Desalination Plant Provides Reliable Process Water for Saudi Arabia Refinery
Saudi Arabia's Rabigh Desalination Plant is located on the Red Sea, about 150km north of Jeddah
By Charles Desportes
One of the key challenges in desalination and other types of water treatment is in investing in new technologies and using new techniques to offer low-cost, reliable, desalinated water to industrial clients and communities. Providing low-cost desalinated water for processes and drinking water was one of the major goals of the Multiple Effect Distillation (MED) Thermal Vapor Compression Seawater Desalination project at the Aramco Refinery in Rabigh, Saudi Arabia.
The project entailed installing a facility for a 10,000 m³/day seawater desalination plant at the Rabigh refinery (currently PetroRabigh Refinery and Petrochemicals Complex) capable of meeting its process and potable water demands. The facility is located on the Red Sea, about 150 km north of Jeddah. The 10,000 m³/day capacity MED plant produces process water and potable water for the Saudi Aramco refinery and the community. The plant has been in operation since mid-2005, and has been consistently producing distillated water at or above the design output and product quality.
|Site view of Rabigh Desalination Plant.|
The facility was designed and supplied on a Design-Build-Own-Operate-Transfer (DBOOT) basis by MUWDC, a joint venture of Hajj Abdullah Alireza & Co (HAACO) Group and Aquatech International Corporation.
The plant has been regularly stopped for scheduled maintenance and put into periodical cleaning using low pH acid solution. The plant has not seen any significant design, operation or maintenance issues since it was commissioned.
The plant’s two 5,000 m³/day units produce about 4,000 m³/day of process water according to the site demand, and the rest of the 6,000 m³/day is used as potable water after remineralization. The high-purity distillate, with a design total dissolved solids (TDS) level of less than 5 mg/L from the MED system, is directly sent to the process water storage.
Aquatech’s MED Thermal Vapor Compression (TVC) design is a multi-effect unit that incorporates SPRAY-FILM® technology, a registered trademark of Aquatech. The MED design is the result of over 60 years of experience in the design, manufacture, start-up and operation of evaporator systems worldwide.
In this system, steam or water vapor condenses inside horizontal tubes. Seawater is sprayed over the outside of the horizontal tubes and acts as a heat sink for the condensing vapor. The system operates under a vacuum with a top brine temperature below 71°C. By operating at these low temperatures, the heat transfer surface is less likely to scale, and the amount of heat required to bring the incoming seawater to its boiling point is minimized.
|Rabigh Desalination Plant.|
The multi-effect plant incorporates the use of a thermocompressor to improve its thermal performance. The thermocompressor is driven by low-pressure (LP) steam, which entrains and boosts lower pressure vapors from the last effect to the moderately higher-pressure condition required in the first effect to operate the process.
By recycling the lower pressure vapors from the last effect with a thermocompressor, less steam is required from an external source than if there were no thermocompressor.
Part of the water vapor from the last effect is converted to liquid form in the condenser and mixed with the seawater distillate collected from all the effects. As per the process requirements, the steam condensed in the first effect is separated from the distillate and reused directly.
The MED-TVC Desalination units described here were designed to provide 5,000 m³/day distillate output each. The net capacity of the plant is 10,000 m³/day available for supplying to the PetroRabigh Refinery with a plant availability factor that remains consistently over 95 percent after eleven years of operation.
The design takes into account the variation in seawater temperature throughout the year and the possible fluctuation in salinity and quality over the 20 years of the concession agreement.
The system is designed to operate with a top brine temperature (TBT) of over 70°C and a final condensation temperature of 46°C in the main condenser. The temperature of saturated vapor 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 the available LP steam from the refinery as input to the thermocompressor and a minimum amount of medium-pressure (MP) steam as motive to the two stage ejectors of the vacuum system (see Fig. 1).
The plant is designed to minimize a carryover of solids into the distillate and achieve a high purity of produced water.
The MED evaporator and distillate condenser units were designed, built, tested and stamped in accordance with ASME standards.
The six-effect evaporator unit is 13 feet in diameter and about 131 feet long. The first challenge was to ensure site delivery. To do so, they were fabricated in the Gulf region and each unit was shipped in three sections of two effects. On site, the three sections were welded into one vessel. Then the main condenser was installed next to each unit, as well as the thermocompressor and all other auxiliary equipment.
A two-stage ejector and condenser vacuum system is provided for each MED unit to continuously remove air and other non-condensable vapors from the evaporator and the main condenser.
Design Concepts: Main Features
The following concepts were used in the design of the MED plants for the PetroRabigh Refinery. The purpose of these design concepts was to provide reliability and availability of the equipment, long equipment life, smooth, stable, economical operation of the equipment, and a process optimized specifically for the utilities available at the plant.
- Six Effect Configuration: Optimizes performance and maintains the MED vessel size within easily transportable dimensions for job site delivery, with optimized cost and lead time.
- Thermocompressor: Used to reduce LP steam flow by recycling vapor from the last effect.
- Capacity Control from 50 to 100 percent: LP steam flow is controlled, allowing smooth operation over the range of 50 to 100 percent capacity.
- Variable Seawater Temperature and Constant Condenser Temperature: The condenser temperature is kept constant by controlling the cooling water flow over the seawater temperature range.
- Controlled Potable and Process Water Production: The amount of distillate used to make potable water is fixed daily by Aramco with the remaining flow of distillate/condensate being sent to the process water header.
- Isolated Condensate: Steam/vapor into the first effect is removed with a separate pump and is used only for the process water as it can contain chemicals unsuited for potable water.
- Constant Make-up Seawater Flow: Ensures even distribution of seawater for all operating conditions from 50 to 100 percent capacity.
- Once-Through Feed Design, No Recycling: Raw seawater is fed in parallel into each effect to prevent over concentration and scaling.
- Bi-annual Maintenance and Cleaning Built into the Schedule: Low pH acid cleaning was scheduled two times per year to remove any build-up of scale. Operating experience and quality of anti-scalant chemicals have substantially reduced the frequency of these cleaning.
- Simple Chemical Treatment
- Sodium Bisulfite Injection: To remove any residual chlorine.
- Anti-scalant Injection: To prevent the formation of scale.
- Antifoam Injection: To prevent seawater contamination into the distillate.
|Spray nozzles and manifold inside MED unit.|
The MED Thermal Vapor Compression design used for the Rabigh Desalination Plant has exceeded its expectations in the following ways:
- Performance has consistently exceeded the design, providing optimization on steam consumption.
- Water purity has been consistently better than guaranteed, with conductivity often measuring below 2 μS.
- Operation has been very flexible, enabling the units to adjust easily to varying water demands or to sudden load changes or outages not related to the MED plant.
- Scheduling the routine maintenance operations has taken into account refinery shutdowns and periods of low steam availability for greater plant availability when needed.
- Scheduled bi-annual cleans have been spaced out, as efficiency of anti-scale and rigorous dosing have shown that tubes were staying clean.
- Plant reliability and availability has confirmed the proponent’s choice of MED as being the right one.
This desalination plant has performed long-term, full scale, parallel tests to further develop the technology using:
- Corrosion testing of various stainless steel alloys
- Comparative testing of newer anti-scale chemicals
The results of these tests have been presented at various IDA World Congress conferences.
|Rabigh Desalination Plant.|
The plant has been operating for the past eleven years with practically no operation and maintenance issues. Importantly, it has consistently produced distillate higher than design maximum flow rate, using less steam and electricity than guaranteed.
The continuing success of the project is due to solid initial design and execution as well as proper and carefully watched operation of the plant by Aquatech-trained operators, and execution of the scheduled maintenance plan.
All these factors helped in optimizing the capital and operating cost of the plant. This, in turn, has helped reduced the cost of water production, allowing the developers to offer a better, cost-effective tariff to the client.
About the Author: Charles Desportes is director of thermal desalination for the Aqua-chem ICD Division of Aquatech International. He can be reached at firstname.lastname@example.org. For more information about Aquatech, please visit www.aquatech.com.