(633f) Experimental and Numerical Study of a Radial Multi-Zone Vortex Chamber Spray Dryer

High-G operation in vortex chambers allows intensifying interfacial mass, heat and momentum transfer [1]. In previous studies, application to particle drying [2,3] and fine particle coating [4] was demonstrated. Combining high-G intensified gas-solids contact, gas-solids separation and solids segregation was also considered [5]. In the present work, a variant called the radial multi-zone dryer (RMD) [6] is experimentally and numerically studied focusing on application in spray drying, characterized by relatively dilute operation. Two distinct temperature zones are created without physical barrier, a radially central zone where hot air with a temperature up to 350°C is injected and a peripheral zone where air injected through vortex chambers at a temperature of around 100°C.

The proposed multi-zone operation allows significant process intensification while preventing degradation of the product. Liquid droplets are injected in the hot central zone, co- or counter-current with the hot air. Fast initial drying is achieved in the hot central zone while burning the product is prevented by rapid evacuation of the produced particles to the colder periphery under the action of the centrifugal force generated by the vortex chamber(s). Typical residence time of the particles in the hot zone can be limited to a few milliseconds. Drying is continued in the colder periphery where high-G operation intensifies interfacial mass, heat and momentum transfer and ensures efficient gas-solids separation.

The experimental set-up that was used is flexible and allows studying various multi-zone concepts, varying the dimension/design of the different zones and the type of spray nozzle. Extension chambers allowing to minimize air consumption and to optimize gas-solids separation can be added. Tests were carried out with total air flow rates of maximum 1000 Nm³/h, liquid flow rates of up to 6 g/s and hot air temperatures of maximum 350°C. Both hollow-cone and full-cone nozzles were tested. Three types of experiments were carried out: (i) in the absence of liquid injection, (ii) injecting water, and (iii) injecting milk. The axial and radial temperature profiles in the device were measured by means of 16 thermocouples. Comparison of the profiles in the three types of experiments and visual observations allowed gaining understanding in the complex flow pattern of the gas and droplets and the extent of evaporation in specific regions. Deposition of powder on the walls and in the gas and solids outlets was also carefully analyzed. Protection of the nozzle from fouling required a new protective sleeve design.

To gain further insight in the flow pattern, CFD simulations were carried out, adopting the RANS-approach and a coarse-grained discrete particle model to track the motion of the droplets. Evaporation was accounted for and the behavior of droplets of different size studied.

Experimental studies and CFD simulations of spray drying in a radial multi-zone spray dryer show that the technology is feasible and promising for intensified two-step drying. Efficient separation of a hot central zone and cold periphery and an extremely short residence time of the droplets/particles in the central hot zone can be achieved. As a result, using 350°C hot air, high-quality milk powder could be produced. A full cone nozzle was shown to be more efficient and stable than a hollow cone nozzle, especially with counter-current injection of hot air.

References

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2. Eliaers, Philippe ; De Wilde, Juray. Drying of Biomass Particles: Experimental Study and Comparison of the Performance of a Conventional Fluidized Bed and a Rotating Fluidized Bed in a Static Geometry. In: Drying Technology, Vol. 31, no.2, p. 236-245 (2013)
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4. 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, In Powder Technology, Volume 258, 2014, Pages 242-251, ISSN 0032-5910
5. Juray De Wilde, George Richards, Sofiane Benyahia, Qualitative numerical study of simultaneous high-G-intensified gas–solids contact, separation and segregation in a bi-disperse rotating fluidized bed in a vortex chamber, Advanced Powder Technology, Volume 27, Issue 4, July 2016, Pages 1453-1463, ISSN 0921-8831
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Acknowledgements

The authors would like to acknowledge the technical support of Luc Wautier, the support of the Dtuch Institute for Sustainable Process Technology (ISPT) and the financial support by the Dutch Rijksdienst voor Ondernemend Nederland (RVO).