Guidance on the Control of Odour and Noise from ... - Defra

• A temperature of 40 o C is considered as an upper limit for an adsorption process.
• The efficiency of activated carbon is reduced at a relative humidity above 75%. The
preferential adsorption of water can lead to condensation within the bed, thus rendering
the carbon inactive.
• Linear velocities through the bed can range from 0.1 to 0.6 m/s. The higher linear
velocities are only suitable for dealing with compound that has higher adsorption rates.
• Particles in the gas stream to be treated seriously interfere with the efficiency of the
carbon bed, as well as increase the operating pressure drop. If particulate or condensing
material is present the use of carbon filtration is very questionable.
Manufacturer’s guidance is available to determine the appropriate operating
parameters for different types of commercial kitchen situations β . Table 4.7
summarises the required residence times for various cooking processes.
Table 4.7 summarises the required residence times for various cooking
processes
Application Capacity Required Residence Time (seconds)
Canteen, normal kitchen and
restaurants
Kitchens producing large
amounts of fried foods or
concentrated cooking of
burgers
Indian restaurants etc. (i.e.
curry, spices etc.)
Excess of onions or garlic
smells from cooking
‘Normal’ 0.1 – 0.2
2 times ‘normal’ 0.2 – 0.4
3 times ‘normal’ but 4 times in
extreme cases
3 time ‘normal’ but 4 times in
extreme cases
4.7.5 Wet scrubbing (absorption) systems
57
0.3 – 0.6 (up to 0.8 in
extreme cases)
0.3 – 0.6 (up to 0.8 in
extreme cases)
The wet scrubbing or adsorption process relies on the preferential solubility of the
odorous components present in the exhaust stream within the absorption medium.
There are a number of different types of absorbed design although many designs are
not suited to the relatively small scale nature of most commercial kitchen situations.
Adsorption is the term applied to a process which involves a mass transfer between a
soluble gas (odour) and a liquid (e.g. water in simplest form) in a gas-liquid tower.
The rate at which an odorant is removed from an air stream depends upon its degree
of saturation of the liquid, which in turn depends upon its solubility and rate of
removal either by reaction (e.g. with a chemical adjunct added to the liquid (e.g.
hypochlorite) or by bleed-off. This rate mechanism determines the efficiency of
removal for a particular size of absorption tower and gas flow rate. Thus the
effectiveness of this type of system will depend on the:
• reaction rate;
β D Collins. Personal communication (2004).