Clear Liquid Height and Froth Density

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- 

Sieve trays, troth regime. Most dear liquid height and froth density correlations (35,68,81-86) are based on the Francis weir formula. A correlation by Colwell (68), based on a model of froth flow over the weir, was demonstrated to agree with experimental data better than other published correlations. Colwell s correlation is recommended by the author and by Lockett (12), and was successfully used as a building block in weeping correlations (56,63,69) and in froth regime entrainment correlation (40). Colwell s correlation is 

Some trial and error is required in this calculation, because the clear liquid height [Eq. (6.61)] and the froth density [Eqs. (6.64) to (6,66)] depend on each other. Under weeping conditions, additional trial and error is required because the weep fraction fw depends on the clear liquid height (Sec. 6,2.12). 

Strictly, the Jeronimo and Sawistowski correlation predicts clear liquid heights at the froth to spray regime transition. However, it has been shown (27,92) that clear liquid height in the spray regime is much the same as clear liquid height at that transition. 

Valve trays. Development of a clear liquid height correlation for valve trays has been inhibited by difficulties in measurement of clear liquid heights on these trays. A number of correlations have been proposed (86,93-95), but questions have been raised about their applicability (12,86,87). None has been tested against a sufficiently large data base 

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Colwell CJ (1981) Clear liquid height and froth density on sieve trays, Ind. Eng. Chem. Process Des. Dev., 20, 298-307. 

The weir height, h, is in centimeters and the weir length, is in meters. The liquid flow rate across the tray, is in cubic meters per minute. The relative froth density on the tray, ( ), defined as the ratio of clear liquid height to froth height, is given by another empirical equation ... 

The Chan and Fair correlation uses the Bennett et al. correlation tSec. 6.3.3) for calculating the clear liquid height and the froth density < > [Eqs. (6.56) and (6.58)]. For calculating Chan and Fair use a correlation by Foss and Gerster (138). 

A simple additive model is normally used to predict the total pressure drop. The total is taken as the sum of the pressure drop calculated for the flow of vapour through the dry plate (the dry plate drop hj) the head of clear liquid on the plate (hw + how) and a term to account for other, minor, sources of pressure loss, the so-called residual loss hr. The residual loss is the difference between the observed experimental pressure drop and the simple sum of the dry-plate drop and the clear-liquid height. It accounts for the two effects the energy to form the vapour bubbles and the fact that on an operating plate the liquid head will not be clear liquid but a head of aerated liquid froth, and the froth density and height will be different from that of the clear liquid. 

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... 

Does downcomer flooding occur when the downcomer clear liquid level reaches the tray above As a matter of fact, the flooding occurs earher than the point of clear liquid reaching the tray above. This is because there is a layer of liquid froth on top of the clear Uquid. As soon as the froth reaches the tray above, flooding happens as froth carries significant amount of liquid to the tray above. Thus, the total liquid height including froth level is H = Ha/ < 1)> where is the downcomer froth density. To be conservative, we assume the downcomer backup limit is 80% and thus downcomer flood capacity is expressed as... 

However, Bernard and Sargent (1966) showed that the estimate of clear liquid height from pressure drop calculations could be unsatisfactory for some applications. Instead, he could be calculated from froth height (Af) based on froth density [Pg.245]

The clear liquid height on the tray, /t is calculated by Equation 14.9, and the relative froth density, 0, by Equation 14.10. The mass transfer coefficients are calculated from the following empirical correlations ... 

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. 

The amount of liquid on the plate increases with the weir height and with the flow rate of liquid, but it decreases slightly with increasing vapor flow rate, because this decreases the density of the froth. The liquid holdup also depends on the physical properties of liquid and vapor, and only approximate methods of predicting the holdup are available. A simple method of estimating h, uses the weir height h, the calculated height of clear liquid over the weir and an empirical correlation factor /3 ... 

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