Liquid purification or separation – Processes – Liquid/liquid solvent or colloidal extraction or diffusing...
Patent
1986-11-17
1989-03-28
Sever, Frank
Liquid purification or separation
Processes
Liquid/liquid solvent or colloidal extraction or diffusing...
210798, 2102572, 2103211, 21033301, 210411, 2104332, 21032169, B01D 1301, B01D 2938
Patent
active
048161608
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to the cooling of porous hollow fibres within a cross-flow separator.
For the sake of convenience, the invention will be described in relation to the use of hollow fibres in the recovery of fine solids from suspensions. However, it is to be understood that the invention is not limited to solids concentrators as it may be readily applied to cross-flow filters and other devices utilising porous hollow fibres.
BACKGROUND ART
The problem of recovery of fine solids from suspensions is complementary to that of recovering clear liquids from suspensions.
A detailed recent discussion of cross-flow microfliltration is given by R. Bertera, H. Steven and M. Metcalfe, The Chemical Engineer, pp. 10-14, June, 1984.
Economically, the ability to cope with strongly fouling solids without filter aids is most pressing. This fouling problem has long been recognised and the art records some attempts to substitute gas for clarified liquid during reverse flow to avoid the recycle of clarified liquid to the feed suspension.
Transmembrane gas reverse flow is impossible in very finely pored filters such as reverse osmosis membranes and ultrafilters because the pressures needed to overcome surface tension are far beyond the strengths of normal hollow fibre membranes used for these purposes; wettable liquids may pass but not gases. Any gas bubbles passing through such a membrane indicate the presence of pin hole defects in the membrane. Hence, this invention has no application to reverse osmosis or to true ultrafilters.
This invention is concerned with microfilters which contain larger pores than those of ultrafilters and which range from 0.001 to 10 microns. Usually, the larger of the pores are so distributed that clarified liquids are free of all visible turbidity. Turbidity of the clarified liquid involves more than pore and particle size, obeying and arising from well known optical laws.
Early microfilters fouled quickly since they treated particles which were not suspended by Brownian motion nor diffusion but which penetrated into the range of similar pore size in the manner of sieve blinding.
One prior art approach to solving this problem was to operate hydrophilic microfilters in a cross-flow mode with clarified liquid transmembrane reverse flow. High crossflow velocities required feed suspension to be directed to the smaller internal filtering surface of the lumen as opposed to the larger external surface of the fibre. Thus, reverse flow pressures had to be limited to avoid fibre crushing. The smaller filtering surface reduced output and this was frequently not a useful solution to the problem.
Another prior art approach is disclosed in Japanese Patent Kokai Publication No. 53-100882 (1978) where a hollow fibre bundle in loose "candle" configuration of hydrophilic "polyvinyl alcohol (PVA)" fibres was made to writhe during long (one minute) lumenal reverse flows with air. Filter "candles" of the kind described in this Japanes specification are more akin to dead-end filters than to cross-flow shell and tube filters in that they are in the form of elongated hollow pots closed at one end.
A development of the above "candle" configuration of hydrophilic "polyvinyl alcohol (PVA) type polymer" porous hollow fibres is to be found in U.K. Pat. No. 2,120,952. The writhing of the fibre bundle was somewhat restrained by taping the fibres loosely and enclosing them in an open sleeve which avoided the tangling and fibre breakage of the earlier mentioned Japanese Kokai Publication No. 53-108882(1978) but the gas reverse flow took 5 minutes.
The prior art in respect of reverse flow cleaning of porous hollow fibres using liquid and gas is very limited and such is also the case in relation to the cooling crossflow sueparators treating hot feed stocks to permit fast operation in the separation mode.
Cooling hot fibres by blowing air on them or by evaporating water from them involves the same thermal principles as shaking a towel soaked in water to cool it; these thermal principles are well known.
REFERENCES:
patent: 3912624 (1975-10-01), Jennings
patent: 3992301 (1976-11-01), Shippey et al.
WO85/01449, 11 Apr. 1985, Ford, "Cleaning of Filters".
"Development Studies of Cross Flow Microfiltration", by Bertera et al, The Chemical Engineer, pp. 10, 11, 13 and 14 (Jun. 1984).
Anderson Eric W.
Ford Douglas L.
Kopp Clinton V.
Memtec Limited
Sever Frank
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