Agc wp-coalescer separator filter system

A Coalescer-Separator Filter System

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AGC REFINING & FILTRATION

A COALESCER-SEPARATOR FILTER SYSTEM 2

Contents The Theory of Coalescence 3

Dynamic Coalescer-Separator Systems 3

The System 8



The Theory of Coalescence

When two liquids are immiscible, or not soluble in each other, they can form an emulsion or a colloidal suspension. In these types of mixtures the dispersed liquid (water) forms droplets in the continuous phase (oil or fuel). For instance, in a water-diesel fuel mixture the diesel fuel is the continuous phase and the water will exist as droplets of various sizes.

Traditionally, gravity separators were used in liquid-liquid systems to promote the coalescing of the dispersed phase. In these systems differences in densities of the two liquids cause droplets to rise or fall according to their buoyancy. The greater the differences in densities the easier the separation is, as rising or falling droplets (water) are acted upon by the frictional forces exerted by the viscosity of the continuous phase (oil or diesel fuel). This mechanism is governed by Stoke’s Law.

When the movement of the continuous phase (oil or diesel fuel) is slow as in a gravity settler, the effect of the inertial force is reduced and the buoyant force and the gravity force are equal. This is termed as the terminal velocity of the droplet.

A critical factor in a gravity (static) settler is the residence time. It has to be long enough to allow for the passive process of merging smaller droplets into larger ones.

Figure 1: Stoke’s Law

Dynamic Coalescer-Separator Systems

In dynamic coalescers the merging of many small droplets to form fewer droplets of a larger diameter is accelerated by interposing a barrier into the fluid flow in the form of fibers. In this method of direct interception the fibers collect the fine droplets as they travel in laminar flow between the fiber mass. This has the general effect of greatly reducing the residence time in the coalescer. An added feature of the fiber mass is that it will also trap solids and thus a pre-filter is required to remove solids from the fluid stream and increase the efficiency and longevity of the coalescer and separator elements.



Figure 2: A Typical Outer Layer of a Coalescer Element Consisting of a Dense Glass-Fiber Matrix (500x Magnification)

Coalescer Element Design

The coalescing process can be thought of as occurring in three steps.

Step 1

The first step in coalescing is the collection of droplets by direct interception on the fibers. It is important that the flow through the glass-fiber mass is as close to laminar flow as possible. The general rule is that the diameter of the glass fibers must be as close to the average diameter of the droplets as possible.

Figure 3: The Inner Layer Also Traps Solid Particles (500x Magnification)

Step 2

The second step is the coalescence of the droplets that have been captured on the fibers. This happens when droplets collide with each other. The critical design factors are the material of the fibers and the speed at which the droplets travel through the fiber mass.

The coalescer medium (fiber) can be either hydrophilic (water-loving) or oleophilic (oil-loving), depending on the solid/liquid interfacial tension between the medium and the dispersed phase (water). In general, an organic dispersed phase will “wet” an organic medium (plastic or glass fiber). This helps in the coalescence step as the droplets adhere to the medium longer and increase the chances of collisions with other droplets.

The density of the medium also influences coalescence. A tightly packed medium increases the number of sites available for coalescing.

When two droplets interact during flight, several events can occur. The droplets may experience bounce,



stable coalescence, temporary coalescence followed by disruption, or temporary coalescence followed by fragmentation.

Droplet Bounce

Droplet bounce will occur if the surfaces of the droplets do not make contact due to the presence of a thin intervening liquid film. The droplets undergo a flattening deformation, but the surfaces do not make contact since the kinetic energy of the collision is not sufficient to rupture the intervening boundary layer of film.

Figure 4: Droplet Bounce

Droplet Coalescence

If the mass of two combining droplets is equal to the sum of the masses of each droplet, the intervening liquid film is ruptured. This contact between the two droplets is followed by coalescence.

Figure 5: Droplet Coalescence

Temporary Coalescence Followed By Disruption

Temporary coalescence will occur when the kinetic energy of collision exceeds the values for stable coalescence. This will result in disruption or fragmentation. In disruption, the droplets surfaces have made contact, but the energy of collision was too great thus temporary coalescence is followed by disruption, resulting in the same number of droplets which existed prior to the collision.



Figure 6: Temporary Coalescence Followed By Disruption

Temporary Coalescence Followed By Fragmentation

Fragmentation is the result of excessive fluid velocities in the fiber matrix. The energy of the collisions between droplet is so great that the result is a fragmentation into smaller droplets.

Figure 7: Temporary Coalescence Followed By Fragmentation

Coalescence is improved in laminar flow conditions since the droplets will stay in the streamlines around the fibers. High fluid velocities overcome surface tension forces and strip droplets out of the coalescer medium. These collisions result in breakup and re-entrainment of the droplets. Laminar fluid flow velocities also result in optimum residence time and allow the droplets to impact with each other without disintegrating.

Step 3

The third step is the collection of coalesced water droplets in the bottom of the coalescer vessel. The optimum degree of separation depends on the geometry of the vessel and timely removal of the collected liquid from the bottom by an automatic, dual-specific gravity drain.

Separator Element Design

Separator elements consist of screens coated with various hydrophobic (water repellent) material. The elements are used as a second stage in a coalescer-separator system. Their sole function is to repel coalesced water droplets produced by the coalescer element while allowing hydrocarbon fluids to pass through.

Water droplets settle into the vessel sump and are prevented from passing downstream of the system. Flow direction is from the outside of the element to the inside and to the outlet of the coalescer-separator



vessel.

Factors Influencing Coalescer Performance

Forms of Water

Two forms of water can exist in oils or fuels, dissolved or suspended as small droplets that range in size from 0.1 to 10 microns. The free water is trapped as an emulsion. The more stable the emulsion is, the more difficult it is to separate the water.

Factors that affect water removal from a water/oil or fuel mixture are:

Interfacial tension

Viscosity

Relative density

Temperature

Interfacial Tension

The ability to coalesce a dispersed phase (water) from a continuous phase (oil/fuel) improves as the interfacial tension increases. The formation of the largest possible stable droplet size by coalescence depends on the interfacial tension. A mixture with a high interfacial tension will result in relatively larger stable droplet sizes. This is important when a coalescer is used to dehydrate turbine lube oil.

Factors that lower the interfacial tension are various additives such as inhibitors and detergents. Solids contamination will also reduce the interfacial tension. For this reason, a pre-filter is often used to reduce the solids content and increase the life of the coalescer elements.

Viscosity

Viscosity has a profound effect on coalescence. Droplets must travel through the continuous phase to collide and coalesce by breaking down the intervening boundary layer. Both of these factors are influenced by increased viscosity. Droplets must overcome the increased drag force of higher viscosities. The breakdown of the intervening boundary layer between droplets is also more difficult to overcome. Therefore, longer residence times and higher differential pressures across the coalescer vessel are required for liquids with higher viscosities. This can be achieved by lowering the flow rate or increasing the coalescer element surface area.

Relative Density

The relative density between the water phase and the oil/fuel phase is important because as the two relative densities approach equality, the separation becomes more difficult.

Temperature

As the temperature of the oil/fuel and water mixture is increased, the interfacial tension is decreased. This results in smaller droplet sizes. In addition, mixtures at higher temperatures can contain higher levels of dissolved water, which cannot be removed by coalescers. As the temperature decreases, water comes out of solution and can be removed by coalescence.

For this reason, coalescer systems should be installed in the coolest possible location. Heaters are not generally used in coalescer systems.

Design flow through the coalescer element is inversely proportional to the fluid viscosity in Cst.

The separator stage is velocity-limited but is not affected by increased viscosity



General Design Considerations

The two-stage coalescer-separator system has the following limitations:

The coalescer stage is limited by the differential pressure and the viscosity of the mixture

When the maximum velocity through the separator is reached, water will break through into the effluent

Generally, a larger coalescer stage is used with the same size separator to enable handling higher flows for more viscous fluids. Because lower viscosity fluids like gasoline are limited by the separator, a larger coalescer stage does not improve the flow rate.

Description of the Process

In liquid-liquid coalescers, the merging of droplets can occur whenever two or more droplets collide and remain in contact long enough for the continuous-phase film, which exists around each droplet, to become so thin that a hole develops in the boundary layer. This allows the liquid droplets to merge and become larger droplets. In a clean system with high interfacial tension, coalescence will occur rapidly.

Particulates and polymeric films tend to accumulate at the droplet surfaces and reduce the rate of coalescence. This can lead to a build-up of a “rag” layer at the liquid-liquid interface in a coalescer system.

Coalescers are mats, beds, or layers of porous or fibers whose properties are especially suited for this purpose. It has been found in studies that coalescence is promoted by decreased fiber diameter. A minimum bed density and residence time is required to achieve complete coalescence depending on the characteristics of the fluids and the coalescer system. Wetting of the fibers by droplets of the dispersed phase is not necessary for good coalescence.

The System

The Allen Coalescer System removes free and emulsified water down to the saturation level. The saturation level is a function of the type of oil, the types of additives in the oil, and the temperature of the oil. For typical ISO 32 Turbine Light Oil at 100°F (38°C), the saturation level is approximately 150 parts per million (ppm).

The complete Allen system consists of a pre-filter vessel, a coalescer-separator vessel, and an optional final filter.



Figure 8: A Coalescer-Separator Filter System

Pre-Filter

The pre-filter is used with solids removal elements to trap the larger solids. Various micron sizes and efficiencies are available. The pre-filter reduces the solids loading on the coalescer and separator elements and allow these to last longer before plugging.

Coalescer Element

From the pre-filter the oil flows to the coalescer-separator vessel where it enters from the bottom, moving through the coalescer elements. The oil travels from the inside to the outside of the element.

The coalescer elements filter any remaining dirt particles down to 10 microns and coalesce (join together) all emulsified and free water particles into droplets large enough to settle out of the oil. The water collects on the outside of the element in the bottom of the vessel where it is continuously drawn off by an automatic water drain.

Coalescer elements are made of a fiberglass material, which separates immiscible liquids with different densities such as water from oil.

Separator Element

The oil continues through the separator elements, which are in the same housing as the coalescer elements. The oil passes from the outside to the inside of the separator elements.

The separator element is a coated screen, which repels 100% of any suspended water and keeps it from passing through it with the oil. The oil is thus separated from any remaining water.

Automatic Water Drain

A dual-specific gravity drain automatically removes water that accumulates in the bottom of the coalescer-separator vessel. The operation of the drain is based on the difference in specific gravity between water and oil.

Optional Final Filter

Various elements are available for the optional final filter.

This element can be a 0.5-micron water-absorbing element, which removes any free or emulsified water remaining in the oil sometimes seen as a fine haze in the fluid. It also functions to filter out any dirt particles over 0.5 microns.



An economical cellulose filter element can also be used for the purpose of adsorbing any water remaining in the fluid. Several micron-size elements can be used to obtain final ISO standard cleanliness grades.

Table 1: The Solubility of Water in Oil



Figure 9: A Schematic of a Coalescer-Separator Vessel

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Agc wp-coalescer separator filter system