(307e) Prediction and Experimental Validation of Oxygen Transfer in a Rocking Disposable Bioreactor

The rocking disposable bioreactor is a novel type of design which distinguishes itself from other common disposable bioreactors such as the stirred-tank and orbitally-shaken bioreactors by its unique geometry and mass transfer mechanisms. In this system, the fermentation medium is contained within a disposable polymer bag that sits on a platform which is mechanically rocked back and forth. The head space is continuously flushed with air or oxygen mixtures, and the rocking motion induces surface aeration and wave action that promotes gas-liquid oxygen mass transfer from the head space to the cells in the medium. No gas sparging into the liquid is used. Compared with stirred-tanks, the shear stresses are much reduced in the medium, which can be beneficial for shear-sensitive mammalian and other cells.

Although the mass transfer characteristics in rocking bioreactors have been studied to some extent, a more fundamental mechanistic approach has not yet been reported. In this work, a fundamental gas-liquid mass transfer (k_La) model was proposed by realizing that the gas-liquid mass transfer in a rocking disposable bioreactor (WAVE^TM Bioreactor from GE Healthcare) occurs through two distinct mechanisms. Oxygen transfer can occur both through surface aeration at the headspace-liquid interface and via air entrainment through a breaking wave at the end of the bag during each rocking cycle. Although both mechanisms can apply simultaneously, their individual contributions will depend on the operating conditions such as rocking frequency, rocking angle, and liquid working volume within the bag.

A fundamental model, incorporating both mass transfer mechanisms, was developed by adapting and combining existing models for surface aeration and air entrainment in breaking ocean waves. The model analysis and sensitivity study revealed that at low rocking intensities surface aeration was the main mechanism, but as rocking frequency and angle increased the wave-breaking mechanism predominated.

Experimental data for oxygen mass transfer (k_La) across the range of possible operating conditions (rocking frequency, angle, and liquid volume) was gathered using the gassing-in method, alternating between nitrogen and air in the bag headspace, while measuring the rate of change of dissolved oxygen in the water. The data confirmed the validity of the modelling approach, with most predictions falling within ±20% of the measured values. No experimentally determined parameters (other than those adapted from literature) or fitting constants were required for the model to successfully predict mass transfer. At low rocking frequencies (up to 20 rpm) the surface aeration mechanism was confirmed to be dominant with k_La of approximately 3.5 h^-1, while at high frequencies (40 rpm) and angles the breaking wave mechanism contributed up to 91% of the overall k_La (65 h^-1).

An electrical method was also tested for its ability to determine power input into the liquid medium. Recognizing the oscillatory characteristic of the power alteration during the rocking, peak, average and base power consumptions were measured for comparison. Results showed that among these three types of power consumption measured, peak power was relatively more capable of reflecting the impact from varied operational parameters including liquid volume, rocking frequency and rocking angle, etc. on the power consumption. Measured averaged power input ranged from 66.45 W/m³ at 10 rpm, 12^o rocking angle with 5 L liquid volume to 730.74 W/m³ at 40 rpm, 12^o rocking angle with 3 L liquid volume. The measured averaged power input data was, to some extent, comparable with the power input values reported by some previous works using either calorimetric method or computational fluid dynamic simulation. The relationship between peak power input and mass transfer performance (represented by mixing time and ) was investigated. The results revealed both similarities and differences between rocking disposable bioreactor and other types of disposable bioreactors, such as stirred-tanks and orbitally-shaken bioreators. The relationship between power input and mass transfer was found to follow a power-law model with reasonable fit.