Hydrocarbon Processing - March 2021 - 55

Process Optimization
Other salt solids can be present in SWS systems, as well.
Salt solids can form if the water feed is hard (i.e., if it contains a
significant amount of calcium and magnesium). Among other
causes, salt solids may occur if low-quality wash water is used in
the equipment generating the sour water.
Ammonium carbamate (NH4CO2NH2), ammonium bicarbonate (NH4HCO3) and ammonium carbonate [(NH4)2CO3]
solids can form if carbon dioxide (CO2) is present. (NH4)2CO3
and NH4HCO3 will sublime from the stripper overhead gas at
temperatures of 54.4°C-75°C (137°F-167°F).7 The deposition temperature depends on the partial pressures of the acid
gas components (NH3, H2S and CO2) and H2O in the stream.
Deposition curves exist in the literature for many of these salts.
It is considered best practice to operate the SWS a safe margin
above the estimated sublimation/deposition temperature.
Takeaway. SWS is a demanding process in a refinery or gas

treating facility. The sour water will contain a multitude of contaminants in addition to the NH3 and H2S stripped out of the
water in the process. These contaminants make reliable operation of the SWS a challenge. Part 1 described the equipment
upstream of the SWS that reduces fouling and foaming issues
in the stripper tower, including the sour water flash drum,
surge/storage tank and solids filtration/coalescing filters. Part
1 also presented design guidance to account for foaming in the
stripper and optimum feed location for the sour water. The design features for trayed columns were also reviewed in detail.

Part 2 focused on the internals specified for SWSs with a
packed tower design. The types of liquid distributors used
in SWS service were discussed, along with a comparison of
HETP values from vendors, actual operation and SMEs. Other
packing recommendations were provided, and operating issues
encountered in SWS systems were also discussed. Even with
all the upstream equipment designed to remove contaminants,
corrosion products can still accumulate on the tower internals
and cause fouling. Salt solids formation-such as NH4HS,
NH4CO2NH2 and NH4HCO3 solids-can also occur at cool
spots in the overheads line, requiring proper temperature management. Regular maintenance and monitoring can improve
SWS performance and extend the run time for the system,
which will benefit overall refinery operations.
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
Weiland, R. H. and N. A. Hatcher, " Sour water strippers exposed, " Laurance Reid
Gas Conditioning Conference, Norman, Oklahoma, 2012.
16
Kister, H., Distillation Operation, McGraw-Hill, 1990.
17
American Petroleum Institute, Division of Refining, Manual on Disposal of Refinery
Wastes: Volume on Liquid Wastes, Washington, DC, 1969.
18
Dobis, J., J. Cantwell and M. Prager, " Damage mechanisms affecting fixed equipment
in the refining industry, " Welding Research Council, Bulletin 489, 2004.
19
Armstrong, T., B. Scott, K. Taylor and A. Garder, " Sour Water Stripping, " Today's
Refinery, June 1996.
15

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