Demonstration of ”creep” during filtration
Advancing the chemical engineering fundamentals
Filtration (T2-11P)
Keywords: Modelling, Creep, Soft materials
The classical filtration theory assumes a unique relationship between the local filter cake porosity and the local effective pressure. For a number of compressible materials, it has however been observed that during the consolidation stage this may not be the case. It has been found that the production of filtrate also depends on the characteristic time for the filter cake solids to deform.
This is formulated in the Terzaghi-Voigt model in which a secondary consolidation is introduced.
The secondary consolidation may be visualized by plots of the relative cake deformation (U) v.s. the square root of time. Even more clearly it is demonstrated by plotting the liquid pressure at the cake piston interface v.s. the relative deformation (to be shown). The phenomenon of a secondary consolidation processes is in short called creep.
Provided that the secondary consolidation rate is of the same magnitude as the primary consolidation (defined by the hydraulic retardation), the creep phenomenon may occur during filtration. This will lead to Ruth’s plots characterized by a concave with two (more or less) distinct slopes. The slopes are defined by the relationship between the porosity and the hydraulic resistance before and after the collapse of the solids structure. This has been observed and modelled for dewatering experiments with activated sludge
It will be argued that although a strong resemblance between blinding and creep exists, these two effects may readily be distinguished by virtue of their dependency of the solid content. Finally a comment on the problem of the scaling of soft materials will be given, in which it will be suggested that the scaling problems may well be related to the existence of creep.
Presented Tuesday 18, 13:30 to 15:00, in session Filtration (T2-11P).
