Rising film evaporator

A rising film or vertical long tube evaporator is a type of evaporator that is essentially a vertical shell and tube heat exchanger. The liquid being evaporated is fed from the bottom into long tubes and heated with steam condensing on the outside of the tube from the shell side. This is to produce steam and vapour within the tube bringing the liquid inside to a boil. The vapour produced then presses the liquid against the walls of the tubes and causes the ascending force of this liquid. As more vapour is formed, the centre of the tube will have a higher velocity which forces the remaining liquid against the tube wall forming a thin film which moves upwards. This phenomenon of the rising film gives the evaporator its name.

Applications: There is a wide range of applications for rising tube evaporators, including effluent treatment, production of polymers, food production, thermal desalination, pharmaceuticals, and solvent recovery.Aschner, F.S. & Schaal, M. & Hasson, D. (1971). “Large Long-Tube Evaporators for Seawater Distillation. In terms of applications within these industries, rising tube evaporators are mainly used as reboilers for distillation columns, or as pre-concentrators or flash evaporators or pre-heaters designed to remove volatile components prior to stripping.

Thermal desalination. A specific application of rising tube evaporators is the thermal desalination of sea water. Sea water is pumped into the long tubes of the evaporator while the heating media (usually steam) heats it up. As vapour forms inside the tubes it flows upwards. This evaporation occurs under vacuum conditions that allow for the use of lower temperatures.

Juice concentration and food processing:- The food industry requires handling of delicate products that are sensitive to high temperature for long periods of time. Rising film evaporators can operate quickly and efficiently enough to avoid having to expose the product to high temperatures which may damage or undermine its quality. Hence, they are suitable to use as concentrators for juices, milk and other dairy products which are products that require delicate handling in the food industry.

Advantages and limitations

Low residence time

The main advantage of the rising film evaporator is the low residence time of the liquid feed in the evaporator compared to other evaporator designs like plate-type evaporators. This is crucial because it allows the usage of the evaporator in higher operating temperatures and gives assurance of high product quality despite the product being heat sensitive. Additional advantage is the availability of the option to operate the evaporator as a continuous process which is overall more energy and time efficient than a batch process operation. [1]

High heat transfer coefficients:- Another significant advantage is the relatively high heat transfer coefficient of this evaporator type. This is essential as it reduces the overall heat transfer area requirement which in turn will lower the initial capital cost of the evaporator. This is accentuated by the fact that the components, which consist of a shell and tubes, are easily obtainable with customized designs making them cost effective for construction and ideal for simple evaporation requirements. Moreover, this type of evaporator also can easily contain those widely available vapour separators for foaming products.

Low efficiency

While the rising film evaporators are relatively efficient and have a several advantages, some literature suggests that they are not as efficient as the vertical or the horizontal tube falling film evaporator. As such, in recent times falling film evaporators are usually chosen in place of rising film evaporators because they have similar advantages as rising film evaporators and have the additional benefits of better efficiency. Moreover, rising film evaporators requires a driving force to move the film against gravity and this causes a limitation because there is a requirement for a sufficient temperature difference between the heating surfaces to provide the driving force. [1]

Limited product versatility:- Another major limitation of rising film evaporators is the requirement for the products to be of low viscosity and have minimal fouling tendencies. Competitive process designs like plate-type evaporators can handle liquids that are more viscous with higher fouling tendencies because the inner parts are more easily accessible for cleaning and maintenance.

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