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To meet the needs of different applications, the design of impeller needs to consider multiple aspects. This article is shared by the Cobetter team about the research, hoping to provide reference for you in process design and selection.
01 Factors affecting mass transfer
1. Types of Impellers
Axial Flow Impeller
Features: There is a certain angle between the blade and the axial direction, and rotation mainly generates axial flow, accompanied by a small amount of radial flow. The liquid circulates up and down along the axis;
Radial Flow Impeller
Features: The blade is parallel to the axis, and the streamline is mainly radial flow. The liquid flows out along the radial direction.
Helical Ribbon Impeller
Features: The blades are spiral shaped and generate both radial and axial flow during rotation.
Rushton Impeller
Features: A single blade has a small area but multiple flat blades, generating strong radial flow and shear forces.
2. Mass Transfer Effect
Impeller improves mass transfer efficiency by enhancing convective mixing of liquids, especially in bioreactors where uniform distribution of nutrients, gases (such as oxygen), products, and metabolites needs to be maintained. The factors that affect mass transfer include:
Rotational Speed: The higher the rotational speed of the impeller, the faster the mixing rate of the liquid, the stronger the mass transfer effect. However, excessive rotational speed may result in high shear forces.
Design of Impellers: Different designs of impeller will form different fluid flow patterns, affecting turbulence, bubble dispersion, and mass transfer interfaces in liquids.
Gas Liquid Mass Transfer (O₂ Mass Transfer): Oxygen, as an important reactant in fermentation and cell culture, promotes the transfer of oxygen from the gas phase to the liquid phase through the dispersion and fragmentation of bubbles caused by impeller.
3. Shear Force Effects
Shear force is the frictional force generated during fluid flow, which plays a double-edged sword role in bioreactors:
Positive effects: shear force can help break large bubbles, increase gas-liquid contact area, and enhance mass transfer efficiency; It can also prevent sedimentation from occurring.
Negative effects: excessive shear force may cause damage to sensitive biological systems, such as cell culture, leading to membrane rupture or cell death.
4. Turbulent and Laminar flow
Turbulent flow: The rotation of the impeller can generate turbulent flow inside the reactor, which promotes rapid mixing and transfer of substances, especially in large bioreactors, which is crucial for maintaining a uniform environment.
Laminar flow: Mass transfer under laminar flow conditions is slower at lower speeds or mild agitation, making it suitable for systems sensitive to shear forces.
02 Finite Element Analysis
Finite Element Analysis (FEA) can be used to simulate the forces acting on the impeller during operation, including shear forces. Shear force is caused by the stress in different directions, usually occurring in the transverse plane of the material. During the movement of the blade, shear force may be generated by the impact of water flow or changes in fluid dynamics. The following are the general steps for conducting finite element analysis:
1. Establish the Geometric Model
Firstly, create 3D geometric model of the blade to ensure that the model accurately reflects the actual structure as much as possible. Geometric models can be created using CAD software such as SolidWorks, AutoCAD, etc. Keeping the ratio of the tank diameter to the blade diameter constant and the blade height constant, only changing the blade shape, establish the following models: (from left to right are axial flow impeller, radial flow impeller, helical ribbon impeller, Rushton impeller.
2. Material Property
The solid of the blade is made of polyethylene material. The watershed is liquid water at 25 ° C
3. Grid Division
Discretize the geometric model into a finite element mesh. The density of the grid determines the accuracy and computation time of the analysis. Complex areas (such as stress concentration areas) require finer grids. The overall grid quality should be maintained with a minimum orthogonal mass greater than 0.1 or a maximum inclination less than 0.95.
4. Define Boundary Conditions and Loads
Boundary conditions: MRF method is used to define the boundary, and the blade rotation speed is 114rpm.
Load: The main loads borne by the blade during operation include fluid dynamics loads, centrifugal forces, and possible gravity loads. Specifically, it is possible to simulate the pressure and fluid dynamics generated by water flow on the surface of the blade, and then calculate the shear force it causes.
5. Solve
Choose an appropriate solver (usually a structural solver) to calculate the stress distribution of the blade under working conditions, with a focus on analyzing the distribution of shear stress.
6. Result Analysis
① Radial Flow Impeller: the impeller generates radial flow in the liquid, thereby achieving uniform mixing inside the liquid. Specifically, the radial flow impeller generates centrifugal force through the rotation of the stirring rotor, pushing the liquid outward in the direction of the stirrer and creating a significant shear force. In this process, the molecules of the liquid are forced to move relative to each other, thereby achieving liquid mixing and mass transfer;
② Axial Flow Impeller: The liquid is pushed in a direction parallel to the rotation axis by the blade, that is, along the axis direction of the agitator (up and down direction); the liquid is pushed towards the axis of the agitator from the blade, and when it encounters the bottom of the container or other obstacles, it will spread outwards and reflux along the container wall, forming a circulating flow. The circulating liquid flow eventually returns to the blade area, and the above process is repeated.
③ Helical Ribbon Impeller: The spiral blade pushes the liquid to move radially and axially, forming a slow and uniform liquid flow; the liquid rises or falls along the spiral direction of the impeller, usually forming a flow path from bottom to top, gradually carrying the lower layer material to the upper layer and mixing with it, thus achieving uniform mixing of the upper and lower layers.
④ Rushton Impeller: The impeller rotates around the mixing shaft, pushing the liquid horizontally outward radially; the liquid flows radially outward under the push of the blades, and when it hits the container wall or baffle, it generates strong turbulence, causing the liquid to circulate in the up and down directions along the container wall; When used in gas-liquid systems, Rushton impellers are more likely to cut gas into small bubbles, while liquid turbulence enhances the contact between the gas-liquid interface, thereby improving gas mass transfer efficiency.
03 Result Analysis
In terms of mixing and mass transfer performance, axial flow impeller, radial flow impeller, helical ribbon impeller, and Rushton impeller each have different characteristics and application areas. Their main differences are reflected in the mixed flow mode, shear force, mass transfer effect, and material viscosity.
1. Axial Flow Impeller
Features: It mainly generates up and down flow along the mixing axis (axial flow), forming a large-scale circulating flow. Low shear force, smooth fluid flow, suitable for liquids with low to medium viscosity.
Mixing and mass transfer performance:
Mixing: Axial flow impeller can form a large circulating flow, which is conducive to the rapid and uniform mixing of large volume materials in the container.
Mass transfer: Suitable for liquid-liquid or solid-liquid mass transfer, and due to its low shear force, it is suitable for situations where material structure needs to be maintained, such as biological fermentation, enzyme reactions, etc.
2. Radial Flow Impeller
Features: It mainly generates radial flow perpendicular to the stirring axis, liquid flows horizontally outward, and generates turbulence after hitting the container wall. High shear force, suitable for enhancing local mixing and turbulent diffusion.
Mixing and mass transfer performance:
Mixing: Due to strong turbulence effects, radial flow blades can quickly achieve local mixing, making them suitable for situations where turbulence and local mixing need to be enhanced.
Mass transfer: Suitable for mixed mass transfer of liquid-liquid and solid-liquid, but due to strong turbulence, the mass transfer effect is limited by the position of the stirrer and local flow, making it suitable for medium viscosity fluids.
3. Helical Ribbon Impeller
Features: Generate a spiral flow that includes both axial and radial flows, with fluid circulating up and down along the spiral direction. Low shear force, suitable for high viscosity fluids, capable of forming smooth flow, and not easily damaging material structure.
Mixing and mass transfer performance:
Mixing: Helical Ribbon Impeller can achieve effective mixing in high viscosity liquids, forming a wide range of circulating flows.
Mass transfer: Suitable for mixing and mass transfer of high viscosity fluids. Due to the small shear force, spiral flow ensures mixing uniformity, but the mass transfer efficiency is relatively low.
4. Rushton Impeller
Features: It mainly generates radial flow and strong turbulence, with high shear force, suitable for dispersing gas into small bubbles. Turbulence effect is significant, suitable for liquids with low to medium viscosity, and has good mass transfer effect in gas-liquid systems.
Mixing and Mass Transfer Performance:
Mixing: It has efficient gas dispersion ability during gas-liquid mixing and can generate a large number of small bubbles, improving the residence time of gas in liquid.
Mass transfer: It has excellent mass transfer effect in gas-liquid systems, suitable for the process requirements of rapid mixing and efficient mass transfer.
