NAMS 2002 Workshop - ICOM 2008

Ultra- and Microfiltration III - Membranes – 5
Friday July 18, 4:30 PM-5:00 PM, Honolulu/Kahuku
Acoustic Investigation of Porous and Membrane Structures
S. Léoni, Ecole Centrale Marseille, Marseille, France
J. Bonnet, Université Paul Cézanne Aix Marseille, Provence, France
Y. Wyart (Speaker), Université Paul Cézanne Aix Marseille, Provence, France -
yvan.wyart@univ-cezanne.fr
J. Allouche, Ecole Centrale Marseille, Marseille, France
P. Moulin, Université Paul Cézanne Aix Marseille, Provence, France
The processes of membrane filtration are in the heart of actual and future
environmental challenges. Nevertheless, in order to increase the impact of
membrane separation techniques in industrial field, the phenomenon of
membrane fouling must be more understood. The membrane fouling can be
monitored by measuring the increase of transmembrane pressure when the
filtration is made at a constant flow rate. This procedure is a good indicator to
estimate the frequency of backwashes or chemical cleanings; but gives no
information about the kinetic of the fouling phenomenon, the fouling location (on
membrane surface or in the membrane bulk), the structure of the cake&
Moreover, it is well known that the membrane fouling depends on the structural
characteristics of the membrane. The aim of the study is to develop an acoustic
method to characterize first new membranes and secondly fouled membranes.
This method must be non- invasive to avoid the destructuration of the cake and
simple for an easy use.
The present work deals with acoustic characterisation of porous media and
membrane using impedance tube. This kind of materials is widely used
throughout industry to measure the sound absorption coefficient and other
acoustic properties of materials. Low frequency acoustic method is used to
determine porosity from acoustic sample properties. The experimental setup is
composed of a sound source, two microphones and a sample holder. The sound
source (speaker), placed at an extremity of a rigid walled tube, generates
incident plane waves which are partially reflected by the material sample, located
at the other extremity of the tube (sample holder). The incident and reflected
waves interfere and create a system of standing waves. For sufficiently low
frequencies, it is a plane wave which is propagated along the tube axis. The
lower limit frequency is dependent on the microphones limitation and on the
spacing between microphones. The higher limit is given by the cut off frequency
of the tube. Placed at the end of the tube, two microphones measure the acoustic
pressure in order to calculate the frequency response function (FRF). This FRF is
used to determine the complex acoustic surface impedance Z of the sample. The
method presented in this paper is based on the theories of Lafarge- Allard which
are based on a low frequency approximation. Using this assumption, porosity Phi