12 ABB


around 1975. One of the first was an electro-optical shive analyser, suitable for the optimisation of screen rooms. A few years later, systems for monitoring refining in mechanical pulping that combined measurements of optical fibre length, an optical shive analyser and a freeness measurement were developed. Ways of measuring average fibre length and radius for online purposes were invented in the 1980s.


In the last 10 years, two major


improvements have taken place. First, high-resolution cameras have delivered a much finer level of detail, enabling the monitoring of fibre fibrillation (as shown in Figure 1). Second, the reliability of the equipment for online sampling and measurement has been greatly improved. The first online instruments were high-maintenance units, often requiring a few hours per day to keep them running smoothly. Modern online fibre sampling and measurement devices, however, have been designed to avoid the build-up of dirt, and an automatic cleaning cycle is incorporated to ensure that plugging does not occur. The only maintenance now required is a weekly or bi-weekly check-up and a maintenance service once or twice per year.


Refiner control Before the fibres reach the paper machine, it is vital that refining generates enough fibrillation and fine material to create proper bonding surfaces between the fibres as they are dried in the process. Too much refining generates additional energy costs (electricity to refiners, drives and vacuum pumps) and slows down the production speed (meaning lost profit). Too little refining


July/August 2021


Pulp Paper & Logistics


Figure 4: Shape factor of typical pulp fibres


generates lower-strength paper that could lead to web breaks at the paper machine or the converting plant. The strength may need to be


increased by other means such as using chemical additives or virgin kraft as a reinforcing pulp, which are more expensive than the cost of refining. Thus, the use of online freeness to control refining can lead to reductions in production costs, web breaks, culled paper and customer claims. An example of such use can


be found at a paper mill in Chile whose ultimate goal was to control the amount of refining of the incoming pulp to meet strength specifications. Located on the heavily-forested Pacific coast near Concepción, FPC Papeles makes packaging grades of paper from a combination of post-consumer and pre-consumer recovered fibre acquired from 27 cities and five customers. They


wanted to improve their ability to adapt their process for the variability of incoming fibres, to both stabilise product quality and reduce costs. Before acquiring an online pulp


freeness sensor, the operators responded to variations in incoming fibre quality by manually modifying the refining load based on lab measurements of pre- and post-refiner freeness taken every two hours. If the strength of the paper, measured once per reel (about every 45 minutes) was insufficient, the refining energy was increased. If that was not successful, other methods were tried, such as adding chemicals, using more pre-consumer double-lined kraft (DLK) or finally, downgrading it to a lower- strength product. Having previously upgraded


their refiners to enable automatic modification of their disk gap based on fibre mass flow, FPC


now needed a way to determine when to open and close the gaps to provide the required refining load, avoiding the delay between lab measurements of freeness. The long delay between lab freeness measurements was masking fibre variation, making it impossible to respond to changes in a timely fashion. In early 2018, FPC Papeles


installed an online freeness measurement system equipped with two sampling devices. This allowed operators to measure freeness every six to seven minutes at the output of the mill’s two refining circuits and more closely monitor the refining process, as shown in Figure 3. Following a two-month


validation period, during which the online and lab readings were regularly compared and correlated, the operators placed their full trust in the new system and began to manually adjust the refiner load based on online


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