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 ʃ A 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. Qsn Nq = 3 ND (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. % pumping in s1 + s4 65 60 55 50 45 40 % Nq 40 35 30 25 20 15 10 s1 s2 s3 s4 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 (%) Vorticity Magnitude (1/s) 1.19125 1.45819 1.85244 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 1.2 Normaliezed Power 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. References 1. L. Liu, X. Ma. Technology-based industrial environmental management: a case study of electroplating in Shenzhen, China. J. Cleaner Prod. 18 (2010) pp. 1731-1739. 2. SEMARNAP, 1997. Nom-001-Ecol-1996, Diario Oficial de la Federación, México, 1997. (In Spanish). 3. Rodríguez, M.G., Aguilar, R., Soto, G., Martínez, S.A. Modelling an electrochemical process to remove Cr (VI) from rinsing waters in a stirred reactor. J. Chem. Technol. Biotechnol. 78, (2003) pp. 371-376. 4. S.A. Martínez-Delgadillo, J. Ramírez-Muñoz, H.R. Mollinedo P., V.Mendoza-Escamilla, C.Gutiérrez-Torres and J.Jiménez-Bernal."Determination of the Spatial Distribution of the Turbulent Intensity and Velocity Field in an Electrochemical Reactor by CFD". Int. J. Electrochem. Sci., 8 (2013) pp. 274-289. 5. R. Thilakavathi, D. Rajasekhar, N.Balasubramanian, C. Srinivasakannan, A. A. Shoaibi. CFD Modeling of Continuous Stirred Tank Electrochemical Reactor. Int. J. Electrochem. Sci., 7 (2012) 1386-1401. 6. M.H. Vakili, M. Nasr Esfahany. CFD analysis of turbulence in a baffled stirred tank, a three-compartment model. Chem. Eng. Sci. 64 (2009) 351-362. 7. J. Derksen. Flow Turbul. Combust. Confined and Agitated Swirling Flows with Applications in Chemical Engineering. 69 (2002) 3-33. 8. A.D. Harvey, C.K. lee, S.T. Rogers, "Steady-state modelling and experimental measurment of baffled impeller stirred tank" AIChE J. 41 (10) (1995) pp. 2177-2186. 9. M. Li, G. White, D. Wilkinson, K. J Roberts. "Scale up study of retreat curve impeller stirred tanks using LDA measurments and CFD simulation". Chem. Eng. J. 108 (2005) pp. 81-90.