CHAPTER 7
DRYING (cont'd)

MASS TRANSFER IN DRYING

In heat transfer, heat energy is transferred under the driving force provided by a temperature difference, and the rate of heat transfer is proportional to the potential (temperature) difference and to the properties of the transfer system characterized by the heat-transfer coefficient. In the same way, mass is transferred under the driving force provided by a partial pressure or concentration difference. The rate of mass transfer is proportional to the potential (pressure or concentration) difference and to the properties of the transfer system characterized by a mass-transfer coefficient.

Writing these symbolically, analogous to q = UA DT, we have

dw/dt = k'_g A DY                                                     (7.3)

where w is the mass being transferred kg s^-1, A is the area through which the transfer is taking place, k'_g is the mass-transfer coefficient in this case in units kg m^-2 s^-1 , and Y is the humidity difference in kg kg^-1.