Breakthrough of MED Technology in Very Large Scale Applications, the Taweelah-A1 Case

Joost W. Vermey, Denis Beraud-Sudreau, United Arab Emirates

Abstract

A very large scale MED type desalination plant is currently being realised at Taweelah, Abu Dhabi Emirate, UAE. For the Taweelah-A1 IWPP, the project developer has made the choice to apply modern MED technology for the new 52 MIGD desalination plant. Next to the implementation of new MED units, four existing MSF will be refurbished and upgraded to a capacity of 8 MIGD each, as part of the project scope.

The MED plant will have 14 units, each 3.77 MIGD in capacity. The applied MED technology is based on horizontal tubes with falling film evaporation and thermal vapour compression. The plant will be operating at a maximum brine temperature of 63 °C to reduce scaling and corrosion potential.

In each MED unit, a steam transformer will separate the potentially contaminated steam from the distilled product water, which will be used for potable water applications. Six effects and a final condenser will be implemented in each unit. In order to prevent up-scaling difficulties and vapour flow rate limits, the first three effects will be executed in two identical rows of three cells. Effects 4, 5 and 6 will be single, which is feasible since the vapour compressor reduces the vapour flow directed into effect no. 4.

The main MED plant performance parameter, the performance ratio, amounts to 8.0 kg distillate/ kg steam, including the steam transformer losses. This MED plant is heat energy efficient, given the unfavourable Gulf conditions due to the high summer seawater design temperature of 35 °C. The low value for the available overall delta T puts a limitation to the possible number of effects of six. A comparison is made of the performance (design) parameters of the new Taweelah-A1 MED plant and two MSF plants at the Taweelah site, viz. the refurbished Taweelah-A1 plant (32 MIGD) and the existing Taweelah-B plant (76 MIGD). It appears that the MED technology is more efficient in specific steam use, specific electric power consumption and specific sea water supply rate.

Due to the favourable efficiency ratios and to considerations of capital expenditure, MED technology has now for the first time been selected for a very large scale desalination plant application, thus competing with MSF technology.

The breakthrough of MED technology in very large scale applications is now a fact.

1. Introduction

2. Desalination Plant Capacity

3. Applied MED Technology

4. Steam Transformer

5. Design Parameters MED Plant

6. Upscaling Aspects

7. Integrated Facilities

8. Comparison New MED Technology With Existing MSF Technology

9. Water Production Costs

10. Conclusion

1. INTRODUCTION

The AL TAWEELAH-A1 Extension Project, in Abu Dhabi Emirate, U.A.E. consists of two lots, each with a different character, viz. the rehabilitation and uprating of an existing power and desalination plant, and the construction of a new power and desalination plant. Both plants will be integrated into the new AL TAWEELAH-A1 POWER AND DESALINATION PLANT.

The project is a so-called Independent Water and Power Project (IWPP), implemented under the privatisation programme in Abu Dhabi Emirate. For this purpose a 20-year Power and Water Purchase Agreement (PWPA) was signed between Abu Dhabi Water and Electricity Company (ADWEC) and Gulf Total Tractebel Power Company (GTTP). Other related contracts that have been signed are an Operation and Maintenance (O&M) contract with Total Tractebel O&M Company, and a turnkey contract with Total Tractebel Emirates EPC Company (TTE EPC).

The consortium currently implementing the project installations is the Total Tractebel Emirates EPC Company (TTE EPC Co). The Owner of the installations will be the Gulf Total Tractebel Power Company (GTTP Co), in which TotalFinaElf from France and Tractebel from Belgium each have a 20% stake. Of the remaining 60%, the Abu Dhabi Water and Electricity Authority is the owner.

Tebodin Middle East in Abu Dhabi is the Independent Engineer assigned to review the design and realisation of the water desalination plant. Tebodin Middle East is a subsidiary of Tebodin Consultants & Engineers, based in The Netherlands. Sargent & Lundy in Chicago is Owner’s Engineer.

© International Desalination Association, 2001 Page 2 of 11 Sidem from France is the desalination plant supplier, being responsible for the Multiple Effect Distillation (MED) plant design and realisation.

2. DESALINATION PLANT CAPACITY

The upgrading of the existing water plant AL TAWEELAH-A involves the rehabilitation and uprating of three Multi Stage Flash (MSF) units of 7.2 MIGD (million Imperial gallon per day) capacity each and one MSF unit of 7.6 MIGD, to an increased capacity of 8.0 MIGD for each of the four units. The new desalination plant will consist of fourteen MED units, each with a capacity of 3.77 MIGD. The total design capacity of the uprated MSF units will be 32.0 MIGD. The new MED plant is designed to produce 52.78 MIGD. The overall AL TAWEELAH-A1 desalination capacity will become 84.78 MIGD or 385,410 m3/day.

After commissioning of the new Taweelah-A1 plant, due at the end of 2002, the total AL TAWEELAH site desalination design capacity will become one of the largest in the world. With 84.8 MIGD for Taweelah-A1, 50 MIGD for Taweelah-A2, 76 MIGD for Taweelah-B and 23 MIGD for Taweelah-B2, a maximum desalinated water production capacity of 234 MIGD or 1,063,000 m3/day will evolve.

Special attention should be given to the new MED plant, which will be by far the largest desalination plant based on Multiple Effect Distillation technology in the world to date, see table 2.

TAWEELAH-A TAWEELAH-A1 PLANT OUTPUT CAPACITY PLANT OUTPUT CAPACITY IN MIGD IN MIGD MSF Plant 29.2 32.0 MSF unit 1 thru unit 3, each 7.2 8.0 MSF unit 4 7.6 8.0

MED Plant - 52.78 MED unit 1 thru unit 14, each - 3.77

Total Plant Capacity in MIGD 29.2 84.78 Total Plant Capacity in m3/h 5531 16,059

Table 1: Plant Capacities Taweelah-A1 Desalination Plants

A comparison of recently commissioned MED plants is given in table 2.

MED PLANT MED PLANT CAPACITY MED UNIT CAPACITY YEAR COMMISSIONED and COUNTRY IN MIGD (m3/day) IN MIGD (m3/day)

Taweelah-A1, Abu Dhabi 52.8 (240,000) 3.77 (17,138) End of 2002 United Arab Emirates Layyah, Sharjah, 10.0 (45,460) 5.0 (22,730) 2001

United Arab Emirates ALBA, 9.5 (43,300) 2.4 (10,824) 2001 Bahrain Umm Al Nar #9, #10 7.0 (31,800) 3.5 (15,911) 2000 Abu Dhabi, U.A.E. AVR, Rotterdam 5.2 (24,000) 2.6 (12,000) 2000 Netherlands Jamnagar 10.6 (48,000) 2.6 (12,000) 1999 India K.A.E., Curaçao, 2.6 (12,000) 2.6 (12,000) 1997 Netherlands Antilles

Table 2: Capacities of Recently Commissioned Large Scale MED Plants

Table 2 shows that the Taweelah-A1 MED unit capacity of 3.77 MIGD is not the largest realised to date, since the MED unit capacity in the Layyah power plant in Sharjah, U.A.E., is 5.0 MIGD, being the largest MED unit capacity in the world. For the Taweelah-A1 project, the Client specified the maximum unit capacity to be applied for the MED plant, in order to limit the up-scaling factor to 1.4, related to the proven capacity of the largest unit of that time. In case the largest unit capacity now being commercially available would have been applied to the Taweelah-A1 plant, the number of MED units could have been reduced from 14 to 10.

3. APPLIED MED TECHNOLOGY

The MED technology applied in the Taweelah-A1 plant is based on horizontal tubes with falling film evaporation, and parallel feed with pre-heaters. The top brine temperature is only 63 °C, in order to minimize scale forming on the tubes. Thermal vapour compression (TVC) is applied to increase the plant Performance Ratio. The vapour compression is effectuated by extracting vacuum vapour from the third effect while mixing the vapour with the supplied low-pressure steam in an ejector device, prior to entering the first effect.

Each MED unit consists of two separate rows of circular modules, or vessels, each comprising three effects and a vapour compression ejector. Positioned as the centre row, there is one circular module comprising the effects 4 to 6 and the final condenser, see figure 1. The MED technology applied yields a very compact installation with a high unit capacity.

MED unit: dimensions 45.5 x 23.7 m

Figure 1: MED Unit View

Steam will be supplied from the adjacent power generation plant, in which plant low-pressure steam is extracted from back pressure turbines. The pressure level of the steam supplied to the steam transformer is 2.8 bar(a), whereas the pressure of the steam supplied through the TVC-ejector into the first effect is only 0.26 bar(a) at a temperature of 65 °C. Due to the seasonal fluctuations in power demand, in wintertime less extracted low-pressure steam is available. To overcome this imbalance in extraction steam availability, additional steam pressure reduction stations will be provided to be able to apply steam from the boilers. In case extracted steam is not used in the MED plant, the steam is condensed in air-cooled dump condensers.

Since the steam originates from a power plant, it can contain low concentrations of chemical additives such as hydrazine. A special design requirement connected to the use of this plant steam in the -potable water- producing MED plant, is that no direct contact is allowed between the plant steam and the distilled product water. Due to the applied thermal vapour compression concept, a physical barrier between supplied steam and produced distillate in the first effect is not possible. This physical division will now be implemented by means of installing a steam transformer unit. In this steam transformer, which is in fact an additional evaporation stage, the supplied low-pressure plant steam flows through horizontal tubes, over which tubes condensate from the first MED effect is sprayed. A thin film evaporation process takes place, thus producing hydrazine-free steam.

5. DESIGN PARAMETERS MED PLANT

In table 3, the main design parameters of the Taweelah-A1 MED plant are summarised.

DESIGN PARAMETER UNITS NOMINAL VALUE

Distillate Production m3/h 9999 Low Pressure Steam to steam transformer t/h 1243 Ejector Steam t/h 40.6 Sea Water Supply rate (summer) t/h 94,584 Make-Up flow rate t/h 32,928

Number of effects - 6 Performance Ratio (incl. steam transformer) kg dist/ kg steam 8.0 Top Brine Temperature °C 63 Seawater temperature (min./max.) °C 18/35 Seawater TDS maximum ppm 47,800 Concentration factor at blow down - 1.44

Distillate Purity (maximum) ppm 25 Antiscale specific consumption to make-up ppm 2.5 Antifoam specific consumption to make-up ppm 0.1

Table 3: Design Parameters Taweelah-A1 MED Plant

The high flow rates required in this high capacity MED plant are self-explanatory. With a Performance Ratio, or Gained Output Ratio (GOR), of 8.0 kg of distillate per kg of steam, the Taweelah-A1 MED plant is an efficient evaporator plant considering the rather unfavourable Gulf conditions, with a high maximum seawater design temperature of 35°C. The overall temperature difference or driving force in the MED evaporation process is only 17.8 °C, or 3.0 °C in each effect. This figure is to be further reduced to obtain the effective temperature difference, due to boiling point elevation and vapour flow pressure loss. Calculated without the application of the steam transformer, the value of the Performance Ratio becomes 8.8.

6. UPSCALING ASPECTS

In table 4, a summary is given of the main design features of the Taweelah-A1 MED evaporators.

DIMENSION / SIZE UNITS THERMO MULTI EFFECT COMPRESSOR SECTION SECTION Number of effects - 3 3 Number of cells - 6 3 Length (total for module) m 40.5 45.5 Unit width m 23.7 Width/Internal diameter m 6.2 5.7 Height of installed modules (top ejector) m 11 Number of tubes per stage - 19,710 9490 Heat transfer surface area m2 41,172 15,900 Fouling factor design value m2 K/W 0.0001 0.0001

Table 4: MED Evaporator Details

The MED evaporator unit has a very compact footprint, viz. 45.5 m x 23.7 m for one unit.

The phenomenon generally considered to be the limiting factor in the up-scaling of MED units, is the internal pressure loss due to the large vapour volumes, causing a limitation to the tube bundle dimensions. In the design of this MED plant this problem was solved by dividing the large vapour flow in the first three effects into two separate vessels, thus creating in fact two rows of effects in parallel, whereas the effects 4 until 6 are constructed in one vessel. This division is possible since the ejector installed for the vapour compression service reduces the vapour flow flowing into the fourth effect.

7. INTEGRATED FACILITIES

The upgraded MSF plant and the new MED plant will have integrated plant auxiliaries such as seawater intake, blending, distillate post-treatment and storage.

© International Desalination Association, 2001 Page 7 of 11 Chlorination of seawater will be achieved by dosing of a sodium-hypochlorite solution, which is generated on-site from seawater by means of an electrolysis process. Distillate post-treatment with remineralisation will be achieved by adding milk of lime combined with dosing of on-site produced gaseous carbon dioxide to the distillate. A safety chlorination will be part of the post-treatment of distillate, executed in the same way the seawater is chlorinated.

8. COMPARISON NEW MED TECHNOLOGY WITH EXISTING MSF TECHNOLOGY

In tables 5 and 6, some design parameters of the new Taweelah MED plant are compared to the design parameters of two existing Taweelah MSF plants, viz. the Taweelah-B MSF plant commissioned in 1996, and the Taweelah-A1 MSF plant after rehabilitation and upgrading (2002).

DESIGN PARAMETER UNIT TAWEELAH-A1 TAWEELAH-B TAWEELAH-A1 MED MSF MSF Nom. Max. Max. Unit capacity MIGD 3.77 10.0 12.7 8.0

t/h 714 1896 2400 1515

Top Brine Temperature °C 63 100 112 108

Performance Ratio kg steam/ 8.04 8.0 7.67 7.56 kg distillate 8.82Note 1) Distillate flow rate t/h 714.2 1896 2400 1515

Total Summer Sea Water Feed rate t/h 6756 24,700 25,800 17,444

Make-up flow rate t/h 2352 7000 8100 6351

Steam Supply flow rate (LPS) t/h 88.8 238 313 200.3 81.0 Note 1) Antiscale Chemical dosing rate ppm 2.5 2 2 2.5 (to make-up) Number of effects/stages 6 20 16

Distillate TDS ppm < 25 10 15-25Note 2)

Note 1: without steam transformer; Note 2: actual values before refurbishment

Table 5: Design Parameters of AL TAWEELAH MED and MSF Plants

The main differences in process parameters and conditions between the Taweelah MED and MSF plants, and possible economic consequences are following.

The unit capacities of the MSF plants are higher than of the MED plant. Highest unit capacities of MSF and MED plants currently available are 12.7 and 5.0 MIGD respectively.

© International Desalination Association, 2001 Page 8 of 11 The most striking, and well-known, difference is the much lower top brine temperature (TBT) of the modern MED technology, 63 °C, versus between 100 and 112 °C for MSF. The positive consequences of this low temperature level on the MED process are the reduced scaling and corrosion rates, lowering maintenance costs and increasing plant availability.

The dosing rates of Antiscale chemical, stated in table 5 are contractual upper levels. In practice, deviations from recommended dosing rates might be encountered for different operational or economic reasons. In general, the dosing rate of Antiscale chemical to make-up water in MED plants varies from 1 to 3 ppm, and in MSF plants from 1 to 5 ppm.

Another main difference between the two technologies is the use of the heat exchanger tubes inside the evaporator. In the MSF process, the seawater with the fouling properties is flowing through the tubes, whereas in the MED process clean steam goes through the tubes. In case of a tube leak, the leaking medium escapes from the tubes to the outside space where the occurring pressure level is lower. As a consequence of tubes leaking, in the MSF plant the distillate quality will deteriorate, due to the seawater leaking from the tubes into the distillate collection area. In the MED plant however, a leaking tube does not cause any deterioration of the distillate quality, since the leaking medium is clean steam and condensate.

The overall temperature difference in the MED evaporation process is only 18 °C, see section 5. This value is much higher for the MSF process, being in the range of 65°C. One aspect of this larger delta T is that fewer stages are possible in MED plants. Another aspect is that the differential pressures occurring in MED plants are lower than in MSF plants. As a consequence of the lower differential pressures, tubes and tube plates in MED plants can be constructed thinner, thus saving on investment cost.

In order to be able to even better compare the performance of MED and MSF plants, in table 6 a comparison is given of specific consumptions and requirements related to a unit quantity of product water (distillate). This comparison enables us to compare the plant performance and the resulting operational costs, per product unit.

PERFORMANCE PARAMETER UNIT TAWEELAH-A1 TAWEELAH-B TAWEELAH-A1 (DESIGN) MED MSF MSF Nom. Max. Max. Specific Steam Use kg steam/ 8.0 (8.8) 8.0 7.7 7.6 (Performance Ratio) kg distillate Net Specific Heat consumption KJ/ 282.5 295 (excl. ejectors) kg distillate Specific Electric Power consumption kWh/ 1.65 4.2 4.0 4.3 m3 distillate Specific Sea Water Supply rate kg sea water/ 9.46 13.03 10.75 11.51 kg distillate Specific Antiscale consumption mg AS/ 8.2 10.5 (contract value) kg distillate

Table 6: Specific Consumption Rates of MED and MSF Plants

© International Desalination Association, 2001 Page 9 of 11 From table 6, the favourable performance of the MED process can be clearly seen. The values of all specific consumption rates are lower for the MED process, which is directly translated in lower operating costs for the MED process under Gulf conditions.

The specific steam use, or Performance Ratio, of the MED process is roughly the same as with the MSF process. The small difference in Performance Ratio between the MED and the MSF processes, as explained in section 5, is due to the Gulf summer conditions with high seawater temperatures of 35 °C or higher, which are rather unfavourable for the MED process.

The specific electric power consumption of the MED process is roughly one third of the value of the MSF process, which is mainly caused by the recycling of brine in the MSF process achieved by pumping around large quantities of concentrated seawater. The absence of brine recirculating pumps and pipelines in the MED process reduces operational expenses and lowers investment cost.

The required seawater feed rate of the MED process is significantly lower than the seawater feed rates of the MSF process. This difference will be even bigger in cases where the high summer seawater temperatures are lower than the ones occurring in the Gulf.

The Antiscale chemical consumption of the MED process is lower than with the MSF process. Furthermore the unit costs of the high temperature additives applied in the MSF process will be higher.

9. WATER PRODUCTION COSTS

The investment level of the Taweelah-A1 Power and Desalination Plant has not been fully disclosed. A total investment of 1.5 billion USD is estimated for the purchasing and refurbishment of the existing Taweelah-A plant, together with the realisation of the new power and desalination plant. For references, the existing power and desalination plant can produce 230 MWe and 29 MIGD, whereas the future AL TAWEELAH-A1 power and desalination plant will have a production capacity of 1350 MWe and 84 MIGD of potable water.

The published first year prices for the produced electricity and water are USD 0.02/kWh and USD 3.16/1000 G or USD 0.70/m3 of water. Actual production costs are of course not known at this stage.

10. CONCLUSION

Very large-scale seawater evaporation applications are now feasible with MED technology, thus becoming competitive with MSF technology. Both types of distiller plants are reliable and well proven. The MED process however, has a number of clear advantages over the MSF process, the most important being the higher water and energy efficiency. This higher plant efficiency and the resulting higher plant output with the available amount of extraction steam, together with considerations of capital investment, have led to the selection of the MED process by the successful bidder for the AL TAWEELAH-A1 project, in which the Client left both MSF and MED technology choices open for this large scale application. © International Desalination Association, 2001 Page 10 of 11