Cleaning of filters

Details Cleaning of filters Cleaning of filters

1988-03-10

1990-06-19

Spear, Frank

Liquid purification or separation

Processes

Liquid/liquid solvent or colloidal extraction or diffusing...

21032169, 21050023, B01D 1300

Patent

active

049351435

DESCRIPTION:

BRIEF SUMMARY
FIELD OF INVENTION

This invention relates to the cleaning and removal of solids from microporous hollow fibre filters.


BACKGROUND ART

International Patent Application Nos. PCT/AU84/00192 and PCT/AU86/00049 describe methods for backwashing elastic microporous hollow fibre filters The filters disclosed in these applications consist of a bundle of polymeric (such as polypropylene) fibres contained within a housing having a feedstock inlet thereto and a concentrate outlet therefrom. The feedstock is applied to the outside of the fibres and some of the liquid contained in the feedstock passes through the walls of the fibres and is drawn off from the fibre lumens as filtrate.
The fibres are cast in resin at both ends of the shell or housing with the ends of the lumens open to constitute a tube-in-shell configuration. Although not described in our above mentioned specifications, the fibres may be cast into one end of the housing with the other ends of fibres free but with the free ends of the lumens sealed to constitute a candle-in-shell configuration.
During the filtering operation, which may be either to recover clarified liquid or to recover concentrated solids, solids contained in the feedstock either pass out of the shell with the remainder of the feedstock carrier stream, or are retained on or in the fibres. These retained solids cause fouling and blockage of the filter. Industrial practice with the more common tube-in-shell microfilters for many years was commonly to apply the feedstock to the inner surface of the fibres by forcing flow through the fibre lumens at such a rate that turbulence scoured the walls of the fibres, retarding blockage by solid material.
In the above mentioned specifications, the feedstock is applied to the outer surface of the fibres, with a penalty of low feedstock flow velocity and consequent low turbulence resulting in a rapid rate of blockage of the pores of the fibres. This is overcome by the application of a two-stage backwashing cycle.
In the first stage a liquid backwash is applied to the lumens of the fibres such that the liquid passes through the porous walls of the fibres and sweeps retained solids out of substantially all of the pores in the walls of the fibres. In the second stage, a gaseous backwash is applied to the lumens of the fibres such that the gas passes through the larger pores in the walls of the fibres, stretching them and dislodging retained blocking solids.
International Patent Application No. PCT/AU86/00049 discloses a method of applying pressure such that the gaseous backwash is applied evenly over the inner surface of the hollow fibres. In this method, the volume of liquid backwash is that volume of liquid trapped in the pores of the walls of the fibres. When the backwash stage is begun, low pressure gas is applied to clear the fibre lumens of liquid, and then high pressure gas is applied so as to exceed the bubble point of the fibres and force gas through the larger pores in the fibre walls.
The application of the two-stage backwashing regime discussed above restores filtrate flux to a high value that is, however, not as high as the initial value At each stage this slight diminution of flux reduces the filtration capacity of the fibres. Eventually chemical cleaning is required. This is expensive and time consuming
Another method of cleaning the fibres is known as reverse flow and is reviewed in "Ultrafiltration Membranes and Application", Edited by A.R. Cooper, a record of a Symposium of the American Chemical Society, Sept. 11-13, 1979, Pages 109 to 127, "Advances in Hollow Fibre Ultrafiltration Technology", by B.R. Breslau.
In the Breslau method the feed is applied to the lumens of the fibres at high velocity so that there is a large pressure drop down the length of the fibres By closing off the filtrate flow at the distal end of the shell, the filtrate pressure climbs within the shell and forces filtrate backwards through the fibre walls in the distal end of the fibre bundle. The direction of flow of feedstock is then reversed and the process re

REFERENCES:
patent: 4552669 (1985-11-01), Sekellick
patent: 4579662 (1986-04-01), Jonsson
patent: 4767539 (1988-08-01), Ford
patent: 4816160 (1989-03-01), Ford et al.
Derwent Abstract Accession No. 85-287044/46, Class J01, JP,A, 60197206, (Daicel Chem Ind KK) Mar. 16, 1984, (16.03.84).
Advances in Hollow Fiber Ultrafiltration Technology, Barry R. Breslau et al., Woburn, Mass.
Anti-Fouling Techniques in Cross-Flow Microfiltration, B. Milisic et al., Institut Francais des Techniques Separatives, France and Lyonnaise des Eaux France. This paper was presented to the IVth World Filtration Congress*, Ostend, Belgium, Apr. 1986.

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