Trickle flow regimes

Figure 3.45 Trickle-flow regime with liquid rivulets (complete wetting of the outer surface of the particle).

Here, issues in relation to the trickle flow regime—isothermal operation and plug flow for the gas phase—will be dealt with. Also, it is assumed that the flowing liquid completely covers the outer surface particles (/w = 1 or aLS = au) so that the reaction can take place solely by the mass transfer of the reactant through the liquid-particle interface. Generally, the assumption of isothermal conditions and complete liquid coverage in trickle-bed processes is fully justified with the exception of very low liquid rates. Capillary forces normally draw the liquid into the pores of the particles. Therefore, the use of liquid-phase diffusivities is adequate in the evaluation of intraparticle mass transfer effects (effectiveness factors) (Smith, 1981). 

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 trickle flow regime and in aqueous solutions, the Goto and Smith equation could be used (Smith, 1981 Fogler, 1999 Singh et al, 2004) ... 

Evaluate the conversion of sulfur dioxide in the trickle flow regime by proposing an appropriate gas and liquid flow rate. 

Model application in the trickle-flow regime In order to assure operation in the trickle-flow regime, the gas as well as the liquid flow rate has to be considerably lowered. At the same time, the conditions (a) and (d) are met in the reactor, while the rest of the conditions have to be checked. 

Operating conditions Following Figure 3.47 in order to operate in the trickle-flow regime, the gas and especially the liquid flow rate should be lowered. We choose a liquid... 

The transition between the trickle-flow regime and the pulse-flow regime is plotted in the Figure 5.2—4 as a function of the superficial gas velocity, uc, the liquid velocity, uL, and the total reactor pressure, P, for the water-nitrogen system [17]. This figure shows clearly that the transition depends strongly on the pressure in the reactor. 

Here k is the permeability of the dry medium and J(S[) characterizes hysteresis behaviour in the trickling regime. It should be noted at this level that Eqn. (5.2-12) was derived by these authors from available data on two-phase imbibition and drainage curves, implicitly identified to the trickling flow regime in trickle-beds. The J function may be multi-valued and depends on the history of the flow, however Grosser et al. [22] as well as Dankworth et al. [23] assume it to be single-valued. 

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. 

In the same way, Larachi et al. [48] evaluated with an important trickle-flow-regime database (4,000 experiments) different phenomenological models for liquid holdup and two-phase pressure drop in TBR. Table 5.2-5 summarizes the respective scatters (mean relative error and deviation) between the experimental values of pressure drop, AP/Z, and liquid holdup, fit, and their predictions by the different models. 

Figures 5.2-26 and 5.2-27 show, respectively, the influence of the total reactor pressure and of the superficial gas velocity on the dynamic liquid hold-up with water-nitrogen and aqueous 40 % ethyleneglycol-nitrogen. Similar trends are observed for the two systems. In the trickle-flow regime, the total operating pressure has no influence on the dynamic liquid hold-up at low liquid flow-rates and at low gas velocity (lower than few mm/s). Note that the influence is however, more noticeable for the viscous system.

Then, in contrast to the operation at atmospheric pressure a small increase in the superficial gas velocity reduces considerably the dynamic liquid hold-up. This effect is more pronounced at higher liquid flow-rate values and total reactor pressures. Because of this high influence of the gas flow on the hydrodynamics, Wammes et al. [34] recommend avoidance of the use of the term, "low interaction regime", for the trickle-flow regime at high pressure. 

Only the trickle-flow regime was investigated, where the mass-transfer resistances are probably the highest compared to other flow regimes. The results are reported mainly qualitatively [32], and only one correlation was given by Larachi et al. [55], In this, the reduced interfacial area a/ac grows by increasing both the gas-and liquid superficial velocity. 

The existing data for dynamic saturation (dynamic holdup divided by bed porosity) in the trickle-flow regime (18, 20, 21, 24, 25) can be correlated by the following equation ... 

Dynamic tracer tests can be used to determine dynamic holdup and catalyst contacting which in trickle-flow regime can be correlated with Reynolds and Gallileo number. A simple reactor model for gas limiting reactant when matched to experimental results for one solvent and one catalyst activity predicts reactor performance well for different catalyst activities and in other solvents over a wide range of liquid velocities. 

Catalytic fixed bed reactors with concurrent downflow of gas and liquid reactants are widely spread in the refining as well as in the chemical industry U) They are generally operated in the trickling flow regime i.e. with the gas as the continuous phase and with the liquid trickling on the fixed bed of catalyst. It is well known that, in such tricklp bed reactors, the performances strongly depend on ... 

A more detailed explanation about the use of tracer methods to evaluate contacting efficiency, the relationships needed to interpret tracer response data, the experimental methodology, and various results are given by Mills and Dudukovic (41). It suffices to say here that the following correlations based upon Reynolds and Galileo numbers were determined by El-Hisnawi to represent the available data on small porous packings in the trickle-flow regime ... 

For hydrocarbon liquids, Midoux et al.S6 recommend the use of Eq. (6-41) for all flow regimes for nonfoaming liquids. For foaming liquids, they recommend the use of Eq. (6-41) for the trickle-flow regime and the use of Eq. (6-42) for all other flow regimes.12... 

For air-water flows through beds of spheres, Sato et al. (S7) proposed two related expressions valid over nearly the same range of X and intended to include the trickle-flow regime ... 

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