Equipment Doctor: How to choose a magnetic stirrer for a reactor

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　　A good method for selecting a magnetic stirrer for a reaction vessel should meet two conditions: first, the selection result should be reasonable; second, the selection process should be simple. However, it is often difficult to have both of these qualities at the same time. 　　Since the viscosity of the liquid has a significant impact on the stirring condition, selecting a model based on the viscosity of the stirring medium is a basic approach. Several typical agitators have different ranges of application depending on the viscosity level. As viscosity increases, the order of use for various agitators is progressive, turbine, paddle, anchor, and screw types, etc. The progressive type is further detailed here, with low speeds recommended for large volumes of liquid and high speeds for small volumes. This selection chart does not absolutely specify limitations on the types of slurries that can be used; in fact, the application ranges of different slurry types overlap. For example, due to its simple structure, the slurry type can have its flow pattern improved by the use of baffles, which is why it is also widely used at low viscosities. Turbine-type pulp, due to its strong convection and circulation capabilities, turbulent diffusion, and shear force, is by far the most widely used type of pulp. 　　Judging the type of slurry suitable for a mixing process based on the purpose of the mixing and the flow pattern created by the mixer is a relatively effective method. Due to the Soviet Union’s own preferences in selecting pulp types, they differ from those commonly used in our country. 　　The recommended slurry type classifies slurry types into fast and slow categories; the former operates in a turbulent state, while the latter operates in a laminar state. When selecting, the slurry type and baffle conditions are determined based on the mixing purpose and flow state; the determination of the flow state is influenced by the viscosity of the mixing medium. 　　Its operating conditions are quite specific; in addition to the slurry type and mixing purpose, there are also recommended ranges for medium viscosity, mixing speed, and tank capacity. 　　The proposed selection table also determines the appropriate option based on the purpose of mixing and the flow conditions during mixing. Its advantage lies in the fact that it defines the scope of use for different types of slurries according to the characteristics of various mixing processes, thereby enabling more specific selection. By comparing the tables above, it can be seen that the criteria for selection and the resulting choices are quite consistent. The following provides further explanation on a few of the main processes. 　　Mixing low-viscosity homogeneous liquids is the simplest type of stirring process; it becomes more difficult only when the volume is very large and a short mixing time is required. Due to its strong propulsive circulation capability and low power consumption, it is the most suitable. Turbine types, due to their high power consumption, although they possess strong shear capacity, are not particularly necessary for this mixing process; therefore, when used for mixing large volumes of liquid, their circulation capacity is insufficient. 　　For dispersed operation processes, turbine types are the most suitable due to their high shear force and large circulation capacity; in particular, straight-blade turbines have a greater shear force than those with folded or curved blades, making them even more appropriate. Push-type and paddle-type turbines can only be used when the amount of liquid to be dispersed is small, as their shear force is lower than that of straight-blade turbines; among these, paddle-type turbines are rarely used for dispersion tasks. Dispersing operations all use baffles to enhance the shearing effect. 　　Solid suspension operations have the widest application range in turbine types, among which the open-turbine type is the best. It lacks a central disc section, which prevents interference with the mixing of liquid phases above and below the blades; moreover, the curved blades offer advantages in driving the turbine – it has good discharge performance and its blades are less prone to wear, making it more suitable for operations involving solid suspension. The push-type has a limited range of application; it is not suitable when the specific gravity difference between solids and liquids is large or when the solid-liquid ratio is above 50%. When using baffles, be careful to prevent solid particles from accumulating in the corners of the baffles. Generally, baffles are used only when the solid-liquid ratio is low; whereas turbines with folding blades and propeller types both have axial flow, so baffles may not be necessary. 　　The disk-type turbine is the most suitable for the gas absorption process, as it has strong shear forces; moreover, some gas can be retained beneath the disk, enabling a more uniform distribution of the gas. This advantage is not present in turbines that are turned on. Paddle and impeller types are basically unsuitable for the gas absorption process; they can only be used in small quantities where the desired degree of dispersion of the absorbed gas is not high. 　　The crystallization process with stirring is very difficult, especially when strict control over crystal size is required. Generally, low-speed stirring with a small diameter, such as turbine-type stirring, is suitable for the crystallization of fine particles, while high-speed stirring with a large diameter, such as slurry-type stirring, can be used for the crystallization of large crystals. 　　There are many ways to classify mixers, and the following types will be introduced here: 1. Based on the structure of the blades, mixers can be divided into those with flat blades, inclined (folded) blades, curved blades, and spiral-blade mixers. Both paddle and turbine mixers have flat-blade and inclined-blade configurations; the blades of propeller, screw, and helical mixers are of helical surface design. Based on installation requirements, they can be further divided into integral and split types, which facilitate direct fixation of the mixer to the mixing shaft without the need to remove components such as couplings. 　　2. Based on the purpose of the mixer, they are divided into mixers for low-viscosity fluids and mixers for high-viscosity fluids. Stirrers used for low-viscosity fluids include: impeller type, paddle type, open turbine type, disc turbine type, Brumagin type, plate-and-frame paddle type, triple-blade backward-curved type, etc. Stirrers used for highly viscous fluids include anchor type, frame type, serrated disk type, propeller type, screw ribbon type, etc. 　　3. Based on the flow pattern of the fluid, they are divided into axial-flow mixers and radial-flow mixers. Some mixers, during operation, generate both axial and radial flow in the fluid; such mixers are known as mixed-flow type mixers. The propeller mixer is a representative of the axial flow type, the straight-blade disc turbine mixer is a representative of the radial flow type, while the inclined-blade turbine mixer is a representative of the mixed flow type. 　　Compared with the straight-blade open turbine mixer, the hexagonal-blade open turbine mixer generates not only radial flow but also axial flow; its shear performance lies between that of the straight-blade and curved-blade types. As a result, it offers superior overall performance and is suitable for tasks involving dispersion, handling of solids, suspension, and solid dissolution. It can also be used in other processes, under the same operating conditions as the hexagonal straight-blade open turbine.