(34d) Sour Water Strippers Exposed

Sour Water Stripping:
The Process Exposed

Ralph
H. Weiland, Nathan A. Hatcher

Optimized
Gas Treating, Inc.

Sugar
Land, TX

ABSTRACT

The
sour water system is often considered to be a refinery's sewer.  It consists of water used for quenching and
scrubbing HDS cold separators, sulphur plant vent gas, blow-down from amine
regenerator reflux sections, and waste water from a variety of other
operations.  Its major components are
ammonia and hydrogen sulphide with usually lesser amounts of carbon dioxide and
heat stable salts (HSSs are mineral acids), and sometimes amine.

Sour
water strippers (SWS) are moderately-large reboiled towers (30 to 60 trays) in
which ammonia and other gases are removed from the sour water by steam
stripping.  Heretofore they have been
designed using equilibrium stages. However, tray efficiencies have remained
obscure with quoted values anywhere from 15% to 40%, a factor of two range. Consequently,
designers have less than complete confidence in the reliability of their final
design. The consequence of uncertainty is either overdesign and, therefore,
excessive costs or sleepless nights because of an underperforming unit.

Recently
a mass transfer rate-based simulation model has become available for designing
and troubleshooting sour water strippers. In this paper, we use the model to
determine tray efficiencies for ammonia and H₂S stripping, how they
vary across the height of a tower, and what operating variables affect them,
and how.  We also predict quantitatively
how the presence of heat stable salts and caustic injection for pH control
affect the treat-ability of sour water to specified residual levels of ammonia
and H₂S.