Presentation on theme: "Dissolved Oxygen (DO) What is ?. It IS elemental oxygen, O 2, physically absorbed in a liquid. It IS NOT the chemically bound O of H 2 O. It IS NOT physically."— Presentation transcript:
It IS elemental oxygen, O 2, physically absorbed in a liquid. It IS NOT the chemically bound O of H 2 O. It IS NOT physically adsorbed or entrained bubbles.
Dissolved Oxygen - the amount of O 2 gas dissolved in a sample of liquid water. Units of measure: ppb ppm % saturation mg/liter( ppm ) What is Dissolved Oxygen?
How does DO get there? Chemical, Electrochemical or Biochemical processes such as Photosynthesis Exposing the liquid to air or O2 Shaking helps
Why is Dissolved Oxygen measured? “ Dissolved Oxygen is one of the most important measurements in the determination of the health of a body of water”
In ppm Applications, DO must stay above a minimal value to support life. Who makes this measurement? Fish & Shrimp Farms Wastewater Treatment Plants
In ppb Applications, DO needs to be below some maximum value to inhibit Corrosion or Food Spoilage. Semiconductor Power Utilities Food & Beverage
POWER PLANT APPLICATIONS-- DISSOLVED OXYGEN-- DO Blowdown Chemical Feed Condenser L-P Turbine H-P Turbine I-P Turbine Boiler H-P Heaters L-P Heaters Deaerator DO 8.25 DO 8.25 DO 8.25 DO 8.25 Chemical Feed Economizer Saturated Steam Cooling Water Drum Super Reheater heater Hotwell Raw water Condensate Polishers Make-up Water Treatment Demineralizer Condensate Storage Tank DO 8.25 Feed Water
Air Saturation in water [25C, 1 ATM]: 8.24 mg O 2 /L [8.24 ppm] Compare to O2 in air: PV = nRT n/V = P/RT = 1 atm/0.082[L-atm/moleK]x 298K = 0.0406 moles air x 0.209 moles O2/mole air x 32,000mg O2/mole O2 = 274 mg O2/L [oxygen in air/ Oxygen in air-sat. water] = 274/8.24 = 33.3
Off-line are too slow to correct sudden upsets Off-line cannot catch momentary fluctuations With Off-line, it is easy to contaminate the sample Have to manually record other influences at time of sample collection – –temperature- weather behavior – –time of day- day of week Currently, no reference standard available for the ppb industry Reasons to use On-line versus Off-line Measurements
sells. Continuous On-line Measurement Membrane Technology - the membrane separates the internal electrolyte and electrodes from the sample, allowing only the gases to penetrate the membrane for measurement. Two Types Diffusion - based on the rate of diffusion of oxygen across a membrane. Example: Polarographic or Galvanic Equilibrium - based on the partial pressure of oxygen.
Electrochemical Measurement of Dissolved Oxygen [DO]
Dissolved O 2 : Typical Diffusion Cell Replaceable Electrolyte Pb Anode Inert Cathode Replaceable Membrane Cathode: O 2 + 4 H + + 4 e - = 2H 2 O Anode: 2 Pb + 2 H 2 O = 2 PbO + 4H + + 4 e - Oxygen Transport Diffusion Rate OC [DO] / path length
in Still Water Dissolved O 2 : Typical Diffusion Cell, Pb Anode Replaceable Electrolyte Inert Cathode Replaceable Membrane Stagnant [still] water Oxygen Transport Diminished Diffusion Rate OC [DO] / path length Diffusion Probe: Fouling Dependent Flow Dependent Requires: Membrane Changing Electrolyte Replenishment Electrode Treatment
Special Requirements of Diffusion Types Minimum Flow - since oxygen is continuously consumed at the cathode, a deficiency of oxygen may result if a steady flow of oxygen is interrupted. Therefore, a continuous minimum supply of oxygen must diffuse through the membrane for an accurate reading. No Membrane Fouling - Fouling reduces the surface area for oxygen transport, less oxygen available for reduction results in lower readings.
Advantages Electrolyte can be restored Lower initial cost Disadvantages Anode needs to be cleaned, recharged or replaced as oxide build-up occurs (electrolyte must be replaced at this time also) Membrane is delicate - needs to be replaced periodically Probe is flow sensitive
sealed in... a tough, permanent, gas-permeable membrane
and bathed in... a permanent internal electrolyte solution.
When the sensor is turned on, + - + - Oxygen immediately adjacent to the CATHODE [the negative electrode] is consumed according to O 2 + 4H + + 4 e - = 2 H 2 O
…and, uniquely for the equilibrium sensor, oxygen is generated - - + + O 2 + 4H + + 4 e - = 2 H 2 O at the ANODE [positive electrode] 2 H 2 O = O 2 + 4H + + 4 e - in an amount equal to that consumed at the CATHODE. The rest of the reaction balances, too.
The current supporting these reactions O 2 + 4H + + 4 e - = 2 H 2 O 2 H 2 O = O 2 + 4H + + 4 e - is directly proportional - - + + to the DO concentration.
Until equilibrium is reestablished Sensor Sample
Fouling or still water only slow reaching equilibrium
And... since the reactions are balanced, O 2 + 4H + + 4 e - = 2 H 2 O 2 H 2 O = O 2 + 4H + + 4 e - No chemical changes occur in the sensor and a permanent sensor is possible. - - + +
Summary: with close-spaced inert electrodes and internal oxygen generation, the equilibrium DO sensor is...
Independent of fouling Independent of flow…and... Permanent, Requiring: NO membrane changing NO electrolyte replacing NO electrode treating For….
Reliable Results Long Life Low Maintenance Low Cost of Ownership
7020 Analyzer DL5000 Equilibrium Probe DirectLine® or + Honeywell DO Instrumentation
7020 Analyzer Honeywell DO Instrumentation Model : 7022 ppm/ 7021 ppb Case : Aluminum NEMA 4X/IP65 Display : Backlit dot matrix LCD Operating Condition : -20 to +40 degC Accuracy : ppm= +/-0.2 ppm ; ppb=2 ppb or 5%of reading Advance feature : Auto Ranging of Input, Auto Calibration & Cleaning relays & software, Control function Operating Voltage : 16-24 Vdc power looped Output Signal : 4-20 mA
DirectLine® Honeywell DO Instrumentation Model : DL424 ppm/DL425 ppb Case : Plastic Display : Backlit dot matrix LCD Operating Condition : -20 to +60 degC Accuracy : ppm= +/-0.2 ppm ; ppb=2 ppb or 5%of reading Advance feature : Integral electronics/sensor Operating Voltage : 16-24 Vdc power looped Output Signal : 4-20 mA
7020 With Automatic Clean and Calibration Example Application
DirectLine® Easy to install/set-up: DirectLine architecture i.e. integral sensor/electronics reduce wiring, cable runs and panel cut-outs Easy-to-use: Local display and keypad provides easy set-up, calibration and operation Calibration can be done right at electrode, no running back and forth to analyzer (convenient + time + money savings) Easy Maintenance: Plug-in sensors allow electrode/probe replacement in few minutes; no wiring required (convenient + time + money savings)
DirectLine® Remote Type Electronic Module Pre-Amp Module DO Electrode Remote Cable Shield Twist Pair 20’/100’