Applications o f Fluidized Beds in Electrochemistry P. LE GOFF F. VERGNES F. COEURET J. BORDET
lectrochemical operations consist generally in a
E combination of two kinds of processes: the motion
of the ions from the bulk of the solution to an electrode and the discharge, or the reaction of thesc ions in contact with the electrode. I t is said that polarization of concentration occurs when the kinetics of the operation are limited by a process of the first type. The motion of the ions is due, in part, to forced migration under thc influence of the electric field, in part to the molecular diffusion caused by the concentration gradient, and in part to convective transfer by the fluid in eddy or laminar flow. The equality between the entire flux transferred to an electrode and the sum of the fluxes due to the migration, the diffusion, and the convection can be written for each ionic species present in the solution. But in the neighborhood of an electrode in the diffusional boundary layer, the term due to the convection is ncgligible and remains ~t
=
),(
- C2W,Z,e
-D, dC2 y=O
r$)
(1)
u=O
where
a2 = C,
=
W, = 2, =
molar flux transferred to the electrode (mole cm-2 sec-I) molar concentration of the transferred ion mobility of the ion valence of the ion diffusivity of the ion
D, = dV _ - electric potential gradient dY
There are as many equations as there are ionic species in the solution, but in the simplest case where a single ion discharges a t an electrode with a Faraday yield equal to one, the measurement of the electric current gives directly the value of the transfer flux of this single ion :
since 8
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
ei=
0 for all the other ions.
If we consider the particular case of the discharge a t the cathode of the cation of a salt in low concentration in a supporting electrolyte with a common anion, the concentration gradients of all the ions at any point in the boundary layer are proportional to each other. Moreover, the electrical neutrality equation, valid in any point of the solution except a t the contact of the electrodes is :
BCtZt = 0 We can verify easily that the terms of Equation 1 are not independent: The whole process occurs as if the gradient of electrical potential does not interfere and the measured current depends only on the concentration gradient of the ion that discharges, that is:
where DE is the effective diffusivity. I t is usual to consider that the concentration gradient in the diffusional boundary layer is constant and to write @$
=
c - c,
-DE. 6 = k(C - C,)
where C is the concentration of the ion in the bulk of the solution, maintained with good homogeneity by the turbulence of the flow, and C, is the concentration in contact with the electrode. The thickness, 6, of the diffusional boundary layer is bound to the thickness 6o of the hydrodynamic boundary layer by the relation
a=----
60
(SC)1'3
The Schmidt number, Sc, is in the order of 1000 for liquids, so 6 is about 10 times smaller than 60, but they are in all cases proportional for a given state of agitation of the liquid phase. When the amount of turbulence grows, 60 and hence 6 decreases, following well-recognized laws of fluid mechanics (2). The mass transfer coefficient k = DE/6 therefore increases correlatively. VOL. 6 1
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Inversely, if, for a given hydrodynamic regime, we want to increase the current density (by increasing the electric potential between the electrodes) it should be possible to reach a limiting value when the concentration C, a t the electrode cancels out: ilim
DEC
= ZtF 6
This simple relation and the related theories are especially important for their applications in the three following fields : In fluid mechanics. T h e measurement of the limiting current for an ion of given nature and concentration is a simple and elegant method for the determination of the local, instantaneous, and absolute value of the mass transfer coefficient in an apparatus where a fluid is flowing. In electrochemical analysis. The measurement of the limiting current, in conditions of known constant hydrodynamics gives a direct determination of the concentration of the discharging ion. This is the basis of polarography. I n industrial electrolytical processes. The theoretical considerations allow the orientation of research toward the best operating conditions for increasing densities of limiting currents and more particularly rates of galvanoplastic deposit. We will study how fluidized beds can be used in these three fields of science and industry. Applications in Fluid Mechanics
The proportionality relation between the mass transfer coefficient and the limiting current density makes electrochemistry an especially simple method for many studies of mass transfer or boundary layers. The required conditions for good determinations are naturally those stated in the introduction-low concentration of electrode reacting species in a large excess of conductive supporting electrolyte. Four other conditions must be fulfilled : The working electrode should not change during the course of the electrolysis The counterelectrode should have a large surface to lessen the effect of any polarization phenomenon The concentration of the transferred ion should not vary too much during measurement The electrochemical process on the electrode should be fast Lin and coworkers (3) first tried many electrochemical systems and they finally preferred the cathodic reduction of ferricyanide ion in 0.5iV sodium hydroxide aqueous solution. They used this system for the determination of the mass transfer coefficients to the walls in a cylindrical coaxial geometry. Following them, many authors have used the reduction of ferricyanide in the same way. 10
INDUSTRIAL AND ENGINEERING CHEMISTRY
2 1
/ */
*' 0
orrvdtagr
I 1CC
I
I
200
300
,C ,I
4m
500
Figure 7 . Polarization curve$ of the electrode
Eisenberg et al. after having studied free convection by copper sulfate electrolysis between copper electrodes (4), took up the ferricyanide reduction for the study of the transfer at the lateral surface of a rotating cylinder (35). Krishna and Jagannadha Raju studied annular electrodes placed at the bottom of stirred cylindrical vessels and rotating electrodes with and without propellers in baffled vessels (5). Iribarne et al. studied with the same method the transfer between the external surface of a fixed vertical cylinder and a liquid film, streaming above it (6). Other authors have used this method with microelectrodes placed in the walls to reach local values of which the fluctuations were of interest, instead of macroscopic values. Reiss and Hanratty (7, 8) studied the local fluctuations of mass transfer with a small electrode set in the wall of a n insulating tube. Van Shaw and Hanratty obtained the same measurements by insulating the microcathode in the middle of a large guard cathode ( 9 ) . LeBouche and Cognet (70) treated the transfer problem for a circular microelectrode in a n infinite neutral plane. They evaluated up to Reynolds number of 100,000, a t which point the kinetics of the electrical phenomenon begin to intervene, and they thoroughly specified the precautions required to eliminate or take into account the parasitic effects. Application to study of fluidized beds. Apart from work in homogeneous liquid phases, other authors have studied the mass transfer between fluidizing liquid and an obstacle immersed in the bed using this method: Jottrand and Grunchard (II), Jagannadha Raju and Venkata Rao (IZ),and the authors ( 7 3 ) . The experimental technique of these authors consisted in fluidizing a bed (usually of ballotini) with the above-defined alkaline ferricyanide solution. A probe, the depth of which can be varied, is immersed in the bed and has two small nickel electrodes of some few square millimeters at its end. One of these is the
measuring cathode, the other is the reference electrode. The anode is a large sheet of nickel stuck to the wall so as not to disturb the fluidized bed. Figure 1 shows a n example of a polarization curve modified by the presence of a fluidized bed: For the same liquid flow rate, the limiting current is increased by the factor 4.3, for the case considered here using glass beads of diameter of 470 1.1 and a bed porosity of 0.60. This means, therefore, that the diffusional boundary layer is continuously torn and restored by the impact of the fluidized particles. Its average equivalent thickness is therefore reduced by the ratio 4.3 because of the presence of the fluidized bed. This increase of the mass transfer coefficient passes through a maximum as the flow rate of fluidizing liquid is increased. T h e existence of this maximum can easily be understood : At low flow rates the number of particles in a given volume is large but the kinetic energy of each grain is still small. O n the other hand, at large flow rates, the bed is greatly expanded, the number of particles in a given volume is small but the average velocity of each grain is large. I t is, therefore, predictable that the average kinetic energy of all the grains in unit of volume should exhibit a maximum. The authors have recently perfected a technique for measuring this value and directly checked the existence of this kinetic energy maximum. They were able to prove that it appears in the same conditions as the maximum of the mass transfer coefficient (36). Very recently, King and Smith (74) studied the mass transfer between the fluidizing liquid and the external wall of a fluidized bed in the same way. Finally, the intermediate case between the homogeneous liquid phase and the fluidized bed may be cited: The study by Krishna et al. of the mass transfer between two concentric cylinders separated by a fixed packing traveled by a liquid flow (15). Applications in Electrochemical Analysis
Polarography and its variant, amperometry, are wellknown analytical methods. They are based on the measurement of the limiting current with concentration polarization. But to obtain a good horizontal plateau of which the ordinate is proportional to concentration requires, among other restrictions, that the hydrodynamic conditions for the existence of the diffusional boundary layer are both well-defined and invariant with time. T h e mercury drop electrode is only an imperfect solution to this problem. Many improvements have been proposed with a view, first, to increase the measured current intensity, second, to make measurement conditions more reproducible and simple. Publications, whether fundamental or directed toward particular analytical applications, describe various devices which try to reduce and to steady the thickness of the diffusional boundary layer.
The rotating electrode has been used bv many authors (28),and the vibrating electrode also (2 ) Roffia and Vianello (76) quote some works which try to change the boundary layer and they propose a n electrode that is brutally and periodically displaced from one steady position to another. Ducret and Cornet (77) have substituted for the usual mechanical convection, a thermal convection obtained by creating a temperature gradient between a steady electrode and the initially motionless solution. T o our knowledge, no author has effected a polarographic analysis or used a similar method in a genuine fluidized bed. This technique would be applicable with difficulty since it entails the use of a large liquid volume for the pumping circuit. But one can conceive the realization in situ of a pseudofluidization of inert grains in a beaker containing the solution to be analyzed. Strafelda and Kozak (78) proposed to improve the reproducibility of amperometric measurements with rotating electrode by placing a suspension of corundum grains in the electrolytic cell which also ensured continual cleaning of the electrode. We have tried to use the two-electrode amperometric method (dead stop end point) for the determination of an N/100 iodine solution with arsenious ion in the presence of isolating grains held in suspension by a magnetic or a propeller stirrer. If the two-electrode probe is suitably placed in the beaker, the presence of grains increases the current by a factor of the order of two. But, in the case of such an easy determination, this does not obviously improve the accuracy of the titration. Moreover it is possible to obtain this result easily by increasing the stirring energy without resorting to this mechanically fluidized bed. It is nevertheless not excluded that other electrochemical determinations could profit by the current density increase allowed by the fluidized bed technique. Coulometry can be especially considered because of the necessity of making the Faraday yield equal to one which leads frequently to the use of very small current densities. I n this particular case the use of conducting particles to increase the active area of the electrode would probably be even more interesting. See the section on fluidized beds of conducting grains. Applications in Industrial Electrochemistry
Generalities. I n industrial electrochemical applications, the phenomena are often less clear and the polarization concentration is not the only factor which tends to limit the current density. The polarizations of all other natures, the necessity not to overstep a certain potential between the cell terminals with a view to avoiding the discharge of undesirable ions and to retaining a n acceptable Faraday VOL. 6 1
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yield, the not negligible resistivity of the electrolytes and the resulting Joule effect and, in the case of metals deposits, the problems of crystallogenesis are factors which contribute to the limitation of current densities. I t would then appear that in many industrial processes where the concentration is generally controlled to a large degree, the concentration polarization is a phenomenon of limited weight. This is true only if the ion itself that reacts on the electrode is present in a large amount a t the electrode. But very often, especially in galvanoplastic baths, the element to be deposited is bound 'for the greater part in a complex ion, often of opposite sign, and the concentration polarization will play a large part. At the border line, it will be always possible to say that this case occurs, since all the ions are hydrated (79). Moreover, other diffusional processes than those of the ions can also intervene. I n the case of metallic deposits, brightening agents are used in very small concentration, and their adsorption on the growing crystalline faces modifies considerably the crystallogenesis of the deposits (79). Therefore, though not generally intervening in ionic form, the brightening agent is consumed on the electrode (ZO), and the whole process can be limited by the transfer of this additive to the electrode. I n sucb a case, an increase in the turbulence will lead to the use of a smaller additive concentration in the bath for the same transfer to the electrode and therefore the same effect on the deposit. The economic problem. Can it be economically interesting to increase the current density of a n electrolysis by degrading simultaneously mechanical energy in the pump or mixer that increases the turbulence of the bath? The transfer coefficient, k , at an electrode (and therefore the electrical energy required by this electrolysis) increases as the exponent 0.55 of the fluid flow rate in laminar regime and as the power 0.8 of this rate in eddy regime (7-3). Moreover, the degraded mechanical energy is proportional to the square of the flow rate, in the laminar, and to the cube, in the turbulent regime. I n such conditions, the reduction of investments obtained by the increase of hourly production rate would be swiftly counterbalanced by the increase of the pumping cost and also the intricacy of the equipment; therefore in each case a cost balance should be justified. But this argument fails if a large increase of the transfer coefficient can be obtained with an artificial turbulence associated with a very small degradation of mechanical energy. Note that this is just the case with the beds of fluidized grains in contact with the electrodes. The mass transfer coefficient can be multiplied by a factor from 4 to 5 when the additional pressure drop, independent of the flow rate is just limited to the weight of the bed of fluidized particles. This economic optimization is not the only aspect to 12
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
I 500
fenrlan
vmy
-
Figure 2. Influence of theflow rate on the electrolysis of copper sulfate in a conventional cell
be considered. It can often be interesting to increase a current density, even if the energy consumption is greater, either with a view to decreasing the whole duration of a n operation (electroplating) or to decreasing the volume of an electrochemical cell (fuel cells). Only definite examples of application of fluidized beds in electrochemical operations are examined here, but it is obvious that many other examples can be a priori expected (37). Electrolysis of an acid solution of copper sulfate between copper electrodes (copper refining). This system is especially simple from an electrochemical point of view to the extent that it does not have counterelectromotive power and it is the background to the industry of electrochemical copper refining. The solution composition is as follows:
0.067M C U S O f ~ 0.067M HzS04
A strongly asymmetrical cell is used to measure the current limited by the diffusion around a single electrode alone. The large electrode is a large cylindrical sheet 20-cm high and 10-cm in diameter stuck to the wall. The small electrode is a cylindrical rod 0.5 cm in diameter and 4-cm high, therefore having 100 times less surface area than the other electrode in order to produce strong current densities. This electrode is immersed along the axis of the tube. I n a first series of experiments, the intensity-tension curves have been determined without a fluidized bed and for different flow rates of solution. For each flow rate, two curves have been plotted, after interchanging of the electrodes.
These curves (Figure 2) show that all the points lie on the same straight line which is independent of the flow rate when the small electrode is the anode; this result is in accord with the general theory of the passage of current through electrolytes ( 2 ) . O n the other hand, when the small electrode is the cathode, a limiting value IL of the current is swiftly reached. I n a second series of experiments, the column was filled by a fluidized bed of glass beads, 470 p in diameter (Figure 3). T h e presence of the fluidized bed of insulating particles (glass) has two effects which act in opposite directions: T h e mean conductivity of the interelectrode space decreases, since the volume fraction filled by the conducting liquid decreases. Thus, it is verified on the figure that the straight line obtained with the small electrode as anode has a slope half of that obtained in the empty cell when the porosity reaches 0.60. This value is in good agreement with the theory (27). The concentration overvoltage 7 remains negligible, even for the strongest current densities used. As in Figure 3, the characteristic obtained with the small electrode as cathode remains linear u p to more than 1100 mV. (We have not been able to detect a bending of the intensity and, a fortiori, we do not know the magnitude of the diffusion current, if it exists.) T o appraise the improvement brought about by the fluidized bed, we have plotted on Figure 3 two curves obtained in the empty cell-Le., without a fluidized bed. Let us compare experimental conditions represented by the three points A , B, C of this figure: The point A is the commencement of the plateau for motionless liquid in the empty cell The point B is the commencement of the plateau in the moving liquid in the empty cell The point Cis the practical extremity of the curve in a fluidized bed for the same flow rate corresponding to the plateau of the point B. The points B and C are roughly aligned with the origin, a fact which indicates the equality of the apparent resistivities of the both baths (the two effects previously pointed out counterbalance). O n the other hand, the current density and therefore the deposition rate a t C is four times greater than a t B and six times greater than at A . I t is, moreover, likely that the electrolysis rate could be extended beyond the point C, without advent of a limitation caused by ionic diffusion or by the release of hydrogen. These results obviously cannot be extrapolated directly to copper electrorefining because they were obtained in a cell in which we have voluntarily increased the weight of the concentration overvoltage by using a very small cathode. Nevertheless a recent paper has shown that the concentration overvoltage plays a not negligible part in copper refining in industrial cells (22).
lic(uid Ibwinq at u.O.O5Cny( in the m w v column
Ka
nnsbnv,,,.,
loo0
Figure 3. Influence of ajuidized bed of glass beads on the electrolysis of copper sulfate
Finally it should be noted that in our experiments the appearance of deposit obtained in fluidized beds is quite different from the conventional deposit. With the same current density, the deposit in the fluidized bed appears qualitatively finer and more adhesive than the conventional deposit. Optimal arrangement of an electrolytic cell with a fluidized bed of isolating grain. As we have shown in the previous section with the example of the electrolysis of the copper sulfate, insulating fluidized grains in a n electrolysis cell present a disadvantage as well as important advantages: The mean conductance of the interelectrode space decreases because the volume fraction e filled by the conducting liquid also decreases. A first approximation of this decrease is given by
as shown elsewhere (30). Practically, in the case of fluidized beds with a porosity of about 0.6 to 0.7 which gives the greater improvement in the mass transfer, the conductivity C of the electrolysis cell is equal to 50 to 60% of its value in the absence of the fluidized bed. But, in the case of a cell geometry assigned by other reasons, one can overcome this disadvantage by not filling the whole interelectrode space but only the small electrode zone that would be the seat of concentration polarization. T o illustrate this fact, the authors carried out measurements with the system Ni/Ni2+/Ni, but with the device shown in Figure 4. T h e cathode is set a few millimeters below the “surface” of the fluidized bed, while the anode is set in the flowing liquid above the bed. The mean conductivity is almost as great as without a fluidized bed (Figure 5 ) . VOL. 6 1
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Figure 4 . Electrolysis cell with fluidization around a single electrode
We can conclude that the mass transfer improvement a t a n electrode can be simply obtained by a thin layer of moving grains which lacerate and renew the diffusional boundary layer. We will not discuss here the technological applications of this device because they will differ widely with various electrochemical systems. Lead dioxide deposit on graphite anode. I n many electrochemical fabrications by anodic oxidation (chlorates from chlorides), the tendency is to replace the very oxidizable graphite or the very expensive platinum anodes by bulky lead dioxide or lead dioxide-coated 14
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
graphite anodes (23). We have carried out a study of the anodic deposition of lead dioxide from a solution of 350 g of lead nitrate and 6 g of copper nitrate by liter on cylindrical graphite anodes with hemispherical ends of 10-mm diameter and 17-mm length. An electrolysis without a fluidized bed with a constant current density, 2.71 A/dm2, requires 24 hours to deposit 33.4 g of P b 0 2 which is grey crackled, not adherent, and dotted with pits. Under the same conditions, but carrying out the electrolysis in a fluidized bed of glass beads with diameters between 430 and 500 microns, 33 g of P b 0 2 were deposited in a black, dense, smooth layer perfectly adhering to the graphite. For the case using the fluidized bed, the copper cathode remained clean. O n the other hand, in the absence of fluidized bed, it was covered by a deposit of lead and copper. The hardness of the deposits was also very different: 75 Rockwell B for the deposit in fluidized bed and 35 for the conventional deposit. Similar results have been obtained by Karasimham (24). The results in fluidized beds are even better than those obtained in a conventional cell using the program of flow rate and current densities recommended by Gibson (25). This spectacular and unexpected result shows that the fluidized bed does not perhaps only intervene by increasing the mass transfer; it is also possible that the continuous bombardment of the growing deposit by the particles of the bed can modify the formation and the growth of the crystalline seeds. I t could also prevent gaseous bubbles arising from a n electrochemical process or from the simple degassing of the solution from being maintained at the electrode surface. Electrolytical polishing of copper by anodic dissolution in phosphoric acid. The first work on this subject was published in 1967 by Thangappan and coworkers (26). The authors used similar conditions and obtained similar results. Summarizing, the limiting current corresponding to the normal conditions of polishing grows from 20 mA/cm2 in a n experiment with motionless electrolyte to 90 mA/cm2 in a typical experiment in fluidized bed (glass beads of 950 p and porosity of 0.80). The time saving that would result for a determined polishing operation, would be considerable. But it is yet increased by this following phenomenon: When the tension between the terminals is set to a convenient value, the current intensity does not immediately attain its limiting value corresponding to the steady regime. For instance, in our cell with the motionless electrolyte, this transition period is about 5 min; it has been reduced to one minute by the presence of the fluidized bed. We have noted, as did Thangappan and coworkers, that the polishing quality is never improved by the fluidization. O n the contrary, faults appear in some conditions. For instance, with glass beads, some beads
1
1
501
Figure 5. Injuence of asuidization around a single electrode
remain stuck to the anode during the passage of the current, forming an inadmissible pitting. This phenomenon can be reduced by the use of the grains of another insulating material but of a greater specific gravity than that of glass (alumina). I t has been wholly eliminated by the use of beads of a conducting and yet more heavy material (bronze). For instance, with bronze beads of 100 to 125 p, with a fluidized bed porosity equal to 0.70 corresponding to the maximal current density, a n excellent surface state is obtained whose quality is comparable to that obtained in a motionless electrolyte. I n conclusion, the use of a fluidized bed allows the division of the duration of an industrial electrolytic polishing by a factor of about 5 to 10. However, a thorough study of the best possible hydrodynamic conditions must be carried out in order to avoid particles remaining motionless in the very viscous boundary layer around the anode. Cathodic deposition of nickel with brightening agents. I n a fluidized bed of glass beads of 300 p, we have studied the nickel plating of polished samples of copper and brass in a Watts bath in which were added various amounts of a conventional brightening agent : the butyne diol 1-4. The quality of the surface of the deposit was defined by measuring its microrelief by interferential microscopy. Our results have been compared with those obtained with identical baths by Froment and Ostrowiecki (27), particularly on a rotating cathode. Figure 6, for a current density of 3 A/dm2 shows the variation of the microrelief of the deposit in Angstrom in terms of the brightening concentration for different modes of stirring of the solution used by Froment and the authors. It is seen from the curves that the use of a fluidized bed (curve 1) allows better results to be obtained com-
pared with the other modes of stirring tried for smaller concentrations in butyne diol. The results obtained in fluidized beds are comparable to those obtained with an electrode rotating at 1800 rpm but they have also the advantage of leading to a pos bl industrial application. Furthermore, the trials carried out in absence of butyne diol have shown that a t small current densities, the microrelief of the deposit was considerably decreased by the use of the fluidized bed. Instead, the microrelief of the conventional deposit is too great to be accessible to our method of measurement and for a current density of 1 A/dm2, the presence of the fluidized bed alone acts as a concentration of butyne diol of 0.45 mmol/l. The authors are not able here to specify the mode of action of the fluidized bed as in the case of the lead oxide deposit: Does it act only by favoring the transfer to the electrode of some reactive species, particularly the brightening agent itself? Do the impacts of the grains on the electrode have direct action on the crystallogenesis of the deposit? Fluidized beds of conducting grains. The presence in the interelectrode space of a fluidized bed of nonconducting grains has the drawback of reducing the conductance of cell by about 50%. This disadvantage can be avoided by fluidized grains of conducting mate-
r
0
I
,
,
I
I
I
,
I
I
AUTHORS P. L e Goff, F. Vergnes, F. Coeuret, and J . Bordet are staff members of the Centre de Cinetique Physique et Chimique, Centre National de la Recherche Scient$que, Route de Vanoeuvre, 54 Villers-Nancy, France VOL 61
NO. 1 0
OCTOBER 1969
15
Figure 7. Influence of afluidized bed of conducting beads
Figure 8. Schema of the current lines
rial. As an example, we repeated the electrolysis described previously, but substituted copper beads of the same diameter (400 p ) for the glass beads. Predictably, because the conductivity of the copper is greater than that of the electrolytic solution, the presence of the fluidized bed enhances the apparent conductance of the cell (Figure 7 ) . But one must also examine whether the conducting material of which the grains are made does not interfere with the electrochemical reactions taking place in the cell. These phenomena are very complex and deserve to be discussed in more detail. We shall consider two extreme cases : that of perfectly particulate fluidization and that of the very aggregative fluidization. (1) Let us assume a perfectly particulate fluidization: at every time and in every point of the fluidized bed, every particle is supposed to be isolated and wholly surrounded by liquid. There exist aggregates of short duration between two particles as they strike one another, but clusters of three or more particles are negligible in number. In these conditions, a line of electrical current going from one electrode to the other passes through a large number of conducting grains and intergranular spaces filled with the electrolyte (see the left schema of the Figure 8). With the system being symmetrical with respect to each grain, nothing should occur statistically from the point of view of the mass transfer at the interfaces. Let us assume, for example, that in the electrolysis of copper sulfate previously considered, copper is deposited on one hemisphere of a copper grain playing the part of the cathode, an equal amount of the other hemisphere playing the part of anode will be dissolved. Because the grains are in movement, each piece of the grain surface will play successively the part of anode and of cathode and no copper deposit will appear on the grains. (2) Let us consider now the other extreme casethat of a very aggregative fluidization. This case is typically represented by experiments of air fluidization of bronze or carbon grains described in previous papers
(37). I n this case, indeed only the grains are conductive. For an explanation of the large value of the conductance measured in these conditions it is not sufficient to use either transfer of charge by diffusional movement of the grains or to assume a series of microsparks between adjacent grains. The existence of unbroken bridges of contacting particles going from one electrode to another must be assumed. In other words, the fluidized bed can be considered as being formed by two phases: a dispersed phase formed of bubbles through which flows the whole of fluid exceeding that at the threshold of fluidization and a “pseudo continuous’’ phase, the mean porosity and the structure of which is close to that of the “prefluidized” bed-this phase in which almost all the grains are continuously in contact is electrically conductive (right schema, Figure 8). Therefore, filling an electrolytic cell without diaphragm with conducting grains in aggregative fluidization would result in a short circuit of the cell because of statistical formation of grain bridges. In conclusion, if it is decided to use grains of conducting material with a view to improving an electrochemical operation by fluidization, conditions giving particulate fluidization must be looked for (such as small specific gravity of the material and small diameters of the grains). A wise precaution would be to set the nonpolarizable electrode outside of the fluidized bed.
16
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
Use of Fluidized Beds as Electrodes
Consider again the case of a bed of conducting grains in aggregative fluidization in which is immersed a cathode, with the anode in liquid above the fluidized bed (Figure 4) or on the other side of a diaphragm. Shce at every moment, all the grains are in contact, we can consider that the cathode is constituted by the whole bed of grains. Such an electrode formed by a fluidized bed appears to present many advantages, especially for fuel cells as shown recently by Fleischmann (32). For these electrochemical generators, two fluidized beds are used by prefcrence, respectively, for the cathodic and anodic
chambers, separated by a porous membrane. Therefore each of the electrodes present the following principal advantages : (1) T h e contact area is very large (100 to 1.000 cm2/cm3) (2) The fluxes of mass and heat transfer between fluid and electrode are very large (3) The electrodes can be regenerated in a continuous manner (4) The system is very versatile and allows control by many variables Fleischmann has shown that the presence of fluidized bed allows a n increase in the current density by a factor of about 10 to 1000. Fluidized particles can also be used advantageously as electrodes in many other processes, especially in chemical synthesis as oxidations and reductions, calling for small current densities such as the reduction of nitrocompounds. These processes are at the present time in the course of industrial development (33). Lastly, a n analogous process, the “Turbojet” (34) recently has been proposed for making galvanoplastic deposits on small brittle pieces. Considered as an improvement on the classical barrel process, the method consists of a local fluidization of a stacking of the small pieces. Conclusion
I t appears from this analysis of recent work, that the association of fluidization and electrolysis can lead to useful effects because of the following factors : I n a large continuous cell, the fluidized bed will act as stirrer and will increase the homogeneity of the electrolyte, and concentration gradients will be reduced. If concentration polarization is a restrictive phenomenon, the fluidized bed allows a considerable increase in the current density. I n a sense, it constitutes a means of control of the diffusional processes that allows the industrial scale-up of a process for which the study in the laboratory with the rotating electrode of Levich has shown a diffusional limitation. This will occur, though it is obviously impossible to assert that a vertical surface immersed in a fluidized bed is as uniformly accessible from the standpoint of diffusion as that of the rotating disk electrode. The thickness of the boundary layer does not depend on a single hydrodynamic parameter, as for the rate of rotation of Levich electrode. Therefore the Levich electrode will remain a n irreplaceable study method in the laboratory, but the fluidized bed could lead to the spreading of some of its advantages to industrial processes. One example is the improvement of the nickel deposition by increasing the diffusion of the brightening agent, a study we carried out following the work of Froment with the rotating electrode (27).
The fluidized bed can modify in an interesting manner the texture of electrolytic deposits, but it is not yet possible to provide a rational explanation of this phenomenon. The mechanism of action of the fluidized bed will obviously vary with the nature of the material to be deposited. The use in diaphragm cells of one or both electrodes constituted by a dense fluidized bed of conducting grains seems to allow, thanks to corresponding increase of surface, a miniaturization of the electrolytic plants especially those used for the electrochemical reduction of organic compounds. This ensemble of characteristics, some of which are not yet fully explored, lead us to forecast a bright future for those processes able to use fluidized beds in electrochemistry. BIBLIOGRAPHY
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