This article presents all the information you need to know about Liquid Filters.
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Liquid filters are equipment used to separate suspended solids from a fluid stream. A physical barrier, called the filter medium, is a prerequisite in the filtration process in which the liquid passes through and where the solids are retained. They are widely used in many solid-liquid mechanical separation processes. They differ from centrifuges, clarifiers, and gravity settlers which do not utilize such media.
Liquid filters are usually found in food and beverage manufacturing, bioprocessing, semiconductor and electronic components manufacturing, pharmaceutical industries, medical facilities, and wastewater treatment. Small-scale filters such as bag filters and cartridge filters are found in some homes, offices, and laboratories.
Liquid filters may be used as pre-treatment equipment to a downstream process, wherein removal of solids is crucial in achieving product quality and safety, and in maintaining the efficiency of the downstream equipment. Filtration of solids is important in upholding the performance of a pipeline and pumping systems. It is also critical in manufacturing products intended for human consumption, such as beverages and drinking water.
Filtration is also applicable to solid-gas separation and works in a similar concept, but this article mainly focuses on liquid-solid separation.
Liquid filtration is the process of removing solid particles, impurities, and contaminants that are suspended in a fluid stream. It is a straightforward operation that involves the flow of the process liquid (in the form of slurries and suspensions) through a permeable filter medium and blocking and retention of the captured solids. The filter medium has tiny, microscopic holes or pores which only allow the passage of the filtrate, leaving the unwanted particles on the other side of the medium. The particles larger than the pores of the filter medium which cannot pass through it are called the oversize. Over time, the oversized particles accumulate to form a solid layer with a significant thickness which is referred to as the filter cake. The clarified liquid free from the oversize is called the filtrate.
During liquid filtration, the fluid encounters flow resistance when it passes through the filter medium and the developing filter cake. The resistance to the flow of the liquid and pressure drop increase as the layer of the cake develops. The flow resistance depends on such properties as porosity and compressibility of the cake and specific surface area of the particle.
Industrial filtration equipment can be operated in the following modes:
In constant rate filtration, the rate of volume of the filtrate (or filtration rate) collected is held constant and the pressure drop gradually increases.
In constant pressure filtration, the pressure drop across the medium is held constant. Consequently, the filtration rate decreases progressively as the filter cake develops.
Some industrial filtration equipment is capable of operating under constant rate and constant pressure filtration. In small-scale filtration applications, the pressure differential is low and the process liquid is simply allowed to flow through a filter medium secured in a filtration vessel.
The filter medium serves as the physical barrier between the raw process liquid to the clear filtrate. The material to be used as the filter medium must satisfy the following requirements to achieve a successful filtration operation:
The filter media can be made of:
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 made from polymeric materials such as polyester, nylon, polypropylene, and fluoropolymers such as PVDF and PTFE. These fabrics may be monofilament and multifilament.
Granular bed made from coarse and fine sediments such as sand, anthracite, and gravel. They are commonly used in water purification and wastewater treatment.
A filter medium is graded based on the following characteristics. These characteristics are critical when selecting the appropriate filter medium for a specific 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.
Common filter aids are diatomaceous earth, perlite, cellulose, asbestos, and activated carbon.
There are two main types of liquid filtration. The filtration method is primarily distinguished from the structure of its filter medium.
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 that have a smaller diameter than the pore width are allowed to pass through the filter medium. At the start of the filtration process, the filter efficiency is around 50-60%. It increases up to 100% as the filter cake builds up, as the filter cake also offers resistance to the flow of the particles.
Surface filters are economical. However, this type of filtration has a lower particle holding capacity and is more prone to blinding. They also require more frequent replacements but they can be reused after cleaning.
Depth filtration is used to retain the particles throughout the depth of the filter medium. Depth filters use a thick, multi-layered filter medium that increases its density towards the direction of the flow. The larger particles are retained on the surface of the filter, which has the least media density, and the particle size progressively becomes finer across the depth of the filter. The high pore volume of the filter presents a twisted and difficult flow path for the solid to pass through. The greater resistance it offers effectively blocks the solid particles from combining into the filtrate.
Depth filters are used when the process liquid with a wide range of particle sizes. They can filter particles smaller than the mean pore size. They have a higher particle holding capacity and can trap a large volume of solids before they become clogged. They can remove gelatinous particles from the process liquid. Lastly, they have a long service life and are less frequently replaced.
Other liquid filtration methods depending on the nature of the process are the following:
The different types of liquid filters are the following:
The construction of a filter bag may be sewn or welded:
Sewn filter bags are constructed from pieces of the filter media that are joined with stitches on their seams. They are known for their mechanical durability, can withstand higher liquid flow rates, and have higher particle holding capacity. However, the holes bored by the needles from stitching the bags can increase the pore size along its seams, therefore decreasing the filtration efficiency. Welded filter bags are constructed from pieces of the filter media that are adhesively bonded on their seams. The weld on its seams joins the pieces more effectively and therefore preserves the filtration efficiency of the bag. However, welded filter bags have a lower holding capacity for filtered solids. Their mechanical strength is also lower and is more prone to rupture if used in higher fluid pressure or higher flow rate applications.Filter bags are cost-effective options for small batch operations. They have a sufficient filtration area to support small-scale demands. They are not easily clogged; hence replacement is done less frequently. Filtration by bags also generates fewer amounts of wastes than 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 come in different types based on their construction:
Pleated cartridges are filter media used in surface filtration. They are constructed by pleating the media bonded at its ends to give a larger filtration area for a minimal volume.
Wound cartridges are filter media used in depth filtration. They are constructed by carefully spinning the strand around the core to establish the layers of the filter and to create the density gradient gradually increasing from the outer to the inner surface.
Spun-bonded cartridges are also a filter media used in depth filtration. They are constructed by thermally bonding the fibers together while maintaining the gradual density gradient, thus increasing the durability and strength of the cartridge.
Specialized types of cartridge filters are also available to cater to a specific need. Some of them are:
Like bag filters, cartridge filters are practical in small-scale operations and used in filtering liquids at lower flow rates. They are more versatile and they are available in varying lengths and porosities. They have higher dirt holding capacity with long service lives. They are commonly used in the filtration of potable water for human consumption. However, the thicker and multi-layered cartridges generate more wastes than filter bags by 15-50%.
Rotary drum filters are industrial filtration equipment used in the filtration of liquid streams with high solids concentration in a continuous process. In these filters, a drum under vacuum pressure is partially submerged in the slurry. The lateral surface of the drum acts as the filtration area. As the drum rotates, the liquid is drawn to the vacuum and the solid is retained on the surface of the drum. Advanced rotary drum filters are equipped with a scraping system to discharge the cake which prevents it from building up.
Filter presses are industrial filtration equipment used in the filtration of liquid streams with high solids concentration in a batch process. In these filters, the slurry is pumped to the plates containing the filter medium, then de-watered under high pressure.
When designing a liquid filtration system and selecting the appropriate liquid filter equipment, consider the following factors:
Filtration efficiency is enhanced by performing multi-stage filtration. The particles are removed in decreasing sizes as the process liquid advances through the stages. To improve the quality of the liquid, additional filtration steps are included to remove residual compounds and microorganisms.
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