Experimental investigation and statistical analysis of pulp vacuum dewatering
Date
2024-03
Authors
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Journal ISSN
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Publisher
Stellenbosch : Stellenbosch University
Abstract
ENGLISH ABSTRACT: Energy consumption in the forming section of a paper machine is high due to the process requirement for vacuum. Thus, making it imperative to explore various energy conservation opportunities in this early stage of water removal. Most of the vacuum generated is utilised in the high vacuum dewatering zone of the section to remove filtrate from pulp slurries at various durations or dwell times. Therefore, the effects of vacuum pressure and dwell time on the achievable solids content of pulp samples refined at different levels of energy were studied. A laboratory suction box was constructed to simulate pulsation effects experienced by pulp slurries in the forming section of paper machines during high vacuum dewatering. A rotating spindle with slots each with a length the same as the diameter of the sample holder was utilised to generate suction pulses at speeds as high as 400 m/min. A pressure profile was generated from data obtained during experimental investigation and compared to the pressure profile from the high vacuum zone of a paper machine. It was determined that the pulses generated by the setup adequately replicated the performance of suction boxes in industrial formers. The dynamic setup could generate suction pulses with a duration of only 6 ms. Three pulp types that each represent a unique pulping process in the South African pulp and paper industry were tested. These included bleached hardwood, mechanical or groundwood and pure K3 (New corrugated kraft waste) recycled pulp. Sample characterisation of the pulps was done before exposure to vacuum, to obtain pulp fibre morphology and analyse their water-fibre interactions, i.e., freeness and the water retention value. Statistical analysis of the fibre characterisation results through ANOVA and ANCOVA revealed that an increase in the refining energy appreciably affected freeness and water retention value. Pearson’s coefficients further revealed that refining had a positive relationship with the water retention value whereas the opposite was true for pulp freeness or drainability. ANCOVA of pulp morphology with respect to virgin pulp drainability and water retention showed that only the shape factor and fibre length was significant for bleached hardwood pulp’s water retention and freeness. For mechanical pulp, only fines content and fibre length affected drainability and water retention. High vacuum dewatering results of the characterised pulps revealed a dry matter/ concentration plateau for dwell times ranging from 30 to 135 ms. However, there was in increase in the dry matter of all pulp types at a dwell time of 250 ms. Diminishing returns exponential trendlines were fitted into the vacuum dewatering experimental data to better characterise and predict the pulp dry matter or concentration of the three pulps. The trendlines effectively showed the dewatering behaviour of pulp samples where a plateau in the predicted pulp concentration or dry matter was observed. This is especially true for dwell times ranging from 30 to 135 ms. Statistical analysis through ANOVA showed that vacuum pressure crucially affects pulp dry matter. However, the linear regression analysis method could not establish an effect of the dwell time on dry matter. This does not signify its absence as the relationship is exponential, making it difficult to analyse through linear means. Lower vacuum pressures were associated with drier pulps. ANCOVA proved that water retention value and freeness had a statistically significant relationship with dewatering rate and plateau pulp concentration of pulps. Pulps with high drainability and water retention capacity performed better during vacuum dewatering, namely bleached hardwood, and recycled pulp.
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar.
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar.
Description
Thesis (MEng)--Stellenbosch University, 2024.