MUWDC Rabigh MED Desalination Plant

MUWDC Rabigh MED Desalination Plant

Operation and maintenance of MED Thermal Desalination Plant in Petro Rabigh.
The end user is Rabigh Refining and Petrochemical Co. (Petro Rabigh).

The facility is located on the Red Sea in Rabigh, about 150 km north of Jeddah. The 10,000 m³/day capacity (2 trains of 5,000 m3/d each) Multiple Effect Distillation (MED) plant produces process water and potable water for Rabigh Refining and Petrochemical Co. (Petro Rabigh) and the community. The plant has been in operation since mid-2005 and consistently producing distillated water at or above the design output and product quality.

The facility was designed and supplied on a Build-Own-Operate-Transfer (BOOT) basis by Modern United Water Desalination Company (MUWDC), and O & M was awarded to Haji Abdullah Alireza & Co. Integrated Services Ltd. (HAAISCO).


The plant has been operating for the past 18 years with practically no operation and maintenance issues. Importantly, it has consistently produced distillate higher than design maximum flow rate, using less steam and less electricity than guaranteed consumption figures.
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.
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.

Technologies and Solutions:

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.
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.
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.
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