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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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.

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