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•The author believes that "low-molecular-weight hydrocarbons" refers to light hydrocarbons at atmospheric conditions or under vacuum. The foaming tendency of light hydrocarbon distillation at medium pressure (> 100 psia) is medium; at high pressure (>300 psia) is high. source: Bolles, W. L. (Monsanto Company), private communication, 1977.

•The author believes that "low-molecular-weight hydrocarbons" refers to light hydrocarbons at atmospheric conditions or under vacuum. The foaming tendency of light hydrocarbon distillation at medium pressure (> 100 psia) is medium; at high pressure (>300 psia) is high. source: Bolles, W. L. (Monsanto Company), private communication, 1977.

creases tray pressure drop (2—4,18) and promotes the formation of stagnant regions near the tray periphery. Such stagnant regions may be detrimental to column efficiency (Sec. 7.3.3).

Generally, downcomers smaller than 5 to 8 percent of the column cross-section area should therefore be avoided (1,5,10,12,30,43,50). This rule is often expressed as a minimum ratio of weir length or downcomer width to column diameter. An alternative rule for small i <10 percent of column area) downcomers (7,51) is to set the minimum downcomer area to either twice the area calculated using the normal downcomer area criteria or 10 percent of the column cross-section area (whichever is smaller). Additional discussion on small downcomers is elsewhere (1).

If liquid flows are extremely small, and the system is nonfoaming, circular or envelope downcomers may be installed within the area subtended by a segmental weir (1,10). These should have twice the area calculated using the normal design procedure.

6.2.10 Derating ("system") factors

With certain systems, traditional flooding equations (e.g., Sees. 6.2.6 to 6.2.9) consistently predict higher flood points than those actually experienced. To allow for this discrepancy, an empirical derating or system factor SF (such that SF < 1.0) is applied. To obtain the actual or derated flood load for a given system, the predicted flood vapor load (entrainment flooding) or liquid load (downcomer choke) obtained from the traditional equation is multiplied by the derating factor. In case of downcomer backup flood, the froth height predicted from the correlation is divided by the derating factor. Derating factors are often vaguely related to the foaming tendency table 6.7 Derating Factors

System

Factor

Reference

Notes

Nonfoaming regular systems

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