Quantitative Measures of Inherent Safety

Although, for example, higher reactor temperature might lead to safer plant if the inventory can be reduced, it is necessary to be able to assess such changes quantitatively. Lowering the inventory makes the plant safer, but raising the temperature makes it less safe. Which effect is more significant  

On the other hand, if the hazard is toxicity, process alternatives can be compared by assessing the mass of toxic material that would enter the vapor phase on release from containment, weighting the components according to then-lethal concentration. 

Example 27.1 A process involves the use of benzene as a liquid under pressure. The temperature can be varied over a range. Compare the fire and explosion hazard of operating with a liquid process inventory of 1000 kmol at 100°C and 150°C, on the basis of the theoretical combustion energy resulting from 

Solution The fraction of liquid vaporized on release is calculated from a heat balance3. The sensible heat above saturated conditions at atmospheric pressure provides the heat of vaporization. The sensible heat of the superheat is given by  

Tsop = temperature of the superheated liquid Tbpt = normal boiling point 

However, in the early stages of design, decisions that have important safety implications must be made based on an incomplete picture. Let us explore simple quantitative measures which can be used to assist decision making in the early stages of design. 

The major hazard from the release of flammable or toxic material 

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The BMD approach has been put forward as an alternative to the no-observed-adverse-effect level (NOAEL) and lowest-observed-adverse-effect level (LOAEL) approach for health effects because it provides a more quantitative alternative point of departure for the first step in the dose-response assessment (International Programme on Chemical Safety, in press). The BMD approach is based on a mathematical model being fitted to the experimental data within the observable range and estimates the dose that causes a low but measurable response (the benchmark response) typically chosen at a 5% or 10% incidence above the control. The BMD lower limit (BMDL) refers to the corresponding lower limit of a one-sided 95% confidence interval on the BMD. Using the lower bound takes into account the uncertainty inherent in a given study and assures (with 95% confidence) that the chosen benchmark response is not exceeded. 

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