Clear liquid height downcomer

The clear liquid back-up is obtained from a tray-pressure balance and is normally taken to be the sum of the tray-pressure-drop, the clear liquid height on the active area of the tray, and the pressure-drop of liquid flowing under the downcomer apron onto the active area. 

Downcomer backup flooding occurs when the backup of aerated liquid in the downcomer exceeds the available tray spacing. Downcomer backup can be calculated by adding the clear liquid height on the tray, the liquid backup caused by the tray pressure drop, and the liquid backup caused by the friction loss at the downcomer outlet. The downcomer backup is then divided by an aeration factor to give the aerated liquid backup. 

The clear liquid height, or the liquid holdup, is the height to which the aerated mass would collapse in the absence of vapor flow. The clear liquid height gives a measure of the liquid level on the tray, and is used in efficiency, flooding, pressure drop, downcomer backup, weep-... 

Seal check. In Sec. 6.5.7, it was decided to go to clearances under the downcomers that exceed the outlet weir heights. For such designs, it has been recommended (1) that the clear liquid height in the downcomer under turndown conditions exceeds the downcomer clearance by at least 2 in. This will be checked here. 

The downcomer backup (clear liquid height basis) should be 40% or less of the tray spacing for high-vapor-density systems (>3.01b/ft, 48.1 kg/m ), 50% or less for medium-vapor-density systems, and 60% or less for low-vapor-density systems (<1.01b/ft, 16.0 kg/m ). 

The liquid and froth flowing over the weir partially fill the downcomer, creating a backup of height h f (Figure 14.1). If the overall relative froth density in the downcomer is (t),i, the equivalent clear liquid height is... 

In a reverse-flow ("half-pass ) tray (Fig. 6.9), liquid is forced to flow around a central baffle. Both the downcomer and the downcomer seal area are on the same side of the tray, and the liquid flow path is quite long. This tray is mainly suitable for low-liquid-flow-rate applications. Making the baffle at least twice the height of the highest calculated clear liquid height on the tray has been recommended (48) in order to avoid short-circuiting of liquid. For the same reason, leakage under the bafile should also be minimized. 

Here, denotes the relative hquid holdup in the downcomer, the height of downcomer clearance, the pressure loss of a tray expressed in clear liquid height, and the clear liquid height on the tray (see Sect. 5.4.1.3). The orifice dis-... 

Clearly, downcomer backup can be estimated if we know the clear liquid height H. However, it is not straightforward in predicting H. As consists of three components, namely, clear liquid height h, tray pressure drop /r and downcomer apron pressure drop h, let us look into each individual components one at a time. 

With relatively high outlet weir height hw, the clear liquid height h increases and vapor and liquid contact time increases. This improves distillation efficiency. However, a too high outlet weir height could affect the downcomer backup and tray capacity. 

Downcomer Backup% According to equation (12.22), the total clear liquid height in downcomer is... 

Downcomer flood is controlled by a combination of tray pressure drop, weir loading, and downcomer apron. Remember that it is the liquid froth level instead of clear liquid height that determines the downcomer flood. This fact is clearly stated in equation (12.24). To prevent downcomer flood in the design, downcomer size must be provided generously. From equations (12.40), (12.42), and (12.44), feed rate and reboiling duty are the major operating parameters to avoid downcomer flood. 

Zda clear liquid height for pressure drop through downcomer apron in. of liquid /Zf froth height on tray in. of froth... 

Zg hydraulic gradient on tray (high minus low clear liquid height) in. of liquid h pressure drop through aerate liquid on tray in. of liquid liquid head over the outlet weir in. of liquid downcomer seal in. tray pressure drop in. of liquid... 

The backup of clear liquid during flowing conditions must be determined in order to set the proper tray spacing. Tray spacing is usually set at twice the liquid height in the downcomer. This can be adjusted to suit the particular system conditions. 

Once foam or froth in the downcomer backs up to the tray above, it tends to be re-entrained in the overflowing liquid, making it apparently lighter, and accentuating this height of liquid-foam mixture in the downcomer. The downcomer must be adequate to separate and disengage this mixture, allowing clear liquid (fairly free of bubbles) to flow under the downcomer seal. 

This is the case with diameter determination. The relation of Equation 8-250 for the perforated tray or sieve tray with downcomers can be used for the plate without downcomers. Generally, the liquid level and foam-froth height will be higher on this tray, hence the ralue of h., clear liquid on the tray, may range from 1-in. to 6-in. depending on the service. 

F = Free height in downcomer above clear liquid level (not froth level)... 

To predict the height of aerated liquid on the plate, and the height of froth in the downcomer, some means of estimating the froth density is required. The density of the aerated liquid will normally be between 0.4 to 0.7 times that of the clear liquid. A number of correlations have been proposed for estimating froth density as a function of the vapour flow-rate and the liquid physical properties see Chase (1967) however, none is particularly reliable, and for design purposes it is usually satisfactory to assume an average value of 0.5 of the liquid density. 

Calculation of the froth height in the downcomer requires consideration of two aspects, the clear liquid back-up and, the so called froth-density, which is actually the volume fraction of liquid in the froth. 

To summarize, the total height of clear liquid in the downcomer is the sum of four factors ... 

Figure 1332. Internals and mode of action of trays in tray towers, (a) Some kinds of bubblecaps (Glitsch). (b) Two kinds of valves for trays, (c) Vapor directing slot on a Linde sieve tray [Jones and Jones, Chem. Eng. Prog. 71, 66 (1975)]. (d) Vapor flow through a bubblecap. (e) Sieve tray phenomena and pressure relations hh is the head in the downcomer, h, is the equivalent head of clear liquid on the tray, hf is the visible height of froth on the tray, and h, is the pressure drop across the tray (Bolles, in Smith, 1963). (f) Assembly of and action of vapor and liquid on a bubblecap tray.

The heights of head losses in Eq. (14-92) should be in consistent units, e.g., millimeters or inches of liquid under operating conditions on the tray As noted, hdc is calculated in terms of equivalent clear liquid. Actually the liquid in the downcomer is aerated and actual backup is... 

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