Development of a method for reliable power input measurements in conventional and single‐use stirred bioreactors at laboratory scale

Abstract

Power input is an important engineering and scale‐up/down criterion in stirred bioreactors. However, reliably measuring power input in laboratory‐scale systems is still challenging. Even though torque measurements have proven to be suitable in pilot scale systems, sensor accuracy, resolution, and errors from relatively high levels of friction inside bearings can become limiting factors at smaller scales. An experimental setup for power input measurements was developed in this study by focusing on stainless steel and single‐use bioreactors in the single‐digit volume range. The friction losses inside the air bearings were effectively reduced to less than 0.5% of the measurement range of the torque meter. A comparison of dimensionless power numbers determined for a reference Rushton turbine stirrer (N P = 4.17 ± 0.14 for fully turbulent conditions) revealed good agreement with literature data. Hence, the power numbers of several reusable and single‐use bioreactors could be determined over a wide range of Reynolds numbers between 100 and >104. Power numbers of between 0.3 and 4.5 (for Re = 104) were determined for the different systems. The rigid plastic vessels showed similar power characteristics to their reusable counterparts. Thus, it was demonstrated that the torque‐based technique can be used to reliably measure power input in stirred reusable and single‐use bioreactors at the laboratory scale.

Power Input Measurements in Stirred Bioreactors at Laboratory Scale

“…Based on our experience with the torque based measurements, the most critical factors to address are: 1) reducing the dead torque by minimizing the friction losses inside the bearings and seals, in particular in laboratory scale bioreactors, and 2) the selection of a suitable torque meter for the desired bioreactor size and agitation conditions. As has been shown earlier 35 , the dead torque can be dramatically reduced by the use of an air bearing. In the present study, a low cost air bushing made of porous carbon material was used.…”

“…As it has been shown earlier 35 , the dead torque, i.e. the torque measured without liquid inside the vessel, which can be a result of friction losses in the bearing, seals and the motor drive or small bends in or imbalances of the impeller shaft (particularly in the single-use plastic shafts), can be substantially reduced by the use of the air bearing.…”

“…The main geometric dimensions of the mixing tank and the agitator are also indicated. This figure has been modified from 35 . Please click here to view a larger version of this figure.…”

“…Both the minimum and maximum feasible agitator speeds can be limiting factors of the method described in this study. In addition to our previous work 35 , this study involved also the bioreactor #3, the smallest member in glass bioreactor family provided by the manufacturer, which is agitated by two-stage impellers with diameters of 42 mm. A comparable power characteristic to that in the geometrically similar bioreactor #4 was obtained with the presented experimental setup.…”

“…In order to close this gap, a reliable method for power input measurements with special focus on laboratory scale stirrers has been developed recently 35 . The torque values measured in the empty vessels, which were caused by the friction losses, were effectively reduced by the use of an air bearing.…”