Factors influencing the drainage and drying of pulp

Abstract

Thermal drying of a pulp sheet on a paper machine is one of the most energy-intensive operations in the paper-making process (Ghosh, 2011). The drying section of a paper machine removes less than 20% of the total water eliminated from the wet sheet, but is responsible for 78% of the energy consumption of the paper machine (Karlsson, 2009 & Biermann, 1996). Any enhancement of the dewatering of the paper sheet in the forming and pressing sections, thereby sending a sheet of lower moisture content to the drying section, would result in significant energy savings in the drying of the paper. Typically, the Canadian Standard Freeness (or freeness) and the Water Retention Value (WRV) of pulp are used as a measurement for the dewatering ability of pulp on the wire of the paper machine (Hubbe, 2007). However, neither of these measurements was designed to predict the water removed from the wet sheet (TAPPI T227, 1999, Scallan & Carles, 1972). The factors that influence the dewatering of pulp on the forming fabric and in the pressing section of a paper machine have been the subject of many studies (Hubbe, 2007). The first objective of the current study was to compile a literature review to build an dewatering of the pulp sheet. Pliable fibres may conform to each other, resulting in a sealing effect that inhibits drainage. Mechanical and enzymatic refining may induce external and internal fibrillation of fibres, and the generation of fines in the pulp. Treating the pulp with an enzyme, specifically endoglucanase, may enhance the susceptibility of the fibres to development by mechanical refining. In the current study, the dewatering of pulp was analysed at pilot scale in order to test which pulp measurements best predicted its forming and pressing performance. A hardwood pulp, composed of Eucalyptus grandis, was refined in a pilot refiner and yielded samples over a range of refining energies. The conventional dewatering measurements, freeness and WRV, were performed on the pulp samples and related to the dewatering performance of the handsheets produced therefrom. The formability (water removed during forming) and pressibility (water removed during pressing) of the handsheets were also determined, and compared to the freeness and WRV of the pulps. It was found that the formability of a handsheet was a much stronger predictor of its dewatering performance than either the freeness or WRV of the pulp. Formability was likely the best predictor of handsheet dewatering because it was a direct measurement of the dewatering performance of a sheet on the forming fabric, whereas freeness and WRV both measured the dewatering of a thicker pad of pulp. The effect of the refining of pulp with an enzyme additive on its dewatering properties was also studied at pilot scale. An endoglucanase was incubated with a hardwood pulp, composed primarily of Eucalyptus grandis, and refined in a pilot refiner. The enzyme was added at four levels, including a control pulp with no enzyme. The dewatering properties of the handsheets formed from the pulp were tested and compared for each enzyme dosage. The morphology of fibres of each of the pulps was determined. It was found that increasing the enzyme dosage of the pulps resulted in the decrease of the dewatering ability of the handsheets. The number of fines in the pulp increased with increasing enzyme dosage. It was likely that fines were responsible for the restriction of pulp dewatering, as suggested by the choke-point hypothesis (Hubbe, 2007). Based on the extent of fibrillation and width of fibres, as well as the number of fines in the pulp, a model of the effect of the enzyme added on the dewatering of the pulp was proposed. Increasing the enzyme concentration in the pulp increased the susceptibility of the fibre to refining. An enzyme dosage of 100 g/t resulted in a more fibrillated fibre than found in the control pulp. However, the fibres of the pulp treated with the enzyme at a concentration of 200 g/t were less fibrillated, suggesting that the fibre surface was weakened to the extent that the fibrils were broke off during refining. The fibres of the pulp treated at 500 g/t were again more fibrillated, suggesting the occurrence of secondary fibrillation of the fibre at this high enzyme dosage. The fibres of the pulps treated at 500 and 200 g/t were also narrower, suggesting the loss of fibre material.