- The Filtration Society - http://www.filtsoc.org -

Volume 17, Issue 2

CHALLENGE TESTING: PART 1 – WET TESTING – INVESTIGATING THE
MEASUREMENT UNCERTAINTIES ASSOCIATED WITH DIFFERENT
DEFINITIONS OF MAXIMUM PORE SIZE
Graham Rideal and Abi Stewart (pages 100-102)

The ‘maximum’ pore size in a sand screen, when challenged by glass microspheres, can be defined as the 97th percentile (the Cut Point), the 99th percentile or the single largest sphere passing the mesh. When analysing the microspheres by Image Analysis, this paper shows that, provided at least 1000 spherical beads (maximum size/minimum size <1.2) are analysed, the measurement uncertainty is approximately 3% for both d97 and d99. However, the uncertainty for the Maximum Penetrating Particle (MPP) went up to 17% and so cannot be recommended as a reliable parameter. Excellent agreement was found between the Ultrasonic wet test method and the Sonic dry test method.

CHALLENGE TESTING: PART 2 – DRY TESTING – INVESTIGATING THE
MEASUREMENT UNCERTAINTIES ASSOCIATED WITH DIFFERENT
DEFINITIONS OF MAXIMUM PORE SIZE
Graham Rideal and Abi Stewart (pages 102-107)

Pre-calibrated, narrow size distribution glass microspheres were used to investigate the most appropriate measure of the ‘maximum’ pore sizes of sand screens. Using a calibration graph of weight passing versus cut point, the measurement uncertainty in a dry challenge test was only 3-4% (this compared with the inherent measurement uncertainty of 1% when the same mesh was analysed 10 times). Analysis of the beads passing three different sand screens by microscopy and image analysis confirmed that the cut point corresponded to the D97 and the measurement uncertainty remained at 3-4% up to D99, herein defined as the ‘maximum’ pore size. The uncertainty for the single largest sphere passing the mesh (D100) was too high (+/-15%) to be of any practical value and could cause confusion in the industry.

FILTERABILITY AND CAKE COMPRESSIBILITY IN DEADEND MEMBRANE
FILTRATION CONTROLLED BY CAKE FORMATION
Eiji Iritani and Nobuyuki Katagiri (pages 108-120)

The cake properties presented as specific cake resistance and cake porosity provide essential information about the flux decline behaviour in deadend membrane filtration such as microfiltration and ultrafiltration controlled by the cake filtration mechanism. Initially, various types of constitutive equations are shown to represent local specific cake resistance and local cake porosity as functions of the solid compressive pressure. It is also shown that their average values can be analytically derived from such local values based upon the compressible cake filtration theory and vice versa.

In the filtration testing methods, the pressure dependence of the average specific cake resistance was available over wide ranges of pressure drop across the filter cake by carrying out a single constant pressure deadend filtration using a membrane with an extremely high flow resistance. Step-up pressure filtration using a filtration cell with a single stage reduction in the effective filtration area allowed us to obtain the pressure dependence not only of the average specific cake resistance but also of the average cake porosity from only one run by making use of the decrease in the cake thickness caused by the cake compression arising from stepped-pressure operation. In sedimentation testing methods such as analytical ultracentrifugation, permeability data were evaluated by measuring sedimentation velocities at various solid concentrations, and the compression data were estimated by measuring the thicknesses of sediments obtained under various rotor speeds of the test cell. These data successfully described the flux decline behaviour observed in the deadend ultrafiltration of nanocolloids.