Evaporators

I N E V A P O R A T I O N P H Y S I C A L A N D C H E M I C A L P R O P E R T I E S O F T H E S O L U T I O N B E I N G

C O N C E N T R A T E D

EVAPORATORS

Properties on which the evaporation greatly depends

Concentration of the liquid : by time liquid becomes concentrated .heat transfer coefficient becomes adequately low so the circulation or the turbulence must be there to prevent the coefficient becomes sufficiently low

Solubility: by the variation in temperature effects the solubility of the solution either .condensation of the liquid from cold liquid causes crystallisation of the substance .effects the evaporation

Temperature sensitivity of the material : some material degrade due to prolong heating

Foaming and frothing: foam and froth forms during boiling accompanies the vapour coming out of the evaporator due to which entrainment loss occurs

Pressure and temperature :B.P. of the solution is related to pressure of the system. The higher the operating pressure of the evaporator the higher the temp at boiling . By increase in the concentration temperature of boiling may rise

Scale deposition and material of construction : by these factors heat transfer coefficient decreases causes the decrease in heat transfer.

Types of evaporators

1. Open kettle or pan

2. Horizontal type natural circular evaporation

3. Vertical type natural circulation evaporation

4. Long tube vertical type evaporator

5. Falling film type evaporator

6. Forced circulation type evaporator

7. Agitated film evaporator

8. Open pan solar evaporator

Open kettle or pan evaporator

Cheap and simple

Heat economy is poor

Paddles and scrappers are used for agitation

Heat is supplied by condensation of stream

Horizontal type natural circulation evaporator

Similar heating tubes such in heat exchangers are used

Vapour leaves the liquid surface often goes through de entrainment device such as baffles are used to prevent carryover of liq. Droplets

relatively cheap then the other higher evaporator devices

Used for non viscous liquid with heat transfer coefficient (since circulation is poor )

Liquids that do not deposit on the surfaces

Stream enters inside the tube Not used for foamy liquids

Vertical type natural circulation evaporator

Types are basket type(liquid inside ) and standard type(liquid outside )

Velocity :1-3 fps Boiling increases de Boiling increases density

liquid rises in the tube by natural circulation and flows downward through a large central open space or downcomer

Natural circulation increases heat transfer coefficient

Not for viscous liquid Also called as short tube

evaporator

long tube vertical type evaporator or kestner evaporator

Heat transfer on stream side is very high as compare to evaporating liquid side so high liquid velocities are desirable

Liquid run inside the tube Length of 12-20 ft. 3-10 mm tubes Formation of vapours inside the

tube causing pumping action which gives quite high liquid velocities

Not re circulated (contact time is low )

Can be adapt for foamy liquids Not for scaling and not for

liquids of high viscosity

Falling film type evaporator

Liquid is fed from top and flow down as a thin film

Vapour liquid separation usually takes place at the bottom

Used widely for concentrating heat sensitive materials such as orange juice and other fruit juice

Holdup time is very small (5-10)

High heat transfer coefficient (due to high velocities )

Forced circulation type evaporator

We use pump in film type evaporator

Increase heat transfer coefficient Useful for viscous fluid External heating provide more

ease to cleaning to tubes more complicated piping is used

For viscous we use positive pumps and for colloidal we use low power pumps

Heating element is placed at the lower level to avoid boiling on heating surface .this reduces the rate of deposition of solids

Agitated film evaporator

Main resistance of the heat transfer is on evaporator side

Heat transfer coefficient and turbulence is increased by mechanical agitation of this liquid film

Modifying falling film evaporator with a single large jacketed tube containing internal agitator

Used for viscous fluids ,high sensitive materials like rubber, latex, gelatine, antibiotics and fruit

Heat transfer coefficient increases

High cost Small capacity

Open pan solar evaporator

Low scale evaporation

Takes longer time

Cheap

Solar energy is used to evaporate the water from the solution

Method of operation of evaporation

Single effect evaporator

Forward feed multiple effect evaporator

Backward feed multiple effect evaporator

Parallel feed multiple effect evaporator

Mixed feed multiple effect evaporator

Single effect evaporator

Required capacity of operation is relatively small

Forward feed multiple effect evaporator

Feed should be near the B.P. of the solution at the pressure in the 1st effect

Previous unit vapour serves as a heating medium for the next effect

Here latent of the vapour can be reused and recovered again

Uses when feed is hot or when the concentration product might be damaged at high temperature

B.P decreases from effect to effect Pressure is 1 atm at 1st effect and

under vaccum in other effects

Backward feed multiple effect evaporator

Uses when fresh feed is cold

Flows from low to high pressure for this to happen we apply pumps at certain places

Temperature increases from effect to effect

Used when concentrated product is highly viscous

High temperature and low viscosity gives high heat transfer coefficient

Parallel multiple effect evaporator

Adding and withdrawal of concentrated product from each effect

Feed almost saturated and solid crystals are the product

Eg- evaporation of brine to make salt

Mixed multiple effect evaporator

When feed moves forward with fresh feed is entering at the 2nd or 3rd

effect k/a mixed type

Used in solutions having considerable change in viscosity with temperature over concentration range

Effects of processing variables on evaporator

Effect of temperature : feed must be at temp greater or equal to the b.p of the solution . Preheating the feed can reduce the size of evaporator heat transfer area

Effect of pressure : a pressure of 101.32 kpa is used in the vapour space which gives the b.p of the feed .if the rise in temperature increases .means large difference in temperature is desirable heating surface area and the cost of the evaporator decreases .if pressure decreases under vaccum condenser and pump are used

Effect of stream pressure :with increase in pressure of the saturated stream difference in b.p of increases giving small size and hence cost decreases

M A D E B Y : S H I K H A T H A P A

C O L L E G E N A M E : I N S T I T U T E O F T E C H N O L O G Y G U R U G H A S I D A S C E N T R A L U N I V E R S I T Y

S T U D E N T O F C H E M I C A L E N G I N E E R I N G

S E M E S T E R : 5 T H

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