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.

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.

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.