3. Literature Survey

Literature Survey

3. Literature Survey:

Methods of Desalination

Desalination is a process is a process that removes dissolved minerals from seawater,

brackish water or treated waste water [31 ].

The main methods of desalination are thermal methods and membrane methods.

Thermal methods are

1. Distillation

2. Solar evaporation

3. Freezing

Membrane methods are

1. Reverse Osmosis

2. Electro Dialyses

3.1 Thermal processes:

Distillation:

According to Working Paper 2003 [25], the oldest desalination process is distillation, which

has been used for over 2000 years. The basic concept behind distillation is that by heating an

aqueous solution one can generate water vapor. The water vapor contains n&ne of the

contaminants, like the salt or other materials originally found in the source of water. If this

vapor is directed towards a cool surface, it can be condensed to liquid water containing very

little of the foreign material. The vaporizing or the condensing temperature and the operating

pressure are process variables.

The only requirement is that, at constant pressure the heated mass must be hotter than the

condensing surface.

The primary problem in making distillation a feasible desalination process is that the amount

of energy required to evaporate water is very high. This energy is recovered when we

condense the water, but it is at a lower temperature.

17

FEED WATER OUT

~

PRODUCT WI. TER IN

CONCENTRATE IN

PRODUCT WI.TER OUT ..

CONCENTRATE OUT

Figure 3.1: Single Stage Flash Evapouration

Source: [25 I

Multi-Stage Flash evaporation (MSF):

Literature Survey

The most widely used distillation process is Multi-Stage Flash evaporation (MSF). A

diagram of a single stage is shown in Figure 3.1 [25]. Water enters at a temperature that is

above the equilibrium temperature for the stage pressure. A fraction of the water, sufficient to ,co.

bring the temperature to the boiling point, flashes (vaporizes rapidly) to steam, or vapour.

Vapour is condensed on tubes running through the flash chamber, heating the water inside

the tubes. These plants are characteristically built along with power plants and use the low

temperature steam from the power plants. The top temperature in the MSF plant is usually

below 250°C.

Advantages and disadvantages of MSF:

Among the advantages of MSF and other distillation processes is that the composition of feed

water feed water has a negligible effect on the energy required to produce a volume of

product water. The process delivers exceptionally high purity water (less than 25mg/l TDS)

and has been successfully operated in very large sizes. Among the disadvantages are high

capital cost ($4-12 per gallon day of installed capacity) and requirement for large input of

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LtBRARY UNIVERSITY 07- r:OO!'li\Tc· .....

MORATUWA ... J\

Literature Survey

heat. The electrical energy of re-circulation pumps alone exceeds the process cost for

seawater reverse osmosis [25].

Multiple-Effect Distillation CMED):

Older than MSF, but currently not as widely used is Multiple- Effect Distillation (MED) [25].

This is similar to MSF except that the water evaporates from the outside of the tubes and

condenses on the inside. Over the years, a greater deal of effort has gone into in to improving

the efficiency and economies of distillation. In Figure 3.2 [26], it shows the absorption of

heat recovered by plate heat exchangers, which are coupled to a multi-effect desalination .s

plant based on falling film evaporation on plate. In that case high rates of heat recovery allow

a larger water production and lead to the lowest life-cycle costs.

In each effect the vapour produced in the previous effect is condensed on one side of the heat

transfer surface and the heat of condensation derived is utilized to evaporate the seawater on

the other side of the heat transfer surface.

The subsequent reuse of the heat of vaporization and condensation is accomplished by

reducing the pressure in each of the effects. The first effect (hot) is the one operating at the

highest pressure and temperature. The subsequent (intermediate) effects operate at lower

pressure and temperature than the preceding effect, and finally the last (cold) effect is the one

operating at the lowest pressure and temperature. The vapour produced in the in the last

effect is condensed in a main condenser, which is typically seawater cooled.

, I •

Figure 3.2: Multi-ef(ect Distillation (MED)

Source: [261

91lui

,~.

!;lA ... ~ lllfl'll

19

Literature Survey

Advantages and disadvantages ofMED:

The major advantage of MED is the ability to operate at significantly higher performance

ratio (PR); in access of 15 pounds of product per pound of steam, where MSF has a practical

PR limit of 10. MED was generally limited in size to about 10 MGD, but Taweelah A-1 was

a break through plant, with 66MED capacity, comprised of 14 x 4.7 MGD units. Generally,

MED capital cost varies from $3.50- $8.00 per GPD installed capacity [25].

Vapour Compression (VC)

In this process [24], flowing water over tubes in a distillation chamber evaporates water.

Vapour from the distillation chamber is compressed, which increases both its temperature

and pressure, and returned to the inside of the tubes where it condenses. There are two

general vapour compression processes, thermal and mechanical, TVC and MVC, which

differ in the manner in which the vapour is compressed.

In MVC, mechanical compressor is used to generate the heat of evaporation. The compressor

creates a vacuum in the vessel and then compresses the vapour from the vessel and

condenses it inside of a tube bundle also in the same vessel. Seawater is sprayed on the

outside of the heated tube bundle where it boils and partly evaporates, producing more water.

VAp<':><

l

~ - - 1 . fe<>•i C l:ln-.<> O .,.J"tf!>:r v~~;J<Jr EI Pro~r l IJI NCG • ~r~l? tnh•b~tor __

Figure 3.3: Mechanical Vapor Compression CVC)

Source: [241

)~·

20

Literature Survey

In TVC, also called steam jet type VC, a venturi orifice at the steam jet creates and extracts

water vapour from the main vessel, creating a lower ambient pressure in the main vessel. The

steam jet compresses the extracted water vapour. This mixture is condensed on the tube walls

to provide the thermal energy (heat of condensation) to evaporate the raw water being

applied on the other side of the tube wall of the vessel.

Advantages and disadvantages of VC:

The multiple effect units of MVC require about 28kWh/1 000 gallons of specific electrical

energy. The capital cost varies from $6.00- $12.00 per GPD installed capacity [25].

~ •

Figure 3.4: Four-effect thermo-Compression (VC)

Source: [241

Solar Distillation

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/o

rw

Figure 3.5: Solar Distillation

Source: [241

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f\r,t("""",

(.):ol;ro /1;1t re-~1·

" ~:.!....- .,..·=:er

• r~r-s!- w;.~cr

''~ {k ·f·r· bt:J#.~ .f,,

,,..

21

Literature Survey

Solar Distillation is the simplest desalination process and is based on the green house effect

[26]. Glass and other transparent materials have the property of transmitting incident short­

wave solar radiation but do not transmit infrared radiation. Incident short-wave radiation

passes through a glass into a still where it is trapped and evaporates the water, which is then

condensed on the glass surface and collected as distillate.

Freezing:

During the process of freezing [24], dis3olved salts are naturally excluded during the

formation of ice crystals. Seawater can be by cooling the water to form crystals under .. controlled conditions. Before the entire mass of water has been frozen, the mixture is usually

washed and rinsed to remove the salts in the remaining water or adhering to the ice crystals.

The ice then melted to produce fresh water. According to [27] the principle behind this is

Eutectic Freezing Crystallization (EFT). It can be described as follows.

Pure water freezes at 0°C, but the freezing point can be lowered by dissolving salt in water. If

the solution is then cooled to below the new freezing point, water crystals (i.e. ice) will start

to form. As a result, the remaining saline solution becomes further concentrated, until the

saturation point is reached. This intersection of the freezing point line and the saturation

point line is referred to as the eutectic point. Below the eutectic point, salt crystals will be

formed in addition to the water crystals. Now water has the special property that its solid

state weighs less than its liquid state. This special property of water is put to good use in a

process called Eutectic Freezing Crystallization (EFT). As salt crystals form near the eutectic ,,.. point, they sink to the bottom of a solution while the ice crystals rise to the surface.

Typically, the difference in density between ice as salt crystals is of the order of magnitude

of one kilogram per liter. The result of the process is a separation by physical means,

allowing the two components of the solution to be readily isolated.

Advantages and disadvantages of Freezing:

Theoretically, freezing has some advantages over distillation. These advantages include a

lower theoretical energy requirement, minimal potential for corrosion, and little scaling or

precipitation. The disadvantage is that it ir.volves handling ice and water mixtures that are

mechanically complex to move and process [24].

22

Literature Survey

3.2 Membrane Methods:

Reverse Osmosis:

In the RO process, the feed is pressurized by a high-pressure pump and is made to flow

across a semi-permeable membrane. The feed pressure should exceed the osmotic pressure of

the salted water in order for the separation to take place [24]. Typical pressures of seawater

range from 50 to 80 atmospheres. Water passes through the membrane, which removes the

majority of the dissolved solids, and the rejected salt emerges from the membrane modules as .. a concentrated reject stream, still at high pressure. In larger plants, the reject brine pressure

energy is recovered in a turbine or pressure exchanger.

Sa!me fccd~.·a:er

Electro dialysis:

High pressure purnp

~mtr;m

mse_mhly

Bri'lC .jischmge

~ Srabihzc'<i

Figure 3.6: Simple Reverse Osmosis Plant

Source: [241 ,,..

In ED, feed water passes between a pair of membranes (an anion permeable and a cation

permeable membrane) in an electrical field applied across the membranes [24]. Impurities are

transported across the membranes and low salinity product water remains between the

membranes. Between each pair of membrane a spacer sheet is placed in order to permit the

water flow along the face of the membrane and to induce a degree of turbulence.

An improved ED process, known as electro dialysis reversal (EDR), consists ofperiodically

changing the polarity of the membrane electrodes alleviating the need for continuous

chemical addition.

23

Literature Survey

N<~gat•ve poi

Bn,.,~

ch;mnel

01recl Lurre,•lt

,,

Elec1rode E j U

~trean1

f-eed •,•,aler chclflr>el

., Cation sctective­membru."'1e

seleCLve membran"

Saline ....._ fPod·~oJ.•.nter

r.heam

o .recl Lll1ft'"ttl sou,·ce

f[R,. ·~~· . • ~·'1* + Positive po;e

Exampl.,,; of

)Cations Na .. Gil .. +

.., Anions Cl. COj

Figure 3. 7: Electrodialysis Process

Source: {241

3.3 Comparison of methods of desalination

Comparison of energy requirement of different desalination technologies

Table 3.1: Comparison o(energy requirement o(different desalination technologies

Source: [241

24

Literature Survey

Comparison of Distillation & Reverse Osmosis Technologies

Table 3.2: Comparison o[Distillation & Reverse Osmosis Technologies

Distillation Reverse Osmosis

Greater potential for economies of scale RO feed water generally does not require

heating, so the thermal impacts of

discharge are lower.

Distillation plants also do not shutdown a RO plants have fewer problems with

portion of their operations for cleaning or corrosion .. replacement of equipment as often as RO

plants, although distillation plants can

and have shut down for tube bundle

replacement and cleaning

Pre-treatment requirements are greater for RO plants usually have lower energy

RO plants, because coagulants are needed requirements

to settle out particles before water passes

through the membranes.

Unlike RO plants, distillation plants do RO plants tend to have higher recovery

not generate waste from backwash of pre- rates about 45%

treatment filters

RO process can remove unwantecj..

contaminants, such as trihalomethane -

precursors, pesticides and bacteria.

RO plants take up less surface area than

distillation plants for the same amount of

water production

Source: [311

3.4 Reverse Osmosis as the Widely Used Process of Desalination

Comparing all methods of desalination, Reverse Osmosis is considered as the method widely

used in the industry as it has the lowest energy requirement for Seawater as well as brackish

water. It also has the lowest capital cost. In addition it takes up less surface area.

25

Literature Survey

3.5 Problems Encountered in Operation ofRO Plants

The majority of RO systems normally will operate under fairly steady conditions over long

periods of time if operating parameters remain constant. Fouling does not occur, and

membrane damage is avoided. Unfortunately, operating parameters (e.g. temperature, feed

TDS, permeate flow, Recovery) do change, and fouling of the membrane of the membrane

and element path can occur. The user has to know whether changes in flow or rejection are

caused by: fouling, damage to membrane, or just due to different operating conditions.

Therefore, a model that can predict the operation parameters .. for varying feed conditions

would be very helpful.

,,..

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