Residence time in downcomer

Residence time in downcomers- Sufficient residence time must be provided in the downcomer to allow adequate disengagement of vapor from the descending liquid, so that the liquid is relatively vapor free by the time it enters the tray below. Inadequate removal of vapor from the liquid may choke the downcomer. 

The residence time in the downcomers is another criterion of adequate tray spacing. 

Allow at least 3 sec residence time in the downcomer for disengagement of vapor for a nonfoaming system, and 6 sec for a foaming system [238]. ... 

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. 

TABLE 14-8 Recommended Minimum Residence Time in the Downcomer... 

Downcomer residence time. This check calculates the apparent residence time in the downcomer and ensures it is within the recommended guidelines in Sec. 6.2.9. 

Example 5 Determination of holdup time in downcomer. A valve-tray tower with 24-in. plate spacing and liquid crossflow contains straight segmental downcomers. The overflow weir at the downcomer entrance is formed by an extension of the downcomer plate. The height of this weir is 3 in. The inside diameter D of the tower is 5 ft, and the weir length is 0. 60. If liquid with a density of 55 lb/ft3 flows across die plate at a rate of 30,000 lb/h, estimate the residence or holdup time in the downcomer from this plate. 

In summery, the downcomer can limit column capacity when liquid flow rains are high, as in absorbers and pressure fractionators. Two viewpoints are used (and these ate not necessarily independent of each other) height of froth baildup in the downcomer, obtained from a pressure balance, and residence time in the downcomer, obtained from an entrainment velocity limitation. When the downcomer backs up liquid, the vapor entrains more liquid, and a flooding condition can be approached. 

More importantly for operation, the gas residence time in the separator controls disengagement such that decreasing the gas residence time in this region would lead to an increase in the downcomer gas holdup, which could decrease the hydraulic pressure differential and liquid circulation velocity (Siegel et al., 1986). The negative effect would be if the gas residence time in the separator is too long such that gas easily disengages and causes a decrease in gas holdup and gas-liquid mass transfer (Al-Masry, 1999). 

The downcomer is designed to give a liquid residence time of three to seven seconds. Minimum residence times are listed in Table 10-2 fKister. 1980e. 1990 Ludwig, 1997). The residence time in a straight segmental downcomer is... 

For sloped downcomers, the critical liquid velocity is at the bottom, insofar as final disengagemmit of vapor is concerned. The total volume of the filled portion of the downcomer can be used in estimating residence time. For downcomers with bottom recesses, where the liquid rmist make an extra turn before entering the tray, the pressure loss under the downcomer may be estimmed as twice that calculated from Eq. (5.7-30). This rule of thumb applies also to the case where an inlet weir is used to distribute the liquid after it has flowed under the downcomer baffle. 

We can double check with the liquid residence time in the downcomer using equation... 

To provide reasonably adequate time for disengagement of foam and froth from the liquid in the downcomer, the total downcomer volume is checked against a minimum allowable average residence time of 5 seconds. 

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