DESALINATION

Process flow diagram

[image] As shown on Process Flow Diagram, seawater desalination process includes 6 major steps: seawater pumping, DAF, filtration, desalination by passing the feed water through reverse osmosis membranes, re-mineralization and product delivery. The above-mentioned steps are typical and optimal for the seawater quality analysis and the unit specification submitted by the client. Optimal process parameters are selected to minimize the power and chemical consumptions. To increase the operation flexibility and redundancy, the subsystems for dosing flocculantm SMBS, antiscalant, acid to seawater, caustic soda for product are added.

67 generic P&ID symbols are described here.

Intake station

[image] As shown on P&ID, flocculant and polymer are admixed to the seawater in the suction of the horizontal centrifugal pump equipped with the vacuum priming system. This low-head pump feeds seawater to the flocculation chambers (divided into fast and slow mixing compartments), from which the feed enters the direct air floatation (DAF) system. The main task of the DAF unit is to remove algae, oil and grease, which might clog the membrane surface. Another target, especially in summer when the raw water quality is potentially more difficult, is the reduction of turbidity, organics and suspended matter to improve the feed water quality to the subsequent dual media filtration (DMF) system.

Pretreatment & SWRO unit

[image] [image] Before reverse osmosis seawater is filtered in the pressurized dual media vertical filters. There is an option to add the flocculant or acid before filtration. This type of filter requires periodical backwashing as the build-up of the filtrated material gradually plugs it. The selected backwashing mode of operation (with brine) does not cause any dips in the plant production during its operation. After backwashing the filter goes through maturing phase during which the filtrated water quality returns to the normal one. The filtration quality is periodically checked through water sampling. Free chlorine, turbidity and pH are monitored as well. The SWRO module is of conventional design built round ERI energy recovery device (ERD). Feed entering the SWRO module through the cartridge filter is split into 2 streams; one goes to ERD, where the stream pressure raised, and the second – to the high pressure pump (HPP). It is the Danfoss APP S 674.38 type of the axial piston construction. The pump is equipped with VSD to match the required production. After ERD the feed pressure is further increased in the ERD booster pump. It is equipped with VSD to accommodate the pressure variation in the SWRO process. The said streams are entered into the membrane vessel array from the same manifold.
Rupture discs are installed on the product and brine lines to protect against the pressure surges during transient operation – startups and shutdowns. Another feature is the pressure-equalizing line connecting the product line to the feed one.
The SWRO trains are equipped with common antiscalant and SMBS dosing systems with the 100% metering pump redundancy and a means to check the metering pump calibration (measuring bucket and/or mass-meter).
The clean-in-place (CIP) system is used to prolong the membrane life before it being replaced for a new one. Fixed percentage of the membrane replacement is a standard clause in the warranty agreement. The system is used for membrane cleaning off scale deposition and for flushing after the unit emergency shutdown. Cleaning is a batch process that may be sensitive to the solution temperature. So CIP scope additionally includes a heater. Before being fed to SWRO membranes the CIP solution is passed through the micron cartridge filter. The CIP interconnecting piping is designed for maximum allowable velocity to cut down the piping volumes to be flushed after each cleaning.

 

Posttreatment

[image] This P&ID describes the remineralization of desalted water in the limestone reactors to make it more stable and less corrosive. The process is controlled by a set of criteria such as alkalinity, hardness, pH and LSI. Only 25-35% of desalted water passes through the limestone reactors. To accelerate the limestone dissolution the HCl acid is injected into the influent. The mineral-enriched water stream is then mixed with the untreated balance. This mixture pH and/or LSI are checked and corrected by injecting caustic soda (NaOH) before the mixture being discharged to the product tank. Its capacity shall be enough to supply the product water for at least 2 hours - time needed to handle most (over 95%) of the plant anticipated failures. The limestone reactors quantity is selected to minimize the impact of the reactor refilling with limestone on the final product quality as at any conditions the flow rate through any isolated reactor is kept constant to keep down the product turbidity index.

Plant layout

[image] The plant layout provides minimum length of interconnecting piping, clearly defines the project areas, and meets work safety and O&M requirements. All chemical storage tanks have direct access for trucks.

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