Heat variation models
During many industrial process operations, temperature evolution (increase) can occur:
- By voluntary heating of the installation (some times necessary for production),
- By progressive heat-up due to process operation (high-speed dispersion, grinding, exothermic reactions, etc).
When a vessel containing a volatile liquid and a non-condensable gas (e.g. air) is heated at constant pressure, the vapour space gas undergoes expansion and a portion of the gas phase leaves the vessel through the vent. Additionally, the saturated vapour pressures for the volatile liquid components increase.
Envmodels propose two standard different models for estimating emissions during temperature evolution processes:
- Heat-up model used for low temperature increase,
- Heating model used when increase temperature becomes important and approaches boiling point (but doesn't reach it).
The two approaches yield similar results when the amount of heat-up is small, and the final temperature is well below the boiling point of the liquid mixture. However, the disparity between the results from the two options increases as the final temperature approaches the boiling point in this case heating model is the better choice).
The hypotheses assumed for the use of these models are following:
- The vessel is closed during the operation, but it is possible for vapours to be vented during operation (with a single vent for example), no forced ventilation is used,
- No material is added during operation,
- The displaced gas is assumed to be saturated with VOC vapour in equilibrium with the process material,
- Total pressure in the vessel is constant at atmospheric pressure.
- The vapour pressure in the vessel never rises atmospheric pressure.
