Spunbondeds as filter media.

Spunbondeds As Filter Media Two very different approaches can be taken to separate particles from fluids. The first is described by Stokes' Law and is referred to as "settling."

[v.sub. t.=g.sub.c.D.sup.2.(d.sub.p.-d.sub.f.)/18[mu].sub.f]

where [v.sub.t]=terminal settling velocity of a particle, [g.sub.c]=gravitational acceleration constant, D=diameter of the particle [d.sub.i]=densities of the particle(p) and fluid(f), [[mu].sub.f]=viscosity of the fluid.

Stokes' law is an excellent design equation and settling is very effective in many different applications, but it has two major disadvantages: Large volume separations of small particles may take too long a time to be cost effective and some post-treatment is usually required to clarify the top fluid stream further and/or to deliquor the bottom sludge.

The second approach, filtration, is defined as the separation of particles from fluids by means of a permeable barrier material known as the filter "medium." Cost effective in an extremely wide variety of applications, filtration is the least well understood of the several engineering unit operations because no practical design equations exist. Even worse, each filter medium must be characterized under the actual conditions of use. Therefore, in most cases, trial-and-error is the most effective and efficient means to select an optimum medium or filter design.

Particle Removal Mechanisms

Particles are separated from fluids by filter media through four different mechanisms. Surface filtration is simply a sieving or screening of particles from a fluid at the upstream surface of the medium. Eventually, any surface medium will become covered with a layer of captured particles and cake filtration becomes the dominant mechanism. When this happens, the dirt cake itself becomes the actual filter medium and the original surface medium performs only a support function. Thick and bulky media capture particles within the interstices by means of the depth mechanism. This can include both depth sieving and electrokinetic attraction between the medium and the particles. In crossflow filtration, part of the fluid passes through the medium to form a clarified stream called the permeate while the remainder of the fluid, the concentrate or retentate, also exists the filter carrying most or all of the original particles but in much high concentration.

Note that crossflow differs from the other three mechanisms in that it is essentially a steady state process; because particles are not arrested on or within the filter medium but exit in the retentate stream, flow is not restricted (the pressure does not increase) and, theoretically at least, a crossflow device should last for years.

The first three filtration mechanisms, those encountered most commonly, are not "clean." For example, almost all surface media possess some degree of depth and this depth will capture particles as well; cake filtration captures particles by surface and by intergranula depth mechanisms. Therefore, more than one mechanism can act at the same time, and the dominant mechanism can

change with time. This is a mixed blessing. On one hand, a theoretical analysis of the filtration process is made much more difficult, but on the more practical side, particles that might not be captured by the surface medium may be trapped within its depth, thereby increasing the overall efficiency of the medium.

Functional Requirements Of Filter Media

Nonwovens offer the widest range of any class of materials for use as filter media. The needle felts serve as surface media in air filtration and as depth media for many liquid applications. Wet laid, resin bonded dry laid and spunbonded nonwovens find many applications as surface media, support septa for other media and in cake filtration and structural substrates for crossflow media. In each application, different functional requirements are important.

Strength

Any filter medium must provide sufficient strength to withstand the pressure drop of the fluid passing through it and sufficient resistance to particle abrasion. Spunbondeds provide the highest level of omnidirectional strength at a given basis weight of any type of nonwoven and have excellent abrasion resistance as well. Yet, even the flow cross-machine direction strength of the carded products does not preclude their use in some applications. Heavy weight, scrim reinforced needle felts can provide the very low elongation required in baghouse dusct collectors.

Particle Size Removal

The size of particles removed by an filter medium is a complex function of several structural variables. First is the denier, really the diameter, of the fibers. (Many filter manufacturers and users have asked about the relationship between the fiber denier and diameter presented in the accompanying graph.) Obviously, the finer the fiber diameter, the smaller the particle that can be captured. In other words, one does not filter golf balls with telephone poles (even through many tee shots end up in the woods).

Second is the cross-sectional shape of the fibers. For any given diameter, a trilobal fiber presents about 30% larger "shadow" diameter than a round fiber. While round fibers might thus be expected to stop smaller particles than trilobal fibers, the trilobal cross section has a greater specific surface are and this can increase small particle capture efficiency because of electrokinetic forces often encountered in depth filtration.

Third is the combination of the basis weight and the degree of compaction of the fibrous structure. The more fibers present in a given area and given thickness of filter medium, the greater the barrier to passage of a particle and the greater the capture efficiency. However, the resistance to flow is greater as well.

Fourth is the directional orientation of the fibers within the structure. Carded unidirectional nonwovens present eccentric, elliptical pore openings to the particle-laden fluid stream and very close fiber packing, while the omnidirectional wet laid and spunbonded nonwovens offer much more circular pores and more open structures. Therefore, the orientation of the fibers can have a very large effect upon the mechanism of capture, particle capture efficiency and initial pressure drop.

Several relationships between the filtration characteristics and the structural variables of filter media have been developed from purelytheoretical considerations. One of the best of these is as follows:

[d=[D/n(1-e)][t(1-e)/(0.00026D + 0.0017)].sup.-0.24]

where d=pore size, microns (absolute when n=1, mean when n=2), D=fiber diameter, microns t=web thickness, cm e=porosity or void fraction.

Note: (1-e) can be calculated simply by dividing the basis weight of a medkium by its thickness and by the density of the polymer expressed in equivalent weight and volume units.

Chemical And Heat Resistance

With the notable exception of wood pulp papers, most nonwovens are made of synthetic polymer fibers and that polymer must be compatible with the chemical and thermal conditions of use. In some cases, the cost of the polymer enters into the selection as well. Thus, while PTFE is functionally ideal for almost any application, the high cost limits its use to high temperatures and very corrosive conditions; polypropylenek has almost the same chemical resistance as PTFE and a very low cost, but its use is limited by a flow meeting point; rayon is an ideal choice for hot oil filtration; the uses of nylon are limited by its dimensional instability due to moisture regain and attack by acids; polyester is widely used except under conditions of high temperature or high alkalinity.

Bulk

Often, the level of bulk of a medium will define the particle removal mechanism and, thereby, predetermine its application. We can imagine two different nonwoven products, each of four denier per filament omnidirectionally oriented polyester fibers and weighing two ozs. sq. yard.

If one product is one inch thick (quite bulky), heating and ventilation air filtration would be a natural application because particles would be capturerd within its thickness or depth and it would offer little resistance to air flow. If a second product is 20 mils thick (very nonbulky), H&V filtration would be ruled out because the air pressure drop would be too high in most cases. Instead, this second product might be excellent surface medium for sediment/water filtration.
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