Volumetric mass transfer coefficient in viscous liquid in mechanically agitated fermenters. Measurement and correlation

- Experimental kLa data for pilot-plant and laboratory fermenters are presented.
- Experimental kLa data for various impeller types in viscous batch are presented.
- Scaling-up based on kLa is improved, based on impeller power number and tip speed.
- Using various impeller types, the kLa correlations suitable for scaling-up are developed.
- A modification of dynamic pressure method for viscous batches is presented.

In the industrial fermentation processes, most liquids are non-coalescent and often exhibit increased viscosity. However, due to the limitations of most measurement methods, there is a lack of reliable data for predicting volumetric mass transfer coefficients (kLa) for viscous batches, especially under high dissipated energies, as the accurate determination of oxygen concentration profile in viscous liquids is not as easy as in low viscosity ones. Our goal is to develop reliable technique for kLa determination in viscous liquids and to establish suitable correlation shapes to describe kLa data. We used the dynamic pressure method (DPM), the experimental set-up of which has been modified for the measurement in viscous liquids. Dissolved oxygen (DO) probes were placed in bypass measuring cells. This set up brings well defined transient characteristics of DO probes, which is crucial for correct kLa evaluation. Measurements were conducted in two phase multiple-impeller fermenters with a non-coalescent viscous Newtonian batch under a wide range of experimental conditions and in the apparatuses of two scales. Using pure oxygen as gas phase, it was confirmed that DPM yields kLa's independent of the driving force of absorption even in viscous batch. The improved set up of DPM enabled to use also optical DO probes as well as polarographic ones. It was confirmed that optical DO probe can be used for kLa values up to 0.4 s−1. Based on the experimental data, correlations were developed to predict kLa in industrial fermenters. Standard correlation kLa = 2.99 ⋅ 10−3⋅(Pg/VL)0.891vs0.556 with standard deviation, SD, 30%, based on gassed power input Pg and superficial gas velocity vs, has low standard deviation but it is scale specific. On the other hand, when the term of impeller tip speed (ND) is used instead of Pg, predicted data exhibit neither over- nor under-estimation of kLa for particular apparatus scale; so the effect of the vessel scale is properly described using this term. In addition, the impeller power number Po was found to be a reliable predictor of kLa in a common correlation for various impeller types, when used together with the impeller tip speed term. The correlation kLa = 0.295⋅(ND)2.083vs0.461Po0.737 with SD 28% suggested in this work can be used for fairly accurate design of industrial fermenters. Both the experimental technique and the correlation shape are ready to be used to obtain the design tool for other batches with various viscosities.

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