Chlorine Dioxide (ClO2) has become the most significant bleaching agent in the pulp and paper industry for Elemental Chlorine-Free (ECF) bleaching. Different bleaching sequences must be used in the production of various pulps. This depends on the pulping process used, the residual lignin content of the pulp, and the target brightness. Key goals at most pulp mills include the optimization of reaction efficiency of ClO2 generators, the proper dosage of chlorine dioxide, and improving the control of vent-gas scrubbers to reduce chlorine dioxide emissions into the atmosphere.
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Chlorine Dioxide gas is commercially generated either by reduction of sodium chlorate in an acidic medium or oxidation of sodium chlorite. It is a synthetic yellowish-green, explosive gas, which is stable as an aqueous solution only if the solution is protected from light and kept refrigerated. For this reason, chlorine dioxide must be produced and consumed on-site by means of a chlorine dioxide generator. The efficiency of these generators and the dosage of chlorine dioxide can be optimized using inline ClO2 analysis.
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Inline Chlorine Dioxide Monitoring - Pulp Bleaching Benefits |
- Optimized ClO2 Generator Performance
- Optimized Chlorine Dioxide Dosage
- Reduced Chlorine Dioxide Consumption
- Better Process Control
- Reduced Costs
- Increased Plant safety
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| Chlorine Dioxide Concentration Control |
In chlorine dioxide generation, gaseous chlorine dioxide is conveyed to the absorber tower and dissolved in chilled water to yield the aqueous chlorine dioxide bleach solution. Chlorine dioxide concentration in this solution can be monitored in the 0 to 15 grams per liter range with an optek AF26 absorption sensor. This will ensure optimum control of the bleach solution strength in the manufacture of pulp.
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| Vapor Phase Chlorine Dioxide Control |
The AF26 sensor can measure Chlorine Dioxide concentration in gaseous process streams at percent levels to control the chlorine dioxide vapor concentrations in chlorine dioxide generators. Concentration measurement in the gas phase is also important in industrial air disinfection.
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| Tail Gas |
For environmental control, residual chlorine dioxide measurements in the vapor phase must be employed to reduce chlorine dioxide emissions and control vent-gas scrubbers. The optek AF26 typically measures chlorine dioxide concentrations ranging from PPM to the very low percent ranges.
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| Inline Chlorine Dioxide Monitoring - Pulp Bleaching Summary |
optek inline ClO2 analyzers installed at strategic locations in the process have helped a host of pulp mills to achieve greater control, increased production and decreased costs.
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The analyzers and sensors meet all requirements according to hazardous locations: II 2G EEx ia IIC/IIB T6/T5 Class I, Div. 1, Groups A, B, C, D. Due to the very aggressive nature of the chemical medium, all wetted parts of optek chlorine dioxide sensors are manufactured from corrosion-resistant materials, such as titanium and sapphire.
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Special NIST traceable filters with absorption characteristics equivalent to a selected chlorine dioxide concentration allow users to validate the analyzer without process intrusion, making validation quick, simple and safe.
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Besides the pulp and paper industry, Chlorine Dioxide is used extensively as a disinfectant agent for industrial water and wastewater treatment, food processing and the treatment of effluents for environmental control. Applications for chlorine dioxide in industrial air cleaning and medical waste disinfection are also becoming more common.
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