Evaporators

Evaporators are used for concentration or crystallization of liquids. In most of the cases evaporators are used to increase the solid contents of the liquid product prior to drying. This is a cost-effective method of removal of moisture. Depending on the number of effects used in an evaporator the quantity of water evaporated per kilogram of steam increases. In case of multiple effect evaporators, steam jet ejectors or thermo-compressors are used to increase the thermal efficiency.

An evaporator consists of either plate type or shell and tube type heat exchanger. The liquid feed is passed through the heat exchanger and indirectly heated with the help of steam. This operation is either done at atmospheric pressure or under vacuum. Evaporation under vacuum is most energy efficient and also ensures that the product does not get over heated. The different types of evaporators are:

The falling film evaporator consists of shell and tube heat exchanger called as calendria mounted in vertical position. The liquid is distributed and fed to the inside of the tubes of the calendria forming a uniform film. The steam in the jacket heats the tubes thus causing the liquid in the feed to evaporate. The vapor is separated from the liquid in a vapor separator and the concentrated liquid is collected at the bottom or transferred to the next stage. The falling film evaporator can be single effect or multiple effect depending on the capacity.

The forced circulation evaporator is used for liquid feeds which have a tendency of scaling or crystallizing. The liquid feed is continuously circulated through the calendrea from bottom to top. The concentrated liquid and the vapors are separated at the top of the calendria with the help of vapor separator. The forced circulation of feed ensures that there is no scaling on the tube walls.

The wiped film evaporator is used for concentration of drying of viscous and heat sensitive products. The feed is distributed on the heated wall of evaporator surface where it is scraped with the help of specially designed rotating scraper thus forming a thin film. The liquid coming in contact with the heated wall gets evaporated as it flows down the wall. The evaporation is normally carried out under vacuum for heat sensitive products. The heating surface area determines the capacity of the evaporator.

Application :

Thin film or wiped film evaporators are used to separate a mixture of liquids having different boiling points. It is also used to increase the concentration of liquids.

Principle of Operation :
The liquid feed is distributed on the heated wall of the evaporator to form a uniform thin film with the help of suitable configuration of blades. The volatile component or the component with lower boiling point gets evaporated and is vacuumed out of the evaporator, while the component with higher boiling point flows down the wall and is collected at the bottom. The vapours are passed through a condenser and collected separately.

Operating features :
The heat transfer in thin film evaporator is very quick and efficient, thus demanding lower surface area and heat input as compared to other types of evaporator. Due to high vacuum distillation and very short residence time the thin film evaporator are suitable for handling wide range of heat sensitive, high boiling and viscous feeds. Due to low rotor speeds in the range of 100 to 150 rpm, the horse power requirements are very low. To reduce the pressure drop between the evaporator and condenser, the internal condensers can be provided to achieve short path distillation. The thin film evaporators are designed to handle various products with operating pressures up to 0.1 torr. Liquids with viscosities above 2,00,000 cps can be handled in these evaporators.

Construction of ATFE :
The agitated thin film evaporator consists of a jacketed shell having a machined surface on the inner side. The rotor assembly consisting of different configuration of blades depending upon the nature of product is mounted in the shell. Feed inlet is provided at the top side. Specially designed feed distributor is integral with the rotor at the top side. The rotor is also fitted with an entrainment separator. Generally the vapour outlet is provided on the top side of the shell. The different types of blade configurations available are:

a) Fixed blade rotor.
b) Spring loaded rotor.
c) Cylindrical wiper rotor.
d) Centrifugal blade rotor.

The construction of the evaporator is such that different types of rotors can be mounted in the same equipment for different products. The evaporators having surface area up to 50sq.m. can be supplied.