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Processing
Feed raw materials sometimes have a pleasant smell, such as by-products from the fruit juice industry. But products can also have a foul odour, especially when oils and fats are involved or products are treated by pressure and temperature.
To avoid complaints of residents around a feedmill these smells need to be neutralised.
The oxygen used is directly extracted from the outside air by means of a blower and then filtered before it enters the injector.
(Photo: Aerox)
Many factors determine whether an odour is pleasant or not, even human perception can
be different to the same odour. The main reason for this subjective perception is because odour can be imagined as a cocktail of possibly hundreds of different organic components and not as just one sin-gle molecule that determines the smell. The weight concentration is mostly very low, but the odour-con-centration (=number of required dilution).
The odour-components with the lowest concentra-tions (sub-ppb-range) cannot be detected by laboratory equipment, but the human nose could very well notice them. So the main reason for a factory or feed plant to do something about odour reduction is the
sensitivity of the noses of the people that live in the vicinity of the plant. This also complicates the matter, because it is impossible to make calculations or pre-dictions on odour-emission. Each odour-component can mask or amplify another component within the cocktail. Odour is dynamic, since the components and their concentrations will vary in time due to differences in process- conditions, raw materials, weather-conditions, etc.
Nothing beats the nose
Paticles can generate odour once they are released in atmosphere, especially particles below 1 micron have a large total surface area. The human smell, although less sensitive than that of other mammals, is a source of information about chemical substances in the environment. Our nose is capable of recognising up until 4,000 smells at very weak concentrations in smelling molecules. Analytical techniques do not always succeed in reaching such a threshold in smelling molecules.To measure a smell, the best instrument still remains the nose with its olfactive mucous, which is a really reliable sensor. The stenches generated by industrial activities contain between 200 and 800 different active substances. The most frequent are sulphur-containing molecules in the smell of rotten eggs, nitrogenous molecules (such as ammonia) and ketones and aldehydes, which cause bitter and rancid smells. The human vocabulary can not exactly define a smell as it can describe a colour, but Table1tries to sum up what can be smelled from different molecules.
Molecule(s) Association smell
Hydrogensulphide Rotten eggs
Mehtylmercaptan Cabbages
Ethylmercaptan Rotten cabbages
Allylmercaptan Garlic
Ammonia Very prickly,irritating
Methylamine Rotten fish
Indole,scatole Excrement
Cadaverine Decaying meat
Acetic acid Vinegar
Butyric acid Rancid butter
Valeric acid Sweat,perspiration
Fomaldehyde Acrid,close
Acetaldehyde Fruit,apple
Acetone Sweet fruit
Dimethylsulphur Decomposing vegetables
Thiolane Gas (product for odourisation of gas)
Olfactometry
It is impossible to calculate odour reduction efficiencies without olfactometric measurements. Olfactometry allows to quantify the concentration of a smelling mixture and to determine the intensity of it. What factually happens is that a sample of air is taken directly from he outlet and in a laboratory is diluted with clean air.A panel of trained people smell the most diluted samples first and continue with smelling samples that are less diluted. As soon as 50% of the panel detects a smell,this is considered to be the odour detectability threshold referring to the minimum concentration that produces an olfactory response or sensation.There are several odour thresholds that can be determined using olfactometry. They are: The detection threshold, which is defined as the lowest concentration that will elicit a response without reference to odour quality. This is reproducible and the most widely reported odour measurement in literature. The recognition threshold is defined as the minimum concentration that is recognised as having a characteristic odour quality. The description threshold is the point at which the panellist is asked to distinguish the odour.
[ 本帖最后由 monica 于 2007-12-24 17:54 編輯 ] |
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