Solid-liquid fluidized beds particle phase

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. 

Fig. 49. (a) Falling bed (liquid phase is stationary), (b) Stationary bed (solid-liquid fluidized bed), (e) Particle in a flowing fluid. 

Viswanathan et al. (V6) measured gas holdup in fluidized beds of quartz particles of 0.649- and 0.928-mm mean diameter and glass beads of 4-mm diameter. The fluid media were air and water. Holdup measurements were also carried out for air-water systems free of solids in order to evaluate the influence of the solid particles. It was found that the gas holdup of a bed of 4-mm particles was higher than that of a solids-free system, whereas the gas holdup in a bed of 0.649- or 0.928-mm particles was lower than that of a solids-free system. An attempt was made to correlate the gas holdup data for gas-liquid fluidized beds using a mathematical model for two-phase gas-liquid systems proposed by Bankoff (B4). 

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. 

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... 

Finally, the conditions of liquid-solid mass transfer were explored by Prakash et al.[66] and Arters and Fan [90] as reported by Muroyama and Fan [5]. Arters and Fan [90] employed cylindrical particles of benzoic acid (diameter 1.5 mm, length 4 mm) fluidized by air and water. Results showed that the liquid-solid mass transfer in a three-phase fluidized bed was higher than that of a two-phase (liquid-solid system) fluidized bed operating at the same liquid velocity. 

Gas-phase reactions catalyzed by solid catalysts are normally carried out in gas-particle operation in either fixed or fluidized beds. The possibility of using gas-liquid-particle operations for such reactions is, however, of interest in certain cases, particularly if the presence of a liquid medium for the transfer of heat or mass is desirable. 

Subsequently, simulations are performed for the air Paratherm solid fluidized bed system with solid particles of 0.08 cm in diameter and 0.896 g/cm3 in density. The solid particle density is very close to the liquid density (0.868 g/ cm3). The boundary condition for the gas phase is inflow and outflow for the bottom and the top walls, respectively. Particles are initially distributed in the liquid medium in which no flows for the liquid and particles are allowed through the bottom and top walls. Free slip boundary conditions are imposed on the four side walls. Specific simulation conditions for the particles are given as follows Case (b) 2,000 particles randomly placed in a 4 x 4 x 8 cm3 column Case (c) 8,000 particles randomly placed in a 4 x 4 x 8 cm3 column and Case (d) 8,000 particles randomly placed in the lower half of the 4x4x8 cm3 column. The solids volume fractions are 0.42, 1.68, and 3.35%, respectively for Cases (b), (c), and (d). 

A reactor model based on solid particles in BMF may be used for situations in which there is deliberate mixing of the reacting system. An example is that of a fluid-solid system in a well-stirred tank (i.e., a CSTR)-usually referred to as a slurry reactor, since the fluid is normally a liquid (but may also include a gas phase) the system may be semibatch with respect to the solid phase, or may be continuous with respect to all phases (as considered here). Another example involves mixing of solid particles by virtue of the flow of fluid through them an important case is that of a fluidized bed, in which upward flow of fluid through the particles brings about a particular type of behavior. The treatment here is a crude approximation to this case the actual flow pattern and resulting performance in a fluidized bed are more complicated, and are dealt with further in Chapter 23. 

Catalytic coal liquefaction processes do not specifically use hydrogen donor solvents although coal is introduced into the liquefaction reactor as a slurry in a recycle liquid stream. Catalyst is used as a powder or as granules such as pellets or extrudates. If powdered catalyst is used, it is mixed with the coal/liquid stream entering the reactor. Pelleted catalyst can be used in fixed bed reactors if precautions are taken to avoid plugging with solids or in fluidized bed reactors. In the latter case, the reacting system is actually a three phase fluidized bed, that is, catalyst particles and coal solids, as well as liquid, are fluidized by gas. 

Pass gas upward through a bed of fine particles. For superficial (or inlet) gas velocities much in excess of this minimum the bed takes on the appearance of a boiling liquid with large bubbles rising rapidly through the bed. In this state we have the bubbling fluidized bed, BFB. Industrial reactors particularly for solid catalyzed gas-phase reactions often operate as bubbling beds with gas velocities Wq 5 30 u. ... 

Fluidized-bed reactor (FLBR) The up-flow gas or liquid phase suspends the fine solid particles, which remain in the reactor (Figure 3.8). This reactor is of tubular shape with a relatively low aspect ratio of length to diameter. The most common application of FLBR is the classical FCC process. 

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. 

Tlie requirement for mechanical agitation can be avoided by using a fluidized bed reactor. In this type of reactor, the agitation and mixing are achieved by means of the moving liquid that carries the solids through the reactor or mixes with the particle phase. Thus, high heat and mass transfer rates are assured. 

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