Ammonia oxidation, reactor fluidized

Some aspects of fluidized-bed reactor performance are examined using the Kunii-Levenspiel model of fluidized-bed reactor behavior. An ammonia-oxidation system is modeled, and the conversion predicted is shown to approximate that observed experimentally. The model is used to predict the changes in conversion with parameter variation under the limiting conditions of reaction control and transport control, and the ammonia-oxidation system is seen to be an example of reaction control. Finally, it is shown that significant differences in the averaging techniques occur for height to diameter ratios in the range of 2 to 20. 

Urea melt is fed into the reactor and is atomized by spray nozzles with the aid of high-pressure ammonia. The reactor is a fluidized bed gas reactor using silica/aluminium oxide as catalyst. The reaction offgas, an ammonia and carbon dioxide mixture, is preheated and is used as fluidizing gas. Conversion of urea to melamine is an endothermic reaction the necessary heat is supplied via heated molten salt circulated through internal heating coils. 

The major part of these catalytic processes is carried out in fixed bed reactors. Some of the main fixed bed catalytic processes are listed in Table 11.1-1. Except for the catalytic cracking of gas oil, which is carried out in a fluidized bed to enable the continuous regeneration of the catalyst, the main solid catalyzed processes of today s chemical and petroleum refining industry appear in Table 11.1-1. However, there are also fluidized bed alternatives for phthalic anhydride— and ethylene dichloride synthesis. Furthermore, Table 11.1-1 is limited to fixed bed processes with only one fluid phase trickle bed process (e.g., encountered in the hydrodesulfurization of heavier petroleum fractions) are not included in the present discussion. Finally, important processes like ammonia oxidation for nitric acid production or hydrogen cyanide synthesis, in which the catalyst is used in the form of a few layers of gauze are also omitted from Table 11.1-1. 

The cobalt oxide catalyst for oxidation of ammonia, worked out in our laboratory, has the form of granules of high mechanical strength, owing to which it may be applied both in stationary and in fluidized beds. The yields of ammonia oxidation to NO measured during laboratory and large laboratory studies of that catalyst exceeded 95%. Optimum temperature of ammonia oxidation process carried out on our catalyst (760-780 ) is lower than that needed for platinum-rhodium wire gauze currently appli in industrial reactors. 

In a typical process, approximately stoichiometric quantities of propylene, ammonia and oxygen (as air) are fed into a reactor containing a fluidized catalyst, such as a bismuth, molybdenum or uranium containing compound. The reaction is conducted at 400-500°C and 1-3 atmospheres. The exit gases are scrubbed with water and acrylonitrile is obtained from the aqueous solution by a series of distillations. In a variation of this process, propylene is treated with nitric oxide (which may be regarded as a product of ammonia oxidation) at about 700°C in the presence of a silver catalyst ... 

Another industrially important reaction of propylene, related to the one above, is its partial oxidation in the presence of ammonia, resulting in acrylonitrile, H2C=CHCN. This ammoxidation reaction is also catalyzed by mixed metal oxide catalysts, such as bismuth-molybdate or iron antimonate, to which a large number of promoters is added (Fig. 9.19). Being strongly exothermic, ammoxidation is carried out in a fluidized-bed reactor to enable sufficient heat transfer and temperature control (400-500 °C). 

Fluidized bed reactors were first employed on a large scale for the catalytic cracking of petroleum fractions, but in recent years they have been employed for an increasingly large variety of reactions, both catalytic and non-catalytic. The catalytic reactions include the partial oxidation of naphthalene to phthalic anhydride and the formation of acrylonitrile from propylene, ammonia, and air. The noncatalytic applications include the roasting of ores and Tie fluorination of uranium oxide. 

In this process HCN is produced when methanol reacts with ammonia and oxygen in the presence of an oxide catalyst that contains iron, antimony, phosphorous and vanadium. The reaction occurs in the vapor phase in a fluidized bed reactor with an oxygen-to-methanol molar ratio in the gas phase that is less than 1.6. The process and the catalyst are described in patents that were issued to Nitto Chemical (now Mitsubishi Rayon) during the late 1990 s (European Patent 864,532 Japanese patents 10-167,721, 10-251,012, 11-043,323 US Patent 5,976,482). 

Ammoxidation refers to the catalytic oxidation of a feedstock with ammonia. When propylene is the feedstock, acrylonitrile is produced. Most of the world s acrylonitrile is based on the Sohio (now BP) process in which stoichiometric amounts of propylene and ammonia are reacted with a slight excess of air in a fluidized bed operated in the turbulent fluidization flow regime. The reactor temperature and pressure are 450° C and 1.5 bar, respectively. The reaction usually... 

Our experiments have shown that the pores of radius within 0.5-1.0 urn are of primary importance for the performance of the catalyst. The above presented results confirm the possibility of obtaining cobalt catalysts for oxidation of ammonia suitable for being applied in fluidized bed reactors. 

In recent years, several modified versions of the two-phase model were proposed for modeling fluidized bed reactors. They include a model proposed by Werther (1980) for catalytic oxidation of ammonia, in which the mass transfer process is expressed in terms of film theory, as described in Danckwerts (1970) a model proposed by Werther and Schoessler (1986) for catalytic reactions a model proposed by Borodulya et al. (1995) for the combustion of low-grade fuels a model proposed by Arnaldos et al. (1998) for vacuum drying and a model proposed by Srinivasan et al. (1998) for combustion of gases. The modifications include the consideration of axial mass transfer profile, the inclusion of a wake phase in addition to the bubble and emulsion phases, and the consideration of the growth of bubbles in the bubble phase. 

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