Key points
Can chemical dosing accuracy contribute to reductions in water usage, energy use and greenhouse gas emissions in the papermaking industry?
Yes, is the answer, says Rens Geskus, Group Sector Business Development Manager at Watson-Marlow Fluid Technology Group; the secret to success is the choice of pumping technology.
​Process problems
​Papermaking uses large volumes of water, 90% of which is used for cooling and 10% as process water. To make paper, fibre is first mixed with water at the stock preparation stage of the process in order to create pulp.
This step is followed by forming and dewatering stages, where vacuum is applied to drain the water. Any water that remains in the product is evaporated in the drying section, most often by gas-heated ovens or steam-heated drying cylinders.
Drying consumes most of the energy in the manufacturing process, and contributes 68% of the total greenhouse gas emissions for the sector. However, better performance at the dewatering stage can deliver the potential for less water to be heated and evaporated.
To enhance the benefits that dewatering brings, additives are introduced that help increase the drainage properties of the paper fibres, while the calcium carbonate content of the raw material also has a significant influence on the dewatering process.
In order to evaluate the feasibility of this thinking, comprehensive trials took place at Huhtamaki OY, a Finnish multinational and global leader in moulded-fibre technology for food and drink packaging. Full scale industrial trials were undertaken at the company’s central research facility in the Netherlands.
The lime factor
At Huhtamaki, an important aspect of production is to ensure neutral pH of the paper pulp through the addition of lime. Introducing lime contributes to lower bacterial growth and decreases the presence of fatty acids in the pulp, which in turn translates into less slime in the system and lower odour of the end product.
The addition of lime also helps to counter a further issue, in that closed-loop water systems, common in paper manufacturing, struggle with a high calcium load. Water evaporates at the drying section, but calcium remains in the system, causing scale formation.
Introducing lime at the wet end means that the calcium will be enclosed in the paper floc (substrate). As a result, the calcium load will be lowered as it leaves the manufacturing process via the end product. Dewatering is also enhanced, resulting in lower drying section temperatures and reduced gas consumption.
Although the benefits of adding lime are numerous, there is a caveat. Lime is challenging to work with, largely because it tends to settle, forming lumps in suspension. Diaphragm pumps, which are often used to dose lime, can cause difficulties as ancillary foot valves tend to get blocked by lime dispersion.
Even after removing the foot valve and pressure-holding valve on the discharge side, diaphragm pumps do not deliver constant flow. As the accuracy of lime dosing has a significant impact on process performance and end-product quality, it is critical to select the optimal pump for the task.
Qdos pump provides the solution for chemical dosing accuracy
​For the trial at Huhtamaki, our ​Qdos 120 peristaltic chemical dosing pump was used to deliver lime prior to dewatering the paper pulp. Qdos pumps deliver accurate flow, despite changing process conditions, and do not require additional ancillaries like inlet and pressure-holding valves. This factor lowers the cost of maintenance and process downtime due to clogged valves.
Huhtamaki recognised the issues with diaphragm pumps some years previously, and actually participated in the development of Qdos peristaltic dosing pumps. The trial project was instigated to optimise the calcium load in its system and reduce CO2 emissions. By accurately dosing lime, Huhtamaki succeeded in achieving complete control over its pH levels.
The trial proved that the dewatering stage was far more effective than it had been, resulting in an increase of 2.5% dry solid content at the entrance of the drying section. Moreover, less water required heating to 100°C for evaporation.
There is a direct correlation here between the dry solid content at the entrance to the oven and the required oven temperature. By accurately metering lime to the pulp, Huhtamaki lowered the average oven temperature over the course of the trial.
Chemical Dosing Accuracy Offers Four-week ROI
Due to the nature of the operation at Huhtamaki (24/7 for more than 350 days a year), the relatively modest temperature decrease had a significant impact. A lowering in oven temperature of 15°C correlates to a 3% reduction in gas consumption and, consequently, a 3% reduction in greenhouse gas emissions.
These outcomes equate to a reduction of 18,000kg in carbon emissions per oven, per year, delivering a return on investment in just four weeks.
Additional benefits associated with the addition of lime include: higher machine throughput (estimated at 5%) due to lower equipment pollution levels; less product shrinkage thanks to lower oven temperatures; and lower product odour and odour emissions.
As can be seen, since the adoption of Qdos chemical dosing pump technology, several process enhancements have been achieved at Huhtamaki, delivering increased efficiency, improved product quality, safer maintenance and significant savings on chemical usage.
