Evaporator selection

Product Quality Considerations of product quahty may require low holdup time and low-temperature operation to avoid thermal degradation. The low holdup time eliminates some types of evaporators, and some types are also eliminated because of poor heat-transfer charac teristics at low temperature. Product quality may also dic tate special materials of construction to avoid met hc contamination or a catalytic effect on decomposition of the product. Corrosion may also influence evaporator selection, since the advantages of evaporators having high heat-transfer coefficients are more apparent when expensive materials of construction are indicated. Corrosion and erosion are frequently more severe in evaporators than in other types of equipment because of the high hquid and vapor velocities used, the frequent presence of sohds in suspension, and the necessary concentration differences. 

The next step is to decide a suitable evaporating temperature. This will be set by the required load condition and the appropriate temperature differential (AT) across the evaporator. In the context of evaporator selection, the AT used is the difference between the evaporating refrigerant and the temperature of the fluid entering the cooler, not the log mean temperature difference (see [1-5]). 

Plasticizers are not chemically bound in most cases, thus they can be separated by either extraction or evaporation. Selection of solvents for extraction includes the following factors ... 

Pressure drop is one of the process variables which have the greatest effect on the size (cost) of an evaporator. Selection of optimum pressure drops involves consideration of the overall process. While it is true that higher pressure drops result in smaller equipment, investment savings are realized only at the expense of operating costs. Only by considering the relationship between operating costs and investment can the most economic pressure drop be determined. 

Application for Thin-Film Evaporators Thermal separation in an evaporator may be conveniently characterized by the viscosity of the nonvolatile stream—the concentrate. Figure 11-27 illustrates various evaporator types and typical viscosity ranges for their useful applications. Unless other considerations are important (thermal stability, fouling tendencies), the terminal viscosity frequently dictates the type of evaporator selected. By far, most evaporation... 

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