Suspensions solid

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. 

Solids suspension three separate suspension conditions are employed in practice  

The key design is the relationship between the vertical flow velocity and the settling velocity of the particles. In general, baffle 4 90° off-the-wall distance = 

015 X tank diameter or about 50 to 100 mm. to allow fluid circulation between the baffle and the wall. For low viscosity 500 mPa s the baffle width = 0.10-0.11 tank diameter for more viscous fluids, baffle width = 0.08-0.09 tank diameter. The baffle should be from the bottom up to or slightly above, the liquid surface. 

Uniform concentration design with the vertical flow velocity 10 X the particle settling velocity. Vertical flow velocity of a circulating axial flow pattern can be evaluated by the impeller s pumping capacity plus its induced flow (adjacent to impeller) divided by its cross sectional area for flow in one direction use the same amount of total flow divided by the rest of the cross sectional area inside the tank for flow in the other direction, then, take the smaller velocity to compare with the largest particle settling velocity. 

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The overall superficial fluid velocity, mentioned earlier, should be proportional to the settling velocity o the sohds if that were the main mechanism for solid suspension. If this were the case, the requirement for power if the setthng velocity were doubled should be eight times. Experimentally, it is found that the increase in power is more nearly four times, so that some effect of the shear rate in macro-scale turbulence is effec tive in providing uphft and motion in the system. 

Using a draft tube in the tank for solids suspension introduces another, different set of variables. There are other relationships that are veiy much affected by scale-up in this type of process, as shown in Fig. 18-22. Different scale-up problems exist whether the impeller is pumping up or down within the draft tube. 

The larger the ratio of impeller diameter to tank diameter, the less mixer power required. Large, slow speed impellers require a low er horsepow er for a given pumping capacity, and solid suspension is governed by the circulation rate in the tank. 

The operating cycle of a belt vacuum filter consists of filtration, washing and dewatering stages. The solids suspension in water is separated under the following set of conditions ... 

Stable solids suspensions in water- The mechanisms involved in keeping solids suspended in water,... 

Table 7-4 shows flow patterns and applications of some commercially available impellers. Generally, the axial flow pattern is most suitable for flow sensitive operation such as blending, heat transfer, and solids suspension, while the radial flow pattern is ideal for dispersion operations that require higher shear levels than are provided by axial flow impellers. Myers et al. [5] have described a selection of impellers with applications. Further details on selection are provided by Uhl and Gray [6], Gates et al. [7], Hicks et al. [8] and Dickey [9]. 

Simullaneous gas dispersion and solid suspension (like mining),... 

Weisman and Efferding [41] in constrast, related the degree of agitation to the height Hg oeeupied by the solid suspension, whieh is expressed as ... 

Reeently, Corpstein et al. [43] found that high-effieieney impellers provide the same levels of solids suspension at redueed eapital and operating eosts. They introdueed the term just-suspended for the most eommonly eneountered level of liquid solid agitation. This oeeurs when none of the solid partieles remains stationary on the bottom of the vessel for longer than 1-2 see. They developed a eorrelation of the speed required to aehieve just-suspended eonditions as ... 

Guichardon etal. (1994) studied the energy dissipation in liquid-solid suspensions and did not observe any effect of the particles on micromixing for solids concentrations up to 5 per cent. Precipitation experiments in research are often carried out at solids concentrations in the range from 0.1 to 5 per cent. Therefore, the stirred tank can then be modelled as a single-phase isothermal system, i.e. only the hydrodynamics of the reactor are simulated. At higher slurry densities, however, the interaction of the solids with the flow must be taken into account. 

Guichardon, P., Falk, L., Fournier, M.C. and Villermaux, J., 1994. Study of micromixing in a liquid-solid suspension in a stirred reactor. American Institute of Chemical Engineers Symposium Series, 299, 123-130. 

Reference [55] provides additional details beyond the bag filter applications, and Reference [60] provides a technical and analytical review of flowing gas-solids suspensions. 

Boothroyd, R. G., Flouring Gas-Solids Suspension, Ghapman and Hall, 1971,... 

Mixer performance is often related in terms of the fluid velocity during agitation, total pumping capacity (flow of the fluid in the system) generated by one impeller, and the total flow in the tank (or sometimes as blending time or a solids-suspension criterion) [25]. 

Primarily for flow controlled requirements, such as solids suspensions, heat transfer, and other high pumping efficiency applications. 

In batch operations, mixing takes place until a desired composition or concentration of chemical products or solids/crystals is achieved. For continuous operation, the feed, intermediate, and exit streams will not necessarily be of the same composition, but the objective is for the end/exit stream to be of constant composition as a result of the blending, mixing, chemical reaction, solids suspension, gas dispension, or other operations of the process. Perfect mixing is rarely totally achieved, but represents the instantaneous conversion of the feed to the final bulk and exit composition (see Figure 5-26). 

Scale-up techniques for using the results of pilot plant or bench scale test w ork to establish the equivalent process results for a commercial or large scale plant mixing system design require careful specialized considerations and usually are best handled by the mixer manufacturer s specialist. The methods to accomplish scale-up will vary considerably, depending on whether the actual operation is one of blending, chemical reaction tvith product concentrations, gas dispersions, heat transfer, solids suspensions, or others. 

A. For blending and solid suspension, use Z/T for minimum power at about 0.6 to 0.7, but recognize that this may not be the most economical. 

In order to achieve uniform solid suspension or pickup of solid particles off the bottom, the upward velocities of the fluid streams in all portions of the vessel must exceed the terminal setding velocity of the particular particles. This can be determined by small scale tests. 

Operations such as blending, solids-suspension, dissolving, heat transfer and liquid-liquid extraction are typical of systems requiring high flow relative to turbulence, while gas-liquid reactions and some liquid-liquid contacting require high turbulence relative to flow. The case of (1) 100% of suspension—requires head to keep particles suspended and (2) 100% uniformity of distribution of particles—requires head for suspension plus flow for dis-tiibution. 

Rheology Ideal requirements low viscosity with the yield point and geis necessary for hole cleaning and solids suspension. Practical recommendation A compromise should be found to minimize pressure losses and bring sand or cutting to the surface at reasonable circulating rate. 

Solids Suspension Turbine Propeller Paddle 1. Circulalion 2. Velaail r 2.0 i to 3.5 i III to l. 2 Depending on Portlelisii Limp. Dio meter Off Bollom 2. On Bollom... 

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