Understanding the Software Options

Environmental Fractionation Column Manager
How long does it take
to reach steady state?
How long does it take a six-tray,
4-ft dia. deisobutanizer to achieve
steady state? That depends on
your definition of “steady state”. Some
control experts regard 95% of “ultimate
value” as a good definition, but
how useful is that definition?
The FRI distillation columns are
located on the Oklahoma State University
(OSU) campus. As a result,
there is a close relationship between
FRI and the OSU chemical engineering
dept., particularly with professors
Rob Whiteley, Jan Wagner and Ken
Bell. A couple of years ago, professor
Whiteley and one of his graduate students,
Anand Vennavelli, performed
a set of time-to-steady-state (TTSS)
tests using the FRI experimental unit.
They found that hydraulic equilibrium
was achieved fastest when
a column was operated at
atmospheric pressure and
two times slower at vacuum
and at high pressure.
They found that the key to
reducing TTSS, was pressure
control.
Several years ago, FRI’s
membership instructed the FRI staff
to study the impact that reflux temperature
has on packing performance.
The membership wondered whether
renegade condensation on the outside
of liquid distributors was shifting the
control of the distribution from the
distributors to Mother Nature. FRI
purchased and installed a new reflux
heater. Professor Whiteley, and OSU,
provided a new control strategy. That
Circle 2 on p. 54 or go to adlinks.che.com/35068-02
40 CHEMICAL ENGINEERING WWW.CHE.COM AUGUST 2011
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 �rst-hand experience with CE readers
strategy has proven to be very effective
at controlling column pressure.
Late this April, the FRI staff decided
to have some “fun” — some “TTSS fun.”
Exactly how long would it take for
steady state to be achieved after an extreme
change in reboiler duty? Different
staff members guessed different times.
Cash bets were collected and stored in a
hermetically sealed mayonnaise jar that
was buried by technician Kenny Martin
in the swamp behind the unit.
On April 27 the column contained 10
ft of structured packing operating at 5
psia. The reboiler duty was increased,
as quickly as possible, from 20 to 80%
of flood. Thereafter, liquid samples
were taken every 15 min. The column’s
pressure was held very steady by —
and thanks to — the new OSU control
strategy. Hydraulic equilibrium was
defined as the point when all of the
levels and pressure drops steadied out.
Hydraulic equilibrium was achieved
within 15 min. Thermal equilibrium,
as evidenced by ten thermocouples,
was achieved at the same time. Mass
transfer, and HETP (height equivalent
to a Theoretical plate), equilibrium
were achieved 30 min. after that.
At the October 2011 AIChE meeting,
FRI will be giving two presentations
associated with the aforementioned
work. The new control strategy
will be described as will the TTSS results.
In fact, between now and then,
several other TTSS tests are planned.
Eventually, the impacts of tower pressure,
tower system and trays-versuspacking
will all be studied.
Meanwhile, in case you were wondering
about who won the contest:
Kenny claimed that he could not find
the buried mayonnaise jar, but he is
wearing a new watch. ■