Temperature profile thermodynamic considerations

Selection of Optimum Reactor Temperature Profiles. . . Thermodynamic and Selectivity Considerations 375... 

SELECTION OF OPTIMUM REACTOR TEMPERATURE PROFILES. . . THERMODYNAMIC AND SELECTIVITY CONSIDERATIONS... 

This value is considerably less than that obtained for pure adiabatic operation (19.7 tons). The heat losses tend to partially remove thermodynamic constraints on the reaction rate and permit a closer approach to the optimum temperature profile corresponding to minimum catalyst requirements. 

Another proof of the importance of temperature is the fact that there is often a strict relationship between the run of isovols and the run of isotherms in deep profiles, both being influenced no doubt by the varying thermal conductivity of the different rocks. The strong influence of temperature on the rank of coal is obvious in the case of contact-metamorphic coals, whose rank increases distinctly when approaching the intrusive body. Apart from these geological observations, all experiments on artificial coalification have shown that temperature is the decisive factor in the coalification process. Thermodynamic and reaction kinetic considerations (9) also support this opinion. 

The complete description of a flame requires the specification of the pressure, the mass flow rate or burning velocity, the initial gas composition, and the appropriate transport coefficients and thermodynamic data. The remaining information is contained in a set of one-dimensional profiles of composition, temperature, and gas velocity as a function of distance (Fig. 2). Other independent variables than distance could have been used, e.g., temperature or time, but distance is common in experimental studies. Not all of these profiles are independent since there are a number of relations between the variables such as the equation of state, conservation of mass, etc. As an example, gas velocity can be obtained both by direct measurement and from temperature measurements using geometrical and continuity considerations. In the example given the indirect determinations of velocity are the more reliable and were used in the analysis. It is general practice to measure as many variables as convenient because the redundant profiles provide a check on the reliability of the measurements. 

If readsorption is important part of surface event, its influence on temperature-programmed desorption is that TPD profile broadens towards higher desorption temperatures the same stands for diffusion limitations. Therefore, the task to neglect or minimize the effects of these processes is imposed and it is relatively easy if TPD experiments are performed in UHV setups. The pumping speed should be sufficiently high to prevent readsorption of the desorbed species back onto the surface. However, in the case of flow systems there are many experimental conditions that have to be adjusted even though, the interpretation of data obtained from TPD experiment is not simple—in the estimation of kinetic parameters, those experimental conditions have to be taken into consideration. Therefore, the derivation of kinetic and thermodynamic parameters from the results obtained in UHV and in the flow system will be discussed separately. 

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