Hydrodynamic regimes three phase

In the design of optimal catalytic gas-Hquid reactors, hydrodynamics deserves special attention. Different flow regimes have been observed in co- and countercurrent operation. Segmented flow (often referred to as Taylor flow) with the gas bubbles having a diameter close to the tube diameter appeared to be the most advantageous as far as mass transfer and residence time distribution (RTD) is concerned. Many reviews on three-phase monolithic processes have been pubhshed [37-40]. 

To understand and ultimately to forecast the performance of a reactor, it is essential to study the coupling of "true" (intrinsic) kinetics with mass and energy transport, and to determine the flow regimes of the three phases (hydrodynamics). Modelling... 

A number of flow regime maps are available for packed bubble columns [see, e.g., Fukushima and Lusaka, J. Chem. Eng. Japan, 12 296 (1979)]. Correlations for the various hydrodynamic parameters can be found in Shah (Gas-Liquid-Solid Reactor Design, McGraw-Hill, 1979), Ramachandran and Chaudhari (Three-Phase Catalytic Reactors, Gordon and Breach, 1983), and Shah and Sharma [Gas-Liquid-Solid Reactors in Carberry and Varma (eds.), Chemical Reaction and Reactor Engineering, Marcel Dekker, 1987]. 

Kinetic regime. The first mechanism corresponds to the so-called kinetic regime (Popel 1994, Grigorenko et al. 1998) which is similar to the adsorption/desorption model (Blake 1993). Contrary to macroscopic hydrodynamic models, the adsorption/desorption model is based on the hypothesis that the motion of the triple line is ultimately determined by the statistical kinetics of atomic or molecular events occurring within the three-phase zone (Samsonov and Muravyev 1998). Such processes may be limiting at the very early stages of spreading of low... 

The modeling and design of a three-phase reactor requires the knowledge of several hydrodynamic (e.g., flow regime, pressure drop, holdups of various phases, etc.) and transport (e.g., degree of backmixing in each phase, gas-liquid, liquid-solid mass transfer, fluid-reactor wall heat transfer, etc.) parameters. During the past decade, extensive research efforts have been made in order to improve our know-how in these areas. Chapters 6 to 8 present a unified review of the reported studies on these aspects for a variety of fixed bed columns (i.e., co-current downflow, co-current upflow, and counter-current flow). Chapter 9 presents a similar survey for three-phase fluidized columns. 

Three-phase fluidized beds and slurry reactors (see Figs. 30g-l) in which the solid catalyst is suspended in the liquid usually operate under conditions of homogeneous bubbly flow or chum turbulent flow (see regime map in Fig. 33). The presence of solids alters the bubble hydrodynamics to a significant extent. In recent years there has been considerable research effort on the study of the hydrodynamics of such systems (see, e.g., Fan, 1989). However, the scale-up aspects of such reactors are still a mater of some uncertainty, especially for systems with high solids concentration and operations at increased pressures it is for this reason that the Shell Middle Distillate Synthesis process adopts the multi-tubular trickle bed reactor concept (cf. Fig. 30e). The even distribution of liquid to thousands of tubes packed with catalyst, however poses problems of a different engineering nature. 

FIGURE 7A.9 Hydrodynamic regimes in three-phase (gas-Iiquid-soHd) stirred tank reactors downflow pitched turbine. A, no dispersion of gas solid settled on bottom B, gas dispersed beginning of solid suspension C, gas dispersed off-bottom suspension of solids D, recirculation of mixture and possible surface aeration. (Reproduced from Rewatkaret al. 1991 with permission from American Chemical Society. 1991, American Chemical Society.)... 

The presence of pulp, even at very low pulp consistencies (0.1%) in the column leads to enhanced bubble coalescence and hence a narrowing of the gas velocity for the dispersed bubble regime as the pulp consistency increases (Reese et ah, 1996). Bubble coalescence inhibitors such as inorganic salts (e.g., sodium chloride and sodium phosphate dibasic) and organic compounds (e.g., ethanol, n-pentanol, iso-amyl alcohol, and benzoic acid) can be effectively applied to the liquid at concentrations up to 200 ppm to inhibit bubble coalescence behavior in three-phase fluidization (Briens et al., 1999). With the addition of the bubble coalescence inhibitor, the bed hydrodynamics at low gas velocities are significantly different from the case without the... 

Bubble dynamics and characteristics discussed above determine the hydrodynamic and heat and mass transfer behaviors in three-phase fluidization systems, which is important for better design and operation of three-phase fluidized beds. In this section, various hydrodynamic variables and transfer properties in three-phase systems are discussed. Specifically, areas discussed in the hydrodynamics section are minimum fluidization, bed contraction and moving packed bed phenomenon, flow regime transition, overall gas holdup and hydro-dynamic similarity, and bubble size distribution and the dominant role of larger bubbles. Later in this section, important topics covering transport phenomena will be discussed, which include heat and mass transfer and phase mixing. 

A clear and operational boundary should be established between slurry reactors and three phase fluid beds. Particle diameter alone (e.g. d < 200 pm for slurry reactors) is not sufficient and boundaries snould be based on hydrodynamic regime, settling velocities and turbulence parameters. 

Sanchez et al. (2008) tested the scale-up method for three-phase fluidized bed hydrodynamics proposed by Safoniuk et al. (1999), based upon the principles of dynamic similitude. The authors matched several systems operated at pressures in the range of 0.79-15.6MPa (prototype) with model systems operated at atmospheric pressure. Experiments were carried out to test this technique by comparing global phase holdups, and flow regime in systems where the five dimensionless groups were matched operated at significantly different pressures. 

As in Section II,A, a set of steady-state mass and energy balances are formulated so that the parameters that must be evaluated can be identified. The annular flow patterns are included in Regime II, and the general equations formulated in Section II,A,2,a, require a detailed knowledge of the hydrodynamics of both continuous phases and droplet interactions. Three simplified cases were formulated, and the discussion in this section is based on Case I. The steady-state mass balances are... 

If we consider the set of six equations on mass and momentum conservation on the one hand, and the characteristic energy extrema for stability of the three broad regimes of operation on the other, mathematical modeling for local hydrodynamics of particle-fluid two-phase flow beyond minimum fluidization needs therefore to satisfy the following constraints ... 

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