Recovering Valuable Chemicals from Air Emissions

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Courtesy of Kenson Associates

Air emissions in many cases are a waste of valuable natural resources that should have become useful chemicals. In some cases the economic payback from recycling/reuse of air emissions can be almost as much a motivation to control air pollution as government regulations. Examples of recycling and reuse of air emissions are sulfur oxides emissions that can be made into sulfuric acid, cement additives, fertilizer or building materials, solvent emissions that can be recycled back into a product manufacturing process or sold to others and alcohol fumes from beverage manufacturing that can be reused.

Non-ferrous metal smelters for recovery of pure copper, lead or zinc metal from naturally occurring ores normally use pyrometallurgical processes to separate impurities from the metal in the concentrated ore. The high temperatures of these processes results in the emission of sulfur oxides that can be recovered to produce sulfuric acid which are then used in the ore leaching process to separate large volumes of ore impurities from the desired metal. Not only does this avoid the emissions of tens of thousands of tons of air pollution from a non-ferrous smelter, but it has a significant economic benefit to the smelter in a sharp reduction, or total elimination, of merchant sulfuric acid purchases for use in the ore leaching process. One process for air pollution control in metal smelters involves gas cleaning by a high efficiency wet scrubber to remove particulate and fume emissions and, in some cases, a final gas cleaning step using a wet electrostatic precipitator to remove any remaining gas contaminants that would poison the catalyst used in a sulfuric acid plant. The cleaned gas is then used as a feedstock for an onsite sulfuric acid plant. Conventional sulfuric acid plants are economically and technically practical when the sulfur oxides concentration in the smelting process emissions is approximately 10% or higher. However, new developments in sulfuric acid plant design can now allow the conversion of sulfur oxides to sulfuric acid to be technically practical and economical at approximately 4% sulfur oxides concentration.

Cement kilns in some cases use locally mined limestone bearing rock as a raw feedstock that contains pyrite (iron sulfide) or they may use kiln fuels that contain high levels of sulfur such as coal or heavy fuel oil. This results in sulfur oxides emissions that exceed applicable air pollution regulations in some cases that requires significant air emission control expenditures. Although not as economical as in the case of non-ferrous metal smelters, high efficiency wet scrubbers can be used to control these sulfur oxides emissions and recover gypsum (hydrated calcium sulfate) for use in the cement kiln as a product improvement additive. Gypsum is a product of the reaction of the sulfur oxides emissions with the limestone used as a neutralizing agent in the wet scrubbing system. Coal fired industrial steam boilers and electrical utility power plants may need to use high sulfur coal to be economically feasible, but at the same time they emit tens of thousands of tons per year of sulfur oxides emissions to the atmosphere. Asian, North American and European air pollution regulations in many cases now mandate stringent control of these air emissions at a high capital and operating cost to do so. In order to do so most economically, wet scrubbers are normally employed that use low cost locally available neutralizing agents to convert the acidic gases into neutral chemical species. For many of these facilities locally available lime or limestone are the most economical neutralizing agents and their reaction with the sulfur oxides leads to very large volumes of semi-solid residues containing calcium sulfite and gypsum. Disposal of these on land is a secondary environmental and cost problem, so some of these facilities use further purification and separation steps to produce high quality gypsum. The gypsum so produced can be used locally to manufacture gypsum board or wallboard that is a desirable building material. A few facilities instead use ammonia as a neutralizing agent for the sulfur oxides emissions and thus produce ammonium sulfate which can be used as a fertilizer. This application has potential economic benefits in areas where fertilizer is in high demand.

Industrial processes that use volatile solvents can be very large sources of solvent air emissions for which air pollution control measures are mandated by regulatory authorities. Some industries that have been able to recycle these solvents economically are auto parts manufacturing, chemicals production, electronics manufacturing, paper coating, pharmaceutical production and rotogravure printing. Carbon, organic resin or zeolite adsorption technology allows the practical capture of these solvent emissions for reuse. Multiple adsorption vessels with vertically or horizontally mounted granular adsorbent beds are usually employed, with the number of vessels governed by the volume of process exhaust gas in many cases. At high solvent concentrations in the exhaust gas, it governs the number of vessels. In order to recover the solvents, the adsorbent must be regenerated by steam, hot air or hot inert gas before it becomes saturated by the solvent. In some cases further purification of the solvent by distillation or by other means must be done before it is reusable in the process or is resold to another solvent user. It is possible, in certain processes that use enormous amounts of solvents such as rotogravure printing, to have a reasonable (2-4 years) payback period for the capital cost of the solvent recovery system by the value of the recovered solvent that can be reused in the process.

In alcoholic beverage manufacturing, alcohol fumes can be emitted to the atmosphere which may be subject to air pollution regulations. Several of these facilities have installed carbon adsorption systems using a unique activated carbon fiber adsorbent which is steam regenerated. The adsorption/desorption speed and the high purity of the carbon fiber adsorbent allow the use of very short adsorption/desorption cycles which results in higher purity of the recovered alcohol than when granular carbon is employed. This allows the alcohol/water mixture so produced by the steam desorption cycle to be recycled into the beverage process. The value of the recovered alcohol will mitigate some of the costs of the carbon adsorption system, but payback of the capital cost of the solvent recovery system may not be accomplished in the 2-4 year time span mentioned above.

Further details on the above mentioned examples may be obtained from the author.

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