Dispersed phase, solid-liquid fluidized

In solid-liquid fluidized beds the particle phase is the dispersed phase and the bed usually operates in the particulate (homogeneous) regime. However, for heavy particles (large size and density or high terminal setthng velocity), heterogeneity sets in. 

Ham et al. (1990) used Eqs. (30) and (31) and estimated the values of the solid phase dispersion coefficient using the experimental results on transition in solid-liquid fluidized beds. However, the estimated values of deviate from the experimental values of obtained by Dorgelo et al. (1985). It may be noted that the RTD based experimental values includes gross nonidealities in addition to the turbulent dispersion. 

This criterion involves the assumption that the gas phase stress terms are negligible. This assumption may not be valid in case of solid-liquid fluidized beds or liquid-liquid dispersions. In this case, the criterion is of the same form as Eq. (172), with different definitions of the parameters Ml, M2, and M3, which are given in Table VII. Table VII also gives the parameters of the criterion when the dispersion terms are not included in the continuity equations of both the phases. 

Countereurrent bubble flow with liquid-supported solids, whieh ean be affeeted by downward liquid fluidization of partieles having a density lower than that of the liquid, has been referred to as inverse three-phase fluidization. The mass transfer potential of sueh a eountercurrent operation is worthy of study, especially for cases in whieh dispersion of the gas rather than the liquid is ealled for and the required gas-liquid ratio and throughput ean be effected without flooding. In contrast, the eorresponding eoeurrent mode has reeeived more attention than all other eases and eonstitutes the majority of the literature on three-phase fluidization. 

In slurry reactors, an attempt is made to realize intensive and intimate contact between a gas-phase component, usually to be dissolved in the liquid phase, a liquid-phase component and a finely dispersed solid. In this respect, slurry reactors are related to packed-bed reactors with the various gas/liquid flow regimes that can be realized (such as trickle flow, pulsed flow and dispersed bubble flow). Also, there is much similarity with three-phase fluidized beds. 

In gas-liquid-solid (three-phase) fluidized beds, solid particles are simultaneously contacted with both gas and liquid. The gas and liquid may flow cocurrently upward, or the liquid may descend, while the gas rises. The liquid usually forms the continuous phase in which the solid particles and gas bubbles are dispersed. The bubbles are larger when the particles are smaller, and bed contraction can occur when gas is introduced into a liquid-fluidized bed of fine particles. Higher pressures lead to smaller bubbles and increased gas hold-ups. 

Advantages of three-phase fluidized beds over trickle beds and other fixed bed systems are temperature uniformity, high heat transfer, ability to add and remove catalyst particles continuously, and limited mass transfer resistances (both external to the particles and bubbles, because of turbulence and limited bubble size, and inside the particles owing to relatively small particle diameters). Disadvantages include substantial axial dispersion (of gas, liquid, and particles), causing substantial deviations from plug flow, and lack of predictability because of the complex hydrodynamics. There are two major applications of gas-liquid-solid-fluidized beds biochemical processes and hydrocarbon processing. 

It is reasonable to present the flow in disperse systems in a general way to avoid repetitions of this topic. Such disperse systems are gas or liquid fluidized beds, bubble or drop columns, and spray columns. In all cases the solid or fluid particles are suspended or moving due to the density difference A/ = p - p ) and the acceleration of gravity. In Fig. 3.5-1 a fixed bed on the left side and several fluidized beds with different flow patterns are depicted. The fluid flow density V , in the fluidized beds is greater than the minimum flow density V(,f necessary to achieve fluidization. The volumetric holdup of the continuous phase increases for Vj > Vjf with the fluid throughput. The relative velocity between the fluid and the suspended particles is inversely proportional to the volnmetric holdup Sg. With... 

While the lower order models described in Section 6.3 are useful for the quick prediction of the overall performance of a reactor, these models often rely on simplified flow approximations and often fail to account for change in the local fluid dynamics or transport processes during the presence of internal hardware or changes in flow regimes. Moreover, these models are also based on empirical knowledge (as discussed in Section 6.4) of several parameters such as interfacial area, dispersion coefficients, and mass transfer coefficients. Some of these limitations may be avoided by using CFD models for simulations of gas-liquid-solid flows in three-phase slurry and fluidized bed. 

A hierarchy of computational models is available to simulate dispersed gas-liquid-solid flows in three-phase slurry and fluidized bed reactors [84] continuum (Euler-Euler) method, discrete particle/bubble (Euler-Lagrange) method, or front tracking/capturing methods. While every method has its own... 

Kato Y, Morooka S, Koyama M, Kago T, Yang S. Longitudinal dispersion coefficient of liquid in three phase fluidized bed for gas-liquid-solid systems. J. Chem. Eng. Jpn. 1985 8(4) 313-317. 

Among engineers, population balance concepts are of importance to aeronautical, chemical, civil (environmental), mechanical, and materials engineers. Chemical engineers have put population balances to the most diverse use. Applications have covered a wide range of dispersed phase systems, such as solid-liquid dispersions (although with incidental emphasis on crystallization systems), and gas-liquid, gas-solid, and liquid-liquid dispersions. Analyses of separation equipment such as for liquid-liquid extraction, or solid-liquid leaching and reactor equipment, such as bioreactors (microbial processes) fluidized bed reactors (catalytic reactions), and dispersed phase reactors (transfer across interface and reaction) all involve population balances. 

Frequently encountered in nature and process industries, multiphase flows may comprise various states of matter, e.g., gas and solid in fluidization gas and liquid in bubble column and gas, liquid, and solid in airlift slurry bed (Mudde, 2005). In this article, the term phase in multiphase flow is related to the aggregative state of flow, which is normally far from equilibrium states. And it is different from the phase for a thermodynamic equilibrium system, where the phase is used to refer to a set of equilibrium states that can be demarcated in terms of state variables by a phase boundary on a phase diagram. As a result, it is possible to have a gas—soHd flow mixture with more than two phases, which can be classified by size, density of particles, or by the states of dispersion, e.g., poly disperse multiphase flow pCue and Fox, 2014) and dilute—dense, gas—soHd multiphase flow (Hong et al, 2012). 

The term three-phase fluidization requires some explanation, as it can be used to describe a variety of rather different operations. The three phases are gas, liquid and particulate solids, although other variations such as two immiscible liquids and particulate solids may exist in special applications. As in the case of a fixed-bed operation, both co-current and counter- current gas-liquid flow are permissible and, for each of these, both bubble flow, in which the liquid is the continuous phase and the gas dispersed, and trickle flow, in which the gas forms a continuous phase and the liquid is more or less dispersed, takes place. A well established device for countercurrent trickle flow, in which low-density solid spheres are fluidized by an upward current of gas and irrigated by a downward flow of liquid, is variously known as the turbulent bed, mobile bed and fluidized packing contactor, or the turbulent contact absorber when it is specifically used for gas absorption and/or dust removal. Still another variation is a three-phase spouted bed contactor. 

Reactors with moving solid phase Three-phase fluidized-bed (ebullated-bed) reactor Catalyst particles are fluidized by an upward liquid flow, whereas the gas phase rises in a dispersed bubble regime. A typical application of this reactor is the hydrogenation of residues. 

Most generally during elution, the liquid flow is reversed and the resin bed is therefore packed. In contrast to conventional complex initial feedstock treatments, fluidized-bed processes combine clarification, expressed product specific capture, and concentration into a single step. Residence time distribution analysis showed a small degree of axial dispersion and the generation of a few dozen theoretical plates that are enough for a good efficiency of the capture step. The efficiency of the separation is, however, dependent on the particle size of the solid phase material. 

Photocatalytic reactions are promoted by solid photocatalyst particles that usually constitute the discrete phase distributed within a continuous fluid phase in the reactor. Therefore, at least two phases, that is, liquid and solid, are present in the reactor. The solid phase could be dispersed (SPD) or stationary (SPS) within the reactor. SPD photoreactors may be operated with the catalyst particles and the fluid phase(s) agitated by mechanical or other means. Depending on the means of agitation, the photoreactor resembles that of slurry or fluidized bed reactors. In numerous investigations, an aqueous suspension of the catalyst particles in immersion or annular-type photo reactors has been used. However, the use of suspensions requires the... 

Recommendations For estimating the liquid- and solid-phase axial dispersion coefficients in a three-phase fluidized-bed column, use of Eqs. (9-37) and (9-39) are recommended. Future work on this subject should include the measurement of axial dispersion coefficients in the gas phase, particularly in large-diameter columns. 

In the previous section, stability criteria were obtained for gas-hquid bubble columns, gas-solid fluidized beds, liquid-sohd fluidized beds, and three-phase fluidized beds. Before we begin the review of previous work, let us summarize the parameters that are important for the fluid mechanical description of multiphase systems. The first and foremost is the dispersion coefficient. During the derivation of equations of continuity and motion for multiphase turbulent dispersions, correlation terms such as esv appeared [Eqs. (3) and (10)]. These terms were modeled according to the Boussinesq hypothesis [Eq. (4)], and thus the dispersion coefficients for the sohd phase and hquid phase appear in the final forms of equation of continuity and motion [Eqs. (5), (6), (14), and (15)]. However, for the creeping flow regime, the dispersion term is obviously not important. 

In fluidization, a suspension of fine solid particles behaves like a liquid during the upflow of a supportive gas or liquid phase. Thus the bed of fluidized solid itself may be analyzed similarly to liquid systems. The gas-lift effect produces internal recirculation, by providing a descending flow of high particle concentration and an ascending flow of low particle concentration. This effect resembles the circulation in bubble columns. Whereas bubble columns contain dispersed gas and a continuous liquid phase, the fluidized bed comprises the bubble phase and the emulsion phase in which particles have gained fluid-like properties by the interstitial gas flow. 

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