Wetting efficiency, trickle flow

Different packed bed reactor designs are illustrated in Figure 6.8. The flow properties are of utmost importance for packed beds used in three-phase reactions. The most common operation policy is to allow the liquid to flow downward in the reactor. The gas phase can flow upwards or downwards, in a concurrent or a countercurrent flow. This reactor is called a trickle bed reactor. The name is indicative of flow conditions in the reactor, as the liquid flows downward in a laminar flow wetting the catalyst particles efficiently (trickling flow). It is also possible to allowboth the gas and the liquid to flow upward in the reactor (Figure 6.8). In this case, no trickling flow can develop, and the reactor is called a packed bed or n fixed bed reactor. 

This fraction could be viewed as a fixed-bed wetting efficiency. If he t = s and hyt= 1, the bed is completely filled with fluid (fully wetted). The term is analogous to the catalyst wetting efficiency/w for trickle beds (Section 3.7.3). However, this equality is valid solely in fixed beds where a single fluid flows through. The active volume of the solid, which is occupied by the fluid, amounts to the fixed-bed volume occupied by the fluid minus the volume occupied by fluid ... 

The Reynolds number is based on superficial velocity. This equation is proposed for applications with organic liquids such as n-hcxane, light petroleum fractions, and similar species. In the trickle flow regime, the increase in the gas flow rate leads to a decrease in the wetting efficiency (Burghardt et al., 1995). 

In the first slit, the liquid wets the wall with a film of uniform thickness the gas being in the central core (wet slit). The second slit is visited exclusively by the gas (dry slit). The high-pressure-and high-temperature-wetting efficiency, liquid hold-up and pressure-drop data reported in the literature for TBR in the trickle-flow regime were successfully forecasted by the model. 

I. Iliuta and F. Larachi, The generalized slit model pressure gradient, liquid hold-up and wetting efficiency in gas-liquid trickle flow, Chem. Engng. Science, 54 (1999) 5039-5045. 

Since dissolved gas concentrations in the liquid phase are more difficult to measure experimentally than the liquid reactant concentration, Equation 8 evaluated at the reactor exit 5=1 represents the key equation for practical applications involving this model. Nevertheless, the resulting expression still contains a significant number of parameters, most of which cannot be calculated from first principles. This gives the model a complex form and makes it difficult to use with certainty for predictive purposes. Reaction rate parameters can be determined in a slurry and basket-type reactor with completely wetted catalyst particles of the same kind that are used in trickle flow operation so that DaQ, r 9 and A2 can be calculated for trickle-bed operation. This leaves four parameters (riCE> St gj, Biw, Bid) to be determined from the available contacting efficiency and mass transfer correlations. It was shown that the model in this form does not have good predictive ability (29,34). 

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... 

On the other hand, Colombo [4] defined the liquid/solid contacting efficiency in terms of the ratio of the apparent diffu-sivity (D.) of a tracer in a porous particle, determined in a trickle-bed iactor (by assuming total wetting of the packing) and the intraparticle diffusivity D., determined in a liquid full reactor. This ratio was found to increase as the liquid flow rate... 

A multiphase reactor design, very similar to the trickle-bed reactor, is the tubular multiphase hollow membrane wall reactor sketched in Eigure 13.2h. In a regular trickle-bed reactor, the liquid flows over a partially wetted pellet as a thin film and supplies the liquid phase reactant to the catalyst pores. This action, however, has the effect of hindering pore access to the gas, thus lowering the reaction rate. On the other hand, in the multiphase membrane reactor, the liquid-filled membrane is directly accessible to the gas flowing in the inside tube. Thus, mass transfer in this reactor is considerably more efficient than in the conventional trickle-bed reactor. 

---