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By Valmor de Almeida, Oak Ridge National Laboratory, Oak Ridge, TN; Rafiqul Khan and David Schowalter, Fluent Inc.
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Pressure contours
on the surface of
structured packing
elements in a
distillation column
Pathlines, colored by
pressure, illustrate
the flow through the
structured packing
elements
Much of the energy required for processing
chemicals in the industrialized
world is used in distillation
columns. Because the multiphase physics and
intricate geometry involved in these operations
are so complicated, their design remains semi-empirical.
CFD modeling offers an opportunity
to improve the performance of distillation
columns, translating into energy savings, better
product purity, and reduced environmental
impact. In fact, the US Department of Energy
(DOE) estimates that optimization could save
trillions of BTUs per year.
The Office of Industrial Technology at DOE
has supported efforts undertaken by a team
of engineers from ORNL, Fluent, and the
Separations Research Program (SRP) at the
University of Texas, Austin to modernize the
modeling practices in distillation columns and
similar chemical processing units. Other partners
on the team include Dow Chemical, John
Zink Co., Praxair, Sulzer Chemtech, and 3D
Imaging and Development Inc. The project has
focused on distillation columns that employ
a structured packing technique, where packings
are fabricated from thin, corrugated metal
sheets, and arranged parallel to one another.
The approach combines detailed geometry
modeling, first-principles simulation, and validation
experiments to deliver a realistic simulation
tool. A collection of macros, GraSPI
(Graphical Structured Packing Interface), has
been developed and integrated into GAMBIT
and FLUENT. GraSPI coordinates the generation
of geometries and meshes for typical
commercial structured packing elements and
flow simulation.
Simulations to date have involved gas flows
through different packing geometries, and comparisons
of predicted pressure drop with experiment
have identified the minimum mesh size
required for a certain level of solution accuracy.
GraSPI is currently being extended to
handle other packing geometries, such as perforated
sheets and cross-stacked cylindrical elements.
FLUENT simulations of gas-liquid
flows are also being carried out using the VOF
model. The ultimate goal is to be able to simulate
gas-liquid countercurrent flows through
as many packing elements as necessary to
produce a realistic model of SRP’s pilot
packed column. Achievement of this goal will
show that accurate, large-scale, first-principles
simulation of distillation in structured packed
columns is finally at hand.
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