Evaporation Process of Food

1. Evaporation
process

2.  Evaporation is an important unit operation commonly employed
to remove water from dilute liquid foods to obtain concentrated
liquid products.
 Essentially an evaporator consists of a heat exchanger enclosed in
large chamber; a noncontact heat exchanger provides the means to
transfer heat from low-pressure steam to the product.
 The product inside the evaporation chamber is kept under
vacuum.
 The presence of vacuum causes the temperature difference
between steam and product to increase and the product boils at
relatively low temperatures, thus minimizing heat damage.
 So that evaporation is the process by which liquid removed from
the product with out significance loss of nutrients.

3. 1. Rate of heat transfer of the liquid
2. Amount of heat required to evaporate each kg of
water
3. Maximum allowable temperature of the liquid
4. The pressure at which the evaporation takes place
5. Any change in food stuff during evaporation process
i.e fouling or scaling , change in boiling point or BPE.
Factors affecting the rate of evaporation

4. Boiling-point elevation of a solution (liquid food) is
defined as the increase in boiling point over that of
pure water, at a given pressure.
 Boiling-point elevation describes the phenomenon
that the boiling point of a liquid (a solvent) will be
higher when another compound is added, meaning
that a solution has a higher boiling point than a pure
solvent.
This happens whenever a non-volatile solute, such as
a salt, is added to a pure solvent, such as water.
Boiling-point elevation

6. Concentration Units
M =
moles of solute
liters of solution
Molarity (M)
Molality (m)
m =
moles of solute
mass of solvent (kg)
12.3

7.  Problem
31.65 g of sodium chloride is added to 220.0 mL of water
at 35 °C. How will this affect the boiling point of the water?
Assume the sodium chloride completely dissociates in the
water. Given: density of water at 35 °C = 0.994 g/mL
Kb water = 0.51 °C kg/mol.
 Solve it quickly…………………

10.  The evaporation process done / occur in a single vessels is called
single-effect evaporator.
 The vapors produced are discarded without further utilizing their
inherent heat, therefore this type of evaporator is called a single-
effect evaporator.
Types of evaporator
Single-effect evaporator

11.  When the evaporation is occur in double vessels is called double
effect, similarly when associate with triple vessels then it is called
triple effect or multiple effect evaporation.
 A triple-effect evaporator, as vapors produced from first and
second effects (or evaporation chambers) are used again as the
heating medium in second and third effects.
Multiple effect evaporation

12. Several types of evaporators are used in the food
industry. In this section, a brief discussion of the more
common types is given.
1. Batch type evaporator:

13. 2. Natural circulation evaporator
 In natural circulation evaporators, short vertical tubes, typically 1-2 m long and
50-100 mm diameter, are arranged inside the steam chest.
The product when heated rises through these tubes by natural circulation while
steam condenses outside the tubes. Evaporation takes place inside the tubes and
the product is concentrated.
The concentrated liquid falls back to the base of the vessel through a central
annular section.
i.e. suger solution

14. 3. Rising – film Evaporator
 In a rising-film evaporator a low-viscosity liquid food is allowed to boil
inside 2.5 -5 cm dia and 10-15 m-long vertical tubes.
 The tubes are heated from the outside with steam. The liquid rises
inside these tubes by vapors formed near the bottom of the heating
tubes.
 A temperature differential of at least 14°C between the product and
the heating medium is necessary to obtain a well-developed film.

15. 4. Falling film Evaporator

16.  In contrast to the rising-film evaporator, the falling-film
evaporator has a thin liquid film moving downward under
gravity on the inside of the vertical tubes.
The falling-film evaporator allows a greater number of
effects than the rising-film evaporator.
 For example, if steam is available at 110°C and the boiling
temperature in the last effect is 50T, then the total available
temperature differential is 60°C.
 Since rising-film evaporators require 14°C temperature
differential across the heating surface, only four effects
are
Falling film Evaporator

17. 5. Rising / falling film evaporator:
 In the rising/falling-film evaporator, the product is concentrated
by circulation through a rising-film section followed by a falling-film
section of the evaporator.
 The product is first concentrated as it ascends through a rising
tube section, followed by the pre-concentrated product descending
through a falling-film section; there it attains its final concentration.

18. 6. Forced – circulation Evaporator
 The forced-circulation evaporator involves a noncontact
heat exchanger where liquid food is circulated at high
rates.
A hydrostatic head above the top of the tubes, eliminates
any boiling of the liquid Inside the separator.
The liquid entering the separator flashes to form a vapor.
The temp. difference across the heating surface in the
heat exchanger is usually 3-5°C.

19. 6. Forced – circulation Evaporator

20. The general equation describing heat transfer from some
heating medium to the product is as follows:
HEAT TRANSFER DURING EVAPORATION

21. 1.Design of single effect evaporator
2.Design of multiple effect evaporator.
Design of evaporator