Nitrogen oxides (NOx) emissions are characterized by visible reddish brown color stack discharges that can trail for miles. Typically, NOx emissions containing Nitrogen Dioxide (NO2) and Nitric Oxide gases in various proportion depending on the producing source processes. For examples, emissions from the silver reclaiming and metal etching operations have high NOx concentrations predominantly containing NO2, while nitric acid plant tail gases typically have a NOx concentration below 2500 ppm but of higher NO/NO2 ratio.
The difficulty in treating NOx emission using aqueous solutions stems from the sparing solubility of NO in water and its relatively inertness to react with oxygen to convert to soluble NO2. Additionally, when NO2 reacts with water to form nitric acid, 1/3 of the reacting NO2 will be released as NO as the by-product, adding it to the NO concentration already in the gas stream. This complication renders a severe limitation on otherwise a simple aqueous scrubbing process for controlling NOx emissions.
In the mid 1980's, through some extensive research and experimentation, Advanced Industrial Technology Corp. (AIT) has developed a catalytic promoted sorption process, CatNOx Sorption Process, that employs an aqueous catalytic medium to bind with NOx gases to form a transient reactive complex which is highly reactive and dissolves readily in water. Using this process technology, NOx containing discharges can be readily reduced to the level to meet EPA standards in a single scrubbing operation. Full commercial scale installations have been in operation since 1986 demonstrating that this process is effective, reliable, and easy to operate. To the left are photos of AIT NOx scrubbers installed at a Texa plant and at a Tennessee chemical plant. The feed gases of these installations were the dark reddish brown color gases drawn from the discharges of original NOx tail gas scrubbers.
Nitrogen Oxides (NOx) Recovery Systems Produce Nitric Acid for Reuse
For operations such as nitration and silver reclaiming that emit high concentration NOx gases, recovery of NOx in the discharge gases is an economic necessity and not just a pollution control requirement.
Depending on the nitrogen dioxide concentration in the influent gases, the recovered nitric acid can be at concentration as high as over 60% by weight. Most recovered acid can be readily recycled back to the manufacturing process. Pre-treatment is some time necessary to remove particulate matters from the process discharge gases to prevent them from contaminating the product acid.
The economics of a NOx recovery system depends largely on the NOx concentration in the discharge gases. As an example, a NOx recovery system for treating a waste air stream of 1,200 SCFM containing 4% by volume of NOx gases can produce 12,000 pounds/day of nitric acid (based on 100% acid). At the nitric acid cost of $0.20/pound acid, the recoverable acid is worth an annual cost savings of $876,000. If the NOx were to be treated by neutralizing with caustic soda solution, the cost of caustic soda would cost about $667,000 annually. Disposal of the neutralized products can cost another$700,000 annually.
The recovery process involves a series of chemical reactions including the absorption and reaction of NO2 with water, the chemical equilibrium between NOx and the acid solution, the evolution of NO from the NO2 reaction with water, and simultaneous oxidation of NO with oxygen in the gas stream. The process becomes quite complex as the reaction rates, chemical equilibrium, NOx concentration and NO2/NO ration are all inter-related. Only through a thorough analysis of the process conditions and several initial setups examined can an optimal system be arrived. Using AIT's catalytic promoted CatNOx Process as the tail end treatment is the simple way of assuring compliance to EPA's emission standards. Shown to the left are pictures of an AIT NOx recovery system installed in a northern New York state plant for treating NOx discharging gases from a nitration reactor. <Return to Home page