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"Coalescing Technology: An Overview"
Coalescing is one of the separation technique used worldwide by the process industry for liquid-liquid and liquidgas separation. This technique involves coalescing of small particles or droplets into larger ones by means of barrier or electrical energy. It is one of the most cost effective methods used today as compared to salt driers, settlers or thermal / vacuum separators. Nowadays, coalescers are often considered preferable to conventional gravity separators. It requires less capital, low operating cost, minimal maintenance and requires less floor space.

Coalescence is the process by which two or more small droplets come together to form larger droplets. It works by attracting miniscule droplets of water or other liquid, possibly even as small as a single molecule. The droplets come together by the force of molecular attraction and surface tension. When they reach a sufficient size, they can be removed from the system by suitable means.
When two liquids are immiscible, or nonsoluble in one another, they can form either an emulsion or a colloidal suspension. In either of these mixtures, the dispersed liquid forms droplets in the continuous phase. Traditionally, gravity separators were used to handle emulsions before the use of coalescer became common. In this equipment, differences in densities of the two liquids cause droplets to rise or fall by their buoyancy. The greater the difference in densities, the easier the separation becomes. Rising or falling droplets are slowed by frictional forces from viscous effects of the opposing liquid. This mechanism of separating liquids by gravity is called Stokes Settling. The various steps of coalescing are:
• Collection of droplets
• Small droplets coming together to form larger ones
• Rise or fall of the enlarged droplets
It is very important to understand the characteristics of the emulsion that has to be treated. The finer the droplets dispersed in an emulsion, the more stable it is, because the buoyancy force diminishes in magnitude as the diameter decreases. The manner in which the mixture is created Coalescing Technology: An Overview Coalescing is one of the separation technique used worldwide by the process industry for liquid-liquid and liquidgas separation. This technique involves coalescing of small particles or droplets into larger ones by means of barrier or electrical energy. It is one of the most cost effective methods used today as compared to salt driers, settlers or thermal / vacuum separators. Nowadays, coalescers are often considered preferable to conventional gravity separators. It requires less capital, low operating cost, minimal maintenance and requires less floor space. effects the droplet size distribution. It is also important to know how much time has elapsed since the mixing/shearing occurred. This is because as time goes on, smaller droplets aggregate or coalesce and larger droplets are more likely to have joined a separate layer so that they are no longer considered to be entrained. An important tool to quantify an emulsion is the Droplet Size Distribution Curve generated by plotting the droplet diameters against the volume. A coalescer is often needed for mature emulsion.
Mixtures of immiscible liquids can generally be separated by a process of settling as a result of the density difference between the two phases. However gravitational settling becomes increasingly difficult as the droplet size of the dispersed phase decreases. Coalescers uses mats, beds or layers of porous or fibrous material whose properties are especially suited for coalescing purpose. The settling process can be enhanced considerably by passing the dispersion through a suitable coalescer pack or providing electrical energy. There are two types of coalescer generally available in the market i.e. Electrostatic and Mechanical Coalescers.

Electrostatic Coalescer
An electrostatic coalescer uses weak electric charges to attract molecules of water to the surface where they undergo collection. Here a weak electric charge is passed through a collection device which imparts a tiny charge that attracts molecules of water or other matter designated for removal.
Here, electrostatic force is used to break oil-water emulsions and subsequently increase in water droplet size. This technology is quite common in offshore production facilities to ensure the maximum allowable water content in oil is less than one percent. A coalescer forces small water droplets to merge and form larger and thus faster separation. Therefore, the settling velocity of water droplets in oil not only depends on viscosity and density, but also on the droplet radius.
The main feature of electrostatic coalescers is the effect its electrostatic field strength has on the conductive droplets such as water in an insulating medium such as oil. In the presence of an electric field, the water droplets become dipoles whose electric charges can overcome the repulsive surfacesurface interactions, resulting in oil film drainage and consecutive coalescence. The coalescing of droplets is mainly dependent on electrostatic induced forces, film rheology, collision frequency depending on laminar or turbulence level and concentration.

Mechanical Coalescer
Mechanical coalescer uses a series of filters or dividers, known as baffles, to induce small drops to move from a mixture and to collect together. A mechanical coalescer relies on a series of barriers, known as baffles or filters, made of very fine mesh, knitted polymer, corrugated sheet or fiberglass fibers, etc. The vapor or liquid passes through the filter, and is attracted to the filter material or the surface of the baffle.
The principle by which this is done is to pass the water-contaminated oil through a thick inorganic fibre bed or filter mat. Water droplets are intercepted by the fibres. The oil on the fibres is thinned by displacement and the effect of viscous drag, until ultimately the oil film ruptures and allows the water droplets to attach themselves completely to the fibre, with the oil film dispersed and passed on through the mat. Other water droplets are now collected by the fibres in a similar manner, and these will join with others, forming streams along the fibres. The droplets continue to grow in size until drag and gravity forces break them away from the fibre, and they drop off from the filter mat.
Mechanical coalescer is used for liquid-liquid or liquid-gas separation. Separating liquid??liquid dispersions can be difficult and costly, depending on the physical properties of the two liquid phases. In liquid-liquid phase, the coalescing phenomenon is dependent upon coalescing media, specific gravity, viscosity and interfacial tension of the two liquid phases. They help in determining how easily two fluids can be separated.
Liquid-liquid coalescers can be used to separate hydrocarbons from water phases such as oil removal from produced water. They are also used for removing quench water in ethylene plants, last traces of contaminants like amine or caustic from intermediate products in oil refineries, last stage dewatering of final products like kerosene or jet fuel, LPG, gasoline and diesel. Typically, a liquid-liquid coalescer can be designed for a process inlet discontinuous phase concentrations up to 10 per cent and reduce them to ppm levels in the outlet for interfacial tensions as low as 0.5 dyne/cm. Coalescers typically will have a service life of 1 to 2 years when protected adequately by prefiltration.
Liquid-gas coalescers are widely used in oil and gas Industry to remove water and hydrocarbon liquids to < 0.01 parts per million by weight and particulate matter less than 0.3 um in size from natural gas to ensure natural gas quality and protect downstream equipment such as compressors, gas turbines, amine or glycol absorbers, molecular sieves, metering stations, mercury guard beds, gas fired heaters or furnaces, heat exchangers or gas-gas purification membranes. Liquids from upstream of the compressor, which may include aerosol particles, entrained liquids or large volumes of liquids called „slugs‰ and which may be water or a combination of hydrocarbon liquids should be removed by a coalescer located upstream of the compressor. The typical service life for liquid-gas coalescer element is 1??2 years. They are generally sized for a clean differential pressure of 2??5 psi, and are replaced with a new element after 15 psi.

Points to Be Considered
It is very important to understand the mature emulsion formation process, the droplet diameter size distribution, selecting right media for the given fluid based on its physical and chemical properties, coalescer media depth, coalescer media placement, coalescer vessel configuration while selecting the coalescer.

Coalescing Media
Selecting the right coalescing media for the duty depends on many factors with initial consideration given to the droplet size range in the dispersion and the target separation performance. Some of the coalescing media used are: polymer, polyester, nylon, fluoropolymer, fiber glass, polytetrafluoroethylene fibres, polyurethane foam, fired boards of saffil fibers, polyamide fibers, matrix of stainless steel wool, multistage filter with cotton polyester and aspen wood fibers, aluminum or steel fibers coated with vinyl acrylic, polyethylene, or PVC, copolymer of acrylonitrile and methyl acrylate matrix, oleophobic/hydrophobic treated media, etc.
There are two types of media i.e. hydrophobic and oleophobic. A hydrophobic media is to separate water droplets and oleophobic media to separate oil and hydrocarbon droplets. Apart from this, there is surface treated coalescer elements used to enhance coalescing property, extend life, reduce fouling and lower saturated pressure drop.

Benefits
• Low Capital Cost
• Low Operational Cost
• Minimal Maintenance
• Low Energy Consumption
• Compact and less floor Space

Limitations
When coalescers used for separating fluids with a very high viscosity and a high solids loading, the highly viscous fluids will plug the coalescer media, reducing its efficiency. Increasing the operating temperature to reduce viscosity can help in overcoming this limitation. Apart from this, solids at high concentration can be problematic. Due to the fine pore structure of the coalescer medium, the solid particles block the pore thereby reducing the coalescing efficiency. It is recommended to install prefilter at the upstream of the coalescer assembly. Sometimes prefilter element and coalescing element comes in the single housing.

Typical Industrial Applications
Liquid-Liquid Coalescer:
• Pipeline Condensate in Gas Production
• Produced Water
• Petrochemical Final Products
• Caustic Treating
• Recover Liquid Catalysts
• Removing water from Lubr icating / Hydraulic Oil
• Fuel / Diesel Purification
• Solvents removal from effluents
• Solvent extraction
• Separation of dispersions formed by azeotropic distillation
• Vegetable Oil

Liquid-Gas Coalescer:
• Compressor Protection
• Amine/Glycol Contactor Protection
• Well Head Hydrate Inhibition
• Molecular Sieve Protection
• Low and Ultra Low NOx Burner Protection