Cellulose (2025) 32:1835–1850
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Mitchell et al. (2002) improved the design of th e moving belt drainage tester by including a nove l mixer that created turbulence in pulp slushes usin g compressed air, which pushed out fine jets of wate r for improved fibre orientation. In an alternative mechanical design, Pujara et al. (2008a, b) simu- lated pulsations through a rotating disc with a slo t width of 0.0127 m whereby the dwell time was con- trolled by varying its rotational speed. Regression analysis of their results revealed a linear depend- ency on the airflow and sheet grammage by th e achievable solids content whereby a plateau is even- tually reached for all vacuum pressures at maximu m dwell time; the plateau of pulp dewatering rate is a well-known phenomenon (Ramaswamy 2003). Gra- nevald et al. (2004) utilised a similar configuration, employing a linearly driven plate with a single slo t beneath a sample holder to administer a pulse. Thei r research focused on the effect of characteristics of forming fabrics on the outlet solids concentra- tion of fibre mats during vacuum dewatering. The y discovered that rewetting was heavily influenced by fabric parameters such as calliper, void volum e and air permeability. The setup was also used in a subsequent study by Nilsson (2014), in which the y concluded that air mass flux is directly related t o vacuum level, especially for low-grammage pul p sheets. It would be beneficial to incorporate th e effects of fibre structure when empirically analysing pulp slurry drainage behaviour during high vacuu m dewatering by a dynamic experimental simulation like those previously discussed. Previously developed setups were evaluated to choose the most appropriate dynamic configuration for the required conditions in the current study. These include the moving belt drainage tester by Räisänen et al. (1995) which utilised a perforated belt to cre- ate pulses, a method that can be prone to friction as the belt loosens over time with usage (Pujara et al. 2008a, b). The formation of the sheet in this setup was achieved on the drainage tester, thereby requir- ing large storage of removed filtrate, as sheet forma- tion is achieved at a very low pulp concentration o r dilute state. This could also lead to excessive air leak- ag e into the storage unit as it was prone to flooding (Mitchell et al. 2002). The setup designed by Pujara et al. (2008a, b) was also evaluated. This design requires a complicated sealing mechanism to pre- vent excessive friction between the rotating disc and
vacuum chamber, which presents significant chal- lenges in fabrication and is likely to have limited robustness against failure. The study aims to develop a novel laboratory suc- tion box layout that can achieve the dynamic condi- tions that exist in industrial formers, thus enabling the evaluation of the vacuum dewatering behaviours of different pulp types during board manufacturing. It is hypothesised that the novel dynamic setup presented will effectively simulate the rapid pulsated vacuum exposures to pulps, much like that exhibited in suc- tion boxes, located in the high vacuum dewatering zone of the forming section of paper machines. The configuration is expected to improve the data collec- tion process during vacuum dewatering of refined pulps to enhance the understanding of water-fibre interactions during the filtrate removal process. The current study presents a design that ca n pro- duce pulses by employing a spindle with five slots encased in a vacuum chamber. The chamber had a single slot that would align with each spindle slot to administer vacuum pulses to hand sheets. The configuration simplified the design of a mechanical seal, a s there was no direct contact to cause friction which was the case in the design utilised by Pujara et al. (2008a, b). Early stages of gravity drainage were achieved by using a hand sheet former thus prevent- ing the need for large storage needed for removed fil- trate. The novel configuration could achieve pulses in the order of a few milliseconds where machine speeds of up to 400 m/min could be simulated at vacuu m pressures as low as − 60 kPa gauge. Three vacuum reservoirs were included to reduce air leakage during dewatering. The novel dynamic configuration criti- cally contributed towards enabling the collection of dewatering data of pulps under various vacuum dewa- tering conditions by simulating the forming section in industrial formers. The relationships that may exist between the structural makeup of individual fibres in pulps, their behaviour in slurries and the achievable mass con- centration or consistency during vacuum dewater- ing by suction boxes in the high vacuum dewatering zone were analysed. There are limited studies where authors statistically relate the changes in water-fibre interactions and morphology of pulps due to refin- ing, to their drainage behaviour in the high vacuum zone when developing correlations to predict solids retention and improve energy efficiency. This issue is
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