Liquid-Liquid Coalescers


Liquid-Liquid Coalescing
Liquid-liquid coalescers are primarily used in separation processes to efficiently separate mixtures of two immiscible liquids such as oil and water. The water dispersed in oil and other hydrocarbon products can interfere with quality specifications. The oil dispersed in process water streams makes additional demands on wastewater treatment systems. Efficient liquid-liquid separations are an integral part of many industrial processes.
Liquid Dispersions Formation
Mixtures of two liquids often occur throughout the process industries as an unavoidable result of the process. In other cases, the mixing of two liquids is required to optimize extraction efficiency or to promote a chemical reaction. When two liquids are mixed together but do not naturally form a solution in each other, they are called immiscible. These immiscible mixtures have common structure with dispersed phase droplets in a continuous phase. The three primary mechanisms for dispersion creation are mechanical energy input, phase condensation or cooling and chemical reaction. The size of droplets to be separated is determined by how the droplets are formed as shown in the chart.
Separating Liquid-Liquid Dispersions
Carried-over liquid dispersions cause off-specification products, excessive solvent losses, and often operating problems in downstream processes. Dispersions and emulsions are often treated by coalescing. Coalescing designs insert device into the flow path of the fluids. The droplets impact the device and collect on the surfaces of the device through surface energy forces. Once captured, these individual droplets combine (i.e., coalesce) to form larger droplets that are much easier to settle downstream.

If the specific gravity of the dispersed phase is greater than that of the continuous phase (i.e., aqueous-from-hydrocarbon separation), the coalesced droplets settle downward by gravity and are collected at the bottom of the vessel. Conversely, if the specific gravity of the dispersed phase is less than that of the continuous phase (i.e., hydrocarbon-from-aqueous separation), the coalesced droplets are collected at the top of the vessel.

When surfactants are not present in an aqueous-hydrocarbon system, coalescing occurs spontaneously. Surfactants, however, can create a stable emulsion that cannot be separated without special equipment, such as a coalescer.

Applications of Liquid-Liquid Coalescers
Efficient liquid-liquid separations are critical to achieve optimum plant performance and cost-effective operations. Several applications described below present solutions to common issues that may occur in the hydrocarbon processing industry.

Protect downstream equipment

Coalescers separate sulfuric acid, caustic or water mixed with hydrocarbon streams to minimize the potential corrosion of downstream equipment.

Reduce wastewater treating cost

Coalescers help reduce effluent contamination, which can reduce the burden on wastewater treatment facilities.

Improve product quality

Coalescers efficiently remove a haze that causes off-specification products.

Increase capacity and efficiency

Coalescers allow higher capacity compared to gravity settling alone by increasing allowable superficial velocities as well as improving separation efficiency.

Minimize loss of valuable solvents

Coalescers minimize the loss of valuable solvents in liquid-liquid contacting towers such as LPG amine treaters.

Plate Pack Coalescers

Plate pack coalescers are typically classified as either parallel-plate or corrugated-plate separator internals. These coalescers work on the principle of gravity settling. Parallel-plate coalescer is the best choice when there is concern about fouling or plugging from solid materials in the feed. If the feed stream is clean, a corrugated-plate coalescer is the better choice. It provides higher separation efficiency per unit of length compared to parallel-plate due to the larger specific surface area.

• Improve flow stability
• Maintain efficiency even with phase inversion of dispersed liquid
• Mechanically durable construction

Materials of Construction
• Stainless Steels
• Carbon Steel
• Nickel Alloys
• Titanium

Separation Mechanisms
The simplest of all liquid-liquid separators is the gravity settler. The most important factors in the settling process are both the droplet size and the settling distance that must reach the interface between two liquid phases.

In cases where the droplet size is larger than 200 microns, it will settle quickly. However, smaller droplet dispersions require a lot of settling time when they have to travel a long distance to the interface.

With the use of plate pack coalescers, each plate provides many interfaces within the vessel. They promote coalescing work by reducing the distance that droplets rise or fall. While passing through the plate pack, the droplet dispersions grow rapidly. And then they settle to the primary interface in the vessel after exiting the plate pack.

Mesh Pad Coalescers

Mesh pad coalescers are used to achieve efficient phase separation where liquid dispersions are being processed. These coalescers are manufactured in the form of mesh pads with the desired thickness, density and surface area. The small droplets of a fine dispersion will be coalesced and enlarged by passing the dispersion through a mesh pad coalescer. And then the enlarged droplets will be settled more rapidly.

• High efficiency with relatively high velocity
• Easy to install in all process equipment
• Most cost-effective solution

Materials of Construction
• Stainless Steels
• Nickel Alloys
• Titanium
• Aluminum
• Copper
• Polypropylene
• Fluoroplastics

Separation Mechanisms
When a wire mesh pad is used, the coalescence mechanism is processed by the following steps :

① Droplets are captured on filaments.
② Droplet coalescence begins and droplets move to filament intersections by liquid stream.
③ Coalescence of two droplets forms one larger droplet, and coalescence of small droplets into the larger droplets is repeated.
④ Ultimately, larger droplets are released from filaments by drag force of liquid stream.
⑤ Continuously repeat of steps ①-④.

The specific surface properties of the coalesce filaments are critical in the adherence of droplets and then coalescing.