Froth height, tray

S" = clear height above foam or froth (equals tray spacing minus foam height above tray floor), ft p = viscosity of liquid, centipoise a = surface tension of liquid, dynes/cm We = entrainment (based on assumed allowance) lbs liquid/ (ft free plate area) (hr) hf = height of top of foam above tray floor, in. 

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. 

Valve hole area 1.65 sq. ft. (separate calculation) = ho Tray pressure drop and froth height... 

In the physical arrangement, make certain that the pressure balance level, plus an allowance for froth, establishes a height that is below the bottom tray of the column to avoid flooding the column. In addition, the estimated froth height on top of the liquid should still be below the level of the vapor return from the reboiler. 

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. 

HFio, HF45, Hf = effective tray froth height, in... 

Step 2. Next the number of gas phase transfer units NG is to be calculated. First froth height in inches HF must be calculated. If the dry tray pressure drop is 0.5 in or more, then use Eq. (3.37). 

For your calculated dry tray pressure drop DPTRAYi (inches of liquid), you may use linear interpolation of the preceding HF values calculated to find your froth height case. Vapor froth tray time may now be calculated from Eq. (3.39) ... 

Before a total sieve tray pressure drop can be summed, the froth pressure in inches of clear liquid over the active area must be calculated. This froth height actually reduces the HHDS value by a factor called the aeration beta correction. This has been done by Smith, who plotted the aeration factor beta vs. FGA (see Eq. (3.120) for FGA). Equation (3.121) is a curve-fit of Smith s beta curve plot [16]. Generally a beta factor of 0.7 to 0.8 is calculated using Eq. (3.121). 

Froth entrainment flooding (Fig. 6.7b). At higher liquid flow rates, the dispersion on the tray is in the form of a froth (Figs. 6.25c and 6.27a), When vapor velocity is raised, froth height increases. When tray spacing is small, the froth envelope approaches the tray above, As this surface approaches the tray above, entrainment rapidly increases, causing liquid accumulation on the tray above. 

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. 

For tray columns the net interfacial area is a = a hfA, where a is the interfacial area per unit volume of froth, hf is the froth height, and A j is the bubbling area. For tray columns the interfacial area is a hA, where a is the interfacial area per unit volume, h is the height of a section of packing, and A is the cross-sectional area of the column. 

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 ... 

FIGURE 12.63 Estimation of froth height on a tray. (J. Stichhnair, J. R. Fair, 1998. Distillation—Principles and Practices, New York Wiley-VCH.)... 

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. 

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