Aerosols are suspended liquid or solid particles ranging in diameter from submicron range to
10 to 20 Ám size. Particles and aerosols of less than 1 Ám are very difficult to remove in
conventional systems like packed scrubbers, venturi scrubbers (even high pressure) or filters,
even with high energy consumption.
Coalescing scrubbers, for the removal of aerosols, function in two stages. In the first stage, water or an absorbent solution) is recirculated and atomized by hydraulic nozzles. Dust laden droplets and bigger dust particles are then separated in the droplet eliminator stage. A mass transfer packing is installed before the droplet eliminator to remove vapor content (absorption stage).
In the second stage, fresh water is atomized into droplets in the saturated gas stream by air nozzles. Liquid and pressure air are mixed together in a mixing chamber at a relatively low velocity and then accelerated to the critical (sonic) velocity of the 2 phase mixture. The pressure jump at the outlet of the nozzles leads to the generation of liquid droplets when the mixture expands in a gas atmosphere.
As the critical velocity of a 2 phase liquid/gas mixture is much lower than that of a liquid or a gaseous phase, the frictional and acceleration pressure drop remains within tolerable limits in the nozzle. The pressure jump required for the atomization can therefore be achieved with relative low energy consumption.
A super saturated gas atmosphere is achieved after atomization of fresh water in the saturated gas stream. The aerosols act as condensation nuclei and are enlarged by condensation of water vapor to a size of approximately 1 Ám. The gas stream is led to a coalescence stage where the aerosol particles are agglomerated by inverse vortices generated at high gas velocity.
Contrary to filters, agglomerators do not plug up due to large clearances between the plates of packing. The flexibility of the system allows optimizing the coalescence packing for a given application. After the coalescence stage, enlarged droplets are removed in the consequent mist eliminator.
The coalescing scrubber has been compared to wet electrostatic precipitators and ionized wet scrubbers by a major European engineering firm and has been found to operate with similar energy consumption to an ESP. However the initial capital expenditure for a coalescing scrubber is dramatically lower, making it an attractive alternative to the others.
|Comparison||Wet ESP||IWS||Coalescing Scrubber|
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