Breakthrough Nitrogen Oxides chemistry saves costly scrubber addition at ShinEtsu America case study

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Courtesy of Bionomic Industries Inc.

By using an innovative chemistry to treat nitrogen oxides (NOx) emissions, silicon wafer manufacturer ShinEtsu America, Inc.. (SEH) avoided adding a costly new scrubber The scrubbing solution, a drop in replacement for sulfide chemistries, enables SEH to meet air and wafer emissions requirements and eliminate nuisance odors at its Vancouver. WA, facility.

The unique scrubbing solution was developed by Bionomic Industries Inc. (Mahwah, NIJ) to meet the need for a safe, easy to handle high efficiency nitrogen dioxide scrubbing solution that eliminates production of toxic and odoriferous hydrogen sulfide gas. It is extremely cost effective since it eliminates the need for a polishing scrubber stage to remove hydrogen sulfide gas. lowering capital equipment, utility and chemical operating costs.

Community Commitment
SEH had been using sodium hydrosulfide (NaSH) in its NOx scrubber to treat emissions from its cut wafer etching process. NaSH use had maintained permitted levels of hydrogen sulfide and opacity requirements, but. even very low concentrations resulted in nuisance odors. The plant's employees as well as the surrounding neighborhood, including an elementary school, often commented on the unpleasant smell.
SEH Facilities Engineer Bryan Trotter explains, 'Since we are concerned about our employees' work environment, and have a close relationship with the school and our other neighbors, we began to explore what we could do pre-emptively to improve the situa ron.

'We investigated the feasibility of adding an additional scrubber at the end of our existing unit to remove small concentrations of hydrogen sulfide gas. We learned we would have to make a large capital investment and gain a very small return. Additionally, the fact that this would have to be done within a small physical area, which would further complicate and add cost to the project.'

SEH considered converting the scrubber fo use hydrogen peroxide. After extensive testing, hydrogen peroxide was eliminated from considerations because it showed vety poor results and had a high operating cost. That led the company to assess the suitability of a solution would be an excellent drop in replacement for NaSH and provide SEH with a host of benefits.

SEH bought a test amount of the chemical and, after contacting the air and wastewater permitting authorities, arranged for on-site testing. When the scrubber was run through all the different production loops, the scrubbing solution controlled the NOx emissions well within the permit limit. Hydrogen sulfide and other sulfur compound testing was done on the exhaust to determine if there was a reduction in the sulfur compounds. Since the company had done similar testing while using sodium hydrosulfide, it was able to see that unlike sodium hydrosulfide, the product virtually eliminated sulfur compounds in the exhaust.

'Since the scrubbing solution has virtually no smell, it eliminated the unpleasant odor that operators were subjected to when even a drop of NaSH contacted their protective clothing. It also eliminated the odor that the NaSH product would infuse throughout the facility when it was pulled into the building's air handlers.'

Solution Means Savings
Trotter explains, 'Our initial test results showed that the product would work well, it would involve minimal modification to our existing scrubber and infrastructure and it would basically be a drop in replacement once we gained permit approval from the air and wastewater agencies. Both organizations were very willing to help us with that transition since we demonstrated that the product eliminated the nuisance odor and the wastewater by-product had less potential for creating sulfur acid gases m the sewer system.

'A key advantage of this solution is that we were able to retain an older manufacturing plant that is close to depreciated, without making a mqor capital investment. If we had to make that investment, it would have been greater than what we are spending for this system.
'The horizontal scrubber, located on the second floor of our facility, has a small footprint that cannot accommodate a vertical scrubber.In addition to the physical and construction constraints, there would have been considerable engineering and design time involved to add a new scrubber. This would have resulted in the challenge of remaining in production during the changeover. In our business, downtime is not an option and construction would have required a significant downtime.

'Since we began using this chemistry in September 2003. we've learned that there are opportunities for future cost reductions of approximately 5 to 10 percent. We are exploring transitioning to obtain the product in bulk supply rather than in small containers. We will investigate running the system at a lower pH to capture some caustic operating cost savings. Currently, we're running at 12.5, but our permit limit is set at no lower than 11.'

The Process
During SEH's cut wafer etching process silicon wafers are etched in a bath including nitric acid, acetic acid and hydrofluoric acid, which creates nitrogen oxide gases.

In the scrubber, the exhaust gas travels horizontally and the scrubber liquor travels vertically. Although the design of the scrubber was not changed, at the same time the solution was introduced. SEH made improvements to the scrubber including reducing airflow to lower the velocity and increase the residence time.It also converted the fan to variable speed and made instrumentation upgrades.

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