Jangam, Mujumdar - Classification and Selection
Drying time (^) • Drying curves
- Effect of process variables
Special requirements • Material of construction - Corrosion
- Toxicity
- Non-aqueous solution
- Flammability limits
- Fire hazard
- Color/texture/aroma requirements (if any)
Foot print of drying system (^) • Space availability for dryer and ancillaries
Drying kinetics play a significant role in the selection of dryers. Aside from simply
deciding the residence time required, it limits the types of suitable dryers. Location of
the moisture (whether near surface or distributed in the material), nature of moisture
(free or strongly bound to solid), mechanisms of moisture transfer (rate limiting step),
physical size of product, conditions of drying medium (e.g., temperature, humidity, flow
rate of hot air for a convective dryer), pressure in dryer (low for heat-sensitive prod-
ucts), etc., have a bearing on the type of suitable dryer as well as the operating condi-
tions. Most often, not more than one dryer type will likely meet the specified selection
criteria. Finally we will discuss a bit on advanced drying techniques as well.
3. 4. COMPARISON OF CONVENTIONAL DRYERS BASED ON FEED
PROPERTIES
As discussed earlier there are various drying options available for food products.
The first way to compare the dryer types for preliminary selection is based on the type
of feedstock and residence time in a dryer to be handled. The type of feed material may
be liquid (fruit juices, milk), slurry, cake (partially dried fruit pulp mixed with additives),
free flowing solids (green peas, grains) or formed solids. Selection of a proper drying
technique based on the physical form of the feed is very useful step. Table 3. 3 shows the
variety of dryers available based on the type of feed.
3.5. SELECTION OF FLUIDIZED BED DRYERS
Fluidized bed dryers are one of the most common dryer types used for decades be-
cause of the high heat and mass transfer rates. Fluidized bed dryers have wide applica-
tions in food industry for drying of variety of products such as, green peas, sweet corn,
coffee, tea, milk powder, grains, etc and more recently for drying of pastes and slurries.
In recent years, there have been numerous improvements in the design of fluidized beds
to enhance the efficiency. Particles/powders can be classified into four major groups
according to their characteristics when exposed to fluidizing gas stream.