Bioreactors are key equipment for converting biotechnology into products and productivity, and they occupy a central position in biological processes. There are various forms of bioreactors used in the biological industry. Traditional bioreactors used in the biological industry are called "fermenters", and the term "bioreactor" began to emerge in the 1970s and 1980s, gradually being widely accepted and widely used. 

          There are many types of bioreactors, including not only traditional fermentation tanks and enzyme reactors, but also animal and plant cell culture reactors such as immobilized enzymes or cell reaction belts, and photosynthetic bioreactors.

Overview of Bioreactor Scaling up Process

          The development of a biological reaction process is usually based on laboratory scale and pilot scale optimization research before being put into production in large-scale production equipment. However, when conducting the same biological reaction in reactors of different sizes, differences in mass, heat, and momentum transfer in the bioreactor may lead to differences in reaction rate and specific processes during the reaction, resulting in reaction alienation. Therefore, the comparative amplification of bioreactors is an important research topic in the biochemical industry.

① In the laboratory stage, bacterial strain screening and culture medium research are usually carried out in shaking flasks or 1-5L reactors.

② In the pilot stage, this stage refers to laboratory data for scaling up experiments to study the optimal environmental operating conditions, which are conducted in small and medium-sized bioreactors.

③ In the industrial production stage, the experimental production is aimed at providing commercial products and achieving economic benefits, usually using large-scale bioreactors.

          Studying the inherent laws and influencing factors of biological reaction systems, with a focus on solving problems related to mass transfer, momentum transfer, and heat transfer, can maintain the growth rate of biological cells and the generation rate of metabolites as much as possible during the amplification process of the reactor.

Scale up criteria for bioreactors

          For the amplification of a bioreactor, the criteria for amplification include: blade tip velocity, mixing time, oxygen transfer coefficient, and input power per unit volume.No matter which amplification strategy is used, the conversion of stirring speed, ventilation rate, etc. at different working volumes will be involved in the amplification process. This requires a lot of time and effort from developers, and often their experience is also important.

① Blade tip velocity

Stirring is an important parameter that should be paid attention to during the amplification process of cell culture. On the one hand, stirring affects the mixing, suspension, and mass transfer efficiency of substances, and increasing the stirring rate is beneficial for improving liquid mixing and gas mass transfer. However, rapid stirring also brings higher shear forces. At present, the tolerance of engineering cell lines to shear force has been greatly improved. Shear force is often reflected through blade tip velocity. The diameter and speed of the mixing blade determine the magnitude of the blade tip speed.Due to the need for tank design, the increase in tank volume will also result in an increase in the diameter of the mixing blade. Therefore, in the mode of constant blade tip speed, the stirring speed of large volume tanks is lower than that of small volumes.

② Mixing time

The two parts that are more related to the mixing effect are: achieving full contact between gas and liquid through stirring and mixing, and forming stable gas mass transfer; By stirring and mixing, the concentration of active ingredients in the liquid is uniformly distributed throughout the entire system, achieving a stable liquid culture environment.Mixing time is a relatively simple amplification criterion. In cell culture processes, small-scale cultivation volumes can quickly reach the mixing time, while larger volumes require higher tip speeds to achieve the mixing time. This will lead to an increase in shear force, causing cell damage.

③ Oxygen transfer coefficient

The oxygen transfer coefficient KLa characterizes the rate at which oxygen enters the liquid phase from the gas phase. The appropriate amount of KLa in the reactor is crucial for process amplification, as oxygen, as an important nutrient for cells, affects their normal growth and metabolism. The constant KLa amplification criterion provides cells with the same oxygen transfer environment. But the determination of KLa is influenced by many factors. For example, during the cultivation process, stirring speed, ventilation flow rate, etc. require a lot of testing and analysis to determine the appropriate KLa. In actual operation, KLa increases with the increase of working volume.

Using a stirred bioreactor, for larger scale or higher density cell cultures, the cell consumes oxygen at a faster rate and requires a large amount of oxygen. Therefore, it is usually necessary to maintain strong oxygen mass transfer through a microbubble distributor ventilation method. Amicrobubble distributor, as a micrometer level gas distributor, improves the oxygen mass transfer coefficient by increasing the surface area of bubbles. In addition, the large bubble distributor, as a millimeter level gas distributor, can bring a large ventilation volume and assist in gas removal. Therefore, it is commonly used to introduce air to remove carbon dioxide. By effectively monitoring and regulating pH and dissolved oxygen values, combined with the overall control of the pump system, a favorable growth environment can be provided for cells.

④ Input power per unit volume

The input power P/V per unit volume is related to factors such as stirring power (Np, stirring blade design), liquid density (ρ), stirring speed (N), stirring blade diameter (d), and working volume (V), which to some extent reflect the mixing effect and affect the mixing and oxygen transfer of the cultivation system.

Constant P/V is recommended as a criterion for scaling up many processes and is currently a widely adopted scaling strategy. The tolerance of comprehensive cell shear force varies, with a common P/V range of 10-40 W/m3.

Methods for Scaling up Bioreactors

① Theoretical amplification method 

The basic theoretical basis for amplification is the similarity theory, which is characterized by the simultaneous transfer of momentum, heat, and mass, as well as biochemical reactions, in either reaction system, which can be described by the same differential equation. The theoretical amplification method is to establish and solve the momentum, mass, and energy balance equations of biological reaction systems. However, due to the complexity of the biological reaction process, the dynamic equations that can fully describe the biological reaction process are extremely complex, making it very difficult to solve certain differential equations, making it difficult to fully follow the ideal process mentioned above to complete the design and scaling of the biological reactor.

② Semi theoretical amplification method 

The theoretical amplification method is difficult to solve the momentum balance equation. To solve this contradiction, the momentum equation can be simplified to only consider the flow of the liquid flow main body, while ignoring the complex flow near local areas such as the stirring impeller or tank wall. There are three types of flow patterns, namely piston flow, piston flow with liquid microelement dispersion, and completely mixed flow.

The semi theoretical method is the most common experimental research method for designing and scaling up bioreactors. However, the main model of liquid flow can usually only be obtained in small-scale bioreactors (5-30L) of experimental scale, and is not based on the actual results obtained from large-scale production systems. Therefore, using this method for amplification carries certain risks and must be verified and corrected through actual biological reaction processes.

③ Dimensional analysis method

Dimensional analysis amplification method is to maintain the dimensionless number group (referred to as the quasi number) composed of biological reaction system parameters constant during the amplification process. When scaling up a bioreactor using dimensional analysis, the first step is to conduct a systematic pattern analysis to determine the dominant mechanism in the reaction system, obtain important parameters, and construct a reasonable number. Once the required standard number is obtained, the same standard number can be taken as an equal value for both small laboratory reaction devices and large production devices, combined with reference to multiple similar conditions, which can be scaled up. However, it is not easy to obtain quasi numbers with certain physical meanings through process analysis, and sometimes balance equations cannot be established.

④ Experience amplification principle

At present, the most commonly used design is still based on experience, but generally only certain criteria can be guaranteed to be equal after amplification, such as the volume dissolved oxygen coefficient kLa, unit volume energy input (P/V), stirring blade tip linear velocity, and mixing time. Therefore, it is necessary to consider whether changes in other criteria will cause changes in flow patterns or damage to microorganisms, and make modifications accordingly.

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