ASSESSING THE INHERENT SAFETY OF CHEMICAL PROCESS ROUTES - IS THERE A RELATION BETWEEN PLANT COSTS AND INHERENT SAFETY

Inherent safety is that which is intrinsic to a plant. It is almost self-evidently true that we should design plant to be inherently safe and it is accepted wisdom that the earlier inherent safety is considered, the better. The choice of route (raw material(s) and the sequence of reaction steps that converts them to the desired product(s)) is the key early design decision that fixes the inherent safety of a plant. However, in order to choose the 'best' from a number of alternative routes, we must quantify their inherent safety. We have developed an index for ranking the inherent safety of routes; low index values indicate an inherently safe route, while high values indicate an inherently unsafe route. Each reaction step is scored using readily-available, safety-related properties of the chemicals involved and the estimated process conditions. The index is the sum of these scores. We have tested the index on a number of routes to methyl methacrylate. We have investigated the assertion that inherently safe plants are cost-effective plants by comparing the index to the capital and production costs of the routes. The results so far are encouraging, but need checking against other assessment methods-for example, expert judgement. The relation between inherent safety and cost-effectiveness is not disproven, but needs further work to establish it.