This preview shows page 1. Sign up to view the full content.
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 bubblecap data,
the Zuiderweg method (Ref. 23) is based on published sievetray efficiency data of FRI. The
correlations for the masstransfer 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 sievetray 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 sievetray columns. It utilizes
volumetric masstransfer 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 masstransfer coefficient in the liquid phase is not new, but is close to that of the
AIChE method. The correlation for the volumetric masstransfer 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...
View Full
Document
 Spring '11
 Levicky
 The Land

Click to edit the document details