modelling drying and particle formation in spray towers

1. modelling drying and particle formation in spray towers Christopher Handscomb Wednesday 9th May 2007

2. outline • Introduction to spray drying • Modelling approach • Continuous phase gas flow • Single particle drying • Conclusions and further work

3. what is spray drying? • An important technology in industry • Used to produce, for example: • Pharmaceuticals • Food stuffs (e.g. milk powder and coffee) • Detergents • Unique drying technology combining moisture removal and particle formation

4. what is spray drying?

5. motivation A computational model would… • predict the effect of process conditions on final product properties • guide the operator towards safe and efficient operating conditions • facilitate the design of new plant based on physics, rather than correlations

6. modelling approach • Adopt an Eulerian-Lagrangian framework

7. continuous phase • Commercial CFD package – STAR CD – used to model the continuous phase • Well known in industry • Easy to test different geometries • Relatively simple to incorporate sophisticated user defined sub models • Test geometry developed representing a generic spray dryer • Counter current dryer • Single spray nozzle • Height: 22m • Diameter: 4m • 118,807 cells in CFD mesh

8. continuous phase z= 4m z=0.5m • Can fairly easily produce plots of the flow field • Consider a single droplet

9. single particle drying • Consider the drying sub-model • Modelling assumptions: • Three component system: • A – solvent; • B – solute; • D – solid • Spherical particles, 1D model • Small Biot number  uniform particle temperature • Allow for a single centrally located bubble Assumed ideal binary solution

10. single particle drying wet bulb temperature boiling temperature Cheyne, A., Wilson, I., and Bridgewater, J. (2002).

11. single particle drying external coordinates internal coordinates advection terms diffusion terms source term • Population balance for solids Cheyne, A., Wilson, I., and Bridgewater, J. (2002). • Spherical symmetry  reduce to 1-D • Solve for the moments of this equation

12. single particle drying assumed independent of internal coordinate (particle size) • Variable of interest is solids volume fraction • Related to the moments of the population balance equation by: • Obtained by solving the moment system:

13. single particle drying Volume Averages Superficial Intrinsic diffusion evolution advection crystallization Total • Volume averaged transport equations for the continuous phase • Advection velocity calculated from volume conservation considerations • Diffusion coefficient from measurements

14. single particle drying • Population balance boundary conditions • Solute boundary conditions

15. moving boundary virtual flux • Moving boundary handled through a standard coordinate transformation r  z: • This adds a ‘virtual flux’ to all equations

16. solution method • Problem is a system of PDEs and coupled ODEs • Solved using Numerical Algorithms Group (NAG) library routines for convection-diffusion type equations • Finite Volume approach with user-defined flux function

17. new drying model – example • Model described so far can simulate up to the point of shell formation • e.g. Consider a system: • Initial 14wt% sodium sulphate solution – no solids • Crystallisation model from Rosenblatt et al. (1984): ‘Kinetics of Phase Transitions in the System Sodium Sulphate-Water’ • Droplet diameter = 1.78mm • Drying air T = 373K • Droplets initially well mixed

18. new drying model – example • Compare with experimental data from Nesic and Vodnik (1990) Kinetics of Droplet Evaporation Chem. Eng. Sci. Experimental data from Nesic and Vodnik (1990) Kinetics of Droplet Evaporation Chem. Eng. Sci.

19. new drying model – example But the new model can give us much more… Experimental data from Nesic and Vodnik (1990) Kinetics of Droplet Evaporation Chem. Eng. Sci.

20. new drying model – example

21. new drying model – example

22. new drying model – example

23. new drying model – example

24. conclusions… • Introduction to spray drying and the associated modelling challenges • Results of continuous phase simulation • Overview of a new drying model • Comparison with experiments for a ‘simple’ case…

25. …work not shown… • Drying after shell formation • Simulation of detergent droplets drying with experimental comparison • Simplified drying models implemented in CFD code

26. …and further work • Obtain data and validate model for high temperature drying • Couple (simplified) model to CFD simulation • Compare with existing drying models when used in CFD

27. acknowledgements