FLUIDIZED-BED ELECTRODES AND RELATED APPARATUS AND METHODS

Abstract: The current-carrying capacity of a fluidized-bed electrode is enhanced by imparting increased velocities to the particles (3) suspended therein while maintaining a relatively low degree of bed expansion or voidage. This is accomplished in one embodiment of the invention (Fig. 2) by means of pairs of opposing jets of electrolyte impinging against each other so as to effect a highly turbulent motion of the fluidized particles while the net flow velocity of the supporting electrolyte is kept relatively low. In a second preferred embodiment (Figs 3 and 4), the electrolyte flow velocity may be as high as desired, but its direction preferably horizontal, is reversed at frequent intervals. The suspended particules get thereby intermittently packed against and retained by filters (10, 11) at the alternating outlet walls (6, 7). The alternating packing and expansion result in improved charge and mass transport, and hence in improved electrode performance. The above improvement is especially applicable to fluidized beds of activated carbon particles and of other materials whose specific gravity is not much higher than that of the supporting electrolyte.

Description: F3_UIDIZ--D-BED ELECTRODES D RELATED APPARATUS AND MEIHCDS --AQ OUND OF THE INVΣHTIQN This invention relates to fluidized-bed electrodes, especially air-depolarized cathodes, and to related apparatus and methods. This is a (Xintinuation-in-part of my co-pending applicaτicn Se- rial Number 813,"+83, filed July 7, 1977 (International Application No. PCT/US78/00030, filed 03 July 1978) which is incorporated herein by refer¬ ence. In said application, I have disclosed improved air-depolarized flu¬ idized-bed electrodes for use in various types of electrochemical process¬ es and apparatus, especially in power sources. One serious limitation of the fluidized-bed electrodes disclosed hereτofore is that the upward flow of the supporting electrolyte through the bed can not exceed a certain optimum value beyond which tne current- carrying capacity of the electrode decreases. This optimum flow corres¬ ponds to rather low bed expansions, usually about 10% or less. Higher bed expansions result in reduced interparticle contacts, and hence in re¬ duced charge transfer. To maintain the expansion sufficiently low and yet permit the electrolyte flow rate to be sufficiently high for adequate mass transDort, the specific gravity of the fluidized particles should prefer- 3 ably exceed that of the supporting electrolyte by at least 2 gm/cm . This liπ tation would preclude the use of activated carbon and of other relative¬ ly light materials in fluidized-bed electrodes. Yet activated carbon has several most desirable features, including a high active surface area per u- nit weight, a high catalytic activity, and low cost, which make it an es¬ pecially outstanding candidate material for the fluidized particles in air cathodes. It is an object of iψ invention to overcome the afore-outlined lim¬ itations of present fluidized-bed electrodes, and thereby increase their CT- rent-carrying capacity. It is also an object of rπy invention to permit the use of activated carton as the chief component of the fluidized-bed electro- des, especially air-depolarized cathodes. SUMMARY OF THE INVENTION Briefly, my invention consists of imparting the desired hijh velo- ( _ ?MPI cities to the particles suspended in the fluidized-bed electrode while maintaining the bed expansion cr voidage optimally low. One way of a- chieving this is by causing opposing horizontal high-velocity inlet jets to impinge against each other, thereby causing their kinetic energy to be dissipated nto turbulent motions while the net vertical flew velocity of the supporting electrolyte is kept relatively low. ' An alternative way is to use a high unidirectional flow at any given time, but to reverse tne flow direction at frequent intervals. In tne latter case, the alternating inlets and outlets may extend over most of τhe length of the bed, so as to impart a substantially uniform average particle motion throughout the bed, and should comprise suitable filters to retain the fluidized particles with¬ in the electrode compartment. The rapid alternating flew causes intermit¬ tent partial packing of particles against the alternating outlets, and a rapid to and fro motion of the particles between the inlet and outlet sides. This yields frequent interparticle contacts and hence a high rate of mass and charge transfer. Since the particle velocities and interparticle contacts imparted by either of these types of flew do not determine the vertical bed expansion or the overall bed voidage, it thereby becomes practical to use activated car- bon and other relatively light materials as fluidized electrode catalysts. Tne high area/weight ratio of activated carbon is especially useful for ef¬ fecting adequate mass transport of oxygen in the fluidized-bed air cathode systems disclosed in my afore-cited co-pending application. BRIEF DESCRIPTION OF THE DRAWING My invention may best be understood with the aid of the drawing, in which: Fig. 1 is a partial schematic magnified cross-sectional view of an electrochemical cell comprising a fluidized-bed electrode; Fig. 2 is a partial schematic view of section S-S of Fig. 1 accord- ing to one embodiment of my invention; Fig. 3 is a partial schematic view of section S-S of Fig. 1 accord¬ ing to an alternative embodiment of my invention; and Fig. . is a schematic diagram of a pump-and-valve system controlling the flow directions of Fig. 3. DESCRIPTION OF THE PREFERRED EMBODIMENTS In Fig. 1 is shown an electrochemical cell 18 similar to those described in my afore-cited co-pending application. A single cαnpart- ment 28 comprises particles 3, preferably of activated carbon, in a flcw- ing electrolyte 16 contained between an outer air-permeable electrolyte- iEπper eable membrane 23 and an inner current-collecting grid 17. Near membrane 23, the surfaces of particles 3 become enriched with oxygen per¬ meating 1-hrough said membrane. This oxygen is electroreducεd as the particles approach grid.17. Electrolyte gap 19-, counter-electrode 20, insulating spacers 21, and end gaskets 22 substantially complete the electrochemical cell. As in πς. afore-cited co-pending application, electrode 20 nay be an anode consuming a hydrogen-rich fuel, a consumable metal anode forming part of a metal-air power source, or, in conjunction with a suitable diaphragm (not shewn) , an anode for the electro-oxidation of chloride ions in the r_-__nufacture of chlorine. " According to one e_--oodi_τ_ant of my invention, the view of section S-S of Fig. 1, perpendicular thereto, would appear as indicated in Fig. 2. Jets of electrolyte, entering through pairs of opposite entrance nozzles 4 and 5 (situated at the eadwalls 6 and 7 of co-roartment 28) ' along the directions indicated by the horizontal arrows 1, 2, impinge against each other, and their kinetic energy is thereby dissipated into a swirling mo¬ tion, of which only a minor component contributes to an upward flow. The bed expansion and voidage can therefore be kept at an optimal low value while the fluidized particles maintain the rapid motions required for high rates of mass and charge transfer. According to an alternative embodiment of my invention, the view of section S-S of Fig. 1, perpendicular thereto, would appear as indicated in Fig. 3. Here the end walls 6 and 7 of corrmartmsnt 28 comprise vertical e- lectrolyte channels 8 and 9 separated from compartment 28 by filters 10 and 11. The latter also serve as flow distributors. During the first portion of a cycle, the electrolyte flews from channel 8 through filter 10 into compartment 28, and thence through filter 11 into channel 9, as in¬ dicated by arrows 12, 13. 3 the second portion of the cycle, the flew is reversed, as indicated by-the arrows 14, 15. Although -the flow directions 12, 13 and 14, 15 are horizontal in Fig. 3, it is also possible to use a configuration in which the entire Fig. 3 is turned around by 90° so as to yield an approximate¬ ly vertical reciprocating upward and downward flow. The reversal of flow directions may be effected by a special reciprocating pump (not shown) or by a unidirectional pump 24 acting in conjunction with an electronically programmed solenoid valve 25, as shown in Fig. 4. In the first portion of a cycle, valve 25 keeps the solid lines 26 and 27 open and the dotted lines 29 and 30 closed, thereby causing the flow through compartment 28 to be from right to left. In the second portion of the cycle, the links 26 and 27 are shut while lines 29 and 30 are opened, whereby the flow through com¬ partment 28 is reversed. Although the embodiments described herein are concerned prima- rily with fluidized air-depolarized cathodes, the improvements dis¬ closed herein are obviously applicable to numerous other types of e- lectrochemical reactors utilizing fluidized-bed electrodes, as is well known in the art. There will new be obvious to those skilled in the art many mo- difications and variations of the above-disclosed embodiments, which, however, will fall within the scope of my invention if defined by the following

Claims: CIAIMS : 1. Apparatus cαπprising a fluidized-bed electrode, and means for in¬ creasing the kinetic energy of the fluidized particles in said elec¬ trode without seriously affecting the average bed expansion or voidage. 2. The apparatus of claim 1 wherein said means comprises pairs of op¬ posing jets of fluid impinging against each other and thereby impart¬ ing swirling motions to the particles of the fluidized bed. 3. The apparatus of claim 1 wherein said means comprises a reciproca¬ ting flow system causing electrolyte to flow through said electrode in a to and fro motion with reversals in the direction of electrolyte flow occurring at frequent intervals in repeating cycles. 4. Apparatus of claim 1 wherein said fluidized particles comprise ac¬ tivated carbon. 5. Apparatus as claimed in claim 1, wherein aid fluidized-bed elec¬ trode is an air-depolarized cathode. 6. A method of increasing the curirent-carrying capacity of a fluidized- bed electrode which comprises increasing the kinetic energy of the flu¬ idized particles in said electrode without seriously affecting the av¬ erage bed expansion or bed voidage. 7. The method of claim S wherein said kinetic energy is increased by causing opposing jets of fluid to impinge against " each other so as to impart swirling motions to the particles of the fluidized bed. 8. The method of claim 6 wherein said kinetic energy is increased by effecting a reciprocating flew of electrolyte "through said electrode, with the direction of said flow reversing at frequent intervals in re¬ peating cycles. 9. The method of claim 6 wherein said fluidized particles comprise activated carbon. 10. The method of claim 6 wherein said electrode is an air-depolarized cathode. . OMPI

Inventors: ZAROMB S
Applicants: ZAROMB S
Application year: 1978
Main classification: H01M8
IPC codes: C25B11, H01M8