AIR POLLUTION SAMPLINGand its equipment-1.pptx

AIR POLLUTION SAMPLING and equipment

1. AIR POLLUTION SAMPLING AND MEASUREMENT Presented by: S. Sudheer (A21126502032) B. Gnana Sai (A21126502004) K. Lokesh (A21126502029)

2. STACK SAMPLING The major problem in stack sampling is of obtaining a representative sample. The important factors are: 1) Selection of Sampling site 2) Number of sampling points required The flow in stack should be of turbulent flow A circular opening of with various transverse points at different radii are located for non turbulent flows

3. STACK SAMPLING

4. SAMPLING SYSTEM Stack sampling is carried out by diverting part of gas stream through a sampling train. Sampling train consists of various parts as shown in figure. A nozzle placed in the gas stream, a sampling probe through which sample is drawn, collector, flow measuring device and a prime mover such as vaccum pump. The nozzle is generally sharp edged and made of stainless steel with glass and Teflon. This type of analysis is mainly used for collecting gases and particulate matter at high temperatures

5. PARTICULATE SAMPLING The common technique employed for sampling particulate gas stream is called isokinetic technique. In the method a sample is drawn into the probe such that the conditions at the tip of the probe are the same as those in free gas stream. At greater velocities errors are small and 20% of errors are acceptable. If a gas velocity (u p ), less than the free stream velocity (u ꚙ ) (Sub-isokinetic), is maintained inside the probe,the static pressure at the tip of the probe is greater than the free stream static pressure at the same cross section. So finer particles approaching the probe tip are carried away with the gas streamlines and larger particles (3 µm or greater) in this portion of the gas travels in straight-line trajectories due to their higher inertia and enter the probe tip and continue into the probe. In this case the particle- capture efficiency of the probe will be less than one and the sample will indicate high concentration of coarse particles.

6. On the other hand if the u p > u ꚙ (Super-isokinetic ), the static pressure at the tip of the probe is less than free stream pressure, and too much gas will be sampled in proportion to the probe area. So the capture efficiency will be greater than one and the sample will contain a large amount of finer particles. Only when the sample is iskinetic , the capture efficiency will be one.

7. GASEOUS SAMPLING Gaseous sampling from stack is similar to the particulate matter sampling, but it is not necessary to sample at isokinetic conditions. The sample can be withdrawn at a constant rate independent of flow rate in the stack. Precautions taken for sampling: Particulate matter must be filtered upstream of the collection system to prevent blocking and reaction with gaseous on cooling. Sampling is heated so as to avoid water from condensation. Tube should be made of sampling materials(stainless steel) Suction through the nozzle draws a known volume of gas from the stack into collection devices, which removes the constituents gas of interest and is sent for analysis.

8. GASEOUS SAMPLING

9. ANALYSIS OF AIR POLLUTANTS Air quality measurement is generally done by continuous automatic analyzers. But convectional laboratory analysis is being carried out for spot checking. It is done by 2 ways Chemical(by chemical reactions): In chemical methods the pollutant being measured undergoes chemical transformations and the product is then with appropriate analytical techniques. Physical principles: In the physical methods of measurement, a physical property of the pollutant is utilized-such as the ability of the gas to absorb infrared radiation. The mount of radiation is detected.

10. TYPES OF GASEOUS POLLUTANTS

11. SULPHUR DIOXIDE( SO 2 ) The most common manual methods for measuring atmospheric SO 2 are based on colourimetry , iodimetry , or turbidimetry . There are several automatic instruments for monitoring sulphur dioxide, the most common of these are based upon conductometric , colourimetric , and flame photometric principles. In the conductometric technique, the sampled air containing SO 2 is passed through a dilute solution of hydrogen peroxide in dilute sulphuric acid. SO 2 is oxidised to H 2 SO 4 thereby increasing the electrical conductivity of the solution. The increase in conductivity, which is measured, is proportional to the concentration of SO 2 in the sample. With this technique, as low SO 2 concentrations as 0.01 ppm can be detected. However, the conductometric analyzers suffer from a wide variety of interferences. Acidic gases such as hydrogen chloride give positive errors, while ammonia interferes negatively. One of the best method used for autometric monitoring of SO 2 is coulometry .

12. SULPHUR DIOXIDE( SO 2 ) In the bromocolourimetric analyser, air containing SO 2 is drawn continuously through an electrolytic cell which contains acidified bromine solution and two sets of electrodes. SO 2 in the air sample is oxidized by bromine, causing a reduction in bromine concentration, this changes in oxidation-reduction potential of the reagent. A voltage is developed and is compared to reference voltage, and is calibrated to SO 2 concentration.

13. NITROGEN OXIDE Colourimetric method ( Griess -Saltzman)– reaction with sulphanilic acid to form diazonium salts, this is coupled with ethylenediamine dihydrochloride to get a pink coloured dye complex, the amount absorbed is measured in a spectrophotometer. It is also used for analysis of NO, but firt it s oxidized to NO 2 . This method is not suitable for the collection of 24-hour samples, because the dye complex is fairly unstable. Continous analyzer: Several continuous analyzers are available for the measurement of NO and NO 2 based on colourimetric , nondispersive infrared and chemiluminescent . Among these chemiluminescent is accurate and cheap. This has wide range of 0.004ppm to more than 1000ppm.

14. In the chemiluminescent technique, NO reacts rapidly with ozone to form NO 2 and oxygen. About 7% of the NO 2 produced is in excited state and reverts to the ground state with the emission of radiant energy. This emitted radiation is received by a photomultiplier tube whose output is amplified and fed to a recorder. The intensity of radiation is proportional to amount of nitric oxide. This reaction is specified for NO only if any NO 2 is present it dissociates into NO before entering.

15. CARBON MONOXIDE Nondispersive infrared spectrophotometry(NDIR) It is based on continuous absorption of infrared radiations by CO molecules It consists of two cells one is reference cell filled with non-absorbing gas(nitrogen) and another is sample cell continuously sent with carbon monoxide. The detector consists of two compartments separate by a thin metal diaphragm and filled with CO. Since more radiation passes to reference cell side compartment, the CO therein expands more compared to other causing pressure difference across the diaphragm, in turn causes it to pulse back and forth. The magnitude of the diaphragm vibration is a function of the absorbing CO concentration in the sample cell.

16. OXIDANTS AND OZONE The standard method for the net determination of total oxidants is the neutral phosphate-buffered KI colourimetric technique of Saltzman. In this method, a sample of air at a known flow rate is passed through a solution of 1% neutral buffered potassium iodide and the liberated iodine is measured colourimetrically at 352 nm. 2KI+O 3 +H 2 O→ O 2 +2KOH+I 2 Since one mole of ozone releases one mole of iodine, the I₂ concentration measured is directly related to the total oxidant concentrations in the rage of 0.01 to 10 ppm (as ozone). Automatic monitoring of ozone, is based on the chemiluminescence of the ozone-ethylene reaction . In this method, ozone is exposed to a large excess of ethylene in a reaction vessel. The chemiluminescent light released during the reaction is then measured using a photomultiplier tube, and related to the ozone concentration. There are no interferences from other atmospheric components and the device is capable of measuring concentrations less than 0.001 ppm. However, regular calibration is required.

17. HYDROCARBONS The total hydrocarbon content is generally reported on a non-methane basis. It is most commonly determined by flame ionisation technique (FID). In this method the sample gas is injected into the flame created by burning hydrogen in either air or oxygen. When hydrogen alone burns, relatively few ions are generated but when hydrocarbons are present, the flame produces a complex ionisation in which a large number of ions are present. An electric field is set up in the vicinity of the flame by making the burner jet positive with respect to a wire loop. The electric field induces ion migration in such a manner that a small ionisation current is established between the electrodes, and this current is proportional to the concentrations of ions in the flame. The data are usually expressed in units of the gas used in calibration, e.g. "ppm of C as methane". The FID is uniquely suitable for hydrocarbon analysis because it does not respond to other gases such as CO, CO2, H2O, SO2 and nitrogen oxides. The recording range of these analysers varies from approximately 2 to 3000 ppm hydrocarbons as methane. Ordinarily, the 0 to 20 ppm range is most useful for ambient-air analysis.

18. The non-methane hydrocarbon analysis can be accomplished by a differential measurement. The sample is passed through a charcoal column which is saturated with methane but is still capable of retaining other hydrocarbons. This column can then be used with FID as an analyser for the analysis of methane only. By differences between the results obtained, the non-methane hydrocarbons can be assessed.

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