Views: 0 Author: Site Editor Publish Time: 2026-04-22 Origin: Site
1. State transition during tank discharging
First note: The situation described below is applicable to certain processes only and does not apply to all processes or operations. If not applicable, please disregard.
Typically, before discharging, the status of the fermentation tank and the fermentation broth is as follows:
1. The fermentation broth has usually reached its maximum working volume. With the aeration and agitation, the liquid level typically rises to over 80% of the total tank volume.
2. The aeration rate (e.g., a common 2 VVM) is typically at its maximum.
3. The agitation speed is usually at its maximum to maintain adequate dissolved oxygen (DO) levels.
4. In some cases, depending on DO requirements, the tank pressure may also be increased to 0.6–1.0 bar.
5. Regarding the properties of the fermentation broth itself, at the late stage of fermentation, parameters such as cell concentration, viscosity, and solid content have typically reached their peak levels.
Etc.
Figure: Reference of Fermentation Tank Status Before Discharging
Typical status required for the fermentation broth/tank during discharging:
Discharging means that the current batch has reached its endpoint, and the system needs to be transitioned from a "high-speed operation" state to a "gentle" state, ready to discharge the material to the downstream system.
The following steps are typically required:
1. For some processes, the temperature must first be reduced.
2. The aeration rate must be reduced to the minimum.
3. The pressure must be either reduced or increased to meet the feed pressure and flow rate requirements of the centrifuge — too low or too high is not suitable.
4. The agitation speed must be reduced to a low level.
5. All feeding must be stopped.
Based on this background, let's discuss a real case and the lessons learned.
2. Case Study and Lesson Learned
In one batch, a 5-ton tank was being discharged. The fermentation broth had met the discharge criteria, with high target protein expression (all other parameters were normal as well). The broth had also reached its maximum harvest volume, and the liquid level in the tank was nearly at the top. If the transfer to downstream had gone smoothly, this batch would have yielded a very substantial output.
The operator began the discharge process by cooling the broth, then reducing the agitation speed, and adjusting the inlet air flow and tank pressure. However, the air flow was improperly adjusted — the reduction was too rapid overall, and pressure control could not keep up, causing the pressure to drop relatively quickly. The on-site personnel reported that the liquid level was no longer visible inside the tank (it had already been high), and the entire tank was filled with foam. At that time, the tank design did not include an exhaust gas filter. Within less than one minute, 2,000 liters of fermentation broth had already leaked out.
The operator quickly switched to manual control, fully closed the exhaust and inlet air lines, and stopped the agitation. However, a large amount of foam mixed with fermentation broth had already escaped through the exhaust line — nearly one-third to one-half of the batch was lost.
Because the tank was kept under dead-end pressure, but the centrifuge still required some pressure to feed properly, it was necessary to maintain a certain level of inlet air and tank pressure. This made continued adjustment very painful. Whenever the air inlet was opened slightly, foam immediately rose at the liquid surface. Whenever the pressure was slightly released, the foam surged even more violently. The only way was to open the valve extremely slowly — a tiny crack at a time — wait for the foam level to subside, then open a little more again. It took nearly one hour of fine-tuning to finally stabilize the system.
3. Cause Analysis
1. First, based on process reasons, the material is inherently prone to foaming in the late stage of fermentation.
a. Some of the bacterial cells "die" or even undergo autolysis, releasing proteins, nucleic acids, cell debris, etc., into the fermentation broth, which act as "foaming agents."
b. Secondary metabolites increase during the late stage of fermentation.
c. In the late stage, the cell concentration increases, viscosity increases, and so on.
2. Additionally, cooling the fermentation broth also makes it more prone to foaming. This is primarily due to a combination of factors such as increased viscosity, stabilization of surface-active substances, delayed gas release, and cell autolysis, all of which cause foam to form more easily and become more difficult to break.
3. The disruption of the steady state is even more dangerous, especially the drop in pressure. First, the pressure drop creates an instantaneous gas flow in the exhaust pipeline, which exerts a certain suction force. Second, and more critically, as the pressure decreases, the gas dissolved in the fermentation broth rapidly flashes into a large number of bubbles. The liquid surface is instantly lifted by the foam, and the foam-liquid mixture overflows through the exhaust line. Once the foam enters the exhaust pipe, the gas-liquid two-phase flow resistance increases significantly, causing the back pressure to drop further. This creates a vicious cycle: "the lower the pressure, the more the liquid escapes; the more it escapes, the lower the pressure becomes."
Ultimately, due to improper operation, the fermentation broth escapes from the tank in an instant — "whoosh."
4. Summary and Prevention
If pressure does not need to be adjusted, the inlet air flow must be reduced gradually.
a. Extend the operation time — preferably at least 10 minutes — and reduce the flow rate to the target value in stages.
b. Do not reduce too much in each step.
If pressure needs to be adjusted at the same time, even more caution is required. Especially when reducing pressure, the approach should be similar to that for air flow reduction — alternate and slow adjustments.
At this stage, process control also poses a challenge for the design of the automation control logic. The response must be fast and timely.
If you are not confident, one alternative during tank discharging is to completely close the inlet air. When the pressure drops, introduce just a small amount of air to maintain pressure — this is also a viable approach.
The core of the matter is recognizing this risk, developing an effective control plan, documenting it in the SOP, providing training, conducting hands-on practice, and implementing dual-operator verification during operations.
5. Extra caution is required when an exhaust gas filter is installed!
It should be noted that some fermentation tanks are equipped with an exhaust gas filter. In such cases, a large amount of liquid escape will indeed not occur. However, if improper operation causes liquid to escape, the filter can become clogged.
Once the exhaust gas filter is clogged, the space above the fermentation broth becomes confined. If not careful, under a high inlet air flow rate, the pressure control may not be able to respond quickly enough. The pressure can instantly rise to the inlet air pressure limit — or even exceed it — causing the rupture disc to burst, and the fermentation broth will be forced out of the tank. This has actually happened in practice. Therefore, this situation is actually more dangerous than liquid escape.
For fermentation tanks equipped with an exhaust gas filter, the following points must be observed:
1. Strictly control the working liquid volume — do not let it be too high. Increase on-site inspections during the late stage of fermentation, and constantly monitor the liquid level and foam conditions inside the tank.
2. During the design phase, ensure a good exhaust gas condensing and reflux device as well as an exhaust gas heating device to keep the exhaust gas as dry as possible before it reaches the filter element.
3. For large tanks, a dual-path exhaust gas filtration system can be used. If one filter becomes clogged, the other can still function, preventing complete blockage.
4. If foam levels are high and an exhaust gas filter is still required, it is recommended to add a bypass line for emergency handling of the exhaust gas filter.
