Scrubbers for Air Pollution Control: What are they?
Technologies for Air Pollution control
When it comes to air pollution control, there are several technologies widely used nowadays that can be installed at production plants to reduce exhaust air emissions generated during industrial processes. Some of those technologies are: Regenerative Thermal Oxidizers (also known as RTO), Direct Thermal Oxidizers, DeNOx systems, Bagfilters, Biofilters, Scrubbers, etc.
What are Scrubbers?
Scrubbers are used as depuration systems that specifically eliminate the emissions of acid gases (SO2) produced in the combustion of pollutants. Wet scrubbers receive their name from the fact that liquids are used as a scrubbing medium and the by-products of the scrubbing system are liquid solution or slurry. Wet scrubbers can serve two simultaneous functions: particulate collection and acid gas control. Gas emissions are absorbed in devices columns that attempt to maximize liquid-to-gas contact to achieve maximum absorption of the pollutants into the liquid phase
How do Scrubbers Work?
The air flow to be treated is introduced into the Scrubber, which has the shape of a silo or cylindrical tank. The Scrubber captures the gas, and thanks to a liquid that can be either water, a chemical reactive or a mixture of both, neutralises the polluting components that are present in the gas. Thus, once pollutants have been neutralized, the clean gas can be issued into the atmosphere, completely safe for people and for the environment.
Derivatives of Nitrogen can be absorbed in an acid media, whereas the derivatives of sulphure can be eliminated by means of alkaline or oxidizing media. There are even some pollutants that are very soluble in water that do not need any chemical reactives in the gas cleaning process.
In the process of eliminating emissions, and usually before an RTO system, there can be a Scrubber unit to eliminate acid gases. Before the RTO, the absorption of the polluting gas is carried out in counterflow inside of the scrubber and within spaces which are stuffed with large specific areas contact elements. That is combined in order to obtain an optimal contact of the gas/liquid stages and a constant distribution of both fluids throughout the process where the washing liquid is dispersed and uniformly distributed by means of high-output full-cone nozzles, easily removable for revision or change.
Retention of drops originated by the liquid distribution system is to be carried out within the tower by means of a high-efficiency low-pressure-drop vertical-flow demister that prevents the carryover and emission of drops to the atmosphere, as well as any loss of washing solution.
The washing liquid contained in the scrubber bottom recirculates through a high-performance (chemical as well as mechanical) centrifugal pump.
All scrubber elements are usually manufactured with polypropylene, polyethylene, fiber glass resin, stainless steel, and are insulated and heated to avoid problems on winter periods. That insulation is usually made by polyurethane with 40 mm thickness and finished with fibreglass, or mineral wool with aluminium covering.
The level of the washing liquid is kept constant by means of a water input through an electro valve controlled by a 3-contact level indicator.
Usually, after the RTO equipment a quench may be installed in order to decrease the gas temperature. The quench system has also neutralizing solution to be used as a first stage neutralizing system, by using the same liquid solution strored in the scrubber.