Basic knowledge of high shear disperser
Successful dispersing depends on a basic understanding of the dispersing equipment and how to use the disperser
High shear mixers typically operate at high rotational speeds for mixing applications that are best described as dispersion processes. They can be used in a variety of applications such as accelerating additions, wet powders, breaking agglomerates and providing process enhancements. Many dispersion problems are caused by improper use of equipment, and simple operational changes may lead to better results. While the inherent strength of high shear dispersers can sometimes provide only limited success even when misused, successful dispersion depends on a basic understanding of how to use a disperser. This article focuses on the use of these dispersers when adding powders and liquids to liquids. Best practices for successful dispersion and different types of dispersing machines are discussed.
The basic principle
The high rotational speed of a high shear disperser is usually the motor speed, including speed at the power frequency. Most dispersers operate at a single speed, but some larger dispersers may have variable speed controls. The dispersion may be a powder dispersion in a low viscosity liquid, or a liquid dispersion in a miscible or immiscible liquid. In a few cases, bubbles can be dispersed in liquids for transportation processes or to create foam. The purpose of dispersion is to quickly form a properly uniform fluid. For processes involving soluble solids, the equipment may be called a dissolver. Fluid viscosity may increase as thickeners disperse or emulsions form.
This lengthy description is needed because different processes and applications can be accomplished with variations of similar equipment (see the High Shear Disperser Types section). The open impeller of a high-speed disperser can be a serrated disc, a short-blade impeller, or a slotted head. Some rotating impellers are mounted inside a stationary cage with holes or slots that force fluid through them. These mixers are often called rotor-stators but may also be called homogenizers. The primary mechanism responsible for dispersion is hydraulic shear, although some mechanical shear is also present, particularly with rotor-stator mixers. Even within the general category of high shear mixers, some are used primarily for rapid initial dispersion, while others use multiple recirculations to improve dispersion and uniformity. Hydraulic shear is rarely sufficient to grind or reduce the size of individual particles, but it can break up agglomerates or clumps. Some particle fragmentation may occur with weak organic or biological particles. Other types of grinders or mills may be used when particle size reduction is required.
Although most high shear dispersers are mounted in the center or slightly off-center, they can be mounted at the top or bottom. To control the fluid vortex created by the rotating impeller, the tank usually has one or more baffles. Excessive swirling flow and surface vortices that draw air into the impeller can reduce dispersion and create unwanted air bubbles in the product. The flow through the stator eliminates most of the rotating flow in the rotor-stator mixer effluent. Single or double baffles can be adjusted to control surface vortices. In other diffuser designs, tanks mounted off-center or with square cross-sections are used to reduce the need for baffles. Deep vortices with rotating flow do not provide good mixing (see High Shear Mixing: Do's and Don'ts box, pp. 41-42).
high speed dispersion
In order to form a "dispersion" of a solid, gas, or immiscible liquid, a disperser must break down the added ingredients into individual particles, bubbles, or droplets. High-speed dispersers can also quickly add miscible liquids or soluble solids. Whether the process is batch or continuous, rapid addition often improves productivity and sometimes even quality.
Solid dispersion. For solid dispersion, individual particles need to be surrounded by a liquid and initially suspended. To ensure that individual particles are exposed to the liquid, the addition rate must be controlled. If solids are added too quickly, particle clusters can introduce trapped air into the liquid and the partially wetted powder may form clumps. If the particles are hydrated in the liquid, the particles must be separated and suspended quickly.
If the solid is soluble, the dissolution rate is determined primarily by equilibrium solubility and particle size. Fluid motion can enhance external mass transfer around particles, but concentration uniformity in the fluid provides the most effective driving force for dissolution.
For insoluble solid particles, the typical goal is to maintain sustained suspension with some degree of homogeneity. Suspension and uniformity are determined by liquid velocity and flow pattern. If the swirling flow is not adequately controlled by baffles or eccentric installations, some solids may never remain suspended at the bottom of the tank. Since most high-speed dispersion impellers are radial flow, they need to be close to the bottom of the tank to create a flow pattern that lifts and suspends settling particles.
Liquid dispersion. For liquid dispersion of immiscible liquids, almost all practical droplet breakup occurs in the fluid velocity gradient near the impeller blades. The rate of addition must be controlled so that the disperser can break the liquid into individual droplets. The final droplet size and dispersion uniformity are closely related to the impeller tip speed or circumferential speed of the disperser. However, viscosity differences and surface tension also affect droplet size. The rate at which the dispersion is formed may depend on the number of passes through the disperser required for uniformity.
Gas dispersion. Gas dispersion by high-speed dispersers is generally not as effective as jet gas dispersion, in which gas enters from below the impeller. Gas diffusion from the surface by vortices can be difficult to control because it is affected by liquid level coverage and baffle adjustment. For the dispersion of solids and liquids, gas entrainment should be avoided or strictly controlled.