Downcomer Apron

The f-factor is the square root of the kinetic energy of the vapor. The velocity in Eq. (6.2) is usually (but not always) based either on the bubbling area Aq or on the net area A^. The user must beware of any data for which the area basis is not clearly defined. In some cases, the hole F-factor, Fh, is used. This parameter is based on the hole velocity, uh, and is given by

A term even more suitable for describing vapor loads than the f-factor is the C-factor, CS) defined as

The C-factor is related to VLOAD and to the f-factor by

The C-factor, Cs, is also usually based either on the bubbling or the net area. The inconsistency regarding the area basis of the f-factor extends to the C-factor. The user must beware of any data for which the area basis is not clearly defined. The C-factor has the same units as velocity (feet per second) and directly relates to droplet entrapment (Sec. 6.2.6). In the author's experience, the C-factor is by far the best vapor load term for comparing capacities of systems of different physical properties.

Tray liquid load definitions. For tray (as distinct from downcomer) design the liquid load is usually defined as

GPM of liquid GPM 1 length of outlet weir, in Lw

In concept, this definition describes the flux of liquid across the tray. An alternative definition sometimes used for describing liquid load is the flow parameter, Flv

The flow parameter is more suitable for packed columns (12), where vapor-liquid counterflow exists, but has also been used in tray columns. For tray columns the author prefers QL [Eq. (6.6)] to the flow parameter.

Downcomer liquid load definitions. For downcomer design, the liquid load is usually defined as q m GPM of liquid = GPM g)

Downcomer entrance area, ft2 AD

Qd is the velocity of clear liquid at the downcomer entrance, expressed in gpm per square foot. In some cases, this liquid load is expressed in feet per second instead of gpm per square foot.

6.2.4 Tray flooding mechanisms

Flooding is excessive accumulation of liquid inside the column. This accumulation is generally caused by one of the following mechanisms.

Spray entrapment flooding (Fig. 6.7a). At low liquid flow rates, trays operate in the spray regime, where most of the liquid on the tray is in the form of liquid drops (Figs. 6.25c£ and 6.276). As vapor velocity is raised, a condition is reached where the bulk of these drops are entrained into the tray above. The liquid accumulates on the tray above instead of flowing to the tray below.

Froth entrapment flooding (Fig. 6.76). At higher liquid flow rates, the dispersion on the tray is in the form of a froth (Figs. 6.25c and 6,27c). When vapor velocity is raised, froth height increases. When tray spacing is small, the froth envelope approaches the tray above. As this surface approaches the tray above, entrainment rapidly increases, causing liquid accumulation on the tray above.

When the tray spacing is large (> 18 to 24 in), the froth envelope seldom approaches the tray above. As vapor velocity is raised, a con-

What Downcomer Apron

.-Backup clue toirav pressure drop, P, clue to iric;ion under DC. hja

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L<quid hacked up due to DC entrance restriction

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Figure 6.7 Common flooding mechanisms in tray columns, (a) Spray entrainment flood; (6j froth entrainment flood; (c) downcomer backup flood; (ci) downcomer choke flood. [Parts a and b reproduced from Dr. D. C. Hausch, Discussion of Papers Presented in the Fifth Session, Proceedings of the International Symposium on Distillation, the Institution of Chemical Engineers (London), i960, reprinted courtesy of the Institution of Chemical Engineers, UK. Parts c and d from H. Z. Kister, Distillation Operation. Copyright C 1990 by McGraw-Hill, Inc.; reprinted by permission. )

dition is reached when some of the froth inverts into spray. Flooding will then take place by the previously described spray entrainment mechanism.

At high liquid rates (>6 gpm/in of outlet weir), high ratio (>2.5) of flow-path length to tray spacing, and a high fractional hole area (> 11 percent), cross flow of vapor in opposite direction to the liquid can build up froth near tray inlet and center. The froth buildup raises the liquid head in the tray inlet and center. This channels more vapor to the tray outlet region, thus accelerating the cross flow. The inlet froth keeps rising until it reaches the tray above.

Downcomer backup flooding (Fig. 6.7c). Aerated liquid is backed up into the downcomer because of tray pressure drop, liquid height on the tray, and frictional losses in the downcomer apron. All of these increase when liquid flow rate is raised, while tray pressure drop also increases when vapor flow rate is raised. When the backup of aerated liquid in the downcomer exceeds the tray spacing, liquid accumulates on the tray above, causing downcomer backup flooding.

Downcomer choke flooding (Fig. 6.7d). As liquid flow rate increases, so does the velocity of aerated liquid in the downcomer. When this velocity exceeds a certain limit, friction losses in the downcomer and downcomer entrance become excessive, and the frothy mixture cannot be transported to the tray below. This causes liquid accumulation on the tray above.

$¿.5 Factors affecting flooding

Effect of pressure and L/V. Figure 6.8 is a rough application chart showing the effect of pressure and L/V on the mechanism of flooding. This chart does not take into account the tray and downcomer geometry, type of system, and operating conditions, all of which strongly influence the flooding mechanism. For this reason, the chart is statable for defining general guidelines only.

Low pressures favor high vapor velocities and low liquid flow rates and, therefore, spray regime dispersions. Flooding in vacuum columns and in columns operating at a low liquid-to-vapor ratio is usually caused by the spray entrainment mechanism.

At high pressures, the difference between vapor and liquid density

Vacuum Distillation Alcohol

Figura 6.8 A rough flooding mechanism application chart. (From H. Z. Kisier, Distillation Operation, Copyright © 1990 by McGraw-Hill, Inc.; reprinted by permission.)

becomes smaller, and separation of vapor from liquid in the down-comer becomes difficult. Because of the more difficult separation, downcomer aeration increases, raising both downcomer frictional losses and froth backup in the downcomer. High liquid flow rates also increase tray pressure drop, tray liquid level, and frictional losses in the downcomer. For these reasons, downcomer flooding is favored at high pressures and high liquid flow rates.

At moderate pressures and moderate L/V ratios, the dispersion on the tray tends to be in the froth regime, and any of the above mechanisms can dominate. Generally, at low tray spacing (<12 to 15 in) froth entrainment flooding is favored. At higher tray spacing, and when conditions do not favor vapor cross flow (see above), the froth regime will turn into a spray as vapor velocity increases, and spray entrainment flooding is favored. Finally, when downcomers are small or downcomer backups are high, downcomer flooding is favored.

Effect of design parameters. A number of design parameters have a far greater effect on one flooding mechanism than on others. These parameters are listed in Table 6.2.

Low bubbling areas or low fractional hole areas enhance the flooding tendency of all types of flooding except downcomer choke. Low bubbling areas and low fractional hole areas generate high vapor velocities, thus enhancing entrainment, pressure drop, and downcomer backup. These parameters have little effect on downcomer liquid velocity or downcomer froth density, and therefore, on downcomer choke flooding.

TABLE 6.2 Effect of Tray Geometry on Various Types of Flooding

Design parameters that lower the flooding point

Spray entrainment flooding

Froth 'entrainment flooding

Downcomer backup flooding

Downcomer choke flooding

Low bubbling area Low fractional hole area (<8%) Low tray spacing High weirs (> 4 in) Small weir length Small clearance under downcomer Small downcomer top area

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