My problem when thinking about scrubbers is that most of the refineries I worked in weren't dedicated to just simple stuff like jewelry scrap. They were large "job shop" refineries that accepted anything and everything coming in the door that could turn a profit. This came from many types of industries. They ran a lot of very large items that were difficult to fit into a reaction vessel and they couldn't afford to have dedicated equipment for each type of scrap. It was much more efficient and required less labor to run a lot this stuff in a variety of multi-purpose open containers than in a closed reaction vessel. When you run maybe 100s of different type material, with each requiring a little different cycle and play-it-by-ear chemical additions, a more fixed process done in a closed reaction vessel often doesn't work that well. Therefore, these companies all had huge fume scrubbers that could handle multiple fume hoods, Pfaudler kettles, etc., etc. At one place, one of the scrubbers was 20' tall and 6' in diameter, enough to handle 40 ft2 of fume hoods, kettles, or other exhaust openings. These giant scrubbers were, in essence, the cheapest way to go in the long run.
From the control tests and calculations I used to make on these scrubbers, the numbers I gave earlier for standard NaOH scrubbers assumed 100% efficiency where, in reality, 70% was closer to the mark.
Whether large or small, though, the same math applies.
With my limited knowledge in this, the only ways I can see to be able to use a smaller scrubber for a given amount of exhaust cfm is to (1) Oxidize the NO to NO2 before it gets to the NaOH scrubber, (2) Somehow, mechanically separate the NO from the air in the scrubber and retain it for a longer period of time, or (3) Use something other than NaOH. There may be other ways of doing these things that I'm not aware of. I would suggest that you listen to 4metals. He knows more about this stuff than anybody, especially for closed systems.
