Partially filled vessel purge

Model definition   Launch model

Filled or partially filled vessels are often purged for following reasons:

  • Maintain vapour concentration below the inferior explosive limit,
  • Inert installation to preserve products quality,
  • Etc.

For the evaluation of emissions that occur during partially filled vessel purge operations, applying the standard surface evaporation model is inappropriate (i.e. transfer coefficient are calculated with a wind speed taken at 10 meters above the surface).

Envmodels propose a standard simple model based on a basic evaluation of the degree of VOC saturation for the vent gas. The saturation level depends on following factors:

  • Purge flow rate in the vessel,
  • Agitation or not in the vessel (quiescent or turbulent liquid surface),
  • Specific vessel geometries.

Example:

An agitated reactor is purge with a 10m3/h inert gas. Mass composition of solvents in the reactor is:

  • Toluene: 30%,
  • MEK: 20%,
  • Xylene: 50%.

Temperature of the material in the vessel an the exhaust gas is 25°C (298.15°K). The purge operates 1000 hours per year.

Calculated emissions (for one year) are:

  • Toluene: 432.3 kg
  • MEK: 980.5 kg
  • Xylene: 202.7 kg

References :
  • EPA. 1994. Control of Volatile Organic Compound Emissions from Batch Processes - Alternate Control Techniques. U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, EPA-453/R-93-017.

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