This article presents all the information you need to know about Liquid Filters.
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Liquid filters are devices designed to remove suspended solids from a fluid stream. They employ a physical barrier, known as the filter medium, through which the liquid flows while retaining the solids. These filters are commonly used in various solid-liquid separation processes and differ from equipment like centrifuges, clarifiers, and gravity settlers, which do not rely on such media.
Liquid filters are prevalent in industries such as food and beverage manufacturing, bioprocessing, semiconductor and electronics production, pharmaceuticals, medical facilities, and wastewater treatment. Smaller-scale filters, including bag filters and cartridge filters, are also utilized in residential, office, and laboratory settings.
Liquid filters are often employed as pre-treatment equipment for downstream processes, where the removal of solids is essential for ensuring product quality and safety, as well as for maintaining the efficiency of subsequent equipment. Effective filtration helps preserve the performance of pipeline and pumping systems and is crucial in the production of consumable products, such as beverages and drinking water.
While filtration is also relevant for solid-gas separation, using a similar principle, this article primarily focuses on the separation of solids from liquids.
Liquid filtration is a method used to separate solid particles, impurities, and contaminants from a fluid stream. This simple process involves directing the liquid, which may be a slurry or suspension, through a porous filter medium. The filter medium features small, microscopic pores that permit only the passage of the liquid (known as the filtrate) while retaining the larger solid particles. Particles that are too large to pass through the filter’s pores are termed oversize. Over time, these oversize particles build up, forming a thick layer called the filter cake. The resulting liquid, now free from these larger particles, is known as the filtrate.
In the process of liquid filtration, the fluid experiences resistance as it flows through the filter medium and the accumulating filter cake. As the thickness of the filter cake grows, both the flow resistance and pressure drop increase. This resistance is influenced by factors such as the porosity and compressibility of the cake, as well as the specific surface area of the particles.
Industrial filtration equipment can be operated in the following modes:
Certain industrial filtration systems are designed to function under either constant rate or constant pressure filtration conditions. For smaller-scale filtration tasks, where the pressure differential is minimal, the process liquid is permitted to flow through a filter medium positioned within a filtration vessel.
The filter medium acts as the physical separator between the raw process liquid and the purified filtrate. To ensure effective filtration, the material chosen for the filter medium must meet the following criteria:
Filter media can be constructed from the following materials:
Metal screens or perforated sheets made from stainless steel, copper, or aluminum. They are suitable for filtering liquids at elevated temperatures and high flow rates, and for corrosive liquids.
Synthetic fabrics can be crafted from polymeric materials like polyester, nylon, polypropylene, and fluoropolymers such as PVDF and PTFE. These fabrics may be available in both monofilament and multifilament forms.
Granular beds consist of coarse and fine materials such as sand, anthracite, and gravel. These are frequently utilized in water purification and wastewater treatment processes.
The filter medium is evaluated according to several key characteristics. These attributes are crucial for choosing the most suitable filter medium for a given filtration system:
The mesh size is the number of openings per inch of the mesh. As the mesh size increases, the greater number of openings are present, and the smaller the holes become. Larger mesh sizes capture finer particles. However, higher mesh sizes do not equate to a higher filtration efficiency.
The strand diameter is the diameter of the strands used to make the weave. Thicker strands and higher mesh sizes have smaller openings and therefore capture the finest particles.
A micron is defined as one-millionth of a meter. It is a unit of measurement used to describe particle sizes. The micron rating refers to the sizes of the holes in a filter and the size of the particle it can remove. A micron rating may be absolute or nominal.
The absolute micron rating refers to the largest size of the particle that can pass through a filter. This means that all particles which are larger than the absolute micron rating shall not pass through the filter. However, this is somewhat an unrealistic standard because it is obtained when perfect spherical particles are filtered on a medium with consistent pore sizes, which is rare.
The nominal micron rating describes the ability of the filter to remove particles at a specified micron size. It is represented by the percentage of the particles which can pass through the micron rating of the filter medium. The percentage ranges from 65-98%. For example, a nominal micron rating of 95% of 10 microns means 95% of particles which are 10 microns are retained in the upstream side of the medium.
Typical filter aids include diatomaceous earth, perlite, cellulose, asbestos, and activated carbon.
Liquid filtration can be classified into two primary types. The distinction between these methods mainly depends on the structure of the filter medium used.
In surface filtration, the particle screening takes place on the surface of the filter medium. Interstitial spaces, called pores, are present between the fibers of the filter medium. Particles that have a larger diameter than the pore width are blocked on the upstream side of the filter medium and form the filter cake.
Particles with a diameter smaller than the pore size of the filter medium can pass through it. Initially, the filter efficiency is approximately 50-60%. As the filter cake accumulates, the efficiency improves, reaching up to 100%, since the cake also adds resistance to the flow of particles.
Surface filters are cost-effective but have a lower capacity for retaining particles and are more susceptible to blinding. They generally need to be replaced more often, although they can be cleaned and reused.
Depth filtration involves capturing particles within the entire depth of the filter medium. Depth filters utilize a thick, multi-layered medium with increasing density in the direction of flow. Larger particles are trapped on the surface of the filter, which has the lowest density, while smaller particles are captured deeper within the filter. The extensive pore volume of the filter creates a complex flow path, making it challenging for solids to pass through. This increased resistance effectively prevents solid particles from entering the filtrate.
Depth filters are employed when the process liquid contains particles of varying sizes. They are capable of filtering out particles smaller than the average pore size. These filters offer a greater capacity for holding particles and can accumulate a substantial volume of solids before becoming clogged. They are also effective at removing gelatinous particles from the liquid. Additionally, depth filters have an extended service life and require less frequent replacement.
Additional liquid filtration methods, depending on the specific requirements of the process, include the following:
The various types of liquid filters are as follows:
The construction of filter bags can be either sewn or welded:
Sewn filter bags are made by stitching together pieces of filter media at their seams. These bags are known for their robust mechanical strength, can handle higher liquid flow rates, and have a greater capacity for holding particles. However, the stitching process creates holes along the seams that can increase pore size and reduce filtration efficiency.
Welded filter bags are assembled by adhesively bonding pieces of filter media at the seams. This welding method ensures a more effective seal, maintaining the bag's filtration efficiency. However, welded bags generally have a lower capacity for retaining filtered solids and are less mechanically robust, making them more susceptible to rupture under high fluid pressure or flow rates.
Filter bags are an economical choice for small-scale operations. They provide adequate filtration area for modest needs and are less prone to clogging, resulting in less frequent replacements. Additionally, bag filtration tends to generate less waste compared to cartridge filters.
A cartridge is a tubular filter medium that is encased inside a housing. The direction of flow in a cartridge filter is from outside to the insides of the cartridge. Cartridges are usually made from synthetic or natural fibers and small metal wires. A core, made of stainless or tin-plated steel or polypropylene, is present on the axis of the tubular cartridge to support the media material. A purer filtrate is collected at its core.
Cartridge filters are available in various types based on their design:
Pleated cartridges are designed for surface filtration. They are made by folding the filter media into pleats and securing the ends, which provides a larger filtration area within a compact volume.
Wound cartridges are designed for depth filtration. They are made by winding a filter media strand around a central core, forming multiple layers. This construction creates a density gradient that increases from the outer surface towards the inner core.
Spun-bonded cartridges are another type of filter media used for depth filtration. They are made by thermally bonding fibers together, which maintains a gradual density gradient. This process enhances the cartridge's durability and strength.
There are also specialized cartridge filters designed to meet specific requirements. Some examples include:
Similar to bag filters, cartridge filters are effective for small-scale operations and are used to filter liquids at lower flow rates. They offer greater versatility and come in a range of lengths and pore sizes. Cartridge filters have a higher dirt holding capacity and longer service life. They are frequently employed in the filtration of potable water for human consumption. However, thicker and multi-layered cartridges tend to produce 15-50% more waste compared to filter bags.
Rotary drum filters are industrial filtration devices designed for continuous filtration of liquid streams with high solid concentrations. In these filters, a drum, operating under vacuum pressure, is partially immersed in the slurry. The drum’s outer surface serves as the filtration area. As the drum rotates, the vacuum draws the liquid through, leaving the solids on the drum's surface. Modern rotary drum filters often include a scraping mechanism to remove the accumulated filter cake, preventing excessive buildup.
Filter presses are industrial filtration devices used for batch processing of liquid streams with high solid concentrations. In these systems, the slurry is pumped onto plates that hold the filter medium and then subjected to high pressure to de-water the liquid.
When designing a liquid filtration system and choosing the right filter equipment, take the following factors into account:
Filtration efficiency can be improved through multi-stage filtration, where particles are removed in progressively smaller sizes as the liquid flows through each stage. Additional filtration steps are often added to enhance liquid quality by eliminating residual compounds and microorganisms.
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