Micro adiabatic temperature rise

In order to show how specific guidelines for the reactor layout can be derived, the maximum allowable micro-channel radius giving a temperature rise of less than 10 K was computed for different values of the adiabatic temperature rise and different reaction times. For this purpose, properties of nitrogen at 300 °C and 1 atm and a Nusselt number of 3.66 were assumed. The Nusselt number is a dimensionless heat transfer coefficient, defined as... 

In order to exemplify the potential of micro-channel reactors for thermal control, consider the oxidation of citraconic anhydride, which, for a specific catalyst material, has a pseudo-homogeneous reaction rate of 1.62 s at a temperature of 300 °C, corresponding to a reaction time-scale of 0.61 s. In a micro channel of 300 pm diameter filled with a mixture composed of N2/02/anhydride (79.9 20 0.1), the characteristic time-scale for heat exchange is 1.4 lO" s. In spite of an adiabatic temperature rise of 60 K related to such a reaction, the temperature increases by less than 0.5 K in the micro channel. Examples such as this show that micro reactors allow one to define temperature conditions very precisely due to fast removal and, in the case of endothermic reactions, addition of heat. On the one hand, this results in an increase in process safety, as discussed above. On the other hand, it allows a better definition of reaction conditions than with macroscopic equipment, thus allowing for a higher selectivity in chemical processes. 

Substantial heat-transfer intensification was also described for a special micro heat exchanger reactor [104]. By appropriate distribution of the gas-coolant stream, the axial temperature gradient can be decreased considerably, even under conditions corresponding to very large adiabatic temperature rises, e.g. of about 1400 °C. 

Xu et al. [124] numerically computed the adiabatic temperature rise in a micro channel due to viscous heating and expressed their results by a correlation based on dimensionless groups. They introduced a dimensionless temperature rise AT = AT/Tjgf with a reference temperature of 1 K. The correlation they found is given by... 

GP 1] [R 1] Numerical simulations prove that isothermal processing is possible in micro reactors even under severe reaction conditions which correspond to an adiabatic temperature rise up to 1400 °C [98]. 

As far as this section is concerned, thermal aspects of the FRRPP process are of most interest, based on the manner at which it was able to store the exotherm locally and apparently able to release it to temperatures much higher than the adiabatic temperature rise in a coherent fashion (Cases 1-2) or catastrophically (Case 3) in micro-, meso-, and macroscales. The following case studies detail such behavior. 

The, for m = 5-20, is an approximation of the Arrhenius temperature dependence. The adiabatic temperature rise is expressed in terms of conversion from the mass and heat balances.) Fig. 12.6.1.A-3 shows how the conversion decreases with imperfect micro-mixing (for A = 15, m = 5). It was also found that the variance divided by the conversion could range from about 20% to zero for very high mixing intensity (j0->-cc). Fig. 12.6.1.A-4 shows that the stability criteria could also be affected by the micro-mixing effects. The latter is even more true for highly exothermic reactions, if m = 15. ... 

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