Process refractometers are optical instruments that measure the refractive index of a substance to determine its concentration. They are widely used in the pulp and paper industry, particularly in the Kraft pulping process, to measure the concentration of essential chemicals in white, green, brown and black liquors.
The kraft process, which employs sodium hydroxide (NaOH) and sodium sulfide (Na2S) to convert wood into pulp, is the predominant pulping technique in the pulp and paper sector. This method is responsible for an annual production of approximately 130 million tons of kraft pulp worldwide, contributing to two-thirds of global virgin pulp output and over 90% of chemical pulp. Kraft pulp's superior strength, the process's compatibility with nearly all types of softwood and hardwood, and its economic benefits stemming from a high chemical recovery efficiency of about 97% make the kraft process more favorable than alternative pulping methods.
Process refractometers apply in the following steps of the Kraft pulping process:
- Green Liquor Control: After burning the black liquor, the resulting green liquor contains sodium carbonate (Na2CO3) and sodium sulfide (Na2S). Refractometers measure the green liquor's concentration, which helps optimize the causticizing process. This process involves converting sodium carbonate to sodium hydroxide by adding lime (calcium oxide, CaO). Accurate measurement of green liquor concentration ensures the right amount of lime is added, thus optimizing the efficiency of the causticizing process and reducing waste.
- Brown Liquor Control: After the causticizing process, the remaining liquor, called "brown liquor," primarily contains sodium hydroxide (NaOH) and sodium sulfide (Na2S). The concentration of brown liquor is critical for achieving the desired pulp quality and yield. Refractometers help maintain the correct concentration of brown liquor, ensuring consistent pulp quality and minimizing chemical waste.
- Black Liquor Evaporation: Black liquor is concentrated through evaporation to increase its solids content before being burned in the recovery boiler. Process refractometers monitor the concentration of the black liquor, ensuring optimal evaporation rates and preventing potential issues in the recovery boiler.
- White Liquor Quality Control: Refractometers can also be used to monitor the concentration of white liquor, helping maintain the desired alkalinity and sulfide levels, directly affecting the cooking process and pulp quality.
- Recirculation and Monitoring: Process refractometers can be installed at various points in the Kraft pulping process, such as in recirculation lines, to monitor liquor concentrations continuously and adjust process parameters accordingly.
Process refractometers play a crucial role in Kraft pulping by monitoring and controlling the concentrations of green, brown, and black liquors. Their accurate measurements ensure the efficient use of chemicals, minimize waste and help maintain consistent pulp quality.
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