Slide 1

Jangam, Mujumdar - Classification and Selection

Figure 3.5. Classification of superheated steam drying

6.3.2. Developments in fluid bed drying

We already have summarized the large number of possible variants of FBDs that are
now used to dry not only particulates (which was the original idea) but also slurries,
pastes, continuous webs and sheet-form materials. Large pieces that cannot be fluidized
by themselves can be immersed in a fluidized bed of smaller fluidizable inert particles
and dried. Most of the variants shown are used in industrial drying applications to vary-
ing extent. Many users seem to be unaware of some of these innovative modifications of
the FBDs.

Interestingly, by fluidizing only parts of the particulate bed at a time, it is possi-
ble to effect a major saving in energy costs, e.g., so-called pulsed fluid beds (Gawrzynski
and Glaser, 1996). In batch fluidized bed drying, a control strategy that keeps the bed
temperature constant by adjusting heat input saves energy (and time) while enhancing
the quality of heat-sensitive products (Devahastin and Mujumdar, 1999). Such a dryer
based on a fuzzy logic control is already on the market. Table 3.7 compares the conven-
tional and innovative fluidized beds based on different operational variants.

6.3.3. Hybrid drying technologies

The use of hybrid technologies is being employed recently mainly to hasten the dry-
ing rate. It was noted earlier that drying of most foods lies in falling rate period and that
the drying rate is very sluggish in the final stages when the interstitial water is to be re-
moved and the performance of convection dryers is very poor which results in to longer
residence time. This can lead to higher energy consumption, loss of important nutrition-
al as well sensory properties of food products making it unacceptable. Use of high tem-
perature to remove this part of water at faster rate may lead to case hardening and simi-
lar problems. The best way to enhance the drying rate is to either to use multi-stage dry-
ing approach or using radiative heat source. Similar limitations are associated with
spray dryer. In many cases it is difficult to reach the expected final moisture content
hence multi-stage drying system is used. Figure 3. 6 shows different options available
for hybrid drying system.

Superheated

Steam Dryers

Low pressure

Near

atmospheric

pressure

High Pressure

Example: potato chip,

tortilla chip, shrimp,

paddy, soybean, noodles

Example: Beet pulp

Example: products

which trend to melt or

undergo glass transition