Gas removal from the papermaking process is currently a standard practice, whereas purificationof the internal water circulation has become common only recently. Both unit processes haveprogressed greatly during recent decades and new concepts are constantly being developed. Theaim of this thesis was to analyse the efficiency and applicability of a channel flow designintroduced by Metso for passive white water deaeration and to study the dynamics of passivebubbly gas removal. In addition, separation of the detrimental process water components byselective flotation during deaeration was studied to add further functionality to the channel flowdesign.
Turbulent mixing at the flow discharge and the consequent air entrainment were seen to limitthe gas separation efficiency. Also, the properties of different white waters notably affect theirdeaeration through viscous forces, the concentration of surface active components and bubble-particle interactions. Thus similar levels of gas separation cannot be achieved with all processwaters. The analysis showed that the drag of small microbubbles is mostly caused by hydrophobiccontamination and the dispersed particles that readily attach to the bubbles. Correlations werederived based on experimental data to provide new information on the drag force experienced bysmall bubbles in white waters.
Chemically unaided flotation of white water in the channel flow was shown to be efficient inseparating hydrophobic contaminants that have adverse effects on paper machine production andproduct quality. Both good reductions in contaminant content and high selectivity in their removalwere achieved. Channel flow with an overflow can be considered well suited for the first stage offroth separation, while further treatment of the channel flow reject may consist of a secondaryflotation or other process that enables the recirculation of fines and fillers. Although a certain levelof losses of fines and fillers must be expected, substantial fraction of these solid components canbe returned to the process stream.
The proposed multifunctional process, channel flow deaeration and frothing of white water,was seen to be straightforward, economical and feasible while also providing benefits in terms oftotal process efficiency that are not delivered by any current process scheme. The experimentalparameters presented here regarding bubble dynamics and flotation efficiency can be used toachieve better models of these processes.
