Analysis of Drying Kinetics of a Slurry Droplet in the Falling Rate Period of Spray Drying

The heat and mass transfer was investigated during the falling rate period of spray drying of a slurry droplet. The effect of the porosity of crust layer formed from primary particles during liquid evaporation was studied numerically using the developed mathematical model which takes into account the heat and mass transfer in the core and crust regions, the movement of the evaporation interface, and the external heat and mass transfer between the drying air and the droplet surface. It was confirmed that the heat transfer through the crust layer was more intense in the case of the dense droplet than the loose one due to the enhanced thermal conduction resulting in the higher average droplet temperature. The mass transfer was facilitated in the crust layer of loose droplet owing to the large pore space available for diffusion of water vapor from the evaporation interface to the outer droplet surface. The longer drying time is required for the droplet of high porosity to reach the final moisture content than that for the dense one due to the larger amount of water to be evaporated during the falling rate.





References:
[1] B. Bhushan, Ed., Springer Handbook of Nanotechnology, 3rd ed. New
York: Springer, 2010.
[2] K. Sollohub, and K. Cal, “Spray drying technique: II. Current
applications in pharmaceutical technology,” Journal of Pharmaceutical
Sciences, Vol. 99, pp. 587–597, 2010.
[3] A. B. Nandiyanto, and K. Okuyama, “Progress in developing spraydrying
methods for the production of controlled morphology particles:
From the nanometer to submicrometer size ranges,” Advanced Powder
Technol., vol. 22, pp. 1–19, 2011.
[4] K. Masters, Spray Drying Handbook, Longman Scientific and
Technical, Harlow, England, 1985.
[5] M. Mezhericher, A. Levy, and I. Borde, “Theoretical models of single
droplet drying kinetics: a review”, Drying Technology, Vol. 28, pp. 278-
293, 2010.
[6] W. Julklang, and B. Golman, “Influence of operational conditions on the
drying kinetics of a slurry droplet in a spray dryer,” Engineering
Transactions, Vol. 15, pp. 59-65, 2012.
[7] W.Woodside, and J.H.Messmer, “Thermal conductivity of porous
media. I. Unconsolidated sands,” J. Appl. Phys., vol. 32, 1688, 1961.
[8] I.H. Tavman, “Effective thermal conductivity of granular porous
materials,” Int. Comm. Heat Mass Transfer, vol. 23, pp. 169 -176, 1996.
[9] G. E. Archie, “The electrical resistivity log as an aid in determining
some reservoir characteristics,” Trans. AIME, vol. 46, pp. 45–61, 1942.
[10] N. Dalmaz, H.O. Ozbelge, A.N. Eraslan, and Y. Uludag, “Heat and mass
transfer mechanisms in drying of a suspension droplet: a new
computational model”, Drying Technology, Vol. 25, pp. 391-400, 2007.
[11] W.E. Ranz, and W.R. Marshall, “Evaporation from drops”, Chemical
Engineering Progress, vol. 48, pp. 141-146, 1952.
[12] J. Crank, Free and Moving Boundary Problems, Clarendon Press,
Oxford, 1984.