New insights into dispersed air effects in brownstock washing

A laboratory washing cell was used to visualize pad formation on the surface of a simulated brown-stock washer drum submerged in a washer vat. Large dispersed air bubbles (diameter>10 mm), obtained in the presence of defoamer or the absence of black liquor, passed through the fiber pad, having little influence on the water flux. By contrast, small bubbles (<2 mm) were trapped in the pad, lowering filtration fluxes by as much as a factor of 100. The detrimental effects of small bubbles on drainage were diminished by defoameter addition, which caused the small bubbles to coalesce and to escape through the pad.