Chemical Engineering August 2023 - 29

Tower Doctor
Can the Cure be as Bad as the Illness?
Henry Kister shares lessons learned from troubleshooting distillation towers
D
uring a course that I was
presenting in Houston one
year, a participant asked
me the following at the end
of one of the sessions: " Do you have
experience with BX-type wire-gauze
structured packings? "
" Sure, " I responded. " Wire gauze
packings are used extensively
in
deep-vacuum clean chemical applications.
In some deep-vacuum
specialty chemical finishing columns,
they are the 'standard' packing
used. "
The course participant further
inquired: " Their vendors claim
they give a very good efficiency -
about five stages per meter. What
is your experience? "
" I have had excellent experience
with them. As long as the tower
and internals are clean and welldesigned,
the liquid loads are not
too high, and the distribution is
good, they usually match the vendor
claims. FRI [Fractionation Research
Inc., www.fri.org] tested them with
excellent results that were published
in the open literature. I plotted the
FRI data in my Distillation Design
book [1], " I explained.
" This is not what we are seeing
in our tower. We are getting one
stage per meter. Nowhere near the
five stages per meter the vendor
claims. The separation is terrible, "
he continued.
" Do you have a drawing of your
distributors? " I asked. Our failure survey
[2] showed that 80% of packed
tower failures are due to poor liquid
distribution. So looking at the distributors
is a good starting point.
He pulled out drawings of the distributors.
I took some time to review
and understand them, then did a
couple of sums. All looked good.
" These are good designs, " I
stated. " I do not see any problems
with them. "
He looked proudly at me.
" These are our own designs. I'm
glad you approve. "
I was at a loss. I expected something
major to be wrong with the distributors.
I found nothing.
" Did you inspect
the installation? " I
then asked.
" You bet. Every bit
of it. Everything was
like it should have
been, " he replied.
" I give up. All looks
good. I have no idea
why you are getting
such low efficiency. "
I started packing
up, but then something
stopped me.
I turned to him and
asked, " Usually the
packing vendor designs
the distributors.
But you said they
were your design. "
" Indeed, " he said.
" The original distributors
were designed
by the vendor. They
did not work. So we
replaced them with
our designs that
you liked. "
FIGURE 1. This diagram shows the tower containing wire-mesh packing
that experienced poor efficiency (Diagram courtesy of H. Z. Kister)
" What was wrong with the vendor's
distributors? " I asked.
" They used to plug all the time with
solids that came in with the feed and
reflux, " was his response.
The exploratory questions continued:
" Why didn't you install filters on
these streams? "
" We had filters. They did not work. "
" What was wrong with the filters? "
" They used to plug up every few
hours. Our operators got tired of
cleaning baskets twice per shift. So
they pulled out the baskets. "
At that, a moment of insight arrived.
I then said, " Do you realize
what happened? [Figure 1] When
you removed the filter baskets, the
solids
traveled into
the distributors
and plugged them. Then you
changed the distributors to a design
that does not plug and allows
the solids through. Do you want to
guess where the solids are now? "
" You think they are in the packing? "
he asked after a short hesitation.
" You got it. BX-type gauze packings
are excellent filters, except
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
AUGUST 2023
that they cannot be cleaned online.
Plugged packings give poor efficiency.
You are lucky that at least the liquid
is going through. "
Takeaway: A cure can be just as
bad as the illness. Always keep the
big picture in mind.
■
Edited by Dorothy Lozowski
References
1. Kister, H. Z., " Distillation Design " , McGraw-Hill, New York 1992.
2. Kister, H. Z., " What Caused Tower Malfunctions in the Last 50
Years? " , Trans. IChemE, Vol 81, Part A, p. 5, January 2003.
Author
Henry Z. Kister is a senior fellow
and the director of fractionation
technology at Fluor Corp. (3 Polaris
Way, Aliso Viejo, CA; Phone: 949349-4679;
Email: henry.kister@
fluor.com). He has over 35 years of
experience in design,
troubleshooting,
revamping, field consulting,
control and startup of fractionation
processes and equipment.
Kister is the author of three books, the distillation equipment
chapter in Perry's Handbook, and over 140 articles,
and has taught the IChemE-sponsored " Practical
Distillation Technology " course 550 times in 26 countries.
A recipient of several awards, Kister obtained his
B.E. and M.E. degrees from the University of New South
Wales in Australia. He is a member of the NAE, a Fellow
of IChemE and AIChE, and serves on the FRI Technical
Advisory and Design Practices.
29
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