Off-gas/Waste Vapour/Hazardous Volatile Treatment
Scrubbers
Typical Scrubber Diagram
Process:
A Scrubber is an air washer (with refinement device) used to control air pollution by cleaning gases from hazardous air pollutants, inorganic fumes/vapours/gases & volatile organic compounds (VOC).
Most scrubbers rely on the mechanisms of impaction and diffusion to achieve sufficient contact between particulate matter and liquid droplets.
They exist in two forms:
Wet Scrubbers:
Configurations of Wet Scrubbers:
1) Venturi scrubber:
The waste gas stream is accelerated to atomize the scrubbing liquid & improve gas-liquid contact:
2) Orifice scrubbers:
3) Fibre-bed scrubbers:
4) Mechanical scrubbers:
5) Spray scrubbers:
6) Condensation scrubbing:
Dry Scrubbers:
Uses:
Limitations:
A Scrubber is an air washer (with refinement device) used to control air pollution by cleaning gases from hazardous air pollutants, inorganic fumes/vapours/gases & volatile organic compounds (VOC).
Most scrubbers rely on the mechanisms of impaction and diffusion to achieve sufficient contact between particulate matter and liquid droplets.
They exist in two forms:
Wet Scrubbers:
- remove air pollutants (by impact inertia) when the pollutants disperse into a sorbent or reagent slurry, or absorb into a liquid solvent.
- Used to control hazardous air pollutants (HAP) (in particulate form (PMHAP )); and inorganic fumes/vapours & gases (e.g., chromic acid, hydrogen sulfide, ammonia, chlorides, fluorides, and SO2 ).
- Used occasionally to control volatile organic compounds (VOCs).
- Also can control Hydrophilic VOC (with an aqueous fluid) and hydrophobic VOC (with an amphiphilic block copolymer in the water).
- Note - When using absorption as the primary control technique, the spent solvent must be regenerated or disposed of in an environmentally acceptable manner (EPA, 1991).
Configurations of Wet Scrubbers:
1) Venturi scrubber:
The waste gas stream is accelerated to atomize the scrubbing liquid & improve gas-liquid contact:
- The “throat” section in the duct increases the velocity & turbulence of the gas stream as the duct narrows then expands.
- The scrubbing liquid is sprayed into the gas stream (either before or in the venturi throat) to avoid a dust buildup & reduce wear by abrasive particles.
- The scrubbing liquid is then atomised into small droplets by the turbulence (in the throat)
- Droplet-particle interaction is increased.
- After the throat section, the mixture decelerates, and further impacts cause the droplets to agglomerate.
- Once the particles have been captured by the liquid, the wetted matter and excess liquid droplets are separated from the gas stream by a mist eliminator.
- The venturi throat is sometimes fitted with a refractory lining to further resist abrasion by dust particles.
2) Orifice scrubbers:
- Uses gas-atomized spray scrubbers, where a tube/or duct forms the gas-liquid contacting zone.
- The contaminated gas stream is forced over the surface a pool of scrubbing liquid at high velocity, en-trained as droplets as it enters an orifice.
- The gas stream (flowing through the orifice) atomizes the entrained liquid droplets (same manner as the venturi scrubber). As the gas velocity and turbulence increases through the orifice, increasing the interaction of particulate matter and atomized liquid droplets.
- A series of baffles after the orifice remove particulate matter and droplets from the gas stream, which drain back into the liquid pool below.
- The scrubbing liquid is fed into the pool and later recirculated by gravity (instead of re-circulation pump used for venturi scrubbers).
- Principal advantage: no need for re-circulation pump.
3) Fibre-bed scrubbers:
- Saturated waste gas passes through beds of packing fibres (e.g. spun glass, fibreglass, or steel).
- If no solid particles present, small fibers may (as only mists are to be collected) - these are often sprayed with the scrubbing liquid so particles can deposit on droplets and fibres.
- To collect solid particles the beds must be composed of coarse fibres with larger voids to minimize blockage.
- The final fibre bed/mat (the gas stream passes through) is dry to remove any existing droplets.
4) Mechanical scrubbers:
- A power-driven rotor produces the fine spray, acts as a turbulence producer and causes the contact of gas and liquid.
- The droplets are the principal collecting bodies for the dust particles.
- An entrainment separator must be used to prevent carry-over of spray.
- The simplest commercial devices of this type are essentially fans upon which water is sprayed.
- Mechanically-aided scrubbers require a pre-cleaner to initially remove coarse dust and larger debris.
- It is only capable of high collection efficiency with high energy consumption.
5) Spray scrubbers:
- Spray nozzles generate liquid droplets for contact with the gas stream in cylindrical/rectangular chambers.
- A common form is a spray tower - gas flows upward (or horizontally) through successive spray nozzles.
- Spray towers have very low gas pressure drops & most contacting power is got from the liquid stream (must be apt combination of liquid pressure and flow rate).
- Physical absorption depends on properties of the gas stream, liquid solvent (density, viscosity) & the pollutant(s) (solubility, diffusivity - temperature dependent).
- Absorption is enhanced by lower temperatures, greater contacting surface, higher liquid-gas ratios, and higher concentrations in the gas stream.
6) Condensation scrubbing:
- Here the particulate matter (PM) acts as the condensation nuclei to form droplets.
- Steam is injected into the saturated gas, which creates a condition of supersaturation and leads to condensation of water on the matter in the gas stream.
- The large condensed droplets are then removed, usually by a high efficiency mist eliminator (EPA, 1998).
Dry Scrubbers:
- Spray dry scrubbers are limited to small-medium coal fired plants.
- Advantages: low waste disposal costs, low water consumption and a low pressure drop.
- Contaminants treated: SO2, PAHs, HF, HCl, and heavy metals.
- A removal efficiency >90% SO2 has been achieved.
Uses:
- To reduce concentrations of certain water soluble acid, base and organic contaminants.
- The target contaminant groups are: HCL, H2SO4, Chlorine, SO2, and acidic & alkaline vapours.
- Primarily for environmental protection, environmental health, and increasing government regulation.
Limitations:
- If the scrubber blow-down liquid is acidified (e.g. by NaOCl produced at waste water treatment plant) the NaOCl breaks down, releasing Cl2 gas.
- Most incinerator scrubbers are sensitive to high temperatures. Prior to scrubbing, the hot gas must be cooled.
- Waste combustion forms small particles which both require additional equipment, can reduce the rate of scrubbing and damage the pump.
- Since the scrubber is so effective at capturing particulates, a (cleanable or disposable) filter is necessary to remove them from the system.