Clearing hazy product
In a clean-fuels desulfurization process such as the one for naphtha in Figure 2, the final processing step for the clean fuel is typically a conventional liquid-liquid coalescer unit that removes suspended droplets of sour water. However, the coalescer output usually contains some very fine water droplets in the size range from 2 to 10 microns, causing the appearance of a faint haze. Even if the hydrocarbon liquid is clear, it may be saturated with dissolved water. Then when the material is pumped to a storage tank at ambient temperature, it may cool considerably, causing some water to emerge from solution as haze.
According to stringent haze requirements in clean-fuel specifications, most if not all of the suspended water must be removed before a product can be blended and sold. The traditional way to clear up haze is to let the product rest undisturbed in a storage tank until the water settles to the bottom. However, gravity settling of such fine droplets may take two to five days occasionally longer.
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Faster removal of the haze requires a high-efficiency, high-capacity liquid-liquid coalescer that is specifically designed to remove water haze from hydrocarbon liquids having the characteristics of the fuel involved. As seen in Figure 8, the hazy fuel passes through special coalescer elements that force the tiny water droplets to merge into larger globules, which efficiently settle out by gravity. Such a coalescer can be provided in the clean-fuels unit, preceded by a cooler if necessary to reduce the product temperature to the ambient range. Alternatively, the refinery can obtain a portable coalescer mounted on a trailer, and haul it from one tank to another in the tank farm as needed. Product in a tank is recirculated through the unit until the haze clears up.
In either case, selecting an appropriate coalescer depends on understanding that the water haziness is associated with low interfacial tension between the water and fuel. Interfacial tension is the name used for surface tension when the two phases involved are both liquid instead of liquid and gas. It is caused by the weakness of the attraction between hydrocarbon and water molecules facing each other across the interface, compared with the much stronger attraction between hydrocarbon molecules on one side and between water molecules on the other.
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The interface serves as a film that behaves somewhat like a stretched sheet of rubber. Unlike rubber, however, the tension—in terms of force per unit of length, or energy per unit of area—is constant rather than increasing when the film is extended. Interfacial tension can be measured by various laboratory instruments in ways similar to measurement of surface tension. It is typically reduced by impurities called surfactants that happen to be present in the water and hydrocarbon phases, either in solution or as very fine particles, and that gather at the interface.
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The lower the tension in the interface film around each tiny water droplet, the less readily the droplets will coalesce—that is, merge into larger ones or into a continuous phase, thus eliminating the haze. This point is illustrated in Figure 9 in the case of one droplet of water approaching a layer of water below. That layer has an interface film of its own. The force driving the droplet—gravity, in this case—presses the droplet’s film against that of the water layer, driving hydrocarbon out of the space between. Soon the two films come so close together that they act almost as one—a very thin layer of hydrocarbon with water on both sides, like the wall of a soap bubble.
If any disturbance allows the two films forming the sides of the hydrocarbon wall to join in a ring around some tiny point, interfacial tension will suddenly pull the film into a flatter shape, collapsing the droplet into the water layer. The same phenomenon occurs when two droplets are driven close to each other, merging to form one larger droplet with less total interface area than the two separate droplets had.
In either case, the higher the interfacial tension, the more strongly the joined films will be pulled flatter, and the more likely that a slight disturbance between the two films will result in such a collapse. Conversely, lower interfacial tension causes greater stability in the hydrocarbon wall between the droplet and the water layer, or between two droplets touching each other. This is why air bubbles are promoted by soap and other detergents, which act as surfactants in water. That same principle applies to microscopic droplets of water in fuel coming from an HDS plant; traces of surfactants reduce the interfacial tension and thus make the haze droplets very slow to coalesce.
Lower surface tension also interferes with the action of coalescer elements in a liquid-liquid coalescer vessel, as suggested in Figure 10 for a mesh-type element. The operating principle is for a haze droplet to strike a filament and be held there until it merges with others to form a much larger droplet. The agglomerated droplet grows until it is too large to be held by the filament, then is swept away. The lower the interfacial tension, the narrower the contact angle between the strand and the surface of the clinging droplet, and the more easily the water droplet will be pulled away by the flowing fuel before coalescing with very many other droplets. Thus, lower interfacial tension causes the agglomerated droplets emerging from a given coalescer stage to be smaller than with higher tension, and thus slower to settle by gravity.
A properly selected coalescer arrangement to cope with low interfacial tension in haze removal will consist of two sections. The first section will mechanically break the emulsion by coalescing the smallest haze droplets as in Figure 10. The resulting larger droplets are still too small to settle efficiently. However, they are easily coalesced in the second section, forming even larger globules that rapidly fall and merge into a continuous water layer. A generic coalescer pad that does not have the first section will simply fail to remove haze.
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