Evaporation
Separation
Process
Presented by
Umer Farooq
BEC-FA11-093
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Evaporation
Introduction
• Vaporization of a solvent from a solution to make it
concentrated.
 Solvent volatile
 Solute non-volatile in nature.
• We are left with thick liquor
• It is different from drying only, as in drying the solvent is
vaporized to have a solid end product
• Distillation and Evaporation
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Evaporation
Feed characteristics
Influencing Evaporation
 Concentration of solution
 Temperature sensitivity
 Foaming
 Scale formation
 Other properties like freezing point, specific heat, gas
liberation, toxicity level, radioactivity etc
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Evaporation
Material of
Construction
• Mostly made of steel
• For highly corrosive fluids, special materials are used like, Cu,
Nickle, Stainless steel, aluminium etc
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Economy of
Evaporator
Economy of evaporator is the total mass of water vaporized per unit
mass of steam input to the evaporator.
Capacity of
Evaporator
Capacity of an evaporator is the amount of water vaporized in the
evaporator per unit time.
Ratio of capacity to economy is called the steam consumption
per hour.
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Evaporation
Single effect
Evaporation
•
Nearly always the material to be evaporated flows inside the tubes.
•
The boiling liquid is subjected under moderate vacuum
•
Reducing the boiling temp of the liquid increases the temperature
difference b/w the steam and the boiling liquid and thus increases the heat
transfer rate in the evaporator.
•
When a single evaporator is used, the vapor from the boiling liquid is
condensed and discarded.
•
Simple but does not use steam effectively.
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Evaporation
Multiple effect
Evaporation
• If the vapor from one evaporator is fed into the steam chest of a
second evaporator and the vapor from the second is sent to the
condenser, the operation becomes double effect.
• The heat in the original steam is reused in the second effect and
the evaporation economy increased.
• Also useful one the feed temp is very low, preheating
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Evaporation
Method of feeding
Variations in the Multiple effect has come from the mode
of feed supply
There are four possible feeding arrangements
• Forward feed
• Backward feed
• Mixed feed
• Parallel feed
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• Figure: (a) Forward feed (b) Backward feed flow patterns in four
effect evaporator.
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• Figure: (c) Mixed feed (d) Parallel feed flow patterns in four
effect evaporator.
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Evaporation
Boiling point
elevation
• Boiling point of solvent increases when some solute is added,
this phenomenon is called boiling point elevation.
• Duhring’s rule states that a linear relationship exists b/w the
temperatures at which two solutions exert the same vapor
pressure. The rule is often used to compare a pure liquid and a
solution at a given concentration.
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• Duhring’s rule is a graphical representation of such a
relationship
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Evaporation
Types of
Evaporators
I.
Evaporators with heating medium in jacket
II. Vapor heated evaporators with tubular heating surfaces
 Horizontal tube evaporators
 Evaporators with tubes placed vertically
 Evaporators with short tubes
 Single effect evaporators
 Multiple effect evaporators
 Evaporators with long tubes
 Climbing film evaporators
 Falling film evaporators
 Forced circulation evaporators
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Evaporation
Steam jacketed kettles/Batch type pan
evaporators
• Simplest and oldest type
• Steam supplied in the jacket gives it’s heat content and
condensate leaves through the outlet.
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Evaporation
Advantages
• Used for both small scale and large scale operations
• Simple in construction and easy to operate
• Low maintenance and installation
Disadvantages
• Heat economy is less
• Not suitable for heat sensitive materials
• Heat transfer rate decreases drastically when the conc. increased
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Evaporation
Horizontal Tube
evaporators
• Steam is passed through the tube which are immersed in the
pool of liquid to be evaporated.
• Feed is introduced into the evaporator until the steam
compartment is immersed. The feed absorbs heat and solvent is
evaporated.
• Use: Best suited for non-viscous liquids because of poor
circulation
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Evaporation
Vertical Tube
evaporators
• Liquid is passed through the vertical tubes and steam is supplied
from outside the tubes
• It consists of short vertical tubes, typically 1-2 m long and 50 –
100 mm in diameter are arranged inside the steam chest.
• The tube bundle is located in the bottom of the vessel
• Provides more heat transfer surface area
• Liquid level is to be maintained above the tubes/calandria
• Not suitable for solution which have solid particles
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Evaporation
Climbing Film
evaporators
• Liquid is passed through the vertical tubes and steam is supplied
from outside the tubes
• It consists of short vertical tubes, typically 1-2 m long and 50 –
100 mm in diameter are arranged inside the steam chest.
• The tube bundle is located in the bottom of the vessel
• Provides more heat transfer surface area
• Liquid level is to be maintained above the tubes/calandria
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Evaporation
Heat transfer
coefficient
• The heat flux and the evaporator capacity are affected by the
overall heat transfer coefficient.
• The heat transfer coefficient is influenced by the design and
method of operation of the evaporator.
• Coefficient is a reciprocal of five individual resistances: the
steam film resistance, the two scale resistances, the tube wall
resistance, and the resistance from boiling liquid.
• Tube side solution, shell side steam
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Evaporation
Steam film
coefficient
• Steam will have high heat transfer coefficient.
• Depends upon the type of condensation.
• No scaling as there is no solute
• The presence of noncondensable gas seriously reduces the steam
film coefficient.
• Tube side high possibility of scaling, thus often cleaning is
necessary
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Evaporation
Liquid-side
coefficient
• Liquid side coefficient depends upon the velocity of liquid.
• In case of viscous materials, the resistance of the liquid side
controls the overall rate of heat transfer to the boiling liquid.
• Forced circulation gives higher heat transfer coefficient.
• Tube side high possibility of scaling, thus often cleaning is
necessary
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• Because of the difficulty of measuring individual coefficients in
an evaporator, experimental results are usually expressed in
terms of overall coefficients.
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Evaporation
Enthalpy balances for single effect
evaporator
• Latent heat of condensation
of the steam is transferred to
vaporize water.
• Two enthalpy balances are
needed, one for steam and
one for the vapor or the
liquid side.
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Evaporation
Industrial
applications
• In sugar industry
• In dehydrating milk, which is then used in many food products
• In fertilizer plants
Evaporation is not only removing water
• In the production of refined petroleum products, more volatile
compounds are evaporated off to separate the more crude
components
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