Froth height, downcomer

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. 

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. 

CP Capacity parameter (packed F dc Froth height in downcomer mm in... 

Downcomer backup. The factors that resist liquid flow from the downcomer onto the tray below are the froth height on the tray, the pressure drop on the tray, and the friction loss under the downcomer apron. These factors cause liquid to back up in the downcomer. Each of these factors can be expressed in terms of clear liquid heads. A tray pressure balance gives... 

The froth height in the center downcomers in the bottom section is only slightly above 30 percent, and increasing the downcomer clearance will suffice to overcome the problem. However, this is unlikely to suffice for the side downcomers in the bottom section. In this example, idle clearance under the downcomer will be increased to 2.0 in in the center downcomers, and to 2.25 in in the side downcomers. The weir height on idle cenler-to-side flow trays in the bottom section will be lowered to 1.5 in to lower tray pressure drop. 

Example 18.6. A sieve-plate column operating at atmospheric pressure is to produce nearly pure methanol from an aqueous feed containing 40 mole percent methanol. The distillate product rate is 5800 kg/h. (a) For a reflux ratio of 3.5 and a plate spacing of 18 in., calculate the allowable vapor velocity and the column diameter. b) Calculate the pressure drop per plate if each sieve tray is in, thick with j-in, holes on a -in. triangular spacing and a weir height of 2 in. (c) What is the froth height in the downcomer ... 

Splash baffles are used in low-liquid-load services. The baffle backs liquid up onto the tray and increases its liquid holdup and froth height (31, 83, 374). The baffle also helps to prevent the tray from drying up and promotes froth regime operation at low liquid loads (374). In small-diameter columns (< 2 ft) it also prevents liquid drops formed at the tray inlet from being flung directly into the downcomer. 

The most common applications of this technique in distillation and absorption columns is for liquid level and liquid level interface detection, especially when normal level-measuring techniques suffer from plugging. Neutron backscatter techniques have also been used for froth height measurements on trays and downcomers, and for measuring the top and bottom of packed beds. One case history has been described (71) where downcomer froth height measurements using the neutron backscatter technique led to a detection of downcomer deposits which caused premature flooding of the column. The author is familiar with one case where this technique successfully detected overflow of a packed tower distributor. 

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. 

The height of froth in the downcomer is H

froth density. Assume the target value of 80% for the froth height in relation to the liquid setting height. For a stable operation, the liquid capacity must satisfy the condition defined in equation (12.23) ... 

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... 

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

A depropanizer normally contains 35 to 55 actual trays and has a uniform diameter. In these systems, the surface tension of the liquid phase is below 6 dyne/cm, the liquid density is near 30 Ib/ft, and the vapor density is about 8% of the liquid density. Under such conditions, the downcomer residence time required can be a significant factor in the specification of the tower diameter for a trayed column. The downcomer normally is designed so that the froth height is no more than 70% of the tray spacing therefore, downcomer area must be large to avoid the need for excessively tall columns. 

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. 

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. 

As the liquid level on a tray increases, the height of liquid in the downcomer feeding this tray will increase by the same amount. Again, excessive downcomer liquid or froth levels result in flooding and loss of tray efficiency. 

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