Table 2. Process/tasks and emission
| Process/task | Potential emission/exposure points | See section | See figures |
|---|---|---|---|
| Production of bulk nanomaterials | Reactor fugitive emissions
Product harvesting Reactor cleaning |
3.4.1
3.4.1 3.4.1 |
7, 8
12 |
| Downstream processing | Product discharge/bag filling
Bag/container emptying Small-scale weighing Machining of products |
3.4.3.1
3.4.3.2 3.4.2 3.4.3.4 |
14, 15, 16
17 10, 11, 12, 13 |
| Product packaging | Small-scale weighing/handling
Large-scale weighing/handling Product packaging |
3.4.2
3.4.3.3 3.4.3 |
10, 11, 12, 13
18 14, 15, 16, 18 |
| Maintenance | Facility equipment cleaning
Air filter change-out Spill clean-up |
3.4.4
3.4.4.1 3.4.4.2 |
19 |
3.4.1 Reactor Operation and Cleanout Processes
Harvesting material from reactors has been identified as a potentially high exposure activity in several manufacturing plants [Demou et al. 2008; Lee et al. 2010, 2011; Methner 2008; Yeganeh et al. 2008]. In addition, cleanout of reactors has contributed to increasing facility concentrations and exposures to operation and maintenance workers. Leakage from pressurized reactors can also contribute to background concentrations and result in exposure to employees throughout the facility. When the reactors are small, some facilities have placed them inside fume hoods to help control fugitive emissions. Two studies have shown that when the reactor is housed in a well-designed and operated fume hood, particle loss to the work environment is low [Tsai et al. 2009b; Yeganeh et al. 2008]. When the reactors are larger, enclosures can be built that isolate the reactor from the environment and seek to reduce fugitive emissions (Figure 7).
Methner et al. [2010] summarized airborne measurements in 12 facilities that processed nanomaterials, including manufacturers and research and development labs. The authors found that some of the highest measured exposures occurred during reactor cleanout tasks, which included brushing and scraping slag material from the reactor walls and during torch cleaning. Demou et al. [2008] evaluated exposure to nanoparticles at a pilot-scale nanomaterial production facility. The major emission source was determined to be the production unit as the airborne particle concentrations rose when the unit was started and fell when production rate was decreased. The other task that resulted in substantial particle release was cleaning of the reactor using a vacuum cleaner not fitted with a HEPA filter. Evans et al. [2010] studied nanoparticle concentrations in a facility that manufactured and processed carbon nanofibers (CNFs). During the thermal treatment of the CNFs in a reactor under positive pressure, elevated concentrations of non-CNF ultrafines were released.
