In paper and pulp production, both the acidic sulfite and the alkaline sulfate processes are used to break down the wood. The wood fibers are dissolved from the natural material and separated from other components, such as lignin. For ecological and economic reasons, the chemicals used must be recycled.
The aim is to regenerate caustic soda (NaOH) and sodium sulfide (Na2S) and the generation of energy, heat, and steam, while minimizing environmental damage.
The lignin-containing cooking liquor (black liquor) produced during the pulping process is concentrated and burned. In addition to energy and steam generation, a melt forms at the bottom of the boiler, which is regenerated in several recycling stages. The green liquor resulting from the water and melt is converted into NaOH-containing white liquor by adding lime in the causticizing process, which is then used again for lignin splitting.
Through the Liquisonic® Measurement technology The green liquor treatment can be monitored inline at various points, and the concentration for the subsequent causticizing can be determined.
Applications in the paper and pulp industry place high demands on all system components. Our application specialists design the sensors to meet the expected conditions. The robust measurement method, which operates without moving parts and seals, impresses with unmatched runtimes. The sonic velocity sensors withstand temperatures up to 130°C, depending on the design. Highly resistant materials and coatings are used to ensure long runtimes.
Through the online analysis of green liquor concentrations using Liquisonic® Measurement technology lime can be precisely dosed. Furthermore, the individual process stages, such as the dilution of the melt to green liquor and its storage and clarification, can be continuously monitored and automatically controlled. Time delays caused by sampling and laboratory analyses are eliminated. This efficient process control leads to an improvement in quality.
With Liquisonic®, the solution from Sensotech, leads to an increase in product quality because the relevant key figures in the green liquor process are known at all times. Measurements are not taken at specific points, as in a sample extraction, but rather every second over the entire process duration. Thus, there is no time delay between the measurement and the result, which means that deviations can be reacted to very quickly, for example, in the form of additional dosing.
In addition to saving time, inline measurement has another advantage: the influence of human error is minimized because the measurement values can be integrated into the process control system. The additional dosing of lime becomes more accurate through automation, making the process more efficient. This reduces lime consumption, leading to cost reduction. Multiple sample extractions and waiting for concentration measurement results are a thing of the past.
Furthermore, the Liquisonic® sensor is not affected by deposited particles or impurities. The Liquisonic® sensors are particularly robust due to the special polymer coating, which allows working with aggressive and corrosive media. After installation and adjustment of the measurement system to the process, no cleaning, maintenance, or calibrations are required!
Application
The starting material in lye recycling is the black liquor in the recovery boiler. The melt formed during combustion sedimentates and must be diluted with water or thin white liquor. The resulting green liquor is Na2S and Na2CO3-containing, hot and is often stored in a separate tank. It is then clarified and stored in a green liquor collection tank. The prepared green liquor is reacted with quicklime in the causticizing plant to form NaOH.
Through the second-fast analysis of the Liquisonic® sensors individual process steps, such as the dilution of the melt to green liquor and its storage and clarification, can be permanently monitored and controlled. This efficient process control thus serves both quality improvement and the economic regulation of lime addition.
Installation of the LiquiSonic® Measurement technology in the green liquor process
The Liquisonic® Pipe sensors are directly installed in the pipes transporting green liquor. The PFA coating ensures long-term measurement accuracy despite alkaline, hot process liquids and tendency to deposit. The Liquisonic® Controller 30 can be connected to up to 4 sensors to monitor the green liquor simultaneously at multiple measuring points:
- after the dissolving tank
- before clarification
- after clarification or before causticizing
Typical measurement ranges:
Concentration: 0 - 30 m% or 950 - 1200g/l
Temperature: 5 - 95 °C
