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.