Friday, March 24, 2017

Gaseous pollutant control by absorption, condensation and combustion

  1. AbsorptionGaseous pollutants that are soluble in aqueous liquids can be removed by absorption. Absorption is one of the main mechanisms used for the removal of acid gas compounds. (Ex: Sulphur dioxide, Hydrogen Chloride and Hydrogen Fluoride) Water soluble organic compounds like alcohols, aldehydes and organic acids can also be removed by absorption. 
    • The contaminant gas or vapour is absorbed from the gas stream as it comes in contact with the liquid
    • All absorption processes operate best when gas and liquid temperatures are low.
    • Gas and vapour phase contaminants are most soluble in cold conditions.
The figure below illustrates the removal of pollutants from a gas stream using a scrubbing liquid. This technique explains the removal of gaseous pollutant by absorption

Water can be used for recovery of water-soluble compounds such
as acetone and low molecular weight alcohols, which can later be separated from water using distillation. Additives are often used to increase the effective mass transfer rate of the pollutant from the gas phase into the liquid phase, affecting the surface tension, reducing interfacial resistance and increasing the apparent solubility.

Gas absorption can be expensive, however it is generally used only to recover VOCs that have a secondary market value. Gas absorption techniques are used for the recovery of a variety of chemicals in the coke manufacturing industry. They are often called scrubbers.

2.   Condensation: Condensation and gas absorption are most commonly used for highly concentrated VOC (Volatile Organic Carbon) streams that are advantageous to recover and the relatively large expense is justified. It employs a drop in temperature and/ or increase in pressure to cause the VOCs in the emission stream to condense. The cleaned air stream is separated from the condensate containing target pollutants. In many cases, very large temperature drops are required to achieve effective condensation, requiring significant energy investment to accomplish cooling.

Condensation is used to recover gasoline and fuel vapors at gasoline loading terminals and in gasoline dispensing facilities. It is also used in the adsorbent regeneration process to separate solvents from the stream to separate solvents from the stream used to regenerate the activated carbon.

3.   Combustion: Incineration or combustion is a commonly used technology to control VOCs. Complete combustion of hydrocarbons produces carbon dioxide and water. Flares, thermal oxidisers and catalytic converters use oxidation chemistry to treat VOC emissions. For example, by using catalytic converters, thermal oxidation of by-products of incomplete combustion can be safely achieved at temperatures much lower than what would be required without the aid of catalysts. Sometimes the gases are moved over a bed of copper oxide, which reacts with oxides of sulphur to form copper sulphate. Copper sulphate acts as a catalyst for reducing NOx to ammonia (NH3). Ammonia may be added to flue gas before passing it over a catalyst. The catalyst enables ammonia to react with Oxides of Nitrogen (NOx) converting it into molecular nitrogen and water. Staged combustion processes significantly reduce NOx emissions

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