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Comment PDF Separation Processes

Facts at your Fingertips: Filtration Process Design

By Scott Jenkins |

Designing a new solid-liquid filtration process or optimizing an existing one requires consideration of a number of factors aside from the type of filter that will be used. This one-page reference reviews two groups of considerations: one related to equipment, and another more related to the materials being separated.

Material-related issues

Solids properties. The physical characteristics of the solids being filtered have a substantial effect on the level of difficulty of the filtration process, and in turn on the appropriate type of filter and operating conditions. For example, particles that are incompressible (rigid) are usually easier to filter than those that are soft and compressible. Solids that are crystalline can be relatively easy to filter, whereas amorphous, slimy or gelatinous solids are more difficult to separate and require more complex techniques.

Particle-size distribution. The solids in a slurry are often characterized by the average particle size, which can be a useful measure. However, the particle-size distribution is also very important. The filter must be designed to retain the smallest particles that need to be removed. The tendency for particles to agglomerate, shifting the distribution toward larger sizes, may also be a factor.

Liquid viscosity. If all else is equal, pressure drop in a filtration system is higher with a more viscous liquid, meaning the flowrate is lower at a given applied differential pressure (a higher liquid viscosity results in a lower filtration rate). To reduce viscosity, liquid filtration processes are often operated at elevated temperatures.

Temperature constraints. The process temperature must not exceed the maximum allowable operating temperature of the equipment being used. Personnel safety must be considered also, with appropriate operating procedures and personal protective equipment (PPE) to minimize the risk of injury. Some products are heat-sensitive, and product degradation will limit the maximum acceptable temperature. Finally, rather than filtering hot, temperatures below ambient must be used in some cases. For example, the solids may be soluble, and reduced temperature may be needed to avoid dissolving them.

Equipment-related issues

Sizing and productivity. The main design specification of a filter is the filtration area. The required area is calculated by dividing the filtration rate per unit area, a number obtained from laboratory or pilot plant experiments, into the desired productivity, as shown in Equations (1–3).

Throughput = V / A                       (1)

Flowrate = (V/Dt) / A                    (2)

Cake thickness = Ws / rsA            (3)

Where:

V = total volume filtered, gal

A = total filtration area, ft2

Dt = total time to filter, min

Ws = total weight of solids filtered, lb

rs = density (ρ) of wet cake, lb/ft3

For example, if the filtrate rate obtained in the pilot plant is 25 gallons per square foot per hour, and the process specification is 12,500 gal/h, the required filtration area is 12,500/25 or 500 ft2.

Filter media. Filter elements consist of a porous or coarsely open support for the filter media. The filter media is the separation point for the flow of clean filtrate into the process. Common filter media include the following: paper (disposable filter sheets made of either cellulose or non-woven synthetics); pads (disposable pads of cellulose fibers or a blend); textiles (cloths made of natural or synthetic fibers, such as polyolefins, polyesters, nylons and others); metallic wire mesh; and porous or sintered metals.

Filter aids. Precoating is a technique whereby an inert solid is coated onto the filter media to avoid plugging or blinding of the media with suspended solids or to facilitate the filter cake release. Filter aids are also used as a body feed to help keep the filter cake open for optimal cycle times (see Figure 1). The most common filter aid materials are the following: diatomaceous earth (made from silica fossils of unicellular organisms); perlite (expanded ground volcanic-lava rock); cellulose; carbon-based aids (used when the chemistry of the process liquid may react with the silica in diatomaceous earth or perlite); blends of diatomaceous earth and cellulose; calcium carbonate.

Figure 1. In cake filtration (a), the separation takes place on a buildup of particles (cake) on the filter medium. In some cases (b), more than simple size exclusion may be involved

Figure 1. In cake filtration (a), the separation takes place on a buildup of particles (cake) on the filter medium. In some cases (b), more than simple size exclusion may be involved

Laboratory testing

Testing in filtration processes is critical for determining the properties of solids being filtered and the ease or difficulty of the filtration. In addition, laboratory testing is important in specifying filter media, filter aids, filter area, cake space needed and cake discharge techniques. Tests are designed to analyze cake depths, operating pressures, filter media, washing and drying efficiencies and cake discharge.

References

1. Sentmanat, Jose M., Clarifying Liquid Filtration: A Practical Guide to Liquid Filtration, Chem. Eng., October 2011, pp. 38–47.

2. Gabelman, Alan, An Overview of Filtration, Chem. Eng., November 2015, pp. 50–58.

3. Perlmutter, B., Filtration Testing and Slurry Conditioning, Chem. Eng., Dec. 2013, p. 40.

4. Engel, D. and Burns, H., Sheilan, M., Filtration and Separation During Chemical Process Operations: Avoid Common Errors, Chem. Eng., June 2015, pp. 52–57.

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