Chemical Engineering November 2013 - 28

Cover Story
Feature Report
Troubleshooting and Solving a
Sour-Water Stripper Problem
A systematic approach
identifies problems that
prevented a stripper
column from achieving
trouble-free operation
Jeremy Ponting
Fluor Ltd.
Henry Z. Kister
and Richard B. Nielsen
Fluor Corp.
A
recent book by Lieberman [1]
stresses that to combat the environmental
crisis that our planet
is facing, it is essential to make
existing units work adequately and
efficiently. Equipment that malfunctions
or limps along limits production
and does not permit good energy and
equipment utilization. This, in turn,
leads to higher environmental degradation,
not to mention plant downtime
and lost production costs. Lieberman
concludes that good troubleshooting
and adequate repair of malfunctioning
units is central for efficiently utilizing
our resources.
Towers that strip components out of
water (so this water can be re-utilized
or disposed of) are common in the process
industries. In petroleum refineries
and gas plants, the main components
stripped out are hydrogen sulfide and
ammonia, and the tower is referred to
as a " sour water stripper. " The technology
for stripping sour water has been
established for decades. It was an
engineer, Beychok [2], who wrote the
leading text on the subject as far back
as 1967. Yet almost five decades later,
there are many sour-water strippers
that do not operate adequately and
efficiently. Many " war stories " of sour
water strippers that failed to achieve
their design objectives are presented
Sour
water
Receiving tank
N2
Vent
Offspec
stripped
water
Oil skim
Sour water
stripper feed
Feeding tank
N2
Vent
LEGEND
Normally
open valve
Normally
closed valve
Figure 1. The sour-water stripping column described in the text is fed from two
sour water storage tanks with an oil skimming facility
in Kister's " Distillation Troubleshooting "
book [3].
Here we present another experience
with a sour water stripper that initially
failed to achieve its design goal due to
startup damage. The problem was correctly
identified in a short, systematic
and effective investigation that combined
field measurements and testing
with good hydraulic analysis. The unit
was subsequently successfully fixed.
This article presents the experience
and the lessons learned.
Process description
A sour-water stripper unit in a gas
processing plant was designed to process
35 m3/h of sour water containing
approximately 650 ppmw H2S. The
column was fed from two sour-water
storage tanks with an oil skimming facility
(Figure 1). The tanks were lined
up such that while one tank was receiving
sour water from upstream separation
units, the other tank was feeding
the sour water stripper column.
The column used heat integration
in the form of 2  100% plate-andframe
exchangers (Figure 2) to ex28
ChemiCal engineering www.Che.Com november 2013
change heat between the column feed
and bottoms (stripped water). The exchanger
was designed to preheat the
sour water from 55°C to about 110°C.
At 110°C and at the normal tower
inlet pressure, the sour water was
calculated to be all liquid close to its
boiling point.
The column (Figure 3) was equipped
with 20 single-pass fixed-valve trays
numbered from the bottom tray up.
Feed entered at the top tray. The column
was designed to operate with an
overhead pressure of 1.4 barg, corresponding
to a top temperature of
126°C. These conditions were sufficient
to achieve a bottoms specification
of less than 5 ppmw H2S. The bottoms
product was routed to the wastewater
treatment plant.
Reflux was produced by the tower air
condenser, and entered a small reflux
drum from which it was pumped back
to the tower on drum level control. Uncondensed
sour gas was vented from
the reflux drum via the tower pressure
controller (mounted on the vent line) to
the acid-gas header. There was no liquid
product. A temperature controller
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Chemical Engineering November 2013

Table of Contents for the Digital Edition of Chemical Engineering November 2013
