(496c) Beyond Kla: How Reaction and Transport Timescales Compete in High-Cell-Density Bioprocesses

In high-density cell culture systems, the competition between reaction kinetics and transport processes defines bioreactor performance. Fast reactions, such as bicarbonate buffering, maintain pH equilibrium on timescales of seconds, while oxygen transfer via sparging and bubble-mediated mass transfer operates on the order of minutes. These transport-limited processes, in turn, constrain metabolic reactions that unfold over hours to days, directly influencing cell growth, productivity, and overall yield. The interplay among these processes is particularly critical in intensified bioprocesses, where high cell densities drive oxygen and nutrient demand to the limits of transport capacity. Misalignment of these competing timescales results in concentration gradients that impact process efficiency, cell viability, and product quality.

We present a mechanistic framework for analyzing these competing dynamics across scales, focusing on the interaction between kla, CO2 stripping, and metabolic uptake in sparged bioreactors. Through computational modeling and experimental validation, we quantify the extent to which mass transfer bottlenecks limit reaction kinetics, particularly in systems with high oxygen uptake rates. Our results highlight the necessity of tuning kla to balance oxygen availability with metabolic demand, demonstrating how poor alignment between transport and reaction timescales can induce local hypoxia, pH swings, and shifts in metabolic pathways. The findings provide a quantitative foundation for optimizing gas exchange strategies and reactor design, ensuring transport processes do not become rate-limiting in high-yield production systems.

This work extends prior CFD-based bioreactor modeling efforts (Oliveira et al., 2023), shifting focus from control strategies to the fundamental competition between reaction kinetics and transport. By contextualizing mass transfer limitations in the broader hierarchy of process dynamics, we provide a framework for engineering bioreactor conditions that maximize productivity. The implications are especially relevant for scale-up, where the balance between fast and slow processes shifts in ways that impact performance unpredictably. Understanding and addressing these competing mechanisms is key to maintaining robust, scalable, and efficient biomanufacturing processes.

Oliveira, C. L., Pace, Z., Thomas, J. A., DeVincentis, B., Sirasitthichoke, C., Egan, S., & Lee, J. (2024). CFD-based bioreactor model with proportional–integral–derivative controller functionality for dissolved oxygen and pH. Biotechnology and Bioengineering, 121(2), 655–669. https://doi.org/10.1002/bit.28598