Chemical Engineering November 2013 - 29

Stripped
water to
wastewater
treatment
plant
Offspec
stripped
water
FC
Sour water
from feed
pump
(Figure 1)
Sour water
filters
2 x 100%
Sour water
feed exchangers
2 x 100%
LEGEND
Normally
open valve
Normally
closed valve
Figure 2. The stripper feed system is presented here, showing heat integration
with the stripper bottoms stream
TC
TC
PC
Acid
gas
20
19
18
Preheated
feed
Cascade
RO
FC
1
Kettle
reboiler
LEGEND
Stripped
water
at the condenser outlet adjusted the
speed of the condenser fan motor to set
the split between the gas and reflux.
Reboil to the tower was supplied by
a kettle reboiler that was gravity-fed
by liquid drawn from a trap-out pan
just beneath the bottom tray. Liquid
from the kettle overflowed a weir into
the kettle draw compartment, from
where it gravity-flowed to the column
bottom sump. Stripped sour water
from the column bottom sump was
cooled in the plate-and-frame feedbottom
exchangers and an air cooler
(Figure 2) and then flowed to the
waste treatment plant.
There was a bypass that permitted
bypassing tower feed around the
Normally open valve
Normally closed valve
plate-and-frame exchangers. If the exchangers
were bypassed, the column
feed temperature was close to ambient
temperature. This no-feed-preheat
case set the design surface area of the
tower kettle reboiler. In case of reboiler
fouling, the column was also equipped
with a live steam injection line.
During startup, column feed preheat
was essential to minimize the temperature
difference between the incoming
feed and the column top temperature.
Column feed preheat minimizes the
chance of sudden condensation in the
top section of the column, which could
lead to tray damage, similar to two
water-stripper-troubleshooting cases
described in detail in Ref. 3.
Condensate
Steam
header
LC
Figure 3.
Shown here is the
stripper column
with its reboiler,
condenser reflux
and products
Stripper
column
20
1
LC
To
condenser
To reboiler
Reboiler vapor
Reboiler liquid
Initial operation
During startup, the column feed flowrate
was gradually increased toward
the 35-m3/h design rate with the feed
preheat exchangers lined up. At approximately
60% of design feedrate the
reflux drum flooded. The reflux-level
control valve was subsequently driven
wide open, increasing the reflux to the
column. The top temperature fell dramatically
and the column pressure fell
from its setpoint of 1.4 barg. The only
way of bringing the tower back under
control was by reducing the column
feedrate. The reflux was then brought
back into line and the column gradually
brought back up to design temperature
and pressure.
There were several attempts (with
small variations) at increasing the
feed to the design flowrate. Each attempt
resulted in the same scenario as
described above. It made no difference
whether the column was operating
with the kettle reboiler or with live
steam injection.
The operating behavior and DCS
(distributed control system) data were
closely reviewed in order to establish
the root cause of the problem. At feed
flowrates below 16 m3/h the column
ran without problems with all parameters
within their correct operating
range. As the column feedrate approached
20 m3/h, there was a sudden
increase in the pressure drop across
the trays. Initially the operators observed
a rapid increase in pressure
drop followed by an equally rapid fall
(Figure 4). These peaks became more
dramatic as the column feed increased
until the reflux drum flooded.
The sudden rise in column pressure
drop is a primary symptom of
flood. Entrainment from the top of the
tower, as evidenced from the overfilling
of the reflux drum and the full
opening of the reflux control valve,
is also a primary symptom of flood.
Pressure fluctuations and drop of the
overhead temperature are also symptoms
of flood. Fluctuations in the column
sump level, which followed the
pressure drop fluctuations, were also
noticed, and provide one additional
symptom of flood. It was concluded
that above a certain feed flowrate, the
column became flooded.
At this stage the cause of flooding
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Chemical Engineering November 2013

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