In this article, we discuss a hypothetical filtration procedure in terms of particles, pores, and liquid flows with a view toward understanding how pores or interstices respond to given particulate challenges. We synthesized several experimental findings, regarding the challenging of filters—depth or membrane—with dilute suspensions.
This article will also differentiate between particle arrests caused by the (normally assumed) size exclusion or sieve retention mechanism, and those retained by the (usually ignored) adsorptive sequestration technique.
A comprehensive investigation of the removal of latex spheres from liquid suspensions by membrane filters has been performed by a group of experimentalists at the Particle Technology Laboratory of the University of Minnesota. Their findings have found practical application in semiconductor manufacturing. They are likewise relevant to the needs of pharmaceutical processing.
These investigations have employed particle suspensions in dilute aqueous systems in which surfactant molecules were ingredients. The particle concentrations, measured by particle counters, were approximately one-fifth those usually utilized in latex bead studies. One particular study by Lee et al. (1993), led to conclusions analyzed by Zeman that are a subject of discussion in this article.
Particle Size and Shape
The stage on which filtration takes place consists of an effective filtration area (EFA) marked by a pore size or retention distribution that is confronted by a particle size distribution. The particles consist of a range of sizes that are differentiated into two groups, each of which is defined by their size relative to the pore sizes.
One portion consists of the larger particles that are too big to fit through any of the distributed pores or fiber matrices, whether large or small. The other size particles are small enough to penetrate the larger pores or fiber interstices but not the smaller.
The pores or fiber interstices also constitute two size groups, only one of which covers a range of pore-size ratings large enough to be passed through by particles of the smaller sizes.
Depending upon the relative proportion of smaller and larger particles, and on the pore or interstice sizes they encounter, whether directed by the flow pattern or by chance, there will be different outcomes to the particle retentions, to the onset of filter plugging, and to the quantity of throughput.
This definition of particle sizes is a bit too neat to apply absolutely to any particle shapes except spheres directed toward circular pores. It begs an invariant sieving action based strictly on particle/pore sizes unaffected by shape factors. However, except for spheres, a particle’s longitudinal and transverse axes may differ in size. In a filtration, it is the particle’s dimensional axis that coincides with the pore that is the functional determinant of the particle’s size.
Probability factors, for example the particle’s axial orientations, governed by the liquid stream’s velocities, viscosities, and drag can result in a more-elongated needle–like shape, either passing through or lying athwart a pore opening. Thus, in a mixture of particles characterized generally as being too large to permeate a pore or fiber matrix interstice, some particles of particular shapes may do so depending upon how their flow pattern is directed by the filtration conditions or by chance.
Not surprisingly, in specific experimental trials, conclusions were reached that some particles of the group size seen as being too large to pass through the filter actually did so. This is not a contradiction of the sieving mechanism, but rather a more realistic description of size as defined by the particle dimensions that are limiting in any actual filtration. It might be simpler to refer to these membranes as being intermediate in size.
One is obliged to describe them as they are referred to in the literature, i.e., as larger particles. The implications are that they are too large to pass through the filter’s smaller pores. However, when properly aligned they can permeate a larger pore, bearing in mind that pore size labels are generally descriptive and the actual pore structure is typically larger than labeled.