Hydrocarbon Processing - February 2021 - 51

Process Optimization
of the different hydrocarbon
changing over time as the refinery
processes different crudes, or
different unit operations are added
to the refinery. Ideally, the SWS can
handle most or all these changes
without major modifications to the
stripper. A conservative estimate
of tray efficiency will provide
more flexibility in the design to
account for the uncertainty of
the feed composition.
As previously mentioned, numerous SMEs use an initial rule-of-thumb
tray efficiency of 33%, or three actual
trays in the SWS for every equilibrium
stage in the process simulation. To further refine the cost estimate or proceed
with detailed design, it may be prudent
to build a mass-transfer rate model of
the SWS. This can be more easily done
once the column internals have been selected, since accurate information about
the tray-such as weir height, active tray
area, etc.-are critical to building an accurate mass-transfer rate model. Reliable
estimates of the sour water composition
will also be necessary to help ensure the
appropriateness of the SWS design.
Another important factor in the design
of the column is tray hydraulics. The actual hydraulics on the tray is dependent
on the tray device such as fixed-valve
trays. The number of valves and size of
the opening is important to maintain liquid on the tray and get proper contacting
of the vapor and liquid; thus, the proper
operating range for the design becomes
important. If the trays are overdesigned,
then the tray may weep or dump liquid,
resulting in poor operation. The design
must account for the low end, as well as
the high end of operations. One reason
24-in. tray spacing is often used is to give
more capacity, especially when fouling or
foaming is expected.
Even when the designer is confident
in the design of the column, some additional precautions are recommended.
These include:
* NH3 will be the more difficult
component to remove in most
sour water streams. NH3 has a high
affinity for water and will almost
always strip out of the sour water
after the H2S is almost completely
removed. It is possible to reach a
stripped water condition where
the remaining NH3 is fixed in

the stripped water, meaning that
it is bound to a non-volatile or
strong acid in the stripped water
and will not come out of solution
regardless of the energy input
into the bottom of the column.
In this case, it is prudent to install
a nozzle in the lower section of
the column to allow for caustic
addition, if necessary, under some
or all conditions. The strong base
will displace the NH3 and allow
it to be more easily stripped from
the column. By placing the
nozzle in the lower section
of the tower, the caustic will not
interfere with H2S stripping.
* Although trays can be designed for
fouling service, some reduction
in efficiency will likely be noticed
over time. Even with adequate
solids removal and hydrocarbon
phase removal, some fraction
of these materials will enter the
column periodically. Some slightlywater-soluble hydrocarbons may
enter the tower and precipitate
in the lower section of the SWS
as the water heats to near boiling.
Other salts may be present in the
water that precipitate in the higher
temperature areas of the SWS.
Adequate access to the column
for quick maintenance and some
additional design margin may
be prudent to address fouling
concerns. FIG. 4. shows an example
of fouling that can occur in SWS
service over a 5-mos period of
operation, which corresponded to
the 10%-15% reduction in vapor
flow area noted by the authors.12,13
Takeaways. Stripping sour water is a demanding process in a refinery or gas treating facility. The sour water will contain a
multitude of contaminants in addition to
the NH3and H2S stripped out of the water
in the process. These contaminants make
reliable operation of the SWS a challenge,
but one that can be realized with appropriate design of the SWS and the equipment
that surrounds it. Proper sizing and level
control of the three-phase separators in
the sour water system are critical to removing contaminants-such as hydrocarbons-that can severely impact SWS
performance by causing fouling and foaming in the column. Solids filtration and liq-

uid/liquid coalescing equipment should
also be considered as additional means to
further clean the sour water prior to the
stripper. The SWS needs to be designed to
handle variations in inlet feed composition
and flowrates, as well as allow a margin for
fouling and foaming. The selection of tray
internals should consider the severity of
the service and the presence (or absence)
of good sour water cleanup steps prior to
the stripper. The tray design should take
into consideration fouling, efficiency and
hydraulics, among other factors.
Part 2. Part 2 discusses the design of

packed tower SWS and details operating problems that can occur in an SWS
system.

NOTE
This article was originally prepared for the
Brimstone Sulfur Recovery Symposium. It has been
edited and separated into two parts for publication in
Hydrocarbon Processing.
LITERATURE CITED
B. Scott, " Processes for Treating Refinery Sour
Waters, " Brimstone Sulfur Recovery Symposium,
Vail, CO, 1995
2
Asquith and A. Moore, " Sour Water Processing-
Balancing Needs, " Brimstone Sulfur Recovery
Symposium, Vail, CO, 2000
3
D. Stevens, A. Mosher and D. Ogg, " Fundamentals
of Sour Water Stripping, " Brimstone Sulfur Recovery
Symposium, Vail, CO, 2007
4
J. Stavros and A. Keller, " Reducing Nitrates in
Refinery Wastewater, " Brimstone Sulfur Recovery
Symposium, Vail, CO, 2013
5
N. Hatcher, R. Alvis and R. Weiland, " Sour Water
Stripper Performance in the Prescence of Heat Stable
Salts, " Brimstone Sulfur Recovery Symposium, Vail,
CO, 2012
6
A. Keller, " Fundamentals of Sour Water Strippers, "
Brimstone Sulfur Recovery Symposium, Vail, CO,
2015
7
P. Le Grange, " Operational Challenges in Sour
Water Stripping, " Digital Refining, online: www.
digitalrefining.com/article/1002335, July 2009
8
D. Stevens and A. Mosher, " Fundamentals of Sour
Water Stripping, " Brimstone Sulfur Recovery
Symposium, Vail, CO, 2008
9
McIntush, K., J. Farone, B. Piggot and C. Beitler,
" Specifying internals in sour water strippers, "
Brimstone Sulfur Recovery Symposium (Virtual),
2020
10
American Fuel & Petrochemical Manufacturers,
" AFPM Process Safety Bulletin-Hazards of Purged
Tanks-Formation of Pyrophoric Iron Sulfide in
Low Oxygen Environments, " No. 14-01, 2014
11
B. Spooner, " Reduce Hydrocarbons and Solids
Contamination in Sour Water Strippers, " VWR
International, 2013
12
R. Hauser and R. T. Kirkey, " Refinery Tests
Demonstrate Fixed Valve Trays Improve
Performance in Sour Water Stripper, " New Orleans,
LA, AIChE, 2003
13
F. Bela, Comprimo, " SWS Fixed Valve Trays, " 2003
14
N. Hatcher and R. Weiland, " Reliable Design of
Sour Water Strippers, " PTQ , September 2012
1

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