Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

In the process industry, packed columns are used in a variety of fluid separation operations, e.g. in distillation and absorption, in order to create a desirable flow pattern of two-phase systems. Due to the high energy requirements of separation processes, the interest on their optimization is vital. In particular, column internals have permanently been the focus of investigations.
An improvement of the separation efficiency can be achieved by the application of sandwich packings. The latter consist of two alternating layers of industrially available standard packings with different specific surface areas, one with lower (the so-called holdup layer) and another with higher (the so-called de-entrainment layer) capacity. Sandwich packings are typically used at operating conditions between the flooding points of holdup layer and de-entrainment layer. Above the holdup layer, a froth sublayer is formed, which reveals high separation efficiency due to intensified phase mixing. In the upper section of the de-entrainment layer, film-like flow patterns can be observed [1]. Under certain conditions, this intensive heterogeneous flow pattern can be used in a beneficial way for fluid separation processes. However, the application of this integrated packing type is hindered by lacking validated design methods [2].
An accurate prediction of the performance characteristics is essential for the design of sandwich packings. In our project, the effects of the individual flow regimes on fluid dynamics and mass transfer are investigated complementarily with both experimental and theoretical studies. In order to determine the impact of each individual flow regime, experiments on an absorption/desorption plant are supplemented by flow imaging measurements of sandwich packings. At the Paderborn University, CO2 absorption is examined in a pilot plant for various design and operating parameters. This plant allows measuring temperature profiles of the gas phase as well as concentration profiles of both phases. At the Helmholtz-Zentrum Dresden-Rosendorf, a detailed view on the phase distribution within the sandwich packings is realized bymeans of an ultrafast X-ray tomography. The measured data from both experimental methods are used to establish correlations for mass transfer coefficients, interfacial area, holdup and pressure drop, which are applied in a rate-based stage model for CO2 absorption processes with aqueous amine solutions.
The absorption experiments were performed under ambient conditions in a column with an inner diameter of 0,1 m and 3,2 m packing height. The influence of different operating and design parameters on the separation characteristics in sandwich packings was studied in order to identify appropriate operating conditions and to provide a sufficient basis for the experimental validation of a model, which is able to describe the heterogeneous flow patterns. In particular, we investigated the impact of the holdup-layer height and its corresponding specific surface area. In addition, the inclination angle of the flow channels in the de-entrainment layer was varied.
The rate-based approach, which accounts for the specifics of different column internals via correlations, was applied. We expected that different fluid dynamic regimes would have different impacts on the mass transfer. Therefore, each fluid dynamic regime was considered individually. The column was represented as a sequence of alternating segments, and each segment was described by a corresponding set of correlations. Experimental data of the CO2 absorption with sandwich packings were then compared with simulation results.
Acknowledgments
The authors are grateful to the Deutsche Forschungsgemeinschaft (DFG) for financial support (KE 837/26-1, HA 3088/10-1).
References
[1] U. Brinkmann, B. Kaibel, M. Jödecke, J. Mackowiak, E.Y. Kenig: Beschreibung der Fluiddynamik von Anstaupackungen, Chemie Ingenieur Technik 84, 36-45 (2012).
[2] Ö. Yildirim, E.Y. Kenig: Rate-based modelling and simulation of distillation columns with sandwich packings, Chemical Engineering and Processing: Process Intensification 98, 147-154 (2015).