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Evaporation

Separation Process

Presented byUmer FarooqBEC-FA11-093

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Evaporation

• Vaporization of a solvent from a solution

to make it concentrated. Solvent volatile Solute non-volatile in nature.

Introduction

• 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

Concentration of solution

Temperature sensitivity

Foaming

Scale formation

Other properties like freezing point,

specific heat, gas liberation, toxicity

level, radioactivity etc

Feed characteristics Influencing Evaporation

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Evaporation

• Mostly made of steel

• For highly corrosive fluids, special

materials are used like, Cu, Nickle,

Stainless steel, aluminium etc

Material of Construction

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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

• 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.

Single effect Evaporation

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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

Multiple effect Evaporation

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Evaporation

• Forward feed

• Backward feed

• Mixed feed

• Parallel feed

Method of feeding

There are four possible feeding arrangements

Variations in the Multiple effect has come from the mode

of feed supply

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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 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.

Boiling point elevation

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• Duhring’s rule is a graphical

representation of such a relationship

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Evaporation

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

Types of Evaporators

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Evaporation

• Simplest and oldest type

• Steam supplied in the jacket gives it’s

heat content and condensate leaves

through the outlet.

Steam jacketed kettles/Batch type pan evaporators

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Evaporation

• 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

Advantages

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Evaporation

• 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

Horizontal Tube evaporators

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Evaporation

• 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

Vertical Tube evaporators

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Evaporation

• 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

Climbing Film evaporators

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Evaporation

• 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

Heat transfer coefficient

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Evaporation

• 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

Steam film coefficient

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Evaporation

• 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

Liquid-side coefficient

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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

• 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.

Enthalpy balances for single effect evaporator

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Evaporation

• 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

Industrial applications