Digital instrumentation key to water conservation in shift to green production
OERLIKON, SWITZERLAND - While the paper industry has done well reducing its water footprint, there still remains more that can be done in today’s age of resource conservation.
Digital pH sensor and analyzer technologies are the key to unlocking new levels of performance, efficiency and quality, while helping the pulp and paper industry to do its part in reducing its water usage and waste.
Out of all the industries, it is known that the pulp and paper industry is one of the heaviest users of water, with a single A4 sheet production requiring up to 20 litres of water.
In addition to the water that goes into the process (influent), special attention also needs to be given to the water that comes out of the process, (effluent). Indeed, in 2015, the Canadian pulp and paper industry accounted for roughly five percent of all industrial waste water released into the environment.
This is clearly not sustainable, particularly against the backdrop of the wider shift towards greener production techniques across all industries. Efficient and effective water management must therefore be adopted across the industry to reduce consumption and help with conservation of the environment.
Achieving this in a cost-effective manner is a challenge for the industry, but it is one that is being met with the advancement of digital sensor technologies which enable more precise measurement of water consumption than ever before, and better control of variables such as pH.
Accurate pH control critical
In the paper making industry, accurate pH control is a crucial parameter which is utilized in a wide range of applications with impacts ranging from paper quality, chemical usage, corrosion control and environmental compliance. Incorrect pH control is therefore one of the more important parameters in the paper making process.
Looking at the bleaching process in particular, this process accounts for the majority of water use in the paper industry with a significant dependency on pH values to ensure paper quality and minimize chemical waste. Low pH levels are required to bleach pulp with chlorine dioxide, while high pH levels are then needed to dissolve and extract the reacted lignin. This means that careful control is required, based on feedback from sensors to detect the effectiveness of the chemical reactions.
Bleach plants in particular present extremely challenging pH conditions. The E stages are often at 12 pH and higher, while temperatures can reach almost 100°C. while D stages normally range between 3.5 and 4 pH at temperatures typically between 60°C and 80°C.
Tall oil extraction is another application that requires careful pH management in order to optimize the process and ensure effective cleaning, while gas scrubbers require pH measurement to minimize the amount of chemicals required while preventing corrosion.
Digitalizing pH measurement
The key to managing pH levels is in effective measurement. Digital sensors can provide continuous real-time data to inform the operator without having to pause or interfere with the process. Such sensors typically incorporate two electrodes: a measurement electrode and a reference electrode. The water or solution to be measured by the electrodes generates, a measurable voltage directly proportional to the pH level of the liquid, thereby providing the pH value.
The challenging conditions typically found in pulp and paper plants, and in particular within the bleaching process, means that sensors must be extremely rugged. However, this is a challenge, as pH sensors need to balance responsiveness with durability which can often be inversely proportional to one another. Sensor lifetimes are improving as the technology develops, but one challenge faced in the industry is endof- life predictions as sensors can fail due to gradual poisoning of the reference electrode, or the glass electrode can break suddenly.
To extend service life, pH sensor manufacturers such as ABB now include advanced diagnostics to allow operators to receive early warning of any potential issues such as electrode poisoning. This allows the problem to be dealt with before the device risks failure, leading to lower maintenance costs.
Better pH measurement means less wastage of chemicals and reagents, while improving quality and consistency of final products. It can also prolong the operational lifetime of machinery, and reduce leakage caused by corrosion. Ultimately both influent and effluent volumes can be reduced, saving vast amounts of water throughout the process, saving money while reducing environmental impact.