The evaporating pan is also called a steam jacketed kettle. In this type of evaporator, the movement of the liquid to be evaporated is due to the convection currents set up by the heating process. It is a type of natural circulation evaporator. It consists of a hemispherical pan surrounded by a steam jacket. The hemispherical shape provides a large surface area for the evaporation to take place. The evaporation pan may be fixed and the contents are discharged from the outlet provided at the bottom of the pan. In some cases, the evaporators are mounted in such a way that they can be tilted on either side to remove the concentrated product. The evaporating pans are heated by the steam which enters the jacket through the inlet and leaves it from the outlet. The heat from the steam in the jacket is utilized for evaporating liquid in the pan.
Principle of Evaporating Pan
The mechanism involved in
this evaporation process is conduction and convection so that the heat is
transferred by this mechanism to the extract. Evaporating pan containing
aqueous extract is provided with steam which gives out heat to a jacketed
kettle. The temperature raises and the escaping tendency of the solvent
molecules into the vapour increases and enhances the vaporization of the
solvent molecules.
Construction of Evaporating Pan
Steam evaporating pan
consists of a hemispherical structure with an inner pan called kettle which is
enveloped with an outer pan called a jacket, Fig.1. These two pans are joined
to enclose a space through which steam is passed. Several metals have been used
for the construction of the kettle. Copper is an excellent material for the
kettle due to its superior conductivity. If acidic preparation evaporated in a copper
kettle, some of the copper gets dissolved in preparation. To avoid this, for
acidic preparation is tinned copper kettle is used. Iron is also used for the
construction of the jacket because it has low conductivity. Rusting of iron
with use is a major problem and to prevent this iron jacket is tinned or enamelled
on the inner surface. An inlet for the steam and non-condensed gases are
provided near the top of the jacket. Condensate leaves the jacket through the
outlet provided at the bottom. The kettle is provided with the outlet for the
product discharge at its bottom.
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| Fig.1: Steam Jacketed Kettle |
Working of Evaporating Pan
The solution to be
evaporated is placed in the kettle and steam which gives out heat to the
content is supplied through the inlet and condensate leaves through the outlet.
For smaller volumes, the contents must be stirred manually and mechanically for
larger volumes. The rate of evaporation is fast in the initial stages. The room
where evaporation is carried must have good ventilation to remove the vapour to
avoid fog formation of condensed vapour. To prevent condensation in the room
and also to accelerate the rate of evaporation fans are fitted. The kettle may
be fixed or made to tilt. A kettle of capacity up to about 90 litres may be
made to tilt. The bottom outlet is used to collect the concentrated products.
Applications of Evaporating Pan
- Evaporating pan is suitable for concentrating aqueous liquids.
- It is suitable for concentrating thermostable liquors, for example, liquorices extracts.
Advantages of Evaporating Pan
- Evaporating pan is constructed both for small scale and large scale batch operations.
- It is simple in construction and easy to operate, clean and maintain.
- Its cost of installation and maintenance is low.
- A wide variety of materials such as copper, stainless steel and aluminium etc can be used for the construction of evaporating pan.
- Stirring the contents in the pan and removal of the products is easy.
Disadvantages of Evaporating Pan
- The natural circulation of the product makes poor heat transfer.
- Deposition of solid may cause decomposition of the product.
- The heating surface is limited and decreases proportionally to increase the size of the pan.
- It is not suitable for the concentration of thermolabile materials.
- It has no provision to operate under reduced pressure.
- No provision to recollect the costly organic solvents.
- Being the evaporating pan open vapours directly pass into the atmosphere. This may cause discomfort to the worker.
- Saturation of the surrounding environment with vapours may slow down the process.
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