Coalescer filters are used to protect industrial equipment and processes from damage caused by liquid pollutants. These contaminants can cause corrosion, clog pipes and equipment, and reduce process performance. Coalescers are an important part of industrial filtration systems. which have various uses in industries such as oil and gas, petrochemicals, power generation, food and beverage, pharmaceuticals, etc. This equipment separates liquid particles from the fluid flow. This fluid can be gas or liquid.
For gaseous fluids, coalescers (gas separators) can be used to separate liquid droplets, moisture and water vapor, or oil mist. Among the applications of this type of coalescer, it can be used for things like separating oil mist or petroleum products from airflow, purifying compressed air, and separating moisture from air in various industries. Other types of coalescers are liquids, which are mainly used to remove moisture from fuel pipeline streams, diesel fuel filtration, separation of liquid organic particles from water streams, environmental cleaning of oily wastewater streams, water purification for drinking, etc.
Coalers use a special mechanism to remove liquid particles. These systems can remove polluting particles through the coalescence of liquid droplets. The full description of this mechanism is given below.
Coalescence is a process in which a filter media can remove immiscible liquid particles from a liquid or gas stream. which includes the separation of oil droplets from water, mist or oil vapors from air, humidity and air vapor, and water pollution from fuel. How coalescer filters remove contaminants depends largely on the size of the contaminants to be removed. Larger particles can usually be removed through direct interception filtration mechanisms, while smaller particles may require diffusion mechanisms. Regardless of the method, however, Coalcer filters operate on the same general principle. Liquid droplets collect on the fibrous material of the filter element and join together at the fiber intersections and create larger droplets. This phenomenon includes the collection of small droplets, both on the surface of the media and on the fibers of the filter media. In this process, a matrix of fine fibers is needed to trap the liquid particles and keep them on the fibers, and then depending on the surface characteristics of the fibers and the liquid droplets, they turn into larger droplets and eventually become so large that they can no longer reach the surfaces. The fibers stick and fall due to the force of gravity, or move away from the environment in other ways.
In coalescing processes, provision must be made to transport the coalesced liquid particles so that they can be removed as a separate stream from the filter structure, for example by a gravity settling mechanism. Overall Coalescer filters use a multi-stage filtration mechanism designed to capture and collect these dispersed particles, resulting in cleaner output streams.
Primary absorption: The immiscible fluid mixture enters the Coalescer filter. In the initial stage of filtration, drops or larger particles are taken.
Diffusion and Coalescence: After capturing the primary contaminants, the droplets or smaller particles that have managed to pass through the primary stage encounter the coalescing media. This media is specifically designed to facilitate coalescence and provide a surface for droplets to adhere to. As the droplets collide and stick together, they form larger droplets through the process of coalescence.
Gravitational separation: Larger droplets, due to their increased size and weight, are attracted downwards in the Coalescer filter. This step separates most of the coalesced droplets from the mixture and allows them to collect at the bottom of the tank or container for easy removal.
Draining and removal: The separated liquid is collected in a collection chamber or compartment, where it can be periodically drained or removed. The purge liquid or gas exiting the Coalescer filter is now substantially free of contaminants. Figure 1 shows a schematic of the particle coalescence mechanism.
The media of the coalescer elements can act in both Surface and depth ways to remove particles.
Surface Coalescer Media: This type mainly has a very hydrophobic or oleophobic surface, which is important to prevent the penetration of liquid droplets. The liquid droplets that reach the surface of the filter either pass through it or collect on the surface and join together somewhere and form larger droplets with other droplets. Eventually, the droplets become large enough to either wash away or fall off the surface due to gravity. This type of media can generally consist of a very low-energy polymer such as a fluoropolymer or silicone.
The process of coalescing in depth is similar to the case where solid particles are removed by being trapped in the fiber structure of the media. When the liquid droplets are trapped in the media fibers, they either aggregate with other nearby droplets or collide with other droplets that approach and penetrate the environment. For this to happen, the fibers must have a relatively high surface energy, contrary to the surface coalescence mechanism. Most media that work with this type of mechanism include fine fibers of fiberglass. Finer fibers filter smaller liquid particles better. It should be noted that for best performance, the fiber surface should be neither too hydrophobic (oleophobic) nor too hydrophilic (oleophobic), but somewhere in between. If the fibers are too hydrophobic or oleophobic, liquid particles will not stick and coalescence will not occur. On the other hand, if they are too hydrophilic or oleophilic, the liquid particles will spread on the fiber surfaces. Figure 2 shows how the droplets coalesce on the fiber surface.
Coalescer elements are mainly designed as a multilayer structure. which have different configurations depending on the offending parameters such as the type of fluid, pollutant, and operating conditions. Figure 3 shows an example of the structure of the coalescer element. Coalescer filter in industrial applications mainly has a cylindrical and so-called sandwich structure, which consists of different layers. Composite filters usually consist of a cover layer and a drainage layer. Droplets carried by the fluid are transported through the coalescing layer to form a liquid layer or enlarged droplets on the back surface of the coalescing layer. The drainage layer provides interconnected channels for the fluid to flow smoothly out of the filter. In this structure, a layer can be considered as a pre-filter to remove solid and larger particles, otherwise, these particles may block the pores of the coalescer media.
Coalescer filter elements are made of specialized materials including microfiber, synthetic fibers, and thermoplastic resins. Each Coalescer media material is designed for specific needs with a variety of filtration grades for different functions. The most common media used in coalescers are fiberglass fibers. This type of media is inherently hydrophobic, as water forms on these fibers in the form of drops instead of a film. Conditions that are favorable for continued filtration efficiency. Microfiber glass fibers in the diameter range of 0.5-0.75 µm typically provide the best results as a Coaler filter media. The depth of the fiber bed and the ratio of the empty space to the fiber space are very important for the proper functioning of the coalescer [2]. Among other fibers used for the elements of the coalescer, we can refer to the borosilicate glass microfibers network. These fibers have biological and neutral chemical properties and have significant advantages. There are other media such as metal, plastic, paper, etc., which do not show the same natural properties as borosilicate glass microfibers, in fact, metal and plastic elements do not have fibers to convert liquid contamination into droplets. And only one movie is made on these elements. Paper elements also tend to retain water droplets like a sponge, even though they have fibers.
While the overall configuration of the element determines the properties of the filter, the selection of the media can also play an important role in achieving the desired properties. Some of the effective factors in the selection of the coalescer media include the required efficiency and the pressure drop of the element in the clean state. compatibility with the chemical used, costs, and operating conditions such as pressure and temperature. In the following table, a comparison of fiberglass and cellulose media is given as Coalescer media mentioned by H&V, a producer of filter media.
Polymer binders are added to reinforce the media structure. A certain amount of binder is required to maintain structural integrity and durability. The coalescer element may be exposed to high temperature, high pressure, and high flow rate conditions. The binder material must provide sufficient structural strength for the media to withstand these conditions throughout its life, but should not make the media inflexible or prone to breakage. Several types of polymers are used as binders. Epoxy, acrylic, and phenol formaldehyde resins are common polymers used as binders. Traditional manufacturing processes use volatile organic solvents such as acetone to apply the polymer adhesive to the fiberglass fibers. Environmental regulations prohibit the use of these solvents because they are considered potentially hazardous. To solve this problem, industries are turning to water-dispersible polymers that can be used as adhesives[4].
Physical Properties of Fluids: Some fluid properties such as viscosity, density, and surface tension of immiscible fluids significantly affect the mechanism of operation of coalescer filter elements.
Particle/Droplet Size Distribution: The size distribution of particles or droplets in the fluid determines how effectively they can be collected and combined. In fact, smaller particles may bypass the initial adsorption stage and reduce overall efficiency.
Flow Rate and Retention Time: The flow rate of the fluid passing through the coalescer affects the residence time of the particles in the system. Slower flow rates can improve the efficiency of coalescers by giving more time for the droplets to collide and adhere.
Filter Element Structure and Configuration: The number of layers of coalescer media and their arrangement and material affect the efficiency of the coalescing process.
Process Operating Conditions: Factors such as temperature, pressure, and fluid properties can vary based on the specific application. Coalescer efficiency may change under different operating conditions.
Drainage Layer: Coalescer filter elements have a drainage channel in the coalescer filter medium, the drainage channel is a woven or non-woven fibrous structure that has pores. The size is larger than the pore size of the coalescer filter medium and has a surface energy lower than the surface energy of the coalescer media, creating suitable drainage channels for the droplets to aggregate at a downward angle plays an effective role in efficiency and performance.
Fiber Diameter: Generally, reducing the fiber size improves the overall separation efficiency of the process. For example, as previously mentioned, microfiber fibers in the diameter range of 0.5-0.75 µm usually provide the best results as a coalescer filter medium.
Research and studies on coalescing filtration are mainly divided into two categories: the first is the individual fiber surface and the second is the entire filter surface. At the fiber surface, a number of studies have analyzed the forces acting on the droplets and their shape on the fibers. Coalescence and droplet motion along fibers have also been investigated, and the interactions between fibers and droplets have been considered. These results can be used to numerically simulate droplet motion in fiber systems, but this simulation cannot describe the actual behavior that occurs in the filtration process. At the whole filter level, studies have focused on planar materials or entire filter elements, which include the effects of slope, fluid velocity, and fluid concentration on filtration performance, and efficiency and pressure drop models are developed based on the results of macroscopic experiments.
One of the parameters in the coalescence process is surface tension. In fact, surface tension plays an important role in the performance of coalescers. There are two types of surface tension. It is better to explain the difference between the two in this section. Surface tension is defined as a single liquid surface, while interfacial tension is defined for the interface between two immiscible liquids. Interfacial tension (IFT) is used to describe the surface forces between liquids and solids. Both surface tension is measured in terms of force per meter (N/m) or energy per unit area (J/m2).
As previously mentioned, the mechanism of coalescence occurs when droplets form on the surface of fibers and grow larger, and a droplet forms when the balance of surface tension and interfacial tension is reached. To better understand this and the mechanism of coalescence, consider Figure 4. There are actually three forces acting at point X:
In fact, the forces acting on a surface or interface act in a direction that reduces the area of that surface. γSG pulls at point X to spread the droplet over the solid surface and reduce the exposed solid surface. On the other hand, γSL pulls in the opposite direction and tries to reduce the interface or contact area between the water droplet and the solid surface. The surface tension, γL, tries to reduce the exposed surface of the droplet by pulling it in an upward direction as if the droplet were trying to lift itself. The force exerted at point X by γL is tangent to the droplet surface at point X where it intersects the solid surface.
The solid surface force γSG is very important. If the force is very strong, it has a high-energy surface and pulls the droplet to wet the surface. If it is very weak, it is a low-energy surface (Teflon is an example of a low-energy surface material), and the solid-liquid force γSL dominates it and pulls the droplet surface inwards. A spherical surface will have a zero contact angle. The spherical droplet will have a non-surface area to adhere to the solid surface and will roll like a ball. It may collide with other droplets and become larger droplets. And so the coalescence process takes place.
In relation to the coalescence process, it separates two liquid (liquid-liquid) emulsions that are completely dispersed and immiscible by passing them through a deep-bed fibrous matrix such as a nonwoven fabric. For example, in an emulsion where water (the discontinuous phase) and contaminated fuel (the continuous or primary phase) pass through the medium, the water droplets break off when the fibers impact the medium. Interfacial tension causes the water droplets to separate from the emulsion and be attracted to each other by the fibers. They separate from the fibers and eventually, as the heavy phase, fall by gravity. As a rule, coalescers lose their efficiency when the interfacial tension difference (IFT) of the two constituent fluids falls below 20 dynes/cm.
For example, surfactants make water removal difficult for conventional liquid/liquid coalescers and other water separation equipment because they reduce the interfacial tension between water and the continuous phase fluid (e.g., fuel). The key is to use an optimal fiber medium when separating liquids with low IFT.
Of course, a liquid-liquid coalescer can be designed to allow for emulsions with low interfacial tension difference (IFT) that are challenging to separate with conventional technologies. High-efficiency coalescers can be formulated with special media, layers, or modifications to enhance the separation of more complex emulsions.
Coalescers are primarily used for the separation of liquid particles and droplets and aerosols. However, these types of elements may naturally absorb some dust and solid particles in addition to liquids, which can increase the pressure drop in the system and clog the coalescer filter in environments with a lot of dust. To prevent such problems, it is necessary to take measures to improve the performance and increase the life of the coalescer element. For example, using a pre-filter before a coalescer can trap larger particles and dust and lead to increased coalescer efficiency. Also, continuous monitoring and observing tips regarding the operation of these types of elements can be useful to prevent problems.
As mentioned earlier, liquid/liquid coalescers are used to separate liquid pollutants from a liquid fluid such as fuel or oil, etc. This type of coalescer is used in industrial processes such as separating water from fuels, removing caustic carried over from refinery fuels, and separating various oils from water.
In fact, the specific gravity, viscosity, and surface tension of the two liquid phases are key parameters for separating two liquid phases from each other.
In liquid-liquid coalescers, for example, to separate water droplets from oil or fuel, oil is passed through the coalescer media, and at the same time, the water droplets hit the fibers and are trapped. At the same time, the oil droplets are also placed on the media until the oil layer thins due to surface tension and eventually breaks. The water droplets are released and will stick completely to the fibers, and gradually these droplets become larger and are separated from the fibers by gravity and poured to the bottom of the chamber. The filtered oil passes through a PTFE-coated metal mesh, which ensures the absence of residual water. The water-free oil is finally discharged from the top of the filter.
As previously mentioned, coalescers are responsible for separating liquid droplets and moisture from other fluids. They have different configurations according to operating conditions. Some can be horizontal and vertical. Figure 5 shows how a gas coalescer filter works and an example of gas-liquid coalescer elements.
Today, as mentioned in the previous sections, coalescer media are mainly fiberglass. However, in recent years, other media have been introduced to improve the performance of coalescer filters. Some of them have become commercial and others are at the laboratory and research level. Examples of new technologies are given below.
The use of nanofibers has also been recommended to improve the performance of the surface of coalescer elements. In fact, the use of nanofibers is one of the latest developments in coalescer media technology. Nanofibers, due to their high effective surface area and high particle adsorption capacity, can improve separation efficiency. And lead to a reduction in pressure drop during the process. Figure 6 shows examples of electron microscope photographs of media with fiberglass and nanofibers.
In a study conducted, four different waste polymer fibers were investigated as filter bed materials for a new coalescer: polyurethane (PU), polyethylene terephthalate of different origins (PET1 and PET2), and polypropylene (PP). These different polymers were tested as coalescer media for the separation of liquid aerosols. In this study, metalworking fluid (MWF) with a 6% MWF emulsion was used as the test liquid aerosol. The results of this study showed that these polymer fibers can be used as coalescer media for specific applications. The findings indicate that the new coalescer can be easily optimized depending on the inlet air quality and the permissible PM1 level in the exhaust air.
Coalescer filters have very important applications in various industries. These devices are mainly used to remove oil and water droplets and aerosols from compressed air systems. By removing unwanted moisture or oil from the air stream, coalescer filters not only improve the quality of compressed air, but also prevent the accumulation of lubricants, condensed water and oil on pipes, fittings and equipment. By doing so, coalescer filters ensure optimal performance, increase the service life of equipment and reduce the need for cleaning and maintenance.
In a natural gas pipeline, the presence of liquid droplets over long distances may cause erosion and corrosion of the pipeline and process equipment. Also, in a gas turbine, the presence of liquid droplets can lead to erosion and deposition on parts and reduce the life of the gas turbine. Therefore, the use of coalescers to remove fine droplets and aerosols can be beneficial. As mentioned earlier, coalescers use a specific mechanism to trap pollutants, and considering parameters such as viscosity, surface tension, fiber diameter, operating conditions, etc. is important for their design and to improve efficiency. Currently, extensive technologies and research are being conducted to improve the quality of these filters and their media.
[1] Sutherland, K. S., & Chase, G. (2011). Filters and filtration handbook. Elsevier.
[2] Hutten, I. M. (2007). Handbook of nonwoven filter media. Elsevier.
[3] Chang, C., Ji, Z., & Zeng, F. (2016). The effect of a drainage layer on filtration performance of coalescing filters. Separation and Purification Technology, 170, 370-376.
[4] https://www.hollingsworth-vose.com/wp-content/uploads/Coalescer-Media.pdf
[5] Vasudevan, G., & Chase, G. G. (2004). Performance of B–E-glass fiber media in coalescence filtration. Journal of aerosol science, 35(1), 83-91.
[6] Bhatta, N., Tafreshi, H. V., & Pourdeyhimi, B. (2024). Toward formulating coalescence filtration: Characterizing wetting saturation via centrifugal force. International Journal of Multiphase Flow, 170, 104641.
[7] Singh, C. J., Mukhopadhyay, S., & Rengasamy, R. S. (2022). Fibrous coalescence filtration in treating oily wastewater: A review. Journal of Industrial Textiles, 51(3_suppl), 3648S-3682S.
[8] Sokolović, D., Laminger, T., Mauschitz, G., & Höflinger, W. (2021). Novel coalescer design with bed of waste polymer fibers for liquid aerosol separation. Separation and Purification Technology, 263, 118187.
Author: Forough Khalili
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