Zhang, Hu; (2003) Application of computational fluid dynamics to micro-titre plate scale bioreactors. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The manufacture of a new drug requires process development at the early stage in the development cycle of the drug to reduce cost and time. Current approaches based on laboratory and pilot-plant operations require large amount of materials which may not be available. A new bioprocess development method based on a miniature bioreactor is proposed. The engineering parameters of the miniature bioreactor are obtained in this study with the aid of computational fluid dynamics (CFD) and the results are compared with data obtained for other small scale systems including micro-titre plates and shake flasks. A new miniature bioreactor with a diameter equal to that of a single well of a 24-well plate is described and its engineering performance as a fermenter assessed. Mixing in the miniature bioreactor is provided by a set of three impellers mechanically driven via a micro-fabricated electric motor and aeration is achieved with a single tube sparger. Parameter sensitive fluorophors are used with fibre optic probes for continuous monitoring of dissolved oxygen tension and an optical based method is employed to monitor cell biomass concentration during fermentation. The local gas and liquid velocity, gas volume fraction and energy dissipation rate are derived from an analysis of the multiphase flow in the miniature bioreactor using CFD. The predictions are compared to experimental observations from the literature. Volumetric mass transfer coefficients are predicted using Higbie's penetration model with the contact time obtained from the CFD simulations of flow in the bioreactor. Comparative measurements are provided from parallel experiments carried out in a 20L (15L working volume) conventional fermenter. Measured volumetric mass transfer coefficients are in good agreement in the miniature bioreactor and 20L bioreactor although the CFD simulation data for the miniature bioreactor are underpredicted. The values of kLa from experiments and simulations in the miniature bioreactor are in the range 100 hr-1 to 400 hr-1, typical of those reported for large-scale fermentation. Mixing in the shake flask is analysed with the aid of CFD. The gas-liquid interface changes are mapped as the shake flask moves in a rotary shaking platform. Local liquid velocity and energy dissipation rate are obtained. The average power input is obtained by integrating the local energy dissipation rate over the entire working volume and results are compared to experimental measurements from literature. It was shown that power input is more sensitive to shaking diameter than to the shaking frequency, providing new insight into the optimisation of shake flask operations. The volumetric mass transfer coefficient obtained from Higbie's penetration model is in the range of the experimental results (30~100 hr-1). The shaken micro-titre plates play an important role in the drug discovery process and have the potential to provide information for process design and development. CFD analysis of flow is made in a 24-well and a 96-well reactor. The predicted flow patterns in the 24 well are very different to those in the 96-well reactor operating under the same shaking conditions. Flow patterns, average power input and volumetric mass transfer coefficients, obtained from CFD, are much more sensitive to changes in shaking diameter rather than shaking frequency. Based on equal power consumption, the volumetric mass transfer rate in the microwell reactor is higher than that in the shake flask. Analysis of the results and comparison with data obtained from the laboratory scale operation indicate that while the mass transfer in different small scale systems (miniature bioreactor, micro-titre plate and shake flask) may be correlated through the use of the concept of equal energy dissipation per unit volume, the fermentation and cell culture growth in these systems are more difficult to relate to each other. In conclusion, based on the results the miniature bioreactor offers the best option for scale-up to laboratory and pilot-scale because of similarity in engineering properties of the two systems.

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