Membranes in an aqueous environment have an attractive or repulsive response to water. The material composition of the membrane and its corresponding surface chemistry determine its interaction with water. A hydrophilic membrane exhibits an affinity for water. It possesses a high surface tension value and has the ability to form "hydrogen-bonds" with water.
Particles that foul in aqueous media tend to be hydrophobic, e.g. colloids, proteins, clays, oily particles (hydrocarbons, surfactants, greases). Hydrophobic particles tend to cluster or group together because this lowers the interfacial free energy (surface tension) resulting from surface area exposure. Spheres of particles are most commonly formed because a minimum surface area results in this shape while limiting exposure to the hydrophilic environment. General tendency will favor particle attachment to any material less hydrophilic than water because less exposure of hydrophobic particles can be achieved by attachment of the particles to these materials, including the membrane surface. To prevent fouling, the membrane requires a surface chemistry that prefers binding to water over other materials. This implies that the membrane surface must be very hydrophilic.
While the separation of oil and water by ultra-filtration (UF) is a well-proven technology, its wide-spread application for minimizing oily wastewater streams or recycling industrial processed fluids has been limited due to either membrane fouling from tramp oil or membrane decomposition from chemical incompatibility. Where conventional membrane technology has failed, FSI has succeeded in creating a system that makes use of a technologically superior membrane for both wastewater minimization and process fluid recycling applications. Based on a patented composition, these Superphilic membranes are not fouled by tramp oils nor degraded by chemical attack. Superphilic membranes come in a wide range of pore sizes from 0.001 microns to 0.2 microns, assuring that just the right membrane is available to meet the objectives of any application (e.g. smaller pore size for wastewater concentration and larger pore sizes for recycling applications).
FSI's Superphilic membranes are made of a chemically modified polyacrylonitrile (PAN) polymer. To avoid fouling by oils, the membrane was engineered to be extremely hydrophilic as compared with conventional polyvinylidene difluoride (PVDF) and polysulfone (PS) membranes which are oleophilic (i.e. oil attracting). The improvement in membrane hydrophilicity is quantified by measuring water-membrane-air contact angles for the various membrane types. A smaller contact angle correlates with a more hydrophilic surface and less fouling by free oils and concentrated oily emulsions as shown in the figure below. In practice, this enhancement results in consistent performance and reliable system operation.
The chemical stability of FSI's Superphilic membrane material relative to conventional PVDF and PS membranes is remarkable and is one key to FSI's long-term success in the environmental market. In addition to being compatible with most hydrocarbons, free oils, acids, and the broad class of polar organic solvents used in many industrial plants, the Superphilic membranes can operate at temperatures up to 170° F and are stable over a broad pH range from 2 to 12. This versatility eliminates the need to neutralize the waste stream prior to filtration and makes it possible to recycle most industrial cleaners and coolants on-line at their operating temperatures.
The complete line of Superphilic ultra-filtration membranes are offered in the SHP Series of spiral-wound cartridges. To meet the widest range of application requirements, FSI supplies three models of the Superphilic cartridge: a standard oil/water separation module; a high temperature configuration for aqueous cleaner recycling; and an open channel design for higher solids capability. All membranes are delivered in completely disposable housings for greater customer convenience.