Improving Oil Separation From Produced Water Using new Compact Flotation Unit Design

Abstract

Produced Water Treatment (PWT) technologies have evolved significantly over the past 15 years. In 2001, the first Compact Flotation Unit (CFU) was introduced to the oil and gas industry on the Norwegian Continental Shelf. Today, this well-proven technology, which separates residual oil from produced water (PW), operates worldwide. CFUs function either in the facility's PW train as a final step downstream from a separator in various configurations, or as a standalone treatment system in the slop/reject treatment process system.Our analysis shows that it is possible to substantially improve conventional CFU oil separation performance. A portion of small gas bubbles that have been in contact with oil droplets will never rise to the top of the CFU vessel due to countercurrent water flow. Instead, they exit at the bottom of the unit. Removing a larger portion of these small bubbles in flotation technology will achieve an additional decrease of oil-in-water (OiW). Because of their high surface-to-volume ratio, removing small gas bubbles helps move more oil away from the PW discharge.Schlumberger development of next-generation CFU technology included theoretical study, benchmark lab testing and field trials. To improve separation efficiency, we implemented several new internal designs within the same external CFU size and design, and analyzed them in a computational fluid dynamics (CFD) model. For manufacturing and further lab-and field-testing, we selected a final design that considerably improved oil-separation efficiency. The new technology delivers two-stage oil separation in a single vessel. Each stage uses a different mechanism to improve oil separation.Results from lab testing show that the new CFU increases oil-separation efficiency up to 45% compared with existing technology. Key benefits of the new design include:• Less impact on the environment through improved OIW discharge figures • Lower skid weight through fewer vessels (one instead of two) • Reduced facility footprint through smaller skid size • Retrofittable technology We based the new CFU design on CFD analysis, testing in a PW flow loop, and an offshore field trial which also verified better oil separation performance. Results from the field trial indicated the new CFU improved performance 27 % compared with conventional technology. This was achieved while maintaining the outer CFU dimensions and design.

Simplified First-Principles Model of a Compact Flotation Unit for Use in Optimization and Control

“…It could also be adapted to account for unmodeled phenomena such as improved separation due to coalescence in the flotation section. By stacking several models of the type we have described, one can also describe more sophisticated designs, e.g., the ones by Schlumberger, , and by Siemens Water Technologies Corp., where the flotation gas and the produced water enter the CFU together from the same inlet and the produced water goes through multiple stages of separation.…”

“…As mentioned above, there are several CFU designs, many of which have resulted due to a continuous improvement over previous designs. − In this paper we consider a simple CFU design that uses IGF with flotation gas fed at the bottom, as shown in Figure . A similar design has been proposed by, e.g., the NATCO Group Inc .…”

Gas Flotation of Petroleum Produced Water: A Review on Status, Fundamental Aspects, and Perspectives

“…This led to the development of the CFUs. These are hydraulic units that can use both IGF and DGF advantages to remove oil droplets from PW . Performing gas flotation in single mode would result in disadvantages: in IGF systems, the oil droplets much smaller than 100 μm may escape flotation, while in DGF systems, the oil droplets larger than 100 μm cannot be floated …”

“…A CFU with internal swirl design has also been reported to improve the gas separation efficiency . The swirl design features a lower terminal velocity in certain areas and a flow eddy that aids collisions between gas bubbles and oil droplets and separates residual crude oil from PW in a second treatment stage, allowing for separation of more of the smaller gas bubbles with attached oil droplets.…”