As a conclusion it can be said that the intake and concentration-based methods are not generally options since the intake-based method is less frequently applicable. It can be used mainly for evaluating the carcinogenic risk. The concentration-based approach is more widely applicable. The point of view of the methods is however different. Intake-based method mostly reflects the risk of cancer whereas the exposure limit-based method reflects different health risks and nuisances.
5. Discussion
This paper discusses how chemical exposure risk can already be estimated in the PFD stages. The chemical concentration and the intake amount are first calculated using a standard process module based method. In the simple PFD phase, the estimates serve as an indicator of the maximum exposure since the stream compositions are unknown, and they are assumed to consist solely of the most dangerous component. In this stage average emission factors are used, which are based on standard technology resulting in a larger emissions estimate. Furthermore, full working time exposure is also assumed for workers in this stage.
The accuracy of the estimation method in simple PFD phase is related to the unknown compositions of process streams. Meanwhile in both PFD stages, the uncertainties are contributed by the applicability of the standard modules for the particular case studied and the accuracy of plot area estimations. Also the wind speed conversion to lower altitude depends on experimental parameters.
6. Conclusions
A method for estimating inhalative exposures is proposed for occupational health risk evaluation during the development and design stages of chemical processes. The risk evaluation can be performed through chemical concentration or intake-based methods.
Both the exposure concentration and the intake amount can be calculated by standard process module based approach in the PFD stages. To depict the realistic exposure scenario local wind speed distribution is used. The results of the assessment may be used to characterize the exposure risk and to compare the design concepts based on health aspects.
The concentration-based method was tested with six alternative processes for MMA production. The result suggests the C3 as the most harmful route to health, whereas the C2/MP is the best. Data on critical wind speed was produced, from which the percentage of time the exposure is above exposure limit values can be determined. This may already provide an idea about the relative exposure level of the process concepts studied.
Both the concentration- and the intake-based risk assessment approaches were applied on the route containing a carcinogen. The study reveals that the acceptable concentration calculated based on 10−4 risk benchmark is smaller than that based on 10% of the exposure limit, indicating the intake-based is stricter than the concentration-based approach. The same trend is apparent for other carcinogenic compounds discussed. This is because of the different background of exposure limit values and slope factors used.
The method developed is simple and flexible for large scale continuous plants involving volatile compounds such as petrochemical plants and oil refineries. The estimation can be done in any process development or design phase (preliminary PFD, PFD or PID stages). The method of fugitive emission calculation however varies depending on which design stage is considered. The method allows foreseeing the potential exposure risk of competing processes or the risk level of a process already in the development stage. This enables early actions on process route selection or choice of dedicated technology, such as leak proof valves or hermetic pumps, to reduce occupational exposure risks.