Audrey Bold describes how the PX-220 PX Pressure Exchanger® energy recovery device from Energy Recovery Incorporated (ERI®) offers excellent savings. This highly efficient and robust energy recovery device (ERD) significantly improves the cost effectiveness of seawater reverse osmosis (SWRO) processes. Based upon rotary positive displacement pump principles, consistently high (>95%) energy recovery efficiencies are guaranteed. The PX-220 offers proven operating endurance and requires negligible maintenance.
Many of the world's largest desalination plants in Australia, China, India, UAE and elsewhere benefit from PX technology, which is becoming the industry standard for medium- to large-scale plants. That’s because patented PX® isobaric ERD technology achieves up to 98% real energy transfer efficiency. The pressure of the concentrated reject from an SWRO process is transferred to low-pressure feedwater. The device (Figure 1) contains a cylindrical rotor that spins inside a sleeve between two end covers. The rotor is driven by the flow so that no power is required to make the device operate.
Rotor speed is self-adjusting, maintaining the interface between the feed streams and thus eliminating mixing. This design results in no pressure spikes or pulsations and does not require pistons, valves or timers. PX technology maintains its exceptionally high energy transfer efficiency regardless of salinity, temperature and recovery rate variations. The PX-220 continues to operate at its excellent efficiency across a broad range (30 – 55%) of reverse osmosis recovery rates.
Figure 1
Schematic diagram showing the pressure difference inlet/outlet system of the rotary positive displacement pump principle of the PX-220 ERD
The PX-220 is extremely robust and reliable. Consisting of only one moving part, the rotor is automatically lubricated by seawater. There is no need for scheduled maintenance. Operation, start-up and shut-down are simplified, requiring only flow controls. When the rotor is stationary (an unlikely unplanned event) flow passes directly through the device. The PX-220 is constructed with ceramic components, tough engineered corundum (aluminum oxide), which is extremely hard and durable (exceeding the hardness of most stainless steels) and does not corrode. Fiber reinforced polymer is used for the housing and other advanced materials are used for internal components. As a result, PX units dismantled for inspection after many years of continuous use show no signs of deterioration.
Figure 2
Perth seawater reverse osmosis (SWRO) desalination plant (144,000 m3/day) – lowest ever recorded energy savings - one of twelve SWRO trains each with a production capacity of 13,350 m3/day
Multiple PX-220 units can be operated in parallel to meet the capacity requirements of any SWRO train size. Each unit handles brine flow rates of 40 – 50 m3/h. While arrays consisting of 10-16 PX units are common, up to 40 PX rotors have been successfully arrayed in a single train. Additional PX units can be readily added to or removed from existing trains, offering superb flexibility for the SWRO plant.
Plant capacities in excess of 200,000 m3/day permeate production can easily be accommodated using PX technology. Multiple PX technology trains of 10,000 m3/day capacity have been operating successfully for more than four years. Plants already commissioned and under construction worldwide with this technology have a total daily permeate capacity in excess of 4,500,000 m3/day. A typical SWRO system is shown in Figure2.
The superior flexibility of PX technology is enhanced by the small unit size (shipping at 1219 x 1016 x 483 mm, 109 kg). See Figure 3. Arrays of PX devices have footprints as little as 10% the size of some alternative systems.
Figure 3
The compact PX-220 ERD unit is readily incorporated into manifold trains and can be flexibly scaled up to suit any plant capacity requirements - units fit to standard 3” flexible coupling connections
Because of ERI's long-standing knowledge and experience base in the ERD field, PX technology has fast been accepted worldwide by the SWRO industry. High confidence can be placed in the quality of ERI’s products because its Engineering and Manufacturing departments administer tight controls across fabrication, construction and performance testing of components and PX units. With more than 10 million unit-hours of proven reliability from more than 5,400 PX units, ERI's device offers vastly more operating experience than any other isobaric ERD.
The PX rotary positive displacement pump ERD offers significantly lower life-cycle costs compared to energy recovery technologies such as Pelton wheels, Francis turbines or turbo chargers. The efficiency of these older designs can vary significantly with day-to-day changes in SWRO process conditions. The consistent high efficiency of PX technology under a variety of conditions, in combination with minimal downtime and high durability derived from the simple design, result in SWRO power consumption as low as 1.6 kWh/m3. For current installations, this represents more than 500 MW real energy savings, with a value to clients of over US$260 million per year.
The PX-220 pressure exchanger from ERI offers a robust, reliable, flexible and scalable energy reduction plant solution to provide genuine savings to the SWRO industry.
Useful Links
ERI website: www.energyrecovery.com
About the Author
Audrey Bold is VP Marketing Communications at ERI. Contact via their website.