L-34GASEOUS CONTROL

TECHNOLOGIES

Air Pollution and Control

Elective- I

CONTENTS OF UNIT- VIII

L-32 Principles of removal of gaseous pollutants, details of incineration, absorption adsorption systems.

L-33 Vehicular pollution, composition, quantity and control.

Status of air pollution in India, Air pollution control act and strategy for effective control of air pollution.

1) ABSORPTION

The removal of one or more selected components from a gas mixture by absorption is probably the most important operation in the control of gaseous pollutant emissions.

Absorption is a process in which a gaseous pollutant is dissolved in a liquid.

Water is the most commonly used absorbent liquid.

As the gas stream passes through the liquid, the liquid absorbs the gas, in much the same way that sugar is absorbed in a glass of water when stirred.

Absorption is commonly used to recover products or to purify gas streams that have high concentrations of organic compounds.

Absorption equipment is designed to get as much mixing between the gas and liquid as possible.

Absorbers are often referred to as scrubbers, and there are various types of absorption equipment.

The principal types of gas absorption equipment include

1. spray towers, 2. packed columns, 3. spray chambers, and4. venture scrubbers.

The packed column is by far the most commonly used for the absorption of gaseous pollutants.

The packed column absorber has a column filled with an inert (non-reactive) substance, such as plastic or ceramic, which increases the liquid surface area for the liquid/gas interface.

The inert material helps to maximize the absorption capability of the column. In addition, the introduction of the gas and liquid at opposite ends of the column causes mixing to be more efficient because of the counter-current flow through the column.

In general, absorbers can achieve removal efficiencies grater than 95 percent.

One potential problem with absorption is the generation of waste-water, which converts an air pollution problem to a water pollution problem.

KEY TERMS

1. Absorbent: the liquid, usually water mixed with neutralizing agents, into which the contaminant is absorbed

2. Solute: the gaseous contaminant being absorbed, such as SO2, H2S, and so forth

3. Carrier gas : the inert portion of the gas stream, usually flue gas, from which the contaminant is to be removed

4. Interface : the area where the gas phase and the absorbent contact each other

5. Solubility : the capability of a gas to be dissolved in a liquid

SPRAY

TOWER

Plate Tower

PACKED

TOWER

PACKING MATERIAL USED IN PACKED TOWER

BUBBLE

CAPPED TRAY

TOWER

2) ADSORPTION

Adsorption is used when

1. The pollutant gas is incombustible or difficult to burn

2. The pollutant is sufficiently valuable to warrant recovery

3. The pollutant is in very dilute concentration in the exhaust system

The technique is based on the reaction of gases on the solid adsorbents.

The adsorption may be physical or chemical.

In this method gas is passed through a bed of adsorbents packed in the specially designed towers to allow the maximum contact between the two

Physical adsorption depends on the temperature and pressure conditions. Adsorption is promoted by increase in pressure and decrease in temperature

Chemical adsorption depends on the reactivity of the gases and their bond forming capacity with the surface of the adsorbent, which provides surface for the reaction.

Adsorbent can be regenerated for continuous reuse.

In some cases if is not economical to regenerate, it better to dispose the pollutant together with the adsorbent

Adsorption is mostly pollutant specific.

e.g – Activated carbon, silica gel and diatomaceous earth are suitable for adsorption of water vapours from a gas phase. It can also adsorb SO2 and NH3.

Activated carbon is most suited for removal of organic gases from gas stream.

Carbon adsorption systems are either regenerative or non-regenerative.

Regenerative system usually contains more than one carbon bed. As one bed actively removes pollutants, another bed is being regenerated for future use.

Non-regenerative systems have thinner beds of activated carbon. In a non-regenerative adsorber, the spent carbon is disposed of when it becomes saturated with the pollutant.

Regenerative Carbon Adsorption System

Non-Regenerative Carbon Adsorption System

PART-IIGASEOUS POLLUTION CONTROL

3) COMBUSTION

In many cases it is not possible to remove the required amount of specific pollutant from an exhaust stream by techniques such as absorption or adsorption.

The other technique available is Combustion

Combustion refers to rapid oxidation of substances (usually referred as fuels) with evolution of heat.

• To summarize, Combustion isdefined as rapid, high-temperature gas-phaseoxidation.

• Simply, the contaminant (acarbon-hydrogen substance) isburned with air and convertedto carbon dioxide and watervapor.

Combustion process involves three distinct components

1. Fuel : -A solid, liquid or gaseous substance with energy rich C-C or C-H bonds among others, which are broken up during combustion

2. Oxidant:- A substance which aids in combustion process by breaking the chemical bonds allowing the release of heat.

3. Diluent:- A substance that does not take part in the combustion process but acts as carrier of the fuel or the oxidants. Most common diluents is Nitrogen present in the air

I) FLARE OR

DIRECT FLAME COMBUSTION

Flare

Combustible gases are burned in open air, which produces flare

The flare is usually employed to remove hydrocarbons and organic vapours, odorous compounds in refineries and chemical works.

It can also burn gases such as NH3, HCN or other toxic or dangerous gases.

If aromatic hydrocarbons are present, they burn with Smokey flame. This can be avoided by injecting a steam into the flame, which reacts and forms hydrogen and CO both burn smokelessly.

However such steam-injected flare are little noisy

ADVANTAGES OF FLARE

1. Can be an economical way to dispose of sudden releases of large amounts of gas;

2. In many cases do not require auxiliary fuel to support combustion; and

3. Can be used to control intermittent or fluctuating waste streams.

DISADVANTAGES OF FLARE

1. Can produce undesirable noise, smoke, heat radiation, and light.

2. Can be a source of SOx, NOx, and CO;

3. Cannot be used to treat waste streams with halogenated compounds; and.

4. Released heat from combustion is lost

4) INCINERATION OR AFTERBURNING

Incineration is method of reducing gases, liquid and solid waste streams by chemically altering the pollutant species once they are formed.

It is used to remove combustible air pollutants (gases, vapours or odours)

I) THERMAL INCINERATION

+Air

In this method combustibles in the gas stream are brought above auto ignition temperatures and burn with oxygen usually present in the gas stream.

If sufficient oxygen is not available, air is added by means of blower fan.

Thermal incineration is carried out in the temperature range of 10000 F to 15000F

Because of this cost is less and NOxformation is also less.

Important considerations in Design of thermal incinerators are the “Three Ts”

Time- residence time should be 0.2 to 0.8 sec with 0.5 sec as a reasonable guideline

Temperature (refer next slide)

Turbulence- complete mixing is very important in case of odour control than hydrocarbons, Less residence time is required if proper mixing occurs

APPROXIMATE AVERAGE TEMPERATURE

REQUIREMENTS

Average temperature range (0K)

Hydrocarbon oxidation 780 – 1030

Carbon monoxide oxidation

950- 1060

Odour control via oxidation

750 - 980

Thermal Incinerator

ADVANTAGES

Incinerators are one of the most positive and proven methods for destroying VOC, with efficiencies up to 99.9999% possible.

Thermal incinerators are often the best choice when high efficiencies are needed and the waste gas is above 20% of the LEL (Low explosive limit).

II) RECUPERATIVE

INCINERATION

Whichever may be the method, Temperature of gases leaving the system vary from 700 to 2000 0F

Thus considerable energy at high temperatures is associated with the gas stream.

So that heat can be used to preheat the contaminated gas entering into the reactor.

Heat exchanger used for this purpose is called as recuperator or regenerator

Use of recuperator reduce use of fuel and makes it economical.

Initial cost is high.

ADVANTAGES

1. 99.9999% efficiency can be achieved

2. Recuperative incinerators usually are more economical than straight thermal incinerators because they recover about 70% of the waste heat from the exhaust gases.

3. This heat can be used to preheat incoming air, and of ten times, sufficient waste heat will be available for process heating, or to generate steam or hot water

DISADVANTAGES

Thermal incinerators, including recuperative types, are not well suited to streams with highly variable flow because of the reduced residence time and poor mixing during increased flow conditions which decreases the completeness of combustion.

Incinerators, in general, are not recommended for controlling gases containing halogen- or sulfur-containing compounds because of the formation of highly corrosive acid gases.

III) CATALYTIC INCINERATION

A catalyst accelerates rate of chemical reaction without undergoing a chemical change itself.

Residence time is in the range of 0.3 to 0.9 sec.

Combustion reaction occurs on the surface of the catalyst

Most gases containing combustible pollutants from industrial processes are at a fairly low temperature. Therefore some type of preheating burner is used to bring waste gas up to temperature , at which catalyst will be effective.

TEMPERATURE RANGES FOR DIFFERENT

POLLUTANT GASES

Temperature range is 590 to 810 0K

Efficiency is 95% to 98%

Effluent gases are CO2, vapours and nitrogen

Industrial pollutant Average temperature range (0K)

Solvents 530 – 730

Vegetable and animal fats 530 - 640

Chemical process exhausts 480 - 670

Catalyst should be

- Cheap

- Long lasting

- Should be able to function at required temperatures.

- Capable of formed into variety of shapes

Examples of catalyst are

Platinum

Palladium

Catalytic Incinerator

ADVANTAGES

1. Lower fuel requirements;

2. Lower operating temperatures;

3. Little or no insulation requirements;

4. Reduced fire hazards; and

5. Less volume/size required

DISADVANTAGES

1. High initial cost;

2. Catalyst poisoning is possible;

3. Particulate often must first be removed; and

4. Spent catalyst that cannot be regenerated may need to be disposed

THEORY QUESTIONS

Write short notes on

1. Absorption 2. Adsorption 3. Catalytic incineration 4. Recuperation 5. Flare

6. Thermal incineration

A. Gaseous control of pollutants (note:- List all and

explain any one or two in detail)

B. Combustion method of gaseous pollution control (note:-list all methods under combustion and

incineration and explain any one with figure)