(5d) From Particle Collectives Characteristics to Cake Permeability: the Use of the Pore-Particle Shape Factor

Although it has been thoroughly recognized that a connection exist between PSD and cake porosity and permeability, a lack of prediction capability usually limits the possibility of forecasting the permeability changes that occurs when the PSD of particles varies. The paper discuss results obtained with limestone, glass spheres, aluminium hydroxide and magnesite, being the limestone experimental matrix broad in both mean particle sizes and polidispersity. Taylor-made particle mixtures were prepared varying mean sizes between 5 and 150 micrometers and geometrical standard deviations between almost 1 (narrow mixtures) and 2.5 in order to ensure a wide data spectrum. Particle size distribution were measured by means of laser diffraction, but it was demonstrated that similar results can be obtained using other sizer devices, by the definition of a novel particle shape factor which is device dependent and relates the actual specific surface to the specific surface estimated from PSD measurements assuming that the particles were spheres. Besides the use of the particle shape factor it was necessary to include an additional parameter to account the fact that the same particle collective can build different porous structures depending on the way the bulk is formed. Therefore a parameter to include the cake building mode was lumped to the particle shape factor to build a pore-particle shape factor, which is a system specific parameter that must be determined experimentally. The use of this approach allows the well-known Kozeny-Carman equation to fit very well the data provided that the connection between PSD and porosity is known. This connection is not discussed in the paper but the approach suggested by Yu et al. was followed. The results can be easily illustrated in a permeability-porosity plot that has the advantage of concentrating all cake parameters in the y-axis, and the particle collective characteristics (namely PSD) in the x-axis, being the shift from the ideal case of spheres forming a random bulk a measure of the pore-particle shape factor.