Rising film evaporator

As the temperature of the miscella exiting the first stage evaporator is low, it is a good heat sink for heat recovery. In various facilities, heat from hot finished oil, heat from steam ejector exhausts, or recovered flash steam is used to preheat the miscella to approximately 75°C in temperature. The preheated, concentrated miscella is then typically heated to 110°C in a steam-heated exchanger prior to entering the second rising film evaporator. 

It is difficult to distribute the entire product liquid to achieve the same thin liquid film in all tubes and to avoid the disrapture of the liquid film Surface tension effects (the surface tension can increase or decrease in the film downward flow) are decisive for the film stability (Zuiderweg and Harmens 1958 Ford and Missen 1968). Cocurrent once-through rising film evaporators can be used for a complete evaporation of the feed. However, fouling may occur at high evaporation rates. 

The 73% caustic is made by feeding steam instead of water to the decomposer, and also by evaporation of 50% caustic. Anhydrous caustic is produced in a rising film evaporator, operating at 385°C under vacuum. 

Since membrane-cell evaporators do not produce solids, forced-circulation evaporators are used less frequently. Rising-film and falling-film types appear in a number of plants. The rising-film evaporator depends on natural circulation of caustic from the bottom to the top of the tubes. Falling-film evaporators, as shown in Section 9.3.S.2, depend on pumps to lift caustic to the distribution system at the top. These units generally have better heat-transfer coefficients and less tendency to foul. Recirculated units in particular allow good control of flow to maintain a proper film on the tubes. This also permits the designer to provide more turndown capability. Liquid velocities are lower... 

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