A scrubber is a waste gas treatment installation in which a gas stream is brought into intensive contact with a liquid, with the aim of allowing certain gaseous components to pass from the gas to the liquid. Scrubbers can be employed as an emission-limiting technique for many gaseous emissions. Scrubbing is also referred to as absorption.
- Gaseous components
- Dust (certain types of application)
- Odour (certain types of application)
During scrubbing there is a transfer of components from the gas phase to the liquid phase. The level of gaseous components that can pass to the liquid phase is determined by the ability of these components to dissolve in the liquid. Henry’s Law is applicable to the solubility of gases in liquids, for low concentrations and components with a partial pressure < 1atm:
p = partial pressure (Pa)
x = mole fraction
H = Henry constant (Pa)
This allows one to calculate the maximum concentration of a particular component in the washing water, for the required end concentration. This also gives an indication about water usage under those circumstances.
The equilibrium concentration in the vapour phase, which corresponds to a certain concentration in the liquid phase, is determined by the temperature – the higher the temperature in the liquid phase, the higher the equilibrium concentration in the vapour phase. Thus a reduction in temperature has a favourable effect on the yield.
It is possible to increase the load by adding chemicals to the washing liquid, which help to convert absorbed components. Thus adding chemicals that react with the absorbed gases has a positive effect on the absorption yield.
Besides water (wet scrubbers), organic liquids are also used as absorption mediums. In many cases chemicals or micro-organisms are added to the scrubbing liquid to convert or neutralise gases that are dissolved in the liquid (conditioned scrubbers). As a result of this conversion, the concentration in the water is reduced, which in-turn allows more gas to dissolve (according to Henry’s Law).
The concentration of polluted substances in out-going gas streams can never become lower than that permitted by the equilibrium between the gas phase and the scrubbing liquid.
In practice, a scrubber consists of three parts: An absorption section, a droplet collector and a recirculation tank with pump.
- Chemicals industry
- Waste incineration installations;
- Pharmaceutical industry
- Storage and transfer of chemicals
- Surface treatment