Comparison of different scraper designs for crossflow filtration with rotating discs and cake limitation by scrapers

Abstract

Abstract Results from filtration experiments comparing the filter cake formation of different scraper designs during multiple-pass operation of the continuous pressure filter at laboratory scale are to be presented. The main interest in this investigation was to clarify the influence of different scraper designs on the filter cake growth depending on the rotation speed of the filter discs. The influence of different scraper designs on flow conditions between the filter discs and the action of the scrapers was also examined. The results of the filtration experiments showed, that the formation of the filter cake essentially depends on the rotation speed of the filter discs. The main mechanism during the formation of the filter cake of finely dispersed solids is the effect of cake thickness limitation by turbulence eddies. The dominating mechanism during the formation of the filter cake of solids with a wide spectrum will be the effect of particle segregation. In experimental studies the comparison of three different scraper designs (straight and curved scraper with different scraper profiles) was done in order to investigate the influence of the scraper shape to the formation of the filter cake at different rotation speeds and to achieve a basis for scraper design optimization. Summing up, a scraper profile with increased overflow velocities (e.g. the ‘plough 120° scraper’-profile) in a curved form (in order to improve the normal flow velocity at the inside part of the filter discs) would improve the filtration behaviour compared to the straight, flat scraper most efficiently.