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Improve suspended water

removal from fuelsA better understanding of molecular forces enhancesfree water separator selection

R. L. Brown, Jr., and T. H. Wines, Pall Corp., East Hills, N.Y.

Article reprint from the December 1993 issue, pgs 95-99. Used with permission.

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HYDROCARBON PROCESSING/ DECEMBER 1993

R. L. Brown, Jr., and T. H. Wines, Pall Corp.,East Hills, N.Y.

Molecular forces, such as interfacial tension, vis-cosity, relative density and temperature, con-trol suspended water removal from fuel/water

mixtures. A better understanding of these physical prop-

erties will assist engineers investigating separation tech-niques. Equations and case histories review several freewater removal methodologies such as salt driers, liq-uid/liquid coalescers, etc., and their effectiveness on emul-sion and surfactant-containing streams.

A big problem. Today, water contamination in refin-ery fuels can be a bigger problem than solids contami-nation. Water in fuel can corrode and plug engine partsand is a significant contributor to tank bottom corrosionand bacterial growth. In addition, water may containcorrosive materials like chlorides that will cause equip-ment damage. Unfortunately, it doesnt take much water

to cause a problem. Water concentrations as low as 100ppm can cause a product to be off-specification due tohaze, color or overall water concentration. Detergentsand additives that are surfactants make water removalmore difficult because they lower the interfacial tensionbetween water and the fuel.

Field tests conducted at two refiner-ies show how a stacked coalescer/sep-arator configuration with polymericmedium outperforms salt driers interms of total water removal fromdiesel fuel and do not disarm (lose effi-ciency to coalesce) when exposed to

surfactants like conventional glassfiber coalescers.

Difficult to remove. Two forms ofwater can be present in fuels: dissolvedor suspended as tiny droplets thatrange in size between 0.1m to 10min diameter. This size is so small thatit cannot be visually detected exceptwhen a highly concentrated haze isformed. The free water is suspendedas an emulsion. The more stable theemulsion, the more difficult it is to

remove the water. Factors that affectwater removal from a water/fuel mix-

ture include interfacial tension (IFT), viscosity, relativedensity and temperature.

Interfacial tension. The ability to remove water improvesas the IFT between the two phases increases. The IFT ()between two liquids is a measure of the attraction forcebetween each phase for its own species. At a two-liquid

interface, a natural surface tension is created as each phaseis repelled by the other phase. A ring-pull method is com-monly used to measure IFT. This method measures theforce required to pull a platinum-iridium ring of known cir-cumference from one discontinuous phase into the next.The typical units of IFT are dyne/cm. The IFT is a criticalfactor when considering liquid/liquid coalescence because thelargest possible stable droplet size that will form by thecoalescence process will be dictated by IFT. A system witha high IFT (i.e., > 20 dyne/cm) can sustain a larger stablecoalesced droplet size, which can be easily separated. Sys-tems with a low IFT (i.e., water in fuels with additives: