As seawater reverse osmosis (SWRO) desalination continues to address increasing global water shortages, plants are increasing in size and capacity. This requires technology innovation to keep pace and ensure that they meet energy efficiency and plant reliability requirements. As a result, one of the most critical technologies within desalination plants is the energy recovery device (ERD).
Through precise engineering, an energy recovery efficiency of 98% can be achieved with isobaric ERDs; devices that have attained nearly universal acceptance within desalination plants across the world. At this optimal efficiency, the ERD can reduce the amount of energy required to desalinate seawater by up to 60%, resulting in more economical production of drinking water and a reduced carbon footprint.
Energy Recovery Inc has achieved this efficiency level in its PX Pressure Exchanger device, the PX-300, which incorporates Quadribaric technology to maximize performance, energy reduction and costs savings.
How the PX device works
The PX device contains a single moving part, the rotor, which is made of high purity aluminum oxide surrounded by a sleeve and two end-covers made with the same tough material, which is three times harder than steel. Inside the rotor are chambers in which pressure exchange occurs. There are no pistons between the concentrate and the feed water. The self-adjusting rotor limits mixing by keeping the concentrate-feed water interface inside the exchange chambers.
In the SWRO process, feed water enters one end of a PX device at low pressure and fills one of the rotor chambers. As the chamber is nearly filled, the rotor continues to turn. The rotating chamber is then exposed to the incoming brine, which is the reject stream from the reverse osmosis (RO) membranes. The brine enters the PX device at high pressure through the opposite end, displacing the feed water and transferring the high pressure to the previously un-pressurized stream. The nearly filled rotor continues to turn and exposes the now un-pressurized brine to a new incoming low-pressure feed water stream, starting a new cycle.
The process is repeated for all the other chambers inside the rotor as it spins within the PX device. This transfer of energy is extremely efficient. The feed water pressurized by the PX device is fed to the RO membranes, reducing high-pressure pump requirements and saving energy.
From an operational perspective, PX technology, including the PX–300 device, does not pulsate like competing isobaric energy recovery devices. These pulsations have been known to complicate and delay plant startups and may damage plant structures, compromise peripheral desalination equipment or adversely affect plant safety. In contrast, hundreds of SWRO plants around the world have started up with PX technology without delays or any performance problems with pulsation-related equipment.
The PX technology is also compact and easy to operate. Up to six times smaller and 90% lighter than competing energy recovery devices in its class, PX device installations require no special foundations or support systems, helping to minimize the overall footprint of a desalination plant. The devices have just one moving part that automatically adjusts to changes in flow rates, requiring no electricity or control system. PX rotors float on hydrodynamic bearings which do not exhibit wear over long device lives. The simple, reliable design requires no periodic maintenance, enabling continuous plant operation.
All of ERI’s PX models, including the PX–300, can be operated in parallel in arrays of multiple devices. Units may be added as required for increased process flows with ‘off-the-shelf’ availability. In addition to its almost unlimited capacity, the redundancy found in PX arrays assures continuity of plant operations when compared to competing technologies. Furthermore, the devices can be installed in any orientation and can be integrated into existing infrastructure, without customization. This ‘plug-and-play capability’ is ideal for new construction as well as for retrofitting or expanding existing plants.
PX-300
The PX–300 operates at 200 to 300 gallons per minute (45 to 68 m³/hour) per unit, a performance level which is suitable for SWRO trains with production capacities of greater than 290,000 gallons (or 1,100 m³) per day.
The PX–300 delivers higher efficiencies and has significant reductions in mixing compared to other isobaric ERDs, a factor which enhances desalination system performance by reducing membrane feed pressure. The PX–300 device is designed to be operated near the top end of its flow range, thereby minimizing the number of devices necessary to meet the SWRO train flow requirement. It also provides operational flexibility, supporting a range of SWRO membrane recovery rates of up to 10%.
The PX–300 device delivers measurable long-term performance. Unlike technologies that are prone to wear and significant degradation of efficiency over time, the PX–300 contains no reciprocating components or constantly actuating valves which can complicate startups and require unplanned maintenance. The internal components are made of highly engineered ceramics, corrosion-proof reinforced polymers and wrought high-alloy stainless steels. In addition, the PX-300 features an industry-standard, side-ported housing to allow easy piping alignment and access to internal components without uninstalling the housing.
Quadribaric technology advantage
At the core of the PX-300 is the breakthrough Quadribaric technology, which stands to revolutionize the way rotary PX devices perform. The Quadribaric technology enables isobaric PX devices to do double the work with even greater efficiency, offering a number of key features, including:
• Higher Efficiency - Quadribaric technology features straighter flow paths than previously possible, improving efficiency.
• Less Mixing - Quadribaric technology, with its reduced cycle speed, achieves minimal mixing between the seawater concentrate and feed water, supporting lower operating pressures in desalination systems.
• Greater Reliability - The slower rotational rate that the Quadribaric technology design provides extends product life cycle.
• Quieter Operation - Quadribaric technology uses a smoother exchange process, reducing the sound level produced by its operation.
The Quadribaric technology allows the PX-300 to operate at 98% efficiency, affording desalination plants a number of short- and long-term benefits. The PX–300 ERD is a cost-effective solution for owners, operators and designers whose desalination plants consume a significant amount of energy. Depending on specifications, PX devices equipped with Quadribaric technology can achieve life-cycle cost pay back in as few as two or three years.
Arrays of PX–300 devices provide unique, fail-safe protection with built-in system redundancy. They also provide full maintenance-free operation and are delivered in a compact size, which means no major additional construction or supports are required, allowing scale-up at minimal costs.
Conclusion
Implementing isobaric ERDs has become common practice at desalination plants across the globe. Selecting the highest performing, most cost-effective and efficient device requires a closer look at the design elements and the technologies embedded into the ERD’s operation.
The Quadribaric technology enables the unique modular design system of the PX-300 to offer a quick and cost-effective solution that requires no customization and can be installed in any orientation. By leveraging the Quadribaric technology, the PX-300 devices achieve a 98% operating efficiency and can reduce the amount of energy required to desalinate seawater by up to 60% compared to a process with no ERDs in place. This savings can result in more economical production of drinking water and a reduced carbon footprint.
