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New stripper design boasts high efficiency with less equipment
A new design for ammonia strippers, which
removes 98% of the NH3 found in wastewater discharge from semiconductor
operations, has been developed by Total Solutions International.
Because of the stripper's high efficiency, it is possible to comply
with discharge limits of less than 10ppm. To reach these removal
levels, previous designs required two trains of a stripper, absorber
and blower operating in series, says process engineer John Strandberg.
The new configuration uses two stripping towers in series
with one absorber tower. This makes it possible to eliminate
a blower and an absorber tower, which reduces costs and
footprint, he says.
Wastewater enters the plant and the pH is
raised to greater than or equal to 10.5 with addition of sodium
hydroxide, which converts amonium ions to NH3. The water is pumped
through the first two towers (packed at random) and the NH3 is stripped
by a countercurrent flow of air.
Air from the first stripper is scrubbed in a
packed-bed absorber, which has used sulfuric acid recirculating the
tower to absorb the NH3 as amonium sulfate. The clean air is used in
the second, then the first, stripping towers, making for a closed
system. This has advantage of preventing evaporation losses since the
heat of reaction and blower energy are recovered. This heat causes the
system to operate at about 10°F higher than ambient temperature, which
allows for a smaller blower and less NaOH compared to open systems,
says Strandberg.
The first commercial application of the new configuration - designed to treat 38 gal/min. at SMIC's
semiconductor fabrication plant in Shanghai, China - was commissioned
in January. The system reduces the NH3 concentration from 1,500 mg/L to
less than 150 mg/L. The total estimated operating cost is $18/h for
chemicals and electricity. The system installed at SMIC cost less than
$300,000. This system would be effective at removing NH3 from other
streams with low suspended solids and low calcium concentrations, and
can be configured for 10-150 gal/min.
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