Wet Scrubbers vs Dry Scrubbers – Which is Better?
Frequently, wet scrubbers fail to achieve expected performance because the incoming hot gas stream evaporates the scrubbing fluid thus producing a “spray-dried” aerosol. In general, the potential for this problem is greatest when the scrubbing liquor is high in dissolved solids (watch out for sodium or potassium salts!) and the incoming gas stream is hot; i.e. over 300ºF (149 ºC). The logical solution to this problem is to quench the incoming gas stream with fresh water prior to contact with any contaminated scrubbing liquor.
Operators should observe the following cautionary note: check the quality of the quenching water. The quench water may actually aggravate the situation; especially when a low outlet particulate concentration is required. The following example, from a real gas-cleaning application, shows the importance of clean quench water.
In an oil-field steam generator application the approach was to remove 95% of the incoming sulfur dioxide through wet scrubbing and 90% of the particulate matter with a wet electrostatic precipitator. The expected particulate outlet concentration was about 0.004 grains/acf (9.156 mg/m3).
Because we were scrubbing with a sodium liquor at a dissolved solids concentration of 100,000 mg/liter we installed a fine spray nozzle upstream of the scrubber to quench the incoming, 450ºF (232ºC) boiler flue gas.
Psychometric calculations showed that we needed approximately 10 gal/min of quench water to achieve adiabatic saturation. Our initial approach was to use the ground water produced along with the crude oil for this quench duty. The dissolved solids concentration of this water was about 500 mg/l.
Assuming that 50% of the droplets in the quench spray evaporated immediately upon contact with the hot gas, and assuming a 200 micron quench spray droplet, size we showed that such an arrangement was adding about .01 gr/acf (22.89 mg/m3) of .25 micron aerosol to the gas stream — two and one half times the outlet loading requirement! We quickly changed to municipal water, which solved the problem.
Watch out for the quality of your quench water.