A southwestern oil refinery had a pair of spent acid tanks for holding Hydrocarbon-Sulfuric Acid mixtures unloaded from barges.  The head space of the tanks contained n-Propane and Sulfur Dioxide (SO2) blanketed with Nitrogen, which would get displaced with inflowing liquid during filling.  The displaced gases could not be directly vented to atmosphere due to limits on allowable Volatile Organic Carbon (VOC) and SO2 emissions, SO2 being one of the six “criteria pollutants” identified by U.S. EPA’s National Ambient Air Quality Standards (NAAQS).  The SO2 would have to be controlled in a wet scrubber, whereas the n-Propane, not treatable with wet scrubbing, would most feasibly be controlled by thermal oxidation.

To avoid SO2 corrosion in the oxidizer and the need for downstream cooling, the vent gas would first be scrubbed, then oxidized.  Variable flows and concentrations would characterize the scrubber gas feed stream.  In addition, subjecting the storage tanks to more than only a very slight vacuum would lead to excessive Nitrogen flows. However, an ambient air bleed-in to minimize vacuum at low vent flows could not be used, since this would create a potentially explosive mixture of n-Propane and Oxygen.