Modelling gas-liquid flow in trickle-bed reactors
Doctoral thesis (article-based)
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AbstractThe performance of a trickle-bed reactor is affected, not only by reaction kinetics, pressure, and temperature, but also by reactor hydrodynamics, which are commonly described by means of global parameters such as pressure drop, liquid holdup, dispersion of gas and liquid phases, catalyst wetting, and mass- and heat-transfer coefficients. Due to the complicated nature of trickle-bed reactor hydrodynamics, there is no straightforward method for the scale-up of these parameters from laboratory to industrial scale. Scale-up has therefore been based on a combination of different procedures: 1) constant dimensionless groups, 2) mathematical modeling, and 3) pilot planting. In general, not all dimensionless groups can be kept constant and pilot planting is both time-consuming and expensive. Thus mathematical modeling is an interesting option for the design of new trickle-bed reactors. In addition, mathematical models can be used in performance and sensitivity analysis. In this work mathematical models have been developed for pressure drop, liquid holdup, dispersion, and catalyst wetting efficiency and different modeling options are also discussed. Global pressure drop and liquid holdup can be estimated from the gas-liquid-solid phase interaction models by formulating a simplified mass and momentum equation for gas and liquid phase. New improved phase interaction models have been developed assuming uniformly distributed gas and liquid flows. Although it is assumed that the liquid is distributed uniformly, the perfect wetting of the catalyst is not assumed. A new model for liquid-solid wetting efficiency has been developed concurrently with the phase interaction models. The liquid-solid wetting efficiency is used to determine the significance of gas-liquid, liquid-solid and gas-solid phase interactions. Phase interaction and wetting efficiency model parameters have been optimized against a large experimental database for pressure drop, liquid holdup, and wetting efficiency. CFD is used to model phase dispersion in non-uniform gas and liquid flow conditions, where plug flow cannot be assumed. Spreading is attributed to three separate factors: overloading, mechanical dispersion, and capillary dispersion. The first of the spreading mechanisms derives from the phase interactions and does not require a separate model. New models have been developed for mechanical and capillary dispersion. The performance of the dispersion models is discussed on the basis of multiple case studies. The ability of the hydrodynamic model developed here is compared to liquid dispersion experiments from four different literature sources, including the author's own experiments. In addition to CFD modeling, an alternative, potential tool for liquid distribution studies is also presented - the cellular automata model. Thanks to single event modeling, a cellular automata model is simpler and thus faster and would be better suited for the modeling of larger reactors, when information on pressure drop and liquid holdup is not required.
computational fluid dynamics, CFD, porous media, capillary dispersion, mechanical dispersion, liquid-solid wetting efficiency
- [Publication 1]: Ville Alopaeus, Katja Hynynen, and Juhani Aittamaa. 2006. A cellular automata model for liquid distribution in trickle bed reactors. Chemical Engineering Science, volume 61, number 15, pages 4930-4943.
- [Publication 2]: Ville Alopaeus, Katja Hynynen, Juhani Aittamaa, and Mikko Manninen. 2006. Modeling of gas–liquid packed-bed reactors with momentum equations and local interaction closures. Industrial & Engineering Chemistry Research, volume 45, number 24, pages 8189-8198.
- [Publication 3]: Katja Lappalainen, Ville Alopaeus, Mikko Manninen, and Juhani Aittamaa. 2008. Improved hydrodynamic model for wetting efficiency, pressure drop, and liquid holdup in trickle-bed reactors. Industrial & Engineering Chemistry Research, volume 47, number 21, pages 8436-8444.
- [Publication 4]: Katja Lappalainen, Mikko Manninen, Ville Alopaeus, Juhani Aittamaa, and John Dodds. 2009. An analytical model for capillary pressure–saturation relation for gas–liquid system in a packed-bed of spherical particles. Transport in Porous Media, volume 77, number 1, pages 17-40.
- [Publication 5]: Katja Lappalainen, Mikko Manninen, and Ville Alopaeus. 2009. CFD modeling of radial spreading of flow in trickle-bed reactors due to mechanical and capillary dispersion. Chemical Engineering Science, volume 64, number 2, pages 207-218.
- [Publication 6]: Katja Lappalainen, Ville Alopaeus, Mikko Manninen, and Sirpa Kallio. 2008. Modelling of liquid dispersion in trickle-bed reactors: capillary pressure gradients and mechanical dispersion. In: Proceedings of the 11th International Conference on Multiphase Flow in Industrial Plants (MFIP 2008). Palermo, Italy. 7-10 September 2008.