Future Prospects

Future Prospects

391 FUTURE PROSPECTS The material gathered and summarized in this monograph demonstrates that the transition to a forced non-steady-state regime ver...

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391

FUTURE PROSPECTS

The material gathered and summarized in this monograph demonstrates that the transition to a forced non-steady-state regime very often allows for a considerable increase in efficiency of heterogeneous catalytic processes. This increase can be accounted for in two different ways. The first is the characteristics of the non-steady-state processes on the catalyst surface, the second is the dynamic properties of the reactor as a whole. The application of an additional (second) converter to ensure intensification of the processes is believed to be the most familiar technique. Let us briefly outline the major trends in the further development of scientific research and practical realization of heterogeneous catalysis under forced non-steady-state conditions. The first and most important task is to construct and investigate kinetic models of the non-steady-state processes on the surface of the catalyst. This task can be tackled on the basis of a thorough investigation of the catalyst state under conditions suggested by the chemical reactions. Updated methods for examination of the catalyst surface can provide the required information about the state of the catalyst. A kinetic model of the process represents the system of integro-differential and algebraic equations which reflect the dynamics of the catalyst cycle and the impact of the reaction medium on the catalyst in a quantitative manner. Then, with such a model one can predict the state of the catalyst under various conditions of variation of the gas phase composition and temperature. There is little doubt that experimental data alone indicate the optimum non-steady-state conditions for the process, because in practice the catalyst is placed in the reactor where the transfer processes significantly affect the conditions of the reaction as compared to a bench-scale simulation. One should not forget that the model of the non-steady-state process obtained must satisfactorily describe the catalyst

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behaviour quantitatively, for example, under varied conditions at the reactor inlet: (a) pressure, composition, temperature, load; (b) circulation of the catalyst in reactors with preudo-fluidized catalyst beds operating in the regime of pneumatic transport; (c) activation and deactivation of the catalyst surface; (d) both rapid and slow changes in the reaction mixture characteristics. The creation of such a model is expensive and requires specialists trained in physics, chemistry and mathematics. This effort seems to be rewarding because it is also in itself an exciting creative process. The second task crucial for successful development of efficient forced non-steady-state processes is the theoretical basis concerning the dynamic of heterogeneous catalytic reactors. The processes of mass, heat and impulse transfer have a greater influence under non-steady-state conditions that in the steady-state. Slight variations in the conditions of mass and/or heat transport in the granular catalyst bed can lead, for example, to noticeable changes in selectiVity or the extent of conversion. That is why a clear-cut understanding of all physical phenomena inside the reactor is reqUired for one to be able to perform the non-steady-state processes. True quantitative knowledge allows for the construction of essential simple mathematical models of the processes to be carried out in reactors of any productiVity rating. In addition, interpretation of all the main laws of mass and heat transfer in the reactors may permit one to create conditions which would favourable affect the performance characteristics of a catalytic process. It is thought that an empirical method of finding these conditions is more often bound to fail than not. Of importance is the necessity to carry out experimental and bench-scale research to investigate and quantitatively describe the behaviour of the solid particles of the catalyst bed in the reactor operating under conditions of pseudo-liquidization, pneumatic transport, circulation of the particles between the reactor and regenerator. This type of the reactor allows for easier organization of operational cop.rlitions at the catalyst non-steady state. As far as the contact apparatus with a fixed catalyst bed is concerned, the technique of intensified and random heat removal

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from the reaction zone is yet to be worked out in detail. The third task is connected with the development of a mathematical theory of the non-steady-state processes: qualitative and numerical analysis of the mathematical models of the process and also the formulation and design of the control optimization for the non-steady-state processes. The significance of qUalitative analysis of the mathematical models constructed (especially under conditions of periodic perturbations of the gas phase condition) is appreciated. Unfortunately, eVen today with the advent of high-speed and large-memory computers there is no effective method to calculate the optimum cyclic regime from systems of differential equations with partial derivatives. Therefore, problems of this kind are often solved numerically with the assistance of personal experience or intuition. The results of the estimation obtained via qualitative analysis are the first step in this process. The fourth task is the development of new, highly efficient non-steady-state methods and apparatus to perform concrete catalytic processes under forced non-steady-state conditions.