Investigation of residence time distributions in electrolytic cells
Term: from 05 / 2007
Minimization of specific energy consumption and optimization of the space time yield at the same time are crucial for the efficiency of electro-chemical processes. Both parameters are significantly affected by the hydrodynamics in the cell volume. Thus, main objective is to establish a minimal invasive measurement method for the investigation of hydrodynamic conditions in the anode and cathode volume of industrial electrolytic cells. Thereby, the study focuses on the measurement of residence time and backmixing.
Modeling and simulation of the hydrodynamics based on measurement results are essential requirements for optimization of cell design and therewith hydrodynamic conditions.
Whereas residence time distributions are mainly obtained by adding tracers whose concentration is measured locally and invasively (e.g. conductivity measurements by adding conductible tracers), the laser induced fluorescence (LIF) is a minimal invasive point-oriented or area-oriented method for determination of species concentrations solved in water for simultaneous estimation of liquid velocity and concentrations.
The dye used in this study is sulforhodamine B (SRB) which fluoresces when induced by a green laser beam with a wave length of 532 nm. SRB is photo-stable and sorptivity is low. Signal isolation is possible due to the emission maximum at a wave length of 583 nm. High signal quality is obtained at high quantum efficiency (high fluorescence intensity emission). The application of LIF requires a direct proportionality between fluorescence intensity and dye concentration (figure 1).
Figure 1: Dye concentration vs. fluorescence intensity
The experimental setup for the measurement of the residence time distribution using LIF in the electrolytic cell is schematically shown in figure 2 exemplarily for the cathode compartment. A 10 second square tracer pulse is introduced at the reactor inlet. The respective volume is laterally exposed to the laser beam. The temporal fluorescence intensity is frontally recorded as a sequence of RGB picture by the camera. A filter is mounted at the camera to block wavelength < 550 nm.
Figure 2: Experimental setup and measurement method (1 – segmented cathode, 2 – cathode volume, 3 – spacer grid, 4 – anode volume, 5 – anode, 6 – membrane, 7 – camera, 8 –laser, 9 – liquid distributor)
Equation (1) is used for calculation of the dimensionless tracer concentration E(t) considering background intensity IB and reference tracer concentration IE=1.
Backmixing (axial dispersion coefficient Dax or Peclét number) and liquid velocity u is calculated using the axial dispersion model (equation 2).
Figure 3 shows exemplarily the results of the parameter estimation for u and Dax based on measured residence time distributions at different cell heights in the cathode compartment.
Figure 3: Residence time distribution, experiments vs. simulation (70 l/h, u = 0.034 m/s, Dax = 0.00068 m2/s)
For the evaluation of the backmixing level the dimensionless Peclét number can be applied. The Peclét number, describing the ratio of convective to diffusive mass transfer, can be calculated using the dispersion coefficients.
Figure 4: Axial dispersion and Peclét number vs. liquid velocity
Characteristic values of the axial dispersion and of the Peclét number in the cathode compartment of an electrolytic cell are shown in figure 4. At low liquid velocities, plug flow behaviour occurs. At higher flow rates, the axial backmixing increases.