Produced water treatment without hydrocyclones

The Corporate Research division of ABB, in co-operation with ABB Offshore Systems, has developed what it believes will be a new solution for the effective separation of oil, gas and produced water for offshore structures.

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The Codeflo separation process
Click here to enlarge image

The Corporate Research division of ABB, in co-operation with ABB Offshore Systems, has developed what it believes will be a new solution for the effective separation of oil, gas and produced water for offshore structures. According to process engineer Brith Claesson, "We have found a design that will be very flexible and will be much cheaper than hydrocyclones". She and her colleagues believe that it will enable produced water to be discharged with sig nificantly lower concentrations of oil and at lower capital costs than is possible at present.

The design dispenses with the use of costly hydrocyclones and centrifuges. Within the next few months, it is expected to achieve its target of producing water that contains oil at concentrations no higher than 10 ppm.

Work on COmpact DEgassing and FLOtation process (CODEFLO) produced water treatment technology was started in the autumn of 1998. It had been realized that the industry's reliance upon hydrocyclones could create an attractive opportunity for ABB if it could develop a system that dispensed with them. This would make produced water systems cheaper to manufacture and more compact to install.

The design consists of three principal components in a compact configuration. The first element is the degasser which works as a buffer tank to maintain a steady flow and, if necessary, retain any produced sand. It also sets the operating pressure that upon depressurization to the downstream pressure will supply the flotation process with flotation gas. The system has been developed as a way of providing a water stream for further treatment without gas as a utility supply.

Short residence time

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Diagram shows projected Codeflo platform installation.
Click here to enlarge image

The produced water continues to a coagulation tank which essentially serves as a mixing device and where the liquid needs only to reside for 8-10 seconds. This is the heart of the new ABB design and is where coagulation and flocculation chemicals are injected into the stream. These serve the purpose of increasing the size of the oil droplets in the water to facilitate separation. The nature of the chemicals to be used will vary according to the type of oil being recovered. The flocculant is typically a polymer that serves to make the droplets larger. The coagulant, which may be a metal salt or a low molecular weight organic compound, is injected into the stream to remove the electrical charge from the oil droplets that inhibits any tendency for them to stick together and form larger and more easily separated particles. Use of these chemicals will typically be at concentrations of just 0.5-5 ppm and their low cost is not expected to affect the economic viability of the system.

The design of the coagulation process is central to the anticipated success of the system. ABB Corporate Research has applied to patent the entire Codeflo process but in the mean time it is keeping the details of its coagulation design a closely guarded secret. However, it is the single element that sets it apart from other separation processes and gives the Codeflo system its unique characteristics and exceptional performance. The addition of the coagulation chemicals serves to increase the size of oil droplets suspended in the water from around 10 microns to between 200 and 400 microns.

On leaving the coagulation tank the produced water is passed into a flotation tank where a design based on lamella packing enables the oil to combine with the gas and allow the water to be successfully separated. The produced water can then be safely discharged and the oil plus any added flocculation and coagulation chemicals may be returned to the production process.

Although work on improving the Codeflo performance parameters is continuing in the ABB laboratories, the system has reached a level of maturity at which the company feels ready to subject it to a field trial. At time of writing the system is producing water which contains between 15-20 ppm residual Troll oil (27 API). This is well within the present limits for the North Sea and the Gulf of Mexico, which stand at 40 ppm and 29 ppm respectively. The ABB Corporate Research team is continuing to refine its design and expects to achieve its ultimate goal of 10 ppm within the next few months. This target is being pursued in anticipation of new legislation that could impose lower limits for oil in produced water.

30% lower cost

The Codeflo system is expected to offer major benefits in comparison to produced water separation systems that use hydrocyclones. It is expected that it will cost around 30% less and unlike those systems that also use mechanical stirrers, Codeflo has been designed to use the kinetic energy within the water flow to create controlled turbulence and the exact mixing conditions required. This has the advantage of enabling it to work without any need for electrical power other than what is necessary for its instrumentation to function.

Laboratory tests are expected to continue on the process until June, following which the team's work will be directed towards development of a field trial unit. When it is in operation, the Codeflo system will be motion insensitive and a flexible system that will be able to handle turndown ratios from 9 to 1 without necessitating any operator intervention. The Codeflo system has been designed to fit within a space measuring just 7 meters by 8 meters by 7.8 meters, including full degassing.

For more information, contact Brith Claesson, ABB Corporate Research, Tel: +46 21 323218, Fax: +46 21 345108, E-mail:brith.claesson@secrc. abb.se or Tor Moxnes, ABB Offshore Systems, Tel: +47 66843060, Email: tor.moxnes@no.abb.com

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