JP2005534806A - Electrochemical cell - Google Patents

Abstract

The present invention relates to a membrane electrolysis process comprising at least one anode space having a metal electrode, a cathode space having a gas diffusion electrode as a cathode, and an ion exchange membrane disposed between the anode space and the cathode space. Electrochemical cell for use. The metal electrode as the anode is immersed in the electrolyte, has an orifice for the flow path of gas generated during operation, and is optionally tilted and / or curved. The orifice has a guide structure that guides the generated gas toward the side opposite to the cathode side of the metal electrode.

Description

  The invention particularly relates to an electrochemical cell used to electrolyze an aqueous solution of hydrogen chloride by membrane treatment using a gas diffusion electrode as a cathode.

  An aqueous solution of hydrogen chloride, also referred to below as hydrochloric acid, is obtained as a by-product in many chemical processes. These particularly include processes in which organic hydrocarbon compounds are oxidized using chlorine. Many of these organochlorine compounds are important intermediates for industrial chemistry, for example the production of plastics. Since chlorine can be used, for example, for further chlorination, recovery of chlorine from these hydrochloric acids is economically important. Chlorine from hydrochloric acid can be recovered, for example, by electrolysis.

  An electrolytic process of hydrochloric acid is disclosed, for example, in US Pat. No. 5,777,0035. For example, an anode space with a suitable anode consisting of a titanium-palladium alloy substrate coated with a mixed oxide of ruthenium, iridium, and titanium is filled with an aqueous solution of hydrogen chloride. Chlorine formed at the anode escapes from the anode space and is sent to a suitable regeneration process. The anode space is separated from the cathode space by a commercially available cation exchange membrane. The gas diffusion electrode is placed on the cation exchange membrane on the cathode side. On the other hand, this gas diffusion electrode is mounted on a current distributor. The gas diffusion electrode is, for example, an oxygen consuming cathode (OCC). When OCC is used as a gas diffusion electrode, an oxygen-containing gas or pure oxygen is usually sent to the cathode space and reacts with OCC.

  The generation of electrochemical gas at the electrode adversely affects the electrolysis process. In addition to hydrostatic and hydrodynamic effects, bubbles increase the ohmic resistance of the electrolyte. In order to mitigate the effects of bubbles, DE-A-3401636 discloses that one or both electrodes have a flow path and the electrolyte flows from top to bottom through one or both half-cells so that the electrodes get wet. An electrolysis process is proposed in an electrochemical cell having a separated anode space and cathode space, configured to do so. Therefore, the electrolyte flows in the direction opposite to the rising gas generated by electrolysis. The resulting bubbles eventually collapse at the phase interface between the downward electrolyte membrane and the adjacent gas space.

  An object of the present invention is an electrochemical cell for a membrane electrolysis process, wherein an anode is an electrochemically active metal electrode having as large a surface as possible, and a gas generated from the side facing the cathode as a counter electrode. To provide a cell with an orifice that allows for delivery to a half-cell space located behind the metal electrode. In particular, this electrochemical cell is used for electrolysis of an aqueous hydrogen chloride solution, and the cathode used is a gas diffusion electrode. The anode space is completely filled with hydrochloric acid, and the hydrochloric acid flows in the anode space from bottom to top.

  This object is achieved by providing a guide structure in the orifice of the metal electrode for guiding the generated gas to the space behind the metal electrode. At the same time, the metal electrode is tilted and / or bent, resulting in an increase in the electrochemically active area of the metal electrode.

  Therefore, the present invention comprises at least one anode space having a metal electrode as an anode, a cathode space having a gas diffusion electrode as a cathode, and an ion exchange membrane disposed between the anode space and the cathode space, The metal electrode relates to an electrochemical cell for a membrane electrolysis process, which is immersed in an electrolyte and has an orifice for the passage of gas generated during operation and is optionally tilted and / or curved. The orifice has a guide structure that guides the generated gas to the side opposite to the cathode side of the metal electrode.

  The electrode orifices can not only be slots or holes, but can also be formed by expanded metal mesh. Here, the guide structure of the orifice is inclined in the direction of the ion exchange membrane in order to guide the gas in a controlled manner to the space behind the metal electrode, also called the back space. Therefore, the gas that is gradually generated on the surface of the metal electrode facing the ion exchange membrane is guided away from the surface, and is released from the narrow gap between the metal electrode and the ion exchange membrane during ascending. This prevents the gas from collecting in a narrow intermediate space and increasing the resistance of the electrolyte.

  In a preferred embodiment of the electrochemical cell according to the invention, the metal electrode orifice has a total cross-sectional area in the range of 20% to 70% of the area formed by the height and width of the metal electrode. When the metal electrodes of the electrochemical cell are in a vertical arrangement, the length of the side of the electrode arranged in the substantially vertical direction is regarded as the height of the metal electrode and is arranged in a horizontal direction substantially parallel to the counter electrode. The length of the opposite electrode is considered the width.

  Where the electrode substantially comprises a sheet metal, it is preferred that the electrode is not flat but curved and / or corrugated. In the case of such an electrode structure, it is preferable to select a corrugated, zigzag or rectangular cross section. This method increases the size of the electrochemically active surface of the metal electrode. Such an electrode structure is particularly important in the case of electrolysis of an aqueous solution of hydrogen chloride (hydrochloric acid). This is because, due to the relatively high conductivity of hydrochloric acid, an electrochemical reaction is observed here even when the distance between the electrode and the counter electrode is relatively large. The electrochemically active surface of the metal electrode is in particular formed by the counter electrode of the metal electrode, in this case the surface facing the cathode. However, further electrochemical reactions are observed on surfaces that do not face the counter electrode. For example, this corresponds to a surface perpendicular to the counter electrode, such as an edge, or a surface opposite the counter electrode, such as behind a metal electrode. Thus, an electrochemically active surface should be understood to mean the proportion of the total surface area of the metal electrode where an electrochemical reaction takes place. The ratio of the electrochemically active surface area to the surface area formed by the height and width of the metal electrode is preferably at least 1.2.

  In a further preferred embodiment of the electrochemical cell, the metal electrode has a depth of at least 1 mm. This depth is regarded as the lateral length of the metal electrode disposed substantially perpendicular to the counter electrode and in the horizontal direction. For corrugated metal electrodes, the depth corresponds to twice the wave amplitude. Expressed in other ways, the depth corresponds to the difference between the minimum and maximum distance formed by the edge of the electrode and the edge of the ion exchange membrane. The same applies to the depth in the case of an electrode with a zigzag cross section.

  With the appropriate mesh size, web thickness, and web width, expanded metal also has the desired characteristics of the electrode structure: a large electrochemically active surface with depth, and an orifice to guide gas away. Have. The mesh allows gas to be guided behind the metal electrode, and when the expanded metal is disposed so that the web is inclined in the direction of the counter electrode, the web functions as a guide structure. If at least one of the expanded metals is installed as a metal electrode, in particular as an anode, in the manner described above, it is also possible to combine two or more identical or different expanded metals. The metal electrode is preferably basically composed of two adjacent expanded metals, and the expanded metal facing the counter electrode has a finer structure than the expanded metal facing the opposite direction of the counter electrode, Further, the expanded metal having a finer structure is rolled flat, and the expanded metal web having a coarser structure is arranged so that the mesh web is inclined in the direction of the counter electrode, thereby serving as a guide structure. It is preferable.

  The cell according to the invention is used in particular for electrolysis of an aqueous solution of hydrogen chloride. The anode half-cell has an electrolyte inlet and outlet that flows through and completely fills the half-cell. The outlet of the electrolyte also serves as an outlet for the generated gas. Suitable anodes are, for example, noble metal coated or noble metal doped titanium electrodes. It is a titanium or titanium alloy electrode, in particular a titanium-palladium alloy, for example based on a ruthenium-titanium mixed oxide or iridium oxide or with an acid-resistant chlorine-generating coating based on platinum Including electrodes. The anode half cell is separated from the cathode half cell by an ion exchange membrane. There is a gap between the anode and the ion exchange membrane. In particular, a gas diffusion electrode that functions as an oxygen-consuming cathode serves as the cathode. On the one hand, the gas diffusion electrode is placed in contact with the ion exchange membrane, and on the other hand, it is placed in contact with the current collector. When a gas diffusion electrode is used as the oxygen consuming cathode, oxygen or oxygen-containing gas can flow through the cathode space. It is also conceivable that the flow direction of oxygen inside the cathode space is influenced by baffles. Oxygen can be fed from below via the inlet and removed again from above via the outlet. However, it is also possible to allow oxygen to flow from top to bottom or to flow laterally in the cathode space, for example from the lower left to the upper right. For the reaction that occurs, oxygen must supply a stoichiometric excess.

It is preferred to use a gas diffusion electrode comprising a catalyst of the platinum group, preferably platinum or rhodium. An example is a gas diffusion electrode from E-TEK (USA) with 30% by weight platinum on activated carbon, which has a 1.2 mg Pt / cm ² noble metal coating on the electrode.

  Suitable ion exchange membranes include, for example, perfluoroethylene containing a sulfo group as the active center. For example, a membrane commercially available from DuPont, such as a Nafion® 324 membrane can be used. Both single-layer membranes having the same equivalent amount of sulfo groups on both sides and membranes having different equivalent amounts of sulfo groups on both sides are suitable. A film having a carboxyl group and a film having not only a sulfo group but also a carboxyl group are also conceivable.

  The cathode-side current distributor can be composed of, for example, titanium expanded metal or noble metal coated titanium, and alternative stable materials can be used.

Hereinafter, the present invention will be described in more detail based on preferred embodiments with reference to the accompanying drawings.
In the embodiments described below, the surface of the metal electrode facing the ion exchange membrane side, and hence the counter electrode side, is referred to as the front portion, and thus the surface facing away from the ion exchange membrane is referred to as the back portion. In the electrochemical cell according to the present invention, the electrode is not arranged on the ion exchange membrane, and there is a gap filled with electrolyte between the anode and the ion exchange membrane, and this gap is from the remaining space of the half cell. Not separated. The gap is generally 1 to 3 mm. It is formed due to the fact that the anode space is maintained at a higher pressure than the cathode space. Thus, the ion exchange membrane is pressed against the gas diffusion electrode, which is then pressed against the current collector. The electrolyte flows freely from bottom to top in the entire half cell. The space of the half cell adjacent to the back portion of the electrode is hereinafter also referred to as the back space.

  In the embodiment shown in FIG. 1, the electrode is composed of metal lamellae 10 arranged in a vertical direction substantially perpendicular to the ion exchange membrane. Inclined in the direction of the ion exchange membrane and with the aid of it, the rising gas is led from the front of the electrode, ie from the side facing the ion exchange membrane, through the orifice 14 to the back space of the electrochemical half cell behind. The baffle plate 12 configured as follows exists as a guide structure in each of two adjacent lamellas. Lamellar contact occurs via the current supply 16. The depth of the lamella is in the range of 1-40 mm, and the distance between the two adjacent lamellae is in the range of 1-10 mm.

  In a further preferred embodiment (not shown here) based on a principle similar to that shown in FIG. 1, the vertically arranged lamellae are at an angle of 50-90 ° to the ion exchange membrane. Yes.

  In the embodiment shown in FIG. 2, the structure of the electrode 20 and the baffle plate is substantially consistent with the embodiment shown in FIG. 1 (thus the same or similar components are represented by the same reference numerals). The substantial difference is that the lamella 20 is connected at its edges facing away from the ion exchange membrane to a metal sheet 28 with an orifice 24 under the baffle plate 22 for expelling gas. It is that you are. The sheet metal 28 attached to the back of the lamella is therefore substantially parallel to the ion exchange membrane.

  In the embodiment shown in FIG. 3, the metal electrode 30 is based on a corrugated cross section. The orifice 34 exists in a region of a wave peak facing in the opposite direction to the ion exchange membrane. The guide structure 32 has a substantially half-wave shape, and is composed of a thin metal plate mounted on the orifice 34 and inclined in the direction of the ion exchange membrane. Such an electrode structure can be formed in a simple manner, for example by punching a substantially triangular orifice 34 from the rear of the electrode 30 in the region of the wave crest. The triangular orifice is not completely punched so that the punched part can be bent as a guide structure 32 in the direction of the ion exchange membrane and disposed at a tilt angle of 10-60 °. Each remains connected to the electrode in the region of the upper vertex of the triangle. Guide structures 32 can optionally weld their edges to electrode 30. However, the orifice 34 is preferably punched from the back of the electrode 30 in a shape corresponding to the shape of the wave crest. This is because the unexpected structure 32 bent toward the front portion of the electrode 30 and inclined in the direction of the counter electrode terminates at the electrode. Therefore, the trouble of further connecting the guide structure 32 to the electrode 30 can be saved. In such a manufacturing method, the area of the punched orifice 34 substantially determines the area of the guide structure 32. It is also possible to reduce the size of the punched metal portion of the electrode that functions as a guide structure, thereby reducing the extent to which these metal portions protrude into the space between the electrode and the ion exchange membrane. Preferably, the size of the orifice 34 is selected so that the area of the guiding structure 32 closely matches the area of the wave peaks. Here, the depth of the electrode is understood to mean the distance from the wave peak to the wave valley, ie twice the wave amplitude, and is 2 to 40 mm. The distance between two adjacent wave peaks or valleys corresponding to the wavelength is 3-30 mm. In this embodiment, the gas flows substantially in the direction indicated by arrow 39.

  A further embodiment (not shown here) is in principle an electrode structure similar to that shown in FIG. 3, but the electrodes basically have a zigzag cross section. Similar to the fabrication method described above, this electrode structure can be fabricated by punching out a triangular orifice from the back at the apex region facing away from the ion exchange membrane.

  The embodiment shown in FIG. 4 is again different from that shown in FIG. 3 only in the basic cross section of the electrode structure 40. Here it is a rectangular cross-section, and the orifice 44 (FIG. 4a) is present on a longitudinal surface facing away from the ion exchange membrane. These are arranged substantially parallel to the ion exchange membrane. Guide structure 42 guides the gas into the back space of the half-cell, which is the direction indicated by arrow 49 (FIG. 4a).

  In a further preferred embodiment (FIG. 5), which also has a rectangular cross-sectional electrode structure 50 (FIG. 5), the orifice 54 is not one of the longitudinal sides, but one of the lateral sides, ie the side of the electrode perpendicular to the ion exchange membrane. One is arranged. In this embodiment, the guide structure 52 is therefore not inclined in the direction of the ion exchange membrane, but in the direction of the opposite lateral side of the electrode. The gas flow from the front to the back of the electrode is marked by arrows 59.

  An electrode structure having a large electrochemically active surface and an orifice with a guide structure for directing gas into the back space of the half-cell is also provided by expanded metal. Therefore, in a further preferred embodiment, it is composed of two different expanded metals adjacent to each other, in particular, the expanded metal facing the ion exchange membrane side is more than the expanded metal facing the opposite direction to the ion exchange membrane, It is preferable to have a fine structure. The finer structure expanded metal is characterized by a smaller mesh width and mesh size and smaller web width and web thickness than the coarser structure expanded metal. Furthermore, the expanded metal with a finer structure is preferably rolled flat, and the expanded metal with a coarser structure is not optional, but is arranged so that the mesh web performs the function of the guide structure. . The mesh is preferably rhombus or square and the coarser expanded metal web, facing away from the ion exchange membrane, is inclined in the direction of the ion exchange membrane. For both finer and expanded metal, the total area of the orifice is 20-70% of the area obtained by the outer dimensions, ie the length of the expanded metal edge. Within range. The following parameters are used to characterize the expanded metal. That is, the web thickness matches the thickness of the sheet metal used for the production of expanded metal. The web width is obtained from the distance between two sections that are parallel to each other but offset. The mesh size characterizes the length of the section and the mesh width characterizes the maximum distance between two adjacent webs as a result of flexion and extension.

  Surprisingly, it has been found that in this preferred embodiment of the invention, a lower operating voltage is observed than in the case of an electrolytic cell with a conventional electrode structure. This is clearly shown by the example described below.

(Example)
Electrolysis of an aqueous solution of hydrogen chloride (hydrochloric acid) was performed using a gas diffusion electrode as the oxygen-consuming cathode. The concentration of hydrochloric acid was 13% by weight, and the temperature of hydrochloric acid during introduction into the anode half cell was adjusted so that the temperature during discharge was 60 ° C. The circulation volume flow rate of hydrochloric acid was adjusted so that the flow rate of hydrochloric acid in the anode half cell was 0.3 cm / s. The anode material consisted of a titanium-palladium alloy activated by a ruthenium-titanium mixed oxide layer (DSA® coating). A cationic membrane Nafion (registered trademark) type 324 manufactured by DuPont was used between the anode space and the cathode space. The cathode used was a gas diffusion electrode manufactured by E-TEK (USA) activated with rhodium sulfide based on carbon. The gas diffusion electrode was fixed to a current collector made of activated titanium-palladium expanded metal. The electrode had a width of 730 mm and a height of 1200 mm. The minimum distance between the anode and the cation exchange membrane was 3.5 mm.

Example 1
A combination of two titanium expanded metals adjacent to each other, that is, an expanded metal with a finer structure applied to an expanded metal with a coarser structure was used as the anode. The finer structure expanded metal had a mesh size of 4.2 mm and a mesh width of 3.1 mm and a web width of 0.6 mm and a web thickness of 0.4 mm. Thus, the total orifice area was 53% of the expanded metal area. This expanded metal was rolled to a thickness of 0.5 mm. Of the two expanded metals, the coarser structure had a mesh size of 13.2 mm, a mesh width of 6.3 mm, a web width of 2.4 mm, and a web thickness of 3.5 mm. Therefore, the total orifice area was 24% of the expanded metal area. The anode was placed so that the rolled expanded metal with a finer structure faces the cation exchange membrane.
The voltage was 1.59 V at a current density of 5 kA / m ² .

Example 2 (comparative example)
The anode consisted of a single coarse expanded metal having a mesh size of 6.2 mm, a mesh width of 3.6 mm, a web width of 1.1 mm, and a web thickness of 1 mm. Therefore, the total area of the orifice was 24% of the total area of the anode.

The voltage is 1.67 V at a current density of 5 kA / m ² , and thus under similar conditions, this is a flatter rolled finer structure expanded metal facing the cathode and a coarser placed on the back It was higher than the case of electrolysis of hydrochloric acid solution by special combination with expanded metal of structure.

1 is a perspective view of a first preferred embodiment of an electrode structure. FIG. 6 is a perspective view of a second preferred embodiment of an electrode structure. FIG. 6 is a perspective view of a third preferred embodiment of an electrode structure. FIG. 6 is a perspective view of a fourth preferred embodiment of an electrode structure. FIG. 5 is a partial view of the embodiment shown in FIG. 4. FIG. 7 is a perspective view of a fifth preferred embodiment of an electrode structure.

Claims (5)

  Ion exchange disposed between the anode space and the cathode space, at least one anode space having a metal electrode (10; 20; 30; 40; 50) as an anode, a cathode space having a gas diffusion electrode as a cathode An electrochemical cell for a membrane electrolysis process comprising a membrane, wherein the metal electrodes (10; 20; 30; 40; 50) are immersed in an electrolyte and for a flow path of gas generated during operation Having an orifice (14; 24; 34; 44; 54) and optionally inclined and / or curved, said orifice (14; 24; 34; 44; 54) An electrochemical cell having a guide structure (12; 22; 32; 42; 52) for guiding the metal electrode to a side opposite to the cathode side.   The total cross-sectional area of the orifice (14; 24; 34; 44; 54) is in the range of 20% to 70% of the area formed by the height and width of the metal electrode. Item 12. The electrochemical cell according to Item 1.   The electrochemical cell according to claim 1 or 2, wherein the metal electrode has a corrugated, zigzag, or rectangular cross section.   The electrochemical cell according to claim 3, wherein the metal electrode has a depth of at least 1 mm.   The metal electrode is based on two expanded metals adjacent to each other, and the expanded metal facing the cathode has a finer structure than the expanded metal facing away from the cathode, and the expanded metal having the finer structure. The rolled metal is flattened, and an expanded metal having a coarser structure is disposed so that the mesh web is inclined toward the cathode and serves as a guide structure. Electrochemical cell.

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