Chemical Engineering May 2012 - 68

Fractionation Column
Environmental Manager
Magic trays, brown solvent
E
very five years or so, a call would
come in and once again I would
be troubleshooting a certain extraction
unit in Canada - usually
in the dead of winter. This particular
unit had a liquid-liquid extractor,
a stripper, a vacuum regenerator and
the usual complement of pumps,
pipes, valves, vessels, heat exchangers,
raccoons and crows. Some of you
might remember the extractor from
my Crow's Nest column (CE, February
2011, p. 21).
The natural state of the extraction
solvent was clear and slightly yellow.
With time, however, oxygen would
creep into the unit, especially via the
vacuum regenerator. Organic acids
would form. The acids would eat any
carbon steel that they encountered.
Iron oxide and carbonaceous materials
would accumulate in the solvent,
which would turn from yellow to
green to brown and then black. Eventually
the solvent would look and feel
like the three-year old engine oil in
my 1974 Ford Falcon.
One troubleshooting visit was in response
to a complaint about reduced
recovery. The unit was shut down for
the annual turnaround. My colleague,
Reese, and I decided to enter and inspect
the extractor at three manhole
locations, starting with the top manhole.
The top manhole was open. According
to our drawings, the top tray
was only about 3 ft below the top
manhole. We inspected the top tray
with our flashlights. It looked fine -
no fouling. Then, Reese entered the
column feet first, on his belly. He almost
fell 20 ft.
Unfortunately, Reese and I failed
to expect the unexpected. The top
Mike Resetarits is the technical director
at Fractionation Research, Inc. (FRI; Stillwater,
Okla.; www.fri.org), a distillation
research consortium. Each month, Mike
shares his fi rst-hand experience with CE
readers
tray was there - but it wasn't. It was
paper thin. It crumbled to dust instantly
under his weight. Fortunately
he was wearing a harness with a lanyard.
It turned out that the top ten
trays were similarly thin. Replacement
trays were provided to the plant
owner on an emergency basis.
Five years later, Reese and I were
called to the plant again. The lean
solvent was no longer lean. The focus
was on the stripper. The pressure drop
across the stripper was too low. We
asked if we could enter the tower, for
an inspection, at the next turnaround.
A few months later, we heard that
the column had been entered without
our presence. Ten of the forty stripper
trays were found to have been eaten
away by acids. Without our input, all
of the trays had been replaced with an
unusual packing technology. We also
heard that the packing technology
was under-performing the acid-ravaged
trays. Again, replacement trays
were provided to the plant owner on
an emergency basis.
In extraction units, solvent maintenance
programs are of critical importance
to the longevity of the units.
Oxygen ingress needs to be minimized.
To neutralize acids, bases can
sometimes be added to the solvent. A
black solvent will seem to work okay
for short time periods, but eventually
the trays, packings and packing distributors
will plug up. Even worse,
sometimes your column internals will
disappear - presto!
■
Circle 5 on p. 61 or go to adlinks.che.com/40267-05
50 CHEMICAL ENGINEERING WWW.CHE.COM MAY 2012
Mike Resetarits
resetarits@fri.org
http://www.fri.org http://adlinks.che.com/40267-05 http://WWW.CHE.COM
Chemical Engineering May 2012

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