Odor Scrubbers Applications with Hydrogen Peroxide
Basis of Control
Hydrogen peroxide may be used in both mist scrubbers and packed tower scrubbers as a replacement for sodium hypochlorite (bleach). Like bleach, the process involves two concurrent mechanisms: 1) absorption of the odors (H2S) into the alkaline scrubbing solution; and 2) oxidation of the absorbed sulfide in solution.
|Step 1:||H2S + NaOH → NaSH + H2O|
|Step 2:||4H2O2 + H2S → H2SO4 + 4H2O|
Typical dose ratios are 5 parts H2O2 per part H2S or, when used in place of bleach, one gallon 50% H2O2 for every 10 gallons of 15% sodium hypochlorite (NaOCl). This generally translates into a break-even cost scenario. Sufficient caustic soda (NaOH) is added to maintain a pH of 10.0 - 10.5 in the scrubbing solution.
There is also in practice a process which uses H2O2 in series with bleach to scrub composting odors. This process relies on a series of three packed tower scrubbers: the first is a pH neutral water wash (to remove ammonia and amine odors); the second uses a conventional caustic/bleach solution in which the bleach is purposely overdosed (to oxidize the complex organic sulfur odors); and the third uses a caustic/H2O2 solution (to remove the unreacted chlorine vapors carried over from the second stage).
|H2O2 + HOCl → HCl + H2O + O2|
Typical dose ratios are 0.5 parts H2O2 per part hypochlorite (OCl-), with sufficient caustic soda (NaOH) added to maintain a pH of 8.5 in the scrubbing solution.
With the elimination of bleach, the benefits realized in using H2O2 include no formation of chlorinated VOC's, no chlorine odors due to overdosing, substantially reduced corrosion of process equipment, and reduced scaling of spray nozzles and packing material. However, because of the different properties of H2O2 versus NaOCl, the substitution is not straightforward. For example:
- Since H2O2 is more concentrated and leaves no salt residues, there is less need for blowdown in packed tower scrubbers. Typically, blowdown rates may be reduced 5-10 fold over those using bleach.
- Since the reaction between H2O2 and sulfide is slower than that between NaOCl and sulfide, a higher working concentration of oxidant is needed in the scrubber solution. Typically, this is 50 - 200 mg/L H2O2.
- Because of the requirement for a high residual concentration of oxidant, conventional control systems which rely on ORP sensing may not be suitable for controlling H2O2 feed. ProMinent Controls has developed an online H2O2 controller which has shown great value in this application.
- Some scrubbing systems which currently use bleach may be operating at a pH < 9 which volatilizes chlorine, thereby allowing direct oxidation of H2S to occur in the gas phase. Since H2O2 will not perform in this manner, changing the operating characteristics of the scrubber may highlight a overstressed design.
- H2O2 is less reactive toward organic odors than bleach and, where this is a concern (e.g., with thiols or mercaptans), the H2O2 must be activated through catalysis. This entails special considerations and the user is encouraged to contact US Peroxide for further guidance.
The evaluation process for using H2O2 in odor scrubbers begins with an initial paper assessment, but inevitably must involve a field pilot test. The following information will greatly assist in completing an initial paper assessment:
- Schematic of unit processes showing the type and number of scrubbers;
- Gas flow rates, including influent / effluent H2S concentrations;
- Comments regarding the relevance of non-H2S (organic) odors;
- Design detail on the scrubbers showing unit volume, packing type/density (if applicable), and gas/liquid contact parameters;
- Recirculation and blowdown rates for the scrubbing solution;
- Current operating and control parameters (e.g., pH and ORP); and
- Current chemical use-rates, costs, and any comments on effectiveness (for both bleach and caustic).
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