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Technical Article On KX Carbon Filters

Big Brand Water Filter Cartridges

Technical Article On KX Carbon Filters

KX INDUSTRIES, L.P. : TECHNOLOGY Activated carbon and particle-reduction filters operate by a combination of adsorption and physical interception. Here is a technical description of how these all work to produce pure, safe, and good-tasting water. "Buckle up -- it's a bit technical." Activated carbon adsorbs contaminant molecules because there is a reduction in the "surface energy" of the activated carbon when adsorption takes place. This reduction in energy is "adsorption energy" and is released as heat when adsorption takes place. There is a gradient of adsorption sites within the carbon, ranging from "high-energy sites" to "low-energy sites." The shape of a curve called an "Adsorption Isotherm" tells us indirectly about the volume of adsorption sites within the carbon at various adsorption energies. (KX Industries routinely performs this test on incoming carbons.) Adsorption first takes place in the higher-energy sites and progressively fills the lower-energy sites. As carbon is activated to create more surface, the capacity of the carbon to hold more adsorbate increases. However, as this happens, the number of high-energy sites decreases. These higher-energy sites are needed to adsorb molecules in low concentration. In some applications, excessively activated carbon will produce a poor result. Since microporous coconut-shell carbons have a large population of high-energy sites, they are very useful in removing low-concentrations of difficult-to-adsborb contaminants, such as Volatile Organic Compounds (VOC�s). As a general rule, the heat of adsorption increases as the size of the molecule increases. For most activated carbons, adsorption is more efficient with nonpolar molecules. Molecules with greater adsorption energy displace molecules of lower adsorption energy. Very large molecules may not be capable of accessing the available adsorption sites, because they are too large to pass into the available pore structure, especially for microporous carbons. Examples of such "sterically hindered" molecules are humic and fulvic acids and color bodies. To best adsorb these large molecules, one needs to use macroporous carbons, whose pores are large and where modest adsorption energy is sufficient to retain the contaminants. The reduction of free chlorine to chloride ion is a catalytic reaction on the surface of the activated carbon. The rate of reaction is extremely rapid and is limited primarily by the rate that free chlorine can diffuse to the carbon surface. This reaction is increased as carbon particle size is reduced, surface area is increased, or carbon interpore diffusivity is increased. Carbon block, which uses powdered carbon, has enormously enhanced dechlorination capability - - often 10 to 100 times greater than a similar weight of granular activated carbon (GAC). Higher activity carbon enhances dechlorination as a result of its increased carbon surface area, but only on a roughly linear basis. Microporous carbons, such as coconut-shell carbon, often exhibit poor interpore diffusivity, and are not necessarily the best carbon for dechlorination. Macroporous or mesoporous carbons often provide a better rate of dechlorination. Increasingly, chloramines are used in residential water treatment systems, because they produce fewer chlorinated organic contaminants than free chlorine and are more stable. Chloramines react slowly with carbon, so the diffusivity of the chloramine is less important than enhancements in the reaction rate. This means that the recommended carbon is different from that used for dechlorination. What is optimal for chloramine reduction is a fine-mesh carbon that has high adsorption energy and maximum surface chemical reactivity. In axial flow, fluid enters one end of the filter and flows the entire length of the cylinder, exiting the other end. Most GAC filters employ axial flow. The area presented to flow is the cross section of the cylinder and the fluid must traverse the entire length of the filter, often about 8" for a nominal 10" filter. Under radial flow conditions, the fluid enters through the entire exterior wall of the cylindrical filter and flows through the wall into an interior hollow space, where it exits the filter. Assuming the same exterior dimensions, the radial flow cylinder presents 10 times the surface area to flow vs. the axial filter and the flow depth is the thickness of the wall, often only about an inch. This combination of higher flow area and shorter flow depth means that the potential pressure required to operate a radial-flow filter can be 100 times less than an axial-flow device. Extruded block carbon filters take advantage of this by using much finer powdered activated carbons without suffering a pressure drop penalty vs. the much coarser GAC axial flow filters. The powdered activated carbons deliver enormously superior performance vs. coarse-grained carbons. The advantage of radial-flow filters is lost if the Inside Diameter (ID) of the filter becomes too small, resulting in dramatically increased pressure drop. The advantage is also lost if the length of the filter becomes very short. Under these conditions, axial flow conditions might become advantageous. For a more detailed insight on Axial vs. Radial flow, read " The Features and Benefits of Extruded Carbon Filters " by Dr. Evan Koslow, CEO, KX Industries, L.P. Particles larger than 3 �m are generally removed by a sieving mechanism. Below 3 �m, particles are removed by a combination of sieving and adsorption. A technique called "Porometry" can be used to characterize the pore structure of a filter. The largest pore determined by this technique is called the "Bubble Point" (KX Industries routinely performs this test on flat sheet prefiltration media used in its filters). Particles intercepted within a filter are generally much smaller than the pores measured by porometry. This is because while the fluid sees a big pore, the particle has to navigate turns, twists, deadends and asperities (things that stick out into the pore space) within the structure. The result is that particles intercepted within a pore are often 0.2 - 0.4 the size of the measured pore. If the particle is much smaller than the pore, it will have a high probability of passing through. If it is nearly the same size as the pore, it will probably never enter the pore. As a rule of thumb, particles efficiently removed from a liquid are about one-third the size of the measured filter pores. For example, if the largest pore of a filter (as determined by Bubble Point) is 1.5 �m, it will generally achieve an effecient reduction of 0.5 �m particles. Particle removal in gases is entirely different from particle removal in liquids. A gas filter�s pores are generally much larger than the particles removed from the flowing gas. This is possible because entirely different mechanisms are available to support particle interception. One such mechanism is "impaction." The particle, while attempting to pass around an obstacle in its path (for example, a fiber or adsorbent particle within the filter), diverts by momentum from the flow stream and hits the obstacle. It "sticks" to the object because viscous drag forces in a gas are small compared to adhesion forces. Another mechanism of particle removal is "diffusion." Small particles vibrate in response to the impact of gas molecules, forcing the particle to divert from the flow path and adhere to the filter. This mechanism favors small particles and low velocities that allow more time to diffuse. A third mechanism is "sieving". If the particles are large enough, the filter forms a traditional sieve and interception is accomplished by physically preventing passage of the particles. Unlike liquid filtration, sieving is generally not a major contributor to gas filter performance. This is the only major mechanism available in water filters down to the sub-micron range where adsorption might occur. Finally, certain air filters can be electrostatically charged so that particles become attracted to their surface by electrostatic forces. Electrostatic charge provides enhanced removal efficiency so that charged filter structures can provide better efficiency for a given pressure drop. Particles sticking to the filter medium eventually form a "dirt cake" that participates in further removal of particles moving through the filter. This means that the filter�s efficiency will change with time. The dirt cake�s contribution to long-term filter efficiency is complex. Most importantly, in a gas filter, efficiency is extremely dependent upon flow rate, while in liquids, interception is generally independent of flow rate for stable filter media like carbon block. Rule 1 You can�t hold dirt if you have nowhere to keep it . Nonwoven materials are used by KX Industries to obtain enhanced filtration. Most nonwovens have 90% open area (high porosity). This means they have much greater capacity to hold dirt than carbon block, which often has an interparticle porosity of approximately 35%. Rule 2 You can either spread dirt around or bury it in depth to keep a clear path open . Failure to keep a clear path open will result in low dirt capacity, prematurely high pressure drops and short filter life. Rule 3 It�s better if you spread the dirt around . Spread the dirt around by increasing the surface area of filter medium presented to the incoming fluid. Pleat the filter, i.e. fold the flat-sheet medium into a compact space. This reduces "flow density," which is the velocity of fluid passing through the prefiltration medium. The optimal number of pleats depends on a tradeoff between two factors. As more pleats are added, the trans-medium pressure drop decreases. However, as more pleats are added, the spaces between the pleats get smaller, adding resistance to flow through the narrow gap between the pleats. As the number of pleats increases, a point is reached where pressure drop increases rapidly due to the effective disappearance of these gaps between the pleats. If pressure drop across the filter is plotted vs. number of pleats, you�ll observe a decreasing pressure drop as pleats are added, until a minimum pressure drop is reached. If more pleats are added beyond this point, overall pressure drop climbs very quickly. Flat-panel pleated filters and radial-flow pleated filters are geometrically very different. KX Industries has computer models to determine optimum pleat structures in both cases. Rule 4 It�s better if you simultaneously spread the dirt around and bury it in depth . To bury the dirt, one needs to create a pore-size gradient where particles of varying size pass to varying depths within the filter medium. This spreads the dirt cake over a large volume of filter medium - a depth filter. If this depth-medium is also optimally pleated, the result should be maximum dirt holding capacity. We often refer to the filter element that actually reduces contaminants as the "filtration software." The filter is designed to interface into the "hardware," which includes the filter housing and any associated plumbing. Just like the computer industry, the hardware varies, but is generally a secondary consideration. It doesn�t matter if the housing is electropolished stainless steel or simple plastic; if the filter you install is of poor quality, the system will perform poorly. The filter, therefore, consummates the investment made in hardware. A poor-quality filter can obviate the hardware investment. A critical element of combining software and hardware is the interface used to create a leak-proof seal. While compression gaskets have been used in very high volumes during the past 50 years, O-rings have become increasingly common and are generally preferred for high-integrity applications. Compression gaskets require tight tolerances on the length of the filter, whereas O-rings often require tight tolerances on plastic parts. The latter is preferred. Pharmaceutical-grade filters use double O-ring seals. The first O-ring serves as a wiper and the second as a secure seal. In most cases, both O-rings seal and a double failure path is essentially impossible. Refined engineering and design is the first step in creating high-quality products that deliver high performance. Basic materials technology assures that the product meets customer requirements and maintains integrity. But to assure quality, we need a design that is easily manufactured and avoids critical failure modes and excessively tight tolerances. An easily manufactured product is simple, fool-proof, self-assembling, requires minimal automation and labor, allows detection of defects, requires minimal tooling, and avoids excessive components and materials. Close coordination between the design teams at KX Industries and the customer at the earliest possible stages increases the probability of high "manufacturability." The application of hard-won experience, careful attention to detail, and free interchange of ideas at the earliest design stages, and continuing throughout the development process, can eliminate costly mistakes.
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