Emulsion regime

In high pressure distillation, tray operation is usually in the emulsion regime. In small diameter (less than 1.5 m) columns, or at low liquid loads, or the low end of the pressure range (towards 10 bar), however, the froth-and spray regimes can be found. 

In the emulsion regime, there is little entrainment of liquid by the vapour. Instead, the high liquid load causes the downcomer to overfill and the tray to flood. 

Spray regime (or drop regime, Fig. 14-20c). At high gas velocities and low liquid loads, the liquid pool on the tray floor is shallow and easily atomized by the high-velocity gas. The dispersion becomes a turbulent cloud of liquid droplets of various sizes that reside at high elevations above the tray and follow free trajectories. Some droplets are entrained to the tray above, while others fall back into the liquid pools and become reatomized. In contrast to the liquid-continuous froth and emulsion regimes, the phases are reversed in the spray regime here the gas is the continuous phase, while the liquid is the dispersed phase. 

Weir Height Taller weirs raise the liquid level on the tray in the froth and emulsion regimes. This increases interfacial area and vapor contact time, which should theoretically enhance efficiency. In the spray regime, weir height affects neither liquid level nor efficiency. In distillation systems, the improvement of tray efficiency due to taller weirs is small, often marginal. 

Weir height. This parameter sets the level of liquid on the tray in the froth and emulsion regimes (Fig. 17a,b). The higher the level, the better is the contact and the efficiency at the expense of a greater liquid backup in the downcomer. Typical absorption weir heights are 2-3 in. (50-75 mm). 

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. 

Figure 6.26 [Continued) Tray action closeupe in various flow regimes, (c) spray. Note the existence of gas jets at tray orifices (orifice positions are marked). Dotted horizontal lines indicate positions of minimum and maximum in the vertical dispersion sensitivity profiles (such as the spray profile shown in Fig. 6,286), id) Inclined gas bubbling under influence of a horizontal liquid flow, typical of the emulsion regime, part c from W) V. Pinczewski and C. J. D. Fell, Trans. Inst. Chem. Engrs. (London), 52. p. 294, 1974 part d from F. J. Zuiderweg. P. A. M. Hofhuis. and J. Kuzniar, Chem, Eng, Res, Des,. 62, p, 39.1984, Parts c and d reprinted courtesy of the Institution of Chemical Engineers, UK.)...

Figure 6.27 (Continued) Tray action in the froth, spray, and emulsion regimes. Horizontal bars indicate height above tray flcor in inches, (c) Emulsion regime. Wall on right is downcomer from tray above. [All parfe courtesy of Fractionation Research Inc. (FR1).]...

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 atmospheric and low-pressure (<100-p i) 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 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 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. 

While the classical hydraulic model provides a reasonable approximation for the froth and emulsion regimes, different mechanisms determine the hydraulics and mass transfer in the spray regime. The transition from froth to spray is gradual, and so is the change in the hydraulic and mass transfer behavior (110,111,113,114). 

Vapor recycle. Because of the large quantity of vapor that enters the downcomer and the difficulty of separation in the downcomer, vapor recycle is a major consideration in the emulsion regime (Sec. 6.2.8) and can lead to reduction of both capacity and efficiency, and to an increase in pressure drop (17). 

Mass transfer. Tray efficiency increases with pressure in the froth regime (118,119), but decreases with increased pressure in the emulsion regime (44,104,105). The efficiency decrease in the emulsion regime is caused by the greater vapor recycle (44,104,105). In general, there is otherwise little difference between mass transfer in the froth and emulsion regimes. 

Comment All the actual flow parameters exceed the flow parameters at the froth-to-emulsion transition. Therefore, all these checks indicate emulsion regime operation. 

Section 6.2.11 recommends the use of Fair s entrainment correlation in the froth (and emulsion) regime. This section also states that entrainment in the emulsion regime is unlikely to be a problem. Fair s correlation (Fig. 6.16) predicts fractional entrainment (pound of entrained liquid per pound of liquid) of 0.0 ll and 0.0075 at 80 percent of flood for the top and bottom trays, respectively. Even at 90 percent of... 

Summary. The third trial checks well against the various hydraulic criteria. Column capacity is limited by downcomer backup flood in the bottom section center-to-side trays (i.e., side downcomers). All trays will operate in the emulsion regime. 

In the emulsion regime [high pressure (> 150 psia) and/or high liquid rates], vapor entrainment through the downcomer (Sec. 6.4.5) is not large enough to affect efficiency. 

O W emulsions, regime TI if rjc is not much higher than that of water for a higher viscosity it tends to be regime TV, especially if the resulting d is very small ... 

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