GAS ABSORBER IN PROCESS INDUSTRY

Gas absorbers are used extensively in industry for separation and purification of gas streams as product recovery devices, and as pollution control devices. Gas absorbers are most widely used to remove water soluble inorganic contaminants from air streams. Absorption is a process where one or more soluble components of a gas mixture are dissolved in a liquid (i.e., a solvent). In gas absorption, soluble vapors are more or less absorbed in the solvent from its mixture with inert gas. The purpose of such gas scrubbing operations may be any of the following;

·         For separation of component having the economic value.

·         As a stage in the preparation of some compound.

·         For removing of undesired component (pollution).

Types of Absorption

·         Physical absorption,

·         Chemical Absorption

Physical Absorption:

 In physical absorption mass transfer take place purely by diffusion and physical absorption is governed by the physical equilibria. Physical absorption occurs when the absorbed compound dissolves in the solvent. Physical absorption depends on properties of the gas stream and solvent, such as density and viscosity, as well as specific characteristics of the pollutant in the gas and the liquid stream. These properties are temperature dependent, and lower temperatures generally favor absorption of gases by the solvent. Absorption is also enhanced by greater contacting surface, higher liquid-gas ratios, and higher concentrations in the gas stream.

Chemical Absorption:                                                                                          

Chemical absorption occurs when the absorbed compound and the solvent react. When oxides of nitrogen absorb in water the chemical reaction take place and nitric acid form this is common example of chemical absorption.

Types Of Absorber:

There are three major types of absorbers which are mainly used for absorption purposes:

·         Packed column

·         Plate column

Packed Column:

Packed towers, which are the most commonly, used gas absorbers for pollution control. Packed towers are columns filled with packing materials that provide a large surface area to facilitate contact between the liquid and gas. Packed tower absorbers can achieve higher removal efficiencies, handle higher liquid rates, and have relatively lower water consumption requirements than other types of gas absorbers. However, packed towers may also have high system pressure drops, high clogging and fouling potential and extensive maintenance costs due to the presence of packing materials. Installation, operation, and wastewater disposal costs may also be higher for packed bed absorbers than for other absorbers.

Plate Column:

Plate, or tray, towers are vertical cylinders in which the liquid and gas are contacted in stepwise fashion on trays (plates). Liquid enters at the top of the column and flows across each plate and through a downspout (down comer) to the plates below. Gas moves upwards through openings in the plates, bubbles into the liquid, and passes to the plate above. Plate towers are easier to clean and tend to handle large temperature fluctuations better than packed towers do. However, at high gas flow rates, plate towers exhibit larger pressure drops and have larger liquid holdups.

The choice between use of a plate tower or a packed tower for a given mass-transfer operation should, theoretically, be based on a detailed cost analysis for the two types of contactors. In many cases, however, the decision can be made on the basis of a qualitative analysis of the relative advantages and disadvantages.