EP1362133A1

EP1362133A1 - Electrolysis cell with gas diffusion electrode operating at controlled pressure

Info

Publication number: EP1362133A1
Application number: EP02701293A
Authority: EP
Inventors: Giuseppe Faita; Fulvio Federico
Original assignee: Uhdenora Technologies SRL
Current assignee: ThyssenKrupp Uhde Chlorine Engineers Italia SRL
Priority date: 2001-02-23
Filing date: 2002-02-22
Publication date: 2003-11-19
Anticipated expiration: 2022-02-22
Also published as: ES2370387T3; ATE518022T1; ITMI20010362A1; WO2002068720A1; EP1362133B1

Abstract

Electrolysis cell containing a liquid electrolyte and at least a gas diffusion electrode (9) with a surface in contact with the liquid electrolyte (10) and the opposite surface connected to at least two gas chambers (13), wherein said chambers are provided with rotameters (14) or equivalent devices and the connection between two subsequent chambers is provided by said rotameters or equivalent devices. Said design is characterized by the fact that it maintains a constant pressure in the chambers also with strong variations of the gas flow-rate. The chambers are further provided with at least an element (15) for discharging the liquid accumulated therein, which may comprise a bundle of hydrophilic fibers.

Description

ELECTROLYSIS CELL WITH GAS DIFFUSION ELECTRODE OPERATING AT

CONTROLLED PRESSURE DESCRIPTION OF THE INVENTION

Several industrial processes are carried out in 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.

In the following description reference will be made to the chlor-alkali electrolysis process which maintains an undoubted industrial importance, but is is understood that everything discussed as the status of the prior art and improvement of the same according to the teachings of the present invention applies in any case also to the other electrochemical processes.

In the case of conventional chlor-alkali electrolysis, 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). In the most advanced type of cell, 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. The design of this type of cell has been exhaustively studied and it can be said the technology is today at the optimum stage as concerns energy consumption. An example of this kind of design is illustrated in the international patent application WO 98/55670. An analysis of the production cost of chlorine and caustic soda obtained with these advanced types of cells however indicates that the impact of energy consumption is still remarkable. This consideration has produced a series of proposals for further improvements, the common element of which is the use of a gas electrode, specifically a cathode fed with oxygen (as such or as enriched air, or simply air free from carbon dioxide) in place of the hydrogen evolving cathode used in the previously discussed technology. 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:

- Conventional cell

2NaCI + 2H₂0 → 2NaOH + Cl₂ + H₂

- Oxygen cathode cell 2NaCI + H₂0 + 0₂ -> 2NaOH + Cl₂

In practice it is observed that a conventional cation exchange membrane cell operating at a current density of 4kA/m², has a cell voltage of about 3 Volt, while a cell equipped with a cation exchange membrane and an oxygen cathode, operating at the same conditions, has a voltage of about 2-2.2 Volt. As it can be seen, the energy consumption thus achieved is in the range of 30% (the production lack of hydrogen, which is usually used as a fuel, has a secondary importance). However so far no industrial application of electrolysis cells incorporating oxygen cathodes is found. The reason for this situation resides in the structure of the oxygen cathode and in the requirements imposed by the operating conditions to ensure a good efficiency of the cathode. 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. By suitably selecting the dimensions of the particles and the chemical nature of the binder, the hydrophobicity/hydrophilicity of the cathode may be suitably controlled. The porous layer may consist of a mesh, a variously perforated sheet, carbon/graphite cloth, carbon/graphite paper or sinterized materials. An electrode of this type, with the relevant production method, is described in US patent No. 4,614,575. When 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. In the first instance, 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. Conversely, 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. With the materials available today and the known production methods, it is possible to obtain cathodes capable of resisting to pressure differential in the range of 30 cm (intended as water column). As a consequence, for the optimum operation of the oxygen cathodes, the cells suitable for housing the same cannot have a height above 30 cm. 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. To overcome the above described inconvenience, it is known resorting to a structure wherein the cathode is kept 2-3 mm spaced from the membrane, the global height is still 1- 1.5 meters but the cell is divided in a number of sub-units, each one having a height of about 30 cm. This design involves a remarkable complexity for the connection pipes among the various sub-units, and on the whole a complexity in the operation and a cost not acceptable for industrial applications. A further structure is described in US Patent No. 5,693,202. The design foresees a unitary structure for the cell which is equipped with oxygen cathodes cut into strips. The pressure of the oxygen which is fed to each strip is automatically controlled by exploiting the hydraulic head of caustic soda by a bubbling system.

A further embodiment of the same concept is disclosed in international publication WO 98/21384.

In the second case of operation, that is with the oxygen cathode in direct contact with the membrane, as for example in US 4,578,159, the only possibility of discharging caustic soda is towards the oxygen atmosphere on the cathode side opposite to the one in contact with the membrane. In this case a series of problems arise, as hereinafter illustrated:

- Caustic soda forced to flow through the cathode tends to fill the porosity, hindering the oxygen diffusion. To avoid this inconvenience it is necessary that 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. Moreover, to further facilitate the caustic soda release and to minimize the risk of total occlusion of the porosity, it has been proposed to cut the cathodes into strips with a porous element interposed between the membrane and the cathode strips, along which part of the produced caustic soda may be released.

- 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. To prevent this negative effect, it is necessary that 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%. Therefore, it has been suggested to operate the cell equipped with an oxygen cathode in direct contact with the membrane with diluted solutions of alkali chloride, as it is known that the quantity of transported water increases as the alkali chloride concentration decreases. However, the operation flexibility permitted by this factor is limited as too low concentrations of alkali chloride negatively affect the membrane efficiency, increase the oxygen content in chlorine and can decrease the operating lifetime of the anodes. For these reasons, it has been proposed, as an additional measure, to saturate the oxygen with water at temperatures close to the operating temperatures; the diffusion of water vapor through the cathode pores permits to further decrease the caustic soda concentration towards acceptable values for the membrane. This measure, however, is only partially efficacious as part of the water vapor is absorbed by the caustic soda released by the back side of the cathode. The comparison between the characteristics of the two cases, that is the oxygen diffusion cathode spaced from the membrane and oxygen diffusion cathode in direct contact with the membrane clearly indicates that in the first case the process is straightforward and the problems are essentially of a mechanical or construction nature connected to the need to introduce actually complex bubbling systems inside the cell structure, while in the second case the process is complicated and the difficulties, in particular the need to ensure an adequate lifetime of the membrane, seem not easy to overcome. An attempt at simplifying the structure described in US 5.693.202 and in WO 98/21384 is disclosed in the patent application DE 19954247 A1 , wherein the oxygen pressure in correspondence of the various strips of the oxygen diffusion cathode is regulated by a restriction or by a valve. In this way the connection between the oxygen chambers corresponding to each strip requires a less complicated design.

However DE 19954247 A1 does not yet present a construction solution to be effectively applied in practice as the proposed design is negatively affected by two problems of substantial seriousness.

The first of these problems occurs when an industrial cell manufactured according to these teachings is operated under variable loads, that is with different current densities in order to produce variable quantities of chlorine and caustic soda as to comply with the market requests. In this case also the oxygen flow-rate must be regulated and this involves a pressure change caused by the restriction. Therefore the pressure compensation is no more observed and the pressure differential between caustic soda and oxygen greatly varies with negative consequences on the performances and mechanical stability of the oxygen diffusion cathode.

The only way to overcome this problem is clearly by maintaining a constant flow rate of oxygen feed also under low electric current density, a situation which is not acceptable from an industrial standpoint due to the costs. The second problem is connected to the fact that gas diffusion cathodes let percolate minimum quantities of caustic soda on the gas side or give rise to condensation of part of the water vapor contained in the oxygen. In any case there is always a small but not negligible formation of a liquid phase which must be removed continuously to avoid flooding of the oxygen chambers corresponding to each strip of gas diffusion cathode. The device illustrated in DE 19954247 A1 does not permit the removal of the condensate phases and therefore its performance are bound to be inevitably spoiled with time.

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.

In this way the pressure differential between the liquid electrolyte and the gas side of the gas diffusion electrode is maintained unvaried as the gas flow rate varies which therefore may be freely optimized depending of the electric load imposed by the production needs. Further the proposed cell design foresees that 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.

Description of the figures

The invention is illustrated in the following figures:

Fig. 1 - Side view, cross section of the cell of the invention of the type separated by an ion exchange membrane, wherein both the anodic and the cathodic compartment are illustrated as well as the anode, gas diffusion cathode, the dynamic pressure drop devices of the invention and the condensate phase discharge elements.

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. 4 - Embodiment of a rotameter with the relevant pressure/gas flow-rate curve.

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.

Detailed description of the invention

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. In an industrial application, a multiplicity of cells as that of Fig. 1 , not necessarily divided by an ion exchange membrane or another diaphragm, are assembled to form an assembly known as electrolyzer, normally according to a preferred configuration called filter-press. In the following description reference will be made to the chlor-alkali electrolysis with air diffusion cathodes, and more particular oxygen. However 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.

In Fig. 1 the main components of the electrolysis cell are identified by numbers, as hereinafter indicated:

1 is the anodic shell which together with the ion exchange membrane 2 defines the anodic compartment 3. 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.

In the case of chlor-alkali electrolysis, 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.

9 represents the oxygen diffusion cathode, which is maintained at a constant distance spaced from the membrane. Caustic soda flows in the gap 10 between the membrane 2 and the cathode 9. With commercial ion exchange membranes, such as Nafion (R), Flemion (R) and Aciplex (R) produced by

DuPont (USA), Asahi Glass (Japan) and Asahi Chemical (Japan) respectively, 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. For this reason 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. In this way 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. In 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 oxygen fed through nozzle 16, crossing the first lower chamber in the longitudinal direction as schematized by the arrow in Fig. 3, partially reacts at the diffusion cathode 9 which constitutes the chamber wall on the membrane side. 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. In 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.

In the preferred embodiment of the invention based on the use of rotameters acting as device 14, the pressure/gas flow rate relationship is reported in Fig. 4 for the specific case of the schematized design. As it can be seen, with the exception of the narrow region with very small flow rates which are of no practical interest, the pressure differential, which is established upstream and downstream the rotameter, thus in the two chambers connected to the rotameter itself, is practically independent from the gas flow rate. Such a behaviour permits to maintain a pressure differential between two adjacent chambers which results constant also when the electric load and thus of the oxygen flow rate are varied.

Using rotameters with equivalent pressure/gas flow characteristics, it is possible to set the same pressure differential in each chamber with respect to the subsequent one for all chambers. Obtaining the same pressure/gas flow relationship is relatively simple as it is sufficient to select rotameters having the same geometrical characteristics (diameter, length, same degree of aperture of the gas diffusing conical section).

In addition, with rotameters having the same characteristics, 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. Whichever is the material selected for the construction of the rotameter, it may be advantageous that both the body and the float be coated by chemically inert and hydrophobic material: this last characteristic has the purpose of preventing the formation of a liquid film due to possible entrapping of microdrops in the gas with possible anomalies in the operation.

By indicating with dP the pressure differential produced by the single rotameter, the pressure Pn in the chamber number n, measured starting from the oxygen discharge nozzle (17 in Fig. 1), results defined by: Pn = P (discharge) + n x dP

The pressure profile 33 of chambers 13, resulting from this relation, is given in Fig. 5, together with the pressure profile exerted by the caustic soda head 34 and with the pressure difference 35 between caustic soda and oxygen as a function of the height of the cell.

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.

In general 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 situation does not change in the adjacent chamber (second chamber) positioned below the first: in fact, if it is true that the pressure exerted by caustic soda, whose head is 60 cm at the base of the second chambers, is doubled, it is also true that the oxygen pressure is doubled due to the additional pressurization due to the second rotameter. If the two rotameters positioned at the outlets of said two chambers, have equivalent characteristics, then the further pressurization is 30 cm and adds to the previous analogous one for a total of 60 cm: as a consequence the pressure differential throughout the diffusion cathode is again substantially nil at the base also of the second chamber and equal to about 30 cm in the upper part.

The above described scenario is repeated unvaried in all the chambers positioned below the two whose behaviour has just been illustrated. Although in the preceding description the pressurization control of each chamber is carried out preferably by means of rotameters, it is obviously evident that other devices may be used as well, provided that they can ensure a pressurization in the range of tens of cm of water column and substantially independent from the gas flow rate For example, devices made of a pipe, whose upper end be closed by a mobile shutter maintained in place by a return spring are probably suitable for achieving the advantages of the invention. The illustrated type of pressure regulation of each oxygen chamber, having the purpose of maintaining the pressure difference between caustic soda and oxygen across the diffusion cathode within moderate limits for all the vertical extension of the cell, is defined, as above mentioned, a pressure compensation system.

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. With this last solution the ion exchange membrane is pressed against the surface of anode 3 and the oxygen diffusion cathode against frame 11 which is in this case provided with a mesh (not shown in Fig. 2) to ensure a supporting surface.

The construction design of the oxygen chambers illustrated in Fig. 1 obviously is not the only one applicable. For example, 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. As an alternative, 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.

In a further embodiment the periphery of the frame 11 is extended to form a flat flange. Onto frame 11 , thus modified, 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. In the case of chlor-alkali electrolysis, taken as an example of the applications of the present invention, 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. When the level 29 of the liquid, which is collected on the bottom of the chamber, reaches the lower end 27 of pipe 24, the pressure differential between the two chambers (generated by the connecting rotameter) causes the upflow of the liquid to the upper end 26.

During all those periods when the liquid phase is absent or is present in an insufficient quantity to reach the lower end 27, the empty pipe 24, represents a path for oxygen in parallel with rotameter 14. To avoid that the operation of the rotameter be substantially spoiled and in particular to avoid that the pressurization value may be affected, it is necessary that the gas flow rate through pipe 24 represent a minor portion of the overall gas flow. With cell sizes corresponding to those of standard industrial use, the overall gas flow of the oxygen feed is about 3 nrVhour and is reduced indicatively to 1 m³/hour at the outlet. Under these conditions, assuming that 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. As a consequence, 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. This material is preferably saturated with water during assembly and provides for an efficient barrier toward the gas passage. On the other end 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.

By said discharge element 15 the pressure differential existing between each couple of adjacent chambers permits the passage of the separated liquid phase from the initial lower chamber to the final upper chamber through the various intermediate chambers. The liquid collected in the last chamber is finally discharged in the caustic soda leaving the cell or alternatively in the residue oxygen flow downstream the last rotameter 14.

Other construction forms are suitable for producing element 15: for example 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.

Claims

CLAIMS:

1. An electrolysis cell containing a liquid electrolyte and at least a gas diffusion electrode having a surface in contact with said electrolyte and the opposite surface connected with at least two chambers containing said gas, said chambers being of the pressure compensated type, characterized in that each one of said chambers is provided with a discharge device of said gas of the dynamic pressure drop type capable of ensuring inside each one of said chambers a pressure value substantially independent from the flow rate of said gas.

2. The cell of claim 1 wherein said chamber are connected to each other by means of said discharge devices.

3. The cell of claim 1 wherein said at least one discharge device is a rotameter.

4. The cell of claim 3 wherein the internal surfaces of said rotameter are hydrophobic.

5. The cell of claim 1 wherein said at least one discharge device comprises a mobile shutter connected to a return spring.

6. The cell of claim 1 wherein the difference between the hydraulic head of the electrolyte and the gas pressure inside said chambers is maintained within a limit of some tens of centimeters of water column along the height of said cell.

7. The cell of claim 6 wherein said limit corresponds to about 30 cm of water column.

8. The cell of claim 1 further characterized in that at least one of said chambers is provided is provided with at least one withdrawal device for the liquid collected on the bottom.

9. The cell of claim 8 wherein said withdrawal device is made of a pipe having a diameter not higher than 1 mm fixed to the ceiling of said at least one chamber with the upper end substantially in contact with said ceiling and the lower end close to the bottom of said at least one chamber but not in contact therewith.

10. The cell of claim 8 wherein said withdrawal device is made of a pipe containing a porous, hydrophilic and chemically resistant material, said pipe being fixed to the ceiling of said at least one chamber with the upper end substantially in contact with said ceiling and the lower end close to the bottom of said cell but non in contact therewith.

11. The cell of claim 10 wherein said porous material consists of a bundle of pressed fibers.

12. The cell of claim 11 wherein said fibers are zirconium oxide.

13. The cell of claim 11 wherein said bundle of fibers is in contact with the bottom of said chamber.

14. The cell of claim 8 wherein said withdrawal device is made of a pipe fixed to the ceiling of said at least one chamber with the upper end substantially in contact with said ceiling and the lower end substantially separated from the bottom of said chamber, said pipe containing a chemically resistant, porous and hydrophilic material having a length sufficient to come out of said pipe reaching a certain distance not higher than some millimeters from said bottom.

15. The cell of claim 8 wherein withdrawal device is made of a pipe fixed to the ceiling of said at least one chamber with the upper end substantially in contact with said ceiling and the lower end substantially separated from the bottom of said chamber, said pipe containing a chemically resistant porous hydrophilic material having a length sufficient to contact said bottom.

16. The cell of claims 14 and 15 wherein said porous material is a bundle of fibers.

17. The cell of claim 16 wherein said bundle of fibers is a bundle of zirconium oxide fibers.

18. The cell of claim 8 wherein said withdrawal device is made of a collar which is fixed to the ceiling of said at least one chamber and supports a stick of ceramic porous material.

19. The cell of claim 18 wherein said ceramic material is sinterized zirconium oxide.

20. The electrolysis cell of the preceding claims wherein said electrolysis is chlor-alkali electrolysis and said gas diffusion electrode is an air or oxygen diffusion cathode.

21. An electrolysis cell containing the descriptive elements of the figures and of the text.