Modelling gas-liquid flow and local mass transfer in stirred tanks

Thumbnail Image
Journal Title
Journal ISSN
Volume Title
Doctoral thesis (article-based)
Checking the digitized thesis and permission for publishing
Instructions for the author
Degree programme
Verkkokirja (4348 KB, 49 s.)
This doctorial thesis offers a guideline for modelling gas-liquid flow in stirred tanks with computational fluid dynamics (CFD). Particularly the effect of varying physical properties and industrial operating conditions is highlighted. The most important thing in modelling mass transfer in stirred vessels is the accurate prediction of local bubble size. Population balances for bubbles are needed for accurate description of the local mass transfer rate. There are many pitfalls in gas-liquid modelling at the transitional turbulence regime, and they need to be recognised and dealt with at a reasonable computational cost. Details of the work are presented in the included publications, this thesis sums up the findings. Backbone of this thesis is the experimental work done on 14 and 200 dm³ vessels. Experimental techniques were compared in making bubble size distribution (BSD) measurements. A variety of experiments were made to investigate: physical properties, vapour-liquid equilibrium, gas hold-up, gas-liquid mass transfer, bubble size distributions, local mixing times, flow fields and bubble swarm interactions. Parameters for a number of phenomenological models were fitted with a computationally less demanding multiblock model and were then used to simulate stirred reactors with CFD. The early systems were lean dispersions of low viscosity; at the end of this work opaque shear thinning G-L dispersions were modelled. The effect of impeller geometry on G-L mass transfer was studied by simulating three impeller geometries. There were no differences in the volumetric mass transfer rate between the impellers, although the flow patters and gas hold-up showed clear differences between the impellers. Heterogeneous behaviour like gas slug creation and reactor dead-spaces were successfully modelled. The simulated dispersions were highly heterogeneous: 50% of mass transfer took place in less than 10% of the reactor volume. A xanthan fermentation batch lasting for days was modelled; the reaction speed was bottlenecked by both mixing and mass transfer. These findings strongly support the use of spatially detailed models over ideal mixing assumption.
computational fluid dynamics, mixing, bioreactors
Other note
  • [Publication 1]: Moilanen, P., Uusi-Kyyny, P., Pokki, J.-P., Pakkanen, M., Aittamaa, J., Vapor-liquid equilibrium for butane + methanol, + ethanol, + 2-propanol, + 2-butanol, and + 2-methyl-2-propanol (TBA) at 323 K. Journal of Chemical & Engineering Data (2008), 53 (1), 83-88.
  • [Publication 2]: Laakkonen, M., Moilanen, P., Miettinen, T., Saari, K., Honkanen, M., Saarenrinne, P., Aittamaa, J., Local bubble size distributions in agitated vessel: Comparison of three experimental techniques. Chemical Engineering Research and Design (2005), 83 (A1), 50-58.
  • [Publication 3]: Moilanen, P., Laakkonen, M., Aittamaa, J., CFD modelling of local bubble size distributions in agitated gas-liquid vessels – Verification against experiments. Proceedings of the 14th European Symposium on Computer Aided Process Engineering (ESCAPE 2004), Lisbon, Portugal (2004), 241-246.
  • [Publication 4]: Moilanen, P., Laakkonen, M., Aittamaa, J., Modeling aerated fermenters with computational fluid dynamics. Industrial & Engineering Chemistry Research (2006), 45 (25), 8656-8663.
  • [Publication 5]: Laakkonen, M., Moilanen, P., Alopaeus, V., Aittamaa, J., Dynamic modeling of local reaction conditions in an agitated aerobic fermenter. AIChE Journal (2006), 52 (5), 1673-1689.
  • [Publication 6]: Laakkonen, M., Moilanen, P., Alopaeus, V., Aittamaa, J., Modelling local bubble size distributions in agitated vessels. Chemical Engineering Science (2007), 62 (3), 721-740.
  • [Publication 7]: Laakkonen, M., Moilanen, P., Alopaeus, V., Aittamaa, J., Modelling local gas-liquid mass transfer in agitated vessels. Chemical Engineering Research and Design (2007), 85 (A5), 665-675.
  • [Publication 8]: Moilanen, P., Laakkonen, M., Visuri, O., Aittamaa, J., Modeling local gas–liquid mass transfer in agitated viscous shear-thinning dispersions with CFD. Industrial & Engineering Chemistry Research (2007), 46 (22), 7289-7299.
  • [Publication 9]: Moilanen, P., Laakkonen, M., Visuri, O., Alopaeus, V., Aittamaa, J., Modelling mass transfer in an aerated 0.2 m3 vessel agitated by Rushton, Phasejet and Combijet impellers. Chemical Engineering Journal (2008), 142 (1), 95-108.
  • [Errata file]: Errata of publications 3 and 8