Froth density, tray

Figure 8-149. Correlation for aerated-tray-liquid pressure drop developed from published data for various valves. Note (j> = relative froth density. Reference numbers are from original article [201 ]. Used by permission, Klein, G. F., Chem. Eng. V. 89, No. 9 (1982), p. 81 all rights reserved.

The work of Calderbank and Rennie (C4) has been criticized by Sargent and Macmillan (S2) on the basis that the liquid flow conditions used by Calderbank and Rennie (C4) are not found in distillation columns. They (S2) consider that cellular foams are formed for dilute aqueous solutions only when low gas flow rates are employed. By using an n-pentane-isopentane system, Macmillan (Ml) found that for all gas flow rates, froths with densities less than 0.15 were formed and the froth densities were independent of the factor vs(pg)112 but dependent on tray geometry. The associated problem of foam stability has also attracted considerable attention (Al, D3, Zl). 

The clear liquid backup is divided by the froth-density to give the froth height if this exceeds the tray spacing plus the outlet weir height, the tray is deemed to be flooded. 

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... 

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 ... 

Sieve trays will be used with 60 cm spacing, 6 cm weir height, 0.6 cm hole diameter, 0.25 cm tray thickness, 5 cm downcomer clearance, and hole area 10% of the total tray area. The foaming factor is 0.80 and the froth density in the downcomer is 0.5. The target fraction of flood velocity is 0.70. 

Even though froth actually flows over the weir (unless calming zones are nsed), h is expressed on an eqnivalent clear liqnid basis, assuming that the Francis relationship also represents froth flow. Later, in connection with mass transfer in tray froths, we will discnss a relative froth density ... 

In the froth regime, an increase in vapor flow reduces tray froth density. Froth height above the weir rises, and some of the tray liquid inventory spills over the weir into the downcomers. The expelled liquid ends up in the bottom of the column, and bottom level initially rises (Fig. 16.5). This is opposed to the expected response, and is termed inverse response. 

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... 

Downcomer Backup Limit The maximum downcomer liquid height allowable is set by the tray spacing and outlet weir height. Downcomer backup flooding occurs when the liquid froth in the downcomer reaches the tray above. Assume the maximum backup limit as 80% and the froth density q) = 0.6 (Kister, 1992). Applying equation (12.23) yields... 

Pm valve metal density Ib/ft a surface tension dyne/in. fji liquid viscosity cP Tr downcomer residence time s (second) downcomer froth density dimensionless (p[ froth density dimensionless (pt relative froth density dimensionless valve open percentage % rjM Murphree tray efficiency dimensionless r]o column overall efficiency dimensionless... 

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

When froth exists, the froth density is a function of the heat transfer. As a result, apparent liquid levels often vary as steam flow varies. This can result in operational problems and flooding of systems, especially if trays are provided in the evaporator body. Excessive entrainment can also result. 

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 ... 

Froth density in the downcomer Relative froth density on the tray Fractional entrainment... 

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