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Types of bioreactor blades, how to choose?

    Bioreactor Blade Types and Applications

    In designing a bioreactor, paddles serve a dual purpose:

    • On the one hand, the rotating paddle can drive the liquid in the reactor to move, thereby realizing the mixing of the substances in the liquid, which is beneficial for the cells in the whole reactor to absorb nutrients.
    • On the other hand, the rotation of the paddle will lead to the formation of a velocity gradient inside the fluid, which will generate shear force. When the shear force reaches a certain value, it will cause cell damage, which is not conducive to cell culture.
    • The structure and rotation speed of the blade determine the mixing effect and the shear force generated in the reactor. These two factors are often contradictory, that is, the shear force generated when the mixing effect is good is large, and the shear force generated when the mixing effect is poor. The key to the design of the bioreactor blade is how to minimize the shear force generated by the stirring while ensuring the mixing effect.

    According to the state of liquid flow caused by stirring, the blades can be divided into:

    • Radial flow
    • Axial flow
    • Mixed flow

    Radial flow blade

    Axial flow blade

    As shown in FIG:

    When the liquid flow is pushed from the shaft of the stirring paddle to the inner wall of the tank, a radial flow is formed. When the liquid flow is pushed up and down along the shaft of the stirring paddle, an axial flow is formed. Similarly, when the stirring paddle drives the liquid to flow both radially and axially is a mixed flow.

    Representative of the current radial flow blade is the Rushton blade

    • Today Rushton impeller has become the name of the flat blade or disc turbine impeller. The blades of this kind of stirring paddle are flat and vertically distributed along the stirring shaft. When there is a baffle plate, two circulating flow fields can be formed from the paddle as the boundary, and the shear force generated is relatively large. Unidirectional flow, so the mixing effect is also poor, and the viscosity range of the material that can be processed is the widest. Usually, the rotation speed range is 100~600rpm.
    • Based on the above advantages, and the gas absorption process, some gas can be stored under the disc, and the gas can be dispersed more smoothly. Rushton and other Rushton type stirring paddles are widely used in cells under aerobic conditions that are not sensitive to shear force. Line cultures such as yeast, bacteria, plant cells, and certain molds.

    Representative blade of the axial flow blade is the propeller stirring blade

    • Also known as propelling paddles, the front of the paddle can be flat or concave, but the back is concave. When rotating, the liquid is discharged forward in an axial flow to form a circulation in the tank for generating axial flow. This kind of blade has a high circulation speed, but a large amount of circulation, and the shear force generated during stirring is small, but like the radial flow blade, it is a unidirectional flow generated, so the mixing effect is poor, and it is commonly used. For low viscosity fluids, the common speed range is 100~500rpm.
    • This type of paddle and propulsion blades are basically not suitable for gas absorption process, and can only be used when a small amount of easily absorbed gas or a low degree of dispersion is required. It is mainly used for the mixing of liquid-liquid systems, to homogenize the temperature, to prevent sludge sedimentation in low-concentration solid-liquid systems, or some specific anaerobic cultivation.

    Representative of the mixed flow blade is the Elephant Ear blade

    • This type of paddle has flat paddles at an angle to the agitator shaft and produces both axial and radial flow. This combined flow can achieve better mixing effect, and the shear force generated is also small, and the rotation speed is generally not high during operation, about 1~100rpm, which is used for low viscosity fluids.
    • When the fan width is 120°, the angle between the blade and the shaft is 45°, and the ratio of the blade diameter to the inner diameter of the tank is 1:3, the Elephant Ear blade has better mixing performance and less damage to cells. , suitable for the culture of animal cells.

    Paddle mixer

    • This type usually has only two blades and is the simplest type of stirring blades. According to the shape and characteristics of the blades, it can be divided into flat paddle agitators and inclined paddle agitators, as shown in the following figure. The flat paddle agitator produces radial force, and the inclined paddle agitator produces axial force.
    • It is mainly used when discharge flow is necessary. Since the power consumption of the axial flow impeller is lower than that of the radial flow under the same displacement, the axial flow impeller is used more.

     Its main uses are:

    • Used to prevent separation and make temperature uniform in liquid-liquid system
    • It is mostly used to prevent solid sedimentation in solid-liquid system

    Slant slurry type (Axial flow impeller) Flat slurry type (Radial flow impeller)

    • However, paddle impellers are not suitable for gas-holding and miniaturization operations such as gas-liquid dispersion. In addition, because it is suitable for making a large blade diameter, it can also be used for stirring high-viscosity liquids. In this case, in order to promote the exchange of liquid up and down, or use 3-5 layers of multi-layer impeller, or use the deformation as shown in the figure below paddle impeller.
    • For low-viscosity liquids, the ratio of the blade diameter of the paddle impeller to the tank diameter is 0.35~0.5, and for high-viscosity liquids, it is 0.65~0.9; the rotating speed used is 20~100r/min.