Technical Breakdown: How Two-Stage CDFU Cyclonic Dissolved Air Flotation Treats Highly Emulsified Sour Water from Refining and Chemical Plants
The treatment of highly emulsified sour water from refining and chemical plants centres on two difficulties: the stability of the emulsion and the special nature of sulphur- and oil-bearing media. The solution proposed by SINOKLE is built around the CDFU cyclonic dissolved air flotation as the core of a two-stage process. This article breaks the process down across four dimensions: equipment principles, the division of labour between stages, index design and engineering characteristics.
1. Equipment principles: multi-technology integration
CDFU is the English abbreviation of the cyclonic dissolved air flotation unit; its technical positioning is a high-efficiency, compact, closed pressurised flotation system. What most distinguishes it from conventional flotation is its degree of technology integration: it organically combines cyclonic centrifugal separation, ultra-fine bubble generation, dissolved air flotation, and coalescence and demulsification within one unit, achieving efficient combined removal of free oil, emulsified oil, suspended solids and other pollutants from the wastewater.
The value of integration lies in complementary mechanisms. Cyclonic centrifugal separation uses the density difference to amplify the separation driving force, solving the separation-speed problem; ultra-fine bubbles provide a vast array of tiny attachment interfaces, solving the problem of fine droplets that cannot rise; dissolved air flotation completes the flotation and skimming of pollutants, solving the outlet problem; coalescence promotes collision and merging of fine droplets, increasing their size; demulsification acts directly on the interfacial film, solving the emulsion-stability problem. Used alone, any one of these covers only part of the process.
2. Division of labour between stages: Stage 1 physical demulsification, Stage 2 final polishing
The first stage CDFU handles purely physical demulsification and separation. Purely physical means that the flow field, bubbles and coalescing interfaces destroy the emulsion stability without any chemical agents. This stage targets the bulk oils and is characterised by stable reliability and high oil removal efficiency. The process yields two key gains: the waste oil is separated and recovered, and no oily sludge is produced at any point.
The absence of oily sludge matters greatly to refining enterprises. Oily sludge is hazardous waste, and producing it means a full set of compliance costs and environmental risks for collection, storage, transfer and disposal. Eliminating this waste at the source in Stage 1 solves the environmental problem and the disposal cost problem at the same time, and the recovered oil brings additional economic benefit.
The second stage CDFU is tasked with final polishing and needs only a small dose of PAC. Only a small dose is needed because Stage 1 has already removed the main load, leaving residual fine suspended solids and oil for Stage 2. The lower load reduces chemical consumption, which in turn reduces scum volume. Data show that the Stage 2 scum volume is more than one third lower than that of conventional processes, and the amount of oily sludge generated and secondary treatment costs fall accordingly.
3. Index design: the boundary between influent and effluent
Process indices reflect the applicable boundary directly. Influent conditions: oil not exceeding 500 mg/L and suspended solids not exceeding 200 mg/L; effluent targets: oil not exceeding 20 mg/L and suspended solids not exceeding 20 mg/L. The roughly one-order-of-magnitude gap between influent and effluent shows that the process is designed to treat highly emulsified oily wastewater to a level acceptable to downstream systems.
4. Engineering characteristics: closed, compact, quick-install
Closed pressurised operation is one of the structural features of the unit. The whole separation takes place inside a pressurised sealed chamber, leaving no escape path for hydrogen sulphide or oil vapour. For sulphur-bearing wastewater this is both environmental and safety design: less odour on site, a better working environment, and no escape of corrosive gas to threaten surrounding facilities.
Compactness comes from retention time. The single-stage retention time is under five minutes — short among comparable oil removal processes — with the direct effect of a small equipment volume and small footprint, especially advantageous for existing plants short on space.
5. Conclusion
The two-stage CDFU process can be summarised in one sentence: use physical means to solve emulsification and a small dose of chemicals for final polishing. The former determines oil removal efficiency and hazardous waste reduction; the latter determines effluent stability. Through multi-technology integration and a two-stage division of labour, SINOKLE balances this contradiction in engineering terms, providing a replicable treatment paradigm for highly emulsified sour water from refining and chemical plants.
