Nitric Acid Selective Catalytic Reduction

Process Licensors. Some of the well-known nitric acid technology licensors are fisted in Table 3. Espindesa, Grande Paroisse, Humphreys and Glasgow, Rhfyne Poulenc, Uhde, and Weatherly are all reported to be licensors of weak acid technology. Most weak acid plant licensors offer extended absorption for NO abatement. Espindesa, Rhfyne Poulenc, Weatherly, and Uhde are also reported (53,57) to offer selective catalytic reduction (SCR) technology. 

Flue gas treatment (FGT) is more effective in reducing NO, emissions than are combustion controls, although at higher cost. FGT is also useful where combustion controls are not applicable. Pollution prevention measures, such as using a high-pressure process in nitric acid plants, is more cost-effective in controlling NO, emissions. FGT technologies have been primarily developed and are most widely used in Japan. The techniques can be classified as selective catalytic reduction, selective noncatalytic reduction, and adsorption. 

Nitric Acid Plant - Nitrogen oxide levels should be controlled to a maximum of 1.6 kg/t of 100% nitric acid. Extended absorption and technologies such as nonselective catalytic reduction (NSCR) and selective catalytic reduction (SCR) are used to eontrol nitrogen oxides in tail gases. 

Nitrous oxide has received increasing attention the last decade, due to the growing awareness of its impact on the environment, as it has been identified as an ozone depletion agent and as a Greenhouse gas [1]. Identified major sources include adipic acid production, nitric acid and fertilizer plants, fossil fuel and biomass combustion and de-NOx treatment techniques, like three-way catalysis and selective catalytic reduction [2,3]. 

A wide range of catalytic materials have been investigated for the selective catalytic reduction of NOx. For stationary emissions, NH3-SCR using vanadium-tungsten oxides supported on titania is the most used method however, when there is a simultaneous emission of NO and NOz (in tail gas from nitric acid plants), copper-based zeolites or analogous systems have been proven to be preferable [31b], In fact, there are two main reactions for NH3-SCR ... 

The primary pollution problem in nitric acid plants is the abatement of NOx in tail gases. Three options exist to reduce these emissions to acceptable levels 1) Capture the NOx and convert it to additional nitric acid, 2) Capture the NOx and convert it to nitrate-nitrite sales, or 3) Render the NOx harmless by converting it to non-polluting compounds. The processes that have been developed to reduce emissions at existing and new plants can be classified into four general categories Absorption, Adsorption, Selective Catalytic Reduction (SCR) and Non-Selective Catalytic Reduction91. 

Selective Catalytic Reduction (SCR) is normally used in new nitric acid plants. In this process ammonia reacts with nitric oxide and nitrogen dioxide but only to a lesser extent with oxygen to selectively reduce the NOx compounds to N2. The reactions are shown below97,104 ... 

The second process is known as selective catalytic reduction (SCR). SCR may also be used in the treatment of nitric acid process tail gas and similar processes, but has achieved prominence through its application to NO removal from electricity-generating power stations, especially those that are coal fired. In SCR, a range of reductants for the NOx can be used the most common is ammonia. The primary reactions involved arc shown in Eqs. (13) and (14). Oxygen is required for this form of NO control, and levels of 2-3% are typically needed for optimum catalyst performance. 

First NO is formed and then it is oxidized in the atmosphere to NOj. Since in combustion, the origin of nitrogen is not only from N-rich fuel but also from air supphed for oxidation, in the elimination of NO, postcombustion methods are important. So far the most effective technique has been selective catalytic reduction of NO . on various catalysts. When the activated carbons are used as removal media, the elimination process includes also adsorption combined either with oxidation or reduction. Oxidation usually leads to the formation of nitric acid whereas Nj is the product of NO, reduction. As in the cases of other pollutants addressed in this review, for NO, removal either unmodified or impregnated (caustics, catalytic metals) activated carbons have been used [101-114]. 

Selective non catalytic reduction (SNCR) with NH3 is limited to industrial boilers in consequence of the relatively narrow temperature range for the reaction. Selective catalytic reduction (SCR) by ammonia has high efficiency and it can be used for many stationary sources, especially for nitric acid plants [1], and it is based on the catalytic pairing of nitrogen atoms, one fi-om nitric oxide, one fi om ammonia. This method, however, is imsuitable for small sources and vehicles. As far as automotive emission is concerned nonselective catalytic reduction (NSCR) by hydrocarbons, CO and H2 from the exhaust stream has been reported over various catalysts recently [1,3,4]. 

Selective Catalytic Reduction (SCR) [17] - Ammonia is used as a reducing agent over a catalyst to convert the NOx gases to nitrogen. These processes can reduce the concentration of NO in tail gas to 100 ppmv and lower. Existing plants with medium-pressure absorption can be economically revamped using this process. The capital cost of the selective catalytic reduction (SCR) process to remove NO from a concentration of 500-200 ppmv would increase the capital costs of a dual-pressure nitric acid plant by 2% and operational costs by US 0.30/tonne of nitric acid. The SCR method needs ammonia for reduction of NOx- The following reactions occur in the process of selective reduction of NO ... 

Briiggemann TC, Przybylski M-D, Balaji SP, Kell FJ (2010) Theoretical investigation of the mechanism of the selective catalytic reduction of nitrogen dioxide with ammonia on H-form zeolites and the role of nitric and nitrous acids as intermediates. J Phys Chem C 114 6567-6587... 

Figure 6.4.23 Single-pressure nitric acid plant (high pressure) using selective catalytic reduction (SCR) for NO f abatement (BFW boiler feed water). Adapted from Moulijn, Makkee, and Van Diepen (2004).

Selective catalytic reduction of N2O and NO,e in a single reactor in the nitric acid industry. Catal Today, 75 (1-4), 227-232. 

Anthropogenic sources of NgO include adipic and nitric acid production, fossil fuel and biomass combustion, land cultivation, and vehicle emissions. " NgO emissions from gasoline-powered engines have been related to the aging of three-way catalysts (TWCs) and NgO is also emitted as a by-product of Pt-based prototypic catalysts for the selective catalytic reduction (SCR) of NO with hydrocarbons in diesel engine exhausts. ... 

Postformation nitrogen oxide emission control measures include selective catalytic and noncatalytic reduction with ammonia, which between them are used by some 900 power station installations worldwide [51]. The catalytic removal methods are 70-90% efficient at NOx removal, but are more expensive to operate than the noncatalytic methods which are 30-80% efficient. Ammonia or methane noncatalytic reduction of NOx to elemental nitrogen is also an effective method which is cost-effective for high concentration sources such as nitric acid plants (Chap. 11). NOx capture in packed beds is less expensive, but this method is not particularly effective [23]. It is also not a very practical method either for utilities or for transportation sources. Two-stage scrubbing has also been proposed as an effective end-of-pipe NOx control measure. The first stage uses water alone and the second uses aqueous urea. 

This paper deals with the hydrothermal deactivation, under an air + 10 vol. % H2O mixture between 923 and 1173 K, of Cu-MFI solids, catalysts for the selective reduction of NO by propane. Fresh and aged solids were characterized by various techniques and compared with a parent H-ZSM-5 solid. The catalytic activities were measured in the absence and in the presence of water. The differences between fresh and aged Cu-ZSM-5 catalysts (destruction of the framework, extent of dealumination...) were shown to be small in spite of the strong decreases in activity. Cu-ZSM-5 is more resistant to dealumination than the parent H-ZSM-5 zeolite. The rate of NO reduction into N2 increases with the number of isolated Cu VCu ions. These isolated ions partially migrate to inaccessible sites upon hydrothermal treatments. At very high aging temperatures a part of the copper ions agglomerates into CuO particles accessible to CO, but these bulk oxides are inactive. Under catalytic conditions and in the presence of water, dealumination is observed at a lower temperature (873 K) than under the (air + 10 % H2O) mixture, because of nitric acid formation linked to NO2 which is either formed in the pipes of the apparatus or on the catalyst itself... 

The catalytic processes are used for the gas purification in the chemical industry and in energy production. They have found extensive applications, for example, in the total or selective reduction for the removal of NO., when nitrogen oxides are reduced to form nitrogen. The efficiency of this process is high, in the case of removing NOj. in the production of nitric acid, it may be as high as 90-95%. 

The Thermal De-NOx process was developed by Richard Lyon at Exxon in the early 1970 s and patented in 1975 [1]. It is one of three SNCR (selective non-catalytic reduction) schemes for nitrogen oxides (the others are RAPRENOx, or cyanuric acid injection, and urea injection). Such after-treatment processes are commonly used on stationary combustion systems to control NOx emissions. The Thermal De-NOx process uses ammonia as the additive, and the complex reaction by which the ammonia reacts with nitric oxide has a number of fascinating properties that have prompted considerable research over the past 15 years or so. 

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