RTOs ASK Piearcey

Technologies


ASK Piearcey have experience and specialise in
Wet Scrubbing, Bio Filtration, Activated Carbon
Adsorption and Oxidisers


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.


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


CCPT

CCPT allows the laden air stream into the base section of the scrubber, just above the liquor sump. Here the gases rise 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.


Venturi scrubbers

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.



Bio-filters


Bio-filters contact bacteria (in a liquid film) directly with the contaminated air. The contaminants pass from the air to the liquid film where they can be acted upon by the bacteria. The bacteria breakdown the molecules of contaminant or react them with other molecules to provide themselves with energy. They may use enzymes in this process. All bacteria require ammoniacal compounds to form amino acids so that they can build cell material.


The more modern systems are much more robust than the earlier open systems. Most are continuously irrigated giving a scrubbing action and water buffer for peaks and troughs. Some of the latest systems are even Counter current (see scrubber section above) and hence have a higher removal efficiency than traditional co-current systems.


All counter current systems need to be "passive" media units. This is because active media units would be vulnerable to bed collapse as the base of the bed would deteriorate first.


In most cases it is advisable to include a down stream polisher (particularly for odour control) as this will protect system performance from bio-filter "glitches".



Carbon Adsorption


Carbon (and indeed, other porous solids) can adsorb contaminants on to its surface by virtue of electrostatic forces. Base carbon is carbon that has been produced by pyrolysis of a base material (e.g. Coal, Wood, Coconut Shell etc). This carbon must then be activated to maximise the potential of carbon as an adsorbant. The process of activation is the use of heat, under anoxic conditions and usually in the presence of steam. Typically activation is around 850 degrees Centigrade.


This technology having a high efficiency removal, also has a forgiving nature and can adsorb wide ranges of compounds over equally wide ranges of concentration. The technology therefore "forgives" mistakes in emission characterisation and is hence an ideal polisher for other technologies.


Adsorbers can be single bed or multi-bed. they can have , vertical, annular or rect-annular orientations. The carbon bed is designed to give the required empty bed contact time, superficial velocity and bed life as necessary.


  • For solvent/VOC applications beds can have between 0.5 and 2 seconds contact time (more if adjusted for longer life)
  • For odour control using impregnated carbon then it is generally recommended that > 2.5 seconds is use


Oxidisers


Oxidisers treat VOCs and other contaminants by thermal or catalytic reaction with oxygen. Thermal oxidisers treat most VOCs at around 750-900 Degrees Celsius, where as catalytic systems can be effective down to 180 Deg C. Dioxins are combusted at 1200 deg C.


Thermal Oxidisers differ in the method used to recover heat. There are two prime categories of heat recovery: primary and secondary.


Secondary heat recovery is where the fuel consumption of some external system is minimised by supplementing it with heat from the oxidiser.


Recuperative Oxidisers

This type of oxidiser utilises mechanical shell and tube type exchangers to pre-heat the incoming air stream with waste heat from the exhaust. This heat exchange mechanism is continuous and hence the operational output is steady. The heat transfer resistance limits the thermal efficiency to around 80% with a practical sized heat exchanger. More typically they operate between 50 and 75% thermal efficiency. Destruction efficiencies depend on residence time, temperature, gas mixing and the chemistry of the contaminant (particularly Dioxins). It is typically >99%.


Regenerative Oxidisers (RTOs)

This is the more popular type of oxidiser in recent years and this is primarily due to the very high thermal efficiencies it can achieve. RTOs use a ceramic media to store and release heat on a dynamic flow cycle. The air flow direction through the unit is changed every 120 seconds. this has the effect of "juggling" the heat within the combustion chamber and minimising the required "top-up" heat from the burner(s). The thermal efficiency of an RTO typically starts at 80% and goes up to 95%. RTO's can therefore operate autothermally at relative low contaminant levels (2000-3000mg/m3 VOC). This means that the y require little or no support fuel to maintain combustion.


If you are interested in a review your problems and quote for their resolution, please email info@askpiearcey.co.uk

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