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 large 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.

We claim:  
     
       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. ;  
       
         the improvement comprising,  
       
       
         each said header having said fibers spaced apart by a support means having a thickness corresponding to a desired lateral spacing between adjacent fibers, said support means extending over only each terminal portion of said fibers near their ends, so as to maintain said ends in spaced apart relationship. 
       
     
     
       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 new 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;  
       the improvement comprising, 
       
         each said header having said fibers spaced apart by a support means having a thickness corresponding to a desired lateral spacing between adjacent fibers, said support means extending over only each terminal portion of said fibers near their ends, so as to maintain said ends in spaced apart relationship; and, 
       
       said gas distribution means having through-pass ages 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 held 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 liquid 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 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;    
         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;    
       
         the improvement comprising,  
       
         an air distribution means within said skein, said air distribution means having through - passages for bubbles rising through said skein, and including an air - tube maintaining said first and second headers in spaced - apart relationship.   
     
     
       24. The device of  claim 23  wherein the upper and lower headers are cylindrical and said air tube is centrally located. 
     
     
       25. 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 herein, 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  0 . 1   %  to less than  5   %  greater than said fixed distance so as to permit restricted displacement of an intermediated portion of each fiber, independently of the movement of another fiber;    
       
         the improvement comprising,  
       
         a gas distribution means having through - passages for bubbles rising through said skein, and said through - passages are integral with said lower header.   
     
     
       26. The device of  claim 25 , wherein said through- passages in said lower header are in open communication with a plenum integral with said lower header.   
     
     
       27. 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 with 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;    
         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;    
       
         the improvement comprising,  
       
       ( a )  a reservoir under essentially ambient pressure containing substrate in which said fibers are immersed;    
       ( b )  a pump to withdraw permeate from within said fibers; and,    
       ( c )  an air distribution means within said skein said air distribution means including through - passages for bubbles which contact said fibers.   
     
     
       28. The device of  claim 27  wherein through- passages are within said skein.   
     
     
       29. The device of  claim 28  including in addition, air- tubes on either side of a skein providing sufficient air to cleanse said skein having less than about  30  arrays of fibers between said air - tubes.