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Reverse Osmosis Desalination Process

The core process of desalination is the Reverse Osmosis Process. It consists of a High pressure pump followed by an Energy Recovery device and the Reverse Osmosis Membranes

 

SWRO

High pressure pump

Before entering the seawater reverse osmosis membranes, clarified seawater is pressurized by the High Pressure Pump typically between 55 and 85 bars, depending on the temperature and the salinity of the water.

Main Seas and Oceans Water composition

Calculate you required applied pressure

Pump types

Plunger or piston pump Centrifugal pump > 5 m3/h

Design Pressure

55-85 bar

Materials

Titanium

 

Bronze

 

CO/ Ni Alloy

SS 904 L

SS 2205


Energy Recovery Device (ERD)

The pressure drop over the RO membranes is about 1.5 to 2 bar, depending on the number of element per pressure vessel, so the concentrate is released at high pressure.

Thanks to Energy Recovery Devices, it is possible to reuse the energy from the concentrate flow. The concentrate is directed to the ERD, where it directly transfers its energy to part of the incoming feed water.

There are two main energy recovery concepts:

- Energy Recovery Turbine (ERT)

SSD MSS
Courtesy of FedCo

- Pressure Exchanger (PX)

Courtesy of Grundfos


Example
:

Seawater Flowrate: 100 m3/h
Applied Pressure: 75 bar
R.O. Recovery: 40%
Permeate Flowrate: 40 m3/h

Power required without energy recovery devices: 300 kW
Specific Energy: 300/40 = 7.5 kWh/m3

Power required with Energy Recovery Turbine: 177 kW
Specific Energy: 177/40 = 4.4 kWh/m3

Power required with Pressure Exchanger: 140 kW
Specific Energy: 140/40 = 3.5 kWh/m3

Device Type Energy savings Costs
Energy recovery turbine (ERT) 30-40% +
Pressure exchanger (PX) 50-60% ++

Desalination costs analysis

 

See complese seawater reverse osmosis flow diagram

Spiral Wound Seawater Reverse Osmosis modules

The most common RO membrane used in desalination are spiral wound Thin Film Composite. They consist in a flat sheet sealed like an envelop and wound in a spiral.

CONCENTRATE WATER containing salta is rejected by the membrane and does not ener the product tube. the concentrate water exits the side of the element opposite of the feed. RAW WATER FEED enters into membrane layers. Applied pressure forces raw water across membrane layers into the product tube. PRODUCT WATER collects in the produt tube and can be output from either end of the membrane element.

Spiral wound Reverse Osmosis membrane

There are 3 typical membrane diameters : 2.5", 4" and 8". Seawater RO membranes have a maximum permeate flowrate ranging from 1.4 to 37.9 m3/d, therefore many membranes are often required to meet the plant permeate production requirements.

Seawater RO membranes in Thin Film Composite are manufactured by Dow Chemical (Filmtec), Hydranautics, Osmonics (Desal) and Toray.

The membranes are enclosed in series in pressure vessels and the number of membrane element per pressure vessels can vary from 1 to 8:



Pressure vessels are then arranged in parallel to satisfy the membranes flow and pressure specifications as well as the plant production requirements:



The total number of membranes and pressure vessels required and their arrangement (the array) depends on permeate flow and applied pressure, i.e., salinity and temperature.


Reverse Osmosis Pressure vessels Rack

LENNTECH engineers design and size the most cost-efficient Seawater Reverse Osmosis Process based on your water specifications.







Lenntech BV

Rotterdamseweg 402 M
2629 HH Delft
The Netherlands

tel: +31 152 610 900

fax: +31 15 261 62 89

e-mail: info@lenntech.com











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