cfd simulation of mixing process in a large crude oil storage

CFD SIMULATION OF MIXING PROCESS IN A
LARGE CRUDE OIL STORAGE TANK WITH
SIDE-ENTRY IMPELLERS
Diana C. Hernández Jaramillo, Aeronautical Engineering School, UPB
César Nieto Londoño, Aeronautical Engineering School, UPB
Nayith Alvarez Sarmiento, Instituto Colombiano del Petróleo, Ecopetrol
Rigoberto Barrero A., Instituto Colombiano del Petróleo, Ecopetrol
Luz Angela Novoa Mantilla, Instituto Colombiano del Petróleo, Ecopetrol
PRESENTATION TOPICS
• Company Overview (2-3 minutes);
• Problem Description;
• Methodology;
• Results;
• Conclusion and next steps.
Company Overview
• Grupo de energía y termodinámica,
Universidad Pontificia Bolivariana
• Instituto Colombiano del Petroleo, Ecopetrol
Problem Description
• Mechanical agitators are used in the petroleum industry
in mixing and homogenization processes.
• Most of the studies are for top-entry impellers and very
few analysis have been done for side-entry impellers.
• There are concerns regarding the effectiveness of such
mixing equipment considering the dimensions of the
tanks with respect to the agitator
• The tank has two side-entry mixers placed on
opposites sides.
Diameter
29” (0.7366 m)
Mounting angle
45° and 90°
Angular velocity 425 rpm
Ecopetrol, Agitador mecánico instalado en un tanque de
almacenamiento, (fotografía), 2014. Archivo del autor
Methodology
• CFD is used to model flow patterns in a tank that has 36.5 m of
diameter, 9.2 m high and 7.7 m of filling level.
• The impeller geometry was made in Solid Edge® , it was imported
to the design modeler of Ansys® where the geometry of the tank
was made.
• In order to simulate the rotation of the impellers, the Multiple
Frames of Reference model (MFR) was used. A cylindrical
volume was defined around the impellers to rotate with
angular velocity of them while they were at rest.
• The k- ߝ RNG model was used to solve the
continuity and the RANS equations (Reynoldsaverage Navier Stokes).
• The VOF model (Volume of Fluid) was used to
simulate the free surface between air and crude
representing the floating roof of the tank.
Crude oil
Grados API
Densidad [kg/m3]
Viscosidad [kg/m.s]
28.03
885.2
0.010773
Results
Three cases were simulated:
• One phase of crude oil - 90° of impeller mounting
angle.
• One phase of crude oil - 45° of impeller mounting
angle.
• One phase of crude oil, one phase of Nafta - 45° of
impeller mounting angle. (Homogenization time)
• One phase of crude oil - 90° of impeller
mounting angle.
Velocity vectors in a horizontal plane at the same level of
the impellers and at the top of the tank
Velocity streamlines in a horizontal plane at the same
level of the impellers and at the top of the tank
Velocity streamlines in a vertical plane
V m/s
Impellers level
Across the tank
V<0.05
0.05<V<0.1
V>0.1
32.6 %
32.4 %
35 %
44.2 %
30.8 %
25 %
V m/s
Top of the tank
V<0.032
0.032<V<0.064
0.064<V<0.096
42.2 %
52 %
5.8 %
Velocity at the impeller level
Velocity
Axial component
Tangential component
Radial component
• One phase of crude oil - 45° of impeller
mounting angle.
Velocity vectors in a horizontal plane at the same level of
the impellers
Velocity streamlines in a horizontal plane at the same
level of the impellers and at the top of the tank
Velocity at the impeller level
Velocity
Axial component
Tangential component
Radial component
V m/s
V<0.05
0.05<V<0.1
V>0.1
Impellers level Across the tank
17,5 %
35 %
21 %
11.5 %
61.5 %
53,5 %
V m/s
V<0.1
0.1<V<0.15
0.15<V<0.25
Top of the tank
20.4 %
46 %
33.6 %
• One phase of crude oil, one phase of Nafta
- 45° of impeller mounting angle.
(Homogenization time)
Crude oil
Nafta
Grados
API
Densidad
[kg/m3]
Viscosidad
[kg/m.s]
Volume fraction
16
957.4
0.4799
92 %
686.84
0.000345
8%
• Nafta volume fraction after 2.5 hours of operation
Conclusion and next steps
• An angle of 90° distributes the flow around the tank in four sections,
where the radial velocity predominates and the axial component
generates a upward recirculation in the middle of the tank.
• An angle of 45° generates a circular pattern where the tangential velocity
predominates near to the wall of the tank, this creates an stagnation zone
in the middle of the tank. This configuration presents the greatest
velocities across the tank, which has a major influence in the mixing
efficiency.
• According to the circular pattern in the 45° configuration, it seems to be
not enough to have an appropriate mixing, that´s why the next step, after
finishing the simulation, is analyze a case with an angle of 60°
REFERENCES
• Dakhel , A. A., & Rahimi, M. (2004). CFD simulation of
homogenization in large-scale crude oil. Journal of
Petroleum Science & Engineering, 151-161.
• Rahimi, M. (2005). The effect of impellers layout on mixing
time in a large-scale crude oil storage tank. Journal of
Petroleum, Science & Engineering, 161-170.
• Wu, B. (2012). Computational fluid dynamics study of largescale mixing systems with side-entering impellers.
Engineering Applications of Computational Fluids
Mechanics, 123-133.