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Show 1.0 INTRODUCTION The residence time of a gas or solid particle is the time during which a particle resides within the boundaries of a given reactor. The mean gas residence time in a statistical sense is useful in reactor design, because it is the primary factor which influences the panicle residence times. Total conversion and hence product yields depend directly on panicle residence times. The mean gas residence time is defined as the first moment of the residence time distribution function. f(t), (1) For a plug flow, i.e. uniform velocity distribution with no mixing, it can be shown (Celik and Chattree, 1988) that f(t) is a dirac-delta function and Eq. 1 is equivalent to (2) provided that the gas density does not change significantly with the axial distance, x, along the reactor. In Eq. (2), L is the reactor length, U is the axial gas velocity, ¥is the volume of the reactor, and Qv is the gas-volume flow rate. Although Eq.(2) does not account for the influence of turbulent mixing or for the presence of recirculation zones, it is commonly used in reactor design and analysis, primarily because it is simple and easy to calculate. A detailed review of residence time distributions can be found in Shinnar (1987). Since the particle residence time itself is difficult to measure, the gas residence time is usually used as a first approximation in practical applications. However, this may not be a good approximation for some cases. The main factors that affect the particle residence time are: the gas flow characterized by the gas residence time and turbulent mixing, gravity, particle size, and the particle size change as a result of chemical reactions. These factors can be accounted for by using a sophisticated two-phase flow model such as that used by Smith et al. (1985). However, the level of effort and computational time is still too high for this approach to be used in practical applications. -2- |