Oily Wastewater Treatment on Offshore Oil and Gas Platforms: A Process Solution

At present the water treatment system on this oilfield production platform uses a combination of hydrocyclones and an oil-water separation tank. The hydrocyclone achieves preliminary oil-water separation by centrifugal action based on the density difference between oil and water; the separation tank in series downstream then removes gas and residual oil further by thermochemical settling, so that the oil content of the effluent meets the discharge-to-sea standard (below 18 mg/L). As the properties of the produced fluids change in future, in particular as crude API gravity falls and viscosity rises, gravity and centrifugal separation will become markedly less effective. Introducing new water treatment technology and integrating it effectively with the existing process has therefore become a necessity.

1. CFU oily wastewater treatment technology — working principle

The CFU process combines flotation, centrifugation and coalescence to separate oil and water efficiently. Gas (usually natural gas or nitrogen) is injected into the inlet pipe, forming large numbers of fine bubbles in the water and creating a three-phase mixture of water, gas and the substances to be removed. Under the combined action of interfacial tension, bubble buoyancy and static pressure difference, the micro-bubbles attach to the surface of fine oil droplets and form aggregates of lower density than water, which rise quickly to the surface of the wastewater. The separated media are then discharged separately through internal guide pipes set at different heights.

2. Process features

(1) No internal moving parts and a small footprint, easy to operate; on an offshore platform it saves operating space, and the degree of automation is high.

(2) By combining flotation with hydrocyclone technology, it raises the treatment efficiency of oily wastewater.

(3) It accommodates a fairly wide range of flow fluctuation and operates stably.

(4) Under most operating conditions it can meet the overboard discharge limit for oily wastewater without chemical agents.

SINOKLE CDFU new technology

1. Introduction to the CDFU (cyclonic dissolved gas flotation unit)

The CDFU cyclonic dissolved gas flotation system integrates three technologies: cyclonic centrifugal separation, ultra-fine bubble generation and dissolved gas flotation. Through their combined action it achieves fast, efficient separation of free oil, emulsified oil and suspended solids from wastewater. The system uses a high-efficiency dissolved gas pump and a dedicated micro-bubble generation and release device, producing bubbles as fine as 5 to 30 microns. Because emulsified oil droplets are mostly between 0.5 and 25 microns, the micro-bubbles can capture them more effectively at a comparable scale, achieving purely physical demulsification. Combined with cyclonic centrifugation, the technology not only promotes collision between bubbles and oil droplets but also, because of the density difference between media, accelerates the migration of bubbles and droplets towards the cyclone core under centrifugal force, thereby strengthening flotation separation.

2. Process advantages

• High oil removal efficiency (over 90% in a single stage), removing droplets as small as 2 microns.

• Skid-mounted design with a short hydraulic retention time (only 1 to 5 minutes) and a footprint of 10% to 20% of a conventional dissolved gas flotation system; small in volume and light in weight.

• Excellent operating stability. There are no internal screens, coalescing plates or other components prone to clogging; because the unit does not rely on internal packing it resists blockage and keeps treatment performance stable.

• Strong adaptability. It copes with changes in hydraulic load, crude oil viscosity and oil content.

• Few moving parts, high reliability, simple installation and low operating and maintenance costs. The system is automatically controlled, runs smoothly and is safe and dependable.