The main body of the glass fermenter is made of stainless steel, usually with a turbine agitator. The connection between the stirring shaft and the tank should be aseptically sealed. The bottom of the tank is provided with an air distributor or nozzle, and the sterile air passing through the air filter is blown into the culture solution from a porous tube with a diameter of several millimeters. The stirrer is driven to rotate at a certain speed by the stirring motor placed on the top of the tank, and the swollen air is broken into small bubbles by the liquid vortex and shear force generated by the stirring turbine, which are evenly dispersed in the culture medium. In this way, the oxygen required for cell growth is provided, and the concentration of the culture medium is uniform at the same time. The charging coefficient of glass fermentation tanks is generally 70 to 80%. The system is usually also equipped with defoaming device, parametric test element, coil or jacket cooling device, etc.
Glass fermenters are suitable for most biological engineering, and have the following advantages: easy to control pH value and temperature; more research on industrial scale-up methods; suitable for continuous culture. The disadvantages are: the power consumption of stirring is relatively large; the structure is relatively complicated, it is difficult to completely disassemble and clean, and it is easy to contaminate bacteria;
The stirring system of the glass fermentation tank consists of a motor, a gearbox, a stirring shaft, a stirring paddle, a shaft seal and a baffle. A brief introduction is given below.
1. Motor and gearbox
The motor and gearbox are placed outside the tank. For small glass fermenters, single-phase electric motors can be used, while large glass fermenters are generally three-phase motors. For large glass fermentation tanks, since the speed of the motor is generally much higher than the stirring speed, the speed must be reduced through the gearbox. The laboratory small glass fermenter can be continuously variable and does not require a gearbox. In batch culture, each stage of cell growth has different requirements for shear force and oxygen transfer. In order to reduce power consumption, it is best to use an adjustable speed motor.
2. Stirring paddles
Can be divided into radial flow stirring paddles and axial flow stirring paddles according to the initial direction of the fluid motion generated by stirring. The radial flow stirring paddle pushes the fluid outward, meets the inner wall and baffle of the glass fermenter, and then turns back up and down both sides to generate secondary flow. Axial flow paddles start the flow in the axial direction. Generally speaking, a glass fermenter with an axial flow impeller has a lower power factor, and the energy consumption required to achieve the same mixing effect is much lower than that of a radial flow impeller. The shear force caused by the radial flow impeller is greater than that of the axial flow impeller, which is beneficial to break up the air bubbles, thereby increasing the overall oxygen transfer rate constant, but can cause damage to some cells. Therefore, radial flow impellers are mostly used for the cultivation of aerobic bacteria and yeast that are insensitive to shear force, while axial flow impellers are mostly used in shear-sensitive biological reaction systems. For large fermenters, the design of the mixing configuration of these two types of stirring paddles can be used to fully utilize the advantages of each.
3. Stirring shaft
The stirring shaft can extend into the tank from the top or from the bottom. The former is called upper stirring and the latter is called lower stirring. Generally speaking, the manufacturing and installation cost of the upper agitator is slightly higher than that of the lower agitator. However, when using lower stirring, the solid particles in the medium or the crystals formed by the soluble components after the water volatilizes will damage the shaft seal and increase the maintenance cost. Different sizes of vented glass fermenters have different layers of stirring paddles. Small vented glass fermenters generally have only one layer of stirring paddles, while large vented glass fermenters generally have 2 to 4 layers of stirring paddles to improve mixing and mass transfer.
Seal The main function of the shaft seal is to prevent microorganisms in the environment from invading the glass fermenter and the leakage of the culture medium. Mechanical transmission parts are often one of the main causes of bacterial contamination, so the key to shaft seal design is to avoid bacterial contamination and leakage, and aseptic sealing materials should be used as much as possible.
In order to prevent large vortexes on the liquid surface during stirring, and to promote the mixing of fluids in the tank in all directions, the baffle is also installed on the tank body corresponding to the stirring paddle. The baffle is designed to satisfy the “full baffle condition”. The so-called full baffle condition means that when a baffle or other accessories are added to the glass fermenter, the stirring power is no longer increased. The number of baffles is usually 4 to 6, and the width is 0.1 to 0.12D. The full baffle condition is the minimum condition to achieve the elimination of liquid surface vortex.