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Process-design, mechanical-design and physical-property parameters must all be taken into account Agitated thin-film evaporators are attractive for the concentration, distillation, stripping or deodorization of liquids in a broad variety of chemical-process-industries (CPI) applications where the process streams are temperature sensitive (and must have only brief exposure to heat), viscous, or tend to foul or foam. When this type equipment does seem to be the right choice in a given situation, the common, and sound, approach is to ask an evaporator manufacturer to conduct an evaluation and make an equipment quotation. The quality of such an evaluation will depend on the type and quality of the data that the manufacturer receives from the potential customer's engineers; and, perhaps to some extent, on those engineers' familiarity with the evaluation process itself.
The thin-film process
That familiarity begins with an understanding of what a thin-film evaporator is and how it works. A vertical thin-film evaporator consists of two major assemblies: a heated body and a close-clearance rotor (Figure 1). The process fluid enters the unit tangentially above the heated zone, and is distributed evenly over the inner surface of the body wall by a distribution ring mounted on the rotor. The rotor blades spread the product over the entire heated wall, and generate highly turbulent flow conditions in the thin layer of liquid (Figure 2).
The product spirals down the wall, while the turbulent conditions developed by the rotor blades generate optimal heat flux, rapidly evaporating volatile components. The resulting vapors flow upward through the unit into a centrifugal separator, which returns entrained droplets or froth directly back to the heating zone. Clean vapors pass through the vapor outlet ready for condensing or further processing. Meanwhile, the concentrated liquid stream leaves the evaporator through its bottom conical outlet. Continuous washing by the bow waves generated by the rotor (Figure 2) minimizes surface fouling of the thermal wall, where the concentrated liquid or residue is most prevalent.
Thin-film evaporators are commercially available in various basic or standard versions. They can have vertical or horizontal designs, with cylindrical or tapered bodies and rotors. The rotor can employ any of several zero-clearance designs, or a rigid fixed-clearance design, or an adjustable-clearance type. The basics for scale-up...