Page 52 - Applied Process Design For Chemical And Petrochemical Plants Volume II

Distillation 41

evaluated by more than one method to obtain some idea a qualitative handling of tray mixing to be considered with
of the possible spread. Even so, in light of the AIChE study overall and local efficiencies. In general it agrees with the
discussed below, some of these empirical methods can be Drickamer correlation at least for towers to seven feet in
off by 1540%. Comparisons indicate these deviations are diameter. Although the effect of liquid path apparently
usually on the safe or low side. The relation of Drickamer must be considered, the wide variety of tray and cap
and Bradford [16] of Figure 8-29 has been found to agree designs makes this only generally possible, and the overall
quite well for hydrocarbon, chlorinated hydrocarbons, gly- correlations appear to serve adequately.
cols, glycerine and related compounds, and some rich
hydrocarbon absorbers and strippers. The American Institute of Chemical Engineer’s Distilla-
tion Tray Efficiency Research [2] program has produced a
The relation of O’Connell [49] (Figure 8-29) has gen-
erally also given good results for the same systems but gen- method more detailed than the shortcut methods, and
correspondingly is believed to produce reliable results.
erally the values are high. The absorber correlation of
O’Connell (Figure 8-29) should be used as long as it gen- This method produces information on tray efficiencies of
new systems without experimental data. At present there is
erally gives lower values than the other two relations. It not enough experience with the method and its results to
can be used for stripping of gases from rich oils provided
care is exercised to not accept too high values. evaluate its complete range of application.
The area of absorption and stripping is difficult to cor- Murphree [85] developed “point” and “overall” distilla-
relate €or the wide range of peculiarities of such systems. tion tray efficiencies, which are examined in detail in Ref-
The correlation of Gautreaux and O’Connell [221 allows erence 2. The expressions are [59]:

Figure 8-29. Empirical correlations of overall efficien-
cies for Fractionation and Absorption.