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Agitated Thin Film Evaporators
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 150rpm, 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 O.1torr. Liquids with viscosities above 2,00,000cps can be handeled 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:
Fixed blade rotor
Spring loaded rotor
Cylindrical wiper rotor
Centrifugal blade rotor
Falling Film Evaporators
A falling film evaporator is a industrial device to concentrate solutions, especially with heat sensitive components.
In falling film evaporators the liquid product usually enters the evaporator at the head of the evaporator. The product is evenly distributed into the heating tubes. A thin film enters the heating tube and it flows downwards at boiling temperature and is partially evaporated. In most cases steam is used for heating the evaporator. The product and the vapor both flow downwards in a parallel flow. This gravity-induced downward movement is increasingly augmented by the co-current vapor flow. The separation of the concentrated product from its vapor is undergoing in the lower part of the heat exchanger and the separator.
Falling film evaporators can be operated with very low temperature differences between the heating media and the boiling liquid, and they also have very short product contact times, typically just a few seconds per pass. These characteristics make the falling film evaporator particularly suitable for heat-sensitive products, and it is today the most frequently used type of evaporator.
Working Principle:
The process fluid to be vaporized is feed to the evaporator at the top of the tube sheet. A suitable distribution unit is necessary in order to achieve an even liquid distribution. It is paramount that for this process minimal wetting rate is achieved.
Application :
Falling film evaporators are used extensively in chemical process industry, food and paper industry. Due to the absence of static head effect caused by liquid column as in other types of evaporators, evaporation can take place at very small effective mean temperature differences. The temperature difference are typically between 3 - 8oC. This is significantly less than in other devices used for evaporation, e.g. forced reboilers or kettle evaporators, here the effective mean temperature difference is between 15
and 300C. The film heat transfer coefficients are in general high, and characterised by surface boiling.
The absence of hydrostatic head allows this type of evaporator to operate at very low absolute pressures.
Product residence time can be very short, especially in one through operation. These characteristic of short retention time low operation pressure and small required effective mean temperature differences makes this type of evaporator particularly suitable for concentration of heat sensitive liquids. The absence of nucleate boiling under normal operation conditions, and low temperature differences also reduces possible fouling tendencies.
Forced Circulation Evaporators
Design Features:
Forced circulation evaporators are most suited for the liquids which tend to crystallize upon concentration and which have tendency to scale.
Evaporators in which circulation is maintained, regardless of evaporation rate or heat duty, by pumping the liquid through the heating element with relatively low evaporation per pass are suitable for a wide variety of applications.
The forced circulation system is the easiest to analyze and permits the functions of heat transfer, vapor-liquid separation, and crystallization to be separated. Forced circulation systems are generally more expensive than natural circulation systems and are therefore used only when necessary.
A choice of forced circulation can be made only after balancing the pumping energy cost, which is usually high, with the increase in heat transfer rates or decrease in maintenance costs. Tube velocity is limited only by pumping costs and by erosion at high velocities. Tube velocities are usually in the range of 5 to 15 feet per second.
Factors which must be considered when establishing the pumping rates include:
Maximum fluid temperature permitted
Vapor pressure of the fluid
Equipment layout
Tube geometry
Velocity in the tubes
Temperature difference between the pumped fluid and the utility fluid, and
Characteristics of pumps available for the service.
Features of RAJ Forced Circulation Evaporators :
RAJ Forced circulation evaporator offer following advantages :
High rate of heat transfer.
Positive circulation.
Relative freedom from salting, scaling, and fouling.
Ease of cleaning and a wide range of application.
Features of RAJ Forced Circulation Evaporators :
Food processing
Effluent treatments
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