A climbing film vertical long tube evaporator is a type of evaporator that is an essential shell and tube, heat exchanger. Thus, it is also known as a rising film evaporator. This evaporator is superior to the falling film evaporator as the upstream film movement causes some particles to remain in the feed stream. The absence of any kind of distributor makes it suitable for such a variety of applications.
Applications of Climbing Film Evaporator
- A climbing film is used for effluent treatment.
- It can be used in t h e production of polymers and for juice concentration and food processing.
- It is used in the thermal desalination of sea-water.
- It has many applications in pharmaceuticals, especially for solvent recovery.
- It is used as reboilers for distillation columns.
- It is used as pre-concentrators or flash evaporators or pre-heaters to remove volatile components before stripping.
Principle of Climbing Film Evaporator
The theory of the
climbing film evaporator is that the ascending force of the steam produced during
the boiling causes liquid and vapors to flow upwards in parallel flow. At the
same time, the production of vapor increases, and the product is pressed as a
thin film on the walls of the tubes, and the liquid rises upwards. This
co-current upward movement against gravity has the beneficial effect of
creating a high degree of turbulence in the liquid.
Construction of Climbing Film Evaporator
A Rising Film Evaporator
(RFE) is a vertical shell and tube heat exchanger with a vapor-liquid separator
mounted at the top, Fig.1. Tubes carrying the steam internally are placed
vertically in the bottom of the cylindrical evaporator chamber. The length of
the boiling tubes is typically not more than 23 ft (7m). This type of unit is
known as the Roberts evaporator in Europe and is the calandria evaporator in
the United States.
Fig.1: Climbing Film Evaporator
Working of Climbing Film Evaporator
The liquid to be
concentrated is fed at the bottom of the heated tube bundle. The feed is heated
with steam condensing on the outside of the tube from the shell side. This produces
steam and vapor within the tube bringing the liquid inside to boil. The product
circulation is by natural convection. The vapor so produced pushes the liquid
against the walls of the tubes and causes the upward force that feeds to move
up. As more vapors are formed, the center of the tube will have a higher
velocity which forces the remaining liquid against the tube wall forming a thin
film that moves to the top above the calandria. The velocities generated by the
vapor lift are quite high, giving a good thermal performance. This is useful
while evaporating highly viscous products and products that tend to foul the
heating surface. Usually, there must be a high-temperature difference between
the heating and boiling sides of the evaporator and is required to convey the
liquid and to produce the rising film. The vapor and balance liquid are
separated in the vapor-liquid separator.
Advantages of Climbing Film Evaporator
- The main advantage of the rising film evaporator is the low residence time of the liquid feed in the evaporator compared to other evaporators.
- It has a relatively high heat transfer coefficient that reduces the overall heat transfer area requirement which in turn will lower the initial capital cost of the evaporator.
- It is easier to clean the tubes.
- Thermosiphon action eliminates the need for a circulation pump.
- Trace quantities of suspended particles in the feed are tolerated.
- Can operate under reasonable vacuum.
- Multiple effect arrangement provides steam economy.
- It is a relatively inexpensive evaporator.
Disadvantages of Climbing Film Evaporator
- It requires large floor space and is heavy.
- It has poor heat transfer at low-temperature differences.
- Not suitable for thermolabile materials.
- It evaporates products of low viscosity and has minimal fouling tendencies.
Applications of Climbing Film Evaporator
- The thermal desalination of sea-water.
- The concentration of dilute solutions such as plant extracts.
- It is used as a reboiler to a distillation column.
- It is an economical alternative to a falling film evaporator for a moderate vacuum.
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