The
significance of interfacial chemistry for the oil removal efficiency
during flotation was demonstrated in a series of laboratory flotation
and pilot-scale compact flotation unit (CFU) tests. Three crude oils
with different physicochemical properties were used in the investigations.
The differences in drop size distributions and densities of the oils
could not fully account for the observed oil removal. However, taking
the time for drainage and rupture (i.e., induction time) of the thin
aqueous film separating the drops and bubbles into consideration resulted
in good agreement with the oil removal. Moreover, it was demonstrated
in a modified CFU setup that water-soluble hydrocarbons adsorbed onto
the bubbles and reduced the oil removal. This was most likely as a
result of increased induction times caused by the adsorbed components.
In general, the operation of produced water treatment systems is often based on manual sampling, visual observations, operator judgment and manual tuning by operation of valves. Work has been conducted to automate the operation of a Compact Flotation Unit (CFU). The objective was mainly to reduce the overall amount of discharged of Oil-in-Water (OiW). As discharge peaks often govern the overall OiW percentage in discharge, a real-time operated and optimized CFU system could immediately detect and reduce the effect of such peaks. Furthermore, the waste generated and utility consumption should both be reduced. The CFU operation would not require operating personnel which is an absolute requirement for an unmanned installation or subsea operation. The work was conducted in a produced water test rig. Manual sampling and visual observations were successfully replaced with online analyzing- and flow measuring technology. This replacement ensured better process overview and allowed implementation of Integrated Operations (IO) philosophy. Advanced control algorithms for real-time CFU operation and optimization were developed based on implementation of this technology. The work has resulted in a new automatically operated CFU system. Testing in the produced water test rig met the goals: increasing OiW content was detected and effective corrective actions were taken without the need for operating personnel. In addition to reducing the OiW content, the waste generation and utility consumption were simultaneously minimized.
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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