The quest to reduce fugitive emissions has taken on new urgency at many sites. This reflects increased attention to operational issues, such as product purity and losses, as well as demands by the U.S. Environmental Protection Agency (EPA), Washington, D.C., for more stringent leak detection and repair (LDAR) strategies.
One company that has successfully lowered leaks is Air Products & Chemicals, Allentown, Pa. It has achieved significant reductions at its two plants that manufacture nitrogen trifluoride (NF3) — in Hometown, Pa., and Ulsan, South Korea. The chemical, which is classified as a greenhouse gas (GHG), mainly is used for cleaning chemical-vapor-deposition chambers for making liquid crystal displays and thin-film solar cells.
Because of the purity demanded by users, leak tightness figured significantly in the design of the NF3 manufacturing process. It features welded systems, bellows valves, diaphragm compressors and sealess pumps to minimize leaks. However, despite the stringent design standards, leak points still existed — so, the company initiated a project to identify them.
"The initial scope of the project was to identify operator actions, through observation and interviews, in order to understand why and how product was being lost, especially during startup, shutdown and upset conditions. The 'non-optimal' operating conditions were noted, and a program was put in place to document and calculate the amount of product which was released," explains Jim Kourpas, global plant process engineering manager.
Air Products relied on a straightforward approach: track times when the units were operating but not producing product, in other words, stable operations, and calculate the amount of product lost. This led to modifications in the work process, simplifying the operators' actions as well as reducing vent losses through automation and reprocessing. The project halved NF3 leakage to approximately 2% of total production.
However, the company wanted to cut leakage to 0.5% and, thus, initiated a further project. "Ideas were generated, further focusing on vent abatement and reclamation. The entire process was again evaluated from production down to the supporting systems. There was debate as to the location of where this next opportunity occurred, in what parts of the process and utility systems this remaining inefficiency existed. Plant engineers performed detailed sensitivity studies of each plant area to arrive at an estimate of the losses with a high confidence of accuracy," says Kourpas.
As part of this detailed evaluation, the engineers looked at every unit operation in the NF3 process, performing an energy and material balance for each. The idea was to close the balance around the entire process from raw material feed to final product. The leak volumes determined via the energy and material balances closely matched those calculated from the amount of product lost during "non-optimal" operating conditions. "This helped us pinpoint product losses through some other previously unexamined leak points like our in-process analytical systems. This also revealed there was a significant margin improvement opportunity by conserving product," he notes.
The overall evaluation also considered fugitive emissions. A research and development team assisted in creating a series of analytical tests around the facility to determine if something was missed in closing the mass balance.