Refining control helps mills improve quality while cutting energy costs
Mixed furnish often incorporates widely varying fiber properties, so that maintaining quality can become a hit-or-miss affair. BTG’s freeness transmitter not only helps take the guesswork out of quality assurance, but better control of the refiner load can translate into substantial savings.
While there are various possibilities for specifying and measuring fiber properties, the instrument currently favored by the majority of mills measures the freeness, or Shopper-Riegler, value.
A freeness transmitter calculates the dewatering speed of a fixed amount of fibers, generating a value which most mills then use to control the refiner load and adjust the fiber properties. Mills which have yet to implement such a control loop face difficult control of the refiner, which has to handle variations originating from the raw material. When these variations cannot be counterbalanced, variations in paper quality result.
Some mills try to counteract this problem by implementing other changes on the paper machine. Online control of freeness with continual real-time monitoring of values is one solution. But when a freeness instrument is used for refiner load control, constant fiber quality can be quickly achieved. That frees up papermakers to focus on other parts of the papermaking process.
Implementing a tight refiner control loop also has proven benefits in terms of savings in money and energy. Take the example illustrated below (see Figure 1), which shows a machine running at a speed of 1000 m/min and producing 33 t/h coated paper. It receives its raw material over a hardwood and softwood fiberline; the variation in form and structure of the different fibers is the reason for separate refining lines. Implementing freeness in combination with specific energy control helps achieve stable refining results independent of variation in fiber grades.
The benefits of reduced energy demand (and hence reduced costs and emissions) is persuading the world’s leading mills to make freeness control loops a standard part of their production process.
In the thick stock area after the refiner, a BTG DRT-5500 Freeness Inline transmitter has been installed to implement freeness in combination with specific energy control. The return on investment (ROI) calculation is based on the installation in the long fiber section.
Figure 1:
BTG’s DRT-5500 Freeness Inline helps mills manage variations in furnish, for more stable, predictable performance and substantial savings on energy.
The specific energy for this stage is 160 kWh/t. Calculations based on one year at 93% capacity usage gives a total energy consumption of 7680 MWh/y. Taking into account an energy cost of EUR 45/MWh, this translates into a yearly cost of EUR 345,600. With a reduced freeness variation of ±1°SR instead of ±3°SR, energy costs can be reduced by 86,000 EUR/y – a very welcome saving for most mills.
Some years ago, when the importance of saving energy was not yet high on the agenda, mills generally implemented freeness control to take advantage of other benefits – for example, improved drainage on the paper machine, and improved sheet properties such as formation and strength. Bulk in the final paper is an important factor, and good drainage properties decrease line pressure before the drying section, which in turn reduces specific steam consumption. Improved drainage also allows potentially increased machine speed.
These benefits are undoubtedly still important. But in a highly competitive, environmentally-sensitive market, the added benefit of reducing costs and cutting energy demand (and hence emissions) in paper and board production is a compelling bonus that is persuading most of the world’s leading mills to make freeness control loops part of their production process.