Desalter Draw-Off Water: Choosing Between One-Stage and Two-Stage Physical De-oiling
Desalter draw-off water from atmospheric and vacuum distillation units has a reputation it deserves. Oil content is high and moves constantly, resin and asphaltene content is substantial, emulsification is severe, and the oil-water density difference is small enough that gravity does very little. Gravity settling, ordinary hydrocyclone de-oiling and standard membrane filtration all struggle with it, and when they struggle the oil load lands on the downstream comprehensive treatment station, which is where exceedances and unstable discharge actually begin.
SINOKLE’s answer is a purely physical train built around two patented units: a high-efficiency coalescing de-oiler and the CDFU hydrocyclone dissolved-air flotation unit. It can be installed and commissioned on-line without stopping production, and it comes in two configurations depending on how much oil is in the feed.
Why density-based settling is the wrong mechanism here
Traditional processes ask the oil to rise because it is lighter than water. With desalter draw-off water that assumption is weak: the oil is heavy, the emulsion is stabilised by resin and asphaltene, and the driving force is small. The SINOKLE’s process instead combines coalescence, cyclonic centrifugation and gas-oil-water three-phase separation, and it separates using several density differences at once — between gas and water, and between oil, gas and water. Because the mechanism no longer depends on the density of the fouling oil, the de-oiling result is essentially unaffected by how heavy that oil happens to be on a given day. For influent oil of 0.5%–5%, overall de-oiling exceeds 90% and treated effluent oil stays below 50 mg/L.
There is a second benefit that finance departments notice: the recovered crude leaves with water content below 1%, good enough to go straight back to the crude-dehydration unit. A compliant discharge and a saleable stream come out of the same vessel.
Two configurations, chosen on feed oil
High-oil, heavily emulsified feed. The full train runs coalescing de-oiler first and CDFU flotation second. The coalescing unit pre-separates large oil droplets, floating oil and heavy impurities, which takes most of the load off the back end; the flotation unit then removes the emulsified oil and fine suspended solids that survived. The two stages are sized to work together rather than each being sized for the whole duty.
Low-oil feed. The front end drops out and the process simplifies to two-stage series CDFU hydrocyclone dissolved-air flotation. Fewer vessels, smaller footprint, easier O&M, and still fully physical. Plants moving between production loads can pick the configuration that matches the water they actually have rather than the worst case they once measured.
What keeps the equipment running
The coalescing de-oiler uses a pressurised coalescing structure with dedicated media and self-cleaning internal flow channels, so long-term clogging is unlikely and shutdowns for cleaning are rare. The CDFU unit relies on cyclonic flow plus micro-bubbles, needs no demulsifier and no flocculant anywhere in the process, and strips emulsified oil physically. Removing chemicals removes chemical cost and removes oily chemical sludge at the source, which is usually the larger of the two savings.
Anti-shock performance matters as much as average performance. When a crude unit changes load, oil content and flow both move hard, and that is when traditional trains go over. The internal flow field and separation system here are designed for exactly that, holding effluent inside the downstream unit's inlet requirement even during sharp swings — which is what protects the refinery from a whole-line shutdown rather than just a bad analysis.
Operation, safety and installation
The system ships with integrated automatic control: online water quality, pressure and flow instruments drive the whole sequence of influent, separation, oil discharge, backwash and effluent, locally or remotely, with parameters adjusting to feed quality. No dedicated operator is needed, and both labour input and the human-error rate come down. Everything is enclosed and pressurised, so wastewater, waste oil and waste gas stay in pipes and vessels, volatile hazardous gas and odour do not escape, and waste gas is collected to treatment while sludge and backwash water are routed to the sludge unit.
Because all equipment is skid-mounted and intensively laid out, there is no large civil structure, and assembly, commissioning and linkage testing are finished before delivery — on site it is pipe connections and control-system linking. The point operators appreciate most is that installation and phased commissioning happen while the desalter stays in normal production.
Track record
The process runs at multiple large petrochemical and refining sites. Two representative installations are the crude-oil de-salter drainage de-oiling skid at PetroChina Yunnan Petrochemical's atmospheric-vacuum unit at 105 m3/h, and a large Shandong petrochemical desalter wastewater project at 30 m3/h. Long-term continuous operation at both shows stable treatment effect and low equipment failure rate.
Put together, the case is: multiple separation mechanisms that handle severe emulsification, a graded configuration that matches the actual feed, anti-shock behaviour that survives crude-unit swings, no chemicals and therefore no oily chemical sludge, oil recovered rather than destroyed, and on-line installation without a shutdown. For refineries trying to stabilise both the desalter and everything downstream of it, that is usually the shortest route to a compliant, boring, uneventful water system.