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Measurement and Analysis of Gas Bubble Evolution from an Anode in Electrolytic Cells Supathorn Phongikaroon,* Robert O. Hoover, and Emily C. Barker Department of Chemical Engineering and Nuclear Engineering Program, UniVersity of Idaho, Idaho Falls, Idaho 83402
Measurement and analysis of oxygen gas bubbles generated from an anode in different aqueous electrolytic solutions have been studied at different applied currents (I). Bubble size distribution (BSD) was measured by using a high-speed digital camera, while corresponding voltage and current were recorded by an electrophoresis power supply. At low aqueous kinematic viscosity (νc < 0.017 cm2/s), the analyzed bubble size (b) is a strong function of νc; here, the slope (∆b/∆νc) below 0.017 cm2/s is an order of magnitude larger than that above it. In addition, at the higher νc, b is dictated predominantly by I. BSD can be described by the lognormal form of the continuous number frequency distribution. Here, the dimensionless median diameter for j ) and the variance (σ0) of the normalized log-normal distribution are 0.98 ( 0.029 and 0.27 the distribution (β ( 0.073, respectively. Further analysis indicates that ohmic resistance decreases as b increases because of the reduction in void fraction in solution. The resistivity of the solution can be described using the creepingflow equation of motion showing that viscosity dictates the initial solution resistance. 1. Introduction The electrometallurgical treatment of metallic spent nuclear fuel is currently carried out in the Fuel Conditioning Facility at Idaho National Laboratory. This treatment involves dissolution of the spent metal fuel at the anode into a molten salt electrolyte along with simultaneous deposition of uranium metal onto the cathode. To adapt this technology for oxide-based fuels, a headend oxide reduction process is necessary to convert the oxide fuel to metal. Details of this process have been discussed and reported in the literature with the relevant electrochemical reactions involving uranium shown in eqs 1a-1c.1-3 cathode: UO2 + 4e- f U + 2O2-
(1a)
2O2- f 4e- + O2(g)
(1b)
UO2 f U + O2(g)
(1c)
anode: net reaction:
be relevant to the real system.3 For the work reported here, the mock-up system is based on water electrolysis of aqueous glycerol solutions providing kinematic viscosity νc ranging from 0.00910 to 0.0378 cm2/s, which covers the actual value (νLiCl ) 0.0102 cm2/s) of the LiCl electrolyte in the oxide reduction process. It is expected that an understanding into the simultaneous effects of both viscosity and density can be achieved by using kinematic viscosity or momentum diffusivity. In addition to the study under different electrolytic solutions, the oxygen bubble sizes and distributions at the anode were also measured under different applied currents. The information from this mock-up study should help predict the bubble size distribution (BSD) in molten salt solutions and to improve monitoring of the electrolytic oxide reduction process. 2. Experimental Facility and Procedure
There is concern that oxygen bubbles generated at the anode may interfere with the process, limiting its efficiency and/or net rate.1,2 If the oxygen bubbles are not adequately dispersed from the anode surface, they can cover some of the anode’s surface, limiting the area available for the oxidation reaction. In addition, the O2 bubbles may react with the Li metal dissolved in the electrolyte or present as pure molten Li to form Li2O, lowering the efficiency of the process. Due to the high temperature and corrosive environment of the current process, it is not currently possible to visually observe bubble formation in the molten salt electrolyte used for oxide reduction.1,2 These issues and the concerns mentioned above become the motivation of the work to gain understanding into the actual system. The approach taken to comprehend these problems is to use a mock-up system containing surrogate fluids at room temperature, anticipating that observations made will * To whom correspondence should be addressed. E-mail: supathor@ uidaho.edu.
Figure 1 shows the experimental system that was operated in a laboratory fume hood. A rectangular vessel (7.5 cm × 8 cm × 19.5 cm) was used as a reaction vessel containing aqueous electrolyte solution. The setup inside the vessel was comprised of an anode and cathode spaced 2 cm apart, both 1 mm diameter platinum wires and immersed 16.5 mm into the electrolyte. They were connected to a Fisher FB200 power supply. A temperature probe and sensor electrode (Ag/AgCl fluid-gel KCl fill solution, Accumet AccuCap) were connected to an Accumet Excel 20 pH/mV/°C conductivity meter. Oxygen bubbles were electrochemically generated at the anode by the electrolysis of water. The electrolysis was performed in a galvanostatic mode at different applied currents with a maximum cutoff voltage of 200 V. Temperature control was achieved by immersing the reaction vessel in a rectangular glass tank filled with water. The water was kept at constant temperature at 25 ( 0.3 °C by using a combination of a heater (a Digital Fisher Isotemp Immersion Circulator) and cooling water flowing through the loop as shown in Figure 1. The cooling water was necessary to remove heat from the water bath as the current was increased by running cold tap water through a coil sitting in an ice-water bath passing through the water tank.
10.1021/ie900908c 2010 American Chemical Society Published on Web 02/16/2010
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Figure 1. Top view of the experimental setup. Table 1. Fluid Physical Properties at 25°C, 1 atm, and Operating Conditions fluid oxygen electrolyte I electrolyte II electrolyte III electrolyte IV electrolyte V LiCl at 650 °C oxygen at 650 °C
glycerol (wt %)
density (kg/m3)
liquid viscosity (mPa · s)
kinematic viscosity (cm2/s)
0 17.14 23.29 33.19 42.58
1.309a 999.4 1031.3 1043.6 1063.4 1082.2 1480b 0.422a
0.0208a 0.909 1.498 1.769 2.592 4.086 1.51b 0.0353c
0.159 0.009 10 0.014 53 0.016 95 0.024 37 0.037 76 0.010 2 0.836
operating current (A)
running temperature (°C)
0.05, 0.10, 0.50 0.05, 0.10 0.05, 0.10, 0.25 0.05, 0.10, 0.25 0.05, 0.10, 0.25
25.59 ( 0.22 25.69 ( 0.13 25.26 ( 0.70 25.31 ( 1.48 26.46 ( 1.48
a Density and visocisty values are obtained from ref 4. b Values are obtained from ref 5. c Value is obtained from the Chapman-Enskog theory from ref 6.
Five solutions of differing kinematic viscosity (νc)s0.00910, 0.0145, 0.0170, 0.0244, and 0.0378 cm2/sswere used to help determine the relative significance of liquid-phase kinematic viscosity on the bubble generation. NaCl was added to all solutions (0.065 M NaCl) to provide an electrolyte to the system. Viscosities and densities were measured by using CannonFenske viscometers and a volumetric technique, respectively.3 Table 1 shows the list of fluid physical properties at 25 °C. The reaction vessel was filled with 330 cm3 of the solution before each run and was placed into the water bath and allowed to thermally equilibrate to a constant temperature. At this point the reaction was initiated by turning on the electrophoresis power supply (Fisher Scientific Model FB200). The reaction was run at three different applied currents (I) of 0.05, 0.10, and 0.25 A, with current densities ranging from 0.0966 to 0.483 A/cm2. When the applied current was increased above 0.25 A, the temperature inside the reaction vessel increased rapidly, causing a temperature control problem. The average reaction cell temperatures for different runs are reported in Table 1. Only the aqueous solution with 0 wt % glycerol was run at 0.5 A. Each experimental run was allowed to reach its equilibrium, which took generally less than 5 min. The main concern is the generated heat at high current conditions; special attention to the cooling water is necessary to keep the temperature at a fixed level. After a stationary stage was attained, bubble images, as shown in Figure 2, were captured at 500 frames/s for approximately 10 min using a high-speed digital system (TroubleShooter 1000 by Fastec, Inc.). A 500 W mercury lamp provided ample light for all digital filming processes. The experiment was performed at least three times for repeatability. After the current was changed, the fluid was allowed to reach the set temperature before any images were recorded. All recorded images were analyzed using the MotionMeasure software suite
Figure 2. Image of bubbles forming at the electrode surface in an electrolyte with a kinematic viscosity of 0.0091 cm2/s and an applied current of 0.1 A.
to obtain bubble sizes and distributions. An average of 199 ( 68 bubbles was measured for each electrolyte/current experiment. 3. Results and Discussion The oxygen bubble generation at the anode appeared to be random. Upon review of all recorded images at 10 frames/s, there was no observation of breakup or coalescence of generated
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Figure 3. Effect of νc on bubble size distribution at an applied current of 0.05 A: (A) number frequency, (B) cumulative number frequency, (C) volume frequency, and (D) cumulative volume frequency.
gas bubbles after departure from the anode surface in the viewed region. While Lessard and Zieminiski7 reported a similar observation showing the decrease of coalescence of gas bubbles within the electrolytic solution by increasing the viscosity and the salt concentration, Prince and Blanch8 illustrated the decrease of breakup rate by increasing NaCl concentration in the system. In addition, it has been shown that the bubble breakup rate decreases significantly by increasing the NaCl concentration in the system.8 This observation provides a similitude to the idea that, by increasing the ionic compound concentration (for example, NaCl), the coalescence and breakup of bubbles within the system would decrease significantly. Since the continuous phase in the electrolytic oxide reduction is mainly composed of LiCl molten saltsanother ionic compound similar to NaClsit is expected that the generated bubbles will not experience breakup or coalescence. Due to this random nature, bubbles of various sizes can be generated providing different spectra of equilibrium size distribution. In this study, the mean bubble size (b10) and its variance (σ10) will be used to facilitate the discussion and to provide a more meaningful interpretation of obtained data.9 3.1. Bubble Size Data. The number and volume distributions are presented in the form of frequency (f(b)) and cumulative frequency (F(b) ) ∑f(b)) with the subscripts “n” and “V” meaning number and volume, respectively. Parts A-D of Figure 3 show different forms of these BSDs at the current of 0.05 A. The results show that the distribution broadens and b10 increases as νc increases to approximately 0.017 cm2/s. At higher νc, the cumulative distributions are almost the same, suggesting that b10 is controlled by the applied current. BSDs at 0.10 A are shown in parts A-D of Figure 4. The trends are similar to those shown in Figure 3. In addition, comparing Figures 3B and 4B reveals that the effect of νc on bubble distribution becomes less pronounced as the applied current increases. Therefore, to observe the competitive influence between νc and I on b10, mean diameter was plotted versus νc in Figure 5. The result shows that at low νc (
