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PH and ORP Control Strategy for H2S Removal in a Packed Bed Scrubber

hehw (Chemical) (OP)
5 Oct 07 15:10
We have a packed bed wet scrubber for H2S removal from our offgas. Two control strategies were installed by the manufacturer: Peroxide to control ORP and NaOH to control PH. Both are independent of each other. Both the ORP and PH are measured in, and NaOH and Peroxide added to the recirculating line going to the top of the column. These two control strategies inteact. The peroxide causes the PH to drop like a rock and the NaOH affects the ORP. My strategy is this: Controlling PH is more important than controlling ORP and Peroxide affects the PH more than NaOH affects the ORP. So I am thinking of controlling the PH to a setpoint and the ORP to a setpoint range. I am also  thinking that I can either use Feed Forward or use a Ratio control strategy. Any thoughts? What are people doing in industry?
kenvlach (Materials)
5 Oct 07 19:59
As concentrated (50%) hydrogen peroxide is very acidic [pH of 1-3 per MSDS from FMC], try neutralizing the make-up solution. Although this will increase the volume of peroxide solution added, the amount of NaOH solution will ~proportionally decrease. Also, you'll get finer ORP control (e.g., less overshoot) using more dilute make-up solution.

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