Depressurisation model

Model definition   Launch model

Depressurisation operations can be realised on various industrial installations: pressure filter, various vessels, centrifuge, etc.

Atmospheric emissions during depressurisation operations are the result of the presence of free solvent vapour in equilibrium with a solvent liquid phase (or residual solvent in cakes for some filtration or centrifuge operations).

In order to apply depressurisation model, following assumptions are made:

  • The system pressure is decreased linearly over time,
  • Air leakage into the vessel during the operation is negligible,
  • The liquid and gas space temperature remains constant during the operation,
  • The vapour space of the vessel remains in equilibrium with the volatile liquid contents or residual solvent during the depressurization process.

The following picture represents an underpressure reactor.

Example 1 :

A 1 m3 pressure filter is used to compress a powder containing acetone at 25°C. At the end of the filtration, pressure system is 4 bars. Residual compressed cake as a 0.4 m3 volume.

The system is then depressurised (to discharge of its contents) with a direct atmospheric vent.

Calculated acetone emissions are: 0.717 kg.

Example 2 :

A 6 m3 reactor contains 4 m3 of mixed solvent at 20°C. Weight fractions are: 20% of toluene, 50% of xylene and 30% of MEK.

In order to make a vacuum distillation, pressure is reduced from atmospheric pressure to 150 mbar.

Calculated atmospheric emissions are as follow:

  • Toluene : 0.097 kg
  • Xylene : 0.066 kg
  • MEK : 0.498 kg

References :
  • Hatfield, J. A. Improved Algorithm for Estimating Process Emissions from Batch Depressurization, Environmental Progress, Vol. 17 No. 3, pp. 195-198 (Fall 1998).
  • EIIP. 2007. Methods for Estimating Air Emissions from Chemical Manufacturing Facilities Chapter 16 in EIIP Volume II.

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