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| Industry | : Chemical | | Product | : pH/ORP Meter |
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Cost Effective and Accurate pH Measurement in Limestone-Gypsum Flue Gas Desulfurization Systems
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In limestone-gypsum flue gas desulfurization systems, the
consumption of a desulfurization agent (lime) is controlled by using
online pH meters. Of great concern in the pH measurement
is heavy lime scaling on the pH electrode. To ensure accurate
measurements, frequent cleaning of the electrodes with acid is
required, adding to both maintenance workload and cost.
The EXA AUTO CLEAN chemical cleaning system automates
the acid cleaning process, which not only saves both time and
expense but also ensures precise pH measurement over long
periods.
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Improves the efficiency of a limestone-gypsum flue gas
desulfurization system |
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Ensures stable, continuous pH measurement |
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Reduces operating costs |
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Eliminates manual cleaning |
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The limestone-gypsum flue gas desulfurization consists of two processes: absorption and oxidation.
Absorption process
Reaction formula; Ca(OH)2 + CO2 -> CaCO3 + H2O, 2CaCO3 + 2SO2 + 1/2H2O -> 2CaSO3·1/2H2O + CO2
In this process, it is important to promote the reaction of calcium (Ca) and to achieve a target desulfurization rate. The absorption
rate is affected by the pH, concentration, and temperature of liquid and the liquid/gas ratio. When the pH of the absorbing solution is
below 5, calcium hydrogen sulfite (Ca(HSO3)2) is generated. Ca(HSO3)2 is converted into sulfur dioxide (SO2) in a reversible reaction,
thereby reducing the desulfurization rate. When the pH of the absorbing solution is 7 or higher, carbon dioxide (CO2) in the flue gas
reacts with calcium (Ca) to produce calcium carbonate (CaCO3). This increases lime consumption. Since the pH has an influence on
the subsequent oxidation process, the pH of the absorbing solution should be kept to between 5 and 6.
Oxidation process
Reaction formula: CaSO3·1/2H2O + 1/2O2 + Aq -> CaSO4·1/2H2O + Aq
In this process, calcium sulfite (CaSO3·1/2H2O) generated in the absorption process is oxidized with air to calcium sulfate (CaSO4)
under a pressure of approximately 500 kPa and a temperature of 50 to 80 °C. The pH of the absorbing solution in the absorber is
approximately 6. It should be kept at around 4 by adding acid from the cooling tower.
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Measurement System & Operation Outline
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Field Data
Deposits of crystalline substances (e.g., CaCO3) on electrodes in limestone-gypsum flue gas desulfurization systems decrease the
accuracy of pH measurements. The cleaning of the electrodes and the reduction of maintenance time and cost are key points to
consider when selecting a pH meter for the system.
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pH System with Chemical Cleaning |
General pH Meter |
| Cleaning |
"Automatic acid cleaning: 2 or 3 times/day, user programmable
Manual acid cleaning: once/day" |
"Manual acid cleaning: approx. monthly" |
| Calibration |
Manual calibration: weekly |
"Manual 2-point calibration: weekly" |
| Other maintenance |
Replenishment of chemical tank: approx. every 2 months |
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Acid cleaning is done with a 4% hydrochloric acid solution
| Utilities |
| Power supply: |
100 V AC, 50/60 Hz |
| Power consumption: |
approx. 60 VA |
| Air source: pressure: |
300 to 950 kPa |
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Maximum consumption: |
approx. 20 L/min |
| Chemical cleaning solution consumption: |
approx. 100 mL |
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Tank effective capacity: |
approx. 17 L |
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| Notes |
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An organic solvent cannot be used for cleaning. |
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Deterioration of materials in chemical solution piping and air piping
When the system is installed in a location where it is exposed to
direct sunlight, the polyethylene resin piping will last approximately
one year. (It is recommended that fluoro resin piping be used.) |
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pH Measurement in Limestone-Gypsum Flue Gas Desulfurization |
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