Holdup efficiency, trickle flow

Liquid holdup, which is expressed as the volume of liquid per unit volume of bed, affects the pressure drop, the catalyst wetting efficiency, and the transition from trickle flow to pulsing flow. It can also have a major effect on the reaction rate and selectivity, as will be explained later. The total holdup, h, consists of static holdup, h, liquid that remains in the bed after flow is stopped, and dynamic holdup, h, which is liquid flowing in thin films over part of the surface. The static holdup includes liquid in the pores of the catalyst and stagnant packets of liquid held in crevices between adjacent particles. With most catalysts, the pores are full of liquid because of capillary action, and the internal holdup is the particle porosity times the volume fraction particles in the bed. Thus the internal holdup is typically (0.3 — 0.5)(0.6), or about 0.2-0.3. The external static holdup is about... 

The reactor is inserted in region A of the solenoid bore (Fig. 11.1). Experiments are first made to measure the liquid holdup, the pressure drop and the wetting efficiency in the absence of magnetic fields. A sufficient time is allowed for the system to reach steady state before measurements are acquired. Experimental data are compared with the predictions of the Holub et al. [20] modeL Eigures 11.3a and b show the experimental data versus Holub s model for the trickle-flow regime with the magnetic field off. The slit model restores the hydrodynamic behavior pretty well in terms of pressure-drop and liquid-holdup variations. 

RTD models for trickle-bed reactors are quite numerous. They are reviewed in part 2 in order to evidence the main fluid flow characteristics that have been considered by the authors developing these models. The fluid mechanics description is based on percolation concepts. The main implications of these concepts are analyzed in part 3 whereas part 4 is devoted to the development of a percolation model describing the liquid flow distribution in a trickle-bed reactor. This model is then applied to derive correlations for the wetting efficiency and the dynamic liquid holdup (part 5) and, finally, for the axial dispersion coefficient (part 6) a classical example... 

---