In bioprocessing, what does KLa represent?

• A. Volumetric Transfer Coefficient
• B. Mass Transfer Coefficient
• C. Diffusion Coefficient
• D. Henry's Law Constant

Answer: (A)

KLa is the volumetric mass transfer coefficient that quantifies how fast a gas, such as oxygen, transfers into the liquid per unit reactor volume. In a bioreactor, microbes consume dissolved oxygen, so the rate of transfer follows dC/dt = KLa (C* − C), where C* is the oxygen concentration the liquid would have at equilibrium with the gas phase at the current temperature and pressure. KLa captures both the liquid-phase mass transfer capability and the interfacial area available for transfer, since it equals the product of the liquid-side mass transfer coefficient (kL) and the interfacial area per volume (a). This is why increasing agitation and aeration, which create more bubble surface area and better mixing, raise KLa and thus the overall oxygen transfer rate. It’s not the diffusion coefficient, nor Henry’s law constant, and it’s not just a simple mass transfer coefficient by itself—the “volume” and interfacial area are essential parts of KLa.