Arsine, Diborane, and Phosphine
These gases (used as dopant gases in the manufacture of electronic circuitry) have been scrubbed
effectively using oxidizing solutions generally stabilized with an alkali to maintain a basic pH.
Since the absorption of these compounds is relatively difficult, it is essential that a deep
counter current packed bed or, if necessary, two beds in series are used to achieve the low
discharge gas concentrations required. Potassium permanganate is used often as the oxidizer but
care must be taken when using this oxidizer because of the relatively low solubility of the
permanganate itself and some of its reaction products in water.
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BF3 and BCl3
These compounds hydrolyze readily and form products of reaction which are water soluble and
generally reactive with alkali compounds. They also tend to form extremely fine aerosols
(sub-micron sized) which are not effectively removed by conventional wet scrubbers. Although
sometimes low in concentration these aerosols produce observable and objectionable plumes.
Croll-Reynolds has developed a multiple staged approach (for low gas flow application) consisting
of two Jet Venturi Fume Scrubbers in series or a single Jet Venturi Fume Scrubber followed by a
Counter Current Packed Tower with an irrigated mesh mist eliminator section and final fiber bed
filter. This ensures complete absorption of the BF3 and/or BCl3 as well as capture of any aerosols
formed.
For higher gas flow applications a configuration, a scrubber/wet ESP combination can be the more
cost–effective solution. The CR-5 Multi-Rod Deck Scrubber can be used to provide high efficiency
removal of the acid gases with the CONDENSING WESP™ vertical tubular wet electrostatic precipitator
employed as a final "polishing" for the remaining sub-micron particulate.
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Halogenated Silicon Compounds: SiCl4, SiClx, SiF4 (Case history available)
Croll-Reynolds’ has found that the combination of multiple air pollution control technologies
is the most effective way to handle this difficult application. The keys to success are the
abilities to scrub acid gases effectively, remove fine particulate efficiently, and handle
the gel-like substances formed with low maintenance requirements.
These compounds hydrolyze with water to form gel like compounds and the halogen acid. For
instance, when SiCl4 is contacted with water it forms silicon gels and Hydrochloric Acid.
The gels will easily adhere to equipment surfaces, spray nozzles, etc. and cause difficult
maintenance problems. The use of an aqueous NaOH scrubbing solution results in the formation
of more easily handled silicon gel complexes than if in the presence of water only. Of course,
the NaOH also reacts with and neutralizes the halogen acids. In addition, if some of the reactions
occur in a moist gaseous phase, extremely fine particulate may form.
The Jet Venturi Fume Scrubber first stage is ideal for the initial contact where the gels are
formed, removed, and acid gas absorption begins. The extremely open body design and the large
open spray nozzle minimizes maintenance as compared to other types of wet scrubbers. The Jet
Venturi Fume Scrubber can also be configured to have a top gas inlet and side liquid inlet
(the reverse of the standard design) which allows for the use of tangential wash nozzles in
the upper portion of the body. This can further reduce down time since the gels can be effectively
washed from the body walls.
The Croll Reynolds CR-5 Multi-Rod Deck Scrubber can then be used to provide the required high
efficiency removal of the generated acid gases (HCl, HF, etc.). The CR-5 design is self-cleaning
and not affected by the residual "gels" that may be present.
Finally a Croll Reynolds CONDENSING WESP™ vertical tubular wet electrostatic precipitator is
employed to "polish" the remaining sub-micron particulate. The self-cleaning action of this unique
technology is extremely advantageous since some of the fine particulate can be extremely "sticky"
creating significant reliability problems for other fine particulate control technologies.
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