Experimental and Numerical Study of a Pilot Scale Radial Multi-Zone Spray Dryer

High-G operation in vortex chambers allows intensification of interfacial mass, heat and momentum transfer [1]. Applications earlier studied include particle coating [2] and particle drying [3,4]. Combined high-G intensified gas-solids contact, gas-solids separation and solids segregation was demonstrated [5,6]. Spray drying is a novel application that is characterized by relatively low particle concentrations. Multi-zone vortex chamber technology was developed aiming at using hot air while avoiding product degradation [7]. This allows drastically reducing the chamber volume and the air consumption, while offering energy consumption comparable to that of conventional technology. Hot air (>350°C) and liquid droplets are injected in the radially central zone of the drying chamber where initial drying is achieved in milliseconds. By the action of the centrifugal force generated by the injection of mild-temperature air (~100°C) through vortex chambers at both ends of the drying chamber, initially dried particles are rapidly evacuated to the periphery for final drying. The design of the vortex chambers depends on the required centrifugal force and mild-temperature air flow rate. Furthermore an axial pressure difference can be generated to control the axial motion of gas and particles in the chamber.

In this work, a pilot scale radial multi-zone spray dryer (RMD) is experimentally and numerically studied. The pilot with a chamber volume of less than 0.5 m³ was designed targeting a production of 100 kg/h of milk powder. A previously used design [8] was linearly scaled up by about a factor of two. A cyclonic extension was added to improve the separation of fine particle from the exhaust air. To generate a sufficiently large and dense cloud of droplets in the radially central zone, an arrangement of four atomizers was used, integrated in a specially designed protective sleeve to protect the nozzle tips from fouling. Experiments were carried out feeding air only or co-feeding water or milk. Detailed axial and radial temperature profiles were measured and comparison of the profiles without and with water injection together with visual observations allowed gaining insight in the flow pattern of both gas and droplets and the extent of evaporation in specific regions of the chamber. With mil spray drying experiments, particles recovered through different solids outlets and the through the air exhaust were sampled and analyzed.

A deeper understanding of the flow pattern and the influence of certain design parameters was obtained through Computational Fluid Dynamics (CFD) simulations. Steady state and unsteady Reynolds-averaged Navier–Stokes (RANS) were combined, using a coarse-grained Lagrangian approach for the discrete phase (droplets/particles) and the SST k-ω turbulence model. Evaporation was accounted for and the behavior of droplets of different size studied. A comparison with experimental observations was made.

References

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[2] Philippe Eliaers, Axel de Broqueville, Albert Poortinga, Tom van Hengstum, Juray De Wilde, High-G, low-temperature coating of cohesive particles in a vortex chamber, Powder Technology, 258, 2014, 242-251.

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[6] Abhishek Dutta, Rahul P. Ekatpure, Geraldine J. Heynderickx, Axel de Broqueville, Guy B. Marin. Rotating fluidized bed with a static geometry: Guidelines for design and operating conditions. Chemical Engineering Science, 65 (5), 2010, 1678-1693.

[7] Axel de Broqueville, Juray De Wilde, Thomas Tourneur, Device for treating particles in a rotating fluidized bed, WO/2018/203745, November 2018.

[8] Thomas Tourneur, Axel de Broqueville, Anton Sweere, Albert Poortinga, Anton Wemmers, Umair Jamil Ur Rahman, Artur 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.