EP1133587A1 - Membrane electrolytic cell with active gas/liquid separation - Google Patents

Abstract

The invention relates to an electrochemical half cell (1) which consists of at least one membrane (4), an electrode (3) as anode or cathode which optionally produces gas, optionally an outlet (8; 16) for the gas and a support structure (12) linking the electrode which optionally produces gas with the back wall (15) of the half cell. The support structure (12) divides the interior (13) of the half cell (1) into vertically arranged channels (5, 9). The electrolyte (14) flows upwards in the electrode channels (9) facing the electrode (3) and flows downwards in the channels (5) facing away from the electrode (3). The electrode channels (9) and the channels (5) facing away from the electrode (3) are interlinked at their upper and lower ends.

Description

Membrane electrolysis cell with active gas / liquid separation

The invention relates to an electrochemical half cell, which consists at least of a membrane, an optionally gas-developing electrode ^' or anode

Cathode, optionally an outlet for the gas and a support structure which connects the optionally gas-developing electrode with the half-cell rear wall. The support structure divides the interior of the half-cell into vertically arranged channels, the electrolyte flowing upward in the electrode channels facing the electrode and flowing downward in the channels facing away from the electrode, and the electrode channels and the channels facing away from the electrode at their upper and lower End connected.

The incomplete or incorrectly performed gas separation in the upper region from the prior art electrolysis cells leads to inadequate wetting of the membrane with an increase in the electrical resistance of the membrane. This causes an increase in the integral cell voltage and also harbors the risk of local membrane damage as a result of so-called "blistering". The damage to the membrane extends to the passage of electrode gas and possibly to the formation of explosive gas mixtures. In addition, incorrect gas separation can cause pulsating pressure surges in the Electrolyte space are triggered, which result in membrane movements with the risk of premature aging due to mechanical damage.

Another problem is to operate the electrolysis cell with the most homogeneous vertical and horizontal temperature and concentration distribution (salt concentration or pH value of the electrolyte) in the area of the electrolyte space in front of the membrane surface, also to avoid premature membrane aging. This is generally desirable for the operation of all gas-developing electrolysers, but in particular for the use of gas diffusion electrodes in which the heat dissipation (dissipation of the lost heat) is predominant or complete must take place via the electrolyte circuit on the other, gas-generating side, depending on whether work is being carried out beyond the membrane with a finite electrolyte gap (finite gap) or with an overlying gas diffusion electrode. This may result in a lowering of the temperature of the inflowing fresh electrolyte for the gas-generating side, which must not lead to local overcooling here.

In the past, there have been some suggestions for reducing these problems, but only for the classic hydrogen-developing NaCl electrolysis. Thus, a system for stimulating an internal natural circulation is described in the European published patent application EP 0579910 AI, in particular in order to make brine acidification more effective for NaCl electrolysis and to reduce excessive foam formation in the upper region of the electrolysis cell.

In the European patent application EP 0599363 AI various methods of treating process-related gas bubbles are dealt with, without mentioning the decisive elements, which include a complete separation of gas and electrolyte with a completely pulsation-free, also common course of the separated phases from the cell and an equalization of Allow temperature and concentration right into the corners of the cell.

The solution to these problems of the known electrolysis half-cell arrangements is achieved by a half-cell according to the preamble with the characterizing features of the independent claim.

The invention relates to an electrochemical half-cell consisting at least of a membrane, an optionally gas-developing electrode as an anode or cathode, and a support structure which connects the optionally gas-developing electrode to the half-cell rear wall, and an inlet for the electrolyte and an outlet for the electrolyte and optionally for the gas, characterized in that the support structure divides the interior of the half cell into vertically arranged channels, the electrolyte in that of the electrode facing electrode channels flows upward and flows downward in the channels facing away from the electrode and that the electrode channels and the channels facing away from the electrode are connected to each other at their upper and lower ends.

In particular, the channels with downward flow and the electrode channels are arranged alternately next to one another or else one behind the other.

The channels with downward flow and the electrode channels can have a trapezoidal cross section.

The channels with downward flow and the electrode channels are preferably formed by a folded, electrically conductive sheet metal as the supporting structure.

In a particularly advantageous embodiment of the half cell, the electrode channels have a cross-sectional constriction at their upper end.

In a special arrangement, a vertically aligned parallel support structure separates the channels open to the electrode, in which the lighter electrolyte-gas mixture rises, from channels open to the rear wall, in which the degassed, heavier electrolyte flows down again. Essential for the improvement of the gas separation is a constriction at the top of the electrolyte channels, which is generated by a wing-like flow deflection profile that is bent towards the electrode. The two-phase flow is accelerated in the constriction between the electrode and the profile, above the backward curved upper edge of the

Profile relaxed and degassed on the back of the profile with separation of the phases. On the back of the profile, openings are opened in the downward channels, so that the heavier, degassed electrolyte flows downwards and, at the bottom of the cell, through connection openings, together with freshly added electrolyte, flows back into the channels open to the electrode as a gas-absorbing fraction and thus the internal natural circulation of the Electrolytes causes. The cross-sectional area of the electrode channels in the narrowest region of the constriction in relation to the cross-sectional area of the electrode channels below the constriction is preferably from 1 to 2.5 to 1 to 4.5.

The narrowing of the electrode channels can be formed, for example, by an angled guide structure.

The narrowing of the electrode channels has in particular an area with a constant cross section, the height of this area being at most 1:

100 in relation to the height of the active membrane area.

The half-cell can be produced in a particularly simplified manner if the guide structure is formed in one piece with the support structure.

An embodiment of the half cell in which the support structure is formed in one piece over the entire height of the electrode channels and the channels with downward flow is also advantageous.

An embodiment in which the gas separation from the electrolyte is advantageous

Electrode channels have an expansion of their cross section above the constriction.

The excess electrolyte leaving the cell can be discharged behind the flow deflection profile either laterally at the top or downwards via a vertical standpipe.

A half cell that has an outlet for the degassing is therefore particularly advantageous

Electrolytes and the gas possibly formed during the electrolysis, in particular a standpipe with passage in the cell bottom or one on a side wall of the Cell arranged outlet, which is arranged just above the upper end of the electrode channels.

As the experimental experience shows, it is particularly advantageous if the overall structure - apart from the connection openings at the bottom and the few mm wide connection gap above the profile at the top - consists of a functional unit in order to fulfill the following functions:

Separation of the gas bubbles from the electrolyte via the so-called "bubble jet" above, in order to remove the electrolyte and product gas separately or phase-separated together, but above all without any pressure pulsations

Uniformity of the vertical temperature profile through a lively natural circulation over the full height to optimize the membrane function

Uniformization of the vertical concentration profile using the same mechanism to optimize membrane function

- equalization of the vertical pH profile e.g. in the targeted acidification of the brine in NaCl electrolysis to improve the chlorine yield and quality. Local acidification of the brine would be harmful to the membrane

In addition to the hydraulic function, the carrying structure takes on the function of mechanically holding the electrode and, moreover, the function of connecting the electrode to the cell rear wall with low resistance.

In a preferred variant, the support structure with the electrode channels and the outflow channels fills the interior of the half cell to at least 90%. The support structure is preferably electrically conductive and is electrically conductively connected to the electrode and in particular to the rear wall of the half cell.

The electrode is then preferably connected in an electrically conductive manner to the support structure of the half cell and fastened on the support structure.

For temperature control of the electrolyte, a heat exchanger is preferably connected upstream of the inlet of the electrolyte, through which fresh electrolyte and, if appropriate, degassed electrolyte returned from the outlet is introduced into the half-cell, so that a temperature-controlling electrolyte circuit is formed, if necessary.

The pressure surge-free and complete separation of the gas bubbles, combined with the equalization of temperature, concentration and pH profile, is of particular importance when using gas diffusion electrodes in one of the half cells, be it on the anode or cathode side, in a gas-developing process on the other Side of the membrane. In these cases, a large part or all of the ohmic heat loss must be removed via the electrolyte from the gas-generating side of the electrolyzer, depending on the type of operation of the gas diffusion electrode.

The electrolyte converted in the anode chamber is, for example, an aqueous sodium chloride solution or a hydrochloric acid solution and chlorine is obtained as the anode gas. The counter electrode is an oxygen consumption cathode.

E.g. If the NaCl electrolysis is operated on the cathode side using an oxygen consumable cathode with a narrow catholyte gap, as described in EP 0717130 B1 and subsequent patents, the heat dissipation on the cathode side can only take place via a plug flow without turbulence; Work warm-up spans that are known not to benefit the membrane. So here either with chilled

Electrolytes in simple feed or, if necessary, also with one cooled anolyte circuit to keep the internal temperature distributions at the optimal level.

E.g. If a NaCl or HCl electrolysis is operated with the oxygen consumption cathode lying on top, the heat dissipation on the cathode side is marginal; the heat has to be dissipated almost completely via the anolyte. This generally requires an external anolyte circuit with cooling.

In all of these cases, an internal equalization of temperature, concentration and possibly pH is of particular importance because the

The amount of electrolyte fed into the cell increases compared to the internal circulation, so that the latter has to be particularly intensive in order to avoid even a local skew. This applies in particular to a highly desirable strong acidification of the brine in the case of NaCl electrolysis, which normally has to be based on the lowest local pH value.

If the half-cell is operated with a finite catholyte gap (finite gap) in front of an oxygen consumption cathode, part of the heat loss can be dissipated on the cathode side through the flow through this catholyte gap and external cooling, while the majority of the heat loss is dissipated with the anolyte flow

If, on the other hand, the half cell is operated with an oxygen consumption cathode (zero gap) resting on the membrane, the entire heat loss is dissipated via the anolyte stream.

Further advantages of the half cell according to the invention are the vertical equalization of the temperature of the electrolyte and the vertical equalization of the electrolyte concentration. The half cell according to the invention can generally be used in all gas-developing electrolyses. It is of particular importance in electrolysis, where electrolyte and gas are more difficult to separate.

The invention is explained in more detail below by way of example with reference to the figures, without the invention being restricted thereby in detail.

Show it:

1 shows a schematic cross section through a half cell according to the invention without a power supply line according to line B-B 'in FIG. 3

FIG. 2 shows a schematic longitudinal section through a half-cell according to the invention along the line A-A 'in FIG. 3

Fig. 3 The front view of the half-cell according to the invention with the electrode removed

4 alternative structures for flow guidance in the half cell according to the invention

Examples

A flow and day structure 12 is welded in an electrically conductive manner in a half cell 1 (FIG. 1). It carries the electrode structure 3, on which in turn the membrane 4 either rests or is positioned at a smaller distance from the electrode structure 3.

The support structure 12 is constructed from trapezoidal-shaped sheets which form vertical channels which are alternately open to the electrodes or are directed to the rear wall 15 as outflow channels 5.

The fresh electrolyte 17 flows through an inlet pipe 10 and through openings 11 into the half-cell interior 13, the openings 11 being distributed such that they supply each of the channels 9 open to the electrode with fresh electrolyte. Depending on the application, the openings 11 can also be arranged under the outflow channels 5 in order to improve mixing between the fresh electrolyte and the electrolyte flowing out in the outflow channels 5 (see FIG. 2).

The gas evolution at the electrode 3 leads to a buoyancy of the electrolyte in the channels 9 open to the electrode. The electrolyte 14, which is interspersed with gas bubbles, flows upwards here and is deflected towards the electrode on a profile structure 2 which emerges from the trapezoidal sheet. It is accelerated in the gap 7 between the electrode 3 and the profile structure 2 and is relaxed in the cross section of the channel 9 which widens again above the profile structure. The alternation between acceleration and relaxation achieves a very effective bubble separation, so that the electrolyte and electrode gas have already been largely separated on the back of the profile structure. The profile structure 2 only protrudes into the upflow channels 9, but is open in the direction of the outflow channels 5. The degassed, heavier electrolyte can flow downward in the outflow channels 5, mix with the fresh electrolyte flowing in at the bottom, and convert the electrode structure again into an upward flow, so that there is an intensive natural convection (see FIG. 3).

The excess electrolyte 18 leaves the half-cell 1 together with the gas separated behind the profile 2 either via a standpipe 8, as shown in FIGS. 1 and 3, or alternatively via a side outlet 16, as in FIG. 2 and in FIG. 3 is drawn.

As an alternative to the flow structure made from trapezoidal sheets, the following variants can also be used with comparable success (cf. FIG. 4). In the event that the gas-developing electrodes 3, be they anodes or cathodes, are connected to the rear wall of the half-shells 1 via vertically inserted structural elements 29, flow guidance structures in semicircular form 28 with the bubble inflow region 20 and the outflow region 21 can be between these structural elements. can be used as a diagonal element 27 with the bubble inflow region 24 and the outflow region 25 or as a separating element 26 running parallel to the rear wall with the bubble inflow region 22 and the outflow region 23. In particular, the separating element 26 can also penetrate the structural elements 29 in a suitable manner as a continuous plate and extend over the entire width of the element. But it can also prove to be advantageous if this

Separating elements are inserted individually between the structural elements 29 before the electrodes 3 are welded in and fix the separating elements.

It is essential that the respective flow channels extend analogously to the trapezoidal structures over the entire height of the element and in the upper area the

Bubbles - narrowed to flow areas - not shown here - analogous to profile structure 2 in order to trigger degassing of the electrolyte after passing through the constriction. Since the separating elements 26, 27, 28 have no electrical function, they can be made not only of metal but also of non-conductive form from suitable plastic molded parts which have suitable chemical stability and temperature resistance. be performed. Depending on the application, EPDF is available here; Halar or Telene on.

example 1

In a NaCl electrolysis pilot cell with 4 bipolar elements, each with an area of 1224 x 254 mm ² , the height corresponding to the full technical height, there are two full and two half up channels 9 and three at a depth of the anode half cell 1 of 31 mm Down channels 5 with a folded sheet 12 as a support structure that divides the half-cell interior 13 have been realized (Figure 1 shows an arrangement with half and four full up channels 9 and half and four full down channels 5). The current contact to the anode 3 was made from the half-cell rear wall 15 via the support structure 12. The profile structure 2 covers the upward channels 9 at the upper end at approximately 60 ° and narrows the flow cross section down to a 6 mm wide gap 7 towards the anode 3. The bent-back part 6 of the profile 2 leaves an 8 mm gap to the upper edge of the half cell 1 for the passage of the two-phase flow to the rear (see FIG. 2). The passage openings to the downward channels 5 are open for an unimpeded outflow of the degassed electrolyte 14. At the lower end there remains an approximately 20 mm wide gap through which the downward degassed brine 14 together with the fresh brine 16 fed from the openings 11 in the line 10 again can flow into the ascending channels 9, where it is enriched again with anode gas. The excess anolyte brine is taken up via a standpipe 8, which ends somewhat below the upper edge of the profile 2, and discharged downward from the cell 1. In the cathode half-shell, not shown, oxygen cathodes are used in the finite gap mode with a catholyte gap of 3 mm.

In a long-term test, it was examined to what extent the phases are separated and whether

Cell can be operated free of pressure pulsations. It was found that the half cells in the working area between 3 and 7 kA / m ² with complete separation can be operated by gas and electrolyte, ie the running anolyte was completely free of bubbles and ran completely evenly and without any palpable or visible pulsation.

Example 2:

An operating mode was tested in which the heat balance over pre-cooled brine was adjusted with an adapted catholyte circuit in such a way that the outlet temperature was limited to 85 ° C. Depending on the set current density, the following warm-up ranges resulted:

It was shown that with the very high current densities for heat dissipation, a moderate anolyte circuit with appropriate pre-cooling is also appropriate. Only in this way and with technically realistic brine inlet temperatures can the catholyte-side warm-up margin be reduced to <10 K.

Claims

claims 1. Electrochemical half-cell (1) consisting at least of a membrane (4), an optionally gas-developing electrode (3) as an anode or cathode, optionally an outlet (8; 16) for the gas and a support structure (12) which, if appropriate Gas-generating electrode connects to the half-cell rear wall (15), characterized in that the support structure (12) divides the interior (13) of the half-cell (1) into vertically arranged channels (5, 9), the electrolyte (14) being divided into the Electrode (3) facing electrode channels (9) flows upward and in the channels (5) facing away from the electrode (3) flows downward and that the electrode channels (9) and the channels (5) facing away from the electrode (3) at their upper and are connected at their lower ends. 2. Half cell according to claim 1, characterized in that the channels (5) with Downward flow and the electrode channels (9) are arranged alternately side by side. 3. Half cell according to claim 1 or 2, characterized in that the channels (5) with downward flow and the electrode channels (9) a trapezoidal Have cross-section. 4. Half cell according to one of claims 1 to 3, characterized in that the channels (5) with downward flow and the electrode channels (9) are formed by a folded, in particular electrically conductive sheet as a support structure (12). 5. Half cell according to one of claims 1 to 4, characterized in that the electrode channels (9) have a cross-sectional constriction (7) at their upper end. 6. Half cell according to claim 5, characterized in that the cross-sectional area of the electrode channels (9) in the area of the constriction (7) in relation to the cross-sectional area of the electrode channels (9) below the constriction (7) from 1 to 2.5 to 1 to 4 , 5 is. 7. Half cell according to claim 5 or 6, characterized in that the constriction (7) of the electrode channels (9) has a region with a constant cross section and that the height of this region is at most 1: 100 in relation to the height of the active membrane surface. 8. Half cell according to one of claims 5 to 7, characterized in that the constriction (7) of the electrode channels (9) is formed by an angled guide structure (2). 9. Half cell according to claim 8, characterized in that the guide structure (2) with the support structure (12) is integrally formed. 10. Half cell according to one of claims 5 to 9, characterized in that the electrode channels (9) above the constriction (7) have an expansion (6) of the cross section. 11. Half cell according to one of claims 1 to 10, characterized in that the support structure (12) over the entire height of the electrode channels (9) and the channels (5) is integrally formed with downward flow. 12. Half cell according to one of claims 1 to 11, characterized in that the half cell has an outlet (8; 16) for the degassed electrolyte and the gas possibly formed during the electrolysis, in particular a standpipe (8) or one on a side wall of the cell arranged outlet (16), which is arranged just above the upper end of the electrode channels (9). 13. Half cell according to one of claims 1 to 12, characterized in that the support structure (12) with the electrode channels (9) and the outflow channels (5) fills the interior (13) of the half cell (1) to at least 90%. 14. Half cell according to one of claims 1 to 13, characterized in that the support structure (12) is electrically conductive and is electrically conductively connected to the electrode (3) and the rear wall (15) of the half cell (1). 15. Half cell according to one of claims 1 to 14, characterized in that the electrode (3) is electrically conductively connected to the supporting structure (12) of the half cell (1) and is fastened to the supporting structure (12). 16. Half cell according to one of claims 1 to 15, characterized in that the electrolyte (14) is an aqueous sodium chloride solution or a hydrochloric acid solution and the electrode (3) is a chlorine-developing anode, while the associated cathode is operated as an oxygen consumable cathode. 17. Half cell according to one of claims 1 to 16, characterized in that the inlet (10, 11) of the electrolyte (14) is preceded by a heat exchanger, through the fresh electrolyte and optionally from the outlet (8; 16) recycled degassed electrolyte into the Halbzellc (1) is initiated. 18. Electrochemical half-cell according to one of claims 1 to 17, characterized in that structural elements 29 installed vertically in the half-shells 1 electrically contact and hold the electrodes 3, in such a way that flow guide structures 26, 27 or 28 are inserted between the structural elements and held by the electrodes become. 19. Electrical half cell according to claim 18, characterized in that the Flow guide structures 26, 27 or 28 consist of metal or plastic. 0. Electrical half cell according to claim 18 or 19, characterized in that the flow guide structure 26 including the profile structure is made in one piece over the entire element surface.