Case Pressure relief system sizing

Problems investigated

• Sizing of pressure relief valve
• Safety system constraints on the process operation
• Safety constraints included in NMPC

Method

• The Divided Difference Kalman Filter updates the model from a logged data series.
• Errors are introduced in the model, and predictions are made without updating from that point forward.

Application to vapour pressure systems

An exothermic reaction may exhibit an accelerating reaction rate if the cooling is insufficient. This is referred to as a “runaway reaction”. The runaway causes the temperature to rise, and possibly also the pressure.

A special case is referred to as “vapour pressure systems”, that is when the pressure generated by a runaway reaction is entirely due to the vapour pressure of the reacting mixture, which rises as the temperature of the mixture increases during a thermal runaway. For vapour pressure systems, the emergency relief system is designed so that the action of the pressure relief system removes vapour (and therefore latent heat) at a rate fast enough to hold the temperature  and the pressure constant.

In practice, the pressure relief system is designed so that the maximum runaway pressure is less than 110 % of the opening pressure of the relief valve.

Results

The scenario typically studied, is loss of cooling at the worst possible time during an exothermic reaction.

For a batch polymerization system,  several studies indicate that loss of reactor cooling is more critical in the middle of the batch than at the beginning and the end.

Another important and related result is that traditional “self heat rate” experiments and calculations, tend not to capture the worst case scenario and may lead to under-dimensioned safety system capacity.