What is the working principle of an air-lift fermentation tank? What are the advantages of an air-lift reactor?

Working principle of airlift loop reactor

The airlift loop reactor has no stirrer inside the reactor, and there is a flow guide tube in the center, dividing the fermentation mash into an ascending zone (inside the flow guide tube) and a descending zone (outside the flow guide tube). An air nozzle or annular air distribution pipe is installed at the bottom of the ascending zone, with many spray holes below the air distribution pipe. Pressurized sterile air is injected into the fermentation liquid through the nozzle or spray holes, with the gas speed from the air nozzle reaching 250-300 (m/s). The sterile air is injected at high speed into the rising tube, where the turbulence of the gas-liquid mixture breaks the air bubbles into smaller sizes, allowing them to closely contact the fermentation liquid inside the flow guide tube, supplying dissolved oxygen to the fermentation liquid. Due to the reduced density of the gas-liquid mixture formed inside the flow guide tube, combined with the kinetic energy of the compressed air jet, the liquid inside the flow guide tube moves upward; upon reaching the upper liquid surface of the reactor, some gas bubbles break, and carbon dioxide is discharged into the upper space of the reactor, while the discharged fermentation liquid flows from the top of the flow guide tube to the outside. The fermentation liquid outside the flow guide tube has a lower gas content, increasing its density, causing it to descend and re-enter the rising tube, forming a circulation flow that achieves mixing and oxygen transfer.

Characteristics of airlift loop reactor

The characteristics of the airlift loop reactor have been briefly mentioned earlier. Since there is no stirrer in the airlift loop reactor and it has directional circulation flow, it has multiple advantages, which will be explained in detail below.

(1) Uniform distribution of reaction solution: The uniform mixing of gas-liquid-solid three phases and the good mixing and dispersion of solution components are common requirements for bioreactors, as their flow, mixing, and residence time distribution are all affected. For many intermittent or continuous aerated fermentations, the substrate and dissolved oxygen should be as evenly dispersed as possible to ensure that the substrate concentration throughout the fermentation tank remains within the range of 0.1% to 1%, with dissolved oxygen at 10% to 30%. This is beneficial for the growth of aerobic biological cells and product generation. Additionally, it is necessary to avoid the formation of a stable foam layer on the liquid surface of the fermentation tank to prevent the accumulation of biological cells on top, which could cause damage or even death. Furthermore, the components of the culture medium, especially easily sedimentable starch-like particles, should be well suspended and dispersed. The airlift loop reactor can meet these requirements well.

(2) Higher oxygen transfer rate and efficiency: The airlift reactor has a high gas holdup and gas-liquid contact interface, resulting in a high mass transfer rate and oxygen transfer efficiency. The volumetric oxygen transfer efficiency is usually higher than that of mechanically stirred tanks, with kLa reaching up to 2000 h-1, and the oxygen transfer power consumption is relatively low.

(3) Low shear force, minimal damage to biological cells: Since the airlift reactor does not have a mechanical stirring impeller, the shear damage to cells can be minimized, making it particularly suitable for the cultivation of plant cells and tissues.

(4) Good heat transfer: Aerobic fermentation generates a large amount of fermentation heat, such as during the vigorous fermentation phase of yeast cultivation, where the fermentation heat can reach 3.0 to 4.0 × 10^5 kJ/(m3.h), while the temperature difference for heat transfer is only a few degrees (°C). Especially in summer, if non-frozen water is used, the temperature difference is only about 3-10°C, thus requiring a large heat exchange area and heat transfer coefficient. The airlift reactor, due to its high liquid circulation rate, is also convenient for installing heat exchangers on the external circulation pipeline to ensure the removal of fermentation heat and control an appropriate fermentation temperature.

(5) Simple structure, easy to process and manufacture: The airlift reactor has no mechanical stirrer inside the tank, so there is no need to install a complex stirring system, and sealing is also easy to ensure, making processing and manufacturing convenient and reducing equipment investment. At the same time, large and super-large fermentation reactors have been successfully designed and manufactured, such as the internationally renowned ICI pressure circulation fermentation tank with a volume of 3000 m3, and another "BIOHCH" reactor also exceeding 3000 m3. Even larger reactors, such as the bubbling tower type "BayerAG" reactor, have a volume of up to 13000 m3, currently used for biochemical wastewater treatment.

(6) Convenient operation and maintenance: Because the airlift fermentation tank has no mechanical stirring system, its structure is relatively simple, energy consumption is low, and operation is convenient, especially as it is less prone to leakage and contamination issues that can easily occur with mechanical stirring shaft seals.

Additionally, since there is no mechanical stirring heat generated, the total fermentation heat is relatively low, facilitating the installation of heat exchange cooling systems.