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.