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 m2. The terminal end portions of fibers in each header are kept free from fiber-to-fiber contact with a novel method of potting fibers.

Description

BACKGROUND OF THE INVENTIONThis invention relates to a membrane device which is an improvement on a frameless array of hollow fiber membranes and a method of maintaining clean fiber surfaces while filtering a substrate to withdraw a permeate, which is also the subject of U.S. Pat. No. 5,248,424; and, to a method of forming a header for a skein of fibers. The term "vertical skein" in the title (hereafter "skein" for brevity), specifically refers to an integrated combination of structural elements including (i) a multiplicity of vertical fibers of substantially equal length; (ii) a pair of headers in each of which are potted the opposed terminal portions of the fibers so as to leave their ends open; and, (iii) permeate collection means held peripherally in fluid-tight engagement with each header so as to collect permeate from the ends of the fibers.The term "fibers" is used for brevity, to refer to "hollow fiber membranes" of porous or semipermeable material in the form of a capillary...

AI Technical Summary

Benefits of technology

It has been discovered that bubbles of a fiber-cleansing gas ("scrubbing gas") flowing parallel to fibers in a vertical skein are more effective than bubbles which are intercepted by arcuate fibers crossing the path of the rising bubbles. Bubbles of an oxygen-containing gas to promote growth of microbes unexpectedly fails to build-up growth of microbes on the surfaces of the fibers because the surfaces are "vertically air-scrubbed". Deposits of animate and / or inanimate particles upon the surfaces of fibers are minimized when the restrictedly swayable fibers are kept awash in codirectionally rising bubbles which rise with sufficient velocity to exert a physical scrubbing force (momentum provides the energy) to keep the fibers substantially free of deleterious deposits. Thus, an unexpectedly high flux is maintained over a long period during which permeate is produced by outside-in flow through the fibers.

It is a specific object of this invention to provide the aforesaid novel gas-scrubbed assembly comprising a bank of vertical skeins and a shaped gas-distribution means for use with the bank, in a substrate in which microorganisms grow, the assembly being used in combination with vertically adjustable spacer means for mounting the headers in vertically spaced apart relationship, and in open fluid communication with collection means for collecting the permeate; means for withdrawing the permeate; and, sufficient air is flowed through the shaped gas-distribution means to generate enough bubbles flowing upwardly through the skein, between and parallel to the fibers so as to keep the surfaces of the fibers substantially free from deposits of live microorganisms as well as small inanimate particles which may be present in the substrate.

It has still further been discovered that a system utilizing a bank of vertical skeins of fibers potted in headers vertically spaced-apart by spacer means, and deployed in a substrate containing particulate material, in combination with a proximately disposed gas-distribution means to minimize fouling of the membranes, may be operated to withdraw permeate under gravity alone, so that the cost of any pump to withdraw permeate is avoided, provided the net positive suction head corresponding to the vertical height between the level of substrate, and the location of withdrawal of permeate, provides the transmembrane pressure differential under which the fibers function in the skein.

Still further, a low cost process has been discovered for treating a multicomponent substrate under pressure ranging from 1-10 atm in a pressurizable vessel, particularly for example, an aqueous stream containing finely divided inorganic matter such as silica, silicic acid, or, activated sludge, when the substrate is confined in a large tank or pond, by using a bank of vertical skeins each comprising restrictedly swayable unsupported fibers potted in headers in open fluid communication with a means for withdrawing permeate, in combination with a source of air which generates bubbles near the lower header.

Problems solved by technology

Bubbles of an oxygen-containing gas to promote growth of microbes unexpectedly fails to build-up growth of microbes on the surfaces of the fibers because the surfaces are "vertically air-scrubbed".

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