Froth flow over weir

Downcomer operation is often described in terms of a non-flooded downcomer, where complications arise from the cascade of froth flowing over the weir onto a froth layer in the downcomer, causing entrainment into-, and gross circulation of the froth layer, in a manner analogous to a waterfall. 

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 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... 

As noted, the weir crest is calculated on an equivalent clear-liquid basis. A more realistic approach is to recognize that in general a froth or spray flows over the outlet weir (settling can occur upstream of the weir if a large calming zone with no dispersers is used). Bennett et al. [AIChE J., 29, 434 (1983)] allowed for froth overflow in a comprehensive study of pressure drop across sieve plates their correlation for residual pressure drop /jf in Eq. (14-87) is represented by Eqs. (14-115) through (14-120) ... 

In fact, the operation of a downcomer at the point of flooding can be simply illustrated froth from the active area flows over the weir onto the downcomer froth. The bulk of the vapour disengagement occurs probably in a very short time, and only the small bubbles in the froth are entrained downwards. Within the downcomer, the bubbles coalesce until they are large enough to rise out from the froth. If coalescence is not fast enough, some of the vapour is carried down to the tray below. Even with this simpler picture, the process is not easily modelled. 

Figure 6.5 illustrates the classical hydraulic model of a fractionation tray. Liquid enters the tray from the downcomer of the tray above. The liquid entering the tray is aerated with vapor rising from the tray below to form froth on the tray. The froth flows across the tray until it reaches the outlet weir. The froth then flows over the weir into the downcomer, where the vapor is disengaged from the liquid,... 

Mechanism, Vapor enters the downcomer with the froth that flows over the outlet weir. Additional vapor is entrained into the liquid due to the impact of the falling liquid on the liquid surface in the downcomer, in the same manner as a waterfall induces air entrainment into the pool below it. Inside the downcomer, vapor disengages from the liquid due to its higher buoyancy. The driving force for vapor disengagement is the density difference between the liquid and the vapor. 

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 order to prevent downflow flooding, the trays must be adequately spaced and the downcomers must be appropriately sized. It is customary in many designs to take the tray spacing S to be equal to twice the liquid holdup H in the downcomer (S = 2H). However, this choice of S may be more conservative than necessary or it may be inadequate. If it is known that the flow over the outlet weir is relatively free of vapor, then the tray spacing may be taken approximately equal to H. On the other hand, if it is known that the downcomer is filled with froth having a much lower density than the liquid, a tray spacing equal to as... 

In a trayed absorber the amine falls from one tray to the one below in the same manner as the liquid in a condensate stabilizer (Chapter 6, Figure 6-4). It flows across the tray and over a weir before flowing into the next downcomer. The gas bubbles up through the liquid and creates a froth that must be separated from the gas before it reaches the underside of the next tray. For preliminary design, a tray spacing of 24 in. and a minimum diameter capable of separating 150 to 200 micron droplets (using the equations developed in Volume 1 for gas capacity of a vertical separator) can be assumed. The size of packed towers must be obtained from manufacturer s published literature. 

In the froth flotation cell used for coal washing, illustrated in Figure 1.48, the suspension contains about 10 per cent of solids, together with the necessary reagents. The liquid flows along the cell bank and passes over a weir, and directly enters the unit via a feed pipe and feed hood. Liquor is discharged radially from the impeller, through the diffuser, and... 

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 ... 

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. 

In principle, a colrrrrm tray can be operated even with very small liquid loads because the necessary height of the two-phase layer (froth) on the tray is provided by the exit weir. At extremely low liquid loads, however, the liquid will flow in an uneven pattern across the tray resulting in some degree of maldistribution of liquid. Accordingly, it is recommended to ensure a ttunimum liquid flow rate over the exit weir larger than Fl/ w 2 m /(m h). In small diameter columns, however, the liquid load can be considerably lower. 

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