Multiscale Modeling of Spray Dryers

A multiscale modelling approach [1,2] is developed to gain understanding in the design and operation of spray dryers. The paper discusses the various phenomena that occur in a wide range of length and time scales and the scale bridging strategies that are needed to come to a coupled simulation of the multiphase flow, the drying and the formation of particles with certain properties. For the simulation of multiphase flows, the description of turbulence, the droplet/particle size distribution and phase interactions have to be dealt with. For the liquid/droplet phase, a coarse-grained Lagrangian approach was taken, allowing to account in a straightforward way for the feed droplets size distribution. Because of the extremely fast initial drying and the typical initial trajectories of droplets from the liquid atomiser nozzle tip, droplet-droplet interactions can be neglected. Particle-particle and particle-wall interactions are, however, to be accounted for. An unsteady RANS approach was adopted and various turbulence models compared. Interfacial mass, heat and momentum transfer were modelled using standard correlations. At the single particle scale, a continuum description was used to describe changing particle diameters and composition and intra-particle diffusion resistance.

The simulation model was used to study a novel radial multi-zone spray dryer (RMD) [3,4]. The spray dryer combines multi-zone and high-G operation to allow use of hot (<350°C) air without degradation of the produced powder and reduction of the required chamber volume by about an order of magnitude. Initial drying can be achieved in milliseconds in a radially central zone where hot air and liquid droplets are injected axially. In a peripheral zone, mild-temperature air (~100°C) is injected through two vortex chambers at both ends of the drying chamber to generate a rotational flow and resulting centrifugal force. Under the action of the latter, initially dried particles are rapidly evacuated to the periphery where final drying is achieved. The drying air is evacuated via a central exhaust in one of the end walls of the chamber. Simulation results are compared with experimental measurements on a pilot-scale RMD unit. As well dry experiments and experiments with water and milk evaporation were carried out and detailed axial and radial temperature profiles were measured that disclose the flow pattern of both air and droplets/particles, as well as the drying performance in different zones of the chamber.

References

[1] M.W. Baltussen, K.A. Buist, E A.J.F. Peters, J.A.M. Kuipers. Multiscale modelling of dense gas–particle flows. In A. Parente, & J. De Wilde (Eds.), Bridging scales in modelling and simulation of non-reacting and reacting flows. Part II (Vol. 53, pp. 1-52). Advances in Chemical Engineering; 53, 2018, Amsterdam.

[2] E. Weinan. Principles of multiscale modeling. 2011, Cambridge, United Kingdom: Cambridge University Press.

[3] A. de Broqueville, J. De Wilde, T. Tourneur, Device for treating particles in a rotating fluidized bed, WO/2018/203745, November 2018.

[4] T. Tourneur, A. de Broqueville, A. Sweere, A. Poortinga, A. Wemmers, U. Jamil Ur Rahman, A.K. Pozarlik, Juray De Wilde. Experimental and numerical study of a radial multi-zone vortex chamber spray dryer. 12th European Congress of Chemical Engineering 2019 - Florence, Italy.

Acknowledgements

The authors would like to acknowledge the technical support of Luc Wautier, the support of the Institute for Sustainable Process Technology (ISPT) and the financial support by the Dutch Rijksdienst voor Ondernemend Nederland (RVO). This project is co-funded by TKI-E&I with the supplementary grant 'TKI- Toeslag' for Topconsortia for Knowledge and Innovation (TKI’s) of the Ministry of Economic Affairs and Climate Policy.