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Types of Application from our mixer
Liquid Blending
Liquid blending is defined as the mixing of two or more miscible liquids to a measurable level of uniformity. It is often quantitatively characterized by a “blend time.” This is the time required from the addition or contact of the ingredients to the point at which any sample taken from the vessel can be expected to show a ratio of mixed components within five percent of a sample taken from any other point. “Crude” blending is achieved when the composition is within ten percent. “Precise” blending is achieved when the composition is within two percent. As a rule of thumb, crude blending takes about half the time of normal blending, precise blending takes about twice as long. Vessel Design Considerations When the viscosities are water-like we expect the flow to be turbulent. Almost any vessel configuration is allowed. It can be short and squatty or tall and skinny. It can be round, square, rectangular, horizontal cylindrical or even irregularly shaped. However, the vessel geometry will restrict where the mixer(s) must be placed for effective blending. For example, you can’t expect to be able to place one mixer close to the end of a long horizontal cylindrical tank. Odd shaped vessels often require more than one mixer as do very shallow pits and sumps. When the liquid exhibits significant viscosity, the flow will be laminar. In this flow regime, your choice of vessel geometry is restricted. Relatively big impellers will be required and these will often run close to the sides of the vessel. Round vessels are the best choice, but square vessels with rounded corners and filleted bottoms may sometimes be used. Tall skinny vessels can be mixed, but as the aspect ratio increases the blend time becomes much longer at the equivalent level of power and cost of the agitator. Solids Suspension Solids suspension is defined as maintaining a slurry of solid particles in a liquid. It usually pertains to systems in which solids would settle to the bottom of the vessel if no agitation were provided. It can also refer to maintaining a slurry of solids that would otherwise float. Suspension or “complete suspension” normally means that no particle will rest more than one second on the floor of the vessel. “Uniform” suspension means that the concentration of solids measured in sample taken at any point in the vessel will be within five percent of the concentration of a sample taken at any other point. Vessel Design Considerations The optimum vessel for solids suspension is round and has an aspect ratio of about 1:1. It must either be baffled with the mixer on center or the mixer can be angle-offset mounted. A center mounted mixer in an unbaffled tank will not effectively suspend solids. Offset mixers in unbaffled vessels will leave a significant heal of solids in one sector of the vessel floor. Angle offset mixers are generally restricted to relatively small volumes although we have used them successfully in 25,000 gallon vessels. It is important to recognize that solids can easily be suspended to a height of about 85% of the vessel diameter with a single impeller. However, even when multiple impellers are used, it is generally impractical to suspend solids higher than about 1.4 times the vessel diameter and it is impossible to suspend them more than 2 times the vessel diameter in a conventional vessel. If these constraints are unacceptable, a draft tube must be installed in the vessel. When the system contains very small solid particles, their tendency to settle out is exclusively controlled by the viscosity of the liquid. This is called Stokes settling. Most of these systems can be treated as liquid blending applications, and many of the restrictions above do not apply. However, if there is a power failure and the solids are allowed to settle, they may be difficult or impossible to re-suspend if the restrictions above were ignored in the design of the vessel. Dispersion Dispersion (liquid-liquid) is defined as the production of a suspension of droplets of one liquid in another immiscible liquid. A dispersion is usually produced in order to enhance mass transfer of a component from one of the liquids to the other. Dispersions are produced by high shear mixing devices that impart a high level of turbulent energy to the system. This energy appears as turbulence which produces droplets of one of the liquids which is “dispersed” into the other. However, the energy level must be controlled so as not to produce a stable emulsion (see Emulsion). When the mass transfer is complete, the liquids will be expected to separate into two distinct phases which can be drawn off for further processing. Vessel Design Considerations These applications almost always require a vertical round vessel. Exceptions may be acceptable when the viscosity is relatively high, but these are unusual. The vessel should be selected so that the static height of the greater component is no higher than the vessel diameter. Angle offset mounting can be used, but we normally expect to see baffles installed in the vessel. Dissolution Dissolution is defined as the solution of solid (particles) in a liquid to form a homogeneous phase. It is generally a sub-category of solids suspension. Note that complete suspension is a sufficient specification for most dissolution operations. When all particles are suspended, mass transfer from the particle surface to the liquid controls the rate, and additional mixing is not cost effective. It is seldom practical to design a mixing system to mechanically break solid particles as an aid to dissolution. Agitated slurries often show evidence of some particle attrition, but this usually has only a small effect on dissolution rate. Specialized attrition devices are available if the application demands it. Vessel Design Considerations The optimum vessel for solids dissolution is round and has an aspect ratio of about 1:1. It must either be baffled with the mixer on center or the mixer can be angle-offset mounted. A center mounted mixer in an unbaffled tank will not effectively suspend solids. Offset mixers in unbaffled vessels will leave a significant heal of solids in one sector of the vessel floor. Angle offset mixers are generally restricted to relatively small volumes although we have used them successfully in 25,000 gallon vessels. Emulsification Emulsification is defined as the production of an apparently homogeneous liquid from two or more immiscible liquids. In an emulsion one of the liquids is generally dispersed in the other in the form of small droplets. Emulsions are relatively stable. That is the liquids will not separate on standing for a reasonable amount of time. What time is reasonable is determined by the nature and purpose of the emulsified product. Emulsions are often produced by high shear mixing device that impart a high level of turbulent energy to the system. The energy appears as turbulence which tears apart suspended drops to produce droplets small enough to remain entrained and to resist coalescence. However, emulsification operations can be aided by “emulsifying agents,” often surfactants, whose chemical properties enhance drop stability. Vessel Design Considerations These applications almost always require a vertical round vessel. Exceptions may be acceptable when the viscosity is relatively high, but these are unusual. The vessel should be selected so that the static height of the greater component is no higher than the vessel diameter. Angle offset mounting can be used, but we normally expect to see baffles installed in the vessel. Many emulsions in particular are very viscous. These applications often require scraped wall agitation in conjunction with a second high speed disperser mixer in the same vessel. Needless to say, these vessels are always round. Washing or Leaching Washing or leaching is defined at the preferential solution of one or more components of a solid (particle) leaving other components behind and undissolved. From the mixer designer’s point of view it is very similar to dissolution. Vessel Design Considerations The optimum vessel for solids suspension is round and has an aspect ratio of about 1:1. It must either be baffled with the mixer on center or the mixer can be angle-offset mounted. A center mounted mixer in an unbaffled tank will not effectively suspend solids. Offset mixers in unbaffled vessels will leave a significant heal of solids in one sector of the vessel floor. Angle offset mixers are generally restricted to relatively small volumes although we have used them successfully in 25,000 gallon vessels. It is important to recognize that solids cannot be suspended to a height of more than about 85% of the vessel diameter with a single impeller. Even when multiple impellers are used, it is generally impractical to suspend solids higher than about 1.4 times the vessel diameter and it is impossible to suspend them more than 2 times the vessel diameter in a conventional vessel. If these constraints are unacceptable, a draft tube must be installed in the vessel. When the system contains very small solid particles, their tendency to settle out is exclusively controlled by the viscosity of the liquid. This is called Stokes settling. Most of these systems can be treated as liquid blending applications, and many of the restrictions above do not apply. However, if there is a power failure and the solids are allowed to settle, they may be difficult or impossible to re-suspend if the restrictions above were ignored in the design of the vessel. Gas Dispersion Gas dispersion is defined as the production of a suspension of bubbles of gas in an immiscible liquid. A gas dispersion is usually produced in order to enhance mass transfer of a component from the gas to the liquid, but occasionally from the liquid to the gas. Dispersions are produced by high shear mixing devices that impart a high level of turbulent energy to the system. This energy appears as turbulence which breaks up gas pockets into bubbles and disperses these into the mass of liquid. This increases the surface area interface between the gas and the liquid. Generally the gas is introduced close to the bottom of the vessel. However, there are a variety of specialized devices and vessel configurations used for specific gas dispersion applications. Vessel Design Considerations These applications almost always require a vertical round vessel. Exceptions may be taken when the viscosity is relatively high, but these are unusual. This is on of the few cases where tall skinny vessels are preferred; some of these have aspect ratios greater than 3.0. Gas dispersion is almost always a turbulent operation. Round baffled vessels are required. Offset mounting is strongly discouraged because irregular unbalanced hydraulic forces will be very large. This requires costly “oversizing” of the mixer in order to prevent destructive vibration. Crystallization Crystallization is opposite of dissolution. It is the production of a suspension of solids from a solution (often called a “mother liquor”) of the solid component in a liquid. Mechanisms include evaporation, cooling and chemical reaction. Crystallization is often a subcategory of solids suspension. Uniform suspension is often required. Minimizing particle attrition is often an important consideration. There are a variety of specialized vessel and mixer configurations used in crystallization operations on an application specific basis. Vessel Design Considerations See Solids Suspension. However, there are many specialized “crystallizers” that use unique vessel designs for individual applications. These must be configured on a case by case basis. Heat Transfer Heat transfer (as mixing application) is defined as the transfer of heat between the contents of the vessel and a heating or cooling surface provided for that purpose. The surface may consist of coils, plates, the wall of the vessel, etc. A heat transfer coefficient can predicted for most systems of interest. This coefficient is called the “inside” heat transfer coefficient. It is a measure of the heat flow capacity at the surface in direct contact with the vessel contents. The coefficient of the wall of the surface and from the heating/cooling medium to the outside wall of the surface must be determined separately, and combined with the inside coefficient to determine an “overall” coefficient. Vessel Design Considerations See Liquid Blending. Liquid blending and heat transfer have much in common. The difference is that a heat source (or sink) is installed in the vessel. These may be pipe coils, plates or the vessel may be jacketed. There are a great variety of coil arrangements used for heat transfer. Some coil assemblies can serve as baffles, but when they cannot, baffles must be installed in the vessel when required. Jacketed vessels are almost always round and usually used when scraped wall agitation is expected. However a jacketed vessel can also be used in most other circumstances. | ||||||||||
