Breakthrough of MED Technology in Very Large Scale Applications, the Taweelah-A1 Case
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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. © International Desalination Association, 2001 Page 1 of 11 CONTENTS 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 © International Desalination Association, 2001 Page 3 of 11 MED PLANT MED PLANT CAPACITY MED UNIT CAPACITY YEAR COMMISSIONED and COUNTRY IN MIGD (m3/day) IN MIGD (m3/day) 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. © International Desalination Association, 2001 Page 4 of 11 MED unit: dimensions 45.5 x 23.7 m Figure 1: MED Unit View © International Desalination Association, 2001 Page 5 of 11 4. STEAM TRANSFORMER 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.