Working principle of airlift fermentor

Working principle of air lift loop reactor
An air lift loop reactor is a type of reactor that does not have a stirrer, but has a central guide tube that divides the fermentation broth into an ascending zone (inside the guide tube) and a descending zone (outside the guide tube). An air nozzle or a circular air distribution tube is installed at the lower part of the ascending zone, and there are many spray holes below the air distribution tube. Pressurized sterile air is sprayed into the fermentation broth through nozzles or spray holes, with a velocity of up to 250-300 meters per second. The sterile air is rapidly sprayed into the riser, and the turbulent flow of the gas-liquid mixture causes the air bubbles to be divided into fine particles, which come into close contact with the fermentation broth in the guide tube and supply dissolved oxygen to the fermentation broth. Due to the decrease in density of the gas-liquid mixture formed inside the guide tube, coupled with the kinetic energy of the compressed air jet, the liquid inside the guide tube moves upward; After reaching the upper liquid level of the reactor, a part of the gas bubbles are broken, and carbon dioxide is discharged into the upper space of the reactor. The fermentation liquid that discharges some gas flows from the top of the guide tube to the outside of the guide tube. The fermentation liquid outside the guide tube decreases due to its low gas content and increased density, and then enters the riser again to form a circulating flow, achieving mixing and dissolved oxygen mass transfer.
Characteristics of Airlift Loop Reactor
The characteristics of the gas lift loop reactor have been briefly mentioned earlier. Due to the absence of a stirrer and directional circulation flow in the gas lift loop reactor, it has multiple advantages, which will be explained in detail below.
(1) Uniform distribution of reaction solution: The uniform mixing of gas, liquid, and solid phases and the good 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 aeration fermentations, the substrate and dissolved oxygen are evenly dispersed as much as possible to ensure that the concentration of the substrate in the fermentation tank is within the range of 0.1% to 1%, and the dissolved oxygen is between 10% and 30%. This is beneficial for the growth and product generation of aerobic biological cells. In addition, it is also necessary to avoid the formation of a stable foam layer on the liquid surface of the fermentation tank, so as to prevent biological cells from accumulating on the top and being damaged or even dead. Moreover, the components of the culture medium, especially the granular materials with starch that are prone to settling, should be able to suspend and disperse. The air lift loop reactor can effectively meet these requirements.
(2) Higher dissolved oxygen rate and efficiency: Gas lift reactors have a higher gas holdup and a higher gas-liquid contact interface, resulting in high mass transfer rate and dissolved oxygen efficiency. The volumetric dissolved oxygen efficiency is usually higher than that of mechanical stirred tanks, with a kLd of up to 2000 hours and relatively low dissolved oxygen power consumption.
(3) Low shear force and minimal damage to biological cells: Due to the lack of mechanical stirring impellers in the air lift reactor, the shear damage to cells can be minimized, making it particularly suitable for the cultivation of plant cells and tissues.
(4) Good heat transfer: Good fermentation produces a large amount of fermentation heat, such as 3.0-4.0 × 105kJ/(m3. h) during the vigorous period of yeast cultivation, while the heat transfer temperature difference is only a few degrees (℃), especially in summer, if non chilled water is used, it is only about 3-10 ℃, so a large heat transfer area and heat transfer coefficient are required. The gas lift reactor has a high comprehensive liquid circulation rate and is convenient for installing heat exchangers on the external circulation pipeline to ensure the removal of fermentation heat and control of suitable fermentation temperature.
(5) The structure is simple and easy to manufacture: there is no mechanical stirrer inside the air lift reactor tank, so there is no need to install a complex stirring system, and the sealing is also easy to ensure, so the processing and manufacturing are convenient and the equipment investment is low. At the same time, the design and manufacturing of large-scale and ultra large fermentation reactors have also been realized, such as the internationally renowned ICI pressure cycle fermentation tank with a volume of 3000m3, and another type of "BIOHCH" reactor with a volume of over 3000m3. Larger reactors such as the bubble tower "Bayer AG" reactor have a volume of up to 13000m3 and are currently used for biochemical wastewater treatment.
(6) Easy operation and maintenance: Due to the lack of mechanical stirring system in the air lift fermentation tank, its structure is relatively simple, energy consumption is low, and operation is convenient. Especially, it is not prone to leakage and bacterial contamination problems that may occur with mechanical stirring shaft seals.
In addition, due to the absence of mechanical stirring heat, the total heat generated during fermentation is relatively low, making it easier to install heat exchange and cooling systems.
Fermentation tank operation technology
1. Technical preparation
Before using the equipment, it is necessary to check whether the power supply is normal, and whether the air compressor, microcomputer control system, and circulating water system can work properly. Check if the valves, joints, and fastening screws on the system are tightened. Start the air compressor and use a pressure of 0.15 MPa to check whether the sealing performance of the seed tank, fermentation tank, filter, pipeline, and valve is good. The temperature meter, speed regulating motor, and capacitive vortex flowmeter should be inspected and calibrated according to the user manual.
2 Empty Elimination
Before feeding, the gas path, material path, seed tank, fermentation tank, and defoaming tank must be sterilized with steam to eliminate all dead corners of bacteria and ensure that the system is in a sterile state.
2.1 Air dissipation of air pipelines
(1) There are three-stage pre filters, refrigerated dryers, and sterilization filters on the air pipeline. Pre filters and refrigerated dryers cannot be sterilized with steam, so before steam is passed through the air pipeline, the valve leading to the pre filter must be closed to allow steam to pass through the pressure reducing valve and steam filter before entering the sterilization filter.
(2) The filter element of the sterilization filter cannot withstand high temperature and pressure, therefore, the steam pressure reducing valve must be adjusted at 0.13Mpa and not exceed 0.15MPa.
(3) During the disinfection process, the exhaust valve at the bottom of the sterilization filter should be slightly opened to remove condensed water.
(4) The air disinfection time should last for about 40 minutes. When the equipment is first used or started after long-term disuse, it is best to use intermittent air disinfection, that is, after the first air disinfection, it should be air disinfected again every 3-5 hours to eliminate spores.
(5) After air disinfection, the filter should be ventilated and dried for about 20-30 minutes, and then the air circuit valve should be closed.
2.2 Empty sterilization of seed tanks, fermentation tanks, alkali tanks, and defoamers
(1) Seed tanks, fermentation tanks, alkali tanks, and defoaming tanks are used to directly introduce steam into the tanks for air disinfection.
(2) When using empty sterilization, the inoculation port, exhaust valve, and material path valve on the tank should be slightly opened to allow steam to be discharged through these valves, while maintaining the tank pressure at 0.13-0.15Mpa.
(3) The idle time is 30-40 minutes, and in special circumstances, intermittent idle can be used.
(4) Before emptying the seed tank, fermentation tank, defoaming tank, and acid-base tank, the water inside the jacket should be drained. After the air disinfection is completed, the condensed water in the tank should be drained and the drain valve should be opened to prevent negative pressure and equipment damage in the tank after cooling. During air disinfection, removing dissolved oxygen and pH electrodes can extend their service life.
3 actual consumption
Disinfection is the process of using steam to sterilize the culture medium after adding it to the tank. The operation process for sterilization in seed tanks, alkali tanks, defoamers, and fermentation tanks is the same. After the empty sterilization is completed, the prepared culture medium should be added to the tank as soon as possible from the feeding port, and there should be no cooling water inside the jacket at this time. The culture medium should be gelatinized before entering the tank. Generally, the formula amount of the culture medium is about 70% of the full volume of the tank (about 65% for the medium with more foam, and 75%. 80% for the medium with less foam). Considering the factors of condensate and inoculation amount, the amount of water added is about 50% of the full volume of the tank. The amount of water added is related to the factors such as the temperature of the culture medium and the steam pressure, which needs to be explored in practice. First, turn on the mechanical stirring device to evenly mix the materials in the tank at a speed of 50-100rpm. Open the jacket steam valve and exhaust valve, preheat the culture medium inside the tank. When the temperature inside the tank rises to 90 ℃, close the jacket inlet valve, open all inlet valves inside the tank, and introduce steam. When the temperature of the tank rises to 105 ℃, slowly open the exhaust valve to exhaust the cold air at the top of the tank. After five minutes, close the exhaust valve. When the tank pressure rises to 0.12Mpa and the temperature rises to 121-123 ℃, control the opening of the steam valve, maintain the tank pressure constant, and stop steam supply after 30 minutes. Open the inlet and outlet valves of the cooling water, and cool the jacket with water. When the pressure inside the tank drops to 0.05Mpa, slightly open the exhaust valve and inlet valve to perform ventilation and stirring, accelerate the cooling rate, and maintain the tank pressure at 0.05Mpa until the tank temperature drops to the inoculation temperature.
4 Vaccination
Using flame sealed inoculation, alcohol cotton, pliers, tweezers, and inoculation rings should be prepared in advance before inoculation. The strain is loaded into a triangular flask, and the inoculation amount is determined according to the process requirements. Wrap the alcohol cotton around the inoculation port and ignite it. Use pliers or an iron rod to unscrew the inoculation port. At this time, ventilate the can to allow air to escape from the inoculation port. Pour the bacterial strain from the triangular flask into the jar in the middle of the fire ring. Sterilize the vaccination port cover on the flame and tighten it. After vaccination, the culture can be ventilated and the pressure in the tank should be maintained at 0.05Mpa