INTRODUCTION

Introduc)on� Manyregionsintheworlddonothaveeasyaccesstofreshdrinkingwater

� Desalinationprocesshelpsremovesaltsfromseawatertomakeitdrinkable

� Pros:Bringsdrinkingwatertomanydryregions� Cons:Expensive

�  InsomepartsofMiddleEast,aliterofwaterismoreexpensivethanaliterofgasoline

PROCEDURES� ReverseOsmosis� Distillation

� Multistageflash(MSF)� Multipleeffectdistillation(MED)�  Vaporcompression

� Electrodialysis� Vacuumfreezing

NaturalDesalina-on:WaterCycle!

MajorStages1.Evapora)on2.Condensa)on3.Precipita)on4.Collec)on

Desalina)onProcesses

Available desalination technologies (II)

ThermalDesalina)onterminology

WaterVapor WaterSteam

Gasphaseofwater. Watervaporathighpressureandtemperature.

SensibleHeat LatentHeat

Heataddedorremovedandcauseschangeintemperature.

Heataddedorremovedandcauseschangeinphase.

SpecificHeatCapacity

Heatrequiredtoraiseaunitmassbyaunittemperatureatconstantvolumeorpressure(J/kg.°C).

Mul)StageFlash

Mul)pleEffectEvapora)on

MED LAS PALMAS IV

Thevaporpressureofanysolutionissmaller

thantheoneofthepurewateratthesametemperature.

Thus,theboilingtemperatureofasolutionishigherthantheoneofpurewater.

Thehighertheconcentrationofthesolution,thehighertheboilingtemperature.

Thisincrement,increasestheenergyneededtoproducetheevaporationofthesolution.

BoilingTemperatureofWateratDifferentPressures

Pressure (bar) Te mperature (deg. C)

2 120

1 100

0.25 65

0.1 45

ThermalDesalina)onWaterSaturationPressure

Pressureatwhichwaterchangesitsphaseatcertaintemperature.

Capacity

Unitelectricenergyconsumption

Heattransferarea

Gainoutputratio(GOR)

Quantityofkgofwaterevaporatedperhour.

It’sproportionaltotheheattransmission

velocitythroughthesurfaceofthetubes.

Thisvelocitydependsonthedifferenceoftemperaturebetweenthevaporandthewater.

ElectricenergyinkWhconsumedperm3of

waterproduced.

MSF:4kWh/m3

MED:2.5kWh/m3

VC:8kWh/m3

Totalareaofheattransferperunitofflowproduced.It’svaluedependsonthemaximumoperation

temperature.

Fortemperaturesof90-110ºC:200-300m2/kg.s

Fortemperaturesof60-70ºC:700-800m2/kg.s

GOR� GainedOutputRatio�  (Measurethethermalenergyconsumedinadesalinationprocess)

� Kgs.ofproductwaterproducedperkg.ofsteamconsumed.MED:GOR=0,8n(n:numberofeffects)MSFcrosstubes:GOR=n/2.5-3(n:numberofstages)MSFlongtubes:GOR=n/3.5-4

Thisvaluedoesnottakeinaccountthe

temperatureofthevapor.

It’sonlyvalidtocomparePlantswithsimilartemperature.

APlantwithaGORof8and70ºCvaportemperatureismoreefficientthananotheronewiththesameGOR,but120ºCvaportemperature.

ECONOMYorPERFORMANCERATIO

�  SameasGOR�  lb.ofwaterproducedperlb.ofsteamconsumed�  1lb.ofsteam=1,000BTU(BritishThermalUnit)�  lb.ofwaterproducedper1,000BTU�  kgofwaterproducedper2,326kJofheatconsumed

Thesteamcanbeobtainedfrom:

Abackpressureturbine

Acondensationturbine

It’sthestandardturbinetype.

It“squeezes”thesteamasmuchaspossible,

producingthemaximumamountofelectricity.

Steamfromthistypeofturbinesisobtainedbymeansofa“bleeding”beforethelaststages.

Extraction / Condensing Turbine

Condenser

Condensate Pump Heater # 1

Deaerator

MSF

Boiler

Fuel

To Ejectors

Extraction- condensing turbine

Power to water ratio 12 to 15 MW/MIGD

It’aspeciallydedicatedturbinewithalower

numberofstages,deliveringsteamatatemperatureupto140ºC.

Thismeansthattheelectricalgeneratorisproducinglessenergythanbeingdrivenbyastandardturbine.Thisdifferenceistheequivalentenergyconsumptionofthedistillationunit.

Power to water ratio 5 to 7.9 MW/MIGD

28 28

Back-pressure turbine t

Condensate Pump

MSF Distillers

Deaerator

Heater # 2 Heater # 1

G

Boiler

Fuel

Back Pressure Turbine

Ejector Moisture Separator

Desalina)onBarriers

� EnergyConsumption� Cost�  Scaling� Corrosion� EnvironmentalImpact

Minimum Energy

11

Salinity, ppm Min. Energy, kWh/m3

35,000 0.71

49,000 0.84

68,600 0.97

104,000 1.16

137,200 1.30

Energy consumption in large

desalination processes

9

Process Thermal energy kWh/m³

Electrical energy kWh/m³

Total energy kWh/m³

MSF 7.5 - 12 2.5 – 3.5 10 – 15.5

MED 4 - 7 1.5 - 2 5.5 - 9

SWRO - 3 - 6 3 - 6

BWRO - 0.5 - 2.5 0.5 - 2.5

Energy effect

In fact, as it can be seen from the energy flow diagram below, the great part of the heat input to the MSF system is returned back to the sea with the seawater drain stream.

Thescalinghasathermalconductivitylower

thantheoneofthemetallictubes.Scalingreducestheamountoftheheat

transferredthroughthewallsofthetubes.

Thethreemainsaltsfoundare:Calciumsulfate(CaSO4)Calciumcarbonate(CaCO3)Magnesiumhydroxide(Mg(OH)2)

0.8

Ant

isca

lant

Dos

e R

ate

(ppm

)

1 1.5

95 100 105

Top Brine Temperature (oC)

2.5 2

Dose Rate Optimized in 22000130

3

4.5

6.5

9

15

6 44

2

Dose Rate recommended in 1981

1.5

1.75

3.5 3

2.5

0

8

10

12

14

16

85 90 110 115 120

Dose Rate Optimized in 1987

ACHIEVEMENTS IN CONTROLLING ALKALINE SCALE FORMATION

Optimization Tests

Improvement of Chemical Formulation

Adoption of On- Line Sponge Ball Cleaning System

Scaled tubes in a distillation plant

CalciumSulfate:CaSO4

Toworkatlowertemperatures

Useofantiscalants

UseofNF

CalciumcarbonateCaCO3andMagnesium

hydroxideMg(OH)2

Acidification(Sulfuricacid)

Antiscalants

Inspiteofthescalingcontrolused,fromtimein

time,anacidcleaningisneededtoeliminatesomelightscalingformedonthetubes.

Thiscleaningisperformedusinghydrochloricacidandacorrosioninhibitor,toavoidtheattackoftheacidonthemetallicsurfaces.

DECARBONATOR

DEAREATOR

ThemetallicpartsofthePlantaremainly

attackedby:

pH

Operationtemperature

Levelofchlorides

Gasesintheinteriorofthechambers

pH,operationtemperatureandchlorideslevel

cannotbechangedinordertoavoidcorrosion.

Levelofgasesinsidethechambersarereducedbyextraction,bydeaeration(Oxygenelimination),bydecarbonation(CO2elimination)andtheuseofmetalswithahighresistancetocorrosion.

COMPARISON AMONG Sea water parameter

Range of Influence on MSF operability

Influence on MED Influence on RO

Temperature 15 – 40 °C Production and quality remain unchanged with aid of sea recirculation increasing TBT

water and

MED production is very influenced temperature

by high since max

top temperature is limited to 64 °C

Membranes can not work for long time at high temperature and water quality is highly influenced. temperature osmotic

At low the

pressure increase.s In general a wide range of temperature is not advisable .

Salinity 30-54 gr/l Production and quality remain unchanged. Little influence on choice since deaerated.

material water is

Production and quality remain unchanged. Some influence on material since water is normally not deaerated

High impact on osmotic pressure and

number of membrane to be used.

Turbidity 1 – 35 NTU Production and quality remain the same. At high value of turbidity antiscale dosage must be increased.

In the low medium range no major problem but at high value the spray nozzle may clog.

Raw water pre- treatment is very much affected by turbidity and at high value plant shut down is possible. High influence

on membrane fouling.

Biological activity na No influence No influence

Oil contamination Few ppb No influence No influence Plant must be stopped

EnvironmentalImpact

� Energyconsumptionandcarbonfootprint.

� Rejectionofbrinebackintosea.

MEDOPERATIONCOST

ROOPERATIONCOST

HYBRID SYSTEMS

Two or more desalination processes coupled together or with power source

• Mainly MSF or MED with RO or VC. This combination can better utilize fuel energy as well as the power produced

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43 43

Seawater

RO DESALINATION PLANT PRODUCT

WATER

Power/Water Hybrid Flow Chart

Seawater intake

Blending

Power Plant Thermal Desalination Plant

steam condensate

Pumping power

Power

Pumping Power Distillate

Permeate

NF Reject

Seawater NF Unit

NF Product

Ca = 481

Mg = 1507

TH = 7406

HCO3 = 145

SO4 = 3257

TDS = 45400

Ca = 72

Mg = 63

TH = 440

HCO3 = 51

SO4 = 23

TDS = 32060

NF/RO/MSF or NF/RO/MED Tri-hybird System

RO Reject

SWRO Unit RO Product Ca = 1

Mg = 2

TH = 9

HCO3 = 4 SO4

= -

TDS = 660

Ca = 281

Mg = 437

TH = 2502

HCO3 = 101

SO4 = 124

TDS = 61080

MSF/MED Product MSF/MED Unit