(586b) Quantification of Fugitive Emissions for Proposed Chemical Plants

Fugitive emissions can be defined as 'leaks' or 'releases' that occur wherever there are discontinuities in the solid barrier that maintains containment. These emissions that cannot be caught by a capture system are originated from unanticipated or spurious leak from anywhere in an industrial site. Commonly, fugitive emissions involve leaks that will rapidly evaporate. Thus, they are difficult to be identified. Despite being very small and mostly invisible to the naked eye, fugitive emissions are the main sources of origin of the continuous background exposure to workers. Cumulative emissions from piping components at a source may comprise a significant portion of the total emissions emitted, simply due to the presence of large number of components.

Information on fugitive emissions especially from chemical processes is substantial for economic, occupational health, and environmental assessment reasons. It is critical to have this information as early a possible, before the plant progresses to the construction phase. It is easier and cheaper to apply changes using an eraser rather than a hammer. Basic engineering is the last step to make the changes at a moderate cost.

In operating plants, fugitive emissions are measured as part of the plant-monitoring program. However, direct measurement is not possible for a plant, which is still ‘on paper'. As a process evolves through its development stages, designers and engineers have better access to more process data. Therefore, we propose a set of methodology for estimating fugitive emissions that are potentially emitted from a chemical process. The methods were developed based on the type and amount of information available in different process design stages; simple process flow diagram (PFD), detailed PFD, and piping & instrumentation diagram (PID). They offer variations in terms of the simplicity, results accuracy, and data requirement.

In general, simple PFD comprehends simplified process diagram and process descriptions found in patents or literatures such as encyclopedias. With this limited process data, the task of quantifying fugitive emissions can be tough. Thus, the emission rates for standard module types in a chemical process (e.g. distillation, absorber) were precalculated. The calculation was based on the Average Emission Factor Approach, as devised by the U. S. Environmental Protection Agency (U. S. EPA). This approach estimates fugitive emissions on the basis of the fluid service types; gas/vapor, light liquid, and heavy liquid. To ensure comprehensiveness, the emissions from each module stream were calculated for all possible types of service. For the purpose of occupational health assessment, which is our research interest, it is important to know the emissions rate of the individual chemicals present in the process. Due to the lack of process data, the most toxic chemical of the stream mixture is determined to represent the stream emission rate.

Detailed PFD offers additional process data of mass and energy balances. The overall approaches of fugitive emissions estimation are similar to those for a simple PFD. However, the estimation approach is tailored with the availability of mass balances data to improve the results accuracy. At this stage, fluid service type is possible to be determined based on the chemical weight composition of the mixture and process conditions. Also, the stream emission rates are corrected with the respective weight composition.

PID provides more comprehensive plant data, which includes the piping and equipment details. Hence, fugitive emissions can be estimated more accurately based on the specific type and number of piping components in the plant. A table of emission factors for various types of piping components is provided to aid the estimation. As for the detailed PFD, the emission rate of each process stream is corrected with the respective chemical weight composition.

The estimation methods will not give exact results as the actual measured values. The application of case study evaluations found that the PID gave estimated values closer to the measured values in comparison to the simple and detailed PFDs. However, these methods do provide a simple and swift estimate that can be used as a general indication of emissions in a proposed chemical plant. We hope, with the introduction of these methods, awareness on the impact of fugitive emissions will be gradually increased and this aspect will be further looked into when developing a chemical process.