Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B15/00—Operating or servicing cells
  • C25B15/08—Supplying or removing reactants or electrolytes; Regeneration of electrolytes
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B9/00—Cells or assemblies of cells; Constructional parts of cells; Assemblies of constructional parts, e.g. electrode-diaphragm assemblies; Process-related cell features
  • C25B9/17—Cells comprising dimensionally-stable non-movable electrodes; Assemblies of constructional parts thereof
  • C25B9/19—Cells comprising dimensionally-stable non-movable electrodes; Assemblies of constructional parts thereof with diaphragms

Definitions

• electrochemical cells such as for example chlor-alkali electrolysis for obtaining chlorine gas and caustic soda or potash, water electrolysis mainly for obtaining hydrogen, the electrolysis of salts for obtaining the corresponding bases and acids, for example caustic soda and sulphuric acid from sodium sulphate, metal deposition, mainly copper and zinc.
• the physiological problem affecting all these processes is the energy consumption, which usually constitutes a substantial portion of the overall production cost. As the cost of electric energy shows in all geograophical areas a constant trend towards increase, the importance of decreasing the energy consumption in the above mentioned electrochemical processes is clear.
• the energy consumption in an electrochemical process primarily depends on the cell voltage: it is therefore soon evident the reason for the efforts directed to the improvement of the cell design, with the use of more catalytic electrodes and with the decrease of the ohmic drops in the cell structure itself and in the electrolytes, for example by decreasing the interelectrodic gap.
• a sodium chloride solution less frequently potassium chloride, is fed to a cell containing an anode, wherein chlorine gas is evolved, while at the cathode hydrogen is evolved with the concurrent of sodium hydroxide (caustic soda - potassium hydroxide when the cell is fed with potassium chloride).
• the caustic soda close to the cathode is maintained separate from the sodium chloride solution present in the anodic area by a membrane made of a perfluorinated polymer containing anionic groups, for example sulphonic or carboxylic groups.
• Said membranes are commercialized by various Companies, such as for example DuPont/USA, Asahi Glass and Asahi Chemical/Japan.
• a chlor-alkali electrolysis cell comprising a cathode fed with a gas containing oxygen has an energy consumption which is physiologically remarkably lower that that typical of the conventional technology.
• the reason for this result is essentially of thermodynamic nature, as the two cells, the conventional one and the one containing the oxygen cathode, are characterized by two different overall reactions:
• the oxygen cathode is substantially made of a porous support, preferably conductive, having applied thereto a microporous layer made of an assembly of electrocalytic particles mechanically stabilized by a binder resistant to the operating conditions.
• This layer may comprise a further film also incorporating preferably conductive but not electrocatalytic particles, and a binder.
• the porous layer may consist of a mesh, a variously perforated sheet, carbon/graphite cloth, carbon/graphite paper or sinterized materials.
• an electrode as the above described is used as an oxygen consuming cathode for chlor-alkali electrolysis, in a parallel position with respect to the cationic membrane, in direct contact or at a limited distance therefrom, indicatively 2-3 mm, the caustic soda produced by the reaction of oxygen onto the electrocalytic particles must be some in some way discharged to avoid a progressive filling of the micro-porosity of the layer. In fact should a complete filling occur, oxygen could not diffuse through the pores to reach the catalytic particles, where the reaction takes place.
• the discharge of the produced caustic soda may be obtained essentially in two ways, either towards the membrane when the cathode is parallel and at a certain distance from the cationic membrane, or towards the oxygen atmosphere, on the opposite side with respect to the one facing the membrane, in the case the cathode is in contact with the membrane.
• a liquid film 2-3 mm thick as already said, is normally kept in circulation upwards (cells are equipped with vertically disposed electrodes) both to withdraw the produced caustic from the cell as well to remove the heat produced by the reaction, and finally to control the caustic soda concentration within predetermined limits which permit to prolong the ion exchange membrane lifetime.
• This situation creates a pressure gradient between caustic soda and oxygen at the two sides of the cathode, which actually acts as a separation wall.
• This gradient may be positive (pressure of the caustic soda higher than that of oxygen) and in this case it increases from top to bottom due to the hydraulic head.
• the gradient may be negative (pressure of oxygen higher than that of caustic soda) and in this case it decreases from top to bottom once again due to the hydraulic head of caustic soda.
• the cathode With higher heights the cathode is either completely flooded with the total filling of the porosity by caustic soda with positive differential or completely gas filled with a heavy loss of oxygen in the caustic soda in the case of a negative differential.
• This fact is dramatically negative in the case of plants of a certain size, as the overall number of cell, each one of small dimensions, should be extremely high with remarkable additional costs for the ancillary equipment (electrical connections, piping, pumps). It must be taken into account that industrial cells of the conventional type, that is equipped with hydrogen evolving cathodes, have normally heights in the range of 1-1.5 meters.
• the cathode structure be provided with two kinds of pores, respectively hydrophobic, available for oxygen diffusion, and hydrophilic, directed to facilitate the caustic soda flow.
• the caustic soda released on the oxygen atmosphere side has a strong tendency to flood the back wall forming a continuous film which again hinder the oxygen diffusion.
• the back side of the cathode be strongly hydrophobic, which fact can decrease the electrical conductivity of the surface with the consequent complications for the electrical contact necessary to feed electric current.
• the concentration of the produced caustic soda is necessarily the one generated by the reaction and no control is possible within predetermined limits, as conversely happens in the first case of oxygen cathode where forced circulation is applied.
• the concentration value of the produced caustic soda is around 37-45% depending on the quantity of water transported through the membrane, a quantity which depends on the type of membrane and on the operating conditions of current density, temperature and concentration of the alkali chloride solution.
• the ion exchange membranes available on the market are irreversibly damaged when in contact, even for relatively short times, with caustic soda at a concentration above 35%.
• the invention intends to disclose the design of an electrolysis cell containing a liquid electrolyte and at least a gas diffusion electrode with a surface in contact with the liquid electrolyte and the opposite surface connected with at least two gas chambers, wherein said chambers are provided with devices characterized by dynamic pressure drop, such as for example rotameters, and the connection between two subsequent chambers is provided by said devices.
• the design is characterized by the fact that said devices maintain a constant pressure in said chambers even under strong variations of the gas flow.
• each gas chamber be supplied with at least a discharge element for the accumulated liquid. Said element, exploiting the pressure differential between two subsequent chambers, permits the flow of the liquid phase from a chamber to the subsequent one until it is discharged in the liquid electrolyte flow leaving the cell.
• FIG. 2 Front view cross section on the membrane side of the cathodic compartment illustrating the supporting frame of the gas diffusion cathode.
• Fig. 3 Front view cross section of the external side (opposite to the membrane) of the cathodic compartment of the cell of Fig. 1 illustrating the gas chambers provided with the dynamic fall devices and the condensate phase discharge elements.
• Fig. 5 Pressure profile of the gas chambers compared with the one generated by the hydraulic head of caustic soda and difference between the caustic soda and the gas pressure as a function of the height of the cell.
• Fig. 6 Condensate phase discharge element between a gas chamber and the adjacent one positioned on top.
• FIG. 1 A preferred embodiment of the device of the invention is illustrated in Fig. 1 , which schematizes a side view cross-section of an industrial cell incorporating at least one gas diffusion electrode.
• a multiplicity of cells as that of Fig. 1 are assembled to form an assembly known as electrolyzer, normally according to a preferred configuration called filter-press.
• electrolyzer normally according to a preferred configuration called filter-press.
• reference will be made to the chlor-alkali electrolysis with air diffusion cathodes, and more particular oxygen.
• this is not to be considered as a limitation of the invention as several other applications may be easily foreseen, for example for hydrochloric acid electrolysis, sodium sulphate and the electrometallurgy field.
• Fig. 1 the main components of the electrolysis cell are identified by numbers, as hereinafter indicated:
• the anodic compartment 3 contains an anode 4 made of a perforated sheet, or a mesh of interwoven wires, or expanded metal, fixed to the shell 1 by conductive supports 5.
• the construction material of the shell 1, of the supports 5 and of the anode 3 is titanium. Further, as known in the art, le anode 3 surface is provided with an electrically conductive and catalytic film for chlorine evolution.
• the anode shell 1 is further provided with nozzles 31 for feeding the sodium chloride solution and 32 for withdrawing the produced chlorine and the diluted solution.
• Numeral 6 identifies the cathodic shell which together with the ion exchange membrane 2 defines the cathodic compartment 7.
• the peripheral gaskets 8, made in chemically resistant elastomeric material, provide for the hydraulic and pneumatic sealing between the anodic shell 1 , the ion exchange membrane 2 and the cathodic shell 6.
• the caustic soda concentration which permits to obtain optimum performances is usually maintained between 30 and 35 % by weight.
• the gas (oxygen in the present case) diffusion cathode is fixed onto a frame, identified by 11 in Fig. 2, by various methods, for example by a purely mechanic method with screws or metallurgically by welding, preferably laser welding.
• This connection performs two functions: electric current transmission to the gas diffusion cathode and sealing between caustic soda and oxygen, to avoid either the caustic soda penetration into the back side of the cathode compartment, filled with oxygen, flooding the same, or, conversely, oxygen bubbling in caustic soda altering the flow uniformity and hindering the electric current passage.
• gaskets not shown in the figure, may be inserted between the oxygen diffusion cathode and the frame 11.
• the frame 11 is fixed to the cathode shell 6 by means of conductive supports 12.
• These conductive supports are made of strips of sheets fixed, for example by linear welding, preferably of the laser type, to the cathode shell 6 and to the frame 11 along their whole perimeter.
• the assembly cathodic shell 6 - supports 12 - frame 11 - diffusion cathode 9 defines chambers 13 insulated one from another, three of them in the specific case of Fig. 1. Obviously a different number of chambers may be obtained in practice.
• Each chamber is equipped with at least a dynamic pressure drop device 14 and at least one discharge element 15 for the accumulated liquid, according to the present invention.
• each chamber the liquid may accumulate with time as a result of both moderate electrolyte percolation (caustic soda in the present case) through micro-defects of the diffusion cathode and condensation of at least part of the water vapor contained in oxygen.
• the residual amount of oxygen crosses then the first device 14 which permits to establish a pressure difference between the first chamber just crossed and the subsequent one.
• the subsequent chamber the above illustrated situation is repeated: longitudinal flow, partial reaction at the diffusion cathode, crossing of the second device. The same situation occurs in the subsequent chambers.
• the residual oxygen is then discharged through nozzle 17.
• the value of the pressure differential is defined by the weight of the float, which in turn may be regulated by adding suitable elements or by adjusting the geometrical dimensions and/or the density of the material used for the construction. Indications on both the theory on which the operation of the rotameters is based and on the type of possible designs are given in the manual "Perry's Chemical Engineers' Handbook, 7.th Edition, McGraw - Hill" at pages 10 -18 ff.
• the purpose of dividing the space occupied by oxygen into a certain number of chambers is to vary, even if in a discontinuous manner, the oxygen pressure in the vertical direction, that is in the same direction wherein also the pressure exerted by the caustic soda head varies. More particularly, the purpose of the design is to maintain a low value of pressure differential between caustic soda and oxygen in every point across the wall represented by the porous film which constitutes the gas diffusion cathode.
• the reason for maintaining this difference at a low value derives from the need to minimize the mechanical stress onto the diffusion cathode to avoid deformation and tearing and to prevent percolation on caustic soda on the oxygen side and oxygen bubbling in the gap 10 between the ion exchange membrane and the gas diffusion cathode.
• the gas diffusion cathodes of the prior art do not tolerate pressure differentials above more or less 30 - 40 cm of water column. If, without introducing limitations to the invention, the height of each chamber is 30 cm and if the rotameter positioned out the outlet of each chamber provides for a gas pressurization equal to 30 cm of water column or slightly more, then the pressure difference between caustic soda and gas results approximately nil at the bottom of the chamber and equal to 30 cm or slightly more in the upper part of the chamber, therefore certainly within the tolerance limits for the diffusion cathode. This situation characterizes the first upper chamber on the hypothesis that that the discharge pressure of caustic soda and oxygen be the same.
• the cathodic shell 6 is provided with caustic soda feeding nozzles 18 and discharge nozzles 19, each one connected to an internal perforated pipe, 20 and 21 respectively, whose purpose is ensuring a homogeneous distribution.
• Caustic soda penetrates inside the gap 10 between the membrane and the oxygen diffusion cathode through an opening 22 obtained in the lower portion of frame 11 and leaves the gap through a further opening 23 made in the upper portion of frame 11.
• the gap 10 may be empty or filled with a spacer (not shown in Fig. 1), for example a large size mesh or other structure, preferably elastic, for example a mattress of interwoven wires.
• a spacer for example a large size mesh or other structure, preferably elastic, for example a mattress of interwoven wires.
• the construction design of the oxygen chambers illustrated in Fig. 1 obviously is not the only one applicable.
• the chambers may be prefabricated as independent boxes and provided with a peripheral flat flange to permit fixing by welding, preferably laser wlding, to the frame 11.
• the assembly frame - boxes is then fixed inside the cathodic shell 6 to the supports 12 already described.
• the bottoms of the boxes may be directly welded to the wall of the cathodic shell, and in this case the supports 12 are no more necessary.
• the periphery of the frame 11 is extended to form a flat flange.
• the independent boxes are fixed as previously mentioned.
• the assembly thus obtained, provided with the necessary above illustrated nozzles, may be directly used as cathodic shell.
• the construction material of the cathodic shell 6, the supports 12, the frame 11 , the rotameters 14 and, in one of the possible construction alternatives, of the boxes for housing the oxygen chambers is nickel.
• Some of these parts may be silver-plated, to ensure a further reduced release of nickel, for example frame 11 , and the mesh which may be applied to frame 11 to maintain the best current distribution to the oxygen diffusion cathode.
• An aspect of the present invention is that the oxygen chambers are capable of release the liquid phases which are collected on the bottom by the accumulation of liquid due to both small leaks of caustic soda through microdefects of the cathode and the at least partial condensation of the water vapor contained in the oxygen.
• the discharge element of the liquid phase identified by 15 in Figures 1 , 3 and 6, is made of a pipe 24 fixed to the ceiling 25 of the chamber with the upper end 26 practically in contact with the ceiling itself and the lower end 27 slightly spaced from the bottom 28 of the chamber itself.
• the empty pipe 24 represents a path for oxygen in parallel with rotameter 14.
• the gas flow rate through pipe 24 represent a minor portion of the overall gas flow.
• the overall gas flow of the oxygen feed is about 3 nrVhour and is reduced indicatively to 1 m 3 /hour at the outlet.
• the gas flow through the pipe 24 be not more than 10 % of the flow-rate in the rotameter 14, a value of 0.1 - 0.3 rrrVhour is obtained.
• pipe 24 must have a very small diameter, in any case not above 1 mm, with the possible risk of stoppage due to the micropowders released by the material of construction of the gas diffusion cathode.
• An alternative and more reliable embodiment is based on the use of a pipe 24 having a substantially larger diameter containing at the inside a hydrophilic porous, chemically resistant material, such as for example a fiber pressed material 30, such as zirconium oxide fibers.
• a hydrophilic porous, chemically resistant material such as for example a fiber pressed material 30, such as zirconium oxide fibers.
• This material is preferably saturated with water during assembly and provides for an efficient barrier toward the gas passage.
• the hydrophilic nature of the filling of pipe 24 facilitates the absorption of the liquid phase separated onto the bottom of each cell, in particular when the hydrophilic material is in contact with the bottom of the chamber.
• element 15 may consist of a short piece of pipe 24, containing the hydrophilic fibers 30 which are prolonged nearly till the bottom 28 of each chamber, for example reaching a distance of some millimeters from the bottom, or lying on the bottom itself. In this case the liquid rises along the fibers by capillarity up to the piece of pipe 24 where the bundle of fibers is pressed and due to the pressure difference, it is transferred above.
• Another embodiment is represented by the use of sticks of porous ceramic material, for example sinterized zirconium oxide, fixed through a suitable collar to a hole in the ceiling 25 of each chamber.
• porous ceramic material for example sinterized zirconium oxide

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• 2001
  • 2001-02-23 IT IT2001MI000362A patent/ITMI20010362A1/it unknown
• 2002
  • 2002-02-22 EP EP02701293A patent/EP1362133B1/fr not_active Expired - Lifetime
  • 2002-02-22 ES ES02701293T patent/ES2370387T3/es not_active Expired - Lifetime
  • 2002-02-22 WO PCT/EP2002/001910 patent/WO2002068720A1/fr not_active Application Discontinuation
  • 2002-02-22 AT AT02701293T patent/ATE518022T1/de active

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