Vertical skein of hollow fiber membranes and method of maintaining clean fiber surfaces while filtering a substrate to withdraw a permeate

Abstract

A vertical skein of “fibers”, opposed terminal portions of which are held in headers unconfined in a modular shell, is aerated with a gas-distribution means which produces a mass of bubbles serving the function of a scrub-brash for the outer surfaces of the fibers. The membrane device is surprisingly effective with relatively little cleansing gas, the specific flux through the membranes reaching an essentially constant relatively high value because the vertical deployment of fibers allows bubbles to rise upwards along the outer surfaces of the fibers. Further, bubbles flowing along the outer surfaces of the fibers make the fibers surprisingly resistant to being fouled by build-up of deposits of inanimate particles or microorganisms in the substrate provided that the length of each fiber is only slightly greater than the direct center-to-center distance between opposed faces of the headers, preferably in the range from at least 0.1% to about 5% greater. For use in a larger reservoir, a bank of skeins is used with a gas distributor means and each skein has fibers preferably >0.5 meter long, which together provide a surface area >10 m 2 . The terminal end portions of fibers in each header are kept free from fiber-to-fiber contact with a novel method of potting fibers.

1. In a microfiltration membrane device, for withdrawing permeate essentially continuously from a multicomponent liquid substrate while increasing the concentration of particulate material therein, said membrane device including:
 a multiplicity of hollow fiber membranes, or fibers, unconfined in a shell of a module, said fibers together having a surface area >1 m 2 , said fibers being swayable in said substrate, said fibers being subject to a transmembrane pressure differential in the range from about 0.7 kPa (0.1 psi) to about 345 kPa (50 psi), and each fiber having a length >0.5 meter;  
 a first header and a second header disposed in transversely spaced-apart relationship with said second header within said substrate;  
 said first header and said second header having opposed terminal end portions of each fiber sealingly secured therein, all open ends of said fibers extending from a permeate-discharging face of at least one header;  
 permeate collection means to collect said permeate, sealingly connected in open fluid communication with a permeate-discharging face of each of said headers;  
 and, means to withdraw said permeate;  
 
       the improvement comprising,
 said fibers, said headers and said permeate collection means together forming a vertical skein wherein said fibers are essentially vertically disposed and terminal end portions of individual fibers are potted in proximately spaced-apart relationship in cured resin;  
 said first header being upper and disposed in vertically spaced-apart relationship above said second header, with opposed faces at a fixed distance;  
 each of said fibers having substantially the same length, said length being from 0.1% to less than 5% greater than said fixed distance so as to permit restricted displacement of an intermediate portion of each fiber, independently of the movement of another fiber.  
 
     
     
       2. The membrane device of  claim 1  wherein each said header is a mass of synthetic resinous material in which said terminal end portions are potted and said fibers are formed from an organic resinous material or a ceramic. 
     
     
       3. The membrane device of  claim 2  wherein each said hollow fiber has an outside diameter in the range from about 20 μm to about 3 mm, a wall thickness in the range from about 5 μm to about 2 mm, and, said fiber is formed from a material selected from the group consisting of natural and synthetic polymers, and pore size in the range from 1000 Å to 10000 Å, and, said displacement is in the lateral or horizontal direction. 
     
     
       4. The membrane device of  claim 3  wherein said transmembrane pressure differential is in the range from 3.5 kPa (0.5 psi) to about 175 kPa (25 psi), said fibers are in the range from 0.5 m to 5 m long, and said terminal end portions of said fibers are potted within said mass of thermosetting synthetic resinous material to a depth in the range from about 1 cm to about 5 cm. 
     
     
       5. The membrane device of  claim 3  wherein said substrate is maintained at a pressure in the range from about 1-10 atm, said fibers extend as a skein upwardly from a fiber-supporting face of each of said headers, each header is a rectangular prism having substantially the same dimensions, said fibers extend downwardly through the permeate-discharging face of said headers, and said permeate is discharged upwardly relative to said upper header. 
     
     
       6. The membrane device of  claim 4  wherein said terminal end portions of said fibers are potted within a mass of thermosetting synthetic resinous material to a depth in the range from about 1 cm to about 5 cm and protrude through a permeate-discharging face of each said header in a range from about 0.1 mm to about 1 cm. 
     
     
       7. The membrane device of  claim 6  wherein said open ends of fibers are bounded by a geometrically regular peripheral boundary around the outermost peripheries of the outermost fibers in the boundary, and the length of a fiber is essentially independent of the strength of said fiber, or its diameter. 
     
     
       8. The membrane device of  claim 7  wherein said fibers together have a surface area in the range from 10 to 10 3  m 2 . 
     
     
       9. The membrane device of  claim 8  wherein said first and second headers are each a rectangular parallelpiped and said first header is disposed parallel to said second header. 
     
     
       10. In a gas-scrubbed assembly comprising, a microfiltration membrane device in combination with a gas-distribution means to minimize build-up of particulate deposits on the surfaces of hollow fiber membranes (“fibers”) in said device, and to recover permeate from a multicomponent liquid substrate while leaving particulate matter therein, said membrane device comprising:
 a multiplicity of fibers, unconfined in a shell of a module, said fibers together having a surface area >1 m 2 , said fibers being swayable in said substrate, said fibers being subject to a transmembrane pressure differential in the range from about 0.7 kPa (0.1 psi) to about 345 kPa (50 psi), and each having a length >0.5 meter;  
 a first and second header disposed in spaced-apart relationship within said substrate;  
 said first header and said second header having opposed terminal end portions of each fiber sealingly secured therein, all open ends of said fibers extending from a permeate-discharging face of at least one header;  
 permeate collection means to collect said permeate, sealingly connected in open fluid communication with a permeate-discharging face of each of said headers; and,  
 means for withdrawing said permeate; and,  
 said gas-distribution means is located within a zone near the base of said skein, having through-passages therein adapted to have sufficient gas flowed therethrough to generate enough bubbles flowing in a column of rising bubbles through and around said skein fibers, to keep surfaces of said fibers awash in bubbles;  
 
       the improvement comprising,
 said fibers, said headers and said permeate collection means together forming a skein wherein said fibers are essentially vertically disposed and terminal end portions of individual fibers are potted in proximately spaced-apart relationship in cured resin;  
 said first header being upper and disposed in vertically spaced-apart relationship above said second header at a fixed distance;  
 each of said fibers having substantially the same length, said length being from at least 0.1% greater, to less than 5% greater than said fixed distance so as to permit restricted displacement of an intermediate portion of each fiber, independently of the movement of another fiber; and,  
 said gas distribution means having through-passages therein to discharge a cleansing gas in an amount in the range from 0.47-14 cm 3 /sec per fiber (0.001 scfm/fiber to about 0.03 scfm/fiber) in a column of bubbles which rise vertically substantially parallel to, and in contact with said fibers, movement of which is restricted within said column;  
 whereby said permeate is essentially continuously withdrawn while concentration of said particulate matter in said substrate is increased.  
 
     
     
       11. The gas-scrubbed assembly of  claim 10  wherein said fixed distance is adjustable, said gas-distribution means includes at least two distribution means disposed, one on each side of said skein, said gas-distribution means generate bubbles having an average diameter in the range from about 0.1 mm to about 25 mm which bubbles contact said fibers, maintain their buoyancy, and maintain said fibers' outer surfaces essentially free from build-up of deposits of said particulate matter. 
     
     
       12. The gas-scrubbed assembly of  claim 11  wherein said through-passages in said gas-distribution means generate bubbles in the size range from 1 mm to 25 mm in relatively close proximity, in the range from 1 cm to about 50 cm, to said through-passages. 
     
     
       13. The gas-scrubbed assembly of  claim 10  wherein said fibers have pores in the size range from about 1000 Å to 10000 Å, each said header is a rectangular prism having substantially the same dimensions, said gas is an oxygen-containing gas, and said particulate matter comprises biologically active microorganisms growing in said substrate. 
     
     
       14. The gas-scrubbed assembly of  claim 10  wherein said particulate matter comprises finely divided inorganic particles. 
     
     
       15. In a process for maintaining the outer surfaces of hollow fiber membranes essentially free from a build-up of deposits of particulate material while separating a permeate from a multicomponent liquid substrate in a reservoir, said process comprising,
 submerging skein fibers within said substrate unconfined in a modular shell, said fibers being securely head in laterally opposed, spaced-apart first and second headers, said fibers having a transmembrane pressure differential in the range from about 0.7 kPa (0.1 psi) to about 345 kPa (50 psi), a total surface area >1 m 2 , and a length sufficiently greater than the direct distance between opposed faces of said first and second headers, so as to present said skein in a swayable configuration above a horizontal plane through the horizontal centerline of a header;    mounting said headers in fluid-tight open communication with collection means to collect said permeate;    flowing a fiber-cleansing gas through a gas-distribution means proximately disposed relative to said skein, within a zone near the base of said skein, and contacting surfaces of said fibers with sufficient physical impact of bubbles of said gas to maintain essentially the entire length of each fiber in said skein awash with bubbles and essentially free from said build-up;    maintaining an essentially constant flux through said fibers substantially the same as an equilibrium flux initially obtained after commencing operation of said process;    collecting said permeate in said collection means; and,    withdrawing said permeate,    
       the improvement comprising:
 introducing a cleansing gas in an amount in the range from 0.47-14 cm 3 /sec per fiber (0.001 scfm/fiber to about 0.03 scfm/fiber) to generate a column of said bubbles alongside and in contact with outer surfaces of said fibers;  
 deploying said skein fibers within said column in an essentially vertical configuration, with said headers in fixed spaced apart relationship at a fixed distance, said skein having fibers of substantially the same length and from at least 0.1% greater, to about 5% greater than said fixed distance, said fibers being independently swayable from side-to-side within a vertical zone of movement with terminal end portions of individual fibers potted in proximately spaced-apart relationship in cured resin;  
 restricting movement of said fibers to said vertical zone defined by lateral movement of outer fibers in said skein;  
 vertically gas-scrubbing said fibers' outside surfaces with bubbles which flow upward in contact with said surfaces;  
 maintaining said surfaces substantially free from said deposits of particulate matter during a period when specific flux through said fibers has attained equilibrium; and,  
 simultaneously, essentially continuously, withdrawing said permeate while increasing the concentration of said particulate material in said substrate.  
 
     
     
       16. The process of  claim 15  wherein each said hollow fiber has an outside diameter in the range from about 20 μm to about 3 mm, and a wall thickness in the range from about 5 μm to about 1 mm; each said header is formed from a mass of thermosetting or thermoplastic synthetic resinous material; terminal end portions of said fibers are potted within said resinous material to a depth in the range from about 1 cm to about 5 cm;
 said particulate matter is selected from the group consisting of microorganisms and finely divided inorganic particles; and,    said gas-distribution means generates bubbles having an average diameter in the range from about 1 mm to about 25 mm.    
     
     
       17. A method of forming a header for a skein of a multiplicity of fibers, comprising,
 forming a stack of at least two superimposed essentially coplanar and similar arrays, each array comprising a chosen number of fibers supported on a support means having a thickness corresponding to a desired lateral spacing between adjacent arrays;    holding the stack in a first liquid with terminal portions of the fibers submerged, until the liquid solidifies into a first shaped lamina, provided that the first liquid is unreactive with material of the fibers;    pouring a second liquid over the first shaped lamina to embed the fibers to a desired depth, and solidifying the second liquid to form a fixing lamina upon the first shaped lamina, the second liquid also being substantially unreactive with either the material of the fibers or that of the first shaped lamina;    forming a composite header in which terminal portions of the fibers are potted, the composite header comprising a laminate of a fugitive lamina of fugitive material and a contiguous finished header of fixing lamina; and,    removing the first shaped lamina without removing a portion of the fixing lamina so as to leave the ends of the fibers open and protruding from the aft face of the header,    whereby the open ends having a circular cross-section are exposed without cutting the fibers.    
     
     
       18. The method of  claim 17  wherein said second light upon solidification forms a thermosetting or thermoplastic synthetic resin, and said first liquid upon solidification forms a solid which has a melting point or glass transition temperature lower than the melting point or glass transition temperature of said synthetic resin. 
     
     
       19. The method of  claim 18  wherein said first liquid upon solidification is flowable at a temperature at which said second liquid upon solidification remains solid. 
     
     
       20. The method of  claim 18  wherein said first liquid upon solidification is soluble in a chosen solvent, and said second liquid upon solidification is insoluble in said solvent. 
     
     
       21. A header in which a multiplicity of hollow fiber membranes or “fibers” is potted, said header comprising,
 a molded body of arbitrary shape striated in a fixing lamina and a fugitive lamina, said fugitive lamina formed from a fugitive potting material and said fixing lamina formed from a fixing material;    said fibers having terminal portions thereof potted in said fugitive potting material which when solidified plugs ends of said fibers, plugged ends having an essentially circular cross-section, said fugitive lamina maintaining said ends in closely spaced-apart substantially parallel relationship;    said fugitive lamina having an aft face towards which said plugged ends protrude, and a fore face through which said fibers extend vertically;    said fugitive lamina having said fixing lamina adhered thereto, said fixing lamina having a thickness sufficient to maintain said fibers in substantially the same spaced-apart relationship relative to one and another as the spaced-apart relationship in said lower portion.    
     
     
       22. The header of  claim 21  wherein said fixing lamina has a cushioning lamina embedding said fibers and coextensively adhered to said fixing lamina, said fixing lamina has a hardness in the range from about Shore D 50 to Rockwell R 110, and said cushioning layer has a hardness in the range from Shore A 30 to Shore D 45. 
     
     
       23. In a microfiltration membrane device, for withdrawing permeate essentially continuously from a multi- component liquid substrate while increasing the concentration of particulate material therein, said membrane device including:      a multiplicity of hollow fiber membranes, or fibers, unconfined in a shell of a module, said fibers together having a surface area > 1  m   2   , said fibers being swayable in said substrate, said fibers being subject to a transmembrane pressure differential in the range from about  0 . 7  kPa  (   0 . 1  psi )  to about  345  kPa  (   50  psi ),  and each fiber having length > 0 . 5  meter;        a first header and a second header disposed in transversely spaced - apart relationship with said second header within said substrate;        a first header and a second header having opposed terminal end portions of each fiber sealingly secured therein, all open ends of said fibers extending from a permeate - discharging face of at least one header;        permeate - collection means to collect said permeate, sealingly connected in open fluid communication with a permeate - discharging face of each of said headers; and,        means to withdraw said permeate;      
       
         the improvement comprising,  
           said fibers, said headers and said permeate collection means together forming a vertical skein wherein said fibers are essentially vertically disposed and terminal end portions of individual fibers are potted in proximately spaced - apart relationship in cured resin;    
           said first header being upper and disposed in vertically spaced - apart relationship above said second header, with opposed faces at a fixed distance;    
           each of said fibers having substantially the same length, said length being from between  0 . 1   %  to less than  5   %  greater than said fixed distance so as to permit restricted displacement of an intermediate portion of each fiber, independently of the movement of another fiber; and,    
           a gas distribution system having through - passages adapted to discharge bubbles near to rise through or around the skein of fibers, the gas distribution system including one or more gas tubes which space the first and second headers apart and which also carry air to the through - passages.   
       
     
     
       24. The device of  claim 23  wherein the upper and lower headers are cylindrical and the one or more gas tubes are a single gas tube located in about the center of the headers. 
     
     
       25. In a microfiltration membrane device, for withdrawing permeate essentially continuously from a multi- component liquid substrate while increasing the concentration of particulate material therein, said membrane device including:      a multiplicity of hollow fiber membranes, or fibers, unconfined in a shell of a module, said fibers together having a surface area > 1  m   2   , said fibers being swayable in said substrate, said fibers being subject to a transmembrane pressure differential in the range from about  0 . 7  kPa  (   0 . 1  psi )  to about  345  kPa  (   50  psi ),  and each fiber having length > 0 . 5  meter;        a first header and a second header disposed in transversely spaced - apart relationship with said second header within said substrate;        a first header and a second header having opposed terminal end portions of each fiber sealingly secured therein, all open ends of said fibers extending from a permeate - discharging face of at least one header;        permeate - collection means to collect said permeate, sealingly connected in open fluid communication with a permeate - discharging face of each of said headers; and,        means to withdraw said permeate;      
       
         the improvement comprising,  
           said fibers, said headers and said permeate collection means together forming a vertical skein wherein said fibers are essentially vertically disposed and terminal end portions of individual fibers are potted in proximately spaced - apart relationship in cured resin;    
           said first header being upper and disposed in vertically spaced - apart relationship above said second header, with opposed faces at a fixed distance;    
           each of said fibers having substantially the same length, said length being from between  0 . 1   %  to less than  5   %  greater than said fixed distance so as to permit restricted displacement of an intermediate portion of each fiber, independently of the movement of another fiber, wherein the headers are rectangular in plan view and the skein has about  30  or less arrays of fibers.   
       
     
     
       26. A device for withdrawing permeate from a multicomponent liquid substrate comprising,
 ( a )  a reservoir under essentially ambient pressure having a feed zone for containing a substrate;      ( b )  a microfiltration membrane device, for withdrawing permeate essentially continuously from the multi - component liquid substrate while increasing the concentration of particulate material therein, said membrane device including:  
   a multiplicity of hollow fiber membranes, or fibers, unconfined in a shell of a module, said fibers together having a surface area > 1  m   2   , said fibers being swayable in said substrate, said fibers being subject to a transmembrane pressure differential in the range from about  0 . 7  kPa  (   0 . 1  psi )  to about  345  kPa  (   50  psi ),  and each fiber having length > 0 . 5  meter;    
   a first header and a second header disposed in transversely spaced - apart relationship with said second header within said substrate;    
   a first header and a second header having opposed terminal end portions of each fiber sealingly secured therein, all open ends of said fibers extending from a permeate - discharging face of at least one header;    
   permeate - collection means to collect said permeate, sealingly connected in open fluid communication with a permeate - discharging face of each of said headers; and,    
   means to withdraw said permeate;    
   said fibers, said headers and said permeate collection means together forming a vertical skein wherein said fibers are essentially vertically disposed and terminal end portions of individual fibers are potted in proximately spaced - apart relationship in cured resin;    
   said first header being upper and disposed in vertically spaced - apart relationship above said second header, with opposed faces at a fixed distance;    
   each of said fibers having substantially the same length, said length being from between  0 . 1   %  to less than  5   %  greater than said fixed distance so as to permit restricted displacement of an intermediate portion of each fiber, independently of the movement of another fiber,    
   the outside of the membranes in fluid communication with the feed zone of the reservoir;    
   ( c )  a pump in fluid communication with the insides of the membranes through the permeate collection means, the pump operable to supply a suction to the lumens of the hollow fiber membranes to draw permeate through the membranes; and,      ( d )  a gas distribution means including a plurality of through - passages for discharging bubbles which rise and contact fibers.     
     
     
       27. The device of  claim 26  wherein at least some of the through- passages have outlets located within the skein.   
     
     
       28. The device of  claim 26  wherein the headers are rectangular in plan view and have about  30  arrays or less of fibers and the outlets of the through- passages are located to the side of the headers.   
     
     
       29. In a microfiltration membrane device, for withdrawing permeate essentially continuously from a multi- component liquid substrate while increasing the concentration of particulate material therein, said membrane device including:      a multiplicity of hollow fiber membranes, or fibers, unconfined in a shell of a module, said fibers together having a surface area > 1  m   2   , said fibers being swayable in said substrate, said fibers being subject to a transmembrane pressure differential in the range from about  0 . 7  kPa  (   0 . 1  psi )  to about  345  kPa  (   50  psi ),  and each fiber having length > 0 . 5  meter;        a first header and a second header disposed in transversely spaced - apart relationship with said second header within said substrate;        a first header and a second header having opposed terminal end portions of each fiber sealingly secured therein, all open ends of said fibers extending from a permeate - discharging face of at least one header;        permeate - collection means to collect said permeate, sealingly connected in open fluid communication with a permeate - discharging face of each of said headers; and,        means to withdraw said permeate;      
       
         the improvement comprising,  
           said fibers, said headers and said permeate collection means together forming a vertical skein wherein said fibers are essentially vertically disposed and terminal end portions of individual fibers are potted in proximately spaced - apart relationship in cured resin;    
           said first header being upper and disposed in vertically spaced - apart relationship above said second header, with opposed faces at a fixed distance;    
           each of said fibers having substantially the same length, said length being from between  0 . 1   %  to less than  5   %  greater than said fixed distance so as to permit restricted displacement of an intermediate portion of each fiber, independently of the movement of another fiber;    
         
           walls extending downwards from a lower header of the first and second header, the walls being adapted to retain a gas below the lower header; and,  
         
           through - passages for gas to pass through the lower header from an area below the lower header bordered by the walls.   
       
     
     
       30. The device of  claim 29  wherein the through passages are located such that gas flowing from the area below the lower header bordered by the walls, exits between fibers. 
     
     
       31. The membrane device of  claim 1  wherein said fibers, said headers and said permeate collection means are all submersible below the surface of the substrate.  
     
     
       32. The membrane device of  claim 31  further comprising a manifold for withdrawing permeate, the manifold extending from the permeate collection means to a point above the surface of the substrate.  
     
     
       33. The membrane device of  claim 31  wherein the permeate collection means further comprises a pan or header enclosure covering each permeate discharging face.  
     
     
       34. A membrane filtration system comprising:
 ( a )  a tank for holding a substrate at ambient pressure during filtration;      ( b )  a membrane device according to    claim 1    immersed below the surface of the substrate;      ( c )  an aeration system for producing bubbles in the substrate which contact the fibers; and,      ( d )  a source of suction in fluid communication with the membrane filtration device.      
     
     
       35. The membrane filtration system of  claim 34  further comprising a backwashing system for backwashing the membrane filtration device with a liquid.