pH/ORP Measurement for Reverse Osmosis

다운로드 (297 KB)

Background Information

Reverse osmosis

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. Feedwater enters 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 forcommercial 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.

 

pH/ORP Measurement for Reverse Osmosis

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 high-purity water with CO2 would generate higher conductivity as well as the reduced pH.

 

Product Recommendations

pH Measurement System

Process Liquid Analyzer

  • 2-wire FLXA21 pH/ ORP Analzyer
  • 4-wire PH450G pH/ORP Analyzer

FLXA202 Two Wire pH and ORP AnlayzerPH450 Four wire pH and ORP Analyzers

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), coupled with the shock-proof measuring electrode (SM21-AG4) and Pt1000 temperature electrode (SM60-T1)

sensor

Option 2:

Holder: PH8HH Flow Thru assembly

Sensor: PH8EHP Flowing reference pH Sensor for High Purity Water

PH8HH Flow Thru assembly

 

Conductivity Measurement System

Process Liquid Analyzer

  • 2-wire FLXA21 Conductivity Analyzer
  • 4-wire SC450G Conductivity Analzyer

EXA450

Sensor Selection

Holders:

  • FF40 Flow Thru assembly
  • FS40 Insertion assembly

Electrodes

  • SC42-SP24
  • SC42-SP34

SC42

Note: For additional information on this application contact the local Yokogawa Process Liquid Analyzer Department

업종

Related Products & Solutions

2-Wire Transmitter/Analyzer FLXA202/21

FLEXA ™ 시리즈 분석기는 산업용 설비의 연속 온라인 측정에 사용됩니다. 단일 또는 이중 센서 측정 옵션이 있어 가장 유연한 2 선식 분석기입니다.

4-Wire pH/ORP Analyzer PH450

PH450 pH와 ORP 분석계인 PH450은 pH와 온도, ORP(Redox) 측정을 결합한 다변형 분석계로, mA 전류 출력 2개, 독립적인 SPDT 접점 출력 및 HART 등 다양한 출력기능을 통해 활용할 수 있습니다. 

Contacting Conductivity Sensors SC42/SC4A(J)

수용액서의 전도도 측정은 물에서 불순물을 측정하는데 점점 더 중요해지고 있습니다. Yokogawa는 극한 조건에서도 이러한 측정에 대응할 수 있는 정밀 센서와 계측기를 설계하고 있습니다.

Flow/NPT Fittings FF20/FS20

Yokogawa는 설치 및 유지보수 시간을 줄이고 결과적으로 운영 비용을 절감하는 설계에 특히 중점을 두어 전체 범위의 피팅을 생산하는데 상당한 설계 및 개발 시간을 투자했습니다.

Four-Wire Conductivity Analyzer

EXAxt450 시리즈는 구성 수를 줄이기 위하여 표준 분석계의 모든 특수 소프트웨어 기능을 포함하여 모든 전도도 어플리케이션을 다룹니다.  따라서 여러 전도도 어플리케이션을 사용하는 사용자에게 필요한 재고를 줄이고 교육 필요성을 줄일 수 있습니다. 

pH Sensor for High Purity Water PH8EHP

pH센서 PH8EHP와 홀더 PH8HH는 보일러 및 반도체 공정에 사용되는 고순도 물에 대해 pH를 안정적으로 측정합니다.

Conductivity Analyzers

전도도 계, 분석기 및 송신기는 전도도, 저항률, WIFI, 탈 염기 수, RO 수, 농도 퍼센트, 보일러 블로우 다운 및 TDS의 연속 공정 측정 및 모니터링에 사용됩니다.

Conductivity Sensors

전도도 센서와 전극은 공정 전도도, 저항률, WIFI, 탈염 수, RO 수, 농도 퍼센트, 보일러 블로우 다운 및 TDS를 측정하는 데 사용됩니다. Retractable, Flow-though, 침적 및 직접 삽입을 포함한 다양한 설치 옵션이 있습니다.

적절한 전극 / 센서 선택은 최적의 측정 결과를 위해 중요합니다.

pH and ORP Analyzers

pH와 ORP 미터, 분석계 및 전송기는 수질/제품 품질을 보장하고 유출물 배출량, 배치 중화, 펄프 스톡, 세정기, 냉각 타워, 화학, 폐수 처리 및 기타 여러 어플리케이션의 연속 공정 모니터링에 사용합니다.

pH and ORP Sensors

pH 전극 및 센서는 pH 측정의 감지 부분입니다. Retractable, flow-though, 침적 및 직접 삽입을 포함한 다양한 설치 옵션이 있습니다.

적절한 pH 전극 / 센서 선택은 최적의 측정 결과를 위해 중요합니다.

×

Have Questions?

Contact a Yokogawa Expert to learn how we can help you solve your challenges.

Top