Material loading in a partially filled vessel with miscible contents

Model definition   Launch model

This loading model is relatively similar to a standard loading model but needs some more calculations.

When a liquid mixture is charged to a vessel that already contains process material from a prior process operation then the vessel contents composition will dynamically change as the charging operation takes place. The equilibrium vapour composition above the batch will also change in accordance to the batch composition.

This model permits to calculate the evolution of the vapour composition at any point in the filling operation.

For that, the degree of dilution of the inlet stream mixture and for the initial vessel contents are calculated. Then, batch composition that exists during the filling operation may be calculated using the composition of each mixture and its corresponding integrated average dilution factor.

The developed model permits two filling operation modes:

  • subsurface,
  • above surface addition.

When the filling operation involves subsurface addition then the inlet stream is exposed to the vessel headspace only as it mixes with the vessel contents. In this case only the average batch composition that exists during the filling operation is used to calculate the average vessel headspace vapour composition.

However, if the operation involves above surface addition then the inlet stream is exposed directly to the headspace in the vessel. The equilibrium vapour pressure of the inlet stream must then be considered as an independent source of vapors in addition to the average batch composition.

For this model it is considered that the temperature of initial products and added products does not change during loading operation. Calculation of vapour pressure for each product is done at the specified temperature.

Example:

1000 kg of acetone at 20°C are loaded in a vessel (vented to atmosphere - system pressure is 760 mmHg) that contains 2000 kg of heptane and toluene.

Initial mixture temperature is 20°C and weight liquid fractions of heptane and toluene are respectively 40% and 60%.

Loading method is subsurface addition.

Calculated emissions are as follows :

  • Acetone : 0.978 kg
  • Heptane : 0.065 kg
  • Toluene : 0.06 kg

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