Analogy amplification of fermentation tank

1、 Compared to enlarged content:
The geometric dimensions of the tank, ventilation rate, mixing power, heat transfer area, and other amplification issues are all interrelated.
2、 Basis for comparison and amplification
1. Mixing power consumption per unit volume of liquid
2. Mixing Reynolds number
3. Dissolved oxygen coefficient
4. Terminal linear velocity of agitator blade
5. Mixing time
6. By providing feedback control conditions, strive to make important environmental factors as consistent as possible.
Three Comparative Amplification and Its Basic Methods
Comparative amplification: It is a means of reproducing the results obtained from scientific experiments conducted in small equipment in large-scale production equipment. It is not proportionally amplified, but rather amplified using the method of similarity theory.
Firstly, it is necessary to identify the various parameters that characterize this system, assemble them into several dimensionless numbers with certain physical meanings, and establish functional equations between them. Then, using experimental methods, the constants and exponents contained in this functional equation can be obtained in the experimental equipment. Under certain conditions, this relationship equation can be used as a basis for analogy amplification. Comparative amplification is one of the commonly used basic methods in chemical process research and production.
Is it applicable in fermentation engineering and the comparative amplification method used in fermentation engineering
The fermentation process is a complex biochemical process, and the parameters that affect this process include physical, chemical, and biological factors. Some parameters have been recognized but cannot be accurately and quickly measured, while others have not yet been recognized.
We have only studied the relationship between a few parameters and this process, and assuming that other parameters are constant, it is actually impossible for all parameters to be constant. Therefore, the improvement of equipment in the fermentation production process relies on a deep understanding of the essence of the fermentation process, similar to the theory and technology of amplification.
The comparative amplification methods used in fermentation engineering include: equal KLa, equal π DN, equal Pg/V, equal Re or momentum factor, similar mixing time, etc.
Control and monitoring of fermentation process
1、 Monitoring content and methods of fermentation process
The significance of parameter detection in fermentation process
During the fermentation process, the process state undergoes constant changes, especially in the case of bulk fermentation where the changes are faster.
Due to their biological activity, substrates and nutrients undergo changes, resulting in an increase in biomass and changes in biomass composition (including physical, biochemical, and morphological changes), while various bioactive products accumulate. The purpose of fermentation process detection and control is to obtain the maximum desired product using as few raw materials as possible.
（1） Main indicators for monitoring the fermentation process
1. Physical detection indicators: temperature; Pressure; Mixing speed; power waste; Foam; Gas flow rate; Viscosity, etc.
2. Chemical testing indicators: pH; Oxidation-reduction potential; Dissolved oxygen; Gas CO2, O2; Sugar content; Compound content, etc.
3. Biological detection indicators: bacterial turbidity; ATP ; Various enzyme activities; Intermediate metabolites.
Of course, not all parameters mentioned above need to be tested during the fermentation process of all products, but some parameters can be selectively tested based on the characteristics and possible conditions of the product.
(Two monitoring methods)
A general monitoring system consists of three parts.
1. Measuring components: such as thermometers, pressure gauges, ammeters, and pH meters, directly measure various parameters of the fermentation process and output corresponding signals.
2. Control part: Its main function is to compare various parameter signals measured by the measuring element with predetermined values, and output signal instructions to the executing element for adjustment and control.
3. Execution element: It accepts instructions from the control part to open or close relevant valves, pumps, switches, and other regulating control mechanisms, so that the relevant parameters reach the predetermined position.
There are two types of control methods: manual control and automatic control.
1. Manual control: This is the simplest control method. For example, adjusting the fermentation temperature by controlling the cooling water (or steam flow) of the fermentation tank jacket to regulate the temperature of the fermentation broth. The manual control method is simple, does not require special additional devices, has low investment costs, high labor intensity, and can also reduce errors if controlled appropriately.
2. Automatic control: When using automatic control, it is necessary to generate output signals from the measuring components and monitor them with instruments. When measuring temperature, a thermocouple can be used instead of a thermometer and connected to the control part, which then generates a signal to drive the actuator for operation.
2、 Routine monitoring of fermentation process
1. Temperature
2. pH value
3. foam
4. Tank pressure
5. Air flow rate
6. Mixing speed
1. Temperature
Due to the metabolic activity of microorganisms utilizing carbon and energy sources, they can produce exothermic reactions,
In addition, stirring can also generate a certain amount of heat, so the speed of heating during the fermentation process can often serve as a rough reference for judging the fermentation speed.
Fermentation is normal, and when the bacterial growth and reproduction are vigorous, the natural temperature rise is faster. In the later stage of fermentation, the temperature rise is slower. In order to maintain the suitable temperature for growth, it is necessary to adjust the cooling water or steam in the heat transfer device of the fermentation tank at any time during the fermentation process to maintain the temperature of the fermentation liquid.
The simplest temperature measurement method is to observe the thermometer inside the temperature chamber on the wall of the fermentation tank. Then, according to the process specifications, when the tank temperature is too high, open the tap water (or cooling water valve) to reduce the temperature of the fermentation broth to the specified temperature. When the heating or cooling is completed, attention should be paid to the occurrence of hysteresis. Timely and reasonable control often requires certain experience and skills.
The temperature self-control method can use thermocouples, thermistor or metal resistance thermometers, which are thermally sensitive
Sensing elements can convert temperature changes into electrical signals, which are then connected to control instruments and transmitted to executing elements through various control switches or circuits. They can also turn on or off cooling or heating devices to maintain a constant tank temperature.
2. pH value
During the fermentation process, the change in pH of the culture medium is mainly determined by the composition of the culture medium and the metabolic characteristics of microorganisms. This is due to the continuous consumption and utilization of nutrients by microorganisms, as well as the secretion of various metabolic products into the culture medium.
The pH value of the culture medium reflects the final hydrogen ion concentration after microbial assimilation and differentiation of nutrients.
Obviously, under fixed cultivation conditions, there is a certain regularity in the pH changes during microbial fermentation. Mastering this change is of great significance for judging and controlling fermentation production.
Method for measuring pH:
a. Usually pH test strips can be used for measurement;
b. Accurate measurements are made using a pH meter.
At present, there are pH electrodes that can be disinfected and installed in fermentation tanks to directly measure the pH of the culture medium at regular intervals. They can also be connected to control instruments to adjust the pH through a circuit system that controls valves or pumps.
3. Detection and control of foam
The simplest detection is to regularly observe the foam generation on the sight hole of the fermentation tank. When foam continues to rise, open the valve of the defoamer storage tank and add a small amount of defoamer to make the foam disappear.
A stainless steel probe can also be installed at the top of the tank and connected to the control instrument to control the opening of the defoaming storage rate valve. The signal generated when the foam rises to contact the top of the probe will be commanded to open the pump switch or valve through the control device, and the defoamer will be automatically added. When the foam disappears, the signal will also disappear, and the valve will be closed.
4. Tank pressure
Fermentation containers are equipped with pressure measuring devices, with the most common being spring pressure gauges. Because both the cultivation process and high-pressure steam sterilization require observation of pressure changes.
During the fermentation process, the influence of air pressure on microbial growth, reproduction, and product synthesis is mainly manifested in the increase of oxygen solubility by pressure, which improves the supply of dissolved oxygen during the fermentation process. However, as the tank pressure increases, it also correspondingly increases the CO2 partial pressure, which may have adverse effects on the normal growth of some microorganisms. Single coil spring tube pressure gauge is the most commonly used pressure gauge, usually installed at the top of fermentation tanks and filters. The number it indicates represents the pressure above atmospheric pressure.
The method of controlling pressure is generally to adjust the inlet or outlet valves, change the amount of air (or gas) entering or exiting, in order to maintain the pressure required by the process specifications.
In an automatically controlled fermentation tank, Hall effect pressure gauges or various remote pressure gauges can be used, which can convert pressure into various electrical signals and connect them with instruments. The latter feedback controls the opening and closing of valves according to the pressure level to achieve the purpose of regulation.
5. Air flow rate
In fermentation production, air flow is generally expressed in terms of ventilation ratio, and is usually expressed in terms of the volume ratio of air passing through a unit volume of culture medium within one minute (V/V • m).
For example, in a fermentation tank containing 2.5m3 of culture medium, if 1.25m3 of sterile air is introduced every minute, it is called a ventilation ratio of 1:0.5, or simply ventilation rate of 0.5 (V/V • m).
The effect of ventilation on the rate of oxygen dissolution
The main manifestation is the surface linear velocity (V) of the gas, which is proportional to the dissolved oxygen coefficient (KLa). Increasing the aeration rate is beneficial for improving the dissolved oxygen rate. However, if the aeration rate is increased without maintaining the original stirring power, the density of the fermentation broth will decrease due to the increase in aeration rate, resulting in a decrease in stirring power consumption. The impact of stirring power consumption on improving dissolved oxygen will be more significant. Therefore, increasing the ventilation rate without maintaining the original stirring power is not very effective in improving dissolved oxygen.
Methods for measuring and regulating air flow rate
The simplest method for measuring air flow is the rotor flowmeter.
It is an instrument with a simple and intuitive structure, low pressure loss, and easy maintenance; Usually installed directly on the exhaust pipe of the fermentation tank.
A rotor flowmeter is basically composed of two components, one is a conical tube that gradually expands from bottom to top; The other component is a rotor placed in a conical tube that can move freely up and down. When the flow rate is large enough, the force generated by the airflow can lift the rotor and raise it. The size of the flow rate determines the position of the rotor at equilibrium. Therefore, the air flow rate can be measured from a known scale.
Air flow regulation is achieved by opening valves.
6. Mixing speed
The stirring speed of the fermentation tank is closely related to the dissolved oxygen coefficient of fermentation. Because the dissolved oxygen coefficient KLa is proportional to the stirring power consumption per unit of fermentation broth, and the power consumption is proportional to the third power of the stirring speed. So under certain geometric structural conditions, such as the ratio of the tank diameter to height, the diameter of the stirring blades, and the baffle, the dissolved oxygen coefficient (or volumetric mass transfer coefficient KLa) of the fermentation tank is mainly affected by the stirring speed.
There are three main ways in which stirring affects the dissolved oxygen coefficient:
① Stir the sterile air into small bubbles to increase the gas-liquid contact area (i.e., increase the inner surface area a), and the small bubbles rise from the bottom of the tank to the liquid level slower than the large bubbles, which also increases the gas-liquid contact time; ② The vortex motion caused by stirring prevents bubbles from directly rising from the bottom to the top of the tank, but instead causes them to spiral upwards, which also increases the rise time of the gas-liquid surface and facilitates the dissolution of oxygen;
③ The turbulent cross-section formed by stirring reduces the thickness of the liquid film, thereby reducing the liquid film resistance and increasing KLa.
At present, small fermentation tanks used in experiments all use variable speed motors, so the stirring speed can be adjusted according to the fermentation process requirements (mainly depending on the dissolved oxygen rate).
Most of the motors used in domestic industrial scale fermentation tanks have a fixed speed, which is reduced by a gearbox or pulley. Therefore, during the fermentation process, it is generally impossible to adjust the stirring speed.
If a variable speed motor is used on a large fermentation tank, although the initial investment cost is higher, the stirring speed can be adjusted according to the oxygen needs of microorganisms at different stages during the fermentation process, which will make the production process more scientific, reasonable, and economical.