ABSTRACT
Introduction ........................................................................................................ 525
Methodology....................................................................................................... 526
Results and Discussion ..................................................................................... 526
Conclusion .......................................................................................................... 529
Acknowledgments ............................................................................................. 529
References ........................................................................................................... 529
The complexity of the spray-drying process makes experimental study of
mass transfer in a real spray dryer impractical (Masters, 1985). Authors have
studied changes in moisture content of the material during the process by
suspending a drop of the liquid to be dried in a fixed position under an air
stream (Ferrari et al., 1989; Adhikari et al., 2000; Dolinsky, 2001). Droplet
diameter or particle size is a parameter of interest, but it is difficult to
estimate during drying given the many variables having an influence in the
spray-air contact stage. At the same time, phenomena related to
evaporation such as shrinkage, deformation, size increase, formation of
crust, and crush start to arise and will, in turn, influence actual heat and
mass transfer (McCormick, 1962; Chawla, 1994; Oakley, 1997). In spray-
drying processes, drying stages can be related to moisture evaporation and
morphology development of the particle (Kieviet and Kerhof, 1997; Oakley,
1997). Drying zones inside the chamber and change of moisture content
during dehydration may help to establish links between drier design and
product quality. An interesting option that would allow evaluation of the
height of drying stages inside the spray chamber is the application of the
concept of the heat transfer unit, which would represent an option and a
contribution to spray-drier design and operation. This method considers
that the longitude of the path that one of the fluids travels through the dryer
is formed by the number of transfer units (NtOG), which is a dimensionless
relationship between the difference of the temperature of the drying air and
the heat driving force. The height of the transfer unit (HtOG) is a function of
the airflow, overall heat transfer coefficient, and the heat capacity of drying
air. The product of the height of the heat transfer unit times the number of
transfer units is equal to the total height of the effective drying stages
(Masters, 1985).
