Separation Process Principles- 2n - Seader & Henley - Solutions Manual

Exercise 128 subject correlations for estimating

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Unformatted text preview: as received little attention and will not be considered here. The method of Hughmark (Ref. 22) has received much attention. It was developed as an improvement over the AIChE method for systems controlled by mass transfer in the liquid phase. It considers the interfacial area for mass transfer in a separate correlation that depends on the F factor, weir height, liquid head flowing over the weir, gas and liquid densities, liquid viscosity, and surface tension. Other factors required are gas and liquid diffusivities, froth height, liquid holdup, and superficial gas velocity. Thus, the disadvantage of the Hughmark method is that it requires more information than the AIChE method. The advantage of the Hughmark method is that it is more accurate for systems controlled by mass transfer in the liquid phase. It predicts three chemical systems for sieve trays well (Korchinsky, Ehsani, and Plaka, 1994). However, it is not accurate for sieve trays with small holes (Dribika and Biddulph, 1992). Unlike the AIChE and Hughmark methods, which are based on AIChE bubble-cap data, the Zuiderweg method (Ref. 23) is based on published sieve-tray efficiency data of FRI. The correlations for the mass-transfer coefficients are very simple, with the only factors being the gas density and liquid diffusivity. However, correlations for the interfacial area depend on the flow regime (spray or mixed emulsion). These depend on the F factor, liquid holdup, flow parameter, Exercise 12.7 (continued) Analysis: (continued) weir height, surface tension, hole pitch, weir length, and liquid density. Although based on distillation data, those data cover only a narrow range of slope of the equilibrium line. Thus, for other distillation systems, the use of the method is questionable. Another disadvantage of the Zuiderweg method is the need for more information than the AIChE method. An advantage of the Zuiderweg method is that it considers the flow regime for sieve-tray columns. Unfortunately, the method greatly underestimates point efficiency for sieve trays with small holes (Dribika and Biddulph, 1992) and also does very poorly on three chemical systems distilled in sieve trays (Korchinsky, Ehsani, and Plaka, 1994). It is difficult to see how the Zuiderweg method could be recommended over other methods. The Chan and Fair method (Ref. 24) for sieve trays has received much attention because it is based on a large amount of experimental data for large sieve-tray columns. It utilizes volumetric mass-transfer coefficients that depend on the F factor, liquid diffusivity, gas diffusivity, gas velocity as a fraction of flooding, and liquid holdup. The correlation for the volumetric mass-transfer coefficient in the liquid phase is not new, but is close to that of the AIChE method. The correlation for the volumetric mass-transfer coefficient in the gas phase is based on distillation data, rather than on absorption data as in the AIChE method. Like the AIChE method, the Chan and Fair method is relatively easy to apply. For sieve trays with small holes (Dribika and Biddulph, 1992), point efficiencies are mostly overpredicted, but no method gives c...
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