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New stripper design boasts high efficiency with less equipment
A new design for ammonia stippers, which removes
98% of the NH3 found in wastewater discharge from semiconductor
operations, has been developed by Total Solutions International.
Because of the stipper'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
sin 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 with random packing) 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 effetive at removing NH3 from other streams with low suspended solids and low calcium concentrations, and can be configured from 10-150 gal/min.
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