LECTURE#3

THERMAL DESALINATION
MULTIEFFECT DESALINATION
(MED) PROCESSS

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 PRINCIPLE OF THERMAL DESALINATION

, thermal desalination is accomplished through two successive stages. In the first stage
sea water feed is heated to be separated into two streams: pure water vapor and highly
saline water.

•In the second and subsequent stage the produced water vapor in the first stage is
condensed to product water in the liquid form by removing the heat content..

”Stage 1 ‘ evaporation :vapor formation

”Stage 2 ‘ vapor condensation
Pure water vapor

Product water
Liquid form

Sea
water

Heat out :cooling

Highly saline water
Heat in

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 METHODS OF VAPORIZATION
Steam Vacuum
Supply
Sea water
Vapor

Flash Method of Vaporization

A Horizontal Falling Film Effect

Process Steam or Vapor Feed or Brine form Effect above

form Effect above

Condensate or
Vapor to Effect below Distillate
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to‫ ل‬Effect below
single effect desalting system
• single effect desalting system
consisting of an evaporator-condenser
combination .
•
A heat source (steam S) heats the
incoming feed F to the evaporator
from its entering feed temperature Tf
Mc-F to its boiling temperature Tb, and
STEAM evaporates part of it equal to D.

• The vapor D is directed to the

condenser where it condenses and
Cooling
water Mc
heats the cooling water Mc from
seawater tem- perature Tc to the feed
condensate temperature Tf.
Distillate D
• Part of Mc leaving the condenser is
used as feed F while the balance B
brine
(=Mc-F) is called brine blow-down and
is rejected back to the sea.
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 ‫‪THE MULTI-EFFECT‬‬
‫‪CONCEPT‬‬

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 Single Effect Desalination
Sea water reject
Sea water make up 3 kg Vent extraction

Sea Water inlet

1 kg
Heating fluid
GOR=1

1 kg 1kg
Distillate pump
Condensate
2kg
Brine extraction pump
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 Multi Effects Distillation

 Distillation
 One effect (cell) principle
Sea water
spraying

Produced vapor

Heating steam
)
Steam
condensate

Brine blowdown

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 Sea Water Desalination

Basics on MED

9kg

3kg 3kg 3kg

or
Steam
Transformer

6kg

GOR=Md/Ms=3
Recovery ratio RR= distillate/seawater
makeup=0.333
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 Multi Effects Distillation

 Distillation
 Multi effects (cells) principle
Sea water
spraying Produced
steam
(to next cell)

Produced steam Distillate

(to next cell)
water
production
Heating steam
(from previous cell)

Distillate water
production
Brine blowdown

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 Parallel feed
system
Conventional
Multi-effect
Desalting Systems

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Parallel feed multi-effect desalting unit with no pre-feed heaters

In the parallel feed arrangement, the feed F leaving the condenser is

divided and distributed almost equally to each effect.

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 Parallel feed
system
Conventional
falling film Multi- Forward feed
effect Desalting system
Systems

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 Forward feed six-effect with regenerative heating distillation system

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 forward feed arrangement
• In the forward feed arrangement, the feed water F (after leaving the bottom condenser at feed
temperature Tf) is supplied to the first effect of the highest temperature. Part of F equal to D1 is
evaporated in the first effect and the balance brine B1 (=F!D1) enters the second effect as a feed
F2, and D2 is evaporated out of it. The brine leaving the second effect B2 (=B1!D2) enters the
third effect as a feed F3, and D3 is evaporated out of it, and so on to the last effect. The brine
leaving the last effect is blown down to the sea. This means that the feed and vapor enter the
effects and flow in the same direction. The temperature distribution for this arrangement is shown
in Fig. 2b.-

Fig.2 (a). Forward feed six-effect distillation system, (b) Temperature

distribution through the effects
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 forward feed arrangement

• In the forward feed arrangement, the feed

water F leaving the condenser at feed
temperature Tf is supplied to the first effect
of the highest temperature.
•Part of F equal to D1 is evaporated in the
first effect and the balance brine B1 (=F-
D1) enters the second effect as a feed F2,
and D2 is evaporated out of it. The brine
leaving the second effect B2 (=B1-D2)
enters the third effect as a feed F3, and D3
is evaporated out of it, and so on to the last
effect.
•The brine leaving the last effect is blown
down to the sea. This means that the feed
and vapor enter the effects and flow in the
same direction. The temperature
distribution for this arrangement is shown
in Fig

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 Parallel feed
system
Conventional
falling film Multi- Forward feed
effect Desalting system
Systems

backward feed
system

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 backward feed arrangement

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 backward feed arrangement
 In the backward feed arrangement, shown in Fig. below, the feed water F is directed
from the end condenser to the last effect n, (of the lowest temperature), and Dn is
evaporated out of it. The brine Bn (=F-Dn) leaving effect n is directed to the
preceding effect (n-1) as a feed, and Dn-1 is generated out of it, and so on to the first
effect. The brine leaving the first effect is blown down to the sea. Thus, the feed and
vapor entering the effects have opposite flow directions. The temperature distribution
of this arrangement is illustrated in Fig..

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 The choice of any of these feed arrangements affects the design and
performance of the MEB desalting system, e.g. the evaporator arrangements,
the required heat transfer areas of the effects, the amount of vapor generated
in each effect (evaporator), the amounts of vapor generated by boiling and by
flashing, the pumping energy, the gain ratio (distillate to heating steam ratio),
and the cooling water to distillate ratio.
The backward feed (BF) has a favorable higher gain ratio and lower specific
heat transfer areas than the forward feed (FF) . However, the BF is seldom
used in desalination since the high salinity in the system occurs at the highest
temperature in the first effect.
Also in parallel feed (PF) system the maximum salinity is reached in each
effect. This increases the risk of depositing scales of inverse solubility salts like
CaCO3, Mg(OH)2, and CaSO4 in the BF system than in the FF system.
Another disadvantage for the BF system is the need for pumping the feed from
one effect to the preceding effect (at higher pressure).

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 Straight Multi Effect Desalination

Sea Water Make up Vent

Heating Q Sea Water

Fluid
Or Steam

Condensate

Distillate
p
Brine
P=Q x n

Production
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 Multiple Effect Distillation Process
 
The MED evaporator consists of several consecutive cells maintained at a decreasing
level of pressure (and temperature) from the first (hot) to the last (cold). Each cell
(also called effect) contains a horizontal tube bundle. The top of the bundle is sprayed
with sea water make-up that flows down from tube to tube by gravity. Heating steam
is introduced inside the tubes. Since tubes are cooled externally by make-up flow,
steam condenses into distillate (fresh water) inside the tubes. The heat released by the
condensation (latent heat)warms up the sea-water outside the tubes and partly
evaporates it. Due to evaporation, sea water slightly concentrates when flowing down
the bundle and gives brine at the bottom of the cell. The vapour raised by sea-water
evaporation is at a lower temperature than heating steam. However it can still be used
as a heating medium for the next effect where the process repeats.In the last cell, the
produced steam condenses in a conventional shell and tubes heat exchanger. This
exchanger, called"distillate condenser" or "final condenser" is cooled by sea-water. At
the outlet of the final condenser, part of the warmed sea-water is used as make-up of
the unit, the other part is rejected to the sea.
Brine and distillate are collected from cell to cell till the last one, where from they are
extracted by centrifugal pumps.
The heating steam of the first effect is generally low pressure condensing steam (as
low as 0.3 bar abs). Other heating media (such as hot water) may be used.

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 VAPOUR COMPRESSION (VC)

VC offers the best potential method of improving the performance of straight

MED desalination plants and achieving high performance ratios and hence
low water cost.

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 Mechanical Compression (MC)
Standard compressor to
increase the vapor’s pressure
VAPOUR
COMPRESSION
Thermal Compression (VC)
Thermo compressor (ejector) to
increase the vapor’s pressure

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 Multi Effects Distillation

 TVC

GOR = 6 Thermo Cell 1 Cell 2 Cell 3 Cond

Heating steam (input) 1kg steam input + 2kg 2kg 2kg 0
1kg recycled steam
Condensate return - 1kg 0 0 0
Distillate production - 1kg 2kg 2kg 1kg
Produced steam (output) 2kg steam output 2kg 2kg 2kg 0

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 MED - TVC
 
The thermo vapour compressor (TVC) uses high-pressure motive steam to
extract and compress low-pressure vapour from the last effect to an
intermediate pressure. The motive steam enters TVC through a
converging-diverging nozzle and expands to a pressure slightly lower than
the suction vapour pressure. The high velocity expanding steam jet
entrains the suction vapour and both get mixed in a mixing chamber of
TVC. The mixed vapour is recompressed to an intermediate pressure
through the diffuser converting kinetic energy to pressure energy. Figure 7
shows a schematic of TVC and Fig. 8 shows a schematic of MED-TVC.

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 Mechanical Compression (MC)
Standard compressor to
increase the vapor’s pressure
VAPOUR
COMPRESSION
Thermal Compression (VC)
Thermo compressor (ejector) to
increase the vapor’s pressure

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 Mechanical Vapour Compression Distiller
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 Multiple effect distillation using mechanical vapour compression
 
•The MED-MVC evaporator is a MED evaporator using only electrical energy. The evaporator itself is
of the MED type and all the features described for the MED evaporator apply for MED-MVC process.
This evaporator is equipped with a mechanical compressor. The compressor allows the vapour produced
in the coldest effect to be brought to the pressure conditions prevailing inside the bundle of the first cell,
thus enabling its latent heat to be re-available for distillation.

The input of energy ranges from around 18 kWh/m3 of distilled water for a single effect evaporator to
around 8 kWh/m3 distillate water for a four-cell evaporator. This energy input being quite small, it is
necessary to recover the heat from the distillate and the brine to preheat the sea water fed into the
evaporator. This is done by means of two plate heat exchangers.

This process is attractive in terms of energy consumption. However, due to the cost of the mechanical
compressor, the investment cost is higher than for a MED or MED-TVC plant. This process is usually
chosen for sites where the cost of energy is high.

From another point of view, due to its high efficiency, this process does not require any additional
cooling other than the feed water. It makes it attractive in places where cooling sources are not available.
The size of a MED-MVC plant is limited by the size of the compressors available on the market. For the
time being, the maximum possible size would be evaporators producing around 5,000 m3 per day of
distillate water.

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 Main Components of the
MED/TVC desalination plant

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 Desalination Unit
Steam from Boiler or Steam Transformer To Reject Min.
Steam from Boiler Flow line
Non Vent/
Condensable gas
From Duplex
Gas
Cell 3
Strainer
s Seawater
Make-up Condens Preheater
exchangers er
To thermo Make up
Sea
Water

Cell 1 Cell 2 Cell 3 Cell Vapor

A&B A&B A&B 6

Cell 1 Brine
water
Make-up

To Steam To Reject To Product

transformer Distillate Treatment
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And Storage Tanks
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 They provide higher overall heat transfer coefficients when
compared to multistage flash (MSF) desalination systems.

 MED does not employ recycling and are thus based on the
once through principle and have low requirements for
pumping energy.
 The power consumption of MED/TVC plants is only around
2kWh/m3 as there are no requirements to recirculate large
quantities of brine.
 The combination of high performance ratio and low power
consumption results in lower overall energy costs.

 Multi-effect distillation also offers the possibility of reducing

the plant size and footprint
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 Drawbacks
 Low temperature operation . Maximum operating temerature
65oC. This results in the decrease of the difference between
the TBT and last effect temperture which in turn increases
the heat transfer area. It also limits the feasible number of
effects

 High frequency of acid cleaning. The configuration is not

suitable for on-line ball .

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 MED NEW CONFIGURATION

■ It consists of three modules

■ Two Med modules with thermal vapor
compression (TVC), installed in parallel
(effect 1 to 3)
■ One module without TVC (Effects 4 to 6)
■ TBT  63oC.

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 Configuration of new MED/TVC desalination plants

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 Configuration of new MED/TVC desalination plants

Dist
Cell Cond Cell
3B enser 3A

Cell
6
Cell Cell
2B Cell 2B
5
Cell
4 Cell
Cell 1B
1B

Steam
Transformer
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 MED MATERIALS

 Top tube rows titanium to avoid erosion from sprays

 remainder usually AI-Brass
 Shell often solid stainless steel, 316L or higher
 Little data on actual plant availability or life

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 Mass and Heat Balance

 Yanbu MED Workshop

Doosan

39
Desalination Technologies
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‫معهد أبحاث تحلية ال‬ Research
‫ لحة‬Institute (DTRI)
Mass Balance Calculation

40
 Mass Balance Calculation
1. Seawater Flow Rate Calculation mSW

ms
Mcd

mSW
mc

MB MD
Qin = Hs – Hc Conversion of Energy Principle

41
 Mass Balance Calculation
2. Brine Flow Calculation from Mass Conservation

mF
ms
mCD
mc mSW

Recovery ratio mD / mF mB mD
=o.28571 given

42
 Mass Balance Calculation
3. Steam Transformer Flow Calculation by Energy
Balance

43
 Mass Balance Calculation
4. Chemical Consumption
Calculation

44
 Thermal Calculation
Heat Transfer
Theory

45
 Thermal Calculation
Example Calculation (1st Effect) – Heat Transfer Area
Calculation

46
 Thermal Calculation
Example Calculation (1st Effect) – Vapor Flow
Calculation

47
 Thermal Calculation
Loss Calculation – Boiling Point Elevation &
Demister Loss

48
 Thermal Calculation
Loss Calculation – Tube side
Loss

49
 Thermal Calculation
Calculation Result

50
 Thermal Calculation
Heat Transfer Coefficient Calculation

51
 Thermal Calculation
Heat Transfer Coefficient Calculation

52
 Thermal Calculation
Heat Transfer Coefficient Calculation – Calculation
Result

53
 ‫‪54‬‬
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