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Evaporator

Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

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Page 1: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Evaporator

Page 2: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

• Evaporation occurs at the

liquid–vapor interface when

the vapor pressure is less

than the saturation pressure

of the liquid at a given

temperature.

Evaporation is a special case of heat transfer to a

boiling liquid.

Page 3: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Evaporation is a unit operation in which solvent

(water) is removed by means of vaporization or

boiling

Evaporation is the removal of solvent as a vapour

from a solution or slurry.

Evaporation is used for concentration of aqueous

solutions, it involves removal of water from solution

by boiling the liquor in suitable vessel called

evaporator and withdrawing the vapour.

Page 4: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Objectives of Evaporation:

To concentrate a non-volatile solute (solute has

negligible volatility) such as organic compounds,

inorganic salts, acids or bases from a solvent.

Common Solutes:

Caustic soda, Caustic potash, Sodium sulfate,

Sodium chloride, Phosphoric acid and Urea

Common Solvent:

Water

Page 5: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Examples of Evaporation:

1. Concentration of cane sugar juice in a sugar

plant

2. Concentration of an aqueous solution of

ammonium sulphate in a fertilizer plant

3. Concentration of dilute recycled sodium

hydroxide in an alumina plant and many

others

Page 6: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Distillation: Solution containing more than one

volatile compound is vaporized (in a reboiler)

and the components are separated thereafter in

distillation column.

Drying: Entire solvent is vaporized out from a

solution leaving a solid residue as the product,

the operation is called drying.

Evaporation: Evaporation of solution is an

essential step in the operation of a

crystallization unit. In crystallization, the

solution is evaporated to make it supersaturated.

Crystal grow in the supersaturated solution.

Page 7: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Evaporation Crystallization

Vaporizing solvent is

the main function

Crystalline product

and crystal growth is

main function

Food, pulp and paper, pharmaceuticals, fine

chemicals, organic and inorganic chemicals,

polymer etc.

Page 8: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Evaporator

Page 9: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Invention of Evaporators

Norbert Rillieux is famous for his invention

of the multiple effect pan evaporator for

sugar refining process in 1881.

Rillieux was born in New Orleans, Louisiana

in 1806.

He used the steam generated from one pan

to heat the sugar juice in the next pan for

energy efficient means of water evaporation.

Page 10: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Equipment, in which

evaporation is performed, is

known as evaporator.

The evaporators used in

chemical process industries are

heated by steam and have

tubular surface.

An adequate number of tubes are provided through

which the solution is circulated and the tubes are

heated by steam.

In general the steam is the saturated steam and thus

it condenses on the outer tube surface in order to

heat the tube.

Page 11: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Classification of Evaporators

The velocity of circulation of the solution through the

tubes should be reasonably high so that (i) a high

inside heat transfer coefficient is attained and (ii)

formation of deposits or scales on the inner surface

is reduced.

Circulation may be caused by density gradient of the

solution in the vertical tubes or by an external

mechanical means like a pump.

Accordingly, most evaporators are broadly

classified as:

(1) Natural circulation and

(2) Forced circulation

Page 12: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Classification of Evaporators

Evaporation can be divided into three categories

on the basis of boiling phenomena

(i) Pool boiling: In this phenomena bulk or pool of

liquid boils. Examples are kettle boiling, natural

circulation boiling units, thermo siphon

reboilers in distillation

(ii) Convection heating and boiling: example is

forced circulation boiling units

(i) Film evaporation: In film evaporation, a thin

liquid film is maintained on the heating surface

Page 13: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Evaporators can be classified as:

Solar Evaporator Batch Pan Evaporator

Natural Circulation

Evaporator

Forced circulation

Evaporator

Horizontal Tube Evaporator Basket Type Evaporator

Long Tube Vertical

Evaporator

Short Tube Vertical

Evaporator

Rising Film Evaporator Falling Film Evaporator

Agitated Thin Film

Evaporator

Horizontal Spray Film

Evaporator

Plate Type Evaporator Vapor Compression

evaporator

Page 14: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Classification of Evaporators

Evaporators are classified by the number

of effects.

In a single-effect evaporator, steam provides

energy for vaporization and the vapor product is

condensed and removed from the system.

In a double-effect evaporator, the vapor product

off the first effect is used to provide energy for a

second vaporization unit.

Page 15: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Classification of Evaporators

This cascading of effects can continue for many

stages. Multiple-effect evaporators can remove

much larger amounts of solvent than is not

possible in a single effect.

Page 16: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Single Effect Evaporators

Single effect Evaporator

In single effect evaporator,

the steam is fed to the

evaporator which

condenses on the tube

surface and the heat is

transferred to the solution.

The saturated vapor comes

out from the evaporator

and this vapor either may

be vented out or

condensed. The

concentrated solution is

taken out from the

evaporator.

Page 17: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Conventional Evaporator

Evaporator is made up of

three functional sections:

(1) Heat exchanger,

(2) Evaporating section,

where the liquid boils

and evaporates, and

(3) Separator in which the

vapour leaves the

liquid and passes off

to the condenser or to

other equipment

Single effect evaporator

Page 18: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

• In many evaporators, all three

sections are contained in a single

vertical cylinder.

• In the centre of the cylinder there is

a steam heating section, with pipes

passing through it in which the

evaporating liquors rise.

• At the top of the cylinder, there are

baffles, which allow the vapours to

escape but check liquid droplets that

may accompany the vapours from

the liquid surface.

• A diagram of this type of evaporator,

which may be called the

conventional evaporator.

Page 19: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

• In the heat exchanger section, called

a calandria in this type of evaporator,

steam condenses in the outer jacket

and the liquid being evaporated boils

on the inside of the tubes and in the

space above the upper tube plate.

• The resistance to heat flow is imposed

by the steam and liquid film

coefficients and by the material of the

tube walls.

• The circulation of the liquid greatly

affects evaporation rates, but

circulation rates and patterns are very

difficult to predict.

Page 20: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Values of overall heat transfer coefficients that have

been reported for evaporators are of the order of

1800-5000 J m-2 s-1 °C-1 for the evaporation of

distilled water in a vertical-tube evaporator with

heat supplied by condensing steam.

Page 21: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Multi Effect Evaporators

Multi effect Evaporator

The saturated vapor

coming out from the

evaporator-1 is used as

steam in the second

evaporator.

Partially concentrated

solution works as a feed to

the second evaporator.

This arrangement is known

as double effect evaporator

in forward feed scheme.

Page 22: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Multi Effect Evaporators

Note: The vapour leaving evaporator-2 is at the

boiling temperature of the liquid leaving the first

effect.

In order to transfer this heat from the

condensing vapor from the evaporator-1 to the

boiling liquid in evaporator-2, the liquid in

evaporator-2 must boil at a temperature

considerable less than the condensation

temperature of the vaporization, in order to

ensure reasonable driving force for heat

transfer.

Page 23: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Multi Effect Evaporators

A method of achieving this is to maintain a

suitable lower pressure in the second effect so

that the liquid boils at a lower temperature.

Therefore, if the evaporator-1 operates at

atmospheric pressure, the evaporator-2 should

be operated at same suitable vacuum.

The benefit of the use of multiple effect

evaporators is that in this arrangement multiple

reuse of heat supplied to the first effect is

possible and results in improved steam

economy.

Page 24: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Solar Evaporator

In this evaporator, solar energy is the heating source.

Production of sodium chloride from seawater or brine by

concentration in large ponds has been practiced all around

the world.

Crystallization is done in large open tanks.

The process depends upon solar radiation intensity,

weather, humidity and wind velocity.

It is the cheapest evaporation process because solar

energy is free of cost.

When small quantities of solutions are to be concentrated,

batch pan evaporator is the choice.

Page 25: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Batch Pan Evaporator

Production of jams and jellies, fruit juice concentration,

production of some pharmaceutical products is done in

these unit.

The pan can be provided with a jacket or coil for circulating

heating medium.

This unit can be provided agitation.

These are small units having restricted heat transfer area.

In these units high temperature difference cannot be used

due to possibility of degradation of product and fouling of

the heat transfer surface.

These are useful for small capacity batch operation.

Page 26: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Natural Circulation Evaporator

As the name indicates, the circulation of the solution is

natural and the density difference derives it.

The solution gets heat up and partially vaporized as it

flows up the tubes.

The heated liquid flows up because of the density

difference.

Vapor-liquid disengagement occurs above the tube.

Thick liquor comes down from this down comer and

withdrawn from the bottom.

The natural-circulation evaporators may be used if the

solution is quite dilute.

Page 27: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Natural Circulation Evaporator

In the dilute solution the natural circulation will be at

sufficient speed.

It may also be used when the solution does not have

suspended solid particles.

As the solution stays in the tube for larger time, the solution

should not be heat sensitive.

The Calandria type or short-tube evaporators have short

tubes as compared to the long tube evaporators.

The short-tube evaporation uses circulation and solution

flows many times in the evaporators. However, in case of

the long tube evaporator the flow is once through.

Page 28: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Natural Circulation Evaporator

Calandria type Evaporator

Page 29: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Short tube Vertical Evaporators

It is called as a calandria or standard evaporator.

It consists of short tubes 1-2 m in length and 50-70 mm

diameter attached in two tube sheets.

The entire assembly is called the calandria of the

evaporator.

The tube bundle has a large down comer at the center that

helps for circulation of liquid in the evaporator.

The driving force for the fluid flow is the density difference

between the liquid in the down comer and in the tubes.

In the tubes there is a two-phase mixture.

Page 30: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Short tube Vertical Evaporators

A short tube vertical evaporator has a short tube bundle

enclosed in a shell. This is called a calendria.

The calendria is of annular construction, i.e. there is an open

region at the center.

The liquid flows down through the central open space of the

calendria called down take or down comer.

Thus, a continuous natural recirculation of the solution occurs.

Thick product liquor is withdrawn from the bottom.

Their STV evaporators are also known as standard

evaporates.

Page 31: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Natural Circulation Evaporator

Long-tubes vertical Evaporators

Page 32: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Horizontal tube Evaporators

In horizontal tube evaporator, the tubes re horizontal.

Example is kettle reboilers in distillation column.

In horizontal standard evaporator, the process liquid is on the

outer surface of the tube and the heating medium is inside the

tubes.

The unit is relatively cheaper and offers moderate to high heat

transfer coefficient.

Horizontal tube unit is not suitable where fouling problem is

serious, because scale build up reduces the performance

drastically.

It is used as re boilers for distillation column and can be used

for boiler feed water preparation.

Page 33: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Basket Evaporators

In basket evaporator the tube bundle can be removed from

the main body.

Circulation of liquid occurs in the space between the shell

and the tube bundle.

Its advantage is that cleaning of tube is easy because the

bundle is removable.

In these units thermal expansion problem does not arise.

The concentrated liquor leaves through an outlet pipe at the

conical bottom of the evaporator.

A basket evaporator uses tubes similar to those of the

calandria type.

Page 34: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Falling Film Evaporators

Highly heat sensitive materials are processed in falling

film evaporators.

They are generally once-through evaporator, in which the

liquid enters at the top, flows downstream inside the

heater tubes as a film and leaves from the bottom.

The tubes are heated by condensing steam over the tube.

As the liquid flows down, the water evaporates and the

liquid gets concentrated.

To have a film inside of the tube, the tube diameter is kept

high whereas the height low to keep the residence time

low for the flowing liquid.

Page 35: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Falling Film Evaporators

Therefore, these evaporators, with non-circulation and

short resistance time, handle heat sensitive material,

which are very difficult to process by other method.

The main problem in falling film evaporator is the

distribution of the liquid uniformly as a thin film inside the

tube.

Page 36: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Falling Film Evaporators

Page 37: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Forced Circulation Evaporator

Natural circulation evaporators have many limitations (as

mentioned earlier) through they are economical as compared

to forced circulation evaporator.

A forced circulation evaporator has a tubular exchanger for

heating the solution without boiling.

The superheated solution flashes in the chamber, where the

solution gets concentrated.

In forced circulation evaporator horizontal or vertical both type

of design is in- practice.

The forced circulation evaporators are used for handling

viscous or heat sensitive solution.

Page 38: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Forced Circulation Evaporator

Vertical tube forced-circulation Evaporators

Page 39: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Agitated Thin Film Evaporators

Agitated Thin Film Evaporators

Page 40: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Plate Evaporators

Plate Evaporators

Page 41: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Characteristics of different types of EvaporatorsEvaporator Typical products handled comments

Calandria Salt, glycerin from spent

soap lye

Suitable for batch or

continuous operation in single

or multiple effects

Forced

circulation

Salting or scale-forming

materials depending on

steam-chest

configuration; caustic

soda solution, sodium

sulphate, tomato juice to

30% concentration etc.

Available with: (1) horizontal

steam-chest with external

vapor separator (less used

now) (2) Vertical steam –chest

with external separator (3)

vertical steam-chest with

integral vapor head. Operates

with either submerged or

partially filled tubes in single or

multiple effect

Falling film Low to medium viscosity

materials, heat sensitive

products, fruit juices and

pharmaceuticals

Single or multiple effects; can

be operated on single pass or

with partial recycle of

concentrated products

Page 42: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Characteristics of different types of Evaporators

Evaporator Typical products handled comments

Natural

circulation

(Thermosyphon)

Foaming liquids, less

viscous materials, black

liquor from the pulp

industry, spent soap lye,

electroplating solutions,

spin bath liquid

External separator

provides some holding

time adjustment, integral

vapor head type with

down comer gives

minimum hold up

Agitated film Handles the full range of

feed viscosities, gelatin,

fruit puree, glue

Available (1) vertical with

integral vapor separator

(2) vertical with external

separator, co-current

flow (3) horizontal with

tapered shell counter

current flow

Rising Film Caprolactum; ammonium

nitrate, fruit juices, for

crystal producing solutions

with suspended solids

Allows single pass

operation with high liquid

and vapor velocities;

minimum liquid hold up

Page 43: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Characteristics of different types of Evaporators

Evaporator Typical products handled comments

Plate type Fruit juices, extracts,

gelatin, condensed and

whole milk

Liquid and vapor flow

essentially as in rising

and falling film

evaporators without

liquid distribution

problems

Page 44: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Typical overall heat transfer coefficient in evaporator

Page 45: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Methods of Feeding of Evaporators

Evaporator trains may receive their feed in several

different ways. The feed order is NOT related to the

numbering of effects. Effects are always numbered

according to decreasing pressure (steam flow).

1. Forward feed

2. Backward Feed

3. Mixed Feed

4. Parallel Feed

Page 46: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Methods of Feeding of Evaporators

Forward feed arrangement in triple-effect evaporator

(dotted line: recycle stream)

Page 47: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Forward Feed arrangements follow the pattern I, II, III.

These require a single feed pump (reduced fixed costs).

They typically have reduced economy (higher operating

costs) since the cold feed must be raised to the highest

operating temperature.

These also tend to have the most concentrated liquour,

which tends to be the most viscous, in the lowest

temperature effects, so their may be difficulties getting a

good overall heat transfer coefficient.

Forward Feed

Page 48: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Methods of Feeding of Evaporators

Backward feed arrangement in triple-effect evaporator

(dotted line: recycle stream)

Page 49: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Backward Feed arrangements go III, II, I.

These need multiple pumps to work against the pressure

drop of the system; however, since the feed is gradually

heated they usually have better economies.

This arrangement also reduces the viscosity differences

through the system and so is better for viscous solutions.

Backward Feed

Page 50: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Methods of Feeding of Evaporators

Mixed feed arrangement in triple-effect evaporator

(dotted line: recycle stream)

Page 51: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Mixed Feed arrangements offer a compromise, with the

feed entering in the middle of the system (i.e. II, III, I).

The final evaporation is done at the highest temperature so

economies are still better than forward feed, but fewer

pumps are required than in a backward feed arrangement.

Mixed Feed

Page 52: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Methods of Feeding of Evaporators

Parallel feed arrangement in triple-effect evaporator

Page 53: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Parallel Feed systems split the feed stream and feed a

portion to each effect.

This is most common in crystallizing evaporators where

the product is likely to be a slurry.

Parallel Feed

Page 54: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Advantages and limitations of different modes of feed

supply to multiple effect evaporator

Mode of

Feed supply

Advantages Limitations

Forward

Feed

Simple to operate; less

expensive; the liquor flows from

one effect to the next driven by

the pressure differential between

successive effects and hence no

pump is required to transferring

the liquor; less chance of

deterioration of heat sensitive

materials because the more

concentrated liquor is vaporized

at a lower temperature

Reduced rate of heat transfer

in the second and higher

effects; feed should not be

below the boiling point

because this reduces steam

economy by consuming

external steam to supply

sensible heat.

Backward

Feed

The most concentrated liquor is

in contact with the highest

temperature steam and thus

lower viscosity and higher heat

transfer rate in the first effect as

a result.

Inter-effect pumps are

necessary; higher risk of

damage of the viscous

product subjected to a higher

temperature; risk of fouling.

Page 55: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Advantages and limitations of different modes of feed

supply to multiple effect evaporator

Mode of

Feed supply

Advantages Limitations

Mixed feed Combines the simplicity of

forward feed and economy of

backward feed; useful for

concentration of a highly viscous

feed.

More complex, piping

instrumentation which make

arrangement more

expensive.

Parallel feed More suitable for use with

crystallizers; allows better

control.

More complex arrangement;

pumps generally required for

each effect.

Page 56: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

There is wide variation in characteristics of liquor to be

concentrated that requires judgment and experience in

designing and operating evaporators.

Some of the properties of evaporating liquids that influence the

process of evaporation are:

1) Concentration:

Initially, the solution may be quite dilute and the properties of

the solution may be taken as the properties of solvent.

As the concentration increases, the solution becomes viscous

and heat transfer resistance increases.

The crystal may grow on the heating coil or on the heating

surface. The boiling points of the solution also rise

considerably.

Page 57: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Solid or solute contact increases and the boiling temperature

of the concentrated solution became higher than that of the

solvent as the same pressure (i.e. elevation in boiling point).

2) Foaming:

Many of the materials like organic substance may foam

during vaporization.

If the foam is stable, it may come out along the vapor known

as entrainment (carry over of a portion of liquid by rising

vapour is called as entrainment).

Heat transfer coefficient changes abruptly for such systems.

Page 58: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

3) Scale:

Many solution have tendency to deposit the scale on the

heating surface, which may increase the heat transfer

resistance that results in reduction of heat transfer

coefficient and hence heat transfer rate.

These scales produce extra thermal resistance of

significant value. Therefore, scaling in the equipment

should not be ignored thus de-scaling becomes an

important and routine matter.

It is therefore necessary to clean the tubes at definite

intervals.

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4) Temperature sensitivity:

The products of many chemical, food, pharmaceutical

industries etc. are very temperature sensitive and they

may get damaged during evaporation (when heated to

moderate temperature even for short times).

For concentrating such materials special techniques are to

be used that reduce temperature and time of heating.

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5) Materials of Construction:

The material of the equipment must be chosen considering

the solution properties so that the solution should neither be

contaminated nor react with the equipment material.

Generally evaporators are made of mild steel whenever

contamination and corrosion is a problem.

Special materials such as copper, nickel, stainless steels may

be used.

Other liquid characteristics that must be considered in design

are specific heat, freezing point, toxicity, explosion hazards

and radioactivity.

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Factors related to process are

• Viscosity

• Thermal sensitivity

• Heat transfer coefficient

• Vapour-liquid separator

• Fouling

• Crystallization

• Evaporator load

• Temperature driving force

• Foaming properties

Criteria for selection of evaporator

The selection of evaporator is done on the basis of

1. Factor related to process

2. Factor related to mechanical design

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Viscosity:

It is the highly influencing parameter.

The higher the viscosity the lower is heat transfer

coefficient and larger the heat transfer surface area.

Fouling:

In most evaporators fouling is due to sedimentation,

crystallization, chemical reaction, corrosion, polymerization

etc.

Fouling tendency is considerably reduced as velocity

increases

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Crystallization:

For solutions which have a tendency to crystallize during

evaporation, tabular heating surface is the best choice.

Thermal sensitivity:

Food, dairy, brewery and pharmaceutical products are

temperature sensitive; Film type evaporators are best for

such operations; usually operated under vacuum.

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Evaporator load:

When heat load is high or heat transfer coefficients are

low, large heating surface area is required. In such

operations multi effect evaporation is a good choice.

Heat transfer coefficient:

Heat transfer coefficient is a function of fluid velocity,

viscosity, density, specific heat and thermal conductivity.

Higher heat transfer coefficient needs smaller heat transfer

surface area for a given duty.

Fouling of the heating surfaces, reduces heat transfer

coefficient and increases pumping cost.

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Heat transfer coefficient:

• Film type evaporators are attractive in such cases.

• The thin film offers less resistance to vapour-liquid

separation.

• Separation of entrained liquid is carried out in the vapour

head to avoid excessive entrainment, product loss and

environmental pollution.

Foaming:

• If foam is formed during evaporation.

• It can cause large material loss due to entrainment.

• Either antifoaming agents has to be used or if it is not

possible special arrangement has to be made for

vapour liquid separation.

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Selection guide of evaporator

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Performance Measures:

There are three main measures of evaporator performance:

1.Capacity (kg vaporized / time)

2.Economy (kg vaporized / kg steam input)

3.Steam Consumption (kg / hr)

Steam Consumption = Capacity/Economy

Capacity: Capacity of an evaporator is defined as the

number of kilogram of water vaporized or evaporated per

hour.

Evaporator economy: Economy of an evaporator is

defined as the number of kilogram of water evaporated per

kilogram of steam fed to the evaporator. It is also called as

steam economy.

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Performance Measures:

Economy calculations are determined using enthalpy

balances.

In single effect evaporator the amount of water

evaporated per kg of steam fed is always less than one

and hence economy is less than one.

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The methods of increasing the economy are:

(i) Use of multiple effect evaporation system

(ii) Vapour recompression

In multiple effect evaporation system, the vapour produced

in first effect is fed to the steam chest of second effect as

heating medium in which boiling takes place at low

pressure and temperature and so on. Thus in triple effect

evaporator, 1 kg of steam fed to first effect evaporates

approximately 2.5 kg of water

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Another method to increase the economy of an evaporator

is to use principle of thermo compression.

Here the vapour from the evaporator is compressed to

increase its temperature so that it will condense at a

temperature higher enough to permit its use as heating

media in the same evaporator.

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Boiling Point Elevation

As evaporation proceeds, the liquor remaining in the

evaporator becomes more concentrated and its boiling point

will rise.

The extent of the boiling-point elevation depends upon the

nature of the material being evaporated and upon the

concentration changes that are produced.

In actual practice, boiling point elevation and liquid head

affect the boiling point of a solution.

As vapour pressure of most aqueous solutions is less than

that of pure water at a given temperature; the boiling point of

the solutions is higher than that of pure water at a given

pressure.

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Boiling Point Elevation

The difference between the boiling point of a solution and

that of pure water at any given pressure is known as boiling

point rise/elevation of a solution will be higher than that of the

water at a given pressure is known as Boiling Point

Rise/Elevation(BPE) or vapour pressure lowering.

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Boiling Point Elevation

The boiling point of a solution is a colligative property -- it

depends on the concentration of solute in the solution, but

not on what the solute and solvent are.

Boiling point elevation is small for dilute solutions and

large for concentrated solution of inorganic salts.

Boiling point elevation of strong solution can be obtained

from an empirical rule known as Duhring’s rule. It states

that the boiling point of a given solution is linear function of

the boiling point of pure water at the same pressure.

Hence when the boiling point of the solution plotted

against the boiling point of the water, straight line results.

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Duhring plot for boiling point

of sodium chloride solutions

Vapour pressure/temperature

curve for water

Page 75: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

Duhring plot for boiling point

of sodium hydroxide solutions

Page 76: Application of Mathematics in Chemical Engineering · Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water Examples of Evaporation:

To use a Duhring plot:

1. For a particular system pressure, determine the boiling

temperature of pure water. This can be done from a

vapor pressure equation or steam table.

2. Enter the plot from the bottom (the water boiling point),

trace up to the diagonal line representing the NaOH

fraction, then trace left to read the solution boiling point

from the vertical axis.

3. The boiling point elevation is the difference between

the two temperatures.

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Vacuum Evaporation

For the evaporation of liquids that are adversely affected

by high temperatures, it may be necessary to reduce the

temperature of boiling by operating under reduced

pressure.

The reduced pressures required to boil the liquor at lower

temperatures are obtained by mechanical or steam jet

ejector vacuum pumps, combined generally with

condensers for the vapours from the evaporator.

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Vacuum Evaporation

Mechanical vacuum pumps are generally cheaper in

running costs but more expensive in terms of capital than

are steam jet ejectors.

The condensed liquid can either be pumped from the

system or discharged through a tall barometric column in

which a static column of liquid balances the atmospheric

pressure.

Vacuum pumps are then left to deal with the non-

condensibles, which of course are much less in volume but

still have to be discharged to the atmosphere.