Unbaffled stirred tanks are increasingly recognized as a viable alternative to common baffled tanks for a range of applications such as biochemical, food or pharmaceutical processes where the presence of baffles is undesirable for some reason. In this work, the mass transfer performance of unbaffled stirred tanks with pitched blade turbine, operating either in up-pumping or down -pumping mode, was investigated. The influence of impeller size and liquid viscosity were also investigated. The mass transfer intensity was measured by means of the Simplified Dynamic Pressure. Method: The measurements concerned both coalescent and non-coalescent (viscous) batches. Results: confirm that increasing apparatus size has a slightly positive effect on gas-liquid mass transfer coefficient. It was also found that when the PBT is operating in the up pumping mode the formation of surface oscillations, which lead to undesired instabilities of the whole apparatus, is conveniently minimized. In the super-critical regime, the unbaffled tanks provide a performance comparable with that of the standard (baffled) bioreactors at the same power dissipation, which makes them a viable alternative for general fermentation operations and other gas-liquid reactions. (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

The interest in the process industry on unbaffled stirred tanks has greatly expanded in the last years because they may bring about significant advantages in a number of applications, including biochemical, food and pharmaceutical processes where the presence of baffles is undesirable for several reasons. Despite their application potential, unbaffled vessels still lack fundamental information, due to the fact that only recently their capabilities have started being dug out. The lack of information on scale up effects is possibly the main reason hindering practical applications.In this work the influence of vessel size and liquid viscosity on the mass transfer performance in unbaffled stirred vessels, is investigated. As regards the first issue (scale-up) quite surprisingly results show that comparable mass transfer coefficients may be obtained in larger vessels, for a given power input per unit volume, with typical kLa values up to 2×10−3s−1 in ungassed conditions and of 2×10−2s−1 in gassed conditions. This is opposite to expectations based on the fact that the exchange surface in subcritical conditions only grows with D2 while volume grows with D3. Notably, the same result is obtained at an increased liquid viscosity (a feature often exhibited by biological cultivation broths) and confirms that unbaffled stirred tanks should be regarded as a strong candidate for industrial applications.Finally, also at the larger scale, in the super-critical regime unbaffled tanks are found to provide a performance comparable with that of standard (baffled & sparged) stirred tanks, hence resulting in a viable alternative to baffled tanks for all gas–liquid processes and bio-processes.

In the design of mechanically agitated fermenters, the volumetric mass transfer coefficient kLa often becomes the key parameter. In case of viscous liquids, kLa prediction based on literature correlations suffers from a wide distribution. The aim is to provide reliable kLa correlations based on a large experimental data set obtained using the dynamic pressure method, the physical correctness of which was verified earlier. Multiple-impeller fermenters of two scales were used. The impeller tip speed, rather than the power input, revealed to be the proper term for prediction of kLa. When the impeller power number is used, kLa values for various impeller types can be predicted by a common correlation which, based on a wide range of operational conditions, can be applied in the design.

Transport characteristics such as volumetric mass transfer coefficients, kLa, power input, P, gas hold-up, γ, and mixing time, tm, are the key parameters in the design of mechanically agitated gasliquid contactors. For their successful design, values of the key parameters can be estimated using empirical correlations. Power input in this case is very often used as the scale of energy dissipation for other characteristics. Our goal was to propose reliable power input correlations for viscous batch processes, which are widely used in industry. The measurements were carried out in a pilot-plant vessel and also results from a laboratory vessel were used to develop the correlations. Different types of impellers and their combinations were used, including radial, axial, and combined liquid flow impellers. The power input was measured in a multiple-impeller vessel at different impeller frequencies and several gas flow rates. Correlation equations describing the behavior of particular impellers were evaluated. In addition, separate correlations for the bottom and upper sections in the multiple-impeller vessel were presented. These correlations can be used for impeller power prediction in industrial scale vessels under a wide range of operational conditions.

Volumetric mass transfer coefficients (k(L)a) and power input (P) are often the key parameters in the design of gas-liquid contactors. However, due to the limitations of most measurement methods, there is a lack of reliable data for predicting k(L)a for non-coalescent batches under high energy dissipation rates. Accurate k(L)a and P correlations are proposed. The reliability of the correlations is ensured by using experimental data from a wide range of process conditions conducted in multiple-impeller vessels of both laboratory scale and pilot scale, and including both non-coalescent and coalescent batches. Applying the proposed correlations, the scale-up and optimization of industrial vessels can be performed more accurately.

Focused on liquid film-controlled processes in mechanically agitated aerated vessels, an attempt is made to develop scale-up correlations based on isotropic turbulence theory. It is shown that including the impeller tip speed fD (impeller frequency * impeller diameter) in data treatment significantly improves the volumetric mass transfer coefficient (k(L)a) prediction abilities in scaling-up applications. Additionally, simplified correlation is developed, which evades the need of impeller power knowledge. Its good usability for k(L)a prediction is shown except for the case when common correlation is made together for axial and radial impellers. The study is based on measurements of volumetric mass transfer coefficient in a pilot scale multiple-impeller vessel with non-coalescent batch and six various impeller types, which expand previous data set from a laboratory scale tank. (C) 2013 Elsevier B.V. All rights reserved.