POLLUTANTS
Acid Gases
Bromine and Fluorine
Bromine (Br2) and Fluorine (F2) are typically scrubbed in a counter current
packed tower to achieve maximum efficiencies using a dilute aqueous alkali solution. For applications
with very high concentrations of these gases it may be useful to use a Jet Venturi Fume Scrubber as a
first stage to remove the bulk of the contaminant then "polish" with a packed tower scrubber. With
respect to F2, the concentration of the alkali in the aqueous solution must be maintained at
a specific level to avoid the generation of undesirable by-products. The alkali generally preferred is
KOH due to the greater solubility of KF in water than NaF.
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Chlorine
Chlorine (Cl2) is often scrubbed with dilute aqueous NaOH in a Counter Current Packed Tower.
Water scrubbing is impractical. The use of a Jet Venturi Fume Scrubber may be advantageous where
Cl2 concentrations are high, however a single stage system is rarely enough used alone.
NaOCl, which may be unstable depending upon its concentration and solution temperature, is one of
the reaction products in this process. A relatively strong alkali solution is preferable therefore,
to maintain stability.
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Hydrogen Bromide and Hydrogen Iodide
The general method of scrubbing described below for Hydrogen Chloride would also apply to these gases.
However, there is little opportunity to recover these acid gases in an aqueous solution.
Hydrogen Chloride
The sources of HCl range from simple HCl holding tank exhausts to process exhausts where
HCl may or may not be mixed with toxic gases, to thermal oxidizer exhausts where chlorinated
organics are incinerated.
In most instances HCl is scrubbed either with a Jet Venturi Fume Scrubber (typically achieving
95% removal efficiency in a single stage), Multi-Rod Deck Scrubber (up to 99% removal) or with a
Counter Current Packed Tower (able to achieve
efficiencies exceeding 99.9% removal). If a gas stream contains high concentrations of HCl, a Jet
Venturi Fume Scrubber may be followed by a Packed Tower or Multi-Rod Deck Scrubber. The Jet Venturi removes the bulk of the
HCl and the Packed Tower or Multi-Rod Deck Scrubber acts as a "polishing" stage achieving the necessary performance to attain
the low limitations required for atmospheric exhaust.
Aqueous Sodium Hydroxide (NaOH) is commonly used as a scrubbing fluid. If, however, HCl is
concentrated in a gas stream, it may be practical to utilize a multiple stage approach. The initial
stage scrubs and recovers usable HCl and the final stage utilizes the NaOH to meet low ppm discharge
concentrations.
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Hydrogen Cyanide
HCN is typically scrubbed with an aqueous NaOH solution however, in certain circumstances, the
addition of Sodium Hypochlorite (NaOCl) to the solution increases performance and/or produces
desirable reaction products.
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Hydrogen Fluoride
The general method of scrubbing described above for Hydrogen Chloride would also apply to Hydrogen
Fluoride (HF). Due to its vapor pressure characteristics in an aqueous solution, however, it is less
likely to be recovered in a usable form. Because the product of the reaction of HF with NaOH (NaF) has
limited solubility in water, it is usually more practical to use a KOH (potassium hydroxide) aqueous
solution for scrubbing. Its reaction product (KF) has significantly greater water solubility.
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Nitric Acid
The scrubbing of Nitric Acid is similar to the scrubbing of HCl described above both in the use
of aqueous NaOH and in the potential for recovering HNO3. Often, NOx gases will be formed in the
presence of concentrated HNO3. NOx gas requires more specific methods of scrubbing. See
NOx
section.
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Sulfur Dioxide
Croll-Reynolds has a wealth of experience in the treatment of Sulfur Dioxide (SO2). Applications are
almost always either process or combustion source related.
SO2 is readily scrubbed in either a Jet Venturi Fume Scrubber (up to 98% efficiency), Multi-Rod Deck Scrubber, or a Counter
Current Packed Tower (99.9+% efficiency) utilizing aqueous NaOH. Croll-Reynolds has also designed
removal systems utilizing Jet Venturi Fume Scrubbers recirculating "lime" slurries or carbonate liquors.
The fact that solids exist in these alkali liquids does not prevent the use of Jet Venturi Scrubbers
since this design is open and difficult to plug.
