Crude Oil Desalter and Dehydrator Systems

Crude oil as received at a refinery contains dissolved salts — primarily chlorides of sodium, magnesium and calcium — in dispersed water droplets. These salts, if carried into the distillation unit, are hydrolysed at elevated temperature to form hydrochloric acid, which causes corrosion of overhead systems and fouling of heat exchangers.

Desalting reduces the salt content of the crude oil to levels acceptable for distillation unit operation. It also removes water, which reduces the hydraulic load on the distillation column and prevents operational issues caused by water flashing in the column feed.

Crude oil desalting involves two main steps: first, mixing the crude with wash water to dissolve the salts from the dispersed water droplets into the wash water; second, separating the wash water from the crude oil using electrostatic coalescence.

The crude oil and wash water are mixed ahead of the desalter vessel, typically using a mixing valve, to achieve good contact between the wash water and the salt-containing droplets. The mixing intensity — controlled by the pressure drop across the mixing valve — must be sufficient to promote salt extraction without creating a stable emulsion that is difficult to separate in the desalter.

Inside the desalter vessel, an applied electric field (AC, DC or AC3) promotes coalescence of the dispersed water droplets, which then settle by gravity to the bottom of the vessel and are withdrawn as brine. The desalted crude is drawn off from the top of the vessel.

Desalting can be carried out in a single stage or in two stages in series. Two-stage desalting achieves lower residual salt levels by using fresh wash water in the first stage and recirculating the less contaminated water from the second stage back to the first stage feed, reducing overall wash water consumption.

The decision to use single or two-stage desalting depends on the crude oil salt content, the required outlet salt specification and economic considerations. Heavy and high-salt crude oils frequently require two-stage desalting.

Desalter performance — measured as salt removal efficiency and residual water content — is influenced by a number of operating variables including wash water rate and quality, mixing valve pressure drop, desalter operating temperature, crude oil properties (particularly density, viscosity and emulsion tendency), and demulsifier chemical addition.

Crude oil properties vary with the crude diet processed by the refinery. Changes in crude slate can significantly affect desalter performance and may require adjustments to operating conditions or the demulsifier programme.

Howe Baker designs and supplies electrostatic desalter and dehydrator systems for crude oil refining and heavy crude applications. The company's technology portfolio includes single and two-stage desalting configurations using AC, DC and AC3 electrostatic separation technology.

Engineering scope covers process design, desalter vessel and internals design, transformer-rectifier specification, mixing system design and instrumentation. Howe Baker also provides spare parts and technical services for operating desalter systems.