Overall Solids Holdup

Fig. 6. Determination of solid distribution at three axial positions (a) cross-sectional distribution (b) azimuthally averaged solids holdup and (c) overall solids holdup. The horizontal line indicates experimentally determined solids fraction.

The overall solids holdup is defined as the particle volume fraction over the entire riser. Figure 10.14 shows the typical effect of the solids circulation rate on the overall... 

Figure 10.14. Overall solids holdup as a function of solids circulation rate for Group A particles (after Jiang et al., 1993).

The prediction of overall solids holdups can be obtained by integrating the axial profile of the solids concentration over the axial length. In terms of Eq. (10.12), the overall solids holdup can be expressed as... 

The exit geometry at the top of the riser influences solids reflection and holdup in the upper part of the reactor, affecting solids flow patterns, overall solids holdup, and hence mixing. 

Padial et al. [91] performed qualitative simulations of three-phase flow in a draff tube bubble column and compared the overall gas volume fraction and liquid circulation time for gas-liquid and gas-liquid-solid systems. Michele and Hempel [92] simulated flow air bubbles and PMMA particles (300 pm, 10 vol%) dispersed in water for superficial gas velocities in the range of 0.02-0.09 m/s. They compared their predictions with measured overall gas holdup and only a qualitative agreement... 

The riser is subject to significant axial gradients in solids holdup, with the extent of variation dependent on such factors as superficial gas velocity (C/g), net solids circulation flux (Gj), and riser overall height/diameter (H/D) or height/width (H/W) ratio. 

The types of equipment used, which range from stirred tanks and mixer-settlers to centrifugal contactors and various types of columns, affect both capital and operating costs [9]. In the decision to build a plant, the choice of the most suitable contactor for the specific situation is most important. In some systems, because of the chemistry and mass transfer rates involved, several alternative designs of contacting equipment are available. In the selection of a contactor, one must consider the capacity and stage requirements solvent type and residence time phase flow ratio physical properties direction of mass transfer phase dispersion and coalescence holdup kinetics equilibrium presence of solids overall performance and maintenance as a function of contactor complexity. This may appear very complicated, but with some experience, the choice is relatively simple. 

Heat removal is accomplished by internal cooling coils or wall jacket exchangers. Hydrodynamic regimes are complex, because of complicated flow patterns, prone to quick and dramatic changes. Usually a few overall parameters are considered, such as gas residence time and holdup, solid suspension, energy input, volumetric mass transfer coefficient (sec 3.2. 

---