1. Wittaya Julklang, Boris Golman School of Chemical Engineering Suranaree University of Technology STUDY OF HEAT AND MASS TRANSFER DURING FALLING RATE PERIOD OF SPRAY DRYING OF A SLURRY DROPLET WITH NANOPARTICLES

2. School of Chemical Engineering Introduction 1/20 Suranaree University of Technology Suranaree University of Technology  Spray drying is used in chemical, agricultural, food, polymer, pharmaceutical, ceramic and mineral processing industries. High energy efficiency comparing to other drying methods -atomization of feed slurry to small droplets generates large surface area for both heat and mass transfer Flexibility in meeting product requirements -free flowing powder Continuous, large-scale operation Design of high-quality product  Currently there is a growing interest in application of agglomerate of nanoparticles. Design of spray drying equipment Optimization of spray drying process  Studying on the drying kinetics of slurry droplet including with nanoparticle is important for

3. School of Chemical Engineering Drying mechanism of a slurry droplet Suranaree University of Technology Suranaree University of Technology 2/20 Fig.1 Typical drying behavior of a slurry droplet. A B C D E F Droplet Average Temperature Drying Time A – B : Initial heating up B – C : Constant rate period C – D – E : Falling rate period E – F : Final heating Fig.2 Variation of average droplet temperature with drying time. A Droplet at initial temperature E End of drying F Particle at final temperature Heating of droplet up to wet-bulb temperature Drying with droplet shrinkage Drying with crust formation Increasing thickness of crust layer B Droplet at wet-bulb temperature C Droplet at final Shrinkage D Droplet at crust formation Sensible heating of dried particle

4. 3/20 Fig.2 Droplet drying in falling rate period.  Heat balance  Mass balance (2) (3) School of Chemical Engineering  Wet core (0 ≤ r ≤ s )  Evaporation interface (r = s) Suranaree University of Technology Suranaree University of Technology (1)  Temperature distribution Mathematical model of drying in falling rate period

5. 4/20 School of Chemical Engineering  Concentration distribution (5)  Dry crust (s ≤ r ≤ R in )  Temperature distribution (4)  Agglomerate surface (7)  Heat balance  Mass balance (6) Fig.2 Droplet drying in falling rate period. Mathematical model of drying in falling rate period

6. Validity of mathematical model School of Chemical Engineering 5/20 Suranaree University of Technology Suranaree University of Technology o Good agreement of model calculation results and experimental data for both weight and temperature of a droplet. Fig.3 Comparison of model calculation results with experimental data. o Experimental data are taken from : Nesic, J. Vodnik, Kinetics of droplet evaporation, Chemical Engineering Science 46 (1991) 527-537.

7. Results School of Chemical Engineering 6/20 Suranaree University of Technology Suranaree University of Technology In this work we studied heat and mass transfer both inside and outside of a agglomerated product by variation of following drying parameters:  Air temperature  Air flow rate  Porosity of agglomerated product

8. Results School of Chemical Engineering 7/20 Suranaree University of Technology Suranaree University of Technology o The evaporation interface moves linearly and slowly to center of droplet in the constant rate period. Moving of evaporation interface Fig.4 The movement of the evaporation interface with drying time o The evaporation interface moves rapidly to center of agglomerated product and the moving is not linear.

9. Results School of Chemical Engineering 9/20 Suranaree University of Technology Suranaree University of Technology Variation of air flow rate Fig.5 Convective heat and mass transfer coefficients with air flow rate o The rates of convective heat and mass transfer rise at high air flow rate  Heat supplied to surface of agglomerate product  Mass transfers from surface of agglomerate product to drying air

10. Results School of Chemical Engineering 10/20 Suranaree University of Technology Suranaree University of Technology o The surface and wet core temperatures increase at high air flow rate Variation of air flow rate o The temperature difference between surface and wet core increases as thickness of dry crust increase Fig.6 Temperature profile inside the agglomerate at different air flow rate

11. Results School of Chemical Engineering 11/20 Suranaree University of Technology Suranaree University of Technology Variation of air flow rate Fig.7 Concentration of water vapor inside the agglomerate at different air flow rates o At the same thickness of dry crust the concentration of water vapor inside the dry crust slightly increase at high air flow rate. o As the thickness of dry crust increases, the accumulation of water vapor inside dry crust rises

12. Results School of Chemical Engineering 12/20 Suranaree University of Technology Suranaree University of Technology Variation of air flow rate o No difference in water vapor concentration is observed at the agglomerate surface. Fig.7 Concentration of water vapor inside the agglomerate at different air flow rates

13. Results School of Chemical Engineering 13/20 Suranaree University of Technology Suranaree University of Technology o The agglomerated product is dried more quickly at high air flow rate. Variation of air flow rate Fig.8 Dimensionless weight of the agglomerate dried at various air flow rates.

14. Results School of Chemical Engineering 14/20 Suranaree University of Technology Suranaree University of Technology Variation of air temperature Fig.9 Convective heat and mass transfer coefficients with drying air temperature. o The rates of convective heat and mass transfer rise at high air temperature  Heat supplied to surface of agglomerate product  Mass transfers from surface of agglomerate product to drying air

15. Results School of Chemical Engineering 15/20 Suranaree University of Technology Suranaree University of Technology o The drying rate of agglomerated product in the falling rate period rise at high air temperature. Variation of air temperature Fig.10 Dimensionless weight of the agglomerate dried at various air flow temperature.

16. Results School of Chemical Engineering 16/20 Suranaree University of Technology Suranaree University of Technology Variation of porosity of agglomerated product Fig.11 Convective heat and mass transfer coefficients with agglomerated product size. o The rates of convective heat and mass transfer declines at large size of agglomerated product  Heat supplied to surface of agglomerate product  Mass transfers from surface of agglomerate product to drying air

17. Results School of Chemical Engineering 17/20 Suranaree University of Technology Suranaree University of Technology o As the porosity of agglomerated product rises, the mass transfer inside the dry crust increases but the heat transfer decreases. Fig.12 The effective diffusivity and thermal conductivity of dry crust with porosity of agglomerated product Variation of porosity of agglomerated product

18. Results School of Chemical Engineering 18/20 Suranaree University of Technology Suranaree University of Technology o The drying time of low porosity agglomerate is shorter in comparison with loose agglomerate Fig.13 Dimensionless weight of the agglomerate dried at various porosity of agglomerated product. Variation of porosity of agglomerated product

19. Conclusions School of Chemical Engineering 19/20 Suranaree University of Technology Suranaree University of Technology  Dry crust and wet core temperatures increased with drying time during the falling rate period due to the accumulation of heat in the dry crust  The difference in temperature between the agglomerate surface and the wet core raised with time as a result of heat transfer resistance of the growing crust layer  The accumulation of water vapor in the crust also increased with drying time owing to the enlarging mass transfer resistance

20. Conclusions School of Chemical Engineering 20/20 Suranaree University of Technology Suranaree University of Technology  The rate of mass transfer enhanced at the same position in the crust layer at higher crust temperature.  The drying rate in the falling rate period is governed by the heat and mass transfer resistances both inside and outside the agglomerate