Background Information
Processes requiring pure water must continually replace the water being consumed. Sources of replacement water are usually local supplies from a river or lake and therefore require pre-treatment and purification before it can be used in the process.
After preliminary purification which may include filtration, clarification and softening, further downstream, a two-pass reverse osmosis system and demineralization operations are typically employed to further purify the water.
Osmosis is the natural tendency of a fluid, usually water, to pass through a semipermeable membrane from a less concentrated solution into a more concentrated one, thus equalizing the concentrations on each side of the membrane.
In reverse osmosis (RO), pressure must be exerted on the side with the concentrated solution to force the water molecules across the semi-permeable membrane to the fresh (pure) water side.
This semi-permeable membrane inhibits the majority of dissolved impurities from passing through to the pure water side. The amount of impurities carried over depends on the type and condition of the membrane (i.e. age, cleanliness) and the amount of pressure applied (energy) to the process.
Not all the feed water passes through the membrane. Some is diverted to flow over them to cleanse away the rejected impurities in a cross-flow filtration mode.
The RO system produces one purified water stream called permeate and a second stream called concentrate, brine, or reject. Feedwaterenters the machine at fairly low pressure and flows through pre-filters to remove suspended particles, such as silt. Pre-filters are typically a replaceable cartridge type which provides a cost effective method for keeping the membrane clean. Typical life expectancy for these membranes is approximately three years.
RO systems are designed for automatic operation and require routine preventative and corrective maintenance. Common problems include membrane fouling and the use of improper flow rates. The result is reduced throughput capacity and shortened runs.
Membranes can fail altogether, resulting in excessive demand on downstream purification systems and poor quality product water.
Both pH and conductivity measurements are used to safeguard the successful operation of an RO system.
Some types of RO membranes are sensitive to feed water pH and can become damaged if the pH is outside the recommended range of 5 to 8 pH.A pH sensor upstream of the membrane can provide a feedback signal to control dosing of acidic or basic reagent to maintain the pH within acceptable limits.
Conductivity measurements are used at both the inlet and outlet of the RO unit to determine whether the total dissolved solids are being filtered effectively.
General Applications
Reverse osmosis systems can remove up to 100% of suspended solids and approximately 90% of dissolved solids, dissolved silica, alkalinity and hardness.
A common use for RO is for purifying water, removing salts and other impurities to improve the color, taste and other properties. It is regularly used for commercial and residential water filtration and is also one of the methods used for desalinization of seawater.
RO systems are capable of rejecting bacteria, salts, sugars, proteins, particles, dyes, and other constituents which have a molecular weight of greater than 150-250 Daltons.
The separation of ions with reverse osmosis is aided by charged particles. This means any dissolved ions which carry a charge, such as salts, are more likely to be rejected by the membrane than those that are not charged, such as organics. The larger the charge and the larger the particle, the more likely it will be rejected.
The majority of RO membranes are negatively charged when they are operated within the pH levels most commonly encountered in water applications.
Pure Water Applications
A two-pass RO system is typically installed upstream of the demineralizer. Its performance is pH dependent with the second-pass section most dramatically affected. While these changes are not significant in the majority of applications, variations become crucial to the success of high-purity water processing.
In addition, the effect of minor feedwater constituents, such as alkalinity and ammonia also play a role in achieving high-purity permeate.
The overall efficiency of dissolved solids removal is usually determined utilizing a pair of conductivity measurements, one at the inlet (cell 1) and one at the outlet (cell 2). This is referred to as % rejection and calculated by the formula:
% rejection = [1-(cell2)/(cell 1)] x 100
For example if the inlet water had 200 ppm of dissolved solids and the outlet water had 10 ppm, the efficiency would be 95% rejection rate. A typical range for this type of application is 80% - 100% rejection.
A final conductivity measurement after the 2nd stage is often used to determine the absolute quality of the outlet water.
Ammonia also affects the production of high purity water and may be present due to municipal chlorination of feedwater or from organic contamination.
Ammonia (NH3) will through the membrane system in either the molecular or ionic (NH4+) form.
Ammonium hydroxide is less conductive than ammonium carbonate [(NH4)2CO3] so it is not uncommon to find off-line samples or storage tank water with conductivity higher than that of on-line readings.
The pH values will be lower. This shift in pH is due to absorption of CO2 from the air and the formation of carbonic acid in the water. Without the presence of ammonia, this type of contamination of highpurity water with CO2 would generate higher conductivity as well as the reduced pH.
Product Recommendations
Process Liquid Analyzer:
- 2-wire FLXA202 Analyzer
- 4-wire FLXA402 Analyzer
pH Measurement Sensor Selection:
Option #1:
Holders
- FF20 Flow-thru assembly with individual measure, reference and temperature electrodes
- FS20 Insertion assembly with individual measure, reference and temperature electrodes
Sensors
- Bellowmatic reference electrode (SR20-AC32)
- Shock-proof measuring electrode (SM21-AG4)
- Pt1000 temperature electrode (SM60-T1)
Option 2:
Holder: PH8HH Flow Thru assembly
Sensor: PH8EHP Flowing reference pH Sensor for High Purity Water
Conductivity Measurement Sensor Selection:
Holders
- FF40 Flow-thru assembly
- FS40 Insertion assembly
Electrodes
- SC42-SP24
- SC42-SP34
Digital SMART Option:
The re-usable smart adapter, SA11, offers full measuring parameter functionality of analog sensors equipped with a Variopin connector and Yokogawa ID chip. The SA11 automatically recognizes the installed sensor and prepares the right configuration.
Note: For additional information on this application contact the local Yokogawa Process Liquid Analyzer Department
Related Industries
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Water & Wastewater
Water resources are finite, and therefore contributing to a sustainable water cycle is one of the Sustainable Development Goals (SDGs). Yokogawa has been providing advanced digital control solutions for the stable supply of clean and safe water, wastewater treatment for protecting the water environment, water loss management and optimization of plant operation for reducing CO2 emissions and running costs. With our leading-edge technologies, dependable products and extensive expertise and experience of diverse water projects around the world, we work with you to provide sustainable water solutions that boost your business and add value throughout the plant lifecycle.
Yokogawa supports a wide range of water control applications in both the municipal and industrial water markets.
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Power
In the mid 1970s, Yokogawa entered the power business with the release of the EBS Electric Control System. Since then, Yokogawa has steadfastly continued with the development of our technologies and capabilities for providing the best services and solutions to our customers worldwide.
Yokogawa has operated the global power solutions network to play a more active role in the dynamic global power market. This has allowed closer teamwork within Yokogawa, bringing together our global resources and industry know-how. Yokogawa's power industry experts work together to bring each customer the solution that best suits their sophisticated requirements.
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Oil & Gas
Yokogawa has a wealth of experience in every part of the oil and gas business, from offshore and onshore facilities to pipelines, terminals, and deepwater operations. We provide solutions that enhance safety, ensure accurate and reliable operation, and increase plant efficiency.
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Oil & Gas Downstream
The oil & gas downstream industry has been facing an increasing number of challenges in recent years. These include the changing characteristics of the feedstock to be processed, aging of process facilities and equipment, rising cost of energy, lack of skilled plant operators who can run a refinery safely and efficiently, and the ever-changing requirements from both the market and the customer.
Over the years, Yokogawa has partnered with many downstream companies to provide industrial solutions focused on solving these challenges and problems. Yokogawa's VigilantPlant solutions have helped plant owners to achieve maximum profitability and sustainable safety within their plants.
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Pulp & Paper
The paper and pulp industry is highly competitive and must meet ever-changing market needs. Yokogawa helps realize energy-efficient plants which are globally sustainable.
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Food & Beverage
At Yokogawa, we understand that today’s food and beverage companies face unprecedented challenges in climate change, consumer demand, and increased global competitiveness. Overcoming these challenges will require innovative solutions that focus on key areas of production, asset management, and food safety and quality.
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Chemical
Chemical plants rely on continuous and batch production processes, each posing different requirements for a control system. A continuous process calls for a robust and stable control system that will not fail and cause the shutdown of a production line, whereas the emphasis with a batch process is on having a control system that allows great flexibility in making adjustments to formulas, procedures, and the like. Both kinds of systems need to be managed in available quality history of product, and to be able to execute non-routine operations. With its extensive product portfolio, experienced systems engineers, and global sales and service network, Yokogawa has a solution for every plant process.
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Refining
In the ever changing marketplace, refineries are seen not only as crude processing units but also as profit centers. At the same time, there is a keen awareness of the need for safety at such facilities. A total production solution that encompasses planning, scheduling, management, and control is required to achieve long-term goals for profitability, efficiency, and environmental protection. With years of expertise in the automation field, Yokogawa can bring you affordable total solutions for improved operability and a cleaner world.
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Offshore (FPSO FLNG & FSRU)
Offshore exploration and production requires maximum uptime under harsh conditions. Manned and unmanned facilities need reliable integrated control and safety systems (ICSS) with advanced remote monitoring capabilities. Yokogawa has state-of-the-art technology and extensive experience in executing offshore projects of all sizes and automation levels of complexity.
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Upstream
The upstream industry includes offshore and onshore activities including wellhead automation, fractionation, completion, and separation to recover and prepare underground or underwater crude oil and natural gas.
As petroleum is brought to the surface, it must be separated prior to transport. Primary and secondary separation stages commonly distribute gas flow, water flow, and oil flow in three phase separation. Gas movement requires pipeline and can include a fractionation process in the upstream stage prior to movement. Liquids can be placed into a tanks or pipelines and sent for processing, requiring accurate level measurements.
Related Products & Solutions
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2-Wire Transmitter/Analyzer FLXA202
The FLEXA™ series analyzers are used for continuous on-line measurements in industrial installations. With an option for single or dual sensor measurement, they are the most flexible two-wire analyzer available.
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2-Wire Transmitter/Analyzer FLXA21
The FLEXA™ series analyzers are used for continuous on-line measurements in industrial installations. With an option for single or dual sensor measurement, they are the most flexible two-wire analyzer available.
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Conductivity Analyzers
Conductivity meters, analyzers and transmitters are used for continuous process measurement and monitoring of conductivity, resistivity, WIFI, demineralizer water, RO water, percent concentration, boiler blowdown and TDS.
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Conductivity Sensors
Conductivity sensors and electrodes are used to measure process conductivity, resistivity, WIFI, demineralizer water, RO water, percent concentration, boiler blowdown and TDS. Various installation options including retractable, flow thru, immersion, and direct insertion. Proper electrode/sensor selection is critical for optimal measurement results.
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Contacting Conductivity Sensors SC42/SC4A(J)
The measurement of specific conductivity in aqueous solutions is becoming increasingly important for the determination of impurities in water. Yokogawa has designed a full range of precision sensors and instruments to cope with these measurements, even under extreme conditions.
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Digital SMART SENCOM™ Adapter, SA11
Reusable SMART adapter, requiring only the analog sensor to be disposed of when it reaches the end of its lifetime. With the SENCOM 4.0 platform, Yokogawa delivers reduced costs and waste while contributing to its long-term business goals of a sustainable future for all.
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Flow/NPT Fittings FF20/FS20
Yokogawa has invested considerable design and development time in producing a full range of fittings with particular emphasis on designs that reduce installation and maintenance time and consequently save operation costs.
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Multi Channel 4-Wire Analyzer FLXA402
The FLEXA™ series analyzers are modular-designed analyzers used for continuous online measurements in industrial installations. They offer single or multi-sensor measurement.
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pH Sensor for High Purity Water PH8EHP
The PH8EHP pH sensor and PH8HH holder perform reliable measurement of pH for high purity water used in boilers and semiconductor process applications.
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pH and ORP Analyzers
pH and ORP meters, analyzers and transmitters are used for continuous process monitoring of pH and ORP to ensure water/product quality, monitor effluent discharge, batch neutralization, pulp stock, scrubbers, cooling towers, chemical, water/wastewater treatment and many other applications.
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pH and ORP Sensors
pH electrodes and sensors are the sensing portions of a pH measurement. Various installation options including retractable, flow thru, immersion, and direct insertion. Proper pH electrode/sensor selection is critical for optimal measurement results.