Flood trays approach

Pinczewski and Fell [Trans. Inst. Chem Eng., 55, 46 (1977)] show that the velocity at which vapor jets onto the tray sets the droplet size, rather than the superficial tray velocity. The power/mass correlation predicts an average drop size close to that measured by Pinczewski and Fell. Combination of this prediction with the estimated fraction of the droplets entrained gave a relationship for entrainment, Eq. (14-202). The dependence of entrainment with the eighth power of velocity even approximates the observed velocity dependence, as flooding is approached. 

Entrainment (Jet) Flooding Froth or spray height rises with gas velocity. As the froth or spray approaches the tray above, some of the liquid is aspirated into the tray above as entrainment. Upon a further increase in gas flow rate, massive entrainment of the froth or spray begins, causing liquid accumulation and flood on the tray above. 

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. 

Dg, Dl = vapor and liquid phase diffusion coefficients, m /s liL = liquid holdup, mm / = fractional approach to flood Fya = active area F-factor, Uap (active area = bubbling area of tray)... 

The tray will be designed so that the flooding velocity within the active area approaches the ultimate capacity ( spray column). The total active area required for 60% of flood = 7023 cm. ... 

Comparison with Trays. The pressure drop for a tray with 0.61-m spacing was found to be 58.4 mm liquid. The comparative packing pressure drop is 66(0.61) = 40.3 mm liquid. The diameter for the packed column is 3.21 m vs. 3.36 m for the trays. If a lower approach to flood for the packing is used, to make the column diameter 3.36 m, the packing pressure drop becomes less than half that of the trays. 

If the separation is satisfactory, the commercial column will require the same or fewer trays than the Oldershaw trays used. Comparisons are made at the same approach to flood for both columns. 

The performance curve of a crossflow tray shows a lowering of efficiency as vapor velocity approaches a flooding value (Figure 12.27). This lowering results from liquid entrainment. Figure... 

Using Figure 13.13 with 24-in tray spacing, 12% downcomer area, 80% approach to flood. 

Reference has been made to liquid entrainment that occurs in increasing amounts as (he flood point is approached. The entrained liquid is recycled back to the tray abuve, negating the efleet of countercurrent contact]ag and decreasing tray efficiency. The recirculation dne to entrainment is shown in Fig. 5.7-11. 

Because of their simplicity and low cost, sieve (perforated) trays are now the most important of tray devices. In the design of sieve trays, the diameter of the tower must be chosen to accommodate the flow rates, the details of the tray layout must be selected, estimates must be made of the gas-pressure drop and approach to flooding, and assurance against excessive weeping and entrainment must be established. 

Design a sieve-tray column for the ethanol absorber of Example 4.4. For alcohol absorbers, Kister (1992) recommends a foaming factor Ff = 0.9. The liquid surface tension is estimated as a = 70 dyn/cm. Take do- 5 mm on an equilateral-triangular pitch 15 mm between hole centers, punched in stainless steel sheet metal 2 mm thick. Design for an 80% approach to the flood velocity. 

Next, enter the data presented above into the Mathcad sieve-tray design computer program of Appendix E. Since the dimensions of the tray are known, the fractional approach to flooding is adjusted until the tray design coincides with the tray dimensions determined in part (a) of this example. Convergence is achieved at a value of/= 0.431. This means that at the bottom of the distillation column the gas velocity is only 43.1% of the flooding velocity. Other important results obtained from the program are as follows ... 

Run the sieve-tray design program of Appendix E using these data. Try different values of the fractional approach to flooding,/, until the program converges to the specified tower diameter D = 1.036 m. When convergence is achieved, the results at the top of the tower are ... 

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