DESALINATION

Process flow diagram

[image] As shown on Process Flow Diagram, seawater desalination process includes 6 major steps: seawater pumping, 2-stage filtration (500 micron and 150 micron), ultrafiltration, 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 SMBS, antiscalant to seawater, caustic soda for product are added.

67 generic P&ID symbols are described here.

Intake station

[image] This P&ID shows implementation of seawater coarse filtration and pumping steps. The former step is implemented with static bar screen removing the debris pieces bigger than 50 mm. As well this screen protects the plant against occasional jellyfish inrush. The screen is equipped with the debris disposal system and is of pool-out design; it may be replaced without stopping the plant. The debris disposal system is a set of interconnecting shoots and conveyers (not shown on P&ID). Ancillaries include the chlorination system. It is simple and rugged in design as it is used only on rare occasions. The shock-chlorination dosing rates are monitored at the station discharge manifold. The pumping requested turn-down ratio and spare (standby and rotating) capacity are provided with a number of pumps connected in parallel, each pump being driven by the variable speed drive (VSD). The main pumps are of horizontal centrifugal design. The pumps are primed with the vacuum system driven by the oil ring pump type. As flocculent is injected into the pump suction line the static mixing before pretreatment unit and the rapid mixing with agitator are not needed. After the pumps seawater goes through the self-cleaning filters of 500 micron before being fed to ultrafiltration system. The said filters backwash does not cause any dips in the seawater flow rate as this process is continuous and consumes less than 3% of the feed.

Pretreatment

[image] From the intake station the seawater is pumped to the self-cleaning filters of 150 micron of the ultrafiltration system. These filters protect the UF fibers from potentially harmful objects. After the filters the water is distributed equally between the ultrafiltration modules. They are operated in dead-end mode with a design flux of slightly more than 72 L/(m2•h) in filtration mode. The filtration takes place from inside to outside. The design filtrate cycle time is 25 minutes until the fibers are backwashed. The filtration quality is periodically checked through water sampling to SDI- monitoring system. The system auxiliaries include the backwash system and CEB one. Additionally the UF modules are connected to the CIP system of the plant.

 

UF dosing system and CIP

[image] CIP system is an auxiliary system used to prolong the life of the membrane 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 or other organic/non-organic matter 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 water heater. The CIP system is equipped with pH-control. 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.

SWRO unit

[image] The SWRO module is of conventional design built round ERI energy recovery device (ERD). The module includes low pressure booster pump feeding the seawater to the cartridge filter. After it antiscalant and SMBS are admixed, and the sample is taken to the SDI analyzer. To decrease the risk of SWRO membrane fouling via scaling formation, the antiscalant may be constantly added to seawater stream. During the intake chlorination, the free chlorine control is engaged, injecting SMBS into seawater if needed. Then the feed is split into 2 streams; one goes to ERD array, where the stream pressure raised, and the second – to the high pressure pump (HPP). It is of the ring-diffuser construction. The pump and motor bearings temperature and vibration are constantly monitored. 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.
The pressure of the feed after the micron filter should be higher than the NPSHR value for HPP. The latter is equipped with VSD – variable speed drive to accommodate for the variations in the seawater temperature and salinity and the train load. The HPP motor is selected oversized to match the future extension. As well the CIP close-looped line connections are shown on P&ID. The product connection additionally contains the non-return valve – protection against the inadvertent CIP in-leakage from the return piping. Rupture discs are installed on the product line and the suction line of the HPP 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.
Each SWRO train contains its own 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 storage systems (charging the dosing system tanks) are common for all SWRO units. Any storage system includes an open tank with a spill berm, the group of transfer pumps with 100% reserve capacity, and the strainer installed at the pumps common suction line.

SWRO membranes array

[image] This P&ID shows SWRO membrane arrays arrangement and manifold fittings. As seen every membrane location is described by row, column, and ordinal number inside the pressure vessel. Each array consists of three sections connected in train via victaulic joints. This membrane address is extensively used by the membrane tracking software.

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 client's product tank. Its capacity shall be enough to supply the product water for at least 4 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. As the permeate water used for the reactor backwashing has high content of solids, it is discharged to the brine collecting tank.

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. Neither SWRO membrane vessel ends nor BWRO ones overlook the maintenance areas which are mostly frequently visited.

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