Estimation of RTD in Tubular Reactors

For the design of tubular reactors an a priori estimation of the axial dispersion is indispensable. The dispersion in tubular reactors depends on the flow regime, characterized by the Reynolds number. Re, and the physical properties of the fluid, characterized by the Schmidt number. Sc. In addition, the presence of internal packings influences the flow behavior and, in consequence, the axial dispersion of the fluid. 

In the literature, a large number of experimental data are available correlating the axial Peclet number (Pe) with the Re and Sc. The axial Pe has as characteristic parameter the diameter in tubular reactors, or the particle diameter dp in packed bed reactors. 

The relation between the Bo characterizing the dispersion in the chemical reactor and the Pe becomes  

In general, axial dispersion decreases with increasing values for Re and Re Sc. An exception is the behavior of empty tubes under laminar flow conditions. For laminar flow a parabolic velocity profile develops. Under these conditions, molecular diffusion in axial and radial directions plays an important role in RTD. The diffusion in the radial direction tends to diminish the spreading effect of the parabohc velocity profile, while in the axial direction the molecular diffusion increases the dispersion. As a result the axial dispersion passes through a minimum (Pe passes through a maximum) as function oiRe- Sc = u- d /D at Re - Sc= (see Equation 3.59). 

Empty tube, laminar flow Empty tube, turbulent flow  

---