Tackle tough separations

There are a plethora of devices in the marketplace that are designed to improve separation and thus improve product efficiency.  Learn how to determine which one is ideal for each particular process application.

By Thomas H. Wines, Pall Corp.

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Based on this mechanism, we can predict that a number of factors will affect the coalescence performance. The surface properties of the coalescer fibers are critical in influencing the adsorption of droplets, as well as the release. The attraction or adsorption characteristics of the fibers must be balanced against the release mechanism. The fact that droplet-fiber adsorption must occur as part of the coalescing mechanism is supported by a number of sources.

Once the droplets have been coalesced, they are now as large as possible for the given flow conditions. The separation stage can be achieved in one of two ways:

Horizontal configuration: The coalescer housing contains a settling zone that relies on the difference in densities between the coalesced droplets and the bulk fluid. This configuration can be used for both hydrocarbon-from-water and water-from-hydrocarbon separation, but the location of the collection sump and outlet nozzle will need to be reversed.
For removing hydrocarbon-from-water, a collection sump is located at the top of the housing and the purified water leaves at the bottom outlet nozzle. The sump can be manually drained on a periodic basis or equipped with an automatic level control and drain system. Estimating the coalesced drop size and required settling zone length are best determined through pilot scale tests at field conditions.

Vertical configuration: Once the droplets have coalesced, they are as large as possible for the given flow conditions (in the range of 0.5 mm to 2 mm in diameter). The separation stage is achieved using hydrophobic separator cartridges that provide an effective barrier to aqueous coalesced drops, but allow hydrocarbons to pass. The separator cartridges can be stacked below the coalescers for the most efficient use of the separator medium. This configuration only applies to the separation of water or aqueous contaminants from hydrocarbons.

After leaving the coalescing stage, the large, aqueous coalesced drops and hydrocarbon then flow axially in a downward direction. The stream is then forced to pass through a separator cartridge located below the coalescer cartridge and the flow direction is from outside to inside. The large, coalesced drops are repelled by the separators and are collected in the bottom sump. The purified hydrocarbon passes through the separators and exits at the bottom of the housing. The aqueous phase in the collection sump can be manually drained on a periodic basis or equipped with an automatic level control and drain system.

Circle of influence
There are several factors that influence coalescing, including interfacial tension, shear history, density difference and viscosity.

Interfacial tension (IFT) is created at the interface between two immiscible liquids. The measurement of IFT is based on the difference between the surface energies of the liquids. The units are dyne/cm (force per distance) or erg/cm2 (energy per area). When surfactants are present, they migrate to the interface and cause a reduction in the IFT.

&ldquoDisarming&rdquo occurs when surfactants concentrate on the coalescer fibers. This shields the fibers from the passing aqueous droplets and results in poor separation efficiency. Generally, the disarming phenomenon does not occur unless the interfacial tension between the water and fuel is less than 20 dyne/cm.

Petroleum naphtha sulfonates have been identified as naturally occurring surfactants that are especially detrimental to conventional glass-fiber coalescers. When a specially formulated polymeric coalescer medium was used instead of glass fiber, disarming was not observed. The coalescing performance of a polymeric medium can be greatly enhanced by modification of surface properties of the medium. This cannot be accomplished with a glass-fiber medium.

Surfactants can also concentrate at the water/hydrocarbon interface, which can lead to the formation of very small droplets and stable emulsions. To separate such emulsions, special consideration must be applied to the size and distribution of pores in the coalescer media. The IFT has an effect on the largest drop size that the coalescing process can create. The coalesced drop size increases with IFT. Larger drops will settle faster and will require a shorter horizontal vessel.

The shear history of the emulsion prior to entering the separation device can have a significant effect on its performance. If the emulsion is passed through a high-speed pump or across a valve with a pressure drop, the shear imparted to the stream will break up larger globules into smaller, more stable droplets. The performance of a separation device can often be improved by simply moving it upstream of the shearing device. The capture ability of high-efficiency liquid–liquid coalescers is much greater than other separation options, and this technology will be least affected by shear history.

The difference in the density between the two phases influences how large the coalesced drops will become and how fast the drops will settle. The practical limitation for coalescers to separate two phases is a density difference greater than 0.03 g/ml.

Viscosity is an important parameter governing the residence time required in the coalescer medium for successful separation. The viscosity affects the rate at which the drops move in the bulk fluid and, in turn, this affects the rate at which drops adsorb to fibers and merge together. As the viscosity increases, the time is required for the coalescing process lengthens and more coalescers will be needed for a given flow rate.

Know the boundaries
Properly designed and sized high-efficiency coalescer systems can process discontinuous phase inlet concentrations of up to 10% and reduce them to ppm levels at the outlet for interfacial tensions as low as 0.5 dyne/cm. For water-from-hydrocarbon separations, coalescer outlet concentrations below 15-ppmv free water can be achieved, whereas concentrations below 20 ppmw have been demonstrated for hydrocarbons-from-water.

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