Axial Dispersion and Residence Time Distribution

The dimensionless group D KuL) is called the vessel dispersion number, and the reciprocal value is called the Bodenstdn number Bo, which can be used in place of Dax as the parameter for axial dispersion and represents the ratio of convective flux to diffusive (dispersed) flux. 

The (axial) dispersion term D jiytl) = 1/Bo can be regarded as the product of 

The dimensionless group ud/Dsx is called the axial Peclet number for mass Pem.ax (named after Jean Claude Peclet, see box), and d is a characteristic length. For empty tubes, d is the tube diameter dt, and for packed beds the particle diameter dp is mostly used. 

Correlations for axial dispersion coefficients in empty pipes and in packed beds are given in the Sections 4.10.6.3 and 4.10.6.4, respectively, so we can also calculate Dax without the need of an experiment (or prove the results of measurements, respectively). 

The dispersion coefficient Dax (iti s ) and the dimensionless numbers Bo and Pem.ax represent the spreading process of a pulse of tracer. Thus a large value of (low Bo and low Pe i,ax) means rapid spreading of the tracer curve (mixed flow), and a low value of Dax means slow spreading. For D = 0 we have no spreading and, hence, plug flow. Note that for a fixed bed we can still use Eqs. (4.10.98) and (4.10.99), but then Bo is related to the interstitial velocity (ratio of superficial velocity Us in the empty reactor to the porosity s of the packed bed with e 0.4). 

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