Liquid-solid fluidized beds

The values of A, B, C, F, G, and Z for gas-sohd fluidized beds, solid-liquid fluidized beds, gas-hquid bubble columns, and three-phase fluidized beds are given in Table I. Let us consider these parameters for gas-solid fluidized beds (refer to Table I). Since palps can be considered negligible,... 

It has been mentioned earlier that the voidage propagation velocity is the same as the sharp front velocity. Thus, the equation for is the same in gas-solid fluidized beds and liquid-solid fluidized beds. However, for bubble columns, the sharp front velocity is given by the following equation ... 

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. 

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

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. 

In bubble columns and gas-liquid stirred reactors, the estimation of parameters is more difficult than in gas-solid or liquid-solid fluidized beds. Solid particles are rigid, and hence the fluid-solid interface is nonde-formable, whereas the gas-liquid interface is deformable. In addition, the effect of surface-active agents is much more pronounced in the case of gas-liquid interfaces. This leads to uncertainties in the prediction of all major parameters, such as the terminal bubble rise velocity, the bubble diameter, the gas holdup, and the relation between the bubble diameter and the terminal bubble raise velocity. 

A continuous process for polymerization of nylon 6,6 in which a fluidized bed solid state polymerization reactor is used as the high polymerizer is represented schematically in Figure 3 (26). In this process the low molecular weight polymer is produced in a filled pipe reactor located just upstream of the spray drier. The liquid product of this step is then sprayed into a hot inert gas atmosphere where the water is flashed off and a fine powder is produced. This powder is fed into an opposed-flow, fluidized bed reactor at 200 °C where the high molecular weight polymer powder is generated at temperatures well below the 255 °C melting point of nylon 6,6. The powder is then melted in the extruder and converted into fiber or chip. 

Catalyst may be packed in a fixed bed within the reactor. Uniformly small particles may also be supported by the upward velocity of the reactant stream (gas or liquid), in which case it is called a fluidized bed. Solid catalyst may also be dissolved or suspended in a liquid reaction media, then separated from the products and recycled. Metal catalysts may be made into screens or other shapes across which the reactants flow. It should be remembered, however, that the reaction takes place on the surface of the catalyst if heat is evolved, cooling should be applied there, or the catalyst could be destroyed or deactivated. Most catalysts also become deactivated due to fouling of the surface with by-products and contamination by impurities in the feed stock, called poisons. The... 

As an example of the novel applications of CFD-DEM, we present results of a spout fluidized bed with liquid injection. In this simulation, both solid particles and liquid drops are modeled as particles. The droplets are injected at a low Weber number such that droplet breakage can be neglected. When a droplet and solid particle collide, the hquid is assumed to form a uniform liquid layer around the particle. When wetted particles collide, the restitution coefficient is as given in the previous subsection. 

Gas—solids fluidization is the levitation of a bed of solid particles by a gas. Intense soflds mixing and good gas—soflds contact create an isothermal system having good mass transfer (qv). The gas-fluidized bed is ideal for many chemical reactions, drying (qv), mixing, and heat-transfer appHcations. Soflds can also be fluidized by a Hquid or by gas and Hquid combined. Liquid and gas—Hquid fluidization appHcations are growing in number, but gas—soHds fluidization appHcations dominate the fluidization field. This article discusses gas—soHds fluidization. 

Solid-gas Mixing of immiscible liquids Pneumatic conveying of solids Fluidized beds... 

This equation has been experimentally verified in liquids, and Figure 2 shows that it applies equally well for fluidized solids, provided that G is taken as the flow rate in excess of minimum fluidization requirements. In most practical fluidized beds, bubbles coalesce or break up after formation, but this equation nevertheless gives a useful starting point estimate of bubble size. 

Fluidized-bed process incinerators have been used mostly in the petroleum and paper industries, and for processing nuclear wastes, spent cook liquor, wood chips, and sewage sludge disposal. Wastes in any physical state can be applied to a fluidized-bed process incinerator. Au.xiliary equipment includes a fuel burner system, an air supply system, and feed systems for liquid and solid wastes. The two basic bed design modes, bubbling bed and circulating bed, are distinguished by the e.xtent to which solids are entrained from the bed into the gas stream. 

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. 

The absorption rate increased with increasing nominal liquid velocity for all particle sizes and decreased with increasing particle size for all liquid velocities. The absorption rates were lower than those measured in an equivalent gas-liquid system with no solid particles present. The difference is explained as being due to a higher rate of bubble coalescence and, consequently, a lower gas-liquid interfacial area in the gas-liquid fluidized bed. 

The results of Massimilla et al., 0stergaard, and Adlington and Thompson are in substantial agreement on the fact that gas-liquid fluidized beds are characterized by higher rates of bubble coalescence and, as a consequence, lower gas-liquid interfacial areas than those observed in equivalent gas-liquid systems with no solid particles present. This supports the observations of gas absorption rate by Massimilla et al. It may be assumed that the absorption rate depends upon the interfacial area, the gas residence-time, and a mass-transfer coefficient. The last of these factors is probably higher in a gas-liquid fluidized bed because the bubble Reynolds number is higher, but the interfacial area is lower and the gas residence-time is also lower, as will be further discussed in Section V,E,3. 

No work on mass transfer across the liquid-solid interface in gas-liquid fluidized beds has come to the author s attention. 

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). 

Measurement of the expansion of a gas-liquid fluidized bed provides a measure of the holdup of solids or of the corresponding combined holdup of gas and liquid. From such measurements, the holdup of liquid may be calculated if the gas holdup has been determined independently. 

Recent research development of hydrodynamics and heat and mass transfer in inverse and circulating three-phase fluidized beds for waste water treatment is summarized. The three-phase (gas-liquid-solid) fluidized bed can be utilized for catalytic and photo-catalytic gas-liquid reactions such as chemical, biochemical, biofilm and electrode reactions. For the more effective treatment of wastewater, recently, new processing modes such as the inverse and circulation fluidization have been developed and adopted to circumvent the conventional three-phase fluidized bed reactors [1-6]. 

Bavarian and Fan [3, 4] reported a similar phenomenon occurring in a three-phase fluidized bed. In their case, the hydraulic transport of a packed bed occurred at the start-up of a gas-liquid-solid fluidized bed. Although the cause was different from the case reported in the present study, similar phenomena were observed in both cases. 

Column reactors for gas-liquid-solid reactions are essentially the same as those for gas-liquid reactions. The solid catalyst can be fixed or moving within the reaction zone. A reactor with both the gas and the liquid flowing upward and the solid circulating inside the reaction zone is called a slurry column reactor (Fig. 5.4-10). The catalyst is suspended by the momentum of the flowing gas. If the motion of the liquid is the driving force for solid movement, the reactor is called an ebullated- or fluidized-bed column reactor (Fig. 5.4-10). When a catalyst is deactivating relatively fast, part of it can be periodically withdrawn and a fresh portion introduced. 

With the rare exception of xenon gas NMR of fluidized beds, which we discuss later, granular flow studies by NMR detect signals from the particles and not the surrounding medium. Because it is technically easier to obtain NMR signals from liquids rather than solids, the majority of granular NMR studies so far use solid particles containing liquids. 

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