Breakthrough engineering is the core of ERI's Pressure Exchanger™ (PX™) energy recovery technology. It is no surprise that the PX is the most efficient energy recovery device (ERD) on the market. ERI has invested years in research and development as well as millions of dollars to build the most efficient ERD available today—up to 98% efficient, producing energy savings of over 60%.
Re-thinking SWRO
Though it has quickly become the benchmark ERD technology, ERI’s PX device was a new paradigm when first introduced to reverse osmosis (RO) systems. There are, consequently, fundamental differences between the way legacy Pelton and Francis turbine devices are designed and optimized and the modern approaches used with PX device-based desalination systems. Some of the new concepts and rules being driven by ERI are:
Though it has quickly become the benchmark ERD technology, ERI’s PX device was a new paradigm when first introduced to reverse osmosis (RO) systems. There are, consequently, fundamental differences between the way legacy Pelton and Francis turbine devices are designed and optimized and the modern approaches used with PX device-based desalination systems. Some of the new concepts and rules being driven by ERI are:
- Hydraulic efficiencies up to 98%
- PX device-equipped SWRO systems use less energy at lower recoveries (between 35% and 45%)
- The high-pressure pump flow equals the systems permeate flow recovery rate
- The high-pressure pump and permeate flows are independent of the reject flow
- PX device efficiency is essentially independent of system flow rate and pressure changes
These fundamental concepts reverse many of the traditional rules that have constrained the RO desalination designer for many years. PX technology opens up new avenues for RO design and optimization using larger train sizes. It also allows the designer or operator to select a recovery rate which is optimum for a given salinity, temperature, and membrane fouling performance. The papers below describe these ideas in detail:
- Development of a Fourth Generation ERD
- Low Energy Consumption in the Perth SDP
- Low Power Bill Makes SWRO Affordable
SWRO Plant Optimization
A major economical benefit of PX device technology is that the PX requires a much smaller high-pressure pump than typically used by conventional technology. Even more crucial, PX technology disassociates the high-pressure pump from the ERD. These two factors combine to give desalination plant designers much more flexibility in optimizing train sizes and selecting the best high-pressure pump for their project.
Design and Installation
ERI’s team of service professionals draws from its experience reviewing designs and supervising successful startups of hundreds of desalination plants around the world. Its people are not just experts in the application of PX technology, but bear comprehensive knowledge of reverse osmosis (RO) plant operation. ERI is dedicated to ensure successful PX device deployments by working closely with operators and partners to understanding each project’s unique needs.
Design
ERI’s expertise comes into play long before RO plant construction begins. Service professionals conduct thorough technical reviews of process and instrument designs and control logic. It is not unusual, for example, that during these reviews the ERI team identifies problems that were overlooked by the plant planner. By helping to address these problems proactively, ERI can help accelerate plant installation and start-up.
ERI’s website contains a wealth of technical information, including operations and service manuals, technical guidance documents, and the ERI Power Model™ to support the design process. The ERI-SIM™ SWRO process simulator and ERI’s Factory Training provide dynamic, hands-on training for designers and operators.
Ceramics
ERI’s PX devices have only one moving part. The heart of the PX device is a high purity aluminum oxide rotor, turning at up to 1,000 rpm in an almost frictionless hydrodynamic bearing. This ceramic material is unaffected by chemicals or aqueous corrosion, is three times harder than steel, and provides unmatched durability in the PX application.
ERI has developed manufacturing capabilities and expertise that range from working with high alloy stainless steels and exotic metals, such as super–austenitic and super duplex stainless steel and titanium, to the synthesis and precision machining of ceramics.
Creating advanced ceramics components requires specialized equipment, a fully equipped materials lab, and optimized processes. Custom formulated spray-dried powders are compacted at extreme pressures to create the machineable blocks that become rotors. The lengthy sintering process occurs at temperatures greater than 1600°C and eventually achieves a hardness of Mohs 9.0 in a material chemically identical to sapphire. Once the ceramics have been properly formed, they are cooled slowly and evenly to avoid cracking. This overall process is critical to achieve a smooth hard surface, which will sustain many years of operation in a harsh seawater environment.
Glossary
Acre-foot (AF). A unit for measuring the volume of water. One acre-foot equals 325,851 gallons (the volume of water that will cover one acre to a depth of one foot) or 1,233 cubic meters. One million gallons equals 3.07 acre-feet.
Bar. A unit for measuring pressure. 1 bar = 14.5 psi = 0.99 atm
Biocide. A chemical used to kill biological organisms (e.g., sodium bisulphite).
Brackish. Water that typically contains less than 10,000 ppm salt-more salt than freshwater but less than the open seawater.
Brine. Water that contains greater than 50,000 ppm salt. Brine discharges from desalination plants may also include constituents used in pretreatment processes. See also "Concentrate".
BTU (British Thermal Unit). A standard unit for measuring a quantity of energy. Electricity, natural gas or any other source of energy can be measured in BTUs. One BTU is the amount of energy required to raise the temperature on one pound of water one degree Fahrenheit at sea level. One thousand BTUs equals 0.29 kilowatt-hours.
Coagulation. A pretreatment process used in some desalination plants. A substance (e.g., ferric chloride) is added to a solution to cause suspended particles to agglomerate and form larger particles that are easier to remove from a solution than small particles.
Cogeneration. A power plant that is designed to conserve energy by using "waste heat" from generating electricity for another purpose, for example to thermal desalination or to warm SWRO feed water.
Concentrate. Water containing concentrated salts rejected by the membrane. See also "Brine".
Deaeration. Removal of oxygen. A pretreatment process in desalination plants to reduce corrosion and fouling.
Dissolved Air Flotation (DAF). A pretreatment process in desalination plants to remove solids and organics.
Distillation. A process of desalination where the water is heated to produce steam. The steam is then condensed to produce product water with low salt concentration.
Efficiency. Energy transfer efficiency expressed as the ratio of the sum of the energy leaving the PX unit (or array) divided by the sum of the energy entering, as calculated with the following equation:
| Efficiency = | ∑(Pressure x Flow)OUT | x100% |
| ∑(Pressure x Flow)IN |
Electrodialysis. Most impurities in water are present in an ionized (electrically charged) state. When an electric current is applied, the impurities migrate toward the positive and negative electrodes. The intermediate area becomes depleted of impurities and discharges a purified stream of product water. This technology is used primarily for brackish waters.
ERD. Energy recovery device.
Feedwater. Water fed to the desalination equipment. This can be source water with or without pretreatment.
Fouling. Contamination or biological growth on the reverse osmosis membranes or pretreatment filters.
Freshwater. Water that contains less than 1,000 milligrams per liter (mg/L) of dissolved solids; generally, more than 500 mg/L of dissolved solids is undesirable for drinking and many industrial uses.
High-Pressure Differential Pressure (HP DP). The pressure at the high-pressure inlet port of the PX device minus the pressure at the high-pressure outlet port of the PX device.
Infiltration Gallery. A method used for the area where seawater enters the SWRD process. Perforated pipes are arranged in a radial pattern in the sand onshore below the water level. Water in the saturated sand enters the perforated pipes.
Ion Exchange. A reversible water treatment process. A charged polymer exchanges Na+, H+, Cl-, or OH- for other ions in a solution.
Isobaric. Literally: same pressure. An isobaric energy recovery device, like the PX, includes chambers wherein the pressure of two volumes of water equalizes.
Low-Pressure Differential Pressure (LP DP). The pressure at the low-pressure inlet port of the PX device minus the pressure at the low-pressure outlet port of the PX device.
Lubrication Flow. The flow of high-pressure brine required to lubricate the PX device's hydrodynamic bearing, measured as a difference in any of the following flows:
- Low-pressure feedwater to the high-pressure pump minus the membrane permeate
- High-pressure brine to the PX unit minus the high-pressure feedwater from the PX device
- Low-pressure brine from the PX unit minus the low-pressure feedwater to the PX device
Kilowatt (kW). One thousand watts.
Kilowatts-hours per cubic meter (kWh/m³). A measure of the power require to produce a cubic meter of permeate.
Megawatt (MW). One million watts.
Minimum Discharge Pressure. The minimum allowable pressure at the low-pressure outlet port of the PX device.
Multiple Effect Distillation (MED). A form of distillation. Evaporators are placed in series, and vapor from one effect is used to evaporate water in the next lower pressure effect. There are several forms of this technology, one of the most common is the Vertical Tube Evaporator (VTE).
Multistage Flash (MSF). A form of distillation. Intake water is heated then discharged into a chamber maintained slightly below the saturation vapor pressure of the incoming water, so that a fraction of the water content flashes into steam. The steam condenses on the exterior surface of heat transfer tubing and becomes product water. The unflashed brine enters another chamber at a lower pressure, where a portion flashes to steam. Each evaporation and condensation chamber is called a stage.
Nanofiltration (NF). A lower pressure membrane filtration technology sometimes used for pretreating reverse osmosis feedwater.
Permeate. Water purified by reverse a osmosis membrane.
Pounds per square inch (psi). A unit for measuring pressure. 1 psi = 0.069 bar = 0.068 atm.
Product Water. The desalted, post-treated water (or permeate) delivered to the water distribution system.
Recovery. Ratio of permeate to membrane feed flows, typically expressed as a percentage.
Reverse Osmosis (RO). A process where pressure is applied continuously to feedwater, forcing water molecules through a semipermeable membrane. Water that passes through the membrane leaves the unit as permeate or product water; most of the dissolved impurities remain behind and are discharged in a concentrated brine or waste stream.
Salinity Increase. The increase in the salinity of the membrane feed stream caused by the energy recovery device. Salinity increase varies with the membrane recovery rate. It is typically expressed as a percentage increase of the membrane inlet stream above the salinity of the system feedwater according to the following equation:
| Salinity Increase = | membrane inlet salinity - system feedwater salinity | x100% |
| system feedwater salinity |
Seawater Reverse Osmosis (SWRO). Reverse osmosis desalination of seawater.
Scaling. Salt deposits on the surfaces of a membrane.
Total Dissolved Solids (TDS). Total salt and calcium carbonate concentration in a sample of water.
Vacuum Freezing (VF). A process of desalination where the temperature and pressure of the seawater is lowered so that the pure water forms ice crystals. The ice is then washed and melted to produce the product water. This technology is still being developed, and is not commercially viable.
Vapor Compression (VC). A form of distillation. A portion of feedwater is evaporated, and the vapor is sent to a compressor. Mechanical or thermal energy is used to compress the vapor, which increases its temperature. The vapor is then condensed to form product water and the released heat is used to evaporate the feedwater.
Volumetric Mixing. The percentage of the volume of concentrate that mixes into the feedwater inside the energy recovery device (ERD). It is expressed as a function of the salinities of the ERD high-pressure and low-pressure inlet and outlet streams according to the following equation:
| Volumetric Mixing = | HP outlet - LP inlet (TDS) | x100% |
| HP inlet - LP inlet (TDS) |
Watt (W). A measure of power used by electricity generating plants. One watt is equivalent to 1 Joule/second or 3.41 Btu/hour.
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