Odour. Odour can be defined as the perception of smell.

Transcription

1 Odour Odour can be defined as the perception of smell. The experience of smell can be taken to mean any perception that results form nasal inspiration or may refer to sensations received through the receptors of the olfactory epithelium, situated within the upper nasal cavity. No instrument or means of chemical analysis available to measure intensity rating of odours.

2 Odour Following are well known facts about odours: 1. Some substances are odours and some are not. 2. Substances of similar or dissimilar chemical constitution may have similar odours. 3. Nature and strength of the odour may change n dilution. 4. Weal odours are not perceived in the presence of strong odours. 5. Odours of the same strength blend to produce a combination in which one or both may be unrecognizable. 6. Constant intensity of odours causes an individual to quickly loose awareness of the sensation and only notices it when it varies in intensity.

3 Odour Following are well known facts about odours: 7. Fatigue for one odour may not affect the perception of dissimilar odours but will interfere with the perception of similar odours. 8. An unfamiliar odour is more likely to cause complaint than a familiar one. 9. Two or more odorous substances may cancel the smell of each other. 10. Odours travel downwind. 11. Man can smell at a distance. 12. Many animals have a keener sense of olfaction than man. 13. Likes and dislikes often depend on the association of the scent with pleasant or unpleasant experiences. 14. The number of different and distinct smells is great.

4 Odour Sources of Odours : An odorant originates from a solid, a liquid, or a concentrated gas. Odour sources: (1) Confined in a space: emission from duct (2) Unconfined: drainage ditches and settling lagoons

5 Odour Typical occurrences: H 2 S, CS 2, mercaptans, products of decomposition of proteins (especially those of animal origin), phenols and some petroleum hydrocarbons are responsible for odour. Generated due to various human activities. e.g. Garbage dumps, sewage works and agricultural activities Bad smell : Decaying of vegetables The exhaust from motor vehicles

6 Table 16.1

7 Odour Measurement of odour: The measurement of odour falls into two categories: 1) Determination of the threshold concentration (minimum identification level) of odoriferous gases 2) Determination of the type and intensity of atmospheric odours

8 Measurement of odour Determination of the Threshold Concentration Threshold concentration: For each odour there exists a concentration below which no perception is possible for most individual. This conc. is called Threshold conc. or minimum identification level. Compound Threshold value, ppm Acetone 100 Acetic acid 1 Pyridine 0.02

9 Osmometers (Odour measuring devices) are based on the Vapour-dilution method. In this method, the amount of odour producing substances added to purified air is increased until an observer or sniffer can detect the odours. The relationship between odour intensity and conc. can be expressed as P = K log S Where P = odour intensity K = constant S = Odour conc.

10 Determination of the type and intensity of odours: 1. Expert panel method The odour intensity is stated according to a predetermined rating system: no odour threshold level (barely perceptible definite odour strong odours overpowering odour Several descriptive words are used to characterize the type or quality of an odour. e.g. fruity, flowery, burnt, aromatic, fragrant, sweet, garlic, foul, repulsive, nauseating etc.

11 2. Control officer 3. Syringe technique -- To measure the conc. of odoriferous compounds in ducts and stacks -- Measured samples of odour-laden gas are taken in hypodermic syringes and brought to an odour-free room. -- A small amount of this gas is then added to a syringe into which odour free air is then drawn. -- The observer holds the syringe to his nostrils and drives the mixture into his nasal passages by means of plunger -- Conc. Is expressed in terms of odour unit which is defined as the amount of odour necessary to contaminate 1 cu ft of air to the threshold level.

12 Methods of Odour control Modification of the process Dilution by ventilation or dispersal Absorption / Adsorption Irradiation Combustion or oxidation (a) Direct incineration (b) Catalytic Incineration Odour masking Odour counteraction or neutralization Injection of a reactive substance

13 Odour masking: It is based on the principle that when two odours are mixed, the stronger one will predominate. Strong odours tend to mask weaker ones. Thus a strong, pleasant odour can be used to mask or cover a weak unpleasant odour. Care must be taken to see that the odorant used for superimposing the pleasant odour is not flammable, corrosive or allergic.

14 Odour masking: Following are the methods used for odour masking 1) Spraying, vaporizing or atomizing the odorant chosen, into air gas streams in stacks. 2) Adding directly to a process wherever possible 3) Adding to scrubbing liquors Odour masking can also be used to control odours in outdoor places like refuse dumps or waste lagoons. The masking compound must vaporize rapidly enough to overcome the unpleasant odour and slowly enough to last for a reasonable length of time.

15 Odour Counteraction: Certain pairs of odours in appropriate relative conc. are antagonistic. Therefore when hey are mixed together, the intensity of each odour is diminished. This effect is known as counteraction or neutralization. e.g. pairs of musk and bitter almond, Rubber and cedar wood

16 Injection of a reactive substance: Odours can be controlled by injecting controlled quantities of chlorine or ozone into the process gas stream. Chlorine either as a gas or in solution form, hypochlorites and chlorine dioxide are used for odour control. Both bactericidal and chemical actions are involved. Hypochlorites are effective for partial oxidation of highly odorous mercaptants to less odorant disulphide.

17 Injection of a reactive substance: Ozone converts organic matter by oxidative degradation usually to aldehydes, acetones and acids. Such conversions can be carried out in stacks carrying odorous exhaust gases, where ozone is produced by corona discharge. Ozone also destroys or prevents the growth of odour producing microorganisms such as fungi and bacteria.

18 Irradiation: Ultraviolet irradiation can be used for control of odours. The action is due to ozone formation or bactericidal effect.

19 Combustion or oxidation: Direct Incineration: The odour generating gases are made to pass through a combustion chamber at a temperature of the order of C, in the presence of excess oxygen. Disadvantages: Cost of fuel Residence time: less than 0.3 sec

20 Catalytic Incineration: The process gases pass through specially designed unit containing catalyst element, on the surface of which oxidation occurs. During the catalytic oxidation, the constituents in the gas stream such as hydrocarbons and other organic malodors react with oxygen to form carbon dioxide and water vapor. Commercial catalyst units are being used for the odour control of effluent gases from chemical plants, varnish manufacture, paint and enamel-baking ovens, refuse incinerators, refineries burning waste cracking gases, and coffee roasters.

21 Catalytic Incineration: In a catalytic unit, oxidation takes place at a much lower temperature than necessary for direct incineration. The process gases pass through specially designed unit containing catalyst element, on the surface of which oxidation occurs. During the catalytic oxidation, the constituents in the gas stream such as hydrocarbons and other organic malodors react with oxygen to form carbon dioxide and water vapor. Catalytic combustion of most of the organic constituents that occur in the effluent gas steam is initiated and self-sustaining at about C.

22 Catalytic Incineration: In case the gas stream is colder than this before coming into contact with the catalyst, it must be pre-heated. However, the fuel cost here for pre-heating will be very much less, when compared to direct incineration. Usually certain metals and metal-oxides are used as catalysts in oxidation reactions. It has been reported that platinum and platinum-rhodium alloy distributed on a suitable support are highly effective as catalysts.

23 Catalytic Incineration: The important properties for a commercial catalyst unit are high specific surface, low resistance to the flow of gases, and arrangement in a manner as to provide turbulent mixing of process gases. Gases entering the unit should be free from particular matter such as dust particles that might cause catalyst loss due to abrasion actions. The gas stream should also be free from fumes of mercury, lead and zinc, that might impair catalyst activity by coating the catalyst or deactivating its active centers. All necessary precautions must be taken to prevent the catalyst from becoming ineffective.

24 Catalytic Incineration: Commercial catalyst units are being used for the odour control of effluent gases from chemical plants, varnish manufacture, paint and enamel-baking ovens, refuse incinerators, refineries burning waste cracking gases, and coffee roasters.