Water. desalination + reuse

PROJECTS | 26 | Desalination & Water Reuse | August-September 2014 at or above the design output and product quality. And it has maintained its availability to the PetroRabigh refinery at consistently greater than 95%. PROCESS OvERviEw The MED thermal vapour compression design is a multi-effect unit that incorporates Aquatech's spray film technology. In this system, steam or water vapour condenses inside horizontal tubes as seawater is sprayed over the outside of the tubes and acts as a heat sink for the condensing vapour. The system operates under a vacuum with a top brine temperature of about 71°C. Operating at such low temperatures means the heat transfer surface is less likely to scale while the heat required to boil the incoming seawater is minimized. The plant deploys a thermocompressor to improve its thermal performance. The thermocompressor is driven by low-pressure steam, which entrains and boosts lower pressure vapours from the last effect to the moderately high-pressure condition required in the first effect to operate the process. Recycling the lower pressure vapours from the last effect with a thermocompressor in this way reduces the amount of steam otherwise required from an external source. Plant power consumption at full capacity Figure 3. The desalination plant was used to carry out a corrosion study of stainless steel alloys during the first years of operation. The aims of the plant's key design features were: to provide reliability and availability of the equipment; long equipment life; smooth, stable, and economical operation of the equipment; and a process optimized specifically for the utilities available at the plant. Six-effect configuration The configuration optimizes steam economy and maintains the MED vessel size within easily transportable dimensions from fabrication shop to job site, to optimize cost and lead time. improved steam economy Recycling vapour from the last effect using a thermocompressor limits low-pressure steam consumption. Low-pressure steam flow to the thermocompressor is controlled to give smooth operation over 50-100% capacity. variable seawater temperature The cooling water control loop maintains the condenser temperature by varying the amount of cooling water going into and out of the system according to the temperature range of the incoming seawater. Potable and process water production control Aramco's demand for potable and process water determines the plant's operating capacity. The amount of distillate sent to the remineralization system to make potable water is controlled with the remaining flow of distillate/condensate being sent to the process water header. Condensate isolated Steam/vapour into the first effect is isolated from distillate produced in the other effects since it contains boiler chemicals not suitable for potable water and is used only for process water. Constant make-up seawater flow Constant flow ensures even distribution of seawater for all operating conditions from 50% to 100% capacity. Constant condenser temperature A steady temperature ensures stable operation. Once-through feed design with no recycling Raw seawater is fed in parallel into each effect to prevent over-concentration and scaling. Six-monthly maintenance and cleaning schedule Acid cleaning was scheduled at twice a year to avoid build-up of scale. Operating experience and quality of anti-scalant chemicals have made it possible to reduce substantially the frequency of cleaning. Simple chemical treatment The cleaning regime comprises: • anti-scalant injection; • antifoam injection to prevent seawater contamination in the distillate; and • sodium bisulfite injection to remove residual free chlorine. Main features

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