Desalter Effluent Without Chemicals: How Coalescing Plus CDFU Cuts Oil From 80000 mg/L to Below 50 mg/L

The effluent that refuses to settle

Every atmospheric and vacuum distillation unit needs wash water to strip inorganic salts out of crude. The water leaving the desalter carries whatever the crude gave up, and what it gives up is rarely easy: oil finely dispersed by the electric field and held there by surfactants that occur naturally in the crude. Operators know the pattern well. Upstream conditions shift, the oil content of the draw-off swings, and the downstream biology takes the hit.

Three characteristics make this stream awkward. Emulsification comes first: droplets sit in the micron range and behave as a stable oil-in-water system, so gravity settling buys very little. Composition comes second, because beside dispersed oil and emulsified oil there are colloids, asphaltenes and suspended solids (SS), each adding its own resistance to separation. Fluctuation comes third. A change of crude slate or a unit upset rewrites the influent specification several times a shift, so any process that depends on a narrow operating window will drift out of it.

The conventional answer has been to add chemicals and wait. Large tanks give the droplets time and area; demulsifiers break the film. It works often enough that few people question it, but it carries two permanent costs, a reagent bill that never stops and a hazardous oily sludge that has to be manifested, hauled and paid for.

What changes when chemistry leaves the loop

The route SINOKLE proposes rests on a different mechanism. Instead of waiting for density difference to do the work, force the droplets together first, then give them something to attach to. Two pieces of equipment carry the load, and the order matters.

Stage one, the coalescing oil remover

The vessel is filled with patented coalescing packing whose surface is strongly oleophilic and hydrophobic. Emulsified oil passing through collides with the media, adsorbs, and merges with whatever else it meets. Droplet diameter climbs sharply. Because rise velocity scales with the square of diameter under Stokes’ law, the growth achieved here is what makes the rest of the process fast rather than marginal. Think of it as converting a hard separation problem into an easy one before any real separation is attempted.

Stage two, CDFU cyclonic dissolved air flotation

CDFU couples cyclone centrifugal separation with micro-bubble dissolved-air flotation in one shell. The bubble field runs at 5-30 μm, smaller than conventional flotation bubbles and therefore far better at catching fine oil. Feed enters tangentially, so the swirl pre-separates heavy particles and simultaneously raises the collision rate between droplets and bubbles. Bubble-droplet aggregates rise immediately, and hydraulic retention time drops to within 5 minutes, a fraction of what a settling basin needs. The CDFU’s enclosed pressurised layout also means there is no open surface and no odour complaint from the fence line.

Numbers from a working installation

At a large petrochemical site, feed to the unit ran as high as 5000-80000 mg/L oil, much of it emulsified crude. The requirement was not polished water but a stream the biological plant could live with, set at oil content under 200 mg/L. After coalescence plus CDFU the effluent held below 50 mg/L. The gap between the target and the result is the point: 200 mg/L was the ceiling the process had to stay under, and it settled well under it.

The recovered phase matters just as much to the economics. Water content in the separated oil came out under 1%, inside the 0.5%-10% band typically quoted for the recovered stream, so the oil went back into the crude system rather than into a sludge contract.

Where this route fits, and where it does not

• Mechanism: physical coalescence and cyclonic flotation replace demulsification followed by gravity separation.
• Footprint: skid-mounted modular construction, roughly one third of a conventional settling and flotation train.
• Secondary waste: no chemical addition, so hazardous oily sludge is never created in the first place.
• Operating cost: dominated by power rather than reagents, with the hazardous-waste disposal line removed entirely.
• Shock resistance: the coalescing stage absorbs swings that would push a chemically dosed system out of specification.

There is a boundary worth stating plainly. If the stream is lightly loaded and stable, the old route may cost less to install. The case for coalescing plus CDFU strengthens as oil content, emulsification and variability rise together, which is exactly the combination refineries describe when they say desalter effluent is the hardest stream in the pretreatment train.

What the operator actually notices

Beyond the outlet number, the day-to-day difference is the disappearance of a chemical logistics chain: no drum store, no dosing optimisation exercise, no sludge manifests. What remains is power consumption and routine inspection. Under a dual-carbon programme that combination, less energy per cubic metre, no hazardous residue, and oil recovered as product rather than waste, is usually the argument that closes the discussion.