Transfer dominated regime

Figure 8. Theoretical regimes for rate dependence on intensity. Rate varies with a power law (intensity ), where x=1.0, x=0.5 and x=0.0 for surface reaction dominant, electron-hole recombination dominant and mass transfer dominant regimes, respectively. Adapted from ref.[13].

Chain transfer always leads to a decrease of P. However, the influence on Pp depends on the relative magnitude of the reinitiation rate constant k compared with kp and on the ratio [M ]/[T ] which is determined by Ct. Ct also determines the relative rate of consumption of the transfer agent as compared to the monomer. Only in the case Ct = 1 the ratio [T]/(M] does not change with conversion and therefore Pn does not change with conversion in a transfer dominated regime (Table 2.2). 

In the case of transfer reactions to monomer. Equation 2.31 simplifies to Equation 2.31a and the average degree of polymerisation is independent of conversion in a transfer dominated regime ... 

When external gradients correspond to substantial differences in concentration or temperature between the bulk of the fluid and the external surface of the catalyst particle, the rate of reaction at the surface is significantly different from that which would prevail if the concentration or temperature at the surface were equal to that in the bulk of the fluid. The catalytic reaction is then said to be influenced by external mass or heat transfer, respectively, and, when this influence is the dominant one, the rate corresponds to a regime of external mass or heat transfer. 

The performance of a fluidized bed combustor is strongly influenced by the fluid mechanics and heat transfer in the bed, consideration of which must be part of any attempt to realistically model bed performance. The fluid mechanics and heat transfer in an AFBC must, however, be distinguished from those in fluidized catalytic reactors such as fluidized catalytic crackers (FCCs) because the particle size in an AFBC, typically about 1 mm in diameter, is more than an order of magnitude larger than that utilized in FCC s, typically about 50 ym. The consequences of this difference in particle size is illustrated in Table 1. Particle Reynolds number in an FCC is much smaller than unity so that viscous forces dominate whereas for an AFBC the particle Reynolds number is of order unity and the effect of inertial forces become noticeable. Minimum velocity of fluidization (u ) in an FCC is so low that the bubble-rise velocity exceeds the gas velocity in the dense phase (umf/cmf) over a bed s depth the FCC s operate in the so-called fast bubble regime to be elaborated on later. By contrast- the bubble-rise velocity in an AFBC may be slower or faster than the gas-phase velocity in the emulsion... 

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