Pollutants from Effluent Streams

There are a variety of approaches to removing gaseous pollutants from effluent streams: absorption, adsorption, condensation, chemical reaction, incineration, and selective diffusion through a membrane. Absorption is an operation involving mass transfer of a soluble vapor component to a solvent liquid in a device that promotes intimate contact between the gas and the liquid. The driving force for absorption is the difference between the partial pressure of the soluble gas in the gas mixture and its vapor pressure just above the surface of the liquid. It is necessary to employ a liquid solvent within which the gas to be removed is soluble. Water is, by itself, quite efficient for removing soluble acidic gases such as HCl and HF and the soluble basic gas NH3 . Gases of more limited solubility, such as S02, C12 , and H2S, can be absorbed readily in an alkaline solution such as dilute NaOH. Thus, when water is used as the solvent, it may contain added species, such as acids, alkalines, oxidants, or reducing agents to react with the gas being absorbed and enhance its solubility. Nonaqueous, organic liquids of low volatility can be used for absorption of gases with low water solubility, such as hydrocarbons. Examples of such solvents are dimethylaniline and amines. Organic solvents are often limited to treating particle-free gases to avoid sludge formation. To provide a large liquid surface area for mass transfer, a means of breaking the liquid stream into small droplets or thin films is provided in the gas absorber. The most commonly used devices are columns containing packing or regularly spaced plates, open spray chambers and towers, and combinations of sprayed and packed chambers. Countercurrent contact of liquid and gas is employed to maximize the driving forces. Adsorption is employed to remove low concentration gases from exhaust streams by causing the gaseous solutes to intimately contact a porous solid to which the solute