Air nozzles, or annular air distribution pipes, are installed in the lower part of the rising area, and there are many nozzle holes below the air distribution pipes. Pressurized sterile air is injected into the fermentation broth through nozzles or nozzle holes, and the air speed injected from the air nozzle can reach 250~300 (m/s). The aseptic air is sprayed into the rising pipe at high speed, and the air bubbles are divided into small pieces by the turbulent action of the gas-liquid mixture, which is in close contact with the fermentation liquid in the guide tube to supply dissolved oxygen in the fermentation liquid.
Due to the reduced density of the gas-liquid mixture formed in the guide tube and the jet kinetic energy of the compressed air, the liquid in the guide tube moves upward; after reaching the upper liquid level of the reactor, a part of the gas bubbles are broken, and the carbon dioxide is discharged into the upper space of the reactor. The fermentation liquid that discharges part of the gas flows from the top of the guide tube to the outside of the guide tube. The fermentation liquid outside the guide tube has a small gas content and its density increases, while the fermentation liquid decreases. It enters the riser again to form a circulating flow to realize mixing and dissolved oxygen mass transfer.
Features of Fermentation Tanks:
The characteristics of the airlift loop reactor have been briefly mentioned above. Since there is no agitator in the airlift loop reactor and there is a directional circulating flow, it has many advantages, which will be explained in detail below.
(1) The reaction solution is evenly distributed: Uniform mixing of gas-liquid-solid three-phase and good mixing and dispersion of solution components are common requirements of bioreactors, because flow, mixing and residence time distribution are all affected. For many aerated fermentations with intermittent or continuous feeding, the substrate and dissolved oxygen are dispersed as uniformly as possible to ensure that the concentration of the substrate everywhere in the fermenter falls within the range of 0.1% to 1%, and the dissolved oxygen is 10% to 30%.
This is beneficial for the growth and product production of aerobic biological cells. In addition, it is necessary to avoid the formation of a stable foam layer on the liquid surface of the fermenter, so as to avoid the accumulation of biological cells bioreactor on it and damage or even death. There are also medium components, especially starchy granular materials that are easy to settle, and should be able to be suspended and dispersed. Airlift loop reactors can meet these requirements well.
(2) Higher dissolved oxygen rate and dissolved oxygen efficiency: The airlift reactor has high gas holdup and specific gas-liquid contact interface, so it has high mass transfer rate and dissolved oxygen efficiency. The volumetric oxygen efficiency is usually higher than that of the mechanical stirring tank, the kLd can reach 2000h, and the dissolved oxygen power consumption is relatively low.
(3) The shear force is small, and the damage to biological cells is small: Since the airlift bioreactor has no mechanical stirring impeller, the shear damage to the cells can be reduced, and it is especially suitable for the cultivation of plant cells and tissues.