Assessing the global environmental impact of airborne releases

This work presents a method for use in the earliest stages of development and design, for assessing the potential environmental impact of airborne releases from a chemical production plant. The principal output of the method is the value of an atmospheric environment friendliness metric called the Atmospheric Hazard Index (AHI). A catastrophic failure of the plant is assumed and the consequent impacts on the atmospheric environment are estimated, scaled and combined. The method is designed for assessing possible alternative process routes (the raw material(s) and the sequence of reaction steps that converts them to the desired product(s)) to make a chemical, in order to determine the route that has the least adverse atmospheric environmental impact. Thus the routes that are inherently environmental hazardous can be identified and avoided, when the selection is made in the early stages of production plant design. The atmospheric impact categories considered are toxicity, photochemical smog, acid deposition, global warming and stratospheric ozone depletion. The magnitude of these impacts are expressed on a scale of 0 (minimum) to 10 (maximum). Each of these impact categories is assigned an importance factor value that depends upon the spatial scale affected, the indirect impacts and the reversibility of the impact. These factors are then used to calculate a Weighted Category Hazard (WCH) value for each chemical. The WCH of all the impact categories and chemicals are combined to estimate the AHI. The AHI has been determined using two methods which use different chemical environmental distribution models. One method assumes the unsteady state distribution of the substance released into the environment and the other assumes steady state distribution. These two methods have been tested on six potential and established routes to methyl methacrylate (MMA). The most atmospheric environment-friendly route based on the overall AHI calculated using method 1 is the ethylene via methyl propionate based route and that for method 2 is the propylene based route. However, the most atmospheric environment-friendly route based on the impact due to separation inventory in both methods is the ethylene via methyl propionate based route because it is the only route which does not show an impact due to this inventory. In all the routes the storage inventory has the potential to cause the most environmental damage compared to the reaction and separation inventories.