Effect Of Tray Spacing

source: From H. Z. Kister, Distillation Operation, Copyright © 1990 by McGraw-Hill Inc. Reprinted by permission.

Low tray spacing also enhances the tendency of all types of flooding other than downcomer choke flooding. As tray spacing diminishes, drops have to travel a shorter distance to be entrained (spray entrain-ment flooding), the froth envelope becomes closer to the tray above (froth entrainment flooding), and a lower downcomer backup is sufficient to cause flooding. Tray spacing has little effect on either downcomer liquid velocity or downcomer froth density, and therefore, on downcomer choke flooding.

High weirs and small weir lengths reduce spray action and therefore slightly decrease the tendency for spray entrainment flooding, but they increase the height of the froth envelope and therefore increase the tendency for froth entrainment flooding. They also increase liquid height on the tray and tray pressure drop and, therefore, downcomer backup. Weir height and length have little effect on either downcomer liquid velocity or downcomer froth density, and therefore, on downcomer choke flooding.

The other two parameters, small clearance under the downcomer and small downcomer top area, have little effect on entrainment flooding, as they are associated with the downcomer only. Downcomer clearance affects downcomer backup, but not downcomer liquid velocity, while downcomer area affects the velocity, but has little effect on downcomer backup.

6.2.6 Entrainment (Jet) flooding

Entrainment flooding can be classified into spray entrainment flooding and froth entrainment flooding. Spray entrainment flooding is far more common. Froth entrainment flooding is only encountered when • Sec. 6.2.4)

■ Tray spacing is smalt: At tray spacing lower than 18 in, either spray entrainment flooding or froth entrainment flooding can restrict tray capacity, with froth entrainment flooding becoming the more likely mechanism as tray spacing diminishes and/or liquid load increases.

■ Conditions favor vapor cross flow: These are the only conditions under which froth entrainment flooding is likely to be encountered when tray spacing exceeds 18 in.

The entrainment flooding prediction methods described below are based primarily on the spray entrainment flooding mechanism. Considerations unique to froth entrainment flooding prediction are presented later in the section.

Spray entrainment flooding prediction. Most available entrainment flooding prediction methods derive from the original work of Souders and Brown (13). Souders and Brown theoretically analyzed entrainment flooding in terms of droplet settling velocity. Flooding occurs when the upward vapor velocity is high enough to suspend a liquid droplet, giving Eq. (6.9)

From Eq. (6.9) the Souders and Brown flooding constant, CSB, can be defined

The Souders and Brown constant, CSB, is the C-factor [Eq. (6.4)] at the flood point. Most modern entrainment flooding correlations retain the Souders and Brown equation (6.10) as the correlation basis, but depart from the notion that CSB is a constant. Instead, they express CSB as a function of several variables (below), which differ from one correlation to another. Depending on the correlation, CSB and uS flood are based either on the net area Ajv or on the bubbling area AB.

Variables affecting the capacity factor CSB

■ Tray spacing: CSB rises with tray spacing. Roughly, CSB is proportional to the tray spacing to a power of 0.5 to 0.6 (7,8,14-17). At low tray spacing (< 15 in), the power may be somewhat higher due to the proximity of the froth envelope and/or excessive splashing from the dispersion at the tray.

■ Liquid load: Figure 6.9a is a plot of flood i1-factor against liquid load. From Eq. (6.5), it also illustrates the effect of liquid load on the flood C-factor, or CSB. As liquid load increases, CSB first rises, then declines. The decline is slow, as illustrated by the cyclohexane-n-heptane curves in Fig. 6.9a. Some of the earlier flood correlations (18-20) predict that CSB rapidly diminishes at high liquid loads (as illustrated for the butane curves in Fig. 6.9a). Later work, first reported by Gerster et al. (21) and then by many others (15,22-26), however, showed that the rapid decline (such as that for the butane system) is associated with downcomer flooding, and not with entrainment flooding.

The increase in CSB with liquid loads at very low liquid loads co-

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  • cassio
    What is the more common type of floodingdowncomer or entrainment flooding?
    8 years ago

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