Entrainment (Fig. 14-33) is liquid transported by the gas to the tray above. As the lower tray liquid is richer with the less-volatile components, entrainment counteracts the mass-transfer process, reducing tray efficiency. At times entrainment may transport nonvolatile impurities upward to contaminate the tower overhead product, or damage rotating machinery located in the path of the overhead gas.

Effect of Gas Velocity Entrainment increases with gas velocity to a high power. Generally, smaller powers, indicative of a relatively gradual change, are typical of low-pressure systems. Higher powers, which indicate a steep change, are typical of high-pressure systems.

Due to the steep change of entrainment with gas velocity at high pressure, the gas velocity at which entrainment becomes significant tends to coincide with the flood point. At low pressure, the rate of change of entrainment with gas velocity is much slower, and entrain-ment can be significant even if the tray is operating well below the flood point. For this reason, excessive entrainment is a common problem in low-pressure and vacuum systems, but is seldom troublesome with high-pressure systems. If encountered at high pressure, entrain-ment usually indicates flooding or abnormality.

Effect of Liquid Rate As the liquid rate is raised at constant gas rate, entrainment first diminishes, then passes through a minimum, and finally increases [Sakata and Yanagi, I. Chem. E. Symp. Ser. 56, 3.2/21 (1979); Porter and Jenkins, I. Chem. E. Symp. Ser. 56, Summary Paper, 1979; Friend, Lemieux, and Schreiner, Chem. Eng., October 31, 1960, p. 101]. The entrainment minima coincide with the maxima in plots of entrainment flood F-factor against liquid load (Fig. 14-29). At the low liquid loads (spray regime), an increase in liquid load suppresses atomization, drop formation, and consequently entrainment. At higher liquid loads, an increase in liquid load reduces the effective tray spacing, thereby increasing entrainment. The entrainment minima have been interpreted by many workers as the tray dispersion change from predominantly spray to the froth regime [Porter and Jenkins, loc. cit.; Kister and Haas, I. Chem. E. Symp. Ser. 104, p. A483 (1987)].

Effect of Other Variables Entrainment diminishes with higher tray spacing and increases with hole diameter [Kister and Haas, I.

Clear liquid velocity in downcomer, ft/s

Foaming 18-in 24-in 30-in tendency Example spacing spacing spacing

Low Low-pressure (<100 psia) light hydrocarbons, stabilizers, 0.4-0.5 0.5-0.6 0.5-0.6* air-water simulators

Medium Oil systems, crude oil distillation, absorbers, midpressure 0.3-0.4 0.4-0.5 0.4-0.5* (100-300 psia) hydrocarbons

High Amines, glycerine, glycols, high-pressure (>300 psia) 0.2-0.25 0.2-0.25 0.2-0.3 light hydrocarbons

*Revised from previous versions.

To convert from ft/s to m/s, multiply by 0.3048; from in to mm, multiply by 25.4; from psia to bar, multiply by 0.0689. source: From H. Z. Kister, Distillation Operation, copyright 1990 by McGraw-Hill, Inc.; reprinted by permission.

TABLE 14-8 Recommended Minimum Residence Time in the Downcomer


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