pH Measurement & ORP Measurement in the Desalination of Seawater
Liquid analysis within the water & wastewater industry
Extended pH/ORP Sensor Life
Desalination is the process of removing dissolved mineral salts from seawater and brackish water for human consumption. Most major markets now employ reverse osmosis (RO) membrane separation in place of energy intensive evaporation/distillation processes as up to 99% of dissolved solids in raw seawater can be separated out by RO membranes. Most RO membranes in pH sensors today are made of polyamides and can tolerate any pH level between 2 and 10. However, pH sensors can be damaged by free chlorine injected upstream in the seawater stream as a biocide. For this reason, sodium sulfite is introduced right before the RO membranes to remove any free chlorine. Residual free chlorine (RFC) is an oxidizer and will register an oxidation reduction potential (ORP) of approximately 500-700 mV. RFC elimination can be controlled by a sodium sulfite pump until the ORP value reaches 50-350 mV. Furthermore, pH can be monitored as well. The Rosemount 389 pH/ORP Sensor features a triple reference junction and is ideal for delaying reference electrode poisoning from chloride ions in seawater, resulting in longer pH sensor life.
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Desalination & pH/ORP Measurement with Rosemount pH/ORP Sensors & Transmitters
Which pH sensor is applicable?
The reference electrode in a pH sensor provides a stable voltage potential independent of the process. The reference electrode degrades over time, requiring eventual replacement. The Rosemount 389 pH/ORP Sensor contains three reference junctions for delay of reference poisoning by chloride ions in seawater, while the Rosemount RBI pH Sensor contains five reference junctions to further slow degradation.
More than one type of pH sensor can be selected for a particular process. While many pH sensors are discarded after use, some pH sensors can be rebuilt and reused. The Rosemount 3500P pH/ORP Sensor contains just two reference junctions, but the reference junction and reference electrolyte are rebuildable.
This video outlines the maintenance process for deciding between a replaceable or rebuildable sensor.
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Desalination Through Evaporation
Older desalination plants still use the evaporation process. Raw seawater is coarse filtered to remove solids and then combined with acid in a treatment tank. The treated seawater is heated in an evaporator, with its resulting vapor collected and recondensed as fresh water. The Rosemount 389 pH Sensor features a triple junction and can be used throughout the process to monitor pH levels to help reduce scale formation on hot surfaces.
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ORP Measurement
The Rosemount 389 ORP Sensor contains a platinum band that can accept and donate electrons to the solution. Over time, a millivolt potential forms across the platinum band, which is the ORP of the solution. Learn more about the fundamentals of ORP measurement.
Oxidizers like free chlorine, ozone and oxygen produce positive ORP values, while reducers like some organic compounds produce negative values.
The sensing element of the pH sensor is the pH glass, whereas the sensing element of the ORP sensor is the platinum band. The remaining elements of both sensors are the same, including the reference electrode.
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Product Solutions
The RBI pH Sensor is designed for extended sensor life when used in applications with sulfides which are typically found in scrubbers.
- Maximize pH/ORP sensor life with multi-junction reference
- Highly chemical resistant rugged Kynar® body
- Retractable option for use with ball valve assemblies
Measurement Range
pH sensor: 0 to 14
ORP sensor: -1500 mV to 1500 mV
ORP sensor: -1500 mV to 1500 mV
Materials of Construction
Wetted Materials: Kynar®, titanium (retractable 547 only), porous PTFE, wood, glass, and choice of EPDM, Viton™ or Kalrez®
Maximum Temperature
248 °F (120 °C) at 40 psig (276 kPa [abs])
The Rosemount 389 pH Sensor / ORP Sensor, with integral cable, is housed in a molded Tefzel™ body with Viton™ o-rings, making each sensor virtually indestructible and chemically resistant. Complete encapsulation eliminates leakage or high humidity problems traditionally found in other pH/ORP sensor designs. The simplified construction, designed with user convenience in mind, does not require electrolyte (KCl) replenishment or any high maintenance troubleshooting procedures.
- Triple junction reference prolongs pH sensor / ORP sensor life and protects against poisoning ions such as ammonia, chlorine, cyanides, and sulfides
- Enhanced performance and increased life with cracking resistant glass
- Available with a glass electrode option for high pH applications
Temperature Range
32 - 182 °F (0 - 85 °C)
Max. Operating Pressure
100 psig (790 kPa) at 150 °F (65 °C)
Materials of Construction
pH sensor body: Tefzel™pH electrode: glassORP electrode: platinumJunction: ceramicO-Ring: Viton™
The Rosemount 1058 Dual Channel Transmitter provides confident liquid analysis measurement in a wide range of industrial, municipal and commercial applications. It is a 4-wire powered device that provides multi-parameter liquid analysis for digital and analog sensors, including inputs from pH/ORP sensors, ISE, flow, current, conductivity sensors (contacting and toroidal), turbidity sensors and amperometric sensors (dissolved oxygen, chlorine and ozone).
- Supports continuous measurement of liquid analysis inputs from one or two sensors.
- Each sensor channel is independently configurable to support a wide selection of digital or analog liquid analysis sensors including pH sensors, ORP sensors, conductivity sensors, dissolved oxygen sensors, chlorine senors, turbidity sensors and more.
- IP66 / Type 4X rated engineered polymer enclosure with universal mounting options provides confident and easy installation indoors or outdoors.
Enclosure Rating
Type 4X, IP66
Dimensions
Overall 160 x 157 x 140 mm (6.3 x 6.2 x 5.5 in); Cutout 138.4 x 138.4 mm (5.45 x 5.45 in)
Weight
3 lb (1.5 kg)
The Rosemount 1066 Single Channel Transmitter supports continuous measurement of one liquid analytical input - pH sensor, conductivity sensor, chlorine sensor, dissolved oxygen sensor or ozone sensor. The design supports easy internal access and wiring connections. The large display gives excellent visibility for live measurements and displayed parameters. Conveniently, live process values are always displayed during programming and calibration routines.
- Fast and easy installation using Quick Start programming
- Intrinsically Safe FM, CSA, and ATEX/IECEx Hazardous Location Approvals
- Visibility of process parameters by utilizing HART® or FOUNDATION™ Fieldbus digital communications
Power Supply
Supply voltage at the transmitter terminals should be at least 12.7Vdc. Power supply voltage should cover the voltage drop on the cable plus the external load resistor required for HART® communications (250 Ω)
Weight
R2 lbs/3 lbs (1 kg/1.5 kg)
Communication Protocol
HART™ Communications & diagnostics: PV, SV, TV, and 4V assignable, temperature
