Evaporation

Dr. Noha Khalifa

 Definition
❖ It is a unit operation involving heat transfer (thermal separation process)

❖ It is the removal of a relatively large amounts of liquid (solvent) to obtain concentrated

solution by application of heat.

❖ Heat is supplied indirectly using steam or hot liquid.

❖ End products of evaporation are A- Concentrate (concentrated solution).

B- Vapor (evaporated volatile component).

 Application
• Conc. Solution (Thick Liquor)
Feed (Dil • Top/Bottom
1- Concentration of sol.)
solutions.
• Top
2- Recovery of Water vapor • Condenser/Separator/Air
volatile component
(solvent).
• Condensate (drips)
Steam
(heating • Collected
element)
 Open

Theory of evaporation
System
Theory of evaporation

Closed
System
Theory of evaporation
Closed
System
Theory of evaporation
(Boiling point)
 Theory of evaporation
(Boiling point elevation)

Conc. Non Conc. Non

volatile volatile
 Factors affecting evaporation rate

1- The exposed surface area (SA): (Directly)

More molecules of water at the

Large SA Faster evaporation rate
surface to be evaporated

2- Temperature (T): (Directly)

Greater number of energetic

Increase T rapid evaporation
molecules
 Factors affecting evaporation rate

3- Air or external pressure : (Inversely)

More heat to reach

High pressure >1 atm B.PT slower evaporation rate
equilibrium

Low pressure<1 atm less heat to reach

B.PT faster evaporation rate
(Vacuum) equilibrium
 Factors affecting evaporation rate

4- The Flow rate of air: (Directly)

More molecules of water can be

High flow rate Faster evaporation rate
borne

5- Humidity of the air: (Inversely)

Increase humidity (amount of Air is filled with W.V. no space to

slower evaporation rate
water vapor in air) hold more
 Factors affecting evaporation rate

6- Nature of the liquid (Inversely)

A- Intermolecular forces
B- Large charged particles
C- Viscous liquid

High only molecules that Additional

intermolecular have sufficient energy energy to Slower
forces between to overcome forces overcome evaporation rate
molecules can be evaporated attraction forces
 Factors affecting evaporation rate

7- Solute solubility (Directly)

Solute with -ve Precipitation Slower
Heat transfer
solubility Temperature Solubility (Scale evaporation
coefficient
coefficient formation) rate

8- Solute Concentration : (Inversely)

Increase solute conc. vapor pressure B.PT elevation Slower evaporation rate
 Problems encountered during
evaporation process
1- Entrainment
❑ Def. : Finally divided liquid carried along with the steam of the vapor.

❑ Problems: 1- serious loss of the liquid

2- Contamination of the condensate (if desired for other purpose)

❑ Solution: 1- Elongation of vapor head

2- Use of baffles
3- Use of cyclone separator
4- Use of entrainment separator
 Problems encountered during
evaporation process
Entrainment separator
v
❑ Location: connected to the vapor outlet.

❑ Principle: Depends on the density difference between vapor (gas) and

entrainment (liquid)

❑ Mechanism: 1- Vapor enters through tangential inlet--→acquires rotation

motion-→centrifugation of the vapor--→heavy liquid droplets thrown to the
walls and settle down---→Light vapor goes to the center, where it changes its
direction and flow throw the central pipe.
Entrainment
separator
 Problems encountered during
evaporation process
2- Foaming
❑ Def. : Formation of stable layer of foam at the surface of evaporating solution.

❑ Causes: Occurs in the evaporation of saponin solutions (liquorice) and colloidal solutions.

❑ Problems: 1- Hinder the evaporation process.

❑ Solution: 1- Anti-foaming agents(sulfonated castor oil, cotton seed oil, octyl alcohol)
2- Use of (long tube vertical) film evaporators
 Problems encountered during
evaporation process
3- Scale Formation
❑ Def. : Materials of negative solubility coefficient tend to form scales upon heating.

❑ Problems: 1- Deposited scales act as a barrier to heat flow, result in slow evaporation
rate

❑ Solution: 1- Use of Forced Circulation evaporators

2- Use of Basket type evaporators
 EVAPORATORS

I- Natural circulation evaporators:

1- Steam jacketed pan

2- Horizontal tubes evaporators

3- Vertical tubes evaporators

i- Standard type

ii- Basket type

iii- Vertical long tubes evaporators

II- Forced circulation evaporator

 I- Steam jacketed pan
❑ Inner wall (line): made of copper (high K)

❑ Outer wall (jacket): made of cast iron (low K) and lined out
with tin or enamel to prevent rusting.

❑ Accessories: Anchor paddle or scrapper may be applied to

the pan to prevent accumulation of deposited solids
(Agitation)
Pan
❑ Operation:

1) Feed enters to the pan, while steam enters to the jacket.

2) Feed solution boils &evaporates to form conc. solution (thick

liquor)→ obtained from the bottom → either removed or fed to 2nd
evaporator. Water Vapor→ collected from the top.

3) Steam converted to-→ condensate.

 Pan
Pressure gauge
Steam
I-Steam
jacketed Boiler

pan Jacket

❑ Advantages: Condensate

1. Simple in construction and operation& cheap

Concentrate
❑ Disadvantages:

1. Low heat transfer coefficient & low evaporation rate (heating surface is limited)

2. Suitable only for aqueous thermostable solutions (open to atmosphere)

3. Not suitable for viscous solutions and solutions tend to form scales or foam upon heating
 **Still (Covered jacketed pan)

❑ Operation: under Vacuum (reduced pressure and low temp.).

❑ Advantages:

1. Widely used in pharm industry

2. Can be used with any solvent
3. Can be used with thermolabile solutions.

Still
II-Horizontal tube evaporator

❑ Operation:

1) Feed is introduced above the horizontal tubes to fill the lower Feed
part outside the tubes.

2) Steam enters inside the tube through the steam compartment.

Steam
3) Heat is transferred from the steam to the feed solution

4) Feed solution boils &evaporates to form conc. solution (thick

liquor)→ obtained from the bottom → When it reaches the
required conc.

5) Steam converted to-→ condensate (drips).

5) Vapor is withdrawn from the top of the vapor chamber. Drips/ Conc.
condensate solutions
II-Horizontal tube evaporator
❑ Advantages:

1. Not expensive and require low head room

2. higher heat transfer coefficient > pan (still low)

3. Can be used for non-viscous liquids

4. Can be used as a batch or a continuous process

❑ Disadvantages:

1. Low heat transfer coefficient due to absence of feed

circulation--→ can not be used for (thermolabile
solutions)→Long evaporation time.

2. Not suitable for viscous solutions

3. Not suitable for solutions tend to form foams or scales upon

heating.
Horizontal
tube
evaporator
III-Standard type vertical tube evaporator

❑ Operation:

1) Feed is introduced from the top to fill (inside) the lower half
of vertical tubes.

2) Steam circulates outside the tubes

3) Heat is transferred from the steam to the feed solution.

4) Feed circulation occurs by natural convection due to the

presence of central down take-→Increased heat transfer
coefficient.

5) Feed solution boils &evaporates to form conc. solution (thick

liquor)→ obtained from the bottom → When it reaches the
required conc (1-3 hrs).

6) Steam converted to-→ condensate (drips).

7) Vapor is withdrawn from the top of the vapor chamber.

III-Standard type vertical tube evaporator
❑ Advantages:

1. Moderate heat transfer coefficient

2. Can be used for low viscous liquids

❑ Disadvantages:

1. Can not be used for (thermolabile solutions)→Long

evaporation time.

2. Not suitable for viscous solutions

3. Not suitable for solutions tend to form foams upon heating

IV-Basket type vertical tube evaporator
❑ Operation:

1) Feed is introduced from the top to fill (inside) the lower half of
vertical tubes.

2) Steam circulates outside the tubes

Feed
3) Heat is transferred from the steam to the feed solution.

4) Feed circulation occurs by natural convection due to the Steam

presence of annular down take-→Increased heat transfer
coefficient.

5) Feed solution boils &evaporates to form conc. solution (thick

liquor)→ obtained from the bottom → When it reaches the
required conc (1-3 hrs).

6) Steam converted to-→ condensate (drips).

7) Vapor is withdrawn from the top of the vapor chamber.

 IV-Basket type vertical tube evaporator
❑ Modifications:

1. Baffles(deflector plate ): fixed close to the steam pipe that reduces entrained
liquid droplets from the vapor.

2. Single unit removable basket: Easy to be separated for cleaning and scale
removal.

3. Annular down take: allow natural convection → increase circulation->decrease

film thickness →increase film coeff.→increase heat flow and evaporation rate

❑ Advantages:

1. Can be used with scale forming (The heating surface is removable) and
solutions tend to form foams upon heating (the foam is broken due to the
liquid/vapor mixture striking an impingement baffle).
❑ Disadvantages:

1. Can not be used for (thermolabile solutions)→Long evaporation time.

2. Not suitable for viscous solutions

 Multiple effect evaporator

Operation:
❖ Connection between 2 or more evaporators.
❖ Heating element is the Steam in the first
evaporator. Then, the produced vapor is
used for the 2nd and etc.

Advantage:
❖ Maximize steam economy as vapor
generated from one effect is used to drive
another.

❖ This type of multiple-effect system is perfect

for continuous feed applications, as fluids
are progressively reduced in each effect to
final concentration discharge
V-Long tube vertical tube evaporator (film)
❑ Operation:

1) Feed is introduced from the bottom at a low rate to fill (inside)

about 2ft of vertical tubes.

2) Steam circulates outside the tubes

3) Heat is transferred from the steam to the feed solution.

4) Feed passes only once, and evaporation takes seconds without

circulation.

5) Feed solution boils &rises in the tubes with high velocity in the
narrow tubes → strikes the baffle in the enlarged head → thick
liquor withdrawn from the periphery tube.

6) Steam converted to-→ condensate (drips).

7) Vapor is withdrawn from the top of the vapor chamber.

Feed
V-Long tube vertical tube evaporator

❑ Modifications:

1. Long narrow tubes: A) Large SA for heat transfer→ increase heat transfer
coef., rate and furtherly evaporation rate (no need for circulation).

B) Feed introduced under high velocity→ decrease contact time between

steam and feed.

2. Enlarged vapor head: narrow tubes then sudden increase in space→

decrease in feed velocity → facilitate separation of entrainment (difference in
density)

3. Baffles(deflector plate ): decrease entrainment and foaming.

4. Connection between feed and down take tube: permit recycling of the
discharge, if more concentrated solution is required.
Long tube vertical evaporator
 V-Long tube vertical tube evaporator

❑ Advantages:

1. Can be used for foamy solutions.

2. Can be used for thermolabile solutions.

❑ Disadvantages:

1. Can not be used with scale forming solutions.

2. Not suitable for viscous solutions.

3. These vertical units require high headroom

VI-Forced circulation evaporators (All-purpose
evaporators)
• Forced circulation evaporators are usually more costly than natural circulation
evaporators. However the natural circulation evaporators are not suitable under some
situations such as:
1- Highly viscous solutions due to low heat transfer coefficient
2- Solution containing suspended particles
3- For heat sensitive materials (thermolabile)
• All these problems may be overcome when the liquid is circulated at high velocity
through the heat exchanger tubes to enhance the heat transfer rate and inhibit particle
deposition. Any evaporator that uses pump to ensure higher circulation velocity is
called a forced circulation evaporator.
VI-Forced circulation evaporator

❑ Operation:

1) Feed is forced by the circulating pump to enter the heater.

2) Feed is first heated (horizontal or vertical heater) without boiling due

to excessive pressure of the head over the heater. (positive pressure)

3) Hot liquid rises in a tube to be feed to the vapor head:

A- excessive pressure is suddenly released resulting in flash evaporation

in the evaporating chamber.

B-through tangential inlet( provides rotation of the mixture causing

centrifugation-→ separation based on density difference:

i. Vapor: withdraw from the top.

ii. Thick liquor: withdraw from the periphery.
iii. Diluted solution: repeat the cycle through connection.
VI-Forced circulation evaporator
❑ Modifications:

1. Separate heating element: (Two pass tubular heater) for easy scale
removal

2. Tangential element: for entry of feed with centrifugation

3. Baffles(deflector plate): decrease entrainment and foaming.

4. Circulating pump: to increase the feed flow (forced circulation)

❑ Advantages:

1. Efficiently transfers heat from steam to liquid.

2. Can be used for foamy, thermolabile, scale forming, and viscous
solutions.

❑ Disadvantages:

1. Expensive
2. Power necessary for circulating pump
Forced circulation evaporator
 All depends on difference in density
Evaporator accessories

Steam Dryer Entrainment separator Steam trap

Connected to vapor outlet Connected to condensate outlet

Connected to steam inlet

Separate entrainment droplets from

Obtain dry steam Separate steam from condensate
vapor

❑ Mechanical steam trap

❑ When condensate fills the device→ ball floats→ valve

opens→ condensate discharged.

❑ When steam fills the device→ ball sinks→ valve closes→

prevent steam leakage.