Large Scale Bioreactors – Insight Into a Black Box
Aerated stirred tank reactors still find widespread use in industries connected to chemical and biochemical engineering today due to their simple operating mode, low investment costs and flexibility. For a variety of applications in these different fields, efficient mixing of two phase flows with high heat- and mass transfer performance is one of the most important challenges [Mid04]. Because of the significance of these apparatuses they have been investigated intensively in the past decades to model and simulate these processes.
Reliable design and scale-up remain a challenging task with many unanswered questions. To expand available data on large scale reactors, a large scale acrylic glass reactor with a volume of VR = 15 m3 and total optical access has been erected at Hamburg University of Technology together with Boehringer Ingelheim Pharma GmbH & Co.KG. The measuring equipment to conduct the experiments in this reactor is supplied by the Institute of Multiphase Flows. The project primarily aims at an improved understanding of the process. Based on this, a better modelling of scale-up and transfer between different systems can be developed. An essential component is the precise and comprehensive determination of the mass transfer rate, the power input and the flow pattern as a function of the operating parameters.
A crucial parameter for the characterization of stirred tank reactors is the mixing time that is necessary to achieve a certain degree of homogenization. Usually the mixing time in large scale systems is measured locally with conductivity probes. The optical access of the acrylic glass reactor to the liquid volume enables us to measure mixing times with the decolourization method with high resolution transient photo series. The advantage of the colorimetry is its ability to visualize dead flow zones as well as local and global flow structures simultaneously. The results are compared to transient computational fluid dynamic (CFD) simulations and used to improve these predictive methods.
Oxygen Mass Transfer
Especially for aerobic processes, the supply of oxygen is of high importance. For this reason, the liquid is being gassed with air or pure oxygen. Resulting from the concentration difference a transfer of mass from the gaseous phase with a high concentration of oxygen to the liquid with the lower concentration is taking place. This process can quantitatively be determined by using the volumetric mass transfer coefficient which can be measured by different methods. The simplest one is the gassing out method where at first the oxygen is stripped out with nitrogen. After that the reactor is aerated with air or oxygen and the increasing oxygen concentration is logged for the desired process parameter. In literature many correlations for the volumetric mass transfer are available and often in the form of
as a function of the power input, and the superficial gas velocity.
Figures 1 and 2 schematically show the experimental setup for (1) mass transfer measurements and (2) for mixing time studies. As shown in bocth figures, the reactor has several test ports over both height and circumference. This allows the measurement of concentration, temperature and gas holdup profiles at almost any point in the reactor. Furthermore, the complete optical access allows a detailed observation of the mixing process, local gas distributions and flow pattern.
|Figure 1: Example of the arrangment of different oxygen probes||Figure 2: Set-Up for decolourisation measurement|
[Mid04] Middleton, J.C. and Smith, J.M.Gas–Liquid Mixing in Turbulent Systems, chap. 11,pp. 585–638. John Wiley Sons, Ltd, 2004. ISBN 9780471451457. doi:10.1002/0471451452.ch11.