Chemical Engineering July 2022 - 21

Tower Doctor
A Quick Diagnosis Makes a Short Assignment
Henry Kister shares lessons learned from troubleshooting distillation towers
O
n an assignment to troubleshoot
a vacuum tower for a
petroleum refinery, my team
was tasked to " run simulations to determine
why the heavy vacuum gas oil
(HVGO) has a very dark color. "
We traveled to the site and met with
the plant personnel in a large meeting
room. The plant's process engineers
carried in large volumes of computer
output. I was not sure what additional
simulations they wanted us to do that
they had not already done.
During the meeting, we discussed
the tower, the plant engineers' experiences
and their analysis. They
had already performed a very extensive
simulation study. The afternoon
agenda focused on contract discussion.
The project manager could not
make it, so I had to fill in for him.
I was traveling with a young engineer,
and did not need him to attend
the contract discussion. So, I dreamt
up something for him to do, to help
answer a question that I had raised
during the meeting, which had not
been fully answered.
" Why don't you go with the unit
engineer to the unit. On the wash
stream going to the tower, downstream
of the filter and the control
valve, the P&I [process and instrumentation
diagram] shows a pressure
gage. Can you please go there
and get me the reading? "
The pressure gage in that location
(Figure 1) is an invaluable troubleshooting
tool. Neglecting line losses,
the pressure balance is such that
the pressure at the gage equals the
tower pressure (which is known from
the tower pressure transmitters, and
is usually very low because this is a
vacuum tower), plus the liquid head
to the column entry (that can be
calculated from the elevation difference),
plus the pressure drop across
the spray nozzles that distribute liquid
to the wash bed. Subtracting the
tower pressure and the liquid head
from the pressure-gage reading
gives the pressure drop across the
spray nozzles.
These nozzles are typically designed
for a 10-20-psi pressure
drop. An even better evaluation
can be obtained by comparing
the measured pressure
drop with the pressure drop
calculated from the sprayvendor's
catalog based on
the metered wash flowrate.
About half an hour later, the
engineers returned.
" What does it read? " I
asked.
" 160 psig. "
Oops. I turned to them.
" I apologize. I misled you.
Please go again, but this time
stop at the instrument shop
on the way to the tower. Ask
the instrument technician for
a new pressure gage, and
make sure that the instrument
technician swears that it
works. Go back to the same
spot. Remove this old pressure
gage that obviously is
" kaput " . Replace it by the new
gage, and get me a decent
reading this time. "
So off they went. They
came back an hour later.
" What does the new one read? " I
inquired.
" 150 psig. "
At that moment, everyone realized
that the problem was not the
gage. To give this kind of pressure
drop, the sprays had to be plugged
solid. With plugged sprays, liquid is
not distributed - the bed cokes,
and the HVGO goes dark. That was
the end of story. There was no need
for any simulations. Our assignment
ended right then.
The takeaway: One good measurement
is worth more than thousands
of calculations.
■
Edited by Dorothy Lozowski
About the Tower Doctor
" The Tower Doctor " is the honorary title bestowed upon
the author of this article in 2002 by Richard Darton, professor
of Engineering in Oxford University and chair of
the European Distillation Network. " When a tower is not
well, " says Darton, " people call Henry to diagnose the illness
and find a remedy. He arrives with his doctor's bag,
examines the patient-tower, measures its temperature
and pulse, gets radiography to get an inside look. Then
comes his diagnosis and cure. Towers treated by Henry
mostly get better very quickly. "
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
JULY 2022
FIGURE 1. The petroleum refinery's vacuum tower is depicted
in this diagram
Being son to two medical doctors who were blessed
with phenomenal diagnosis ability, the author aspired
to live up to this special honorary title. Like with
medical doctors, some illnesses were a struggle to
diagnose, others were easier. All were exciting. This
column will reminisce through some of the more entertaining
cases. They may not have seemed entertaining
at the time, but looking back at them, they leave
unforgettable memories and raise a smile or two. One
great aspect of being a tower doctor, one gets to work
with and learn from some of the greatest engineers
and operators that contributed so much to the chemical
industry. We hope that this column can pass some
of the fun, excitement and lessons learned to future
troubleshooters and tower doctors.
Author
Henry Z. Kister is a senior fellow
and the director of fractionation
technology at Fluor Corp. (3 Polaris
Way, Aliso Viejo, CA; Phone:
949-349-4679;
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 130 articles, and has taught the IChemEsponsored
" Practical Distillation Technology " course
more than 530 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 Committees.
21
Email:
http://WWW.CHEMENGONLINE.COM
Chemical Engineering July 2022

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