Chemical Engineering August 2014 - 65

Fractionation Column
Environmental Manager
Mega columns, mega issues
N
ew distillation columns are getting
much larger. The history
of column sizes, and design and
construction issues associated
with large columns were recently addressed
at the AIChE Spring Meeting
(New Orleans, La.; March 30 to April 3)
by Dan Summers of Sulzer Chemtech
(Winterthur, Switzerland; www.sulzer.
com), Bob Miller of UOP (Des Plaines,Ill.;
www.uop.com) and Henry Kister of Fluor
Corp. (Irving Tex., www.fluor.com). They
focused on trayed, rather than packed
columns. It is entirely possible that the
largest-volume distillation column in the
U.S. was recently commissioned - a C3
splitter with a diameter of 28 ft and a
tangent-to-tangent height of 309 ft. That
column included four-pass trays.
When high liquid flowrates are encountered
at such large diameters,
tray designers increasingly consider
six- and eight-pass crossflow configurations.
Many tray designers avoid odd
numbers of passes. As the number of
flow passes is increased, the number
of column inspection manways should
increase accordingly. With very long
flow-paths, froth height gradients
can cause vapor maldistributions
and even vapor crossflow channeling.
With large bubbling areas, froth stagnancies
can occur. Push devices can
correct such maladies, but those devices
must be selected and positioned
by experienced experts.
The mechanical design work associated
with mega-sized trays is even
more challenging. Tray parts need
to fit through column manholes that
are usually only 24 in. (nominal)
in diameter. Those tray parts need
to overlap support rings in column
shells where 1% out-of-roundness
can yield installation difficulties
if the rings are not wide enough.
Many engineers know the importance
of ring and tray levelness,
but with diameters as large as 50
ft, target levelness is more difficult
to achieve, especially if the rings
are shop-installed with the column
shell in a horizontal position. Similarly,
trays must not sag excessively.
Miller stated that with UOP Multiple
Downcomer (MD) trays, I-beams
are required at diameters greater
than 32 ft. Downcomers can sometimes
help to support large trays.
Propylene towers are always tall.
Columns designed prior to 2000
were often excessively slender and
required guy wires to prevent swaying.
Today's mega-sized columns are
so large in diameter and shell thickness
that guy wires are not needed.
The weight of these towers, however,
demands deep foundations. The design
and fabrication of the shells are
challenging even to the best of shops.
Huge shells need to be shipped to the
production site, then righted and positioned
on the base studs. The feed
and product streams of huge towers
are similarly huge. Many manholes
are recommended. Nozzles must
be properly sized and feeds must
be properly distributed, especially
With 38 years of experience, Mike Resetarits
consults on distillation, absorption
and extraction processes. Each month,
Mike shares his first-hand experiences
with CE readers
where a second phase is involved.
In 1990, I designed a set of trays
for a propylene column that was
14 ft in diameter. Back then, that
column was considered to be huge.
Back then, I knew of no columns that
were taller than 300 ft. These days,
column heights are regularly approaching
330 ft. In the U.S., hydraulic
fracturing is making ethane and
propane very available. In response,
very large olefin plants are being designed
and constructed. Column sizes
are setting records and engineers are
entering uncharted areas.
■
Mike Resetarits
The Chemical Engineering bookstore offers a
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ChemiCal engineering www.Che.Com august 2014 65
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Chemical Engineering August 2014

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