Chemical Engineering September 2022 - 32

Tower Doctor
Downcomer Unsealing: A Correct Diagnosis
Brings a Correct Cure
Henry Kister shares lessons learned from troubleshooting distillation towers
F
ollowing a distillation conference
hosted by a simulation
vendor, the delegates
were transported by bus to
a venue where the simulation vendor
hosted a delicious barbecue. A
gentleman sitting next to the author
introduced himself. After exchanging
a few greetings, he asked:
" Do structured packings work in
the wash section of an atmospheric
crude tower? "
" They do as long as you have good
liquid and vapor distribution and they
do not plug, " I replied. " Some refiners
swear by them. They experience
occasional plugging episodes, but
are comfortable living with them. "
" I was afraid you'd say that. We
have to put these in the wash section
of our tower. "
" Why? What is wrong with trays? "
I asked.
He replied, " Trays just do not work
for us. We are struggling with dark
gas oil " (the presence of dark oil indicates
poor separation in this section).
" Did the trays ever work for you? "
" At one time they did. But not any
more, " the gentleman replied.
That reply got my attention and I
inquired further, " You went up in rates
and then ran into a bottleneck? "
" Not really. We have always run
50,000 bbl/d [barrels per day]
crude, " he responded.
" This is getting interesting, " I said.
" Why would trays all of a sudden
stop working? "
" If you really want to know, I will
be happy to tell. At one time, trays
worked for us. " Referring to a drawing
showing the tray details (Figure
1), the gentleman explained: " The
12-in. nozzle at the bottom is the
tower feed. Nozzle 14 is the atmospheric
gas oil (AGO). It is a total
draw. Reflux is pumped on flow control
and returned above it. We ran
1,000 bbl/d reflux, which is about
2% of the crude feed. "
" I would not complain about this.
This is a 1-1.5% overflash on the
crude, which is excellent. You won't
do much better with packing, " I said.
" Yes, but those days are gone and
we cannot get back to them. "
" Why? " I asked.
He explained further: " About six
years ago we had a vendor make improvements
to our tower. They were
alarmed by the small feed nozzle, so
they enlarged it from 12 to 24 in., and
added a vane vapor horn distributor
on it. They also added splash baffles
on the trays to make sure they do not
dry up, and downpipes on the spent
wash to avoid re-entrainment [Figure
2]. When we came back on line, we
needed to go to 6,000 bbl/d reflux to
get the AGO on-color. Actually, once
it got on-color, we could cut the reflux
down to 1,500 bbl/d. But if we went
down below this, it would go dark
and we will need to
go up to 6,000 bbl/d
to get the AGO back
on-color. "
FIGURE 1. This is the initial setup of the crude tower wash section
32
He continued, " We
complained to them.
They then reconsidered
the vapor horn,
and in the next turnaround
removed it.
They also changed
the trays. The ones
we had before were
the Venturi moving
valves that have
smooth orifices and
give low pressure
drops. They were concerned that
these trays will excessively weep,
and changed them to moving valves
with sharp edged orifices [Figure 3]
that are more weep-resistant. "
" Did this improve things? " I asked.
" Quite the contrary. Now we cannot
get the AGO on-color, even if we
run a reflux as high as 15,000 bbl/d.
We went to different vendors. They
all say we need structured packings. "
I replied, " There are many towers
that have trays working in the wash
zone. And the trays worked for you
at one time. What I believe you need
is troubleshooting to find why the
current trays are not working. I smell
a downcomer unsealing problem,
but need to take a close look at the
dimensions and do a few sums. "
" If you can find a way to keep trays,
you will be a hero. But you'd better
come up with a good proof that trays
will work, because most people in
our refinery, including management,
are believing the vendors and have
given up on trays, " he said.
Once we drew the sketches to
scale, the problem became apparent.
The 2-in. clearance under the
center downcomer was just too large
for the low liquid rates experienced in
the wash section, that typically operates
at very low liquid loads. With
this large clearance, vapor would
break through into the downcomer.
With the splash baffles 1.5 in. away
from the weirs, and with the draw
pan restricting the vapor exit from
the center downcomer, the vapor
breaking into the downcomer would
not allow the liquid in. The middle
tray would flood, and the bottom tray
would not receive liquid and dry up.
There would be no separation, and
the AGO would go dark.
The problem was introduced
by adding the splash baffles. The
vendor was right about the Venturi
valves weeping. This weep was
actually beneficial, as it allowed the
liquid a path downwards by weeping
through the valves. Once enough
weeping occurred, it would seal the
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Chemical Engineering September 2022

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