High combustion temperature reactants

In the case of strong forced gas flux (ir> 1), the so-called inverse wave structure of FC occurs (Fig. 31b). The unreacted solids in zone I are heated by a gas flux to a high combustion temperature T combustion front, limiting final conversion. Decreasing combustion temperature by strong gas flow alleviates this problem (Aldushin, 1993). 

When visualizing a combustion process, it is useful to think of it in terms of the three Ts time, temperature, and turbulence. Time for combushon to occur is necessary. A combustion process that is just initiated, and suddenly has its reactants discharged to a chilled environment, will not go to completion and will emit excessive pollutants. A high enough temperature must exist for the combustion reaction to be initiated. Combushon is an exothermic reachon (it gives off heat), but it also requires energy to be inihated. This is iUustrated in Fig. 6-5. 

C. S. Raymond, A. Bayliss, B. J. Matkowsky, and V. A. Volpert, Transitions to chaos in condensed phase combustion with reactant melting, Int l J. Self-Propagating High-Temperature Synthesis, 10 (2001), pp. 133-149. 

Heat Recovery and Feed Preheating. The objective is to bring the reactants to and from reaction temperature at the least utihty cost, and to recover maximum waste heat at maximum temperature. The impact of feed preheating merits a more careful look. In an exothermic reaction, preheated feed permits the reactor to act as a heat pump, ie, to buy low and sell high. The most common example is combustion-air preheating for a furnace. 

Adiabatic Reaction Temperature (T ). The concept of adiabatic or theoretical reaction temperature (T j) plays an important role in the design of chemical reactors, gas furnaces, and other process equipment to handle highly exothermic reactions such as combustion. T is defined as the final temperature attained by the reaction mixture at the completion of a chemical reaction carried out under adiabatic conditions in a closed system at constant pressure. Theoretically, this is the maximum temperature achieved by the products when stoichiometric quantities of reactants are completely converted into products in an adiabatic reactor. In general, T is a function of the initial temperature (T) of the reactants and their relative amounts as well as the presence of any nonreactive (inert) materials. T is also dependent on the extent of completion of the reaction. In actual experiments, it is very unlikely that the theoretical maximum values of T can be realized, but the calculated results do provide an idealized basis for comparison of the thermal effects resulting from exothermic reactions. Lower feed temperatures (T), presence of inerts and excess reactants, and incomplete conversion tend to reduce the value of T. The term theoretical or adiabatic flame temperature (T,, ) is preferred over T in dealing exclusively with the combustion of fuels. 

The fluidized-bed process for this reaction has several advantages over a fixed-bed process. First, the process is highly exothermic, and the selectivity to C3H3N is temperature dependent. The improved temperature control of the fluidized-bed operation enhances the selectivity to acrylonitrile, and substantially extends the life of the catalyst, which readily sinters at temperatures in excess of 800 K. Furthermore, since both the reactants and products are flammable in air, the use of a fluidized bed enables the moving particles to act to quench flames, preventing combustion and ensuring safe operation. 

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