Mechanically Agitated Thin-Film Evaporators

These evaporators, sometimes called oxscraped-film evapo- 

The rotors may be one of several zero-clearance designs, a rigid fixed clearance type, or in the case of tapered rotors, an adjustable clearance construction type (Fig. 16). One vertical design includes an optional residence time control ring at the end of the thermal surface to hold back product and thus build up the film thickness. The majority of thin-film evaporators in operation are the vertical design with a cylindrical fixed-clearance rotor shown in Fig. 14. 

Mechanically agitated thin-film evaporators are used for four general types of applications  

Liquids containing a large amount of dissolved or suspended solids 

The one-pass, plug flow operation of a thin-film evaporator is an advantage for minimizing thermal degradation of a heat sensitive product in an evaporation step. The mean residence time in the evaporator can be just seconds, rather than minutes or hours in a recirculating evaporation system. For this reason, thin-film evaporators are widely used for heat sensitive food, pharmaceutical, and other chemical products. Also, it should be noted that the thin-film evaporator can be operated at a higher temperature to make a better separation, whereas care must usually be taken to keep the product temperature lower in an evaporation system with longer residence times (see Fig. 21, later in this chapter). 

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Figure 14. Mechanically agitated thin-film evaporator, vertical design with cylindrical thermal zone. (Luwa Corporation.)...

Freese, H. L., Mechanically Agitated Thin-Film Evaporators, from the short com Evaporation Technology, p. D-2, The Center for Professional Advancement, East Brunswick, New Jersey (May 2-24, 1978)... 

In agitated thin-film evaporators, a thin layer of solution is spread on the heating surface by mechanical means. Wiped-film evaporators are used for very viscous materials and for producing solid products (see Figure 10.34). The design and applications of this type of evaporator are discussed by Mutzenburg (1965), Parker (1965), and Fischer (1965). 

T iquid films, whether in the form of natural falling films or in the more advanced form of mechanically agitated films in thin-film apparatus, offer convenient conditions for the solution of many difficult processing problems because of the films favorable surface-to-volume ratio and good heat transfer. This is especially so with an agitated film between heating wall and film. The agitated-film principle has previously found its broadest application in thin-film evaporators. 

In mechanically aided film evaporators, a thin film of material is maintained on the heat transfer surface irrespective of the viscosity. This is usually achieved by means of a rotor, concentric with the tube, which carries blades that either scrape the tube or ride with low clearance in the film. Mechanical agitation permits the evaporation of highly viscous materials or those that have a low thermal conductivity. Because the temperature variations in the film are reduced and residence times are shortened, the vacuum evaporation of viscous thermolabile materials becomes possible. 

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