(64e) Analyzing Hazards of Sour Water Spills

Sour water in petrochemical plants can contain a significant concentration of hydrogen sulfide and ammonia. Design of sour water storage tanks should account for the hazards of a large leak. The dike area and location are important, as is predicting the extent of toxic vapor plumes.

Analyzing the risk of a large spill of sour water from a sour water storage tank requires applying dispersion analysis to predict the contours of both H2S and of NH3 plumes out to ppm concentrations. This requires first estimating the vapor pressures of the toxic gases using Henry's law constants. Fortunately, the vapor pressure of both H2S and of NH3 are reduced substantially by the ionization of the molecular species to the ionic form (hydrosulfide and ammonium ions). The equilibrium concentrations of these ions is a strong function of pH since a hydrogen ion is lost in forming the hydrosulfide ion and is gained in forming the ammonium ion. This suggests that pH adjustment can be an effective mitigation measure for a spill event.

This paper makes use of chemical equilibrium constants and of rigorous thermodynamic formulas, coupled with dispersion modeling to show how the plume contours of H2S and of NH3 vary with initial concentrations and pH.