Noxious gas removal

NOXIOUS GAS REMOVAL. Gaseous pollutants can be removed from air streams either by absorption, adsorption, condensation, or incineration. A list of typical gaseous pollutants that can be treated with these four methods is given in Table 9. Generally, condensation is not utilized as a method for removing a solvent vapor from air or other carrier gas unless the concentration of the solvent in the gas is high and the solvent is worth recovery. Since condensation cannot remove all of the solvent, it can only be used to reduce the solvent concentration in the carrier gas. 

Fluidized-bed combustion uses air-classified municipal solid residues to provide heat for a conventional gas turbine to produce power. Several stages of cyclone separators are also used to remove particulates from the gas prior to its expansion through the turbines. An advantage of the process is reduction of noxious gas emission. 

ABSORPTION (Process). Absorption is commonly used in the process industries for separahng materials, notably a specific gas from a mixture of gases and in the production of solutions such as hydrochloric and sulfuric adds. Absorption operations are very important to many air pollution abatement systems where it is desired to remove a noxious gas, such as sulfur dioxide or hydrogen sulfide, from an effluent gas prior to releasing the material to the atmosphere. The absorption medium is a liquid in which (1) the gas to be removed, i.e., absorbed is soluble ill the liquid, or (2) a chemical reaction takes place between the gas and the absoibing liquid. In some instances a chemical reagent is added to the absorbing liquid to increase the ability of the solvent to absorb. 

A significant advantage of the fluidized bed reactor over conventional incinerators is its ability to reduce noxious gas emission and, finally, the fluidized bed is unique in its ability to efficiently consume low quality fuels. The relatively high inerts and moisture content of solid wastes pose no serious problem and require no associated additional devices for their removal. 

For some applications, an adsorbent may be impregnated with a material that enhances its contaminant-removal ability. The improved effectiveness may be related to any of several mechanisms. The impregnating material may react with the vapor contaminant to form a compound or complex that remains on the adsorbent surface. Some impregnants react with the contaminant, or catalyze reactions of the contaminant with other gas constituents, to form less noxious vapor-phase substances. In some instances, the impregnant acts as a catalyst intermittently, for example, under regeneration conditions. In this case, the contaminant is adsorbed by physical adsorption and destroyed by a catalytic reaction during regeneration. 

Similar to Black, Daniel Rutherford (1749-1819) studied gases for his medical degree dissertation. Rutherford found that common air contained a part that supported respiration and a part that did not. Initially, Rutherford assumed the part that did not support respiration was contaminated by fixed air. Rutherford experimented and removed the fixed air, and he discovered the uncontaminated air still did not support life or combustion. Rutherford assumed the gas he had isolated was ordinary air saturated with phlogiston hence, it was phlogisticated air, which he referred to as noxious air. What Rutherford had isolated was nitrogen, and he is given credit for its discovery. 

Purely adiabatic fixed-bed reactors are used mainly for reactions with a small heat of reaction. Such reactions are primarily involved in gas purification, in which small amounts of noxious components are converted. The chambers used to remove NO, from power station flue gases, with a catalyst volume of more than 1000 m3, are the largest industrial adiabatic reactors, and the exhaust catalyst for internal combustion engines, with a catalyst volume of ca. 1 L, the smallest. Typical applications in the chemical industry include the methanation of traces of CO and CO2 in NH3 synthesis gas, as well as the hydrogenation of small amounts of unsaturated compounds in hydrocarbon streams. The latter case requires accurate monitoring and regulation when hydrogen is in excess, in order to prevent complete methanation due to an uncontrolled temperature runaway. 

The problem of the removal of aerosol particles from gas streams has become of increasing importance from the standpoint of public health and the recovery of valuable products. Technology of controlling the aerosol particles or improving the liquid phase of aerosol is very important in many industrial processes such as oil and petroleum, electronic, mining, and food, as well as waste products like noxious emission of aerosol in chemical plants. There are several ways for this purpose among which fibrous filters are more popular so that it is obvious to try to improve their efficiency. The efficiency of collection and the pressure drop are the most important practical considerations in the design of these fibrous filters [2], Various... 

The term ionizing wet scrubber was first used by the Ceilcote Co., located in Berea, Ohio, and has found wide application in the air pollution control field. This system is a proven means for the removal of pollutants from industrial process gas streams. The IWS combines the established principles of electrostatic particle charging, image force attraction, inertial impaction, and gas absorption to collect submicron solid particles, liquid particles, and noxious and malodorous gases simultaneously. The IWS system requires little energy and its collection efficiency is high for both submicron and micron size particles. 

Several processes have been used to combat acid-rock drainage that has already been formed. Metal precipitation and acid neutralization with lime or limestone is the most common technique used to limit the environmental impact of an acidic, metal-bearing solution from waste material. Metal hydroxide formation is quite simply achieved with moderate changes in the pH of the solution. Metal sulfide formation is typically preferred because the sulfide precipitates are more stable, more quickly formed, and much easier to remove by filtration or sedimentation. However, the sulfide sources (such as hydrogen sulfide gas, sodium sulfide, and thiourea) are very expensive on a per pound of metal removed basis, and can produce a noxious hydrogen sulfide gas emission from the mixing tanks. The chemistry of precipitation is discussed in more detail later. 

In 1772 the Scottish botanist Daniel Rutherford found that a mouse enclosed in a sealed jar quickly consumed the life-sustaining component of air (oxygen) and died. When the fixed air (CO2) in the container was removed, a noxious air remained that would not sustain combustion or life. We now know that gas as nitrogen. 

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