Multifluid Population Balance Model - A Tool for Designing Multiphase Reactors

Multifluid Population Balance Model – A Tool for Designing Multiphase Reactors

The design of multiphase reactors can be a challenging task. Several phenomena occur simultaneously and influence each other (see Fig. 1). Therefore, these complex interactions must be taken into account in the design of reactors. In principle, multiphase CFD simulations are well suited for this purpose. However, they are still too slow and unfeasibly complex for modeling in the concept phase of reactor design, where large parameter and sensitivity studies need to be performed.

We present an efficient alternative modeling approach for such sensitivity studies - 1D multifluid population balance (MPB) models [2]. These models allow to describe properties (in our case the size distribution) of disperse phases, which are e.g. bubbles, droplets, solid particles. The size distribution of disperse phases is important because it determines local mass transfer rates and can also affect reaction and even local heat transfer rates, and in some cases flow behavior. The MPB approach assumes that the transient 3D turbulent flow can be simplified to a steady 1D flow along the main flow direction, which is valid for basic reactor designs or compartments within a more complex reactor. As with many simplified models, especially multiphase ones, the limitations in predictions and scale-up must be considered and will be discussed. Model parameters, such as those describing particle breakage and coalescence, must be calibrated against experimental data. Similarly, the size distribution of the disperse phase at the reactor inlet must be known. We used two different measurement techniques for that purpose: optical needle probes and a telecentric imaging method developed in-house.

For bubble columns as a first example, we will show that it is possible to predict bubble size distribution, interfacial area and gas holdup within a given chemical system over a wide range of compositions, operating points and geometry variations (see Fig. 2 for an example of a bubble column). The results prove the strength of the modeling approach and guide the way of extending it to reactive operation with mass transfer. Based on such extensions, the model approach can be used to describe complex multiphase reactive systems – allowing sensitivity studies which give indications of overall conversion and selectivity in reactor variations. In our presentation we will show that this is a valuable tool for the design of multiphase reactors and also discuss further open questions in the broader application of such models for real systems and scale-up.

REFERENCES

[1]          Breit, F, Weibel, C., von Harbou, E., 2024, Application and parameterization of a one-dimensional multifluid population balance model to bubble columns, AIChE J. e18424, https://doi.org/10.22541/au.170010367.75931252/v1

[2]          Breit, F., Mühlbauer, A., von Harbou, E., Hlawitschka, M. W., & Bart, H. J., 2021, A one-dimensional combined multifluid-population balance model for the simulation of batch bubble columns, Chemical Engineering Research and Design 170, 270–289. https://doi.org/10.1016/j.cherd.2021.03.036