The stirrer is the core component of the mixing tank. The mixing tank can realize the process of emulsifying, shearing, mixing, dispersing, reacting, preparing, blending and filtering for materials. It covers the petrochemical industry, fine chemical industry, metallurgy, daily chemical, food, medicine, pesticides, fertilizers, building materials, inks, dyes and other chemical industries, widely used. This article mainly introduces the principle, function and types of the stirrer.
An agitator is a device that forces liquid and gaseous medium to be forced convection and mixed uniformly.. The type, size and speed of the stirrer have an effect on the distribution of the stirring power between the overall flow and the turbulent pulsation. In general, the power distribution of the turbo agitator is favorable for turbulent pulsation, while the propeller stirrer is advantageous for overall flow. For the same type of stirrer, under the same power consumption conditions, large diameter, low speed of the stirrer, the power is mainly consumed in the overall flow, is conducive to macro-mixing. Small diameter, high speed agitator, power consumption mainly in turbulent pulsation, is conducive to micro-mixing.
During the mixing process, for low-viscosity media, with a small diameter, high-speed stirrer can drive the surrounding fluid circulation, and to the far. And high viscosity medium fluid is not, need to be directly driven by a stirrer. Suitable for low viscosity and medium viscosity impellers are paddle type, open turbine type, propulsion type, long blade propeller type, disc turbine type, Brumagin-type, plate frame paddle type, clover curved type, MIG type, etc.. Impellers suitable for high viscosity and extra high viscosity liquid are ribbon impeller, screw type, anchor type, frame type, propeller type, etc.. Some of the fluid viscosity changes with the reaction, you need to use the wide viscosity impeller, such as fullzone impeller.
The function of agitator
1, Mutual dissolution of liquids
Two or more liquids dissolving and mixing, but the mixing of the homogeneous liquid should distinguish whether threr is chemical reactions in the homogeneous mixture. For the absence of a chemical reaction, it is usually called the intermixing or mixing of the miscible liquid. For the case of chemical reactions between two or more miscible liquids, such as some transposition reactions, addition reactions, stirring should be carried out in order to accelerate the reaction or to complete the reaction, and this stirring is different with the mixing without chemical reaction. The stirrer is good or not, is to evaluate the mixing effect, the general evaluation index of the mixing effect is the mixing time, the shorter the mixing time, the stirrer the better.
2, Dispersion of immiscible liquids
The purpose of this operation is that the immiscible liquids in contact with each other and sufficiently dispersed with each other to facilitate mass transfer or chemical reactions, or to prepare suspensions and emulsions. When the extraction, mass transfer or chemical reaction under the action of stirring, the evaluation index is the mass transfer speed and reaction time, and the effect of stirring is to make the liquid phase dispersing and disintegrating, increase the liquid phase contact area, increase the mass transfer coefficient and reaction speed, in the preparation of suspensions and emulsions, the stirring makes the droplet refining to improve the relative contact area.
3, Gas and liquid contact
This kind of stirring is similar to the contact of immiscible liquids, the gas becomes fine bubbles, evenly dispersed in the liquid phase to form stable dispersate, or improve the mass transfer coefficient, enhance the liquid absorption of gas, with gas-liquid phase chemical reaction and so on. The evaluation index is when the gas flow rate is constant, the gas in the liquid phase dispersion effect is good, high mass transfer rate.
1, Propeller stirrer
Propeller agitator is a widely used axial-flow type high-performance stirrer. It is composed of 2~3 propeller blades, with higher working speed, and the circumferential speed of the blade outer edge is generally 5~15m/s. Convective circulation at low speed, turbulence at high speed, the larger blade angle and blade twist can make the mixer in the transition flow or even turbulence can reach a higher flow field, the discharge capacity is 30% higher than the traditional propeller type stirrer. Propeller-type agitator mainly produces axial flow, resulting in a large amount of circulation, suitable for mixing low viscosity (<2Pa·s) liquid, emulsion and solid particles less than 10% of the suspension mixing, dissolution, solid suspension, heat transfer, reaction mass transfer, extraction and crystallization operations. The rotating shaft of the stirrer can also be inserted horizontally or obliquely into the tank. At this time, the circulating circuit of the liquid flow is asymmetrical, which can increase turbulence and prevent the liquid surface from sagging.
2, Turbine agitator
Turbine agitator is a kind of widely used mixer, which can effectively complete almost all mixing operations, and can handle a wide range of viscosity of the fluid. Composed of 2~4 flat or curved blades mounted on a horizontal disc. The ratio of the blade outer diameter, width and height is generally 20:5:4, the circumferential speed is generally 3~8m/s, rotate speed is 300~600r/min. Turbine in the rotation causes highly turbulent radial flow, its main advantages are low energy consumption, high mixing efficiency and the mixing produces a strong radial flow. With large shearing force, the fluid micelles can be dispersed very fine. which is applicable to the dispersion of gas and immiscible liquids and the liquid-liquid phase reaction process. The viscosity of the stirred liquid is generally not more than 25Pa·s.
3, Paddle stirrer
There are two kinds of flat paddle and oblique paddle. The flat paddle stirrer consists of two straight blades. The ratio of diameter and height of the blade is 4:10, the circumferential velocity is 1.5 ~ 3m/s, and the radial flow velocity is smaller. The two blades of the oblique paddle stirrer are reversed by 45° or 60°, resulting in axial flow. The paddle stirrer is simple in structure and often used for the mixing of low viscosity liquids and the dissolution and suspension of solid particles.
4, Anchor type stirrer
The anchor stirrer is simple in structure and is suitable for fluid stirring at a viscosity of 100Pa·s. When the fluid viscosity is between 10~100Pa·s, a horizontal blade can be added to the anchor paddle, called gate stirrer, in order to increase the mixture in the container. The outer edge of the blade is consistent with the inner wall of the mixing tank, with only a small gap between them, which can remove the viscous reaction product attached to the groove wall or the solid material deposited on the bottom of the tank to maintain a good heat transfer effect. The circumferential speed at the outer edge of the blade is 0.5 ~ 1.5m/s, which can be used to agitate the Newtonian fluid with viscosity up to 200Pa·s and the pseudo plastic fluid (see viscous fluid flow). When mixing high viscosity liquid, there is a large stagnation area in the liquid layer.
5, Ribbon agitator
The blade of the ribbon stirrer is spiral, and the number of ribbon is two to three, installed on the middle screw of the mixer, the outer diameter of the ribbon is equal to the pitch. It is specially used for mixing high viscosity liquid (200~500Pa·s) and pseudo plastic fluid, and usually operates in laminar flow.
6, Magnetic stirrer
The digital heater has a closed-loop knob to monitor and adjust the stirring speed. The microprocessor automatically adjusts the motor power to accommodate water quality, viscous solution and semi-solid solution.
7, Magnetic heating stirrer
The digital heating stirrer comes with an optional external temperature controller that monitors and controls the temperature in the container.
8, Folding blade stirrer
According to the physical properties, capacity and mixing purpose of different media to choose the corresponding agitator is very important to promote the speed of chemical reaction and increase the production efficiency. The folding blade turbine mixer is generally suitable for the reaction of gas and liquid phase, and the rotate speed of the agitator shall generally be more than 300r/min.
9, Variable frequency double stirrer
The base, support rod and motor of the variable frequency mixer are fixed by patent technology. Patent chuck, no loosening, no swing, no fall off, safe and reliable. Chromium plated support rod, tapering, strong rigidity, reasonable structure. Has the advantages of convenient movement, light weight, etc., and suitable for various small containers.
10, Side entry mixer
The side entry mixer is the mixing device installed on the side wall of the cylinder. The agitator on the mixer is usually of axial flow type, mostly propeller agitators, and the maximum stirring effect can be obtained under the same power consumption, the power consumption is only 1/3~2/3 of the top entry mixer, the cost is only 1/4~1/3 of the top entry mixer. Speed can be 200~750r/min. Widely used in desulfurization, nitrification and the mixing of a variety of large tanks or storage tanks. Especially in large storage tanks, one or more side entry mixers work together, in the case of low consumption of energy can get a good mixing effect.
Applicability of the stirrer
Propeller stirrer - for low viscosity fluid mixing, high circulation capacity, low power consumption, can be applied to a large volume of the mixing container.
Turbine stirrer - a wide range of applications, all kinds of mixing operations are applicable, but the fluid viscosity should not exceed 50Pa·s.
Paddle stirrer - simple structure, widely used in small volume of fluid mixing, lack of circulation capacity in large volume of fluid mixing.
Anchor type, screw type, ribbon type - suitable for high viscosity fluid mixing.
Measures to improve the mixing effect
1, Install baffles
Install the baffle, not only can improve the degree of turbulence of the liquid, but also change the tangential flow into axial and radial flow, to stop swirling phenomenon. After the installation of the baffle, the liquid surface subsidence disappeared, the liquid flow inside the tank to form turbulence, so that significantly improved mixing effect.
2, Eccentric installation of the stirrer
The eccentric installation or eccentric and slanting installation of the agitator not only can effectively prevent the swirling phenomenon, but also increase the turbulence of the liquid so as to improve stirring effect.
3, Set the draft tube
The draft tube is a cylinder, its function is to cause the liquid discharged from the blades to form an axial circulation flow inside and outside the draft tube. The draft tube can restrict the liquid flow path in the kettle, forcing the liquid in the kettle to pass through the strong mixing zone in the draft tube, which not only improves the circulation flow and the mixing effect, but also helps to eliminate the short circuit and the flow dead zone.
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