Question

Explain the working of a simple stirred-tank bioreactor.

Hint:

Imagine a large mixing bowl with ingredients (nutrients, cells, oxygen) being constantly stirred to ensure everything is evenly distributed and the conditions (temperature, pH) are just right for growth.

Answer 1

A simple stirred-tank bioreactor is one of the most commonly used types of bioreactors for industrial bioprocesses, including the production of pharmaceuticals, enzymes, and biofuels. Its working principle relies on creating a homogeneous and controlled environment for the growth of microorganisms or cells. The key components and their functions are:
(A) Vessel: The bioreactor consists of a cylindrical vessel, typically made of stainless steel, which can range in size from a few liters to thousands of liters. The vessel is designed to be sterilizable to maintain aseptic conditions.
(B) Agitation System (Impeller/Stirrer): A mechanical stirrer or impeller is centrally mounted inside the vessel. It is driven by a motor and rotates to mix the contents of the bioreactor. The stirring ensures: \begin{itemize}
(C) Uniform Distribution of Nutrients: The stirrer helps to evenly distribute the culture medium, containing essential nutrients required for the growth of the microorganisms or cells, throughout the vessel, ensuring that all cells have access to the necessary resources.
(D) Oxygen Transfer: For aerobic processes, oxygen is supplied into the bioreactor, usually through a sparger. The stirring enhances the dissolution of oxygen in the liquid medium and its uniform distribution to the cells. Oxygen transfer is often a rate-limiting step in large-scale cultures, and efficient mixing is crucial.
(E) Temperature Control: The bioreactor is equipped with a jacket or internal coils through which heating or cooling fluids can be circulated to maintain the optimal temperature for the biological process. Uniform temperature distribution is achieved through stirring.
(F) pH Control: Probes are used to monitor the pH of the culture, and acids or bases can be added in a controlled manner to maintain the optimal pH for cell growth and product formation. Stirring ensures rapid and uniform mixing of these pH-adjusting agents.
(G) Prevention of Sedimentation and Aggregation: Mixing helps to keep the cells in suspension and prevents them from settling at the bottom of the vessel or forming large clumps, which could hinder nutrient and oxygen transfer. \end{itemize}
(H) Sparger: For aerobic cultures, a sparger (a porous device) is located at the bottom of the vessel to introduce air or oxygen into the culture medium in the form of small bubbles, increasing the surface area for gas transfer.
(I) Baffles: Baffles are usually attached to the inner walls of the bioreactor. They are stationary plates that help to prevent the formation of a vortex during stirring, which can reduce mixing efficiency and gas transfer. Baffles promote more turbulent flow, leading to better mixing.
(J) Control Systems: Modern bioreactors are equipped with sophisticated control systems to monitor and regulate parameters such as temperature, pH, dissolved oxygen, nutrient levels, and agitation speed, ensuring optimal conditions for the bioprocess. In essence, the stirred-tank bioreactor provides a well-mixed and controlled environment that supports the large-scale cultivation of biological entities for the production of desired products. The stirring mechanism is central to its function, ensuring homogeneity and efficient mass transfer.