40 migd potabilization plant at ras laffan: design and ... · 2 msf vent gases addition followed by...

40 MIGD potabilization plant at Ras Laffan: design andoperating experience

Giorgio Migliorini, Renzo Meinardi

Fisia Italimpianti, Genoa, ItalyTel. þ39 010 6096 429; Fax þ39 010 6096 410; email: [email protected]

Received 21 February 2005; accepted 15 March 2005

Abstract

The potabilisation plant is an essential part of evaporative desalination plants facilities. Remineralization

systems are designed and installed in the past years using different process approaches finalized to achieve

satisfactory potable water properties with respect to corrosiveness and health aspects. The process developed for

40 MIGD Ras Laffan plant is based on calcium carbonate and CO2 MSF vent gases addition followed by

sodium hypochlorite from sea water electrochlorination blending and can be considered the most efficient

system to match the best potable water quality. In this paper the process model is described and the simulation

results are examined and compared with the field operating data.

Keywords: Potabilisation; Remineralization; pH adjustment; Acid dosing; Sea water salinity; Carbonate system;

Mass balance

1. Introduction

The aim of a remineralization plant is toconvert the product water from a desalinationprocess units into potable water for civil con-sumption [5]. The treatment process includesthe following stages:� CO2 recovery from the MSF plant vents� limestone dissolution (not lime dosing)

� corrosion inhibitor dosing� potable water chlorination

The remineralization plant can be divided,from the process point of view into threemain parts:a) Carbon dioxide compression and feeding

systemb) Carbon dioxide purification and absorptionc) Limestone dissolution, degasifying and

chemical dosing system

Presented at the Conference on Desalination and the Environment, Santa Margherita, Italy, 22–26 May 2005.

European Desalination Society.

0011-9164/05/$– See front matter � 2005 Elsevier B.V. All rights reserved

*Corresponding author.

Desalination 182 (2005) 275–282

2. Process description

2.1. Carbon dioxide recovery

In order to recarbonate the distillate pro-duced from the evaporators, the necessary CO2

is recovered from the same evaporator units. Thevent gases containing CO2, steam and othergases (mainly oxygen and nitrogen), are col-lected from the final condenser of the vacuumgroup connected to the evaporator vent linesand sent to the vent gas gathering system.

2.2. Carbon dioxide compression

The CO2 gas recovered from each eva-porator is collected by the suction headersof the liquid ring compressors.

The compressors increase the pressure ofCO2 to a sufficient level in order to send themixture of gases to the absorber columns.

Compressed gas is further purified in acti-vated carbon filters to remove pollutantstraces that could be dissolved in water.

2.3. Carbon dioxide absorption

The CO2 absorber is a packing tower,where a part of distillate water is fed in coun-ter current with the evaporator vent gasescontaining CO2.

The main requirement in the design ofabsorption equipment is to bring the gasinto intimate contact with the liquid, that is,to provide a large interfacial area and a highintensity of interface renewal.

For the action of pressure, the CO2 con-tained in the vent gas stream, is dissolved intothe water.

Solubility of CO2 into distillate water isextremely high and to increase the reactionrate of the following process step, that is thelimestone dissolution by means of CO2, dis-solved in the distillate stream, is necessary tomaximize the CO2 content at the limestonefilters inlet.

This result is obtained operating at pres-sure of approx 5 bar(a) and increasing theuncondensable load feeding the tower alsowith the air present in the vent gases comingfrom the evaporators.

At the outlet of the adsorption tower, thedistillate containing a large amount of CO2 ismixed with an other portion of bypassed dis-tillate and the is sent to limestone dissolutionfilters to obtain an appropriate hardness andalkalinity.

2.4. Calcium carbonate dissolution

Reaction of dissolution takes place intofilters that are periodically re-filled withfresh limestone, to continue the process.

Filling of filters with limestone shall bedone by a proper hydraulically operated fill-ing system.

The limestone is a chemical compound(usually not pure) that contains not lessthan 92% of CaCO3.

The reaction in the limestone dissolutionfilters is represented by the following equation:

CO2 þH2Oþ CaCO3 $ CaðHCO3Þ2 ð1Þ

The stoichiometric CO2 quantity necessaryto obtain a final bicarbonate content (or inother words the final alkalinity required) iscalculated by means of the previous equation.

Fig. 1 shows the relationship between theCO2 concentration and the alkalinity duringthe reaction process [6].

2.5. Post treatment

Water from limestone filters contains somequantities of unreacted CO2, that must bereleased into a degasifier to limit the causticsoda consumption for the neutralization.

CO2 removal is achieved in a packed strip-ping tower where the water enters in contactwith stripping air to remove the residual CO2.

276 G. Migliorini, R. Meinardi / Desalination 182 (2005) 275–282

The treated water is then mixed to abypassed distillate in a blending chamber toobtain the final product characteristics.

Sodium hydroxide solution to increase thepH in the potable water is dosed pH-con-trolled to the by-pass distillate line beforethe blending chamber.

Hypochlorite solution dosing is alsoaccomplished in the blending chamber pro-portionally to the potable water flow andthe residual chlorine content in the blendedwater.

Corrosion inhibitor is fed into the blend-ing chamber proportionally to the potablewater flow to prevent corrosion phenomena.

At the end of the treatment water shallcontain the required alkalinity at pH around8 as required.

3. Ras Laffan remineralization plant main

characteristics

The Ras Laffan potable water blendingplant has been designed with a nominal

capacity of 182,100 m3/d (40 MIGD) to treatdistillate from four desalination units of RasLaffan desalination project to produce waterof potable and low corrosive quality.

In order to optimise the sum of operatingand capital costs, according to the processdescription of section 2, the plant has beendesigned in by pass mode, so only part ofdistillate is processed into the blending plant.

4. Plant description

4.1. Gas gathering system

The four desalination units are served by thevent gas gathering system, that is composed bytwo identical lines, each one gathering the gasfrom a different couple of evaporation units.

The gas extracted from the distillers isgathered in a CO2 receiver positioned at thesuction of the CO2 liquid ring compressors.

In order to keep constant the pressure inthe vent gas inlet line, part of the compressedgas from the drain separator is recycled backto the compressor suction line.

The compressed gas passes through adrain separator, in which the water is sepa-rated from the gas and collected.

The vent gas from two desalination unitsjoins with the vent gas from the other twodesalination units after the drain separator.

The compressed gas flows to the carbonfilters one in operation and one in stand by;after the carbon filters the compressed gas isconveyed to the CO2 absorption section.

In Fig. 2 a scheme of the plant is reported.In Fig. 3 a plant overview is shown.

4.2. Distillate system

The raw distillate flow is divided into threepart streams:to the absorbersto the absorber by-passto the blending chamber.

150

100

50

0 100 200 300

Alkalinity (ppm in CaCO3)

Co 2

(m

g /l)

CS

Co

C

Temperature 40°C

A

B

C

Operation path in

limestone filter

Equi

libriu

m li

ne

Fig. 1. Remineralization process diagram in lime-

stone dissolution filter.

G. Migliorini, R. Meinardi / Desalination 182 (2005) 275–282 277

distillatewaterfrom

distillers

vent gasfrom

distillers

productwater

GasGathering

CO2Absorbers

LimestoneFilters

Degasifier

Sea water hypochloritedos.system

NaOHdosingsystem

BlendingChamber

Fig. 2. Ras Laffan remineralization plant—block diagram.

Fig. 3. Ras Laffan remineralization plant—general overview.


The streams coming from the absorberand the one coming from the absorber by-pass will join and they will be sent to thelimestone filters section.

The general principle on which the totaldistillate flow rate is distributed in these threestreams is based on the setting of twopercentages:� the percentage of distillate to the absorber

by pass within a range of 20–30% of thetotal distillate water flowrate.

� the percentage of distillate to the absorberswithin a range of 5–10% of the total dis-tillate water flowrate.These two percentages will be fixed by the

operator from DCS.The distillate flow rate to blending chamber

depends on the streams through the absorbersand the absorbers by-pass flow rate.

In the pipe conveying distillate to theblending chamber the dosages of sodiumhydroxide solution, Hypochlorite solutionand corrosion inhibitor solution has beenforeseen.

From the absorber, the acidified waterjoins with the absorber by-pass stream andthe total flow rate is conveyed to the lime-stone filters and from there to the degasifier.

The following parameters are monitoredon limestone filter section inlet header:� Pressure� Temperature� Conductivity

The system is designed to have six filters inoperation.

The carbonated water coming from thelimestone filters is fed to the degassing tower.

On the incoming water from the limestonefilters the following parameters aremonitored:� pH� Conductivity� Temperature transmitter

The atmospheric CO2 degassing tower pro-vide to remove some remaining free carbondioxide and to increase the content of oxygen.

The air necessary for degassing is suppliedby two air fans one operating and one in hotstand by.

The water coming out from the degasifiertower is gravity-conveyed to the blendingchamber situated below the tower; the by-passed raw distillate also is directed to theblending chamber.

5. Carbonic acid equlibrium

All the phase of the remineralization processcanbe followedbymeans of the salts balance andtrough the carbonic species equilibria [2].

The pH of most natural waters is generallyassumed to be controlled by the carbonic acidsystem.

The applicable equilibrium reactions are:

CO2 þH2O$ ðH2CO3Þ $ Hþ þHCO3�

ð2Þ

HCO3� ! Hþ þ CO3

�� ð3Þ

The CO2/HCO3�/CO32� equilibrium inthe water can be calculated starting fromalkalinity and pH values through the calcu-lation of the following equations:

kw ¼ Hþ½ � � OH�½ � ð4Þ

k1 ¼Hþ½ � � HCO3

�½ �CO2½ � ð5Þ

k2 ¼Hþ½ � � CO3

¼½ �HCO3

�½ � ð6Þ

where:Kw = ionisation product of water; K1 = firstdissociation constant; K2 = second dissoc-iation constant.


The following relations have to be takinginto account:

The following ionic balance can be alsowritten [1], . . . [3]

ALK½ � þ Hþ½ � ¼ HCO�3½ � þ 2 � CO3¼½ �

þ OH�½ �ð7Þ

And using the ionisation and dissociationconstants defined above, the following finalrelations are obtained:

HCO3�½ � ¼

Alk½ � þ Hþ½ � � kwHþ½ �

1þ 2k2

Hþ½ �ð8Þ

CO3¼½ � ¼ k2

Hþ½ �� HCO3�½ �: ð9Þ

CO2½ � ¼ Hþ½ �k1

� HCO3�½ � ð10Þ

For a defined pH and alkalinity is possibleto determine the bicarbonate content thatcontribute to the CO2 formation.

6. Limestone filters kinetics

The residence time necessary to achieve arequested CO2 content at the outlet of thelimestone dissolution filters can be calculatedby the following relationship deriving fromthe work of Yamauchi, Tanaka and others[6]:

t ¼ lnC0 � Cs=C� Csð Þ

kð11Þ

where C0 is the inlet CO2 content, Cs is thesaturation CO2 content, C is the actual outletCO2 content, K is reaction rate constant, t istime where the reaction rate constant can becorrelated to the limestone specific area of thelimestone bed.

A different approach can be developedaccording to the reversible reaction of thesecond order model for the reaction:

CO2 þH2O, H2CO3 , Hþ þHCO3 ð12Þ

where the kinetic constant of the hydrationrate kCO2

can be assumed in the range 0.025–0.04 s�1 and dehydration rate KH2CO3

is in therange between 10 and 20 s�1 [4].

By the integration of the relevant kineticequation the following expression can beobtained:

KCO2� kH2CO3

¼ 1

t� 1

m1�m2

ln

m2ðm1� reacted�molesÞm1ðm2� reacted�molesÞ

ð13Þ

where m1;2 ¼ initial�moles

KCO2

kH2CO3

�ffiffiffiffiffiffiffiffiffiffiffiKCO2

kH2CO3

q� �

KCO2

kH2CO3

� 1

By the resolution of Eq. (13) the relevantresidence time can be obtained with excel-lent correspondence to the actual registereddata.

7. Comparison between performance tests data

and model results

Two performance tests have been carriedout on the Ras Laffan remineralization plantin April 2004 and December 2004. During thetests all the process parameters have beenrecorded and some samples at the limestonefilters outlet and on the final product waterhave been taken and analysed in a chemicallaboratory.

In the following tables have beenreported the results from the calculationmodel simulating the characteristics of thewater at two different location of the plant


and the actual values resulting from thelaboratory analysis.

The kinetic equations implemented withinthe model, describing the reaction rate of thelimestone dissolution with the CO2, fit wellthe operating results as shown from the com-parison at the limestone filters outlet.

In fact the M-alkalinity value resultingfrom the model is in compliance with themeasured values and the same is for theCO2 excess at the limestone filters outlet eval-uated by means of the pH value.

The carbonic species equilibrium reactionconstants implemented within the model fitwell the operating results as compared forthe final product water stream.

8. Conclusions

The remineralization plant is an essentialpart of the potable water production systemsstarting from distillation plants.

For this reason, in the early 2000 ourcompany decided to develop his own process

Limestone filters outlet

Parameter Unit of measurement Potable watersample no.1

Potable watersample no.2

Results frommodel

M-alkalinity ppm as CaCO3 220 217 220PH — 6.8 6.8 6.75Final product waterParameter Unit of measurement Potable water

sample no.1Potable watersample no.2

Results frommodel

Caþþ ppm 28 28 28HCO3� ppm 88 88 89M-alkalinity ppm as CaCO3 72 72 73TDS ppm 188 189 192PH — 7.7 7.7 7.7

Date of the test run: April 2004.Water production: 44.7 MIGD.Temperature of distillate: 36.5�C.

Limestone filters outlet

Parameter Unit of measurement Potable watersample no.1

Potable watersample no.2

Results frommodel

M-alkalinity ppm as CaCO3 220 217 216PH — 6.6 6.6 6.8Final product waterParameter Unit of measurement Potable water

sample no.1Potable watersample no.2

Results frommodel

Caþþ ppm 26 26 26HCO3� ppm 92 92 91M-alkalinity ppm as CaCO3 75 75 75TDS ppm 186 184 196PH — 7.7 7.7 7.7

Date of the test run: december 2004.Water production: 40.9 MIGD.Temperature of distillate: 30�C.


to assure a optimal final product water for hisclients.

The Ras Laffan remineralization plant hasfully performed the guaranteed data and canbe considered on of the best plant of thistechnology installed in the world.

References

[1] Industrial Water Treatment Practice, Butter-

worths, London, 1961.

[2] H.E. Homig, Sea Water and Sea Water Distilla-

tion, Vulkan Verlag, ESSEM 1978.

[3] High temperature scale inhibitors for sea water

distillation, Watson Desalination Consultants,

October Naxassas 1979.

[4] W. Stumm and James J. Morgan, Aquatic

Chemistry—An Introduction Emphasizing Che-

mical Equilibria in Natural Waters, John Wiley

and sons, New York 1981.

[5] H. Ludwig and M. Hetschel, Treatment of dis-

tillate and permeates from seawater desalination

plants. Desalination, 58 (1986) 135–154.

[6] Y. Yamauchi, K. Tanaka and K. Hattori, Remi-

neralization of desalinated water by limestone

dissolution f1ilters. Desalination, 66 (1987)

365–383.


40 migd potabilization plant at ras laffan: design and ... · 2 msf vent gases addition followed by...

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