Water. desalination + reuse

PROJECTS | 28 | Desalination & Water Reuse | August-September 2014 is 0.965 kWh/m 3 and steam consumption is less than 120 kg/m 3 . Water vapour from the last effect is converted to liquid in the condenser and mixed with the seawater distillate collected from all the effects. The steam condensed in the first effect is separated from the distillate and reused directly. DESign The design accommodates the variation in seawater temperature throughout the year and the possible fluctuation in salinity and quality over the 20 years of the concession. The system is designed to operate with a top brine temperature of over 70°C and a final condensation temperature of 46°C in the main condenser. The temperature of saturated vapour at the inlet of the first effect tube bundle is about 74.5°C. The six effects operate entirely under vacuum within this temperature range. The plant uses low-pressure steam available from the refinery as input to the thermocompressor and a small amount of medium-pressure steam to drive the two stage ejectors of the vacuum system. Carryover of solids into the distillate is kept to a minimum to achieve a high purity of produced water. The design holds total dissolved solids to less than 5 mg/l from the MED system from which the distillate is sent direct to process water storage. The six-effect evaporator unit is about 4 m in diameter and more than 39 m long. The first challenge was its installation. It was shipped from the fabrication shop located in the Gulf region in three sections with each section including two effects to ease transportation to the job site. At the site, the fabricator welded the sections into one vessel. Then the main condenser was installed next to each MED unit, as well as the thermocompressor and all other auxiliary equipment. Each MED unit has a vacuum system to remove air and other non-condensable vapours from the evaporator and the main condenser. Each vacuum system includes two plate-type condensers: one intercondenser, one aftercondenser, and two steam ejectors. SuCCESSful PERfORmanCE The MED design used for Rabigh desalination plant has exceeded its expectations. • Performances have consistently outstripped specification in optimizing steam consumption. • Water purity has consistently exceeded guarantees with conductivity often below 2µS. • Flexibility in operation has enabled the units to adjust easily to varying water demands or to sudden load changes including outages not related to the MED plant. • Scheduling routine maintenance operations has taken into account refinery shutdowns and periods of low steam availability. • Scheduled six-monthly cleans have been spaced out as the design's inherent resistance to scaling and the efficiency of anti-scaling injections have kept the tubes clean. The plant has been operating for the past nine years with practically no operational or maintenance issues. Furthermore, its flow rate of distillate has been greater than its design specification while its consumption of steam in evaporating the feed water has been less than its specification. Various factors contribute to the continuing success of the Rabigh project, its better-than-design performance and its excellent condition as it approaches the end of its first decade in operation. They include: • optimized selection of different steel alloys for different effects of the MED units; • an efficient sourcing plan for the vessel fabrication and the other major components of the plant; • an experienced engineering, procurement and construction company for the turnkey installation and an experienced local operation and maintenance company for the plant; • a local financial partner in the consortium; • carefully monitored operation of the plant by Aquatech-trained operators; • careful execution of the scheduled maintenance plan; and • optimization of the steam available. All these factors helped in optimizing the capital and operating cost of the plant. Reining in the cost of water production has enabled the developers to offer a tariff that is favourable to the client. Aquatech's MED design is the result of over 40 years of experience in all aspects of MED evaporator systems worldwide. The choice of this cost-saving, advanced MED technology has been proven correct in the performance of the Rabigh plant. And the choice of this cost-saving, advanced technology coupled with forward planning in the plant's design and optimization of its operation have maintained the plant's reliability and availability at a level that has outstripped expectations.l Figure 4. The multiple effect thermal vapour compression seawater desalination units at Rabigh were each designed to provide 5,000 m³/d distillate output. Testing the mettle of steel This desalination plant was used in a corrosion study of stainless steel alloys during its first years of operation. Test racks of different grades of stainless steel were installed in different parts of the first effect of the MED unit to study their corrosion over more than a year. The results are documented in a paper Stainless Steels for LT-MED Plants and were presented at the International Desalination Association's (IDA's) World Congress-Masplalomas, 2007. The findings have helped in the further development of MED. The paper is available to IDA members – details are at www.idadesal.org.

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