Solid suspensions, mixing

A bioreactor is a typical three-phase reactor. Chapman et al. (1983a, b, c, d) carried out a systematic investigation on the different mixing processes of importance in stirred three-phase reactors. This was followed by various studies on gas-liquid mixing and SOM-liquid mixing (solid suspension), as outlined in Chapter 7A. 

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. 

Batch tests (i. e., tests on individual waste materials) are conducted with the provided solid suspensions (e.g., soils such as Woodburn, Sagehill, and Olyic, as well as two bottom sediment samples) prepared with previously air-dried solids (i. e., soils and bottom sediments), ground to a uniform powdery texture for mixing with the eluates from the 24-h batch leaching test of the different SWMs/COMs. The concentrations of eluates in solution were designed to evaluate the capability of different environmental solids to adsorb available contaminants. The solid particles were fully dispersed with the aqueous phase to achieve complete adsorption. Common practice is to use a solid solution ratio of 1 g 4 ml [ 1 ], together with proper tumbling of the samples at a constant temperature (e.g., at least 24 h in a constant temperature environment of 20°C). 

Several advanced filtration systems are finding applications in treating refinery wastewaters. Examples include the HydroClear filter (Zimpro, Rothschild, WI) and the Dynasand filter (Parkson Corporation, Fort Lauderdale, FL). The HydroClear filter employs a single sand medium (0.35-0.45 mm) with an air mix (pulsation) for solids suspension and regeneration of the filter surface. Filter operation enables periodic regeneration of the medium... 

Mixed solid fertilizers can be made by either direct granulation methods (40%) or bulk blending (40%). Bulk blending is made by mechanical mixing of the separate granular intermediate materials. It is usually done in small plants near the point of use. This technique is employed because the fertilizer can be tailor-made to fit the exact requirements of the user. Fluid or liquid fertilizers (clear, suspension, and slurry) account for 20% of all NPK mixed fertilizers. 

Birch, D J., Ahmed, N., Solids suspension in aerated agitated vessels Role of sparger design, 9th European Conference on Mixing Mixing 97, Vol. 11, 177 (1997)... 

Complete mixing, of course, is not practically realizable and in any event may have a drawback in that intense agitation will cause much secondary nucleation. Some rules for design of agitation of solid suspensions are discussed in Chapter 10, notably in Table 10.2 internal velocities as high as l.Oft/sec may be desirable-... 

The study of solids suspension in quantitative terms normally involves a method of sampling. Typically, samples are withdrawn from the side of the mixing tank through openings or tubes inserted into the vessel wall. It may also be done by submerging a container and quickly removing... 

Many processes involve criteria other than solids suspension, for example, crystallization, precipitation, and many types of leaching and chemical reactions. In crystallization, the shear rate around the impeller and other mixing variables can affect the rate of nucleation, and can affect the ultimate particle size. In some cases, the shear rate can be such that it can break down forces within the solid particle and can affect the ultimate particle size and shape. There are some very fragile precipitate crystals that are very much affected by the mixer variables. 

Arena, U., Marcozhella, A., Bruzzi, V., and Massimilla, L. Mixing Between A Gas-Solids Suspension Flowing in a Riser and a Lateral Gas Stream, in Circulating Fluidized Bed Technology IV (Amos A. Avidan, ed.), pp. 660-664. Somerset, Pennsylvania (1993). 

For gas velocities ranging 4.1 to 6.3 m/s, the measured axial solids dispersion coefficients Ds varied from 0.39 to 0.91 m2/s for dilute solid suspension, while for the whole fast bed, in the same gas velocity range, Ds was 0.22 to 1.67 m2/s. Axial solid dispersion coefficient generally increases with both gas velocity and solids circulation rate. Milne and Berruti (1991) have also given their own model for solids mixing. 

---