Jangam, Mujumdar - Basic Concepts and Definitionways less than ρv,∞. The surface area for mass transfer can also change as the drying
proceeds.
Chen 1998 has suggested that the surface vapor concentration can be written as
휌푣,푠= 푅퐻푆 ∙ 휌푣,푠푎푡(푇푆) (1. 16)
where, RHS is the relative humidity at the interface, and it becomes 1 if the surface is
covered by a liquid. Hence the equation becomes
푚푆푑푋푑푡�=푘푚 퐴 �푅퐻푆∙ 휌푣,푠푎푡(푇푆)−휌푣,∞� (1. 17)
RHS and ρv,sat can be approximated as follows
푅퐻푆=exp�−푅푇∆퐸푣푆� (1. 18)
휌푣,푠푎푡=퐾푣exp�−푅푇퐸푣푆� (1. 19)
where, ∆Ev is correction factor in apparent activation energy for drying due to the
increasing difficulty of removing water, Ev is termed as activation energy for pure water
evaporation (similar value of latent heat) and Kv is the apparent reaction frequency (for
moist air at 1 atm, Kv ≈ 2.262236 × 10^5 kg.m-3 and Ev ≈ 40.207 × 10^3 J.mol-1. After modifi-
cations the equation becomes
푚푆푑푋푑푡�=− 푘푚 퐴 �퐾푣exp�−퐸푣푅푇+^ ∆퐸푣
푆
�−휌푣,∞�= − 푘푚 퐴 퐾푣exp�−퐸푣푅푇+^ ∆퐸푣
푆 �
+ 푘푚 퐴휌푣,∞ (1. 20)The first term on the right hand side is a zero order drying reaction while the second
term is the first order wetting process. Chen, 200 8 elaborates that if there is no hystere-
sis observed then the equation describes both the drying and wetting process. However,
if there is a hysteresis, then different values of ∆Ev and thus Kv may corresponds to ad-
sorption and desorption processes. There are various suggestions to modify these basic
equations of the reaction engineering approach and can be found elsewhere. The REA
has been successfully used to model drying of various products.
1.4.3. Advanced Drying Techniques for Foods
Although the conventional techniques used for food drying are well established,
there is still need to develop new and innovative drying techniques (Kudra and Mujum-
dar, 2009; Mujumdar and Wu, 2008 ). Major reasons to trigger attempts for development
of advanced drying techniques are: making the process cost effective, reducing the ener-
gy consumption, intensifying the drying rates (in turn reducing the size of the dryer re-
quired), improving the quality of dried food products, reducing the carbon foot print,
increasing safety in operation and making the drying process easy to control. There are
few facts that need to be highlighted with respect to food products. The drying of fruits
mainly falls in the falling rate period hence it becomes a diffusion control process. How-
ever, higher temperatures cannot be implemented to enhance diffusion which may lead
to thermal deterioration of the product. Higher temperature can also lead to unaccepta-
ble quality changes such as case hardening, flavor loss, nutritional loss and color degra-
dation. Presence of microorganisms in drying medium can also lead to unfavorable qual-
ity of dried product; hence it needs to be filtered before using for drying. Freeze drying