(203a) Flow Simulation of Bioreactors Using Entropic Lattice Boltzmann Method

In the pharmaceutical and biopharmaceutical industry, stirred-tank reactors are extensively used for production of Active Pharmaceutical Ingredients (APIs). Characterization of the stirred-tank reactor and proper understanding of the mixing process is essential for streamlining the production of APIs. Several experimental techniques like Laser Doppler Anemometry (LDA) and Particle Image Velocimetry (PIV) have been successfully used for predicting the large scale flow structures, but these methods fail to provide information about small scale flow structures [1]. Computational Fluid Dynamics (CFD) techniques are more commonly used to study the flow at any desired scale with a fraction of cost as compared to experiments [1][2].

Stirred-tank reactors are typically characterized by their power number, which can be accurately and inexpensively predicted by steady-state Reynolds-averaged Navier-Stokes (RANS) simulations coupled with Multiple Reference Frame method (MRF) used to account for impeller rotation [3]. In order to study the inherently turbulent, multi-phase, non-stationary flow characteristics of the mixing process, and to accurately predict the mixing time; however, efficient transient schemes are needed. Direct Numerical Simulation (DNS) of such a complicated setup involving moving boundaries is compute-intensive. RANS sliding mesh approach and Large Eddy Simulation (LES) [4] are the standard transient schemes for mixing time simulations. These techniques typically involve ad hoc turbulence modeling for small scales. The objective of the current work is to simulate flow in a single-phase stirred-reactor by employing the Entropic Lattice Boltzmann Method (ELBM) [5][6], amicable to moving boundaries. A major feature of ELBM is that it does not require any explicit turbulence modeling.

A novel body centered cubic (BCC) structured grid [7] is used for efficient spatial discretization and resolving the complex moving geometries, as against the regularly used simple cubic (SC) structured grid. An in-house solver is used to demonstrate the novel ELBM on a BCC lattice by simulating two variations of a single-phase stirred tank reactor. The reactor is agitated by 1) a single impeller of diameter 50 mm [1] and 2) double rushton impellers of diameter 36 mm [2]. In both the cases, dominant flow features like the shape and positioning of circulation loops, first order quantities like phase-averaged radial and tangential velocity profiles are accurately captured with resolutions comparable to LES and are compared against RANS, LES and experimental results [1][2]. The advantage of the BCC grid, which uses a lesser number of grid points compared to the SC grid for a given accuracy, is demonstrated by comparing the second order quantities like the turbulent kinetic energy.

References

[1] Hartmann, Hugo & Derksen, J.J. & Montavon, Christiane & Pearson, J & Hamill, I.S. & Van den Akker, Harry. (2004). Assessment of Large eddy and RANS Stirred Tank Simulations by Means of LDA. Chemical Engineering Science. 59. 2419-2432. 10.1016/j.ces.2004.01.065.

[2] Kuschel, Maike & Fitschen, Jürgen & Hoffmann, Marko & Von Kameke, Alexandra & Wucherpfennig, Thomas & Schlueter, Michael. (2021). Validation of Novel Lattice Boltzmann Large Eddy Simulations (LB LES) for Equipment Characterization in Biopharma. Processes. 9. 10.3390/pr9060950.

[3] Haringa, Cees & Vandewijer, Ruben & Mudde, R.F.. (2018). Inter-compartment interaction in multi-impeller mixing part I: Experiments and Multiple Reference Frame CFD. Chemical Engineering Research and Design. 136. 10.1016/j.cherd.2018.06.005.

[4] Haringa, Cees & Vandewijer, Ruben & Mudde, R.F.. (2018). Inter-compartment interaction in multi-impeller mixing part II: Experiments, Sliding mesh and Large Eddy Simulations. Chemical Engineering Research and Design. 136. 10.1016/j.cherd.2018.06.007.

[5] Kolluru, Praveen & Atif, Mohammad & Namburi, Manjusha & Ansumali, Santosh. (2020). Lattice Boltzmann model for weakly compressible flows. Physical Review E. 101. 10.1103/PhysRevE.101.013309.

[6] Atif, Mohammad & Kolluru, Praveen & Thantanapally, Chakradhar & Ansumali, Santosh. (2017). Essentially Entropic Lattice Boltzmann Model. Physical Review Letters. 119. 10.1103/PhysRevLett.119.240602.

[7] Namburi, Manjusha & Krithivasan, Siddharth & Ansumali, Santosh. (2016). Crystallographic Lattice Boltzmann Method. Scientific Reports. 6. 27172. 10.1038/srep27172.