Criteria for axial dispersion effects in adiabatic trickle bed hydroprocessing reactors

This paper first outlines an approximate solution to the governing equations for an adiabatic hydroprocessing trickle bed reactor operating in the presence of axial dispersion. The approximate solution agrees very well with the rigorous numerical solution for Peclet numbers greater than approximately three.Using the approximate solution, criteria for significant axial dispersion effect are obtained. These criteria indicate that at high conversions, an adiabatic operation produces a larger axial dispersion effect than the isothermal operation. At low conversions, opposite results are obtained.The derived criteria are used to evaluate the orders of magnitude of Peclet number required to avoid axial dispersion effect in pilot scale adiabatic reactors for (a) residual hydrodesulfurization (b) hydrocracking of gas oils and (c) denitrogenation of shale oils. The calculations indicate that the axial dispersion effect is of less importance in case (c) than in cases (a) and (b).Finally, the role of heat effects on the axial dispersion in a vapor phase fixed bed adiabatic reactor is evaluated.