(98an) CFD-Aided Characterization Of A Spinning Disc Reactor (SDR)

Background

In the last years spinning disc reactors (SDR) are
becoming more and more popular in the field of chemical reaction engineering.
The radial acceleration of the liquid caused by the centrifugal forces
generates a very thin film on the disc, typically in the range of 20 ? 300
μm. This thin film not only has superior micromixing properties when
adding reagents to the film but also creates large heat transfer coefficients
from the disc to the film and mass transfer coefficients from gas to liquid.
These properties make the SDR technology attractive for many reactions such as
polymerization, organic synthesis, photocatalysis and precipitation reactions.
The film thickness and regime and as a result the performance of a SDR is
strongly affected by the operating conditions, e.g. rotational speed,
volumetric flow rate, feeding point locations, disc diameter and disc design. Although there
is an increasing research in academia on the use of SDRs for various reactions,
still very little is known about the film properties and most researchers rely
on either experimental methods, which only provide film heights but do not
disclose local film properties and factors affecting the film structure and
regime, or very simplistic models such as the Nusselt film model [1].
Understanding and predicting the film properties is crucial for optimization of
the operating conditions.

Aims

In this work we aim at characterizing and predicting
film properties with CFD. In a recent publication of our group the micromixing
efficiency was experimentally determined by use of the iodide-iodate reaction
[2]. Focus in this work is also set on testing the predictability of CFD in
terms of describing the micromixing efficiency by numerically modeling the
reaction kinetics. 

Fig. 1: Computational mesh
(left); waves and ripples at n = 500 rpm (right).

Results

Creating a computational mesh is very challenging due
to the very large disc radius compared to the very small scales, which need to
be resolved. One of the meshes used in this study is shown in Fig. 1
(left). In Fig. 1 (right) we present the top view of the liquid film at a
rotational speed of n = 500 rpm. The
disc diameter is d = 5 cm and the
volumetric flow rate is Q = 3 mL/s.
Close to the inlet spiral waves can be found, while further away from the inlet
ripple structures prevail. The mean film height is generally in good agreement
with the Nusselt film thickness at all rotational speeds for r ≳ 2 cm, see
Fig. 2. For r ≲ 2 cm the
influence of the inlet is still large. The height of the waves and ripples can
be depicted from the error bars in Fig. 2 denoting the radial minimum and
maximum film height. For higher rotational speeds the height of the waves and
ripples decreases.

Conclusions

The hydrodynamics of the liquid film on a SDR were predicted with CFD.
We point out that, to our knowledge, this is the first CFD study on SDRs to
present a mesh convergence study to determine the influence of the numerical
grid on the results. Current investigations focus on validation of the model
with experimental film heights and on determining the micromixing efficiency by
modeling the reaction kinetics of the iodite-iodate reaction.

References
[1]    Wood,
R.M., and Watts, B.E.: Trans. Inst. Chem. Eng. 51, p. 315 (1973).

[2]    Jacobsen, N.C., and Hinrichsen, O.:
Ind. Eng. Chem. Res. 51, p. 11643 (2012).