Adsorption: A Simple and Cost-effective Solution to Remove Varnish

Physisorption depends on weak physical attraction of the solid phase (adsorbent filter material) for components in the fluid phase (varnish molecules). It is characterized by the fact that physisorption is: (a) sensitive to temperature, (b) relatively nonspecific among constituents in the fluid phase, (c) relatively fast because there is no significant activation barrier, (d) possibly occurs in multiple layers on the solid surface, and (f) the magnitude of the heat of adsorption is relatively small.

Conversely, chemisorption occurs by chemical bonding via electron transfer. It is characterized by the fact that it is: (a) specific, (b) relatively slow due to the existence of an activation barrier (with associated chemical kinetics), (c) exhibits monolayer formation on the solid surface, and (d) the magnitude of the heat of adsorption is relatively large.

Though certain aspects of lubricant filtration imply that the uptake of varnish particles might involve chemisorption, it is more likely that physisorption governs their uptake. Thus, the remainder of this section is devoted mostly to that topic.

The forces of physisorption fall into two main categories: van der Waals (or dispersion) forces and electrostatic forces.

Van der Waals forces are weak molecule-to-molecule forces that cause a small particle to adhere to a surface, for example, as dust sticks to eye glasses. Electrostatic forces can be subdivided into polarization forces, field-dipole interactions, and field gradient-quadrupole interactions. These forces occur only if the surface is polar and are represented by a Coulombic potential function.

The last type of electrostatic effect is called hydrogen bonding. It is a specific form of electronic interaction which is important for polar, partially oxidized molecules, such as varnishes and surfaces, such as cellulose. On account of the generic chemical structures for varnish identified earlier, it is likely that hydrogen bonding is important for removing varnishes from lubricants.

In summary, adsorption of varnishes from oil likely occurs mostly by physisorption instead of chemisorption. The phenomena that contribute to physisorption are van der Waals (or dispersion) forces and electrostatic forces. Of these, van der Waals forces depend on the polarizability of the varnish molecules.

Electrostatic forces that are important for varnish in contact with cellulose include polarization forces, field-dipole interactions, and especially hydrogen bonding. While these provide a means for understanding the interactions and might someday be a basis for computer simulations, today we are limited to using our understanding merely as a basis for choosing materials and subsequently performing empirical analysis.

Adsorption Kinetics (Movement within the Oil) 
In order to adsorb varnish molecules (or particles), they must be brought into contact with the adsorbent surface. That sounds simple, but there are some complications. Most importantly, there is resistance to motion from the oil to the adsorbent surface. The rate of uptake, or the adsorption kinetics, depends on that resistance. In addition, instead of being a single resistance, it is a combination of resistances.

Figure 1. Filter Outset

To illustrate visually, Figures 1 and 2 show a before and after rendition of a loose bundle of cellulose fibers (pale yellow) in the midst of oil containing varnish particles (gold). Figure 1 shows the contaminated oil approaching the fibers early in the life of the filter. Figure 2 shows the nearly loaded filter still producing clean oil, at some later time, having nearly exhausted the adsorbent. One of the features is that the varnish particles shown are too small to be captured by physical forces such as impaction or sieving, but rather are captured by adsorption, due to the molecular forces mentioned earlier.

Figure 2. Filter Exhaust

What happens inside the fiber can be viewed according to Figures 3 and 4, which show a cross-section of a typical fiber, comprised of dozens or hundreds of cellulose molecules, shown as smaller not-quite-circular sections. The space between those molecules inside the fiber is shown in pink, but it is intended to be open space. To the extent there is binder, it may, of course, penetrate the space, but a skin is shown, along with a fictitious film (blue).

Figure 3. Overview

Let's consider what happens to the varnish molecules (or particles). First, there is the transport from the oil to the boundary of the adsorbent (for example, a bundle of fibers). This is frequently referred to as film diffusion because the resistance is pictured as a fictitious stagnant film. Second, there is diffusion within the adsorbent (sometimes referred to as macropore diffusion).

Figure 4. Inside the Filter Media

This could be viewed as among or between fibers. Finally, there is diffusion from the pore fluid to the adsorption sites at the adsorbent surface (sometimes referred to as micropore diffusion). This could be viewed as among the molecules. All of these depend on geometry, temperature, fluid properties, etc.

Figure 5. Governor Filter Varnish Removal