Filtration is arguably the most common unit operation in the chemical process industries (CPI). Filtration processes can be divided into three broad categories: cake filtration, where the incoming slurry contains enough solid material to form a cake on the filter medium; clarifying filtration, which involves feeds with solids levels that are too low to form a cake and where the solids become embedded in the filtration medium; and crossflow filtration, where the feed flows parallel to the filtration surface, rather than the conventional perpendicular flow. This one-page reference summarizes the basic operation and mechanism for cake filtration processes.
In cake filtration, feed is introduced upstream of the filter, and a layer of solids is deposited onto the surface of the filter medium. Some of the particles in this layer of material bridge the gaps between the fibers of the filter medium, a process known as bridging. Subsequently, new particles are deposited onto this existing layer, forming a second layer of solids. The concept is depicted in Figure 1a, where the dark circles represent the solid portion (for example, fibers) of the filtration medium, and the gaps between the circles denote the flow path for filtrate. The process continues, with new solids forming additional layers adjacent to solids already deposited, and in this manner, a cake forms. Rather than the actual medium, the cake itself acts as the filtration medium, determining the quality and flowrate of the filtrate. The role of the actual medium is only to support the cake.
Cake filtration is suitable only for feeds containing enough solids to form a cake — at least 1 vol.%. Higher levels of feed solids lead to better results, including improved bridging and more porous cakes.
There are a variety of media available, including paper, textiles, polymers, and even wire screens. Among the materials used are polyolefins, nylon, polyester, acrylics and fluorocarbons. Both woven and nonwoven media are employed, over a wide range of porosity. When selecting a medium, consider mechanical strength, chemical compatibility with the process material, temperature tolerance, ease of cleaning and porosity. If the medium is too coarse, solids may become lodged in the openings, leading to blinding. In other cases, solids may not be retained and bridging may not occur. Conversely, a medium that is too tight will impose an unnecessary restriction to liquid flow, leading to reduced productivity.
Filtering soft solids
Solids that are soft, slimy or gelatinous tend to pack tightly, forming cakes of low permeability. This problem can be alleviated by the addition of a small amount of filter aid to the feed slurry. Filter aids are inert, highly porous materials that act to separate blinding solids, leading to a more open cake and in turn a higher filtration rate. Filter aid added directly to the slurry is known as body feed. Alternatively, prior to introducing the feed slurry, a layer of filter aid can be deposited onto the filtration medium to form what is known as a precoat.
Several types of filter aid are available. Most common is diatomite (diatomaceous earth), the skeletal remains of single-cell algae, composed primarily of silica. Diatomite offers the highest clarity of all types of filter aid. Another option is perlite, which is milled volcanic glass composed mainly of potassium aluminum silicate. Because perlite is not as tortuous as diatomite, the high clarity levels achievable with the latter are not possible. Nevertheless, perlite may be more cost effective than diatomite for separation of coarse particles. This is because the density of perlite is lower, so that less material is needed to form a precoat of a given thickness. Other types of filter aid include expanded cellulose.
Cake filtration mechanism
A filter precoated with diatomaceous earth (DE) is represented in Figure 1b. This diagram shows how filtration by simple size exclusion may be an oversimplification. At the top of the figure, rigid particles larger than the openings in the DE precoat are retained, while smaller ones pass through, consistent with expectations. However, the compressible nature of the large, soft particles shown in Figure 1b allows them to squeeze through, even though the particles are smaller than the openings.
The sub-micron, non-haze particles shown in the figure also are not retained, but this is expected because these particles are smaller than the pores in the cake. On the other hand, the sub-micron haze particles seen at the bottom of the diagram are retained even though they are larger than the openings. This is attributable to some mechanism other than physical exclusion, perhaps electrostatic or hydrophobic interaction.
Editor’s note: This column is an excerpt of the following article: Gabelman, A., An Overview of Filtration, Chem. Eng., November 2015, pp. 50–58.