How to treat polluted exhaust air
Many technical processes generate chemically aggressive and corrosive exhaust air streams that may only be released into the atmosphere in a purified state for environmental protection reasons. In many countries, legislation regulates the maximum permissible pollutant limits. Exhaust air purification systems made of plastic are used to treat chemically aggressive exhaust air and gases, as these materials are highly resistant to corrosion.
Exhaust gases when producing microchips
Such exhaust gases are also produced during the production of microchips in the semiconductor industry. Microchips are installed in a large number of electrical devices. They are used in computers and smartphones, for example, and therefore form the basis of modern digitalisation. The production of microchips generates large volume flows of polluted exhaust air. The exhaust air contaminated with acid and ammonia vapours is cleaned with exhaust air scrubbers.
An exhaust air purification system for the semiconductor industry
An existing production facility for microchips has been expanded. The investment for a new production building amounts to several hundred million euros and several hundred new jobs are to be created.
The exhaust air flows are collected separately and fed into the control centre via a coated pipe network. A total of up to 100,000 m3/h of acidic and 100,000 m3/h of ammonia-containing exhaust air must be purified. In addition to purification and compliance with the applicable limit values, energy is also to be recovered from the exhaust air flows. The volume flow is variable; for redundancy reasons, the volume flow is divided between three exhaust air scrubbers. Depending on the operating point, the scrubbers can be switched on or off.
Execution of the treatment
The exhaust air is treated by horizontal packed-bed scrubbers with integrated heat exchangers for heat recovery. All components in contact with the medium are made of corrosion-resistant plastic on both the gas and liquid sides.
Exhaust air purification is achieved by absorbing the pollutants from the gas phase into the water phase. This requires the largest possible contact surface. The surface area is achieved by using special packing with a high specific surface area. The air flow is channelled horizontally through the packing.
A circulation pump continuously sprays washing liquid from the reservoir above the filler pack. It is distributed via nozzle lances. The washing liquid flows over the tower packing and collects in the lower part of the scrubber.
In order to set the optimum pH value of the washing liquid for the cleaning process, a complete pH value regulation and chemical dosing system is installed. This means that the pollutants are not only dissolved in the water, but are also chemically neutralised. The level in the scrubber is kept constant via a level measurement. Evaporation losses and the absorption of pollutants from the air increase the salt concentration in the scrubbing liquid. To avoid unacceptably high concentrations, the conductivity in the liquid is measured. If a specified value is exceeded, water is removed from the system via the waste water valve. A droplet separator is installed after the cleaning stage to separate the entrained liquid droplets.
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