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By Chang-Ming Lee and Ted Frankiewicz, NATCO Process Solutions Group, Houston, TX
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When crude oil is pumped from wells, water is
also produced, and this water must be
cleaned of oil and suspended solids prior to
disposal. Induced gas flotation (IGF) systems are separators
that can be used to clean the oily water to the
required purity level. These systems work by introducing
small gas bubbles, 100 to 500 microns in diameter,
into the vessel along with the contaminated water. The
hydrophobic oil droplets and oil-coated solids attach to
the water-gas interface and float to the surface, where
they are removed by a skimming mechanism. By controlling
the gas flow rate, vessel internals, and gas inlet
geometries, the gas bubble sizes can be optimized for
the most efficient lifting of contaminants.
In the original design, air entering the vessel was not well distributed, as illustrated by the photograph (above) and CFD simulation (below)
Traditional IGF systems consist of four flotation cells
in a horizontal tank. Oily water enters the unit from
one end and passes sequentially through the series of
cells. To save weight and space on offshore platforms,
the industry is moving towards a single-cell vertical column
configuration. These devices pose a major design
challenge because the same level of separation – the
removal of 90% of the oil – must be accomplished in a
single cell rather than in four cells. To meet this need,
the gas bubbles must be uniformly dispersed in the
vessel so that they contact all of the contaminated
water. Unfortunately, most vertical IGF systems suffer
from short-circuiting and as a result, poor water cleaning
efficiency.
At NATCO, a series of Eulerian multiphase calculations
using FLUENT were done to improve the performance
of a vertical IGF system. The initial work
focused on improving the performance of an off-theshelf
eductor, a gas bubble injection device. The results
showed that the injected bubbles created a straight,
fast-rising plume up to the top of the vessel. The plume
strengthened recirculation zones that further increased
the tendency towards short-circuiting. Physical tests
have validated the CFD results.
The introduction of a radial eductor (above) resulted in much better gas distribution
in the separator (below)
A number of design variations for the vessel and
eductor were subsequently studied using FLUENT.
Based on the results, NATCO engineers developed a
new eductor design with a radially-directed discharge
that offers performance far superior to conventional designs used in other IGF systems. They obtained additional
performance improvements by testing different
configurations of internal baffles, which control the
downward flow of oily water, prevent the development
of recirculation zones, and promote a more uniform
gas bubble distribution.
NATCO engineers went one step further by using
the volume of fluid (VOF) model with user-defined
functions to simulate the six degrees-of-freedom,
ocean wave motion-induced sloshing effect, and to
study how it affects the performance of IGF vessels
installed on floating offshore platforms. Based on the
results, improved wave motion suppression baffles
were installed in the skimming region inside the offshore
version of the IGF system. The final system configuration,
under the trade name of VersaFlo™, with
US and PCT (Patent Cooperation Treaty) patents pending,
dramatically improves upon existing designs by
providing a uniform distribution of gas bubbles across
the vessel while effectively eliminating the recirculation
zones. An initial installation of VersaFlo on a floating
platform in the Gulf of Mexico is very effectively cleaning
the produced water and easily meeting overboard
discharge quality requirements. Several additional
installations will be operational in the short term.
From left to right: Patrick McCarthy, President, Robert Curio,
Senior VP of Technology, Kevin Juniel, Product Manager of
Water Treatment Solutions, and Chang-Ming Lee, Engineer, of
NATCO Group Inc.; and Heshmat Massah of Fluent Inc.
Editor’s Note: The VersaFlo™ vertical column IGF was one
of fifteen innovations to receive an Offshore Technology
Conference (OTC) Spotlight on New Technology Award
in 2004.
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