H2S CONTROL
odor scrubbers intro
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.
Practical Considerations
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.
Evaluation Process
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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