9J O

Figure 6,25 Common flow regimes on distillation trays.

bubbles, still with a very definite top or surface to the dispersion. Bubbles formed at the tray perforations rise unbroken to the top of the foam, and little bulk circulation occurs. The bubbles are distorted to polyhedra. This model is described in detail elsewhere (101,102).

Cellular foam occurs at low vapor velocities in small columns, where the wall provides foam stabilization. It occurs with some systems or tray designs but not with others and is promoted by surface tension effects such as the Marangoni effect (99). Cellular foam is uncommon in industrial columns. The foam that causes problems in industrial installations is mobile foam, where the bubbles are in turbulent motion. Mobile foam is associated with the froth and emulsion regimes. Cellular foam is encountered in bench-scale and pilot-scale columns. If cellular foam occurs in the test unit, caution is required when scaling up the results.

Froth (Figs. 6.25c, 6.26b, and 6.27a; sometimes referred to as the "mixed" regime). As vapor rates are increased in the bubble and cellular foam regimes, the dispersion changes to froth. In this regime each perforation bubbles vigorously, with chain bubbling common; the bubbles circulate rapidly through the liquid, are of a wide size range, of nonuniform shapes, and travel at varying velocities. The froth surface is mobile and not level, and generally covered by droplets. Often, waves or oscillations are present. Bubbles are formed at the tray perforations and are swept away by the froth.

At low liquid depths (i.e., very low liquid loads), bubbles break the liquid surface before detaching from the tray orifices, causing nonuni-formity and some unique performance characteristics (40). This region )low vapor loads and very low liquid loads), however, is infrequently encountered in industrial practice.

As vapor load increases in the froth regime, jetting begins to replace bubbling in some holes. The fraction of holes that are jetting increases with vapor velocity. When jetting becomes the dominant mechanism, the dispersion changes from froth to spray. Prado and Fair (103) showed that this change is gradual and that the transition from froth to spray takes place as jetting replaces bubbling in 45 to 70 percent of the tray holes.

The froth regime is the most common operating regime in distillation practice, and its hydraulics is reasonably well approximated by the classical hydraulic model (Sec. 6.2.1; Fig. 6.5).

Spray (Figs. 6.254, 6.26c, and 6.27b; sometimes referred to as the "drop" regime). As vapor load is increased at relatively low liquid rates, the spray regime is reached. While in the previous three regimes (and also

Figure 6.26 Tray action closeups in various flow regimes, (a) Cellular foam; (6) froth {Part a from P. H. Calderbank and M. B. Moo-Young, International Symposium on Distillation, p. 59, the Institution of Chemical Engineers, UK, I960; part b from M. J. Lockett, Distillation Tray Fundamentals, Cambridge University Press, Cambridge, 1986. Part a reprinted courtesy of the Institution of Chemical Engineers, UK. Part b reprinted courtesy of Cambridge University Press, Cambridge, UK.)

Figure 6.26 Tray action closeups in various flow regimes, (a) Cellular foam; (6) froth {Part a from P. H. Calderbank and M. B. Moo-Young, International Symposium on Distillation, p. 59, the Institution of Chemical Engineers, UK, I960; part b from M. J. Lockett, Distillation Tray Fundamentals, Cambridge University Press, Cambridge, 1986. Part a reprinted courtesy of the Institution of Chemical Engineers, UK. Part b reprinted courtesy of Cambridge University Press, Cambridge, UK.)

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