Finally, independent of the nature of the surface, HOG values will be expected to fall off at concentrations of around 2% amine since the slope of the equilibrium line changes rapidly and the contribution of the liquid-phase resistance will be significantly reduced. Conclusions Contrary to the statement of Francis and Berg, i t has been shown that the addition of a surfactant can improve the column performance when separating surface tensionpositive systems, a t near-terminal compositions. The results indicate that surfactant addition has definite industrial potential for separating pollutants which are present a t low concentrations in water and which are surface tension-positive in character. Acknowledgment One of us (P. Trauffler) acknowledges financial assistance from Fonds d’Encouragement aux Recherches Scientifiques, Berne. Nomenclature = average concentration of the higher volatile component in the packing (mole fraction)
HOG = height of overall gas phase transfer unit, cm y = mole fraction in vapor x = mole fraction in liquid
Literature Cited Boyes, A. P., Ponter, A. B., Chem. Roc. Eng.. 50, 70-73 (1969). Boyes, A. P., Ponter. A. E., A.6Ch.E. J., 18, 935-940 (1972). Boyes, A. P., Ponter, A. E., Chem. hg. Tech., 45, 1250-1256 (1973). Boyes, A. P., Ponter, A. E., J. Chem. Eng. Data, 15, 235-238 (1970). Charpentier. J. C., Prost, C., Van Swaaj, W., Legoff, P.. Chim. h d . G n i e Chim., 99, 803 (1968). Francis, R. C., Berg, J. C., Chem. Eng. Scb, 22, 685-692 (1967). Jackson, M. L.,Geaglske. N. H., Ind. Eng. Chem., 42, 1188-1198(1950). Komarov, V. M., Krichevtsov, E. K., Z.Prik/. Khim., 39, 2834-2838 (1966); 43, 112-115(1970). Liang, S.Y., Smith, W., Chem. Eng. Sci., 17, 11-21 (1962). Norman, W. S., Cakalor. T., Fresco, A. Z., Sutcliffe, D. H.. Trans. Inst. Chem. Eng.. 41, 61-71 (1963). Ponter. A. E.. Boyes. A. P., Houlihan. R. N., Chem. Eng. Sci.. 28, 593-596 (1973). Ponter, A. B., Hassanien, S.,Landau, J., Chem. hg. Tech., 47, 775 (1975). Porter, K. E., Templeman, J. J., Trans. Inst. Chem. Eng., (part /I), 46, T86T94 (1968). Pritchard. M. L.. Ph.D. Thesis, University of Birmingham, 1963. Sawistowski, H., Smith, W., Ind. Eng. Chem., 51, 915-918 (1959). Trauffler, P., D.Sc. Techniques Thesis, Swiss Federal Institute of Technology, Lausanne, 1975. Vanwiik, W. R.. Thijssen. H. A. C., Chem. Eng. Sci., 3, 153-160 (1954).
Receiued f o r reuiew June 6, 1975 Accepted August 26,1975
Aerosol Scrubbing by Foam Tlbor G. Kaldor and Colin R. Phillips’ Department of Chemical Engineeringand Applied Chemistry, University 01 Toronto. Toronto, Canada M5S l A 4
Extraction of particles from an air stream by foam was evaluated by varying throughput rate, residence time, and particle concentration. Dusty air was bubbled into a pool of surfactant solution (0.3 g/l. of ethylhexadecyldimethylammonium bromide). The dust/foam system travelled up a 4 in. diameter by 4.5 ft Plexiglas column, and was then broken centrifugally. The concentrations and size distributions of both the input fly-ash and the particles extracted into the foamate were measured and an efficiency of extraction was calculated. Qualitative comparison of results with the mechanisms postulated was good. Sedimentation was fully effective on particles greater than 2 p , and diffusive deposition accounted for about 30% extraction in the 0.5-1.0 p range. At the cross-over (1-2 p ) from the sedimentation mechanism (coarse particles)to the diffusive mechanism (fine particles), removal was about 5%. Aggregation in the foamate is a probable cause of the apparent low extraction of small (0.3-0.5 p ) particles and the high extraction of large ones. Extraction increased with residence time (1-3 min), and decreased with gas rate (5-15 Wmin). Heavier dust loading resulted in poorer extraction, leading to a hypothesis of interfacial saturation and/or repulsion.
Introduction It is of interest to examine, theoretically and experimentally, the application of foam to the removal of particulates from an air stream. The large surface-to-volume ratio and low pressure drop of such a system, combined with the fact that when a foam is broken, the particulates are concentrated into a small volume, make the process promising. The surfactant film inside the bubble also aids in wetting hydrophobic particles. On the other hand, the static atmosphere inside the bubble is not ideal for dust precipitation: also, work must be expended in breaking the foam. Few systematic studies of foam scrubbing exist in the literature. Bransky and Diwoky (1940) scrubbed sulfuric acid mist (particle size 2-11 w ) in foam bubbles (diameter 0.6-1.2 cm)
produced from a sulfuric acid solution. About 93-95% of the total H&Ol was removed in 10-11 sec. The acid concentration of the incoming mist had very little effect on the efficiency of removal, which was found to be directly related to contact time. Pozin et al. (1955) correlated the fractional removal of dust, F , in a foam produced above a perforated plate with the Stokes number defined as
-
Nst = D P V gdo%
where D , is the particle diameter, pp is the particle density, V is the gas velocity, do is the perforated plate orifice diateter, g is the acceleration due to gravity, and vg is the gas viscosity. The correlation was of the form F = a(N& Ind. Eng. Chem.. Process Des. Dev., Vol. 15, No. 1, 1976
I99
I
II---
I-
I
I
1
1
I
1
I
.
Figure 1 Apparatus.
where (Y and (3 had values depending on the ranges of D, and Nst. Mukhlenov and Demshin (1955) determined that the effect of an increase in the height of a sodium oleate foam was more pronounced for the removal of a hydrophilic dust (FeS2) than for a hydrophobic dust (BaSOJ. Quon and Phillips (1966), however, found that their foam column was more efficient for removing carbon black (hydrophobic) than iron oxide (normally hydrophilic). They suggested that the dominant removal mechanism was impaction, because it was found that efficiency of dust removal increased with gas velocity, that is, decreased with increasing residence time. Quon and Phillips used a right-angle light scatter device to make measurements of particle concentration in the inlet and outlet air streams. They found that the wetted particles leaving the foam column gave rise to erroneous readings. Related work on the removal of aerosol by froth produced in a packed tower has been carried out by Yano et al. (1955). Complete theoretical treatment of the process of foam scrubbing requires consideration of particle properties, gas properties, particle size, bubble size, and properties, and residence time. No previous work has taken into account all of these variables.
Experimental Section The pilot-plant scale “foam scrubber” apparatus used here consisted of a vertical Plexiglas column (4 in. diameter by 4.5 in. high), surmounted by a centrifugal foam breaker. A stream of air contaminated with dust was generated by dispersing fly-ash from the pan of an ultrasonic bath (l), Figure 1. The aerosol was passed through a surge chamber (1 1.) to damp out fluctuations in concentration and to remove any very large particles (2). In the metering sector, it was sampled isokinetically with a pitot tube (3) and diluted suitably for counting (4). The volumetric flow rate was measured (5). The dusty air stream was then fed to the bottom of the foam scrubber (6). As it was bubbled through 2 1. of 0.3 g/l. ethylhexadecyldimethylammonium bromide surfactant solution by means of a sparger (45-0.33 mm holes), it formed a column of foam wherein scrubbing of the aerosol took place. The overflowing foam was centrifugally broken by straining through a stainless steel wire mesh in a rotating (5000 rpm) basket (7). A small y50 h p 200
Ind. Eng. Chem., Process Des. Dev., Vol. 15, No. 1, 1976
motor was used. The dust was supplied by Ontario Hydro from electrostatic precipitators on their generating stations. T o make it amenable for experimentation on a small scale, the coarse fraction was removed by sieving. Thus only -400 mesh (
