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 varying 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 possible 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.
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 conventional and innovative fluidized beds based on different operational variants.
Hybrid drying technologies
The use of hybrid technologies is being employed recently mainly to hasten the drying 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 removed 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 nutritional as well sensory properties of food products making it unacceptable.
Use of high temperature to remove this part of water at faster rate may lead to case hardening and similar problems. The best way to enhance the drying rate is to either to use multi-stage drying 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.
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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 J
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