Application of the multi-effect process at high temperature for large seawater desalination plants

Desalination, 45 11933) 81-92 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

81

APPLICATIONOF THE MULTI-EFFECT PROCESS AT HIGH TEMPERATURE FOR LARGE SEAWATER DESALINATION PLANTS

B. FRANQUELIN - F. MURAT - C. TEMSTET Societe Internationale

de Dessalement (SIDEM), Paris,

France

process at high A large sea water desalination plant, using the multi-effect temperature has been described and a comparison is given with the other distillation processes. The plant is easily adaptable in ,^a wide range of operating condi. multi-effect _. tions. It has a low total energy consumption (8.75 KWh per m3 produced (expressed as electrical energy). A high GOR is obtained (12.4). Compared with a MSF of the same production capacity, the investment costs are smaller. This leads to a substantial decrease in cost of the produced water. The technology of horizontal sprayed tubes is a proven one and has been executed more than a hundred times on small and medium units. APPLICATION DU PROCEDE DES EFFETS MULTIPLES A HAUTE TEMPERATURE POUR DE GRANDES UNITES DE DESSALEMENT B. FRANQUELIN - F. MURAT - C. TEMSTET Une grande unite de dessalement (25000 m3/jour de production ; 5.5 MIGPD) utilisant le pro&d6 des effets multiples a haute temperature est d&rite. Une comparaison est faite avec les autres pro&d& de distillation. L'unite est facilement adaptable a de nombreuses conditions op@ratoires. Elle a une faible consonnnation totale d'energie (8.75 KWh par m3 produit, exprim& en energie electrique). Un GOR eleve est atteint (12.4). L'investissementest inferieur a celui d'une unit6 MSF de mOme capacite. Tout ceci conduit .!l un prix plus faible de l'eau produite. La technologie des tubes horizontaux arroses est eprouv6e‘et a donn6 satisfaction sur plus d'une centaine de petites et moyennes unites. ANWENDUNG DES MEHREFFEKTVERFAHRENSMIT HOHER TEMPERATUR FUER GROSSE MEERWASSERENTSALZUNGSANLAGEN B. FRANQUELIN - F. MURAT - C. TEMSTET Es wird eine grosse Meerwasserentsalzungsanlage, die nach dem Mehreffektverfahren arbeitet, beschrieben. Ein Vergleich mit anderen Distillationsverfahren wird angestellt. Die Anlage kann problemlos Uber einen weiten Bereich betrieben werden. Der Gesamtenergiebedarfist niedrig (ausgedrUcktin el. Energie 8.75 KWh pro m3 prod. Wasser) was einem-hohen Verhlltnis zwischen Distillat und Heizdampf entspricht (GOR 12.4). Dabei sind gegeniibereiner MSF-Anlage gleicher Leistung die Investitionskostenserinqer. Hieraus resultiert insoesamt fUr die vorgestellte Anlage die niedrigiten Kosten fltir produziertei Frischwasser. Das Technik der horizontal besprijhtenRohrbUndeln (HTME) hat sich bereits bewlhrt und wurde an Uber hundert kleinen und mittleren Anlage erprobt. OOll-9164/83/$03.00

0 1983 Elsevier Science Publishers B.V.

82

In our previous publication (1) we have compared four desalination plants with an output of 5.5 MIGPD, combined with a steam power plant and, as an example, installed in the Middle East :

- a multistage flash unit operating with polyphosphate treatment (top brine temperature : 9O'C) - a multistage flash unit operating at high temperature (115'C) - a multi-effect type unit operating at low temperature (70dC) - a mechanical vapour compression unit operating at 1OO'C. The purpose of the present communication is to present a multi-effect type unit operating at high temperature with acid treatment, this unit being also designed for the Middle East and also combined with a steam power plant. Multi-effect evaporators are based on a proven and well known process (horizontal falling film) which has been used for many years and has demonstrated excellent reliability. This process can be applied to large desalination plants capable of operating in a wide range of conditions with regard to the sea water salinity and temperature with the additional advantages of a low investment cost and a low energy consumption. To illustrate and outline these advantages we have herein studied a 16 effects unit producing 25000 m3 per day (5.5 MIGPD), (16ME25000)operating at a temperature of 106°C with acid treatment and a GOR of 12.4. 1. CHEMICAL TREATMENT The sea water has been assumed to have a salinity of 50g/kg and a temperature of 35°C. A typical analysis of such a water (from the Arabian Gulf) is given in table 1. After treatment with sulphuric acid, the bicarbonate ions are removed and the sulphate content is increased, leading to a new analysis also given in table 1. The limitating factor of such a system being the solubility of the sulphate species when concentrating,thewater and increasing the temperature, the solubility curves (concentrationfactor versus temperature) have been drawn up for the given sea water after acid treatment. The curves have been established by a compilation of the latest works on the subject, including Marshall's correlation (2) and our own developments (fig 1.).

83

2. HEAT BALANCE DIAGRAM The heat and mass balance diagram of the evaporator is given in figure 2. The incoming sea water after being heated up in preheaters located at the top of each cell, is sprayed on the tube bundles of the twelve hottest effects, while the spraying in the four last effects is realised by means of a brine recirculation from cell 12. This configuration has been adopted in order to have in all the effects a brine concentration sufficiently low to avoid any scaling by the sulphates. Since the cold effects can accomodate a relatively high concentration of the sprayed water, the purpose of the brine recirculation for the last four effects is to minimize the total required amount of sea water, thus limiting the cost of the sea water pretreatment. Naturally other configurations are possible leading to the same result. Figure 1 shows for each cell the maximum sea water concentration (i.e. concentration at the lowest part of the tube bundle). It can be seen that the operating points are still far away from the precipitation curves of the two scaling species Ca S04, l/2 H20 and Ca S04, 2H20. The cinetics of formation of the anhydrite is very low and does not affect the operation of such a plant, particularly if the recirculating brine is taken in conditions of no precipitation for these species. Representativepoints of sea water or brine at the spray nozzles level have been drawn for each cell on figure 1.

3. ENERGY CONSUMPTION The heating steam in the first effect is taken at the outlet of the medium pressure body of the turbine. Figure 2 gives a schematic diagramm of such a turbine. As indicated in this diagram, one ton of heating steam, if completely expanded in the turbine would have produced an energy of 90 KWH. That same quantity of steam, when extracted for desalination plant, generates about 12.4 tons of fresh water. One can draw the conclusion that one ton of fresh water corresponds to an energy loss of 7.25 KWH. Since this type of process avoids the operation of large brine recirculating pumps, its consumption of energy for auxiliary equipment is low compared with

84

a similar MSF plant and will be of about 1.5 KWH per m3 produced, compared to an energy consumption of more than.3.8 KWH/m3 for the auxiliaries of an MSF type plant. It can be seen therefore that the total energy consumption of such a multieffect evaporator stands at 8.75 KWH/m3 which is obviously the best performance in sea water desalination today.

4. TECHNOLOGY OF CONSTRUCTION The main guideline in selecting the materials of the evaporator has been the use of materials that have proved their compatibility in sea water and brine handling, at high temperature and with acid treatment such as : - evaporator Shell : carbon steel cladded with Cu-Ni 90/10 - water boxes, tubes plates, tube bundles : Cu-Ni 90/10 - preheater, final condenser, top layers of tube bundles : Titanium

5. HEAT EXCHANGE AREA It is well known that the heat treansfer coefficient of tubes is better when using horizontal falling films outside the tubes than with brine circulating inside the tubes. Nevertheless, the evaluation of heat transfer surfaces have been done assuming a conservative coefficient of 2700 kcal/hmPC (314G W/mZK). This value is a current requirement of specificationsfor MSF desalination units operating with acid treatment. The surface of the tube bundles calculated under these conditions would be 65600 m2. The accumulated surface of the preheaters and final condenser will be 9780 m2, which gives a total heat exchange area of 72.34 m2/m3 h produced. The typical arrangement of one cell is given in figure 4. If necessary, demisters can be installed to obtain the desired salinity for product water in the range of 2 to 25 pRm. It can be seen from figure 4 that the overall dimensions of the evaporator will be 5.5 x 54 x 7.2 m. As a comparison, a MSF plant of the same capacity and the same heat input will require at least 36 cells. Its consumption of energy for the auxiliaries will be more than 4 KWH/m3.

86

On the other hand, it is impossible to design a MSF plant having the same total energy requirements (the necessary number of cells would be more than 80 cells.

6. INVESTMENT COSTS Compared to an MSF plant, the main advantages of the multi-effect plant with regard to investment are : lower number of stages, relatively small volume smaller water boxes no brine recirculation pump smaller installed power (cabling, panels...) This leads to important savings in materials and equipment costs, construction, transportation,erection and civil engineering cost. The cost of the pumping station will also be decreased. As can be seen from figure 2, the flow of cooling water for the evaporator will be 5407 t/h (i.e. a saving of 20 % compared with a MSF plant of same capacity and GOR working under the same conditions). It has been calculated that an overall saving of 20 % for the investment cost will be made comparing a multi-effect plant with a MSF plant (115'C maximum temperature, GOR 9.5).

7. TOTAL COST COMPARISON The above considerations lead to the comparison of the five distillation processes :

- multiflash operating at 90°C, GOR 8 - multiflash operating at 115'C, GOR 9.5 - multi-effect operating at 70°C, GOR 6.2 - vapour compression at 100°C - multi-effect operating at 106'C, GOR 12.4 This comparison is given in table 2. The values relating to the first four processes have been previously exposed (1) and are here updated. It can be seen that the cost of distilled water is lowered by 30 % (on average) using multi-effect technology at high temperature instead of MSF at high temperature.

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8. CONCLUSIONS As shown above, the advantages of a multi-effect plant are numerous :

- based on a well proven technology, it is applicable whatever are the seawater salinity and temperature. Separate feed of each cell allows the modulation of the brine salinity permitting by this way the minimisation of the sea water pretreatment. - high GOR are obtained with a relatively low number of stages leading to important savings in energy consumption (thermal and mechanical) and in investment cost - high purity water can be easily produced even from sea water with high salt content. No other desalination process can up to now guarantee such qualities. This entitles us to consider this type of plant as the most suitable method for the future in sea water desalination.

TABLE 1 - SEA WATER ANALYSIS

1

!

!Before acid ! treatment

!

! ! ! ! ! ! ! ! ! ! !

. . . . . * .

Sodium Calcium Magnesium Potassium Chloride Sulphate Bicarbonate

! I i ! ! ! ! ! ! !

15 200 580 2 070 670 27 980 3 970 200

!

! After acid ! treatment ! I i 15 200 ! 580 ! 2 070 670 ! ! 27 980 ! 4 127 ! 0 !

!

! ! ! I ! ! ! ! ! ! ! !

REFERENCES 1

B. FRANQUELIN, Future in distillation processes, InternationalCongress On Desalination and Water Reuse, Nice 1979

2

MARSHALL W.L., SLUSHER R. , J.

Chem. Eng. Data, 13(l), (1968)

pp 83-93.

Vapour pressure

at

/m3

! ! ! ! ! ! ! ! ! ! ! ! ! !

/m3

!

! ! ! ! ! ! ! ! !

!

263

200

! I

! I

268

I

!

! I

I

!

I

! I

190

I

I

1050

!

I

!

I

! ! ! ! ! ! ! ! !

!

I

1150

3.72 kg

15.5 kWh

3.8 kWh

11.7 Kwh

9.5

!

I !

3.72 kg

15.5 kWh

4 kWh

!

11.5 Kwh

! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! !

Investment related to va-! pour extraction USB/m3/d i

Investment related to the auxiliaries power consumption USS/ m3/ d

Direct investment US $ / m3 / d

Fuel consumption

/m3

I Total power consumption

!

I consumption

! Auxiliaries

I

power

Power loss due to vapour ! extraction /m3

I

8

3.2 ata

282

75

950

3.31 kg

13.8 kWh

1.5 kWh

12.3 kWh

6.2

! ! !

2 ata ! ! !

I

! I

Acid or X

! ! ! !

! ! Water/Vapour ratio or ! Grain Output Ratio (GOR) 1 !

I

! turbine outlet

I

!

I Antiscaling treatment

! ! Polyphosphate !

i Maximum temperature

!

! 70°C (158 OF) ! ! Polyphosphate ! 1.2 ata !

i 115°C (239 OF)

MULTI-EFFECT

! 90°C (195 OF)

I

I

MULTIFLASH HIGH TEMP.

WATER

I

! !

I

OF COST OF DISTILLED

I

I

I

MULTIFLASH

2 : COMPARISON

I

i

I

i

!

! !

I

TABLE I

I-

!

! I

! I

! I

! I

!

!

I

! !

550

900

2.64 kg

11 kWh

11 kWh

Acid

!

166

75

850

2.10 kg

8.75 kWh

1.5 kWh

7.25 kWh

12.4

2.85 ata

Acid

I

I

! I

! !

I

!

!

!

I

!

i ! ! i

!

! I

!

! !

!

!

I

i ! ! ! ! !

!

i !

i 106'C (223 OF)

MULTI-EFFECT HIGH TEMP.

i 100°C

!

I

I

(212 "F)

VAPOUR COMPRESSION

I

I

i !

!

I

1.180 !

!

0.08

0.648

I

!

! ! ! ! ! ! ! ! ! ! ! ! !

: USS 550 / kW i.e. 22.9/kWh

1 kWh requires 0.24 kg of fuel.

I

; i !

0.008

0.452

1508

: USg 1200 / kW i.e. S 50/kWh

1.142

! ! ! !

! ! ! ! ! ! ! ! ! ! ! !

!

i !

MULTIFLASH HIGH TEMP.

Extra investment due to vapour extraction

i

I

i I ! I

Power investment

BASIC DATA

! 2. Fuel at 172 USS / m3 1

!

1 ! ! I MULTIFLASH I ! ! ! ! ! ! Total investment i 1613 ! USg / m3 / d 1 ! ! 0.484 ! Investment cost per m3 ! ! ! USg / m3 ! ! ! Chlorine cost 15 g/m3 ! 0.008 ! ! 0.010 ! Polyphosphatecost ! ! 10 g/m3 ! ! ! ! Cost of acid treatment ! ! ! (USg 286 / t) ! ! ! Cost of water (investment! ! +fuel + pretreatment) i ! 0.610 ! 1. Fuel at 29 US% / m3 1

I

: US$ 286 / t

0.776

0.476

0.08

0.008

0.327

1091

Acid local price

! !

I

!

!

I

!

i ! ! ! ! ! ! ! ! ! I ! ! !

.

: uss 571 / t

II .

I-

! ! ! !

!

! ! ! ! !

!

! I

!

!

! ! ! ! ! I

!

MULTI-EFFECT i HIGH TEMP. ! !

: usg 057 I t

0.949

0.572

0.06

0.435

1450

I ! !

I

Chlorine local price

I ! ! !

! ! ! ! ! ! ! I ! ! !

! ! ! !

i !

I

; i COM%;ON !

Polyphosphate local price

0.979

0.506

0.010

0.008

0.392

1307

MULTI-EFFECT

E

89

--

--

_---_

----__

-m-w-

91

Low pressure body

Medium Pressure Body

2.85 bar-

301°C

2721.3 kJ/kq

n llO°C

I

I

Deaerator

seater 129.3"C

T

Heater 55.5"C

I .

FIGURE 3 - TURBINE DIAGRAM This diagram

shows the equivalence of one kg of steam taken for desalination

(2.85 bar) when fully expanded in low pressure body of the turbine for power production. One kg of steam would have produced 0.09 KWh.

92

eSECT1 5PRA.Y riozzLE5

5EA WATER PRCMEKTER

.I

.

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