Glitsch Downcomer Backup Equation

^"Distillation Theory and Practice—an Intensive Course," University of New South Wales/University of Sydney, August 9-11, 1977.

f Glitsch, Inc., Ballast Tray Design Manual, 6th ed., 1993; available from Koch-Glitsch LP, Wichita, Kans. ^ R. H. Perry and D. W. Green (eds.), Perry's Chemical Engineers' Handbook, 7th ed., McGraw-Hill, 1997. § The author believes that low-molecular-weight hydrocarbons refers to light hydrocarbons at near atmospheric pressure or under vacuum. The foam stability of light-hydrocarbon distillation at medium and high pressure is best inferred from the Glitsch criterion.

To convert from lb/ft3, to kg/m3, multiply by 16.0.

source: From H. Z. Kister, Distillation Design, copyright © 1992 by McGraw-Hill, Inc.; reprinted by permission.

^"Distillation Theory and Practice—an Intensive Course," University of New South Wales/University of Sydney, August 9-11, 1977.

f Glitsch, Inc., Ballast Tray Design Manual, 6th ed., 1993; available from Koch-Glitsch LP, Wichita, Kans. ^ R. H. Perry and D. W. Green (eds.), Perry's Chemical Engineers' Handbook, 7th ed., McGraw-Hill, 1997. § The author believes that low-molecular-weight hydrocarbons refers to light hydrocarbons at near atmospheric pressure or under vacuum. The foam stability of light-hydrocarbon distillation at medium and high pressure is best inferred from the Glitsch criterion.

To convert from lb/ft3, to kg/m3, multiply by 16.0.

source: From H. Z. Kister, Distillation Design, copyright © 1992 by McGraw-Hill, Inc.; reprinted by permission.

upflow are greatly reduced. With sloped downcomers, the downcomer bottom area is normally set at 50 to 60 percent of the top downcomer area. This taper is small enough to keep the downcomer top area the prime choke variable.

There is no satisfactory published correlation for downcomer choke. The best that can be done in the absence of data or correlation is to apply the criteria for maximum velocity of clear liquid at the downcomer entrance. Kister (Distillation Operation, McGraw-Hill, New York, 1990) surveyed the multitude of published criteria for maximum downcomer velocity and incorporated them into a single set of guidelines (Table 14-7). The values for 30-in spacing were revised to reflect the author's recent experiences. The values given in Table 14-7 are not conservative. For a conservative design, multiply the values from Table 14-7 by a safety factor of 0.75. For very highly foaming systems, where antifoam application is undesirable, there are benefits for reducing downcomer design velocities down to 0.1 to 0.15 ft/s.

Another criterion sometimes used is to provide sufficient residence time in the downcomer to allow adequate disengagement of gas from the descending liquid, so that the liquid is relatively gas-free by the time it enters the tray below. Inadequate removal of gas from the liquid may choke the downcomer. Kister (loc. cit.) reviewed various published criteria for downcomer residence times and recommended those by Bolles (private communication, 1977) and Erbar and Maddox (Maddox, Process Engineer's Absorption Pocket Handbook, Gulf Publishing, Houston, 1985). Both sets of guidelines are similar and are summarized in Table 14-8. The residence times in Table 14-8 are apparent residence times, defined as the ratio of the total downcomer volume to the clear liquid flow in the downcomer.

As a segmental downcomer becomes smaller, its width decreases faster than its length, turning the downcomer into a long, narrow slot. This geometry increases the resistance to liquid downflow and to the upflow of disengaging gas. Small downcomers are also extremely sensitive to foaming, fouling, construction tolerances, and the introduction of debris. Generally, segmental downcomers smaller than 5 percent of the column cross-sectional area should be avoided. Additional discussion of small downcomers is available (Kister, Distillation Operation, McGraw-Hill, New York, 1990).

Derating ("System") Factors With certain systems, traditional flooding equations consistently give optimistic predictions. To allow for this discrepancy, an empirical derating or system factor (SF < 1.0) is applied. To obtain the actual or derated flood load, the flood gas load (entrainment flooding) or flood liquid load (downcomer choke) obtained from the traditional equations is multiplied by the derating factor. In the case of downcomer backup flood, the froth height from the traditional flood equation is divided by the derating factor.

Derating factors are vaguely related to the foaming tendency, but are also applied to nonfoaming systems where standard flooding equations consistently predict too high. Sometimes, derating factors are used solely as overdesign factors. Brierley (Chem. Eng. Prog., July 1994, p. 68) states that some derating factors actually evolved from plant misop-eration or from misinterpretation of plant data. Kister (loc. cit.) compiled the derating factors found in the literature into Table 14-9.

The application of derating factors is fraught with inconsistent practices and confusion. Caution is required. The following need to be carefully specified:

TABLE 14-7 Maximum Downcomer Velocities

1. The flooding mechanism to which the derating factor applies (entrainment, downcomer backup, downcomer choke, or all these) must be specified.

2. Avoiding double derating. For instance, the values in Table 14-9 may apply with Eq. (14-81) because Eq. (14-81) does not take foami-ness into account. However, they will double-derate a flood calculation that is made with a correlation or criteria that already take foaminess into account, such as the criteria for downcomer choke in Tables 14-7 and 14-8. Similarly, two different factors from Table 14-9 may apply to a single system; only one should be used.

3. Derating factors vary from source to source, and may depend on the correlation used as well as the system. For instance, some caustic wash applications have a track record of foaming more severely than other caustic wash applications (see note in Table 14-9). The derating factors in Table 14-9 are a useful guide, but are far from absolute.

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