packing chosen to promote coalescence of secondary hazes, followed b y layers designed to promote coalescence and separation of the larger drops formed, which would be chosen such that the dispersed phase wetted the surface. For successful operation of fiber beds it is important that solid particles are removed from the dispersion before being passed through the bed. Solid matter deposited in the bed will not only change the voidage and local velocities of fluid in the bed, but, more important, effectively change the surface properties of the bed. Thus fiber bed coalescers should not, as is often stated (Hazlett, 1969), be used as filters. The exact mechanism of initial attachment of micron- and submicron-sized drops on fiber packings is difficult to elucidate. The simple interception mechanism based on data from aerosol filtration, assuming laminar flow conditions around fibers suggested by Hazlett, cannot be entirely true. Even a t t h e fairly low over-all flow rates used in this type of packing, 1 t o 1.5 cm per second, Davies and Jeffreys (1969) and Kintner (Kintner, 1969; Sareen et al., 1966) have observed and
obtained photographs of local turbulence. These results confirm the conclusions of Hazlett that the efficiency of separation increases with decreasing fiber diameter. literature Cited
Davies, G. A,, Jeffreys, G. V., Filtr. Separ. 6,349 (1969). Haxlett, R. N., IND. ENG.CHEM:FUNDAM. 8,625,633 (1969). Juma, S., M. Sc. thesis, University of Manchester, 1968. Kintner, R. C., private communication, 1969. Lawson, G B., Chem. Proc. Eng. 48,45 (1967). Padday, J. F., “Proceedings of 2nd International Congress on Surface Activity,” Vol. 3, Butterworths, London, 1957. Robinson, J. W., U. S. Patent 2,611,490(1952). Sareen, S. S., Rose, P. M., Gudesen, R. C., Kintner, R. C., A . I . Ch. E. J. 12, 1045 (1966). Treybal, R. L., “Liquid Extraction,” 2nd ed., pp. 447-50, McGraw-Hill, New York, 1963. G. A. Davies University of Manchester Nanchester., Enaland ” G. V . Jeflreys University of Aston Birmingham, England
Coalescence of Droplets in Fiber Beds SIR:Davies and Jeff reys have presented recent information on the subject of coalescence in fiber beds which is worthy of comment. I agree that the mechanism of coalescence of small droplets ( < l o microns) is not understood and that many coalescence events occur between a n attached droplet and a dispersed droplet (Bitten, 1969; Beatty, 1969). I also agree that the arguments b y Hazlett (1969) concerning film drainage were circumstantial and were useful only in emphasizing that film drainage for the drop-collection step was not a controlling element in beds of a reasonable depth for water-in-oil emulsions. The data presented by Hazlett (1969) on untreated and coated glass fibers, confirm the thesis of Davies and Jeffreys, that the wetting properties of a fiber are not important for the collection of small water droplets. Recent studies a t N R L show, however, that the wettability of fibers a t the release site is important for some water-in-oil emulsions. This latter point illustrates the complex nature of separatdroping two phases in a fibrous bed. Coalescence-the collection step-is but one of the critical processes; passage and release of the discontinuous phase can frequently be more important in over-all efficiency. The statement by Davies and Jeffreys (1969) that secondary dispersions coalesce repeatedly and drops grow continuously as they move through a fiber bed cannot be accepted for water-in-oil emulsions. Bitten (1969) has shown that water distribution is localized primarily in the forepart, and to some extent at the downstream face, of a bed. The forepart can contain over 60% water b y volume. Thus, coalescence occurs at this stage and pools of water fill most of the pores. Hydrodynamic forces direct the collected water into channels that carry the water to release sites. Movies taken b y Brown (1966) have shown water threads snaking through a fiber bed. More recent movies taken a t N R L have shown these threads feeding droplets attached to bonded glass fibers at the downsteam face. These drops grow to 3 m m for a flow velocity of 5 feet per minute, if the kerosine is free of surfactants. The same site is active repeatedly, once a channel has been established. If certain surfactants are present, the thread may become detached from the pooled water in the forepart of the bed. The thread can then behave as a free jet and break into a series of uni520 Ind. Eng. Chem. Fundam., Vol. 9, No. 3, 1970
formly sized and spaced drops by the Rayleigh mechanism (1879). The same site remains intermittently active in this situation also. These observations are closer to those presented b y Davies and Jeffreys for so-called primary dispersions (drops >lo0 microns) rather than secondary dispersions (drops
