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Since 1968
This article does not have approximations
of HTUOL for the wide variety
of strippers represented in this
category of mass transfer. However,
high volatility and few stages are not
indicators of a trivial task, and an accurate
NTUOL count and a reasonable
HTUOL approximation are needed. A
2007 paper by Lopez-Toledo [7] and
others at The Dow Chemical Co. presents
a general correlation for kL values
that can be used to build HTUOL
for stripping service.
If a stripping column is large and a
conservative approximation is too expensive,
the situation could justify research
and testing to measure HTUOL
values. This situation occurred several
years ago when an entire industry
needed design data for stripping
References
1. Treybal, Robert, " Mass-Transfer Operations, "
McGraw-Hill, Third Edition, 1980.
2. Seader, J.D. and Henley, Ernest J., " Separation
Process Principles, " John Wiley & Sons,
1998.
3. McCabe, W. L. and E. W. Thiele, " Industrial
Engineering Chemistry, " 17, 605 (1925).
4. " INTALOX® Packed Tower Systems - Structured
Packing, " Koch-Glitsch Brochure
KGSP-1, page 5.
5. Koshy, T. D. and Rukovena, Frank, Reflux
and Surface Tension Effects on Distillation,
Hydrocarbon Process., May 1986.
6. Strigle, Ralph F., " Packed Tower Design and
Applications, " Gulf Publishing, Second Ed.,
1994.
7. Lopez-Toledo, Sawant, Castillo, Au-Yeung,
and Pendergast, A.I.Ch.E. Annual Meeting,
Salt Lake City, UT, November 2007.
8. O'Connell, H. E., Trans. A.I.Ch.E., 42, 741
(1946).
VOC (volatile organic compounds)
from contaminated groundwater. The
magnitude of the need justified many
packing tests.
Distillation trays
This article has not addressed trays,
as opposed to packing, as the mass
transfer device. However, nothing
in the analysis of driving forces and
nothing in the counting of transfer
units is specific to the type of mass
transfer device. As with packing, equilibrium
stages are a poor representation
of the mass transfer task of trays,
and tray efficiencies must account for
this. For that reason, trays with good
bubbling activity and tall weirs can
exhibit tray efficiencies of 25% in absorption
service. The 1946 O'Connell
[8] tray efficiency correlation has a
relative volatility effect, because high
relative volatility implies low " apparently
poor " tray efficiency.
There may be a perception difference
between packing and trays, in
that 25% tray efficiency in absorption
service is not perceived to indicate
poor performance of the tray, but a
packing HETP of 2,000 mm is often
perceived as poor performance. Designers
may accept without question
the premise that 12 trays are needed
for one or two absorption stages, or 20
trays for three absorption stages. An
understanding of transfer units will
help explain tray efficiency under
high-relative-volatility and absorption
conditions.
■
Edited by Rebekkah Marshall
Author
Kenneth Graf is a principal engineer at KochGlitsch,
LP (4111 East 37th Street North, Wichita,
KS 67220; Email: kenneth.graf@kochglitsch.
com). Ken graduated from the University of
Akron (Akron, Ohio) with a B.S.Ch.E. degree is
1977 and an M.S. degree in 1984. Since 1977,
he has worked with distillation equipment (column
trays and packing) at the Standard Oil Co.
(Sohio), Norton Co., Saint-Gobain NorPro, and
Koch-Glitsch, LP. Graf has designed many columns
for the petroleum refining, ethylene and
chemical industries.
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