Solid-liquid mixing floating solids

Solid-liquid mixing is used to float solid particle in a liquid and is important for solid solutions, solid catalyst reactions, crystallization, and so on. 

Solid-liquid mixing involves the suspension, distribution, and the drawing down of solids by agitation. In addition to vessel geometiy, impeller variables include type, diameter, number, speed, and location. Process results include the desired level (quality) of suspension, such as just off-the-bottom, complete uniformity, or any intermediate condition. The slurry properties, density difference (solid/liquid), viscosity, and solids concentration all determine how difficult the task may be. As alternatives to stirred vessels, jets (see Section 9.10) can be used for light-duty suspension. Literature references deal mainly with settling solids as opposed to floating solids. We will try to address both conditions. 

Basket USP 1 EP basket JP method 1 BP 1 Rotating stirrer Dosage form confined leads to consistent solid/liquid interface Floating products kept immersed Limited sink conditions Trapped bubbles in basket can slow dissolution rate Inadequate mixing at slow speed Floating dosage forms Dosage forms that tend to adhere to the vessel surfaces... 

Many operations treat particle-liquid mixing in chemical industry. The first aim of solid-liquid mixing is to make a solid particle float. However, mixing performance of operations/equipment is not clear. Additionally, when many kinds of particles are involved, it is not known whether there is any difference in the mixedness between the following two cases the case where all particles are treated as a particle (two-phase mixing—particle and continuous liquid phase) and the case where every particle is treated individually (multiple-phase mixing—each particle and continuous liquid phase). Therefore, a solution to this unsolved problem is not imperative. 

Now pour in an equal amount of strong alcohol (at least 95%) and shake daily to insure the two liquids mix, as the lighter alcohol will float on top. If the two liquids do not separate, it means there was too much water in the herb or the alcohol. Slowly add solid dry potassium carbonate to absorb the excess water. If separation does not occur, you will have to start again. 

For suspension of free-settling particles, circulation of pseudoplastic slurries, and heat transfer or mixing of miscible liquids to obtain uniformity, a speed of 350 or 420 r/min should be stipulated. For dispersion of dry particles in liquids or for rapid initial mixing of liquid reactants in a vessel, an 1150- or 1750- r/min propeller should be used at a distance Df/4 above the vessel bottom. A second propeller can be added to the shaft at a depth Da below the liquid surface if the submergence of floating liquids or particulate solids is otherwise inadequate. Such propeller mixers are readily available up to 2.2 kW (3 hp) for off-center sloped-shaft mounting. 

In operations such as crystallization or solid-catalysed liquid reactions, it is necessary to suspend solid particles in a relatively low viscosity liquid. This can be achieved in mechanically agitated vessels where the mixer is used to prevent sedimentation of the solids and to provide conditions suitable for good liquid-solid mass transfer and/or chemical reaction. If agitation is stopped the solids will settle out or float to the surface, depending upon the relative densities of the solid and liquid phases. The suspension of solids in mixing vessels and the design of mixing vessels for solid-liquid reactions are treated in Chapters 16 and 17 respectively. 

The Froude number, Npr, is a predominant correlating parameter in these systems, where liquid surface behavior is so important. Joosten et al. (1977) have developed a correlation that has been used successfully to design a commercial mixing system for suspending floating solids in a 50 m vessel. The correlation is... 

Miscible. The above terms refer principally to solutions of solids in liquids. When one liquid dissolves in another liquid to form a solution, the term miscible is used. The solvent will be the component that is present in the larger quantity. Ethanol is completely miscible with water - a property used to produce the wide strength range of alcoholic drinks. Immiscible. When one liquid is insoluble in another hquid, i.e. the hquids do not mix, the term used is immiscible. The two hquids will form separate layers with the one with the lower density on top. For example, a puddle in the street often has an iridescent or rainbow-like appearance due to a thin layer of oil on the top. Oil is immiscible with water, and has the lower density, so it floats (see Figure 4.6.1). 

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