The design of bioreactors is primarily determined by the method of energy supply and aeration of the medium. Based on these criteria, bioreactors can be categorized into the following types:
- Bioreactors with energy supply to the gas phase
- Bioreactors with energy supply to the liquid phase
- Bioreactors with combined energy supply
Bioreactors with Energy Supply to the Gas Phase
In this type of bioreactor, aeration and mixing of the culture medium are carried out using compressed air, which is introduced into the system under specific pressure. These include:
- Bubbling Reactors: Air is supplied through bubbling devices located at the bottom of the apparatus, ensuring efficient gas exchange.
- Airlift Bioreactors (Diffuser-Based): These contain an internal cylinder-diffuser that facilitates mixing of the substrate and air, which enters through distribution pipes at the lower part of the system.
- Tubular (Gas Lift) Bioreactors: These consist of a shell-and-tube structure, where liquid moves upward through an air stream, enters a separator, and returns for recirculation.
- Nozzle Air Distribution Bioreactors: Equipped with air supply nozzles at the lower section and a diffuser above, ensuring internal liquid circulation.
- Column-Type Bioreactors: Cylindrical columns segmented by horizontal plates, where air bubbles pass through each liquid layer, allowing for countercurrent movement of the liquid and gas phases.
Bioreactors with Energy Supply to the Liquid Phase
In these systems, energy is introduced directly into the liquid phase, with notable designs including:
- Self-Priming Turbine Apparatus: Comprising a cylindrical diffuser and an agitator with hollow blades, this system generates a vacuum during rotation, drawing in air to promote liquid circulation.
- Turbo-Ejector Agitator Bioreactors: Featuring vertical partitions that divide the bioreactor into sections, each containing a self-priming turbine agitator and a diffuser, facilitating liquid movement between sections.
Bioreactors with Combined Energy Supply
These bioreactors integrate energy supply to both the gas and liquid phases, ensuring optimized aeration and mixing. The system typically consists of a cylindrical vessel equipped with both a mechanical stirrer and a bubbler, positioned beneath the lower tier of the stirrer.
Bioreactor Classification by Mixing Method
Bioreactors can also be classified based on their mixing mechanisms:
- Mechanical Mixing: Utilizes a central shaft and variously shaped blades. Aeration can be enhanced by bubbling, with a mechanical vibrator aiding in the dispersion of air into fine bubbles.
- Pneumatic Mixing: Mixing and aeration are enhanced through rotating discs with perforations or bottom propellers. Some designs incorporate a diffuser positioned above the bubbler.
- Circulating Mixing: Features pumps or ejectors that generate a directed liquid flow through a closed circuit. Some models combine pneumatic and circulating mixing, such as “falling jet” and “submerged jet” systems, as well as ejector-based mixing. Circulating systems often contain solid packing materials to improve mixing efficiency.
Structural and Operational Considerations
Bioreactors are typically cylindrical, hermetically sealed containers, with a height-to-diameter ratio of 2:1 or 2.5:1. Stainless steel is the preferred material for construction, ensuring durability and sterility. Temperature regulation is achieved through a double casing or coil-type heat exchanger.
Sterility and Performance Considerations
Maintaining sterility is crucial for bioreactor operation. To achieve this, bioreactors are designed to be airtight, with all pipelines and internal surfaces accessible for high-temperature steam sterilization. The working volume of a bioreactor typically does not exceed 70% of the total capacity.
The choice of bioreactor depends on the specific requirements of the biotechnological process, including the nature of the microorganism, medium properties, and economic feasibility. For aerobic processes, aeration efficiency is critical. Oxygen transfer rates must be optimized by evaluating the balance between oxygen intake from the gas phase and its mass transfer into the liquid medium, as well as oxygen consumption by microorganisms and its removal from the system.
The rate of oxygen transfer from the gas phase to the liquid phase is defined by the volumetric absorption rate and can be expressed by the equation:
dC/dt = KLa (Cp – C),
where:
- KLa is the volumetric mass transfer coefficient at the gas-liquid interface,
- Cp represents the equilibrium oxygen concentration in the medium,
- C is the actual instantaneous oxygen concentration in the medium.
By optimizing these parameters, bioreactors can achieve efficient mass transfer, ensuring optimal microbial growth and bioprocess performance.
