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UBC Theses and Dissertations

Understanding rapid dewatering of cellulose fibre suspensions Paterson, Daniel Thomas


Rapid dewatering of cellulose fibre suspensions is a fundamental process in many unit operations in the production of pulp and paper. Understanding dewatering behaviour can be applied to optimizing designs of industrial equipment. In this project, we assess the suitability of a well-established modeling approach, referred to as the base model, at capturing the one dimensional dewatering behaviour of cellulose fibre suspensions seen experimentally. This modeling approach requires two closure relationships determined experimentally, i.e. compressive yield stress and permeability. Experimental equipment has been designed, constructed, and operated to obtain the closure relationships and collect dewatering results for validation of the model. Two experimental techniques, with close agreement, have been developed for the collection of compressive yield stress. Permeability results are obtained through Darcian permeation experiments. Two approaches, neglecting and accounting for flow induced compaction, were developed. Results were found to fall within values seen in the literature. The base model provided good representation of ideal nylon fibre suspension trials. These solid fibres are representative of the base models constitutive equation for an infinite solid phase rearrangement rate constant. The base model poorly represents the cellulose fibre suspensions' dewatering behaviour. The suggested source of discrepancy is the further dynamic due to the dewatering of the individual porous cellulose fibres which results in a finite solid phase rearrangement rate constant. The base model is expanded upon in hopes of capturing this rate dependent behaviour. This extended model, with the determined closure relationships, captured load versus solid volume fraction profiles at varying dewatering rates better than the base model for cellulose fibre suspensions. Further improvements in representation were seen through close representation of the solid phase velocity profiles found experimentally during dewatering. Various cellulose fibre suspensions were investigated to begin a catalog of different dewatering behaviours seen through variations in pulp production variables. Investigations included varying fibre species, pulping processes, levels of low consistency refining, and impacts of dewatering chemical additives.

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