The Vent Sizing Package 2 (VSP2™) is the most widely used low thermal-inertia (low phi-factor) adiabatic safety calorimeter. The VSP2 is designed for laboratory simulation of runaway chemical reactions, which are typically unintended and often unknown. The VSP2 is fundamentally different from a reaction calorimeter, which is typically operated under isothermal conditions (where thermal inertia does not matter) and measures heat flow and gas evolution, usually for the intended process chemistry. VSP2 results include adiabatic rates of energy and gas release as well as total adiabatic temperature rise, and these data are directly applicable to process conditions.
The commercial version of the original DIERS bench-scale apparatus, invented at FAI in the 1980's, the VSP2 sees daily use in FAI’s own fully equipped thermal hazards laboratory and in the top process safety laboratories around the world. Although originally conceived as a means to develop the runaway reaction data for pressure relief design (vent sizing) the versatile VSP2 provides a plethora of data for safe process design.
Introducing the VSP2
The VSP2 is essentially a 120 ml bench-scale chemical reactor housed within a protective 4-liter containment vessel. The instrument is able to run experiments in true adiabatic mode or with controlled application of external heating (e.g., fire simulation) or cooling. The operator can add or withdraw liquid or gaseous reactants at any point during an experiment.
The reactants are well mixed in a 120 ml thin-wall cylindrical test cell which is ideal for reactions requiring good agitation (e.g., emulsions and suspensions). Each test cell is tailor-made with appropriate materials (typically stainless steel or Hastelloy) and configured to mimic process conditions. The VSP2’s sample size is typically 40-80 ml. This ensures a representative sample and good accuracy when adding ingredients such as catalysts that make up a relatively small percentage of the mixture.
Image Credit: Fauske & Associates
The VSP2 utilizes an innovative automatic pressure-balancing system to prevent rupture of the thin-walled test cell from high pressure during a runaway reaction. The test cell is surrounded by a heater assembly, so the reactants are kept adiabatic throughout an experiment. In a VSP2 test essentially all the reaction energy stays in the reacting mass, rather than being absorbed by a heavy container or lost to the environment. The closed-cell operation yields continuous pressure-temperature data during a runaway reaction; this information is key to performing detailed vent sizing calculations.
The VSP2’s versatile and innovative design enables the accurate simulation of virtually any upset (abnormal) condition with the potential to lead to a runaway chemical reaction, so that process changes can be implemented to prevent an accident or to mitigate the possible consequences.
Image Credit: Fauske & Associates
The most important VSP2 data are the adiabatic rates of temperature and pressure change, which can increase exponentially with temperature, and which are critical input for any emergency relief system (ERS) design. The versatile design lets users directly simulate process upset conditions which might be identified during a Process Hazard Analysis (PHA), including:
- Loss of agitation or cooling
- Mischarge or accumulation of reactants
- Contamination of batch
- Resident incubation time
- Thermally initiated decomposition
A wide range of possible testing configurations allows for:
- Solids, liquids or two-phase mixtures
- In-situ liquid/gas sampling or dosing
- Closed or open (vented) tests
- Scaled blowdown simulation
- Determination of two-phase flow regimes
VSP2 data empowers users to comprehensively characterize chemical reaction hazards and determine essential process safety parameters. For example:
- Required size of emergency relief system (ERS)
- Adiabatic temperature and pressure rise rates (dT/dt, dP/dt)
- Total adiabatic temperature rise (ΔTad)
- Vapor pressure data
- Heat of reaction or mixing
- Time-to-maximum rate (TMR)
- Temperature of no return (TNR)
- Self-accelerating decomposition temperature (SADT)