Foam fractionation

Most of the work on foam fractionation reported in the literature is exploratory and on a laboratory scale. A selected list of about 150 topics has been prepared with literature references by Okamoto and Chou (1979). They are grouped into separation of metalhc ions, anions, colloids, dyes and organic acids, proteins, and others. 

Stable foams that leave the fractionator are condensed for further processing or for refluxing. Condensation may be effected 

TABLE 19.3. Properties of Membranes for Reverse Osmosis (a) Cellulose Acetate Membranes 

Membrane Type Mfgr Volumetric flow rate/pressure (ml/min cm2 bar) Molecular weight range Conditions 

BM Rcrghof-Mcmbran DDS De Danske Sukkerfabriken m PCI Paterson Candy International 

Membrane Manufacturer Volumetric ftow rata l/m d) NaCI retentton (%) Pressure (bar) NaCI concentration %) 

CTA Ultra-thin wound module Univ. Oil Prod. (Gulf) 550 99.3 70 3.5 

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Foamed polymers Foamed sheet Foamed silicone rubber Foam fractionation... 

Foam Fractionation. An interesting experimental method that has been performed for wastewater treatment of disperse dyes is foam fractionation (88). This method is based on the phenomenon that surface-active solutes collect at gas—Hquid iaterfaces. The results were 86—96% color removal from a brown disperse dye solution and 75% color removal from a textile mill wastewater. Unfortunately, the necessary chemical costs make this method relatively expensive (see Foams). 

Among the methods of foam separation, foam fractionation usually implies the removal of dissolved (or sometimes colloidal) material. The overflowing foam, after collapse, is called thefoamate. The solid lines of Fig. 22-42 illustrate simple continuous foam fractionation. (Batch operation would be represented by omitting the feed and bottoms streams.)... 

FIG. 22-42 Four alternative modes of contimioiis-flow operation with a foam-fractionation column (1) The simple mode is illustrated hy the solid lines. (2) Enriching operation employs the dashed reflux line. (3) In stripping operation, the elevated dashed feed line to the foam replaces the solid feed line to the pool. (4) For combined operation, reflux and elevated feed to the foam are both employed. 

A separation can sometimes be obtained even in the absence of any foam (or any floated floe or other surrogate). In bubble fractionation this is achieved simply by lengthening the bubbled pool to form a vertical column [Dorman and Lemlich, Nature, 207, 145 (1965)]. The ascending bubbles then deposit their adsorbed or attached material at the top of the pool as they exit. This results in a concentration gradient which can serve as a basis for separation. Bubble fractionation can operate either alone or as a booster section below a foam fractionator, perhaps to raise the concentration up to the foaming threshold. 

The droplet analogs to the adsubble methods have been termed the adsoplet methods (from adsorptive droplet separation methods) [LeiTilich, Adsorptive Bubble Separation Methods, Ind. E/ig. Chem., 60(10), 16 (1968)]. They are omitted from Fig. 22-41, since they involve adsorption or attachment at liquid-liquid interfaces. Among them are emulsion fractionation [Eldib, Foam and Emulsion Fractionation, in Kobe and McKetta (eds.). Advances in Petroleum Chemistry and Refining, vol. 7, Interscience, New York, 1963, p. 66], which is the analog of foam fractionation and droplet fractionation [Lemlich, loc. cit. and Strain, J. Phys. Chem., 57, 638... 

Of all the adsubble methods, foam fractionation is the one for which chemical engineering theoiy is the most advanced. Fortunately, some of this theory also apphes to other adsubble methods. 

Excess collector can also reduce the separation by forming micelles in the bulk which adsorb some of the colhgend, thus keeping it from the surface. This effect of the micelles on Ki for the colhgend is given theoretically [Lemhch, Principles of Foam Fractionation, in Periy (ed.). Progress in Separation and Purification, vol. 1, Interscience, New York, 1968, chap. 1] by Eq. (22-44) [Lemlich (ed.). Adsorptive Bubble Separation Techniques, Academic, New York, 1972] if F, is constant when C, > C-... 

FIG. 22-43 Graphical determination of theoretical stages for a foam-fractionation stripping column. 

Both types of coalescence can be important in the foam separations characterized by low gas flow rate, such as batchwise ion flotation producing a scum-bearing froth of comparatively long residence time. On the other hand, with the relatively higher gas flow rate of foam fractionation, the residence time may be too short for the first type to be important, and if the foam is sufficiently stable, even the second type of coalescence may be unimportant. 

Overflow at the rate of 2700 m (713,000 gal) per day from a zinc-concentrate thickener is treated by ion flotation, precipitate flotation, and untrafine-particle flotation [Nagahama, Can. Min. Metall. Bull., 67, 79 (1974)]. In precipitate flotation only the surface of the particles need be coated with collector. Therefore, in principle less collector is required than for the equivalent removal of ions by foam fractionation or ion flotation. 

By using an anionic collector and external reflux in a combined (enriching and stripping) column of 3.8-cm (1.5-in) diameter with a feed rate of 1.63 ni/n [40 gal/(h ft )] based on column cross section, D/F was reduced to 0.00027 with C JCp for Sr below 0.001 [Shou-feld and Kibbey, Nucl. AppL, 3, 353 (1967)]. Reports of the adsubble separation of 29 heavy metals, radioactive and otheiwise, have been tabulated [Lemlich, The Adsorptive Bubble Separation Techniques, in Sabadell (ed.), Froc. Conf. Traces Heavy Met. Water, 211-223, Princeton University, 1973, EPA 902/9-74-001, U.S. EPA, Reg. 11, 1974). Some separation of N from by foam fractionation has been reported [Hitchcock, Ph.D. dissertation. University of Missouri, RoUa, 1982]. 

The numerous separations reported in the literature include surfactants, inorganic ions, enzymes, other proteins, other organics, biological cells, and various other particles and substances. The scale of the systems ranges from the simple Grits test for the presence of surfactants in water, which has been shown to operate by virtue of transient foam fractionation [Lemlich, J. Colloid Interface Sci., 37, 497 (1971)], to the natural adsubble processes that occur on a grand scale in the ocean [Wallace and Duce, Deep Sea Res., 25, 827 (1978)]. For further information see the reviews cited earlier. 

Foam fractionation Fractionation, see Distillation Metal separation... 

Limiting Equations If the height of a foam-fractionation column is increased sufficiently, a concentration pinch will develop between the counterflowing interstitial streams (Brunner and Lem-lich, loc. cit.). For an enricher, the separation attained will then approach the predictions of Eq. (20-48) and, interestingly enough, Eq. (20-43). 

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