Section 7.3.1 Solids Suspension

Mechanism only for thickener, c/s including drive, access platform, central shaft, bridge, rake and scraping arm, handrail, surface skimmer, excluding inlet pipe and overflow launders, FOB cost 98000 for cross-sectional area = 100 m with n = 0.62 for the range 60-7000. Factors sandblast and paint, X 1.2. L+M = 1.35. 

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The experiments of Dou et al. (1991) also indicate that the heat transfer coefficient varied with radial position across the bed, even for a given cross-sectional-averaged suspension density. Their data, as shown in Fig. 20, clearly indicate that the heat transfer coefficient at the bed wall is significantly higher than that for vertical surfaces at the centerline of the bed, over the entire range of suspension densities tested. Almost certainly, this parametric effect can be attributed to radial variations in local solid concentration which tends to be high at the bed wall and low at the bed centerline. 

The data of Fig. 20 also point out an interesting phenomenon—while the heat transfer coefficients at bed wall and bed centerline both correlate with suspension density, their correlations are quantitatively different. This strongly suggests that the cross-sectional solid concentration is an important, but not primary parameter. Dou et al. speculated that the difference may be attributed to variations in the local solid concentration across the diameter of the fast fluidized bed. They show that when the cross-sectional averaged density is modified by an empirical radial distribution to obtain local suspension densities, the heat transfer coefficient indeed than correlates as a single function with local suspension density. This is shown in Fig. 21 where the two sets of data for different radial positions now correlate as a single function with local mixture density. The conclusion is That the convective heat transfer coefficient for surfaces in a fast fluidized bed is determined primarily by the local two-phase mixture density (solid concentration) at the location of that surface, for any given type of particle. The early observed parametric effects of elevation, gas velocity, solid mass flux, and radial position are all secondary to this primary functional dependence. 

The computation performed in this study is based on the model equations developed in this study as presented in Sections II.A, III.A, III.B, and III.C These equations are incorporated into a 3-D hydrodynamic solver, CFDLIB, developed by the Los Alamos National Laboratory (Kashiwa et al., 1994). In what follows, simple cases including a single air bubble rising in water, and bubble formation from a single nozzle in bubble columns are first simulated. To verify the accuracy of the model, experiments are also conducted for these cases and the experimental results are compared with the simulation results. Simulations are performed to account for the bubble-rise phenomena in liquid solid suspensions with single nozzles. Finally, the interactive behavior between bubbles and solid particles is examined. The bubble formation and rise from multiple nozzles is simulated, and the limitation of the applicability of the models is discussed. 

Much attention should be given to correlations for liquid-solid suspensions or fluidizing systems derived experimentally. If the experimental data have been correlated to particle density, this kind of density and not the hydraulic density should be used. For instance, this is the case of the Liu-Kwauk-Li criterion for determining the fluidization pattern (Section 3.8.2). However, for correlations that have been derived using nonporous particles, the hydraulic density should be used. This is because the correlation accounts for the whole mass included in the volume of the particle, which is the sum of the solid mass and liquid mass in the pores for porous particles. 

Suspension of solids is maintained by upward movement of the liquid. In principle, use of a draft tube and an axial flow impeller will accomplish this flow pattern most readily. It turns out, however, that such arrangements are suitable only for low solids contents and moderate power levels. In order to be effective, the cross section of the draft tube must be appreciably smaller than that of the vessel, so that the solids concentration in the draft tube may become unpractically high. The usually practical arrangement for solids suspension employs a pitched blade turbine which gives both axial and radial flow. 

Solid Suspensions. The category of solid suspensions is split into two sections. The... 

Without losing generality, in this section we only consider case (3), where the pipe bend is located in the vertical plane with a vertical gas-solid suspension flow at the inlet, as shown in Fig. 11.10. It is assumed that the carried mass and the Basset force are neglected. In addition, the particles slide along the outer surface of the bend by centrifugal force and by the inertia effect of particles. The rebounding effect due to particle collisions with the wall is neglected. 

Chapter 8 provides practical guidance on the use of widely used extraction and isolation techniques from the sample preparation perspective. The first two sections, solid-phase extraction and liquid-liquid extraction deal with liquid samples. The sections on supercritical fluid extraction and accelerated solvent extraction focus mainly on solid samples while the centrifugation and filtration sections handle suspensions. A successful sample preparation protocol accounts for specificity and homogeneity as well as recovery and final physical state of the targeted material. The ultimate aim is to produce a sample that is compatible with the desired analytical technique to assure generation of maximum information. 

The model based on the concept of pure limiting film resistance involves the steady-state concept of the heat transfer process and omits the essential unsteady nature of the heat transfer phenomena observed in many gas-solid suspension systems. The film model discounts the effects of thermophysical properties such as the specific heat of solids and hence would not be able to predict the particle convective component of heat transfer. For estimating the contribution of the particle convective component of heat transfer, the emulsion phase/packet model given in a subsequent section should be used to describe the temperature gradient from the heating surface to the bed. 

The following section discusses crystallizer design from the perspective of mixing and solids suspension. See Chapter [5], Crystallizer Selection and Design for more information. 

Three different but connected problems must be studied (i) the reaction kinetics model (ii) the development of the rate of electron-hole generation in a material particle of the solid suspension and (iii) the model for characterizing the radiation field to evaluate the local volumetric rate of photon absorption (LVRPA). Point (iii) has been already described in section 6.6.1 for quantum yield determinations. In the first part of this section, we will concentrate on problems (i) and (ii). 

Sections 7.3.1 to 7.3.4 consider solids suspension, solids dispersion, solids dissolving, and solids flocculating respectively. General issues related to mixing using a fluidized bed are given in Section 7.3.5. 

Creation of dispersions, slurries, pastes and compounds. For concentrations of solids in liquid < 50 % see Section 7.3.1 on solids suspension to achieve a uniform concentration. For more concentrated solids and more viscous liquids, see Solids, Section 7.4 where pastes, melts, plastics and extrusion compounding are discussed. 

For readily soluble solids, use the principles of solids suspension, in Section 7.3.1, where flow and shear are supplied by the impeller to provide the mass transfer. However, select for flow because the resistance to mass transfer is usually low. A unique challenge is polymer powder to be dissolved in a solvent. 

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