Solids suspension scale

Erosion-corrosion. Generally, all types of corrosive media can cause erosion-corrosion, including aqueous solutions, organic media, gases, and liquid metals. The corrodent can be a bulk fluid, a film, droplets or a substance adsorbed on or absorbed on another substance. For example, hot gases may oxidize a metal at high velocity and blow off an otherwise protective scale. Solid suspensions in liquids (slurries) are particularly destructive from the standpoint of erosion corrosion.16 31... 

For equivalent solids suspension, an exponent of JA will be used to scale up from the small-scale solids suspension speed of 465 r/min ... 

Weetman, R. J. (1994). Development of an erosion resistant mixing impeller for large scale solid suspension applications with CFD comparisons, presented at the 8th European Conference on Mixing, Cambridge. 

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. 

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. 

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. 

Divilio, R. J., and Boyd, T. J., Practical Implications of the Effect of Solids Suspension Density on Heat Transfer in Large-Scale CFB Boilers, Circ. Fluid. Bed Tech IV, 334-339 (1993)... 

Agitation for a desired level of solids suspension is based on an overall appearance of the solid-liquid system. Results of the empirical correlations have been summarized for most types of solids-suspension scale-up cases... 

Agitation levels 1 and 2 are characteristic of applications requiring minimal solids-suspension levels to achieve the process result. Agitators capable of scale levels of 1 will Produce motion of all of the solids of the design settling velocity in the vessel Permit moving fillets of solids on the tank bottom, which are periodically suspended. 

Agitation levels 3 and 5 characterize most chemical process industries solids-suspension applications and are typically used for dissolving solids. Agitators capable of scale levels of 3 will Suspend all the solids of design settling velocity completely off the vessel bottom Provide slurry uniformity to at least 1 /3 of fluid-batch height Be suitable for slurry draw-off at low exit-nozzle elevations. 

Figure 3 Solid-suspension scale value versus (p.

Agitation levels 6-8 characterize applications where the solids-suspension levels approach uniformity. Agitators capable of scale levels of 6-8 will a. Provide concentration uniformity of solids to 95% of the fluid-batch height. b. Be suitable for slurry drawoff up to 80% of fluid-batch height... 

Scaling for a dilute gas-solid suspension may represent a simple case of using the nondimensionalization method. The following example is a simplified version of the work of Chao (1982). 

Much of the literature correlations for solids suspension are based on the so-called critical impeller speed. Attempts to duplicate experiments between various investigators often yield deviations of 30-50% from the critical speed shown by other investigators. Because power is proportional to speed cubed, power varies on the order of 2 to 3 times, which is not sufficiently accurate for industrial full-scale design. Therefore, many approximate, conservative estimates have been made in the literature as general guidelines for choosing mixers for solids suspension. Table IV is one such guideline for solid particles of a closely sized nature. 

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