Electrostatic Coalescers in Hydrocarbon Processing

Water droplets dispersed in a continuous hydrocarbon phase can be difficult to separate by gravity settling alone when the droplets are small or the oil-water interfacial tension is high. Electrostatic coalescers apply an alternating (AC) or direct (DC) electric field across the hydrocarbon stream, which causes the dispersed water droplets to become polarised and migrate towards each other.

As the droplets coalesce into larger droplets, the settling velocity increases — following Stokes' law, settling velocity increases with the square of droplet diameter — making gravity separation more effective and allowing a compact vessel design.

Electrostatic coalescers are designed using either alternating current (AC) or direct current (DC) electric fields, or a combination of both. AC systems use a transformer to generate a high-voltage alternating field between electrode grids within the vessel. DC systems rectify the supply to produce a unidirectional field.

The choice between AC and DC configuration depends on the properties of the crude oil or process stream, particularly water cut, conductivity and the nature of the emulsion. Conductive crude oils and streams with high water cut can cause electrical shorting in AC systems, and DC or combined AC/DC configurations may be required in these cases.

Howe Baker's AC3 electrostatic coalescer design incorporates an enhanced three-zone electrode configuration that provides improved performance on crude streams that present difficulties for conventional AC designs.

In upstream oil production and pipeline operations, electrostatic coalescers are used to improve water removal from produced crude oil ahead of pipeline export, reducing BS&W (basic sediment and water) to meet pipeline specification and protecting downstream processing equipment from water and salt ingress.

Upstream coalescers must handle variable feed compositions, including fluctuating water cuts and the presence of production chemicals such as corrosion inhibitors and emulsion breakers, which influence emulsion stability and coalescer performance.

In crude oil refining, electrostatic coalescers are used as the separation stage within crude oil desalter units. After mixing the crude oil with wash water to dissolve and extract salts, the mixture passes through the coalescer section of the desalter vessel where the applied electric field promotes water droplet coalescence and settling.

Effective water removal in the desalter is important for protecting downstream distillation equipment from corrosion caused by chloride salts, which hydrolyse in the distillation column to form hydrochloric acid.

Electrostatic coalescers are used in distillate treating service to remove water and contaminants from kerosene, jet fuel and diesel fractions. Distillate treating typically follows a wash or extraction step and requires the coalescer to handle a lighter hydrocarbon phase at lower viscosity than crude oil.

Product quality requirements for jet fuel and kerosene include strict water content and particulate limits, and the coalescer design must be matched to the specific product stream properties and treating chemistry.

Howe Baker has engineering experience in electrostatic coalescer and desalter design for upstream, pipeline and refinery applications. The company's technology portfolio includes AC, DC and AC3 coalescing configurations.

Engineering scope covers process design, vessel and internals design, electrical transformer-rectifier specification and control system design. Replacement transformer-rectifier units and electrode internals are available as spare parts for operating Howe Baker units.