Liquid Residence time Downcomer

In current design practice for downcomers, three parameters are commonly considered liquid residence time, liquid velocity, and downcomer backup. 

The residence-time concept is commonly misunderstood. The residence time is defined here as the downcomer volume divided by the liquid flow-rate. Typically, it is said that a liquid-residence time is required to allow adequate disengagement of vapour. Generally, two mechanisms are at work in a downcomer to provide the separation of vapour from liquid. The more obvious one is the relative velocity of the phases. If the downward velocity of the liquid exceeds the bubble-rise velocity, it does not matter how much residence time is provided, separation will not occur. This is true unless there is coalescence (the second mechanism), which there always is, to some extent. Coalescence is time dependent and therefore a residence-time criterion has some relevance. 

To avoid downcomer choke flooding, downcomers are sized to give a liquid residence time of not less than 3-7 sec, depending on the tendency of the liquid to form a stable foam. 

Downcomer unaerated liquid velocity 0.5 m/s head loss via underflow clearance <0.3 kPa. Allow 3 s liquid residence time and extend to 6 s for foaming systems. Weir overflow velocity = 5 to 20 L/s m of outlet weir. 

If the liquid is unable to flow down the downcomer fast enough, the liquid level will increase, and if it keeps increasing until it reaches the top of the weir of the tray above, the tower will flood. This downcomer flooding must be prevented. Downcomers are designed on the basis of pressure drop and liquid residence time, and their cost is relatively small. Thus, downcomer design is done only in the final equipment sizing. 

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... 

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. 

Sufficient residence time must be allowed in the downcomer for the entrained vapour to disengage from the liquid stream to prevent heavily aerated liquid being carried under the downcomer. 

The downcomer area required for trays not only increases with the liquid-flow-rate, but also with the difficulty in achieving separation between the vapour and the liquid phases. The volume required for the downcomer (downcomer residence time) increases at a lower surface tension and a smaller density difference between vapour and liquid. Because of the large downcomer area required to handle the high liquid flow rates typical of high-pressure distillations, a trayed column cross-sectional area may be 40% to 80% greater than the active tray area calculated from the vapour flow rates for such distillations. Thus, the downcomer area becomes a significant factor in the determination of the diameter of a tray column. 

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. 

Liquid mixing time decreases sharply for an initial increase in the gas sparging rate and approach an asymptotic value that is determined by the height and diameter of the downcomer and the liquid properties [5]. A higher liquid velocity shortens the gas residence time and results in a decrease of gas holdup and interfacial area. The radial profile of the liquid is parabolic. These are disadvantageous for mass transfer. The mounting of internals in a fixed bed is often used to improve the radial profile of the liquid velocity. This motivates us to mount internals in an EL-ALRs to improve the radial profile of the gas holdup and the liquid velocity and to intensify turbulence. 

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 apparent residence time is the ratio of the downcomer volume to the clear liquid flow in the downcomer, The downcomer volume is the tray spacing times the average downcomer cross-section area. The true residence time is the ratio of froth volume in the downcomer to the frothy liquid flow in the downcomer. The true residence time can alternatively be expressed as the ratio of the clear liquid volume in the downcomer to the clear liquid flow. The definition adopted here is that of the apparent downcomer residence time. The author found this definition to be better and to be consistent with riublished criteria for downcomer residence times (1]. 

Weir heights of 2 in. are fairly standard and weir lengths about 75% of the tray diameter. For normal conditions downcomers are sized so that the depth of liquid in them is less than 50% and the residence time more than 3 sec. For foaming and foam-stable systems, the residence time may be two to three times this value. The topic of tray efficiency is covered in detail in Section 13.6, but here it can be stated that they are 80-90% in the vicinity of F = = 1.0(ft/sec)(lb/cuft) for mixtures similar to water... 

When the liquid has foaming tendencies, additional residence time must be allowed for phase disengagement. System discount factors, recommended by Koch-Glitsch, Inc. on the basis of field experience, are shown in Table 12.7. As an example, the maximum allowable velocity of 0.1 m/s would be multiplied by the discount factor so as to provide adequate downcomer volume. 

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. 

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