Scrubbers

                                                           Scrubbers

These are a diverse group of air pollution control devices that can be used to remove some particulates and gases from industrial exhaust streams. Traditionally, the term "scrubber" has referred to pollution control devices that use liquid to wash unwanted pollutants from a gas stream. Recently, the term is also used to describe systems that inject a dry reagent or slurry into a dirty exhaust stream to "wash out" acid gases. Scrubbers are one of the primary devices that control gaseous emissions, especially acid gases. Scrubbers can also be used for heat recovery from hot gases by flue gas condensation.There are several methods to remove toxic or corrosive compounds from exhaust gas and neutralize it.

1. Wet Scrubbers:

                   The exhaust gases of combustion may contain substances considered harmful to the environment, and the scrubber may remove or neutralize those substances. A wet scrubber is used to clean air, flue gas or other gases of various pollutants and dust particles. Wet scrubbing works via the contact of target compounds or particulate matter with the scrubbing solution. Solutions may simply be water (for dust) or solutions of reagents that specifically target certain compounds.
Process exhaust gas can also contain water soluble toxic and corrosive gases like hydrogen chloride HCl or ammonia NH₃. These can be removed very well by a wet scrubber_.

Removal efficiency of pollutants is improved by increasing residence time in the scrubber or by the increase of surface area of the scrubber solution by the use of a spray nozzle, packed towers or an aspirator. Wet scrubbers may increase the proportion of water in the gas, resulting in a visible stack plume, if the gas is sent to a stack.

Wet scrubber systems generally consist of the following components:

• Ductwork and fan system
• A saturation chamber (optional)
• Scrubbing vessel
• Entrainment separator or mist eliminator
• Pumping (and possible recycle system)
• Spent scrubbing liquid treatment and/or reuse system
• An exhaust stack

• Venturi Scrubber:

     It is designed to effectively use the energy from the inlet gas stream to atomize the liquid being used to scrub the gas stream.This type of technology is a part of the group of air  pollution controls collectively referred to as wet scrubbers.

A venturi scrubber consists of three sections: a converging section, a throat section, and a diverging section. The inlet gas stream enters the converging section and, as the area decreases, gas velocity increases (in accordance with the Bernoulli equation). Liquid is introduced either at the throat or at the entrance to the converging section.Particle and gas removal occur in the throat section as the inlet gas stream mixes with the fog of tiny liquid droplets. The inlet stream then exits through the diverging section, where it is forced to slow down.

Venturis can be used to collect both particulate and gaseous pollutants, but they are more effective in removing particles than gaseous pollutants.

·         Rectangular throat venturi scrubber:

Ventur is with round throats  can handle inlet flows as large as 88,000 m³/h (40,000 cfm). At inlet flow rates greater than this, achieving uniform liquid distribution is difficult, unless additional weirs or baffles are used. To handle large inlet flows, scrubbers designed with long, narrow, rectangular throats have been used.

·         Mechanically aided scrubbers:

They are a form of pollution control technology. This type of technology is a part of the group of air pollution controls collectively referred to as wet scrubbers.
In addition to using liquid sprays or the exhaust stream, scrubbing systems can use motors to supply energy. The motor drives a rotor or paddles which, in turn, generate water droplets for gas and particle collection.Systems designed in this manner have the advantage of requiring less space than other scrubbers, but their overall power requirements tend to be higher than other scrubbers of equivalent efficiency. Significant power losses occur in driving the rotor.

                 

    2. Dry Scrubbers:

A dry or semi-dry scrubbing system, unlike the wet scrubber, does not saturate the flue gas stream that is being treated with moisture. In some cases no moisture is added; while in other only the amount of moisture that can be evaporated in the flue gas without condensing is added. Therefore, dry scrubbers do generally not have a stack steam plume or wastewater handling/disposal requirements. Dry scrubbing systems are used to remove acid gases (such as SO₂ and HCl) primarily from combustion sources.

There are a number of dry type  scrubbing system designs. However, all consist of two main sections or devices: a device to introduce the acid gas sorbent material into the gas stream and a particulate matter control device to remove reaction products, excess sorbent material as well as any particulate matter already in the flue gas.

Dry scrubbing systems are often used for the removal of odorous and corrosive gases from wastewater treatment plant operations. The media used is typically an activated alumina compound impregnated with materials to handle specific gases such as hydrogen sulfide. Media used can be mixed together to offer a wide range of removal for other odorous compounds such as methyl mercaptans, aldehydes, volatile organic compounds, dimethyl sulfide, and dimethyl disulfide.

3.      Absorber:

          

Many chemicals can be removed from exhaust gas also by using absorber material. The flue gas is passed through a cartridge, filled with one or several absorber materials, adapted to the chemical properties of the components that should be removed from the exhaust gas .This type of scrubber is sometimes also called dry scrubber. The absorber material has to be replaced after its surface is saturated.

Scrubber Waste Products:

One side effect of scrubbing is that the process only moves the unwanted substance from the exhaust gases into a liquid solution, solid paste or powder form. This must be disposed of safely, if it can not be reused.

As an example of reuse, limestone-based scrubbers in coal fired power plants can produce a synthetic gypsum of sufficient quality that can be used to manufacture drywall and other industrial products.