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Principles of Climbing Film Evaporators

Climbing film evaporation is an energy saving method used in the chemical engineering field. Its application involves liquid vaporization to concentrate solutions consisting of volatile solvent and nonvolatile solute mixtures, including waste water, protein extracts, food products and chemicals. Climbing film evaporator types include short tube, long tube and agitated film evaporators.

  1. Principle

    • Climbing film evaporators function either continuously or in batches, using simple evaporation principles. Liquid is fed into the base of the evaporator tube, and heated steam is applied to induce circulation. This occurs using either a priming pump or via self-circulation. Vapor is evaporated from the mixture during this process, and the liquid concentrate is collected in its respective receiver.

    Other Separation Methods

    • Climbing film evaporation operates via different principles than other separation methods, such as distillation, crystallization and drying. In evaporation, the desired product is the liquid concentrate -- hence, the vapor phase is not separated into individual components as with distillation. The solute remains in solution rather than formed into crystals as in crystallization. Also, the final product is a liquid, compared to drying, which produces a solid residue.

    Evaporator Performance

    • The capacity of climbing film evaporators is determined by the amount of liquid vaporized each hour, which is measured against the amount of steam fed into the unit to determine the economical feasibility. The rate of heat transfer is dependent on the heat transfer surface area, the overall heat transfer coefficient and the overall temperature driving force, based on the physical properties of both the steam and the boiling liquid.

    Problems

    • Several problem areas have to be considered during the designing of climbing film evaporators. For instance, some liquids are less viscous than water, making them more difficult to evaporate. Other mixtures may foam or precipitate into crystals, leaving behind deposits on the heating surface. Some liquids are denatured by high temperatures whereas others have extremely high boiling points.

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