In industrial production, mixing and homogenizing operations are often an indispensable unit operation. As a more efficient mixing device, the shearing homogenizer has been widely used in more than 30 industries such as chemical, food, pharmaceutical, cosmetic, paint, bioengineering, petrochemical, printing and dyeing, and has achieved certain results. The biggest advantage of the shear homogenizer is that it provides high-strength shearing action, mixing, pulverizing, dispersing, dissolving, homogenizing and emulsifying as an integral factor for effective handling of the material. However, the rational selection of shear homogenizer has not been discussed thoroughly. It is only selected by experience or small-scale test, which may cause some unnecessary losses in industrial production.
This paper mainly discusses the rational selection of shear type homogenizer from the aspects of mixing process, parameter and practical application.
1, Types and applications of shear type homogenizer
There are many types of shear homogenizers, but their function is to convert the two miscible (or immiscible) phases into uniform distribution system under the action of mechanical energy. In the emulsion homogenization effect, the following types of shear homogenizer in the actual operation has been widely used.
1.1 Colloid mill
Colloid mill is a rotor-stator system, which is a homogenizer for emulsification and dispersion with mechanical shearing force. Rotor & stator is its main component, the rotor speed is generally 1000-10000r/min, up to 20000r/min, and the gap between stator and rotor can be adjusted according to the needs of production, in order to control the strength of the shear force and the cycle time. The disadvantage is that the residence time of material in the gap is short, generally several cycles or several colloid mill to operate together. In the unit operation, the auxiliary pump is needed to reduce energy consumption and increase the amount of processing, and because of the high speed, the shaft part needs to be equipped with a cooling water jacket to cool down.
Colloid mill is suitable for slurry, soft and high viscosity material grinding and can make the emulsion liquid droplet particle size decreases. Can be used for the production of dairy products such as ice cream, condensed milk and milk powder, and can also be used for the production of cocoa drinks and chocolate products.
1.2 High pressure homogenizer
The main components of the high pressure homogenizer are high pressure pumps and narrowed valves with holes. In the industry, high-pressure homogenizer, including constant pressure pump with variable valve and variable pressure pump with sizing hole valve two combinations. The material is forced to pass through a small orifice valve under high pressure to obtain high velocity, so that the liquid becomes strong turbulence and vortex, and the mixture is sheared and dispersed into smaller particles. High-pressure homogenizer needs to consume more energy, and consumes the most energy in several homogeneous machines discussed.
The high pressure homogenizer is suitable for the continuous emulsification and dispersion process of the less viscous liquid. Can be used in food, pharmaceutical and other departments, to process emulsion mixed or emulsified by colloid mill, so that dispersed phase droplets size homogenization.
1.3 Single stage rotor-stator homogenizer
Rotor-stator homogenizer is composed of many gear rings, the gap between each other is very small, single-stage rotor-stator homogenizer is composed of a pair of rotor and stator. When working, the rotor rotates at high speed and sucks a large amount of material through the narrow gap into the mixing zone, quickly grind solid particles and hydraulic shear droplets under action of rapid shear, collision, compression and stretching. The Single-stage units have been produced over 30 years ago and are still widely used in various industries today. Its advantages include high-strength mechanical and hydraulic shear and high flow rates at low viscosities. The mixing cycle time is only half of the conventional impeller mixing, but it can only handle materials with not very high viscosity, and the material capacity should not be too large.
The single-stage rotor and stator homogenizer is suitable for the treatment of requiring crushing particles or droplets into the size of 4-10μm low-medium viscosity materials. Typical applications include detergents, dyes, inks and rubber polymers treatment.
1.4 Multistage rotor-stator homogenizer
There are two types of multi-stage rotor-stator homogenizer, that is, many single-stage rotor and stator units connected in series and several sets of rotor and stator combined into one inline homogenizer. Their shear mechanism is similar to the single-stage rotor and stator homogenizer. As the particles or droplets through the rotor and stator homogenizer, the shear frequency increases between the stator and rotor, resulting in a higher shear force and reduce the mixing cycle time. The latter design is multi-pair rotor and stator set into each other, this homogenizer can reduce energy consumption, capital investment and maintenance costs. As the mixed material flows outwardly from the center of the mixing zone, the stator and rotor groups produce high-speed continuous shearing action to form a rapid mixing cycle, resulting in a homogeneous material with a particle size or droplet size of 5μm or less.
1.5 X series multi gear rings rotor-stator homogenizer
X series multi gear rings rotor-stator homogenizer is a relatively new rotor-stator homogenizer, can produce a very large shear rate, and the homogeneous effect and dispersion stability are better than the expensive colloid mill and multistage rotor-stator homogenizer. This unit is not equipped with conventional blades on the rotor, but the stator and rotor are all rows of staggered gear teeth. The material flows in from the center of the homogeneous head and flows out through the radial stator and rotor slot. The material is subjected to strong mechanical and hydraulic shear through each row of gear teeth. And each time through the mixing area will be subject to shearing action. Since this homogenizer can operate at high speeds, its shearing action will be widely used.
The output rate and yield of this homogenizer are much greater than colloid mill and other rotor-stator homogenizers, and are not easy to form dead zones, short residence time, easy to clean and sterilize. Suitable for obtaining submicron particles or droplet size operation, to achieve satisfactory stability, uniform appearance and texture. Can be used for homogeneous emulsification of salad dressings, fruit juices, beverage and cosmetics.
2, Selection of shear type homogenizer
With the increasingly fierce competition in the process industry at home and abroad, the selection of shear homogenizer has become more and more accurate and analytical than before. Mainly from the process improvement of mixing system, with more advanced systems to replace the traditional equipment; and also from the small aspects improvement, speed up the mixing cycle speed, improve product quality and reduce energy consumption. All of these have played an important role in the rational selection, the following from the process, parameters and practical application of the homogenizer for a reasonable selection.
2.1 According to the mixing process
There are two kinds of mixing process: batch process and continuous process. The selection of the homogenizer in the mixing process is shown in Figure 1. As you can see from the diagram, there are two kinds of batch mixing, namely low shear and high shear. Low shear homogenizer is generally used stirrer; high shear homogenizer commonly used stator and rotor structure, two mixing methods have a large number of equipment can be used. There are also two kinds of continuous mixing, also low shear and high shear. The low shear homogenizer consists of static mixer, spiral and scroll mixer.
Only when the mixed liquid has a similar viscosity, the Figure 1 can be conducive to the decision of batch or continuous homogenizer. However, the production unit is generally used continuous mixing process, because the continuous process can increase output and productivity, reduce the production cycle, cost consumption and by-product production. The combination of reaction and flow time will reduce the overall process time, and the continuous process is also beneficial to play its role and reduce costs.
2.2 According to the relevant parameters
The design of the shear homogenizer is still based on experience, there is no mature formula for reference, but must fully consider the relevant parameters. In general, the relevant parameters are mainly geometric factors, performance parameters and fluid parameters of three aspects.
The geometrical elements of shear homogenizer are the size and geometry of stator and rotor, the depth of the container and the material fluid, etc..
The performance parameters of shear homogenizer are rotor speed (motor torque), shear rate and cycle time. Rotor speed can be controlled by the frequency control of the motor, but the shear rate of the material and the final particle size of material can only be controlled within a certain range.
The fluid parameters of shear homogenizer are the viscosity of the fluid, the Reynolds number (Re) and the Weber number (We) associated with the fluid. The viscosity of the fluid to a large extent determines the choice of homogenizer, different viscosity of the material must be used different forms of homogenizer to achieve the desired homogenization effect and the least cost of consumption. And the rational selection of rotor and stator is also very important, different types of rotor and stator suitable for different viscosity of the material, it can be determined in the reasonable design of the rotor and stator, and strive to meet the homogeneous needs of various viscosity material.
2.3 According to practical application
Shear homogenizer is widely used, and different types of homogenizer have different application range in the processing of materials. Some typical practical applications of homogenizer selection are shown in Table 1.
|Practical application||Optimum selection of homogenizer||Other optional homogenizer|
|Similar viscosity liquids mixing||General agitator||Static mixer|
|Different viscosity liquids mixing||Rotor-stator homogenizer||Agitator and static mixer|
|Low viscosity solid-liquid mixing||Rotor-stator homogenizer||Agitator|
|High viscosity solid-liquid mixing||Serrated blade agitator||Rotor-stator homogenizer with stirrer|
|Low viscosity dispersion||Rotor-stator homogenizer||Agitator|
|High viscosity dispersion||Rotor-stator homogenizer with stirrer||Agitator, Colloid mill|
|High contact reaction||Rotor-stator homogenizer||Agitator|
|Immiscible liquid emulsification||High pressure homogenizer||Rotor-stator homogenizer, Colloid mill|
We should consider the viscosity of the fluid and the output of the mixing equipment in order to achieve higher homogeneous and smaller particle size. Whether the liquid mixing or solid suspension, solid-liquid mixing, from the product homogenization, stability and production efficiency, costs and so on to consider the accuracy and rationality of selection.
For different applications, of course, the advantages of all kinds of homogenizer will inevitably be brought into play, and through the combination of each other, to achieve the desired purpose.
Shear-type homogenizer as a very necessary operating unit in industrial mixing, the best choice in addition to consider some of the above parameters, and must also consider the specific problems encountered in the process of using, such as sealing, testing and so on. Although the shear homogenizer has been widely used, there is no basic for theoretical or experimental evaluation, to obtain information about their characteristics, to help users make a reasonable choice. Therefore, the development, scale-up and operation of the mixing process should be tested by experiments, relying on mechanical models, computer simulation and experiments to understand the relevant parameters of homogeneous machine. In this way, we will be very convenient to choose a reasonable operating unit for industrial production.