5 illustrates the flow of fluids in the gaps such as 40A and 40B and along the feed conduit 82.
• feed fluid 90 fills the feed conduit 82 and the gaps such as 40A, 40B.
• the membrane pack 32A is rapidly rotated, fluid lying adjacent to the membrane pack surface 92 rotates with the membrane pack.
• Such rotation results in centrifugal force which causes feed fluid lying adjacent to the surface 92 to move radially outwardly along an outward path 94 (which also includes circumferential components in the direction of membrane pack rotation).
• the radially outward flow along outward paths 94 results in a lower pressure at the radially inward end 100 of the gap, and this causes a radially inward flow of feed fluid along inward paths 102. The result is that there is a circulating flow of feed fluid along each gap.
• This circulating flow causes fresh feed fluid from the feed conduit 82 to repeatedly flow across the membrane pack surface 92.
• Permeate of the feed fluid moves along membrane-crossing paths 104 through a membrane 105 or other filtering element into porous backup sheets 106 of the membrane packs, and in inward directions at 108 to the centers of the pack from which the permeate is removed.
• the membrane or filtering element may be a polymeric membrane, a screen (woven or matt or etched), a porous ceramic, a sintered metal, or other construction that passes only small particles or molecules.
• Applicant uses the term "membrane pack" for all of such elements. Common processes include dialysis, electrodialysis, reverse osmosis, and various size filtration. Applicant's system is especially useful for microfiltration and is useful for ultrafiltration, although it possibly can be used for nanofiltration and even possibly for reverse osmosis.
• fluid moving along the outward paths 94 is mixed with the feed fluid, and some of the fluid (at least one percent) that has moved along the outward paths 94 is returned as indicated by path 112, while some of it (at least one percent) is moved into the feed conduit as indicated by path 114.
• the fluid that is not recirculated within the same gap 40A can move along the feed conduit 82 and into another gap such as 40B.
• Much of the fluid passes in a loop indicated at 120 back along the gap.
• Some of the fluid passes in paths 122 between adjacent gaps such as 40A and 40C, but since there is substantially the same pressure at the radially inner ends of both gaps 40A, 40C, there is little flow in the directions 122.
• Fig. 6 indicates the velocity profile, in a circumferential direction, (the radial velocity component is not shown) of feed fluid passing along a gap 40, by the length of the arrows.
• the circumferential direction is perpendicular to the radial direction and is parallel to the membrane pack surface motion.
• the path at 93A is very close to the membrane pack surface 92, and the fluid moves at almost the same speed as the rotating surface 92.
• the velocity at path 93 is much less than the velocity at 93A, and the difference component 93D tends to sweep away particles 111 from the surface 92 of the membrane pack. This phenomenon can be referred to as the localized shear that tends to sweep particles from the surface.
• the magnitude of such shear for a given velocity of the surface 92 with respect to the stationary separator element surface 113 of the separator element, depends upon the thickness 116 of the gap. The smaller the thickness 116, the greater the localized shear effect, or ratio of velocity difference 93D with distance 118 along the gap.
• Applicant prefers to make the gap 116 as small as possible, but must prevent the surfaces 92, 113 from touching since this could cause damage to the membrane pack. Applicant is able to obtain a gap thickness 116 of about two millimeters without causing membrane pack damage in a stack of many tens of membrane packs. In addition to the velocity differential per unit distance, the small gap results in large turbulence at the surfaces, and such turbulence near surface 92 also tends to sweep away particles that might otherwise block the pores of the membrane. It is noted that the large circumferential fluid movement at 93A and 93 results in fluid near the membrane pack flowing radially outwardly.
• Applicant avoids disadvantages of a serial connection, because the present system results in a substantially homogenous fluid throughout the apparatus, in that the pressure is about the same everywhere and the mixing of fluid near the outer ends of the gaps results in the feed fluid everywhere having about the same concentration of solids.
• the substantially uniform pressure in the system allows applicant to apply an optimum pressure to the feed fluid. For example, in one situation, a pressure of 40psi will produce a high flow of permeate into the membrane packs while obtaining minimal fouling of the membrane packs, while a pressure of 80psi could cause fouling and a pressure of 20psi could result in a low permeate flow rate.
• the optimum pressure depends upon the concentration of solids. Applicant can adjust the rate at which concentrate is drawn off, based on the permeate flow, to vary the concentration of solids so as to minimize membrane fouling while still obtaining a considerable permeate flow.
• each separator element 36 with apertures 131 - 138, to leave spokes 141 - 148 that form wall portions on circumferentially (perpendicular to a radial line) opposite sides of each aperture.
• One advantage of the apertures is that they result in the same pressure on opposite faces 160, 162 of the element.
• Applicant prefers that the membrane packs have a small thickness such as 8mm, and that the spacer elements have a small thickness such as 6mm, with the gaps each having a thickness such as 3mm, for a system using membrane packs of 410mm diameter.
• the small thickness of a spacer element would allow locations on it to be axially deflected, when there was a small pressure difference between its opposite faces. Any such axial deflection which results in direct contact of a spacer element with a rapidly rotating membrane pack, would cause destruction -lo ⁇
• each imaginary 90° sector 164 of a separator element By providing at least one aperture in each imaginary 90° sector 164 of a separator element, and with the apertures occupying at least 10% of the area of each spacer element and of each sector and separator walls (e.g. spokes) lying in each sector, applicant avoids such a pressure difference.
• each spoke such as 144, 145 with a leading edge 174 that is designed to interfere with circumferential fluid movement. The result is a large change in fluid velocity over a short distance near the membrane pack face, which helps sweep away particles.
• each spoke such as 144 in Fig. 7 (in which the thickness is exaggerated) is selected so the spoke is self-centering. That is, if the spoke approaches the face 93 of one membrane pack, the reaction of the spoke with fluid moving in the circumferential direction 170 is to move the spoke away from the membrane pack surface 93.
• Membrane packs of 410mm diameter are rotated at at least 200rpm, and usually at about 760rpm to l OOOrpm.
• the average surface velocity (at a point four inches from the axis) is about 2 meters per second.
• the system operates with an average membrane pack velocity of at least 2 meters per second, and a velocity at the pack periphery of at least 4 meters per second.
• the most effective rotational speeds for membrane packs usually create centrifugal forces that increase pressure by at least 20 psi (140 kPa)
• the rotary filtration apparatus 10 can be operated in a batch or continuous process, or in a combination.
• feed fluid with a predetermined concentration of solids, such as 200ppm (parts per million) is pumped into the container.
• Inlet and outlet valves 220, 221 are closed.
• a pump 222 may be connected to a recirculation conduit 230 to maintain a more uniform concentration of particles, although this can be accomplished within the container 24 (e.g. by dividing the toroidal feed conduit 82 of Fig. 3 into two parts and pumping fluid up in one part and down in the other).
• the inlet and outlet 12, 20 form axially spaced locations of the feed conduit, and the recirculation conduit 230 lies outside of the feed conduit.
• a sensor 224 is connected to the container to sense the concentration of particles.
• the motor 26 is energized so the rotor 22 rotates at a predetermined speed, and permeate is constantly drawn from the feed fluid, while the concentration of particles in the feed fluid increases.
• the sensor 224 senses this, and can control the pump 222 to change the pressure of the feed fluid, and also control the motor 26 to change the speed of rotation of the rotor, for optimum conditions (large permeate flow into the membrane pack and low fouling of the membrane pack surfaces).
• feed fluid can flow from a source 232 continually, though at a slow rate, into the feed fluid inlet 12. Concentrate flows out of the outlet 20, for use or for further processing.
• the present invention can prevent any build up, and also reduces the rate of fouling. Selection of the proper membrane material for a particular application is of great importance in reducing the rate of fouling. Once fouling occurs, the membrane can be cleaned as by the use of chemicals to dissolve or loosen particles that have adhered to the interstices of the membrane. Measurement of the permeate flow rate enables a determination of the degree of fouling and indicates when cleaning is needed.
• Applicant prefers to connect an accumulator 240 (FIG. 1) to the feed conduit 80 at all times.
• the accumulator assures that the pressure of feed fluid will change only slowly, despite a possible interruption of supply or other phenomenon that might rapidly decrease feed fluid pressure. If the feed fluid pressure should suddenly fall, but the pressure of permeate in the membrane packs falls only slowly, then the larger pressure of permeate in the membrane packs could cause the thin membranes to burst.
• the accumulator 240 is of a common type, which include a membrane, bellows, or piston divider member 242 lying in a container 244, and separating the container into an air chamber 246 that contains air under pressure, and a liquid chamber 248 that contains liquid under the same pressure.
• any other means for maintaining feed fluid pressure such as a valve connected to a high pressure liquid source such as a city water main, and opened only when a rapid pressure drop is sensed, can be used.
• the motor is preferably automatically deenergized.
• the rotor will substantially stop from perhaps 700 rpm, in a period such as eight seconds, and the fluid pressure should be maintained during perhaps six of those seconds.
• Fig. 2 shows that permeate exits the apparatus in opposite directions A, B through opposite ends of the shaft at 14 and 16. Where there is a large filtrate flow, this allows for the use of a smaller shaft and bearings 54, 56, 58 and seals such as 59', which reduces cost.
• Fig. 4 indicates that holes 250 can be formed in the radially inner ends of the membrane packs to allow feed fluid to flow largely axially from one gap to another at their inner ends. However, there would be only small flow through such a hole, and providing such a hole can increase the cost of the membrane pack because of the need to seal against the migration of feed fluid into the permeate.
• Fig. 8 illustrates a portion of another rotary filtration apparatus 260 which is similar to that of Figs. 1 - 6, except that the separator elements 262 which lie between adjacent rotating membrane pack 264 A, 264B, also form membrane packs. That is, each separator member 262 has membrane sheets 266 and flow sheets 268 for removing permeate. Permeate passing through a membrane pack separator member 262 passes through a hole 270 in a hollow tying member 272 which is comparable to the tying members 70 of Fig. 2, to remove the permeate.
• the membrane packs had diameters of 16 inches (40cm), and the gaps 40 were 3mm thick, with the other dimensions being relative to the diameter, as shown in Fig. 2.
• aluminum oxide particles originally used for dyes
• the aluminum oxide particles were to be concentrated from four percent to twenty percent of the volume of the stream, by removing the salt water and dissolved solids which constitute the permeate of the waste stream. Larger particles had been previously removed by settling and screening, so the largest particles were no more than about 10 microns in diameter.
• the feed fluid is initially maintained at a pressure of 40psi (270 kPa) and the membrane packs are rotated at 800rpm.
• the pressure can be maintained constant, or can be increased slightly.
• the speed is increased to 1,000rpm and the pressure is increased to 50psi (340 kPa)
• the invention provides a filtration system wherein feed fluid moves through axially thin gaps between membrane packs and separator elements.
• the system obtains advantages of a serial connection of the gaps, of a long flow path along the membrane surfaces, while avoiding disadvantages of widely varying pressure and particle concentration.
• the system includes a feed conduit that connects to the radially outer ends of the gaps, to allow fluid to flow in a loop through each gap, and to promote homogeneous fluid throughout the system.
• the separator elements preferably have through apertures to leave spokes.

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• Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Separation Using Semi-Permeable Membranes (AREA)

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• 1996
  • 1996-09-06 AU AU69688/96A patent/AU6968896A/en not_active Abandoned
  • 1996-09-06 EP EP96930747A patent/EP0948391A1/de not_active Withdrawn
  • 1996-09-06 WO PCT/US1996/014352 patent/WO1998009720A1/en not_active Application Discontinuation
  • 1996-09-06 CA CA002264932A patent/CA2264932A1/en not_active Abandoned

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