The VSP is a laboratory tool which allows a hazards assessor to collect precise ‘off-normal’ process design data. Using the VSP calorimeter, experiments can be performed to model upset scenarios such as loss of cooling, loss of stirring, mischarge of reagents, mass-loaded upset, batch contamination and fire exposure heating.
As these experiments are performed prior to taking a chemical process from the small lab scale onto the pilot plant and manufacturing scales, the data needs to replicate the cooling properties of a large scale vessel. This is achieved by the calorimeter being centered around a 120ml thin walled metal test cell which is kept truly adiabatic. A set of fast acting heaters prevent heat loss from the system. Due to the low mass of the test cell relative to the reactive mass, the system has a low Phi factor which allows the direct collection of quality data that does not need to be manipulated after the experiment to correct for the heat energy that is adsorbed by the cell walls.
As the metal test cell is made with thin walls, the pressure that is developed inside will eventually bulge the cell. However, this issue is overcome by compensating for the pressure inside the vessel cell by applying an equal pressure to the outside of the can, thus keeping the ?P across the cell wall to an acceptable level. This ingenious design, first implemented by FAI in adiabatic calorimetry, allows a system with a high pressure rise rate to be studied without the test cell rupturing.
It is these three crucial factors (truly adiabatic, low phi and high pressure rise rate), that make the VSP the perfect apparatus for modelling upset scenarios on a large scale. Tests can be run in a ‘closed’ or ‘open’ test cell mode, allowing for direct venting simulation.
The data that is directly collected from the reactive system is:
- adiabatic rate of temperature rise
- adiabatic rate of pressure change
This data then allows the calculation of:
- heat of reaction or mixing (DELTAHr)
- total adiabatic temperature rise (DELTATadiabatic)
- onset temperature
- vapour pressure – temperature relationship
- self-accelerating decomposition temperature
- time to maximum rate (TMR)
Some of these parameters are then used to allow the direct calculation of process scale relief vent sizes, quench tank designs and other relief system design parameters related to process safety management.
The VSP2 is a true adiabatic safety calorimeter for process hazard characterisation.
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