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Abstract
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Inherently safer design (ISD) concepts have been
with us for over two decades since their elaboration by Kletz [Chem. Ind. 9
(1978) 124]. Interest has really taken off globally since the early nineties
after several major mishaps occurred during the eighties (Bhopal, Mexico city,
Piper-alfa, Philips Petroleum, to name a few). Academic and industrial research
personnel have been actively involved into devising inherently safer ways of
production. The regulatory bodies have also shown deep interest since ISD makes
the production safer and hence their tasks easier. Research funding has also
been forthcoming for new developments as well as for demonstration projects.A
natural question that arises is as to how to measure ISD characteristics of a
process? Several researchers have worked on this [Trans. IChemE, Process Safety
Environ. Protect. B 71 (4) (1993) 252; Inherent safety in process plant design,
Ph.D. Thesis, VTT Publication Number 384, Helsinki University of Technology,
Espoo, Finland, 1999; Proceedings of the Mary Kay O'Connor Process Safety
Center Symposium, 2001, p. 509]. Many of the proposed methods are very elegant,
yet too involved for easy adoption by the industry which is scared of yet
another safety analysis regime. In a recent survey [Trans. IChemE, Process
Safety Environ. Prog. B 80 (2002) 115], companies desired a rather simple method
to measure ISD. Simplification is also an important characteristic of ISD. It is
therefore desirable to have a simple ISD measurement procedure.The ISD
measurement procedure proposed in this paper can be used to differentiate
between two or more processes for the same end product. The salient steps are:
Consider each of the important parameters affecting the safety (e.g.,
temperature, pressure, toxicity, flammability, etc.) and the range of possible
values these parameters can have for all the process routes under consideration
for an end product. Plot these values for each step in each process route and
compare. No addition of values for disparate hazards (temperature, pressure,
inventory, toxicity, flammability, etc.) is being suggested to derive an overall
ISD index value since that conceals the effects of different parameters.
Further, addition of numbers with different units ( degrees C for temperature,
atm/bar for pressure, t for inventory, etc.) is inappropriate in scientific
sense. The proposed approach has a major advantage of expanding consideration in
future to incorporate economic, regulatory, pollution control and worker health
aspects, as well as factors such as the experience one has or 'the comfort
level' one feels with each of the processes under consideration.
Additionally, it would also guide the designers and decision makers into
affecting specific changes in the processes to reduce the unsafe features.We
demonstrate our simple approach by using the example of six routes to make
methyl methacrylate as documented by Edwards and Lawrence [Trans. IChemE,
Process Safety Environ. Protect. B 71 (4) (1993) 252; Quantifying inherent
safety of chemical process routes, Ph.D. Thesis, Loughborough University,
Loughborough, UK, 1996] and show that the decision could well have been
different if addition of disparate hazards had not been done. |