Real-Time Process Monitoring in the Pulp and Paper Industry Using Raman Spectroscopy

Competence Center CHASE
Mar 17
3 min read

Updated: Mar 19

Closing the loop in pulp and paper production with Raman spectroscopy

Chemical recovery is one of the most critical processes in the pulp and paper industry. The chemicals used to dissolve lignin from wood must be recovered efficiently to ensure both economic viability and environmental sustainability.

Improving this recovery process can significantly increase resource efficiency, operational stability, and overall production performance. A research collaboration involving CHASE has demonstrated how advanced process analytics can help solve long-standing challenges in chemical recovery. By implementing Raman spectroscopy directly in an industrial environment, researchers showed that real-time monitoring of key process parameters is possible even under extremely demanding conditions.

The challenge: harsh process environments

In sulfite pulp production, magnesium bisulfite is a key component of the cooking liquor used to dissolve lignin from wood. After the cooking process, the spent liquor must be treated to recover the chemicals involved. This recovery step includes the incineration of the spent liquor and the subsequent recovery of sulfur dioxide from hot flue gases using a cascade of Venturi scrubbers.

At the same time, magnesium oxide is washed and hydrated to form a magnesium hydroxide slurry. When the flue gas interacts with this slurry, the magnesium bisulfite cooking liquor is regenerated in a two-step reaction process.

However, the process conditions inside the scrubbers are extremely challenging. Temperatures exceed 60 °C, the pH ranges between 4 and 7, and the system contains highly concentrated ionic solutions. These conditions can lead to the formation of insoluble salts, which reduce recovery efficiency, increase maintenance requirements, and may even cause unplanned plant shutdowns.

To address these issues, a better understanding of the process chemistry and its dynamics is essential.

Raman spectroscopy for real-time process monitoring

In the project, Raman spectroscopy was investigated as a non-destructive, in-situ analytical tool to monitor the chemical composition inside the Venturi scrubbers. The goal was to bridge the gap between thermodynamic process modelling and the real industrial process.

Raman spectroscopy provides detailed spectral information about the chemical components present in the process medium. Combined with multivariate regression models, these spectra can be translated into important process parameters.

The system was able to monitor key variables such as free SO₂, total SO₂, and monosulfite concentrations in real time. These parameters are crucial for effective process control but are typically determined through manual sampling and laboratory titration.

Demonstrating industrial feasibility

One of the key challenges in process analytics is that many measurement probes cannot withstand the harsh conditions found in industrial processes. Even robust sensors often degrade quickly when exposed to high temperatures, aggressive chemical environments, and complex multiphase systems.

Despite these challenges, the project successfully demonstrated that Raman spectroscopy can be applied as a viable online Process Analytical Technology (PAT) tool in this demanding environment.

Measurements collected along the cascade of Venturi scrubbers revealed concentration profiles that closely matched the results obtained from conventional off-line titration methods. This strong agreement confirmed the reliability of the spectroscopic approach.

The method was then implemented in test operation directly in a Venturi scrubber at the Sappi mill in Gratkorn, Austria. The results clearly demonstrated the potential of Raman spectroscopy as an efficient tool for process monitoring and optimization.

Improving efficiency and reliability

Accurate real-time monitoring enables operators to better control the chemical recovery process. By observing changes in key parameters immediately, it becomes possible to prevent the formation of insoluble salts and maintain optimal operating conditions.

This leads to several important benefits:

improved chemical recovery efficiency
reduced loss of valuable process chemicals
fewer maintenance interventions
lower risk of unplanned downtime
more stable plant operation

The successful demonstration of Raman spectroscopy in this application highlights the potential of advanced process analytical technologies to support more efficient and sustainable industrial processes.