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2003; Sjöstrand 2020). The forming section draws fil- trate from pulp slurries through gravity and vacuum (Åslund 2008; Sjöstrand 2017, 2020). This i s fol- lowed by the press section where mechanical force is used to further dewater pulps (Adanur 2017). The last stage, referred to as the dryer section, utilises evapo- ration to achieve the desirable product (Sjöstrand 2017, 2020). Dewatering by a vacuum pulse is highly energy demanding, particularly in the high vacuum zone of the forming section (Åslund 2008). It would be beneficial to investigate ways of optimising vac- uum usage in the forming section to lower the overall energy consumption of the dewatering process. By assessing the outlet dryness achieved by pulp slurries, vacuum consumption in the forming section may be inferred. Fibre web formation through drainage of pulp sus- pension in the high vacuum dewatering zone of the for ming section of a paper machine has been studied by various authors (Åslund 2008; Pujara et al. 2008a, b; Rahman et al. 2018; Sjöstrand et al. 2020). In the first zone of the forming section of a paper machine, water is removed through low vacuum filtration to accelerate gravitational dewatering (Ramaswamy 2003). The paper product is subsequently exposed to higher levels of vacuum in the form of suction pulses to further induce dewatering of the fibre mat s in the second zone (Ramaswamy 2003); this process is referred to as high vacuum dewatering. High vac- uum dewatering occurs at pressures ranging between − 15 and − 40 kPa gauge, although vacuum pressures as low as − 65 and − 70 kPa gauge can be imple- mented at high-speed paper production (Åslund 2008; Åslund and Vomhoff 2008a). The vacuum pulses are achieved by employing slotted suction boxes that act as vacuum chambers to administer a pressure differ- ential over each slot as the wet paper product or pulp slurry moves over these boxes. This pressure dro p across the fibre mat triggers mechanisms by which water is removed, namely, fibre mat compression, and displacement by air (Åslund and Vomhoff 2008a). Web compression occurs due to the fibre mat deform- ing which draws the filtrate out (Åslund and Vom- hoff 2008b). Once the pressure differential exceeds the capillary pressure in the fibre mat, air penetrates the mat to displace the existing water (Ramaswamy 2003; Åslund and Vomhoff 2008b). Fibre mat s ma y expand after compression, during which removed filtrate may re-enter t he mat, a phenomenon called
rewetting (Åslund and Vomhoff 2008a; Sjöstrand 2017). Although it is not an explicit vacuum dewater- ing mechanism, it is often included in the discussion as it affects the overall dryness of fibre mats. The phe- nomenon is highly undesirable as it reduces the dry- ness of pulp mats which will require more energy to counteract in the previously mentioned subsequent dewatering stages, namely, the press and dryer sec- tion. The combined duration of individual pulses i s referred to as the dwell time, which is dependent on paper speed through the machine and the slot dimen- sion. Slurries are transported across suction boxes by a filter medium called the forming fabric, whic h i s permeable to water and air while retaining valuabl e fibres and additives (Bajpai 2018). Vacuum pressure and dwell time are usually the focal point of stud- ies that aim to predict conditions in the high vacuum dewatering zone. Experimental simulations of the high vacuu m dewatering process can be categorised into stati c and dynamic setups. Montgomery (2008) modi- fied a conventional hand sheet former to study th e effect of vacuum dewatering of pulp on retention and filler migration. The simple filtration setu p was modified to allow the collection of data dur- ing the mat formation process, thus providing a static setup to determine the impact of different vacuum pressures on the retention of fibres, fines , and filler. However, such static setups are incapa- ble of accurately simulating the vacuum pulsation effects that are applied in industrial paper machines. More complicated dynamic setups have been previ- ously explored to better simulate the vacuum pul- sation effects, achieve more accurate dwell times o r paper machine speeds and attain fibre orientations in pulp slurries, comparable to industrial formers . For example, Räisänen et al. (1995) reported on th e application of a pilot-scale moving belt drainag e tester in which pulsation effects were achieved b y using a slotted conveyor belt rotating around a suc- tion box that supplied vacuum to hand sheets. Th e authors developed a mathematical model by modi- fying a wet press model by Jönsson and Jönsso n (1992) in which they replaced a press pulse with a suction pulse to better explain the results obtained. It was concluded that a vacuum pulse had a more prominent effect on the dryness of pulp when com- pared to airflow where experimental data was com- parable with predictions provided by the model.
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