Chemical Engineering June 2014 - 42

Feature Report
Engineering Practice
Fine Tune the Design of
Sour-water Strippers Using
Rate-based Simulation
A look at the benefits of using simulations
in the design of SWS units,
and some practical tips on SWS operation
Soumitro Nagpal
Fluor Daniel India Pvt. Ltd.
S
our-water strippers (SWSs) are
common in petroleum refineries,
as well as gas-processing
and gasification plants, and are
used to remove ammonia, hydrogen
sulfide and carbon dioxide from contaminated
process waters. The design
of these units is typically done
using the equilibrium or theoretical
stage method. The actual number
of trays is determined from the required
theoretical stages, using tray
efficiencies. The tray efficiencies
are not based on actual tray performance,
but on experience-based
rules of thumb. These rules of thumb
are usually conservative to compensate
for reduced tray efficiency -
due to tray malfunction, foaming or
plugging - and to add some design
margin for future capacity expansion.
Reliable rate-based process
simulators are now available for
the design of SWS units, and can be
used for improved estimation of tray
efficiency and for estimating the
actual number of trays for optimal
design of these units. For existing
columns, this approach can be useful
for troubleshooting and evaluating
available spare capacity.
Equilibrium-stage models, which
form the basis for rate-based calculations,
are first evaluated by looking
at the impact of sour-water feed
H2S and NH3 content, stripper column
equilibrium stages on stripped
water NH3 and H2S content, and
stripping energy consumption. Two
commercial process simulators are
used and the results are compared
with SWS operating and design
data from several sources. Subsequently,
a commercially available
rate-based model was used
to simulate stripper performance
and estimate tray efficiencies. The
rate-based model calculated significantly
higher NH3 tray efficiency
(~60%) compared to the common
industry practice of using NH3 tray
efficiencies of of 20-25%.
Stripper and associated equipment
design and operational guidelines
are also discussed.
Conventional methodology
The prevailing design methodology
for SWSs is the equilibrium-stage
approach. The method determines
the number of equilibrium or theoretical
stages required to attain a
required ammonia level of treated
water for a given feed sour-water
condition and specific steam consumption.
Phenol and hydrogen
cyanide removal are also evaluated.
The following two commercial process
simulators are used:
* HYSYS v7.0 with PR-SOUR property
package (HYSYS)
* AspenPlus v7.0 with ELECNRTL
property package (ELECNRTL)
A typical SWS unit configuration
42 ChemiCal engineering www.Che.Com June 2014
Table 1. Sour-waTer feed and
STripper condiTionS
Flow, Nm3/h
Temperature, °C
Composition, ppmw
NH3
H2S
Phenol
HCN
H2O
Column top tray pressure,
bara
Column bottom tray pressure,
bara
Reflux drum pressure, bara
100
45
8,000
12,000
100
100
balance
Column feed temperature, °C 110
2.4
2.7
2.1
Reflux drum temperature, °C 85
is shown in Figure 1. The stripper
column comprises a steam-heated
bottoms reboiler and an overheads
condenser. A pump-around type
overhead condensing arrangement
is another common SWS configuration,
and is sometimes preferred,
because it can reduce plugging and
corrosion problems. This approach,
however, entails higher capital expenditure
than the overheads-condensing
configuration [1].
The sour-water feed conditions selected
for the evaluation are shown
in Table 1.
Equilibrium model evaluation
Gantz [2] presents data from an
API-sponsored survey of a large
number of SWS operating plants.
Data from " questionnaire No. 34 "
are used. The data represented a
packed column with 15 feet of 3-in.
Raschig rings, and 1,400 ppmw
(parts per million by weight) NH3 in
feed. The packed column was found
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Chemical Engineering June 2014

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