For light and medium crude oil produced waters, the compact flotation unit (CFU) technology has been widely used as the final cleaning step in the process during the last 10 years, especially in Europe. In general, more traditional methods such as induced gas flotation (IGF) and dissolved gas flotation (DGF) are more commonly applied on heavy crude oil-produced waters. The CFU technology has been preferred over the more traditional technologies due to substantially reduced weight and footprint combined with superior cleaning efficiency. This paper describes efforts related to examining the effectiveness and applicability of using CFU technology in heavy crude oil produced water (PW) polishing and comparing the results with polishing produced water from light and medium crude oil. Experimental work was conducted in a water test setup in which synthetic produced water with different crude oils (13.7–39.7° API gravity) was tested for flotation efficiency using a proprietary compact flotation unit. A literature study was performed to support the experimental results. Furthermore, field data were collected from field trials using the proprietary compact flotation unit technology with produced water containing crude oils in the 17–43° API gravity range. Experimental results showed that for a given crude oil type, the oil removal efficiency is dependent on oil droplet size, and these results are supported by the literature. Comparing different API gravity crude oils with the same droplet size showed, in general, a slightly reduced flotation efficiency with increasing gravity, which could be explained by the correlation between increasing gravity and increasing coverage time. However, field data for 17–43° API gravity crude oil show the ability for the proprietary compact flotation unit technology to meet normal effluent requirements (≤ 25 ppm) over the full API gravity range. Field data further emphasizes the importance of preparing optimum process conditions for the flotation unit. Despite of the low retention time compared to traditional IGF and DGF technology, the compact flotation technology has proven to be robust for heavy-oil treatment. This result is partially explained by the fact that induction and coverage times are shorter in turbulent flow regimes than in traditional laminar flotation environments. Furthermore, the proprietary compact flotation unit technology combines both induced and dissolved flotation with vessel internals that effectively minimizes the rise path. It should be emphasized that these findings relate to the examined CFU models only because the various models have different technologies that will lead to different performances.
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.
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