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.

As chemical processing becomes more demanding, improved separation can help increase production efficiency and meet increasingly stringent environmental regulations. For separation of emulsions, myriad devices exist in the marketplace, so it can be confusing to determine which one should be used for a particular process application.

Industrial equipment available for separating immiscible liquids includes decanters, plate separators, mist pads, hydrocyclones, centrifuges, gas flotation and high-efficiency coalescer cartridges. High-performance liquid-liquid coalescers are finding expanding roles to meet separation challenges. An overview of commercially available technologies is given below:

Decanter: An open vessel containing baffles or plates that relies on gravity for separation. These separators are often large and typically are used as a first-stage treatment unit.
Mesh pad/parallel plate: These separators operate by the principle of inertial impaction whereby the droplet momentum is great enough so that the globules leave the streamline of the fluid flow and impact either metal fibers in the mesh pads or corrugated plates, depending on the type of internals used. The droplets then coalesce into large drops that separate by gravity. The inertial separation force is greatly reduced as flow rates are lowered below design, which leads to poor separation.

Electrostatic precipitator (ESP): This equipment separates salt or caustic water from hydrocarbons by means of an electric charge that is created by a high-voltage source. They are used extensively to desalt crude oil in refineries. Electrostatic separators are not effective for stable emulsions.

Packed bed: This system consists of a vertical vessel filled with sand or other packing that enhances drop coalescing. This separator provides a low-cost option, and is a less-efficient separation method.

Salt or molecular-sieve tower: When operation is optimal, such towers will break emulsions and reduce water content in hydrocarbon streams below saturation levels. These separators rely on adsorption of water to salt molecules or molecular-sieve material. Salt towers are prone to compaction cracking and channeling of flow, which reduces practical efficiency. Additional salt must be added regularly, and the system can generate corrosive ions that can affect downstream processes. A larger, two-stage system is required for molecular sieves, and energy is required to thermally regenerate one of the beds while the other is online.
Vacuum dehydrator: A vacuum is pulled across a tower where the emulsion contacts trays or packed beds. The more volatile liquid is vaporized and separated. The vacuum dehydrator can achieve removal levels well below saturation, but has high operating costs.

Centrifuge: This equipment has a motor that spins a shell, using the centrifugal force created by the swirling liquid to separate the discontinuous phase. Rotating equipment might require significant capital investment and has high operation costs for electricity and maintenance. This technology is difficult to economically scale up.

Hydrocyclones: The conical-shaped internals use the momentum of the feed stream to create centrifugal force and resultant separation. Hydrocyclones will create a significant pressure drop and may require installation of booster pumps. The equipment is not prone to fouling, but is not effective at separating very stable emulsions.

Gas flotation: Oil drops and solids are separated from water using air bubbles. This technology requires the use of flocculation agents and chemical additives, and can process high water flow rates, but loses efficiency during upsets. Gas flotation is also known as dissolved air flotation (DAF) and induced gas flotation (IGF).

Conventional glass-fiber coalescer: Constructed of glass-fiber coalescer media, this equipment works adequately for emulsions with interfacial tensions greater than 20 dyne/cm. It is known to disarm and lose efficiency in the presence of surfactants. These coalescers are widely used to dewater jet fuel for the aviation industry.

High-efficiency liquid-liquid coalescer: This is the newest generation of coalescers. They are constructed from optimized polymer and fluoropolymer materials to separate the most difficult emulsions with interfacial tensions as low as 0.5 dyne/cm. This coalescer can be used in a variety of applications. It can process aggressive chemicals and handle demanding operating conditions while providing the highest level of performance.

Set the stage
A liquid-liquid coalescing system operates in three stages: separation of solids/pre-conditioning, coalescence and separation.

Solids can increase the stability of an emulsion. Removing them can make coalescing easier, and lengthen coalescer service life. Solids can be removed by a separate disposable-cartridge-filter system, or by a regenerable backwash-filter system for high solids levels.
The next step in the process is the primary coalescence. In this stage, the pore dimensions initially are very fine and then become more open to allow for void space of the coalescing droplets. In the primary coalescence zone, the inlet dispersion containing fine droplets (0.2 μ to 50 μ) is transformed into a suspension of enlarged globules (500 μ to 5,000 μ).

This coalescence mechanism can be described by the following steps:

1. Droplet adsorption to fiber.

2. Translation of droplets to fiber intersections by bulk flow.

3. Coalescence of two droplets to form one larger droplet.

4. Repeated coalescence of small droplets into larger globules at fiber intersections.

5. Release of droplets from fiber intersections due to increased drag on adsorbed droplets caused by bulk flow.

6. Repeat of steps 1-5 with progressively larger droplet sizes and more open media porosity.
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.
The use of polymer and fluoropolymer materials of construction allows for expanded use of coalescers since they can withstand an array of aggressive chemical applications over a range of temperatures from -40° F (-40° C) up to 300° F (149° C).

Whereas liquid–liquid coalescers have many benefits in breaking tough emulsions, there are some limitations to consider. Higher concentrations of solids can be problematic and lead to excessive changeout of disposable prefilters. Generally, the solids range at which liquid-liquid coalescers can economically operate with disposable filters is less than 10 ppm. Above this concentration, further pretreatment will be required, such as backwash cartridge filters, mixed-media packed beds, or hydrocyclones.

The operational limits for removing free liquids must also be understood. The coalescer will not be able to remove contaminants that are in solution. If the clarified outlet stream is cooled, condensation of previously dissolved contaminants can occur, resulting in a hazy product. You must carefully consider the location of the coalescer, as well as changes in downstream process conditions.

Coalescers typically will have a service life of one to two years when adequately protected by prefiltration. Despite the long life, the equipment will eventually require disposal and replacement.

The problem of surfactant disarming must be considered for liquid-liquid coalescers constructed from glass-fiber medium, as well as for low-IFT-emulsion systems (less than 20 dyne/cm). An efficient separation cannot be achieved under such circumstances. For these conditions, non-disarming fluoropolymer or polymer coalescers should be considered. Such materials also have wider compatibility for chemical streams, in addition to a wider operating-temperature range.

Coalescers at work
Liquid-liquid coalescers can be applied to a number of processes to protect equipment, recover valuable streams and to meet environmental discharge limits. Some examples are given below.

• Improve extraction efficiency: High-efficiency liquid-liquid coalescers can allow a higher degree of mixing in extraction processes since they can break stable emulsions, thereby enhancing the mass transfer and reducing the size of equipment for extraction.

• Guard solid-bed catalysts/desiccants: Catalysts and desiccants are often sensitive to free water since it may contain dissolved salt, metals or caustic.

• Purify final product: Water condensation originating from steam stripping in refinery operations can lead to off-specification, hazy products.

• Recover liquid catalysts: Liquid or homogeneous catalysts will form emulsions in product streams. Such contamination can foul downstream equipment and poison solid-bed catalyst so that it has to be replaced.

An example is the recovery of caustic used to remove mercaptans (organic sulfur compounds) from gasoline. Caustic and air are injected into the gasoline feed upstream of the fixed-bed catalyst reactor. In the reactor, mercaptans are extracted into the caustic and are then converted to disulfide oils by oxidation and catalytic action. The reactor effluent typically contains some caustic that results in off-spec, hazy gasoline, high costs of caustic makeup, and corrosion of downstream piping.

Installing a high-efficiency liquid-liquid coalescer system was found to be an effective solution to recover the caustic carryover.

• Recycle Solvents: A number of unit operations require solvents, including extraction, crystallization and leaching. These solvents can be purified and reused. High-efficiency liquid-liquid coalescers have been used for solvent recovery in the manufacture of active pharmaceutical ingredients (API).

• Protect distillation/stripper columns: Distillation columns can be adversely affected by aqueous contaminants. The distillation energy requirement will be increased by having to vaporize the free aqueous contaminant, and any dissolved solids will plug the trays or packing. For sour-water stripper columns, oil and other hydrocarbons, if present, can lead to fouling of the internals.

In an ethylene plant, a liquid hydrocarbon feed stock is thermally cracked in a furnace along with steam. The effluent passes through oil and water quench towers where solids and heavy organic compounds are separated. The gas is further compressed, treated and distilled to obtain the final products. 

The quench water removes primarily pyrolysis gasoline (py-gas) from the cracked gas stream along with solid contaminants. Significant amounts of py-gas in the quench water can cause fouling and reduced efficiency of process equipment, including heat exchangers, strippers and boilers. A high-efficiency liquid-liquid coalescer system was found to be an effective way to protect the equipment and recover the py-gas.

Emulsions are present in many processes and require efficient separation to meet industry&rsquos increasing demands. High-efficiency coalescers can tackle these separation challenges.

Dr. Thomas H. Wines is senior marketing manager of the fuels and chemicals group for Pall Corp., East Hills, N.Y. He has more than 16 years of experience troubleshooting filtration systems in the refining, gas processing and chemical industries. E-mail him at Tom_Wines@Pall.com.