(119g) Numerical Simulation of Two-Phase Flow and Interfacial Species Transfer in Structured Packings

To optimise the separation performance of structured packings in distillation columns, a thorough understanding of the fluid dynamics as well as the transport processes within these column internals is vital. Conversely, this means that only simulations comprising both phenomena allow a comprehensive analysis of the process. This study presents the setup to simulate the fluid dynamics, the GCST model to consistently incorporate the interfacial species transfer and the combined setup which merges both fields into a single simulation run for determination of the separation efficiency.

Introduction

Distillation columns have a wide application in many sectors of process industry. By them it is possible to separate the individual components of a mixture. This is done by multistage continuous distillation where the separation is enabled through evaporation and condensation.

To optimise the efficiency of this process the interface between the gas and the liquid phase must be maximised, whereas the pressure drop should be minimised. Both requirements are met by so called structured packings. Against the background of further improvement, a thorough understanding of the fluid dynamics and transport processes inside the mentioned column internals is vital. To approach this target, numerical simulations of single- and multiphase flow inside structured packings, including species transfer, are conducted and closely evaluated.

Simulation of fluid dynamics in structured packings

In a first step the fluid dynamics in structured packings are investigated and the results achieved are presented in Hill et al. (2016). In the course of spatial modelling the smallest possible periodic domain could be identified which in combination with cyclic boundary conditions is an efficient calculation region. First, the case of single-phase gaseous flow is analysed, focusing on the visualisation and understanding of the transient flow structures occurring at higher flow rates. Second, a two-phase flow simulation is conducted to identify the local distribution of the liquid phase whereby the results are compared to experimental data.

GCST model to simulate interfacial species transfer

To enable the simulation of interfacial species transfer in the context of finite volumes, a new model is derived and validated. The model is based on the Continuous Species Transfer (CST) method introduced by Marschall et al. (2012) and Deising et al. (2016), which is generalised in a way that not only absorption but also distillation can be simulated and is therefore termed Generalised Continuous Species Transfer (GCST) model. The derivation and validation of the model is published in Hill et al. (2018).

Combined setup for simulation of distillation in structured packings

In this last step the two previously described fields of research were combined and two-phase flow and species transport in structured packings are simulated simultaneously. Before doing so new boundary conditions simulating the head and the bottom of a column had to be developed. The conducted simulations cover different liquid and gas loads and as a parameter to evaluate the results the Height Equivalent to a Theoretical Plate (HETP), a parameter describing the separation efficiency, was calculated.

Conclusions

Combining the GCST model with a 3D transient direct numerical simulation of the fluid dynamics in a structured packing enables the prediction of the separation performance (HETP) of the packing. Hereby, only physical properties are required, and no fitting parameters are part of the model. In-depths analysis of the simulation data will not only help to optimise packing geometries but also to gain a deeper insight into the process itself. In addition, important parameters like the specific effective surface area, degree of wetting or mass transfer coefficients can be extracted.

Acknowledgments

The authors gratefully acknowledge the Gauss Centre for Supercomputing e.V. (www.gauss-centre.eu) for funding this project by providing computing time on the GCS Supercomputer SuperMUC at Leibniz Supercomputing Centre (www.lrz.de).

References

1. Deising D., Marschall H., Bothe D., A unified single-field model framework for Volume-Of-Fluid simulations of interfacial species transfer applied to Bubbly Flows, Chemical Engineering Science, Vol. 139, pp. 173-195 (2016)
2. Hill S., Acher T., Hoffmann R., Ferstl J., Rarrek A., Klein H., CFD Simulation of the Hydrodynamics in Structured Packings, Conference Paper, 9th International Conference on Multiphase Flow, Florence (2016)
3. Hill S., Acher T., Hoffmann R., Ferstl J., Deising D., Marschall H., Rehfeldt S., Klein H., Numerical Simulation of Two-Phase Flow and Interfacial Species Transfer in Structured Packings, Chemical Engineering Transactions, Vol. 69 (2018)
4. Marschall H., Hinterberger K., Schüler C., Habla F., Hinrichsen O., Numerical simulation of species transfer across fluid interfaces in free-surface flows using OpenFOAM, Chemical Engineering Science, Vol. 78, pp. 111-127 (2012)