Wet Scrubbing

The scrubbing of gases utilising an aqueous or solvent based liquor has been long established as an environmental abatement technique. It exploits the phenomena of MASS TRANSFER, whereby chemicals will pass across a boundary between two or more contacting phases, until the concentration of the chemicals is at parity in each of the phases.

Scrubbers bring together a gas phase (typically air) and a liquor (typically aqueous). The contacting of these two phases allows contaminant to move from the air into the liquor where it is either mineralised as a salt by reaction or purged to drain via a liquor replenishment process.

The most common types of scrubber that we design are the Counter Current Packed Tower (CCPT) and Venturi scrubbers.

The CCPT allows the laden air stream into the base section of the scrubber, just above the liquor sump. Here the gases rises through a packing support plate into the packing or mass transfer zone. The scrubbing liquor, which is often re-circulated, trickles down over the packing surface, counter current to the rising gas stream. This ensures that there is always a difference in concentration of contaminant in the 2 phases throughout the tower and hence mass transfer is maintained.

Chemical scrubbing requires controlled dosing of reagents which render the gaseous contaminant non-volatile. This means that knowledge of the highest level of contaminant rate is required to size the dosing system. This also enables the associated storage facilities to be adequately sized. It is important to note however, that the control methods (ORP and pH) are affected by concentrations of salts and free contaminant in solution. It is important to check on how the system reliability will be affect by the solution 'activity' so that a sensible purge rate can be determined.

Click here to see a basic CCPT scheme.

The Venturi system utilises high velocity jets to fill an accelerated gas stream with tiny droplets of liquor. These droplets provide the surface area for mass transfer to occur. As the venturi is a co-current scrubber it is inherently less efficient than a CCPT, however, stage efficiencies of 90% are readily achieved. The venturi scrubber is typically used where the liquor is a slurry or the gas stream has a significant solids concentration.

The high pressure drop associated with the venturi approach can be overcome by designing the system as an Eductor. Here the pump is providing motive force for the air as well as the liquid and this is typically sized to match the throat, gas side, pressure drop. This type of venturi is known as the liquid jet venturi scrubber.

Click here to see a basic Venturi scheme