Air Pollution and Control: Absorber and Catalytic Convertor

 Absorber:

Absorption is also called as gas absorption, gas scrubbing or gas washing.

  In absorption there is transfer of one or more species from gas phase to liquid solvant.

  Absorption involved no change in chemical species present in the system

  Absorption is used to

1. To separate gas mixtures.

 2. To remove impurities.

 3. To remove or recover valuable chemicals.

   The operation of removing the absorbed solute from the solvent is called stripping.

Catalytic Convertor:

    A catalytic converter is a device that uses a catalyst to convert harmful exhaust gases into less harmful ones. It's used in a variety of industries, including steel, manufacturing, and refineries, where harmful gas emissions are high catalytic converter is a gas  emissions control device that is used to convert toxic byproducts of combustion to less toxic substances by performing catalysed chemical reactions. The reactions tend to vary depending upon the type of catalyst installed.

Construction of catalytic converter

 

 A catalytic converter consists of several key components designed to facilitate the chemical reactions that convert harmful exhaust gases into less harmful substances. The construction of a typical catalytic converter involves the following main parts:

1.      Casing: The outer shell of the catalytic converter is usually made of stainless steel or another heat-resistant material. It provides protection to the internal components and ensures that the high temperatures generated during the catalytic reactions are contained within the unit.

2.      Catalyst Support: Inside the casing, there is a substrate or support material that holds the catalyst. This substrate is often a ceramic or metallic honeycomb structure with a large surface area. The honeycomb design provides ample contact area for the exhaust gases to interact with the catalyst.

3.      Catalyst Coating: The catalyst, composed of precious metals like platinum, palladium, and rhodium, is applied as a thin washcoat onto the substrate. These metals act as the catalysts for the chemical reactions that occur within the converter. The washcoat enhances the surface area available for reactions to take place.

4.      Oxygen Sensors: Modern catalytic converters often have oxygen sensors positioned before and after the converter. These sensors monitor the oxygen levels in the exhaust gases, providing feedback to the engine control unit (ECU) to optimize the air-fuel mixture for efficient catalytic conversion.

5.      Heat Shield: Catalytic converters generate substantial heat during their operation. A heat shield made of insulating materials is often placed around the converter to prevent excessive heat from affecting surrounding components and to protect against potential heat-related damage.

 Working :

catalytic converter works by facilitating chemical reactions that transform harmful pollutants produced during the combustion of fuel into less harmful substances.

At first, it reacts with the carbon monoxide generated by the combustion of gasoline. It also reacts with the hydrocarbons formed by unburned fuel and other nitrogen oxide emissions. Thus, the cat-con converts these gases into less harmful by-products which are the carbon-di-oxide, water vapor, and nitrous emissions.

  The catalysts used for effective pollution control are the precious metals, primarily platinum and palladium or their alloys. These are arranged in such a way as to provide the maximum possible surface area for contact with the gas. The catalyst is coated onto suitable elements such as metal ribbons, ceramic rods or alumina pellets. These elements are then packed into the Catalyst bed. A catalytic combustion unit consists of a reaction vessel or converter in which the catalyst is arranged in single or multiple fixed beds proceeded by a preheat section, if necessary. In the preheat section, only the pollutant gas stream is heated to the temperature required to support catalytic combustion. The preheated gas is then passed through the catalyst bed where the combustion occurs. To maintain the catalyst in an active state and to achieve complete combustion about 1% excess oxygen is required. Product gases are simple compounds like carbon dioxide and water.

1.      Exhaust Gas Flow: The process begins with the flow of exhaust gases from the engine into the catalytic converter through the exhaust pipe.

2.      Catalyst Activation: Inside the catalytic converter, there are catalysts made of precious metals like platinum, palladium, and rhodium. These catalysts are coated onto a substrate, often a ceramic or metallic honeycomb structure. The catalysts are specifically designed to promote chemical reactions that occur at lower temperatures than would normally be required.

3.      Oxidation Reactions: As exhaust gases pass through the catalytic converter, they come into contact with the catalysts. In the case of carbon monoxide (CO) and unburned hydrocarbons (HC), the catalysts initiate oxidation reactions. Oxygen from the exhaust combines with these pollutants to form carbon dioxide (CO2) and water (H2O): CO + ½O₂ → CO₂
HC + O₂ → CO₂ + H₂O

4.      Reduction Reactions: Nitrogen oxides (NOx) are targeted through reduction reactions. A different catalyst, often rhodium, promotes the reduction of nitrogen oxides using unburned hydrocarbons present in the exhaust gases. The result is the conversion of nitrogen oxides into nitrogen (N2) and oxygen (O2): NOx + HC → N2 + CO2 + H2O

5.      Conversion of Pollutants: Through these oxidation and reduction reactions, the harmful pollutants—carbon monoxide, unburned hydrocarbons, and nitrogen oxides—are converted into less harmful substances—carbon dioxide, water, and nitrogen. These products are then expelled from the catalytic converter as exhaust gases.

6.      Temperature Management: Catalytic converters require a certain operating temperature range to function efficiently. Modern vehicles are equipped with oxygen sensors that monitor the oxygen levels in the exhaust gases. The engine control unit (ECU) uses this information to adjust the air-fuel mixture to maintain the optimal temperature for catalytic converter operation.