EVALUATION OF THE EFFECT OF PITCHED BLADE TURBINE ON
MIXING IN AN ELECTROCHEMICAL REACTOR WITH
ROTATING RING ELECTRODES.
Sergio A. Martinez, Universidad Autónoma Metropolitana, México D.F. Mex., Jorge Ramírez
Universidad Autónoma Metropolitana, México D.F. Mex., Helvio Mollineado, Instituto
Politécnico Nacional, México D.F. Mex., Oliver Huerta, , Instituto Politécnico Nacional, México
D.F. Mex, Victor Mendoza, Universidad Autónoma Metropolitana, México D.F. Mex.
Introduction
Hexavalent chromium, Cr(VI) is one of the most hazardous pollutants discharged into
wastewaters (1). Cr(VI) is generally considered to be 1000 times more toxic than Cr(III), so
it is extremely harmful to human health. Therefore, this hazardous heavy metal must be
removed to reach an acceptable concentration level of less than 0.1 mg/L and comply with
the environmental regulations for wastewater disposal (2).
The electrochemical process is an alternative method used to remove Cr(VI) from
industrial wastewaters. However, in electrochemical plug flow reactors with no liquid mixing
or static electrodes, an iron salt film is formed on the electrodes surface (electrode
passivation) leading to poor mass transfer that reduces the Cr (VI) removal efficiency and
causes greater energy consumption (3). Therefore, the electrochemical reactors require to be
designed to provide high mass transfer between the bulk liquid and the electrodes..
To increase mass transfer and mixing, an electrochemical reactor with rotating ring
electrodes has been proposed and improvements were evaluated (3). However, further
research, using state-of-the-art computational fluid dynamics (CFD) tools, showed that the
flow velocity field and turbulence intensity were not homogeneous in the reactor (4). There is
a zone inside the rotating ring electrodes, which accounts for nearly 35% of the reactor
volume, where velocity, turbulence and vorticity have considerably lower values than in the
rest of the vessel, reducing mass transfer and process efficiency (4). In this work, two
pitched blade (PB4) impellers were placed inside the rotating ring electrodes to improve the
mass transfer and mixing.
The performance of an electrochemical reactor with a volume capacity of 18 L, was
evaluated by measuring power consumption. Moreover, parameters such as turbulence
intensity and vorticty magnitude, were calculated by using CFD tools (5-7) at different
rotational speeds in order to evaluate the effect of the impeller on reactor performance.
Experimental
The rotating ring electrodes consist of an arrangement of 14 iron steel rings, 7
cathodes and 7 anodes. Electrode rings are evenly separated by 17 mm, in a sequence of
one cathode followed by one anode. The main shaft is driven by a variable speed motor
(servomotor) to control the speed of the ring electrode arrangement. The cylindrical tank has
a torispherical basis with four baffles arranged symmetrically. Tests were performed at
different electrode rotational speeds of 150 rpm and 230 rpm. Four baffles are symmetrically
disposed inside a cylindrical vessel of 18 L. The reactor height was 0.367 m with and internal
diameter of 0.27 m.
The reactor performance was evaluated with two PB4 impellers placed inside the
rotating ring electrodes. In one case, the blades of both impellers were aligned (case A), as
seen in figure 1b. For the other case, the blades of one impeller were rotated 45° (Case B),
as shown in figure 1c. The results were compared against those of the reactor without
impellers (figure 1a, case N).
The power consumption were evaluated at the different rotational speeds for all three
cases. Tests were carried out at 150 rpm and 230 rpm. The numerical simulation was
performed using Fluent©. A complete three dimensional model was prepared for each
geometrical configuration of the reactor. Due to the complex geometrical shapes, all these
models were meshed using tetrahedral cells.
The grid independence was tested and results compared with simulations of the same
reactor models reported in previous work (4). The pressure-based segregated algorithm
solver was used for CFD simulations, for which the governing equations are solved
sequentially.
Discretization was accomplished using the standard scheme for pressure discretization,
while a second order upwind scheme was used for momentum discretization. The semi
implicit pressure-linked equation (SIMPLE) algorithm was used for the pressure-velocity
coupling. Simulations were carried out using the κ-ε realizable turbulence model, that is the
most recently developed variation of such κ-ε models. Realizable κ-ε model uses a new
equation for the turbulent viscosity and the dissipation rate transport equation is derived
from the equation of transport of the mean-square vorticity fluctuation. The form of the eddy
viscosity (turbulent) equations is based on the realizability constraints; the positivity of
normal Reynolds stresses and Schwarz' inequality for turbulent shear stresses. This is not
satisfied by either the standard or the RNG κ-ε models which make the realizable model more
precise than both models at predicting flows such as boundary layers under strong adverse
pressure gradients or separated flows, rotation, recirculation, strong streamline curvature
and flows with complex secondary flow features. Finally, a 0.001 tolerance was chosen for
the convergence criterion.
To facilitate analysis, the volume of the electrode was divided in four sections (s1 to
s4) as shown in figure 2a. Also, five surfaces, with the same diameter as the electrode, were
located at different positions (1 to 5) along the electrode axis as seen in figure 2b.
The power consumption P was calculated as the product of torque on the rotating
rings electrodes, with and without impellers, and shaft angular velocity (8, 9) according to
the equation 1.
P = ω ʃA r
X
(Ƭ dA)
(1)
Where A is the overall area of the rotating electrodes and the shaft surface area, ω is the
angular velocity vector, r the position vector and Ƭ the stress tensor.
The pumping number Nq was calculated for each section (s1 to s4) with equation 2.
Nq =
Qsn
ND3
(2)
Where Qsn is the mass flow rate in each section, N the revolutions per second and D the
diameter of the electrode.
(a)
(b)
(c)
Figure 1. Electrochemical reactor: (a) without impellers (Case N); (b) aligned impellers (case
A) and (c) one impeller rotated 45° (Case B).
a)
b)
Figure 2. a) Electrode sections and b) surfaces at different positions.
Results
The CFD simulation results for the flow field, at a rotational electrode velocity of 150
rpm, for the three cases are shown in figure 3.
(a)
(b)
(c)
Figure 3. Flow field for the three cases: a) case N, b) case A and c) case C.
As can be seen, dominant circulation loops, located at the bottom and top of the
reactor, are observed in case N. There is a radial flow from the rotating rings where the
liquid is pumped towards the wall that returns to the inner volume of the electrodes
arrangement through sections s1 and s4 as depicted in figure 3a. In case A (figure 3b), loops
at the bottom and top are formed as in case N, and the impellers also caused loops under
the impellers. In this case, the loops at the top have higher velocity values than in case N. In
both cases, the pumping in section s1 and s4 is about the 60% of the total pumping, while in
case B is about 52%, as shown in figure 4a. In case B, more loops are formed inside and
outside the rings electrode, but in this case the pumping is better distributed than in the
other cases, as represented in figure 4b.
40
65
s1
s3
s4
30
55
25
50
45
% Nq % pumping in s1 + s4
s2
35
60
20
15
40
10
35
5
30
0
Case N
Case A
Case B
a)
b)
Figure 4. a)% of pumping in section s1 and s4 and b) pumping distribution in
the different sections.
In cases N and A, the pumping in s2 and s3 is lower than the other sections, and
lower than in the same sections of case B. In this case, although the pumping in s4 is
reduced, the pumping in s3 increased. Based on the flow fields shown in figure 3, the areaweighted average velocity and vorticity magnitude and turbulent intensity (TI) were
evaluated (table 1). As shown, the values of all three parameters were higher for cases A and
B than for case N.
Table 1. Area-Weighted Average parameters for the three cases at 150 rpm.
Parameter
Case N Case A Case B
Velocity Magnitude (m/s) 0.06118 0.07827 0.07124
Turbulent Intensity (%) 1.19125 1.45819 1.85244
Vorticity Magnitude (1/s) 6.54579
13.3511 8.69605
This means that the impellers inside the rotating rings electrode improved the mixing
in the reactor, in comparison with the reactor without impellers (case N).
In figure 5, are shown the results of the turbulent intensity in the different positions
along the rings electrode (figure 2b).
Figure 5. Turbulent intensity at the different positions in the rotating rings electrode.
As it was mentioned before, the pumping is the lowest in sections s2 and s3, for the
cases N and A. These sections are located between position 2 and 3 (s2) and 3 and 4 (s3).
Figure 5 shows that the lowest TI was in position 3 for case N. At the same position,
cases A and B, have higher values of TI, which can be explained by the loops formed by the
impellers in this zone (s2 and s3).
The highest TI values were found in case B, which agrees with the higher number of
loops formed by the impellers, between position 2 and 3 (s2) and 3 and 4 (s3), as it was
mentioned before.
In addition, the power consumption was evaluated and the highest value was obtained
in case A: almost 20% more power is consumed than in cases N and B, as shown in figure 6.
1.4
Normaliezed Power 1.2
1
0.8
0.6
0.4
0.2
0
case N
case A
case B
Figure 6. Normalized power consumption for the three cases.
For all the rotating electrode speeds, higher values of vorticity and TI were obtained
when the reactor was operated with internal impellers (cases A and B). The contours of
vorticity magnitude obtained at 230 rpm are shown in figure 7.
As can be noted, in comparison with case N, the vorticity increased inside and outside
the volume enclosed by the electrode rings (dark zones) for cases A and B.
(a)
(b)
(c)
Figure 7. Contours of vorticity magnitude at 230 rpm, for the three cases; a) case N, b) case
A and c) case B
Conclusions
It was shown that, when impellers are installed, higher TI, velocity and vorticity
magnitude values are reached inside the rotating rings electrode. Also, the number of
circulation loops is incremented inside and outside the electrode, improving mixing in the
reactor in comparison with the reactor without impellers
When aligned impellers (case A) were used, the power consumption increased up to
20% more than the reactor without impellers (case N). On the other hand, when one of the
impellers was rotated 45° (case B), the power was higher than in case N, by only about 4%.
Also, higher TI inside the rotating electrodes was reached in case B, in comparison to that of
cases A and N.
Acknowledgements
Financial supports of this work by the Consejo Nacional de Ciencia y Tecnología
(CB2011/169786) are gratefully acknowledged.
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