By Shagufta Patel, John Krogue and Michelle Hewitt, PECOFacet (USA), Inc.
Natural gas processing is a complex system designed to clean raw natural gas by removing impurities and separating and recovering various non-methane hydrocarbons and fluids to produce dry, pipeline quality, natural gas. The composition of natural gas varies in different parts of the world. Its chief component, methane, usually makes up 80% to 95% of its composition. The balance is composed of varying amounts of ethane, propane, butane, and other hydrocarbon compounds. When processed and purified into finished by-products, all of these are collectively referred to as Natural Gas Liquids (NGL). In addition, raw natural gas contains water vapor, hydrogen sulfide, carbon dioxide, helium, nitrogen, and other compounds including solid and liquid contaminants. Clean natural gas is required for improving plant performance, pipeline performance and minimizing damage to compressors and other critical process equipment.
Several technologies are used to remove solid and liquid contaminants from natural gas, including: cyclone separators, gravity separators, mist eliminators, filter-separators and gas-liquid coalescers. Gas-liquid coalescers are high efficiency filtration systems, which provide effluent cleanliness down to parts per billion. These are used to polish the gas and are often present downstream of bulk removal separators and filter-separators. These coalescers contain filter cartridges with a media matrix of fine fibers, which are highly efficient in capturing fine aerosols and coalescing (growing) them into larger droplets, which can then be drained using gravity. Traditionally, gas-liquid coalescer cartridges have used glass fibers in a pleated or depth style. Newer technologies have been able to create a polymeric, depth media using a concept called Saturated Depth Coalescing™.
Saturated Depth Coalescing is a mechanism that utilizes a thick, polymeric fiber matrix. It has a larger pore size and optimized porosity versus a traditional glass matrix. Most coalescers using glass fibers, due to the smaller pore sizes, require fluorocarbon treatment to have droplets drain quickly in order to keep pressure drop low. Saturated depth coalescers do not require this treatment, which allows the droplets to remain for a longer period of time within the fiber matrix and saturate it. This saturation of the matrix is important because it allows droplets carried by the gas to not only collide with fibers, but also with previously captured droplets. Another benefit of saturated depth coalescers is that many solids that come in with the gas stream adhere to the liquid droplets that are held within the fiber matrix. As the droplets drain they carry these solids with them. This “self-cleaning” effect keeps the matrix clean and minimizes the pressure drop. Figures 1 and 2 show the solid contaminants and droplets captured on the fibers.
The separation efficiency of a coalescer depends upon the viscosity of the liquid, drop size, gas velocity, pressure, temperature of the gas, structure of the filter medium (fiber diameter, fiber orientations and packing density), surface properties of the fibers, binder content, and filter thickness. The surface energy, or wettability, of the fibers in the coalescing filter media strongly affects the performance of coalescing filters. High surface energy fibers capture and hold onto droplets, slowing their movement through the filter and hence increase coalescence between drops. Low surface energy fibers allow drops to slip through the filter with little or no hindrance, but do not contribute significantly to the coalescence. One of the more common ways to determine
the surface energy of a filter media is by measuring the contact angle of the known surface tension liquid on the surface of the media using the Sessile Drop Method. The shape of the drop is then fitted with a polynomial to obtain the contact angle. The test liquid can be selected based on the filtration application. Many different liquids can be tested. Figure 3 shows an example using water where a small contact angle (< 90┬░) is observed when the water spreads on the filter media. This means the filter media is hydrophilic. Figure 4 shows an example using water where a large contact angle (> 90┬░) is observed when the water beads on the filter media. This means the filter media is hydrophobic. As an example, glass fiber media completely wets with water and the contact angle is ~0┬░ as the droplet turns into a puddle. Contrast that to a water droplet on a fluoropolymer, such as PTFE, which beads up with a contact angle near 120┬░. A similar terminology would be applied to oil and would be referred to as oleophilic and oleophobic, respectively. A small contact angle is very good at assisting in coalescing by holding onto the droplet so it can grow. A large contact angle does not assist in coalescence but does promote drainage. The effective design of a coalescer has to perform coalescence and drainage at the same time. A polymeric saturated depth coalescer has an intermediate surface energy providing effective coalescing with adequate drainage within the fiber matrix. This allows the efficient capture, growth and release of the liquid droplets without the need for hydrophobic coatings or separate hydrophilic materials.
Performance of the coalescing filter depends upon its liquid saturation. Liquid saturation of a filter medium is defined as the volume fraction of the pores filled by the liquid phase and largely depends on the surface energy of the fibers. Saturation directly affects the pressure drop and local gas velocity within the medium. Since higher liquid saturation results in higher pressure drop, optimizing the surface energy of the coalescing filter is very important. Efficient saturation and drainage of saturated depth coalescers are responsible for their increased on-stream life with moderate pressure drop without compromising capture efficiency. This leads to longer operational life of the filter, which benefits the economics of the natural gas cleaning process.
PECOFacet is a global filtration and separation products and services company. Committed to delivering innovative solutions, PECOFacet collaborates with its customers to help them realize improved plant availability, reduced environmental impact and enhanced bottom line performance. PECOFacet wants to be the customer’s first choice in providing innovative, contaminant management technology that drives safe, cost-effective, reliable and sustainable solutions. PECOFacet is a subsidiary of CLARCOR Corporation.
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