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Spray Regime Distillation
Figure 6.28 Characteristic features of the spray regime, (a) Generation of drops by tearing liquid sheets at tray orifices (Part o from W. V. Pinczewski and C. J. D. Fell, Trans. Inst. Ckem. Eng. (London), 52, p. 294, 1974. All parts reprinted courtesy of the Institution of Chemical Engineers, UK.)

the vapor bubbles. In this regime, most of the gas is emulsified as small bubbles within the liquid. In the emulsion regime, the mixture behaves as a uniform two-phase fluid, which obeys the Francis weir formula (17,85,104,105). In industrial practice, the emulsion regime often occurs in high-pressure and high-liquid-rate operation. This regime is well-approximated by the classical hydraulic model (Sec. 6.2.1).

6.4.2 The flow regime likely to exist on Industrial trays

Figure 6.29 suggests guidelines for the occurrence of each flow regime on commercial distillation trays. As stated earlier, the bubble and eel-

System : air/water Liquid load : 0.00524 m2)s Ug [m/s]

System : air/water Liquid load : 0.00524 m2)s Ug [m/s]

Dstilling Tray Parts

Rgur* 6.28 (Continued) Characteristic features of the spray regime, (b) Vertical dispersion density profiles, x axis is liquid fraction (test tray, dH = 0.25 inches, Af = 0.07, <?l = 2 gpm/in. o, x froth regime; A froth regime, close to froth to spray transition; * spray regime. (Part b from F. J. Zuiderweg, P. A, M. Hofhuis, and J. Kuzniar, Chem. Eng. Res. Des., 62, p. 39, 1984. All parts reprinted courtesy of the Institution of Chemical Engineers, UK.)

lular foam regimes are not common in industrial applications, and will not be discussed further.

Effect of pressure. In vacuum columns, vapor velocities are generally high and liquid flow rates are low, which coincides with an operating point in the spray regime. If the column operates at high liquid loads, it may operate in the froth regime. The emulsion regime is unlikely to occur in vacuum columns.

In high-pressure (>200-psi) distillation, vapor velocity is low and liquid flow rate is relatively high, which coincides with an operating point in the emulsion regime. If the column operates at low liquid

Figure 6.28 (Continued) Characteristic features of the spray regime. (c) Liquid transport mechanism by trajectory ("jumping") over the outlet weir. (Part c from F. J. Zuiderweg, P. A. M. Hofhuis, and J. Kuzniar, Chem. Eng. Res. Des., 62, p. 39, 1934. All parts reprinted courtesy of the Institution of Chemical Engineers, VK.)

Figure 6.28 (Continued) Characteristic features of the spray regime. (c) Liquid transport mechanism by trajectory ("jumping") over the outlet weir. (Part c from F. J. Zuiderweg, P. A. M. Hofhuis, and J. Kuzniar, Chem. Eng. Res. Des., 62, p. 39, 1934. All parts reprinted courtesy of the Institution of Chemical Engineers, VK.)

Froth Regime Tray

Flgur* 6.29 The flow regime likely to exist on a distillation tray, as a function of vapor and liquid loads.

loads, it may operate in the froth regime. The spray regime is unlikely to occur.

In atmospheric and low-pressure (<100-psi) distillation, the column is likely to operate in the froth regime, but depending on the liquid and vapor rates, it may also operate in either the spray or emulsion regime.

Effect of L/V ratio. In the rectifying section, the liquid load is often lower and the vapor load higher than in the stripping section. Therefore the rectifying section tends toward the spray regime, while the stripping section tends toward the emulsion regime. Similarly, a high-

liquid-load absorber or stripper tends toward the emulsion regime, while a low-liquid-load wash section tends toward the spray regime.

Effect of column diameter (at constant UV and percent of flood). As column diameter increases, both the liquid and vapor flow rates increase as the square of the diameter. The area for vapor flow also increases as the square of the diameter, so the vapor load remains unaffected. On the other hand, the area available for liquid flow only increases in proportion to the diameter. Therefore, the liquid rate per unit of weir length increases, the increase being proportional to the column diameter. The operating point on Fig. 6.29 will therefore shift horizontally to the right, toward the emulsion regime. Increasing the number of liquid passes on the tray reverses the above action, and shifts the operating point back to the left.

Effect of tray spacing (at constant L/V and percent of flood). Lower tray spacing reduces flooding vapor rate, and therefore, the operating vapor velocity. This reduction in vapor velocity shifts the operating point on Fig. 6.29 toward the froth or emulsion regime. Conversely, an increase in tray spacing shifts the operating point toward the spray regime. Columns operating at very low (< 12 in) tray spacing almost always operate in the froth regime.

Effect of fractional hole area. Low fractional hole areas increase the tendency of trays to operate in the spray regime (92,106), In terms of Fig. 6.29, lower fractional hole areas tend to shift the spray-froth boundary to the right. No effect of fractional hole area was observed (17,85) on the transition from the froth to the emulsion regime.

Effect of hole diameter. Large holes increase the tendency of trays to operate in the spray regime (88,89,92,106-108). In terms of Fig. 6.29, larger holes tend to shift the spray-froth boundary to the right. No effect of hole diameter was observed (17,85) on the transition from the froth to the emulsion regime.

Effect of weir height. Pinczewski and Fell (90,92) showed that weir height has little effect on the transition from the froth-to-spray regime. More recent work by Prado and Fair (103) and Lockett (12) suggests that the tendency of a tray to operate in the froth regime increases slightly with an increase in weir height. No effect of weir height on the transition from froth to emulsion was observed (17,85).

Effect of tray type. Most flow regime work was carried out using sieve trays. Some of the preliminary work reported for valve trays suggests that valve trays may have the same or a somewhat greater tendency (86) to operate in the froth regime than sieve trays.

Summary. Spray regime operation is favored in vacuum and low-pressure columns, under low liquid loads, when column diameter is relatively small, and when tray spacing is relatively high. Spray regime operation is also favored by trays with large hole diameters and low fractional hole areas. Emulsion regime operation is favored in high-pressure columns and under high liquid loads.

6.4.3 Transition between flow regimes

Froth-spray. Froth-spray transition has been investigated for sieve trays using a variety of techniques (12,24,88-92,103,106-110). The gradual nature of this transition bred a large number of criteria for defining it, and made its correlation difficult. Excellent overviews of the state of art were given by Lockett (12) and Prado and Fair (103).

Porter and Jenkins (24) presented a simple correlation for the froth to spray transition. ,

This correlation is based on the premise that froth to spray transition occurs when the entrainment versus liquid load relationship passes through a minimum (Fig. 6,15, Sec. 6.2.11). This premise has been questioned, and it was argued that the minimum may represent a transition from the froth regime to a partially developed spray region (40,53). This argument implies that whenever the correlation predicts froth, it is highly unlikely that the column operates in the spray regime. If, however, it predicts spray, it may well be that the column is still operating in the froth regime.

A second correlation is by Pinczewski and Fell (52,90)

This correlation is based on transition data obtained from orifice jetting measurements (92,106) for the air-water system and on entrainment minimum data (22) for some hydrocarbon systems. A third recent correlation by Johnson and Fair (18,103,111) gives

NpAB

Equation (6.73) applies to 2-in weirs; for 1-in weirs, uB should be multiplied by 0.92; for 4-in weirs it should be multiplied by 1.12 (18).

Froth-emulsion. Froth-emulsion transition occurs (17,45,85,105) when the aerated mass begins to obey the Francis weir formula. Using this criterion, the latest version of this transition correlation is

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