Oxidation, ammonia

Catalytic oxidation of ammonia to nitric oxide is the basis of production of nitric acid. It is also used in other processes, of which may be mentioned hydroxylamine synthesis (Section XIII) and the chamber process for the production of sulfuric acid. The products of the reaction are nitric oxide, water, and nitrogen, so that the reaction can be described by the equation 

In this equation s is the fraction of the whole amount of ammonia reacted that is converted into nitric oxide i.e., the selectivity. If the process is carried out properly, selectivity is near to unity. 

In the production of nitric acid the reaction is accomplished with mixtures of ammonia and air or sometimes with mixtures of ammonia, air, and oxygen, containing about 10 vol % NH3 at pressures of 1-10 atm (below 

Besides platinum, certain metal oxides and their mixtures may serve as catalysts of reaction (397). Cobaltous-cobaltic oxide, Co304, is the best among nonplatinum catalysts. The reaction on this catalyst is carried out at 700-750°C, selectivity is near 0.95. 

Until now we limited ourselves to the discussion of reaction rates in the kinetic region even though in the commercial realization of the reactions the effect of transfer processes could have been essential. This was justified since the allowance for transfer effects would mostly be of the character of corrections. 

Because of the challenges in obtaining high conversion for the methane oxidation reaction, the oxidation ofNHj was subsequently studied. For safety reasons, NHj was maintained as the limiting reagent. 

The feed composition was 8.8% NHj, 25.6% O2, 6.9% Ar, 14.3% Kr, and the balance He. This microreactor was placed in Reactor Board 1. Error bars represent the 95% confidence interval of each value. 

The selechvity data shown in Fig. 12.22 indicate that significant amoimts of NO are produced by the microreactor during the oxidation of NHj. The data indicate that selectivity improves dramatically, from almost zero at 250 C to approximately 30% at 500 C. 

Similarly, small amounts of N2O are detected at approximately 400 C, and this product was observed during most of the ammonia oxidation trials. Srinivasan et al. (1997) and Srinivasan (1998) did not report the presence of N2O during studies on the oxidation of NHj in a prototype MIT microreactor design. This may have been due to an overlap with CO2 since these species have the same molecular weights. 

Overall, the AIMS functioned comparably to the conventional catalyst test stations used at DuPont. Individual reactors could be started or shut-down independently of the other reactors in the system. In addition, the safety systems responded appropriately during testing and prevented the occurrence of any safety incidents during this project. 

At high pressures the heat evolved is sufficient for adiabatic operation of reactors. Since the normal operating temperatures of industrial converters lie between 800 and 1100°C and pressures between 1 and 10 atm, homogeneous gas-phase reactions may contribute to the observed product distribution. The initiation temperatures for homogeneous oxidation are of course a function of pressure, so that in the laboratory the catalytic reaction may be isolated by working at low pressures under molecular beam conditions. In this way the reaction kinetics can be observed over very wide temperature ranges (usually up to 1200 °C). Studies of ammonia oxidation carried out in this way range from 0.1 to 10 Torr total pressure.  

Ammonia Oxidation.—In normal industrial practice reactors operate within the range 1 atm/800 °C to 10 atm/950 °C with about 10% NHs/air mixtures. Using 10% Rh/Pt gauzes yields of NO are about 95% with 100% conversion of ammonia. In general, ammonia oxidation has been observed over the temperature range 200—1400 °C. At low temperatures N2 and N2O are the main products but are superseded between 400 and 1200 °C by NO and N2 above 1200 °C N2 again becomes the major product 

Working at 10 Torr and an NH3/O2 ratio of either 1/3 or 2/1 Fogel et al. ° detected the secondary ions O, 02 , H20, C02, N +, NO, N2, and NH3 + over the whole of the temperature range 20—1200 °C. No peaks could be detected for any of the hypothetical intermediate species HNO, NH2OH, HNO2, or N2O. They also examined co-adsorption of ammonia and oxygen and showed 

Finally he showed the reaction to be inhibited by H2S and that this coincided with a substantial change in the secondary ion spectra due to strong H2S adsorption (species detected were HS , H2S, SO, S02, and PtS ). 

Otto et al. at the Ford Motor Company have published two papers ° on the reaction of NO with NH3 over supported platinum at 200—250 °C in a recirculating reactor. Working at total pressures of about lOOTorr they were able to show that the overall stoicheiometry was virtually independent of NH3/NO ratio and temperature. The approximate product distribution was given by 

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Dual-Pressure Process. Dual-pressure processes have a medium pressure (ca 0.3—0.6 MPa) front end for ammonia oxidation and a high pressure (1.1—1.5 MPa) tail end for absorption. Some older plants still use atmospheric pressure for ammonia conversion. Compared to high monopressure plants, the lower oxidation pressure improves ammonia yield and catalyst performance. Platinum losses are significantiy lower and production mns are extended by a longer catalyst life. Reduced pressure also results in weaker nitric acid condensate from the cooler condenser, which helps to improve absorber performance. Due to the spHt in operating conditions, the dual-pressure process requires a specialized stainless steel NO compressor. 

Above pH 9, decomposition of ozone to the reactive intermediate, HO, determines the kinetics of ammonia oxidation. Catalysts, such as WO, Pt, Pd, Ir, and Rh, promote the oxidation of dilute aqueous solutions of ammonia at 25°C, only two of the three oxygen atoms of ozone can react, whereas at 75°C, all three atoms react (42). The oxidation of ammonia by ozone depends not only on the pH of the system but also on the presence of other oxidizable species (39,43,44). Because the ozonation rate of organic materials in wastewater is much faster than that of ammonia, oxidation of ammonia does not occur in the presence of ozone-reactive organics. 

The main areas of commercial apphcation are automotive emission control catalysts (autocatalysts), oil refining, ammonia oxidation, hquid-phase ... 

The reaction is exothermic reaction rates decrease with increased carbon number of the oxide (ethylene oxide > propylene oxide > butylene oxide). The ammonia—oxide ratio determines the product spht among the mono-, di-, and trialkanolamines. A high ammonia to oxide ratio favors monoproduction a low ammonia to oxide ratio favors trialkanolamine production. Mono- and dialkanolamines can also be recycled to the reactor to increase di-or trialkanolamine production. Mono- and dialkanolamines can also be converted to trialkanolamines by reaction of the mono- and di- with oxide in batch reactors. In all cases, the reaction is mn with excess ammonia to prevent unreacted oxide from leaving the reactor. 

Dutch State Mines (Stamicarbon). Vapor-phase, catalytic hydrogenation of phenol to cyclohexanone over palladium on alumina, Hcensed by Stamicarbon, the engineering subsidiary of DSM, gives a 95% yield at high conversion plus an additional 3% by dehydrogenation of coproduct cyclohexanol over a copper catalyst. Cyclohexane oxidation, an alternative route to cyclohexanone, is used in the United States and in Asia by DSM. A cyclohexane vapor-cloud explosion occurred in 1975 at a co-owned DSM plant in Flixborough, UK (12) the plant was rebuilt but later closed. In addition to the conventional Raschig process for hydroxylamine, DSM has developed a hydroxylamine phosphate—oxime (HPO) process for cyclohexanone oxime no by-product ammonium sulfate is produced. Catalytic ammonia oxidation is followed by absorption of NO in a buffered aqueous phosphoric acid... 

Toray. The photonitrosation of cyclohexane or PNC process results in the direct conversion of cyclohexane to cyclohexanone oxime hydrochloride by reaction with nitrosyl chloride in the presence of uv light (15) (see Photochemical technology). Beckmann rearrangement of the cyclohexanone oxime hydrochloride in oleum results in the evolution of HCl, which is recycled to form NOCl by reaction with nitrosylsulfuric acid. The latter is produced by conventional absorption of NO from ammonia oxidation in oleum. Neutralization of the rearrangement mass with ammonia yields 1.7 kg ammonium sulfate per kilogram of caprolactam. Purification is by vacuum distillation. The novel chemistry is as follows ... 

Rates and selectivities of soHd catalyzed reactions can also be influenced by mass transport resistance in the external fluid phase. Most reactions are not influenced by external-phase transport, but the rates of some very fast reactions, eg, ammonia oxidation, are deterrnined solely by the resistance to this transport. As the resistance to mass transport within the catalyst pores is larger than that in the external fluid phase, the effectiveness factor of a porous catalyst is expected to be less than unity whenever the external-phase mass transport resistance is significant, A practical catalyst that is used under such circumstances is the ammonia oxidation catalyst. It is a nonporous metal and consists of layers of wire woven into a mesh. 

The catalyst temperature is about 1100°C. Precious metal catalysts (90% Pt/10% Rh in gauze form) are normally used in the commercial processes. The converters are similar to the ammonia oxidation converters used in the production of nitric acid (qv) although the latter operate at somewhat lower temperatures. The feed gases to the converter are thoroughly premixed. The optimum operating mixture of feed gas is above the upper flammabiUty limit and caution must be exercised to keep the mixture from entering the explosive range. 

At low temperatures the SCR reactions dominate and nitrogen oxide conversion increases with increasing temperature. But as temperature increases, the ammonia oxidation reactions become relatively more important. As the temperature increases further, the destmction of ammonia and generation of nitrogen oxides by the oxidation reactions causes overall nitrogen oxide conversion to reach a plateau then decreases with increasing temperatures. Examples are shown in Figure 7 (44). 

The most popular SCR catalyst formulations are those that were developed in Japan in the late 1970s comprised of base metal oxides such as vanadium pentoxide [1314-62-1J, V20, supported on titanium dioxide [13463-67-7] Ti02 (1). As for low temperature catalysts, NO conversion rises with increasing temperatures to a plateau and then falls as ammonia oxidation begins to dominate the SCR reaction. However, peak conversion occurs in the temperature range between 300 and 450°C, and the fah-off in NO conversion is more gradual than for low temperature catalysis (44). 

Tests on pig gut contents using molecular probes to detect the presence of (aerobic) ammonia oxidizers proved negative. Recently, the anaerobic oxidation of ammonia coupled to nitrate reduction has been demonstrated in... 

The first observation of sensitivity-stability was reported by Liljenroth (1918) in connection with the autothermal operation of ammonia oxidation reactors. Papers of Damkdhler (1937) and Wagner (1945) went unnoticed. At Union Carbide Corp. Perkins (1938) used zero order kinetics to define a safe range for ethylene oxidation in an unpublished report. His result,... 

Energy recovery has been standard practice from the early days of ammonia oxidation plants with escalating energy costs energy recovery is becoming increasingly important. 

Absorption of pollutant gases is accomplished by using a selective liquid in a wet scrubber, packed tower, or bubble tower. Pollutant gases commonly controlled by absorption include sulfur dioxide, hydrogen sulfide, hydrogen chloride, chlorine, ammonia, oxides of nitrogen, and low-boiling hydrocarbons. 

Ammonia oxidation Test drawn during manufacturing process to evaluate the ammonia oxidation rate for the nitrifiers. 

F.G. Liljenroth, ChemMetEng 19, 287—393 (1918) (Starting and stability phenomenon of ammonia oxidation and similar reactions)... 

Eng 20, 470-477 (1919) (Description of ammonia oxidation process beginning with Kuhl-mann s method of 1839 and ending with the cyanamide process at Muscle Shoals) 7) C.L. Parsons, 1EC 11,541 (1919) (Oxidation of ammonia to nitric acid as well as the prepn of nitric acid from Chile saltpeter) 8) F.C. Zeis-berg, ChemMetEng 24, 443-45 (1921) (Manuf of nitric acid from Chilean saltpeter brief description) 9) G.B. Taylor, IEC 26,1217-19 (1922) (Some economic aspects of ammonia oxidation) 10) Ministry of Munitions, Manufacture of Nitric Acid from Nitre and Sulfuric Acid , London (1922) (Book No 7 of Technical Records of Explosives Supply, 1915—1919)... 

IndChem 23, 17—24 (1947) (Ammonia oxidation process and concentration of nitric acid) 37) O.A. Hougen K.M. Watson, Chemical Process Principles , J. Wiley, NY, Combined volume (1947), 224 (Heat capacities of nitric acid) 38) W.M. Latimer J.H. Hildebrand, Reference Book of Inorganic Chemistry , Macmillan, NY (1947), 202-207 39) S. 

Ammonia Oxidation Kinetics in a High Temperature Flow Reactor , Univ California, Berkeley UCB-TS-71-6, AFOSR (1971)... 

C. Sigal, and C.G. Vayenas, Ammonia Oxidation to Nitric Oxide in a Solid Electrolyte Fuel Cell, Solid State Ionics 5, 567-570 (1981). 

It has been reported that titanium supported vanadium catalyst is active for ammonia oxidation at temperatures above 523 K [2,3]. Also, supported vanadium oxides are known to be efficient catalyst for the catalytic reduction of nitrogen oxides (NO ) in the presence of ammonia [4]. This work investigates the nanostructured vanadia/Ti02 for low temperature catalytic remediation of ammonia in air. 

Ammonia oxidation over Cu-based metal oxides under microwave irradiation... 

Closed symbols in Fig. 1 show the effect of reaction temperature on ammonia oxidation over CuO by heating with a conventional electric furnace. The reaction started at about 400 K and the conversion of NH3 became 1 at temperatures higher than 500 K. Fig. 1 also indicates that selectivity to N2 was high at low temperatures but it decreased as the temperature increased. Both N2O and NO increased instead of N2, except at 623 K, at which N2O decreases. NO was detected above 583 K, and it monotonously increased by the temperature. High reaction temperature seems to tend deeper oxidation to NOx. Considering that oxidation of N2 to N2O and NO is difficult in the tested temperature range. 

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