Variable volume filter or concentrator

Details Variable volume filter or concentrator Variable volume filter or concentrator

1986-12-05

1988-12-27

Sever, Frank

Liquid purification or separation

Processes

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

210637, 210651, 210798, 210324, 210351, 210412, 210414, 2104332, 2103218, B01D 1301

Patent

active

047939320

DESCRIPTION:

BRIEF SUMMARY
FIELD OF THE INVENTION

This invention relates to a variable volume filter or concentrator containing a bundle of porous hollow fibers.
For the sake of convenience, the invention will be described in relation to the use of hollow fibers in the recovery of fine solids from suspensions. However, it is to be understood that the invention is not limited thereto as it may be readily applied to cross-flow filters and other devices using bundles of porous hollow fibers.
The problem of recovery of fine solids from suspensions is complementary to that of recovering clear liquids from suspensions.
The producers of clear liquids usually regard all visible traces of solids as wastes. The methods used often involve the addition of flocculants and filter aids which contaminate the solids. The solids content tends to be low, encouraging the use of methods which remove clear liquid from a continuously fed feed suspension tank in which the solids content increases until some deleterious effect arises, necessitating the dumping of the contents of the feed suspension tank into some other device. Invariably, the accumulating solids have been steadily slowing production and productivity could benefit from some device which continuously rejected concentrated solids.
In contrast thereto, the producers of finely divided solids are usually the food, mining or manufacturing industries for which the solids are desired and the liquid is best recycled. Also, the solids have specifications for size and purity, often need further processing and mostly need to be obtained at high solids content as concentrates. Filter aids will, of course, contaminate the product.
A detailed recent discussion of cross-filtration is given by R. Bertera, H. Steven and M. Metcalfe, The Chemical Engineer, pp. 10-14, June, 1984.
As shown in FIG. 8 of the above publication even the latest 1984 commercial Enka Membrana A.G. filter module rapidly fouled and the clarified liquid flux continued to decline when backwashed with transmembrane clarified liquid in the constant concentration cross-flow (diafiltration) mode on a fine inorganic filler.
Economically, the ability to cope with strongly fouling solids without filter aids is most pressing. This fouling problem has long been recognized and the art records some attempts to substitute gas for clarified liquid during backwashing to avoid the recycle of clarified liquid to the feed suspension. Thus, Japanese unexamined Patent Kokai Publication No. 53(1978)-108,882 states:
"Since the filtrate is not used in the present invention for membrane reverse cleaning, the serious defect of the prior art method, that is, returning the filtrate substantially to the crude liquid, is eliminated, with obvious industrial merits."
Transmembrane gas backwashing 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 fiber membranes used for these purposes; wetting 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 0.01 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 pores of a similar size range as the particles in the manner of sieve blinding.
One approach to solving this problem was to operate hydrophilic microfilters in a cross-flow mode with clarified liquid transmembrane backwash. High cross-flow velocities required feed suspension to be directed to the sma

REFERENCES:
patent: 3442002 (1969-05-01), Geary, Jr. et al.
patent: 3912624 (1975-10-01), Jennings
patent: 3992301 (1976-11-01), Shippey et al.
patent: 4629568 (1986-12-01), Ellis
"The Chemical Engineer" Development Studies of Crossflow Microfiltration, by R. Bertera et al., pp. 10-14, Jun., 1984.
WO 85/01449, Apr. 11, 1985, Douglas Ford, "Cleaning of Filters".

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