Te Helado

Distillation

Effects of Design on
Tray Efficiency in
Commercial Towers
Henry Z. Kister
Fluor Corp.

When designing a new distillation column
or revamping an existing one,
it is important to understand
the factors that affect overall tray efficiency.

D

espite decades of work on tray efficiency, it is still
unclear which design variables affect tray efficiencies of commercialcolumns and to what extent.
Many of the efficiency studies have been contradictory,
and others relied on data derived from laboratory-scale
towers that do not extend to commercial towers. Based on
much experience gathered in the industry over the years,
and a large amount of commercial-scale data recently
released by Fractionation Research Inc. (FRI), it is now
possible to get a clearerpicture of the factors that affect
overall tray efficiency.
Previous work showed that errors in vapor-liquid equilibrium (VLE) data and reflux ratio measurements can have
a major effect on tray efficiency calculations. Past work also
showed that overall tray efficiency increases with lower viscosities and relative volatilities. This article presents an
analysis of FRI data and quantitativelydefines the effects of
tower geometry — flow path length, fractional hole area,
hole diameter, and weir height — on overall tray efficiency
in commercial-scale fractionators.

The pitfalls of obtaining
tray efficiency from test data
Overall column efficiency. The overall column (or
overall tray) efficiency, EOC, is the ratio of the number of
theoretical stages required for the separation (notcounting
the reboiler(s) and condenser(s)) to the number of trays in
the tower. Since tray efficiencies may vary from one tower
section to another, the overall column efficiency concept is
often applied separately to the stripping section and the rectifying section. It is easy to apply, and generally provides a

good characterization of tray efficiencies in commercial
towers. This conceptis, therefore, preferred by most industrial practitioners. Alternative definitions of tray efficiency
are more often employed to relate tray efficiency to masstransfer fundamentals, as discussed by Lockett (1).
Overall column efficiency is normally obtained from
test data by matching a stage-to-stage calculation, usually
by means of a commercial tower simulation, to test data.
Based on themeasured mass and component balances,
boilup, reflux, temperatures, pressures and compositions,
the number of stages in the calculation is varied until the
closest match to data is obtained. Good practices for
obtaining reliable test data and for correctly deriving
overall column efficiencies from them are discussed in
Refs. 2 and 3.
Reliability of field and test data. Deriving reliable datafrom industrial operating towers is difficult (2–4); flowmeter and analyzer errors present some of the biggest challenges (4). Tabulations of measured efficiency data obtained
from operating columns are available (e.g., Ref. 5), but their
limitations and reliability are not always well-defined.
An excellent source of commercial-scale test data is
measurements by FRI. For many years, onlylimited FRI
data have been published in the open literature, but recently
much more data have been released (6–8). The FRI tests
were conducted in a research facility that permitted close
monitoring, using standard test systems whose physical
properties are well-known. This arrangement overcomes
many of the limitations encountered in the testing of operating plant towers. Most of the datadiscussed here was
obtained by FRI.
CEP

June 2008

www.aiche.org/cep

39

Distillation

% Error in
Efficiency
α = 1.1

αtrue

50
48

αapparent < αtrue

40

α = 1.5
30

α = 2.0

20

α = 3.0
–10

–8

% Error in
Relative Volatility

10
2
–6

–4

4

–2

6

8

10

α = 3.0

–10

α = 1.5

–30

α = 1.2

–40

α = 1.1

–50

■ Figure 1....