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1. The flooding mechanism to which the derating factor applies (en-trainment flood, downcomer backup, downcomer choke, or all).

2. Avoiding double derating. For instance, the values in Table 6.7 apply with the equations in Sec. 6.2.6, since these do not take foami-ness into account. However, they will double-derate the flood calculation if applied jointly with a correlation or criterion that already takes foaminess into account (e.g., the criteria for down-comer backup flooding or downcomer choke in Tables 6.4 to 6.6). Similarly, two different factors from Table 6.7 may apply to a single system; only one of them should be used.

3. Choosing the appropriate value for the derating factor. Derating factors vary from source to source, and may depend on the correlation used as well as the application. For instance, some caustic wash applications have a track record of foaming more severely than other caustic wash applications (see note in Table 6.7). The derating factors in Table 6.7 are useful as a guide, but are not absolute.

6.2.11 Entrapment

Entrainment (Figure 6.14) is liquid transported by the gas to the tray above. This liquid contains more of the less-volatile material than the tray above, and therefore it counteracts the mass transfer process and reduces tray efficiency. Other undesirable effects of entrainment are carryover of nonvolatile impurities upward to contaminate the overhead product and the possibility of damage to rotating machinery located in the path of the column overhead vapor.

Figure 6.14 Entrainment. [Reprinted courtesy of Fractionation Research Inc. (FRI).}

Mechanism. In the froth regime, most tray holes are bubbling; entrainment is produced by breakup of liquid sheets defining emergent bubble. The mechanism forms small drops (typically <200 p.m) at low projection velocities (52), resulting in low entrainment. In the fully developed spray regime, most holes are continuously jetting; entrainment is produced by atomization of liquid by gas jets passing through the tray holes. This mechanism forms large drops (typically >1000 p.m) at high projection velocities, resulting in high entrainment. When the spray regime is only partially developed on the tray, both bubbling and jetting occur simultaneously at adjacent tray holes. Entrainment produced by the holes undergoing jetting far outweighs that from holes which are bubbling. Entrainment in the partially developed spray region is therefore similar to entrainment in the fully developed spray regime (40).

Effect ct vapor velocity. Entrainment increases with vapor velocity to a high power (19,22,23,26,27,36,38-40,53,54). Most investigations report a power between 2 and 5, but in some cases as little as a 10 percent change in vapor rate results in a tenfold change in entrainment (22,36). Generally, smaller powers, indicative of a relatively gradual change, are typical of low-pressure systems, while high powers, which indicate a steep change, are typical of high-pressure systems.

At high pressure, the vapor velocity at which entrainment becomes significant tends to coincide with the flood point. Due to the rapid rise of entrainment with vapor velocity, it only takes a small additional velocity rise to escalate entrainment past the point of flooding initiation. Conversely, a small vapor velocity reduction lowers entrainment to a negligible extent. At low pressure, the rate of change of entrainment with vapor velocity is much slower, and entrainment can be significant even when the tray operates 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 (unless operated right near the flood point).

Effect of liquid rate. At low liquid rates, entrainment diminishes with higher liquid loads, while at high liquid rates entrainment increases with liquid loads (22,24,26,38,40,53-55; Fig. 6.15). When most of the dispersion is in the form of a spray, entrainment diminishes with higher liquid loads (22,24,27). The point at which the trend reverses, and entrainment begins to increase with liquid rate, has been interpreted either as the point where the dispersion changes from partially developed spray to froth (40,53), or where the dispersion changes from the spray to froth regime (22-24,45,55).

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