Drying indirect

Figure 4.22 Indirect, dry-type cooling tower condensing system employing a natural-draft tower.

SECTION I INDIRECT DRYING (by Giovanni Bellini and Laura Pellegrini)... 

The drying operation is often the final step of a manufacturing process. Indirect drying will be discussed in this section it is the process of removing liquid by conductive heat transfer. 

In indirect or nonadiabatic drying, the heat is transferred by conduction from a hot surface, first to the material surface and then into the bulk. This chapter discusses only indirect drying. 

Avoid agitation of material conveyor, tunnel, tray dryers Inert drying medium vacuum dryer, superheated steam dryer As above lower oxygen concentration in drying indirect drying Pre-condition solids backmixing, pelletization Vacuum or freeze dryers Freeze dryers (to retain aroma)... 

Kress Indirect Dry Cooling System, Bethlehem Steel s Coke Plant Demonstration at Sparrows Point, Maryland... 

The effects of water on the destruction of the protective food structure in some specific dehydrated foods is probably involved in prevention of lipid oxidation in heated meat systems (Karel, 1986). In systems in which there are both surface lipids and lipids encapsulated within a carbohydrate, polysaccharide, or protein matrix, the surface lipids oxidize readily when exposed to air. The encapsulated lipids, however, do not oxidize until the structure of the encapsulated matrix is modified and/or destroyed by adsorption of water as shown by Simatos and Karel (1988). In some IM meats, muscle may be considered as being encased in or surrounded by a humectant matrix. However, free lipid may be left on the surfaces. The unwarranted overuse of lipids, which often happens in the indirect drying process to improve heat transfer and to prevent burning, is detrimental to the products. Prevention of this structural change is of considerable... 

Under a wider definition of indirect dryers one may technically include radiant and microwave/radio frequency (RF) dryers as there is no direct contact between the drying solid and the heat source (or medium). However, these dryer types are covered elsewhere in the handbook. Drum drying of thin pastes also involves indirect drying but it is covered under a different chapter in this handbook. 

Source Adapted from Walsh, J.J., Indirect Drying of Solids Particles, Bepex Corporation, Minneapolis, MN, 1992. 

Kimball G. Direct vs. indirect drying Optimizing the process. Chemical Engineering, 108, 74—81, May 2001. 

FIGURE 14.34 Solar-assisted indirect drying system for alfalfa (a) scheme of system (b) reduced mass flow network model of the fluid (c) reduced mass flow network model of the airflow system. 

No commercial dryers exist for direct drying of paper webs with superheated steam. For the first time, Mujumdar proposed a variety of possible configurations for direct and direct and indirect drying of paper using superheated steam [15]. These include... 

Aside from conventional heat sources used for direct and indirect drying, an interesting option is direct combustion of organic solvent inside the drying chamber, especially when the material to be dried contains the flammable solvent left over from the technology process. An example is the Rcmallam technology developed for textile industry where the fabrics soaked with aqueous solution of methanol is bnmed under carefully controlled conditions (see Section 21.3.10). 

Therefore, in an indirect drying system, the dried sludge is frequently mixed with raw sludge to have a solid content of about 70% in order to avoid the sticky zone. With further removal of moisture, the sludge becomes crumbly in nature and can mix much more freely. 

Indirect dryers have several distinctive operating features (1) the risk of cross-contamination is avoided since the product does not contact the heating medium (2) since a limited amount of gas is encountered, solvent recovery is easier than with an adiabatic dryer (3) dusting is minimized because of the small volume of vapors involved in indirect drying ... 

Althongh this classification of dryers has some importance, it is quite difficult to apply it in more than a general way. Both types of dryers are commonly used in polymerdrying processes. Often a combination of direct and indirect drying is economically the most efficient solution to some polymer-drying problems. 

In Australia, indirect drying has been used for many decades to prepare brown coal for briquetting. In this process, low-pressure steam is condensed on the outside of tubes conveying brown coal to provide process heat the more water removed in liquid form, the higher the efficiency. After reducing the moisture to nominally 15%, dried brown coal is compacted into strong briqnettes nsing stamp presses. 

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