The dirty gas stream is drawn into the portable wet scrubber and is forced down through an inner packed bed column.
Packed-bed scrubbers consist of a chamber containing layers of variously shaped packing material, such as Pall rings, Raschig rings or Berl saddles that provide a large surface area for liquid-gas contact.
The packing is held in place by wire mesh retainers and supported by a plate near the bottom of the scrubber. Scrubbing liquid is evenly introduced above the packing and flows down through the bed.
The liquid coats the packing and establishes a thin film. The pollutant to be absorbed must be soluble in the fluid. In vertical packed bed scrubber, the gas stream flows up the chamber (countercurrent to the liquid).
The moist clean gas stream is drawn up through the unit by an internally fitted blower, passes through a stainless steel mist eliminator, where the moisture is removed from the clean air and is discharged from the scrubber.
The byproduct which have settled to the bottom of the water reservoir are removed through a drain valve.
Portable wet scrubber
The effectiveness of the scrubbing solution in removal of harmful gases depends on the concentration of the scrubbing chemical, continuously depleted in the process. Scrubbing chemical concentration must be maintained to ensure effectiveness of the scrubber.
Control of the scrubbing solution strength can be achieved by either of the following methods:
Continuous Scrubbing:Continuous replenishment and blow down. Scrubber solution composition at any time will depend upon how it is being controlled, the scrubbing chemical, and gases being scrubbed.
Batch Scrubbing: Starts with an initially high concentration of scrubbing chemical that is allowed to reach near depletion, followed by blow down and replenishment by fresh, full strength scrubber solution.
The choice whether to control scrubber solution strength with pH or conductivity will depend on the scrubbing solution composition and how it changes during the scrubber operation. While each application should be reviewed separately, the applications best suited for pH and conductivity are as follows:
pH control is often the choice for scrubbers using continuous blowdown and replenishment. Acidic gas scrubbing, such as hydrochloric acid (HCl), is controlled by maintaining an excess concentration of a basic scrubbing chemical, such as caustic (NaOH) or lime (Ca(OH)2). pH is specific to hydrogen ion (H+) concentration, which is related to the concentration of the basic scrubbing chemical. This specificity allows pH to be used to control scrubbing with minimal effects from scrubber by-products (unlike conductivity, which is nonspecific and will be influenced by by-products).
If the scrubber uses strong caustic (10-15% NaOH), pH control is not recommended because the exposure of the pH glass to high concentration of caustic will attack the pH glass, thus destroying the electrode.
