L 37 final

L-37Sampling of gases/vapours and Stack

Monitoring

Air pollution and control

(Elective -I)

Devices for general use

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Meters

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• They are used to determine accurately the volume of the gas collected. They are fitted with manometers and thermometers to indicate the pressure and temperature of the gas stream sampled.

Probes

• These are tubes suitable for penetrating into the gas stream and should be constructed of materials which are non-corrosive and which can withstand special temperature conditions.

• Also, they should be constructed of materials which do not react with the substances to be sampled.

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• Therefore, they should be made of stainless steel or preferably of glass or quartz.

• A probe should have suitable length and diameter.

• To ensure isokinetic sampling conditions, the opening of the probe should face the gas stream to be sampled.

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Suction Devices

• Any suction device which has the required volumetric capacity can be used. Vacuum pumps driven by electric motors are very commonly used.

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Gaseous sampling methods

• Sample methods include

1. Wet chemical (Absorption),

2. Condensation or freeze out

3. Gas sampling into bags, syringes

4. On sorbent tubes(Adsorption )

5. Direct read instruments and,

6. Isokinetic methods (Particulate sampling)

1. Absorbers • In this process, effluent gases are passed through

absorbers (scrubbers) which contain liquid absorbents that remove one or more of the pollutants in the gas stream. The efficiency of this process depends on

• (1) amount of surface contact between gas and liquid

• (2) contact time

• (3) concentration of absorbing medium

• (4) speed of reaction between the absorbent and gas.

• Absorbents are being used to remove sulphur dioxide, hydrogen sulphide, sulphur trioxide and fluorides and oxides of nitrogen.

• The equipment using the principle of absorption for the removal of gaseous pollutants includes: (I) packed tower, (2) plate tower, (3) bubble cap plate tower, (4) spray tower, and (5) liquid jet scrubber absorbers. Selective chromatographic absorption of gases on small pellets may offer much higher rates than those achieved in packed towers.

• A gas can be sampled by means of a suitable absorption reagent. For this purpose, V-shaped absorbers are used. These absorbers are filled with a certain measured amount of reagent and fitted with a porous glass partition, so that the air or gas led into them passes through the reagent solution in the form of fine bubbles thus ensuring intimate contact.

Absorption or bubblers or impinger tubes

• All-glass impingers, commonly called midget impingers, are of 35 ml capacity, 22 cm in length and 2.6 cm wide, impinging end of the tube 1 mm bore size or preferably with fritted disc with porosity 50 microns or less.

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2. Bags and Containers and tubes • The use of bags and containers allows the

collection of air in its natural state without any concentration of pollutants. The technique is not suitable for collection of samples of large quantities of air, but may be convenient for the collection of pollutants those are present in relatively larger quantities or those permit the determination by chromatographic or infrared gas analysis or other such instrumental techniques where only a small, quantity of pollutants is required.

• For example, carbon monoxide can be collected in small bags from different locations and analysed in the laboratory by infrared gas analyser

Gas Sampling tubes

• The Gas Sampling Tube shown opposite has two stopcocks at either end to allow the flow of gas through the tube.

• The gas can be extracted by piercing the septum port on the side arm with a syringe. The stopcocks can be either glass or other synthetic material and can be manufactured a range of capacities.

Gas sampling by using tubes

Gas sampling syringe• These jumbo syringes are designed for holding and

dispensing large volumes of gas. Easy access to sample for the addition of standards or removal of subsample via secondary port.

3. Adsorption• Activated charcoal or silica gel or any other

suitable adsorbent is filled to separate out desired gases.

• Organic pollutants thus separated are analysed by gas chromatography.

• After adsorption charcoal is washed with solvent and then analysed by gas chromatography.

• Used for ozone and light hydrocarbons

4. Freeze out or condensation• Here the gas stream sampled is cooled in

suitable containers, thus bringing about the condensation of the volatile substances present. As in the case of absorption devices, here also the condensation traps can be arranged either in series or parallel, at decreasing temperatures.

• By using various coolants, e.g., ice, liquid air, or liquid nitrogen-the components can be separated by fractional condensation.

• This method is used in particular for the sampling of odoriferous substances.

Various coolants used for condensation

Stack Gas Monitoring

Types of MonitoringSource Monitoring

General Objectives

• Source monitoring may be carried out to:

– Determine emission rates, and assess how these are affected by process variations

– Evaluate pollution control devices

– Evaluate emission compliance with NAAQS

29

Prof S S Jahagirdar, NKOCET

STACK

IDFan

• Stack or source emissions monitoring involves taking direct measurements of an emission source, usually at an industrial or manufacturing facility.

• Often the source emission sampling is a requirement for annual monitoring in the site environmental licence.

• Hence, the purpose is often to demonstrate compliance of emissions with the legislative requirements and the environmental performance commitments of a company.

• Other circumstances where stack testing is required include development of new processes and products, and testing to allow optimisation of existing processes.

• For example, stack emission testing data can provide an important indication of the efficiency of a combustion process, or a particular production, as this type of testing allows the compounds that are being emitted as a waste product via the stack to be quantified.

• The process of stack testing is complex, and generally involves collection of samples at the source of emissions (often a stack) with the samples subsequently lodged with an appropriately certified laboratory for analysis.

• Given the range of compounds that can be tested, the methods and sampling instrumentation involved in source emission testing is varied.

Traverse points

• For the sample to become representative it should be collected across at various points across stack. This is essential as there will be variation in velocity and temperature across cross section of stack. Traverse points are to be located to achieve this.

Reference table for selecting traverse points

Circular stack

Rectangular stack

Sampling Train

• For sampling of particulates and gases in stack monitoring all components of stack monitoring kit are to be attached in logical series.

• Such assembly is known as ‘sampling train’

ISOKINETIC SAMPLING

• To obtain a representative sample independent of particle size, it is necessary to remove the sample stream isokinetically, i.e. with the same velocity as the main stream.

• The following Figure shows the pattern of the flow lines in the vicinity of a thin-walled sampling probe.

(Vs = Vg)

(Vs < Vg)

(Vs > Vg)

Vg VsVg

Vs

• In the isokinetic case (Vs=Vg), all particles flowing toward the intake opening are equally collected.

• If the sample taking velocity is too low (Vs>Vg), heavy particles can enter the probe even if the flow line on which they were located passes by the probe. Thus too many large particles are collected. (Over isokinetic sampling)

• If the sample taking velocity is too high (Vs<Vg), heavy particles fail to adhere to the flow lines and end up bypassing the probe. So too few large particles are collected. (Under isokinetic sampling)

Mathematical equations used in Stack Monitoring

Vs

Vs

Theory QuestionsQ 1. Write short notes on

1. Stack monitoring.

2. Iso-Kinetic sampling. (May 2011,Dec 2009, 6 marks)

3. Selection of Traverse points in stack monitoring

Q2. Draw neat diagram of a ‘Sampling Train’.Give equations for determination of Pressure and Stack gas velocity.

Q3. Explain various principle of waste gas sampling. (Dec 2010,8 marks)

Q4. List various devices used for sampling of gases and vapours. Explain any two. (Dec 2010,9 marks)

Q5. Discuss devices used for sampling of gases and vapours (May 2009, 8 marks).

Q6. List various devices used for sampling of gases and vapours. Explain any one in detail. (Nov 2008, May 2011,9 marks)

Q7. Draw neat sketch of a particulate ‘Sampling Train’. Write formula for calculation of stack gas velocity inside the stack. (Dec 2008, May 2011, 8 marks)

Q8. What is stack sampling? How to collect a representative sample? (Dec 2010, 8 marks)

Q9. Explain various devices used for sampling of gases and vapours. (May 2011, 8 marks)