Eliminating axial dispersion effects

Table 8.4 Minimum Bed Length to Eliminate Axial Dispersion Effects...

Eliminating axial dispersion effects To eliminate axial dispersion effects a reactor that is sufficiently long should be used. For this purpose, a length of at least 40-50 pellet diameters is recommended. 

The reduction in solution time for these simulations is minimal and in some cases the elimination of the axial diffusion terms actually increases the solution time. Simulations show that neglecting the axial dispersion of mass has little effect on numerical computation time, whereas eliminating axial dispersion of energy may significantly increase computation time and only rarely decreases it substantially. 

According to Mears criterion, the minimum bed-length required to eliminate the axial dispersion effect can be expressed as... 

Large-scale hydroprocessing trickle-bed reactors normally operate under adiabatic conditions therefore, heat effects caused by the reaction must also be included. Shah [61] showed that in this case the critical Bodenstein number for elimination of axial dispersion effects is a function of a heat parameter as well as a modified Damkohler number. For low Damkohler numbers smaller critical Bodenstein numbers than in isothermal reactors are sufficient to eliminate axial dispersion in adiabatic reactors, whereas the inverse is true for large Damkohler numbers. 

Further, selective Pd-catalyzed hydrogenation of 3-methyl-l- pentyn-3-ol to PI without over-hydrogenation to P2 was used to prove the advantageous characteristics of segmented flow in eliminating diffusional effects and axial dispersion. 

In the cases where one diffusion type is dominant, another diffusion effect could be eliminated. Gorke et al. [30] formulated a plug flow model with a very low axial dispersion (diffusion) for the production of hydrogen by ethanol reforming on... 

---