Discuss evaporation characteristics and equipment

1. ARBAMINCH UNIVERSITY
COLLEGE OF NATURAL SCIENCES
DEPARTMENT OF CHEMISTRY
Advanced Chemical Engineering
IChem 4181
Unit-4
Unit operations
4.1 Evaporation
Prepared By Dr.Ballekallu

2. Evaporation
Distillation is the technique of heating a liquid to create
vapor which is collected when cooled separate from the
original liquid. Distillation is a method of separating
mixtures based on differences in their boiling points. It
is a unit operation, or a physical separation process, and
not a chemical reaction.
Examples: Pure water can be separated from salt water
through distillation. Salt water is boiled to create water
steam, but the salt remains in the solution. The steam is
collected and allowed to cool back into salt-free water.

3. Vaporization (or vapourization) of an element or
compound is a phase transition from the liquid phase
to vapor. There are two types of
vaporization: evaporation and boiling. Evaporation is
a surface phenomenon, whereas boiling is a bulk
phenomenon.
In physics a vapor (American) or vapour (British and
Canadian) is a substance in the gas phase at a
temperature lower than its critical temperature, which
means that the vapor can be condensed to a liquid by
increasing the pressure on it without reducing the
temperature.

4. • An evaporator is a device used to turn the liquid form
of a chemical into its gaseous form
• The equipment used for evaporation is known as
Evaporator.
Applications
• Manufacturing of bulk drugs
• Manufacturing of biological products
• Manufacturing of food products
• Manufacturing of de mineralized water. Product
concentration
• Dryer feed pre-concentration ;Volume reduction
• Water / solvent recovery ;Crystallization

5. Properties of evaporating liquids that effects on the
process of evaporation
• Concentration. The viscosity and density of solution
increases with increase in concentration of the solution.
The boiling point of the solution increases considerably
with concentration increase ( Boiling Point Elevation ,
BPE) under the same pressure.
• Foaming: Some materials have a tendency to foam that
causes heavy entrainment ( Rising solvent vapor take away
the solute liquid) and thus loss of solute.
• Economic factors:
The recovery of solvent and utilization of waste heat are
also important as they involves considerable reduction of
cost.

6. Scale: Some solutions deposit scale on the heat transfer surfaces.
This reduces the heat transfer coefficient and rate of heat
transfer. It is necessary to clean the surfaces at regular intervals
of time.
Temperature sensitivity: Some materials such as
pharmaceuticals and food products get degraded (damaged)
when heated to moderate temperatures even for some period.
Corrosiveness: Whenever contamination and corrosion is a
problem, special materials such as copper, nickel, stainless steel
may be used.
Film and deposits:
- Some solutions deposit solid materials called scale on the heating
surfaces.Results in the overall heat-transfer coefficient decreases
and evaporator must be cleaned.

7. Performance of evaporators:
Performance of the evaporators evaluated in terms of
(i) Capacity (ii) Economy
• Capacity : The capacity of an evaporator is defined as the
number of kilogram of water vaporized per hour. If the
feed solution is cold then initially the heat given is used for
the liquid to reach the boiling temperature and then
evaporation starts, so the capacity is low.
• If the fed is at boiling temperature then capacity is
proportional to the heat transfer. If feed is at more than
boiling temperature for the corresponding pressure then
capacity is more. It is called flash evaporation.
• Economy: The economy of the evaporator is defined as the
kilogram of the water evaporated per kilogram of the steam
fed to the evaporator. In single effect economy is low. In
the case of multiple effect evaporator the economy is more.

8. Selection criteria
• Operating capacity
• Degree of concentration required
• Heat sensitivity of product
• Requirement for facilities to recover volatiles
• Ease of cleaning
• Reliability and simplicity of operation
• Size of evaporator
• Capital and operating costs

9. Classification of evaporators
The major requirement in the field of evaporation
technology is to maintain the quality of the liquid
during evaporation and to avoid damage to the
product.
• This may require the liquid to be exposed to the lowest
possible boiling temperature for the shortest period of
time.
• In almost all evaporators the heating medium is
steam,which heats a product on the other side of a heat
transfer surface.
• The following list contains the descriptions of the most
common types of evaporators.

10. 1.Open kettle or pan
2. Horizontal-tube natural circulation evaporator
3. Vertical-type natural circulation evaporator
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

11. OPEN KETTLE EVAPORATOR

12. Steam is supplied to the jacketed kettle in which the
aqueous extract is placed. Heat is transferred to the
extract by conduction and convection.
Construction
Copper is the excellent material for the kettle because of
its conductivity.
Iron is used for construction of jackets since it has
minimum conductivity.
• Working
Aqueous extract to be evaporated is placed in the kettle.
Steam is supplied through inlet. Steam gives out its heat to
the contents and the condensates leaves through the
outlet.

13. ADVANTAGES
1. Used for both small scale & large scale operation.
2. Simple in construction and easy to operate.
3. Low maintenance & installation cost.
4. Wide variety of materials.
DISADVANTAGE
1. Heat economy is less.
2. Not suitable for heat sensitive materials.
3. Heat decreases on product concentration.
4. Since, open type so vapor passes to atmosphere.
USES
• Concentrating aqueous and thermo-stable liquors.
• Eg. Cooking pickels, liquorice extract etc.

14. (a) HORIZONTAL TUBE TYPE
(b) VERTICAL TUBE TYPE EVAPORATOR

15. HORIZONTAL TUBE EVAPORATOR
• Steam is passed through the horizontal tubes, which are
immersed in a pool of liquid to be evaporated. Heat transfers
through the tubes and the solvent evaporates. Concentrated
liquid is collected.
CONSTRUCTION
• Large cylindrical body with doomed shaped at the top and
bottom, made of cast iron or plate steel. Stainless steel tubes
are used in steam compartments.
WORKING
• Feed is introduced into the evaporator until the steam
compartment is immersed. The horizontal tubes receives the
heat and conduct it to the liquid. The feed absorbes heat and
solvent gets evaporated. Concentrated liquid is collected.
USES
Best suited for non-viscous solution. E.g. Cascara extract.

16. VERTICAL TUBE EVAPORATOR
• Liquid is passed through the vertical tubes and steam is
supplied from outside tubes. Heat transfer takes place
through the tubes and the liquids inside tube gets
heated. The solvent evaporates, vapor escapes from the
top and concentrated liquid is collected from bottom.
CONSTRUCTION
• Consist of long cylindrical body made up of cast iron with
dome shaped top and bottom.
• About 100 such tubes are fitted in the body of 2.5 mtr.
Inlets are provided for steam and feed. Outlets are
provided for vapor, concentrated products, non-
condensed gases and condensate.

17. ADVANTAGES
1. Increases the heating surface 10-15 times than steam
jacketed kettle.
2. Vigorous circulation enhances rate of heat transfer.
3. More units can be joined.
DISADVANTAGES
1. Liquid to be maintained above calandria.
2. Complicated- increased installation cost.
3. Pressure has to maintain.
4. Cleaning and maintenance is difficult.
USES
Manufacture of cascara extract, sugar, salt, caustic soda etc.

18. FALLING FILM EVAPORATOR

19. ADVANTAGES
1. Suitable for high viscous
liquids
2. Liquid hold up time is less
3. Liquid is not overheated
4. Highly acidic and corrosive
feeds can be concentrated
DISADVANTAGES
1. Not for suspensions,
salting and scaling liquids
2. Poor feed distribution
in tubes
3. Feed ratio is high
USES
1. Separate volatile and non volatile liquids
2. Concentration of yeast extracts
3. Manufacture of gelatin
4. Extracts of tea and coffee

21. Methods of Operation of Evaporators
• Single effect evaporator
• Forward feed multiple effect evaporator
• Backward feed multiple effect evaporator
• Parallel feed multiple effect evaporator
• Single effect evaporator

22. • The feed enters at TF
• Saturated steam at TS enters the heat- exchange
section
• Condensed steam leaves as condensate or drips
• The solution in the evaporator is assumed to be
completely mixed .Hence, the concentrated product
and the solution in the evaporator have the same
composition
• Temperature T1 is the boiling point of the solution.
• The temperature of the vapor is also T1, since it is in
equilibrium with the boiling solution
• The pressure is P1, which is the vapor pressure of the
solution at T1

23. • If the solution to be evaporated is assumed to be dilute
and like water, then 1 kg of steam condensing will
evaporate approximately 1 kg of vapor (if the feed
entering has TF near the boiling point)
• Single-effect evaporators are often used when the
required capacity of operation is relatively small
and/or the cost of steam is relatively cheap compared
to the evaporator cost
• However, for large-capacity operation, using more
than one effect will markedly reduce steam costs

24. FORWARD FEED MULTIPLE-EFFECT EVAPORATORS

25. If the feed to the first effect is near the boiling point at the
pressure in the first effect, 1 kg of steam will evaporate
almost 1 kg of water.
The first effect operates at a temperature that is high
enough that the evaporated water serves as the heating
medium to the second effect
Here, again, almost another kg of water is evaporated,
which can then be used as the heating medium to the third
effect
As a very rough approximation, almost 3 kg of water will
be evaporated for 1 kg of steam in a three-effect
evaporator
 Hence, the steam economy, which is kg vapor
evaporated/kg steam used, is increased.
This also holds approximately for more than three effects

26.  However, the increased steam economy of a multiple-
effect evaporator is gained at the expense of the
original first cost of these evaporators
 In forward-feed operation as shown in above Fig, the
fresh feed is added to the first effect and flows to the
next in the same direction as the vapor flow.
 This method of operation is used when the feed is hot
or when the final concentrated product might be
damaged at high temperatures
 The boiling temperatures decrease from effect to effect.
This means that if the first effect is at P1 = 1 atm abs
pressure, the last effect will be under vacuum at a
pressure P3

27. BACKWARD FEED MULTIPLE-EFFECT
EVAPORATORS
In the backward-feed operation shown in Fig. for a triple-effect
evaporator, the fresh feed enters the last and coldest effect and
continues on until the concentrated product leaves the first
effect.

29. • CHEMICAL EVAPORATOR
• Evaporators mostly used in power plants and chemical
process industry. In power plants, the water is
vaporized from raw or treated water, for the purpose of
the steam production. Steam is used to produce the
power using steam turbines.
• In chemical industry, in the manufacture of the heavy
chemicals like caustic soda, table slat, sugar etc., the
process starts with dilute solution and evaporation large
quantity water takes place before final crystallization.
Chemical evaporation means the evaporation of water
for the purpose of concentrating the chemical solutes.

30. • Differences between the power plant evaporation and
chemical evaporation.
• Power plants operate with continuous blow down
(blow down removal of the concentrate from
evaporator, normally waste). Where as the chemical
evaporators do not operate with blow down because the
concentrate is the main product.
• In power plant evaporation, the vaporized water
(steam) is the main product, the concentrate in the
evaporator is waste and removed. In chemical
evaporators, the vaporized water is waste and removed,
the concentrate in the evaporator is the main product.

31. • Chemical evaporator require more heat transfer
area than power plant evaporators for the same
amount of water removal. This is due to “ Boiling
Point Rise (BPR)” in chemical evaporator.BPE
means, the increase in the boiling point of water
due to presence of chemicals.
• The design of the chemical evaporator is varies and
depends mainly on the properties of the chemical
( density, viscosity, foaming properties etc), where
as the design of the power plant evaporator is
unique.