EP0946789B1 - Process for carrying out chemical reactions in an electrochemical cell - Google Patents

Description

The invention relates to a method for converting a gas or gas mixture in the presence of an ion-conducting Liquid in an electrochemical cell with at least two electrodes, namely at least one anode and at least one a cathode, with a between the cathode and the anode external DC voltage acts and through the ion-conducting liquid flows a direct current.

A method of this kind is described in German Patent 195 04 920 described. The electrochemical cell contains one aqueous ammonium sulfide solution, which the Electrode surfaces almost completely covered. Outdoors Gas containing oxygen comes through a Gas diffusion cathode in contact with the solution, whereby Forms ammonium polysulfide as a product. Here it shows that the fluid level in the cell is not arbitrarily high can be chosen because otherwise annoying leaks occur. Furthermore, due to a high fluid level the current-voltage characteristic of the cell is unfavorable influenced.

The invention has for its object the electrochemical Reaction of gases with liquids even in the presence of Catalysts in a cost-effective manner with high sales to be able to perform reliably even at high pressure. This is achieved according to the invention in the method mentioned at the beginning in that a swamp forms in the lower area of the cell the ion-conducting liquid, in which the Immerse electrodes that are at least 20% of the total Surface of at least one of the electrodes outside the Sump in an upper one through which the gas or gas mixture flows Area and that the upper area with the ion-conducting liquid is sprinkled or sprayed, whereby the electrode surface is at least partially wetted, while the gas or gas mixture on the electrode surfaces flows along. In this way, different gases and liquids are reacted. Usually the gas or gas mixture is oxidized or reduced.

All or at least some of the electrodes are perpendicular in the Bottom of the ion-conducting liquid, being through this bottom the current flow between the electrodes is secured. Mostly 20 - 95% of the total surface will cover at least one of the electrodes are above the sump. One of the It is also possible that either the anode or the Cathode completely covered by the liquid of the sump is. The electrodes can not only plate or be cylindrical, you can also use an electrode as an electroconductive bed or ordered packing by itself contacting, current-conducting elements. Such Filling or packing can also have a coating Have catalyst.

Between the anode and the cathode of the cell is from the outside a DC voltage is applied, which is chosen in a wide range can be. The voltage between adjacent anodes and Cathodes can be between 0.01 and 100 V, usually these voltages are in the range of 0.1 to 10 V.

A large part of the entire electrode surface is outside the liquid sump and is sprayed or sprinkled by the liquid serving as the electrolyte. At the same time, the gas fed into the cell comes into contact with the surfaces of the electrodes, which are located outside the sump. It is not important in which direction the gas flows. The gas can first be introduced into the liquid sump in the lower region of the cell and flow upwards, or the gas can be conducted without leading through the sump to the upper region of the cell to the sprayed or sprinkled electrodes. The gas can be used to introduce a component for the reaction to be carried out in the cell, for example oxygen or hydrogen. For example, air, O ₂ , H ₂ S, NH ₃ , SO ₂ , SO ₃ or a synthesis gas mixture (CO + H ₂ ) or also mixtures of these gases can be passed into the cell as the gas.

With the ion-conducting liquid in the cell, which also serves as an electrolyte, it will usually be about an organic or inorganic solution or around a melt act.

The electrodes can be made of different materials consist of, for example, metal alloys,

Mixed oxides or carbon-containing. If that Electrode material itself does not have a catalytic effect Catalyst, for example, as a coating on an electric conductive carrier can be applied. That way you can both cathodes and anodes for different reactions be specially trained. It is also possible that Consume electrodes during the conversion and as so-called sacrificial electrodes act. If you are with carbon-rich electrodes works, it may be appropriate be to hydrophobize the surface of what is known in Way by partially covering the surface with Polytetrafluoroethylene succeeds.

If you divide the cell into several reaction rooms with partial If you want to divide fluid exchange, you can do this by reach one or more diaphragms that are in in a known manner, porous and permeable to liquids are. Another way of subdivision is to use ion-selective membranes, which are also in themselves are known.

The desired product of implementation in the cell can be found in the liquid withdrawn from the cell or in the withdrawn liquid Exhaust gas or both in the exhaust gas and in the Liquid. The separation and concentration of the Product then takes place in a manner known per se.

The regulation of the desired implementation or implementations takes place e.g. B. by varying the gas and / or liquid supply as well as the current flow in the cell and from outside applied electrical voltage. You can also do that Measure the redox potential in the electrolyte sump and as Use control variable.

Fig. 1

eine erste Variante der elektrochemischen Zelle in schematischer Darstellung,

Fig. 2

eine zweite Variante der Zelle,

Fig. 3

eine dritte Variante der Zelle,

Fig. 4

einen horizontalen Schnitt nach der Linie IV-IV in Fig. 3,

Fig. 5

den horizontalen Schnitt durch eine Zelle ähnlich Fig. 3,

Fig. 6

eine Zelle mit bipolaren Elektroden und

Fig. 7

eine Zelle mit einer Gasdiffusionselektrode.

Design options of the method are explained with the aid of the drawing. It shows:

Fig. 1

a first variant of the electrochemical cell in a schematic representation,

Fig. 2

a second variant of the cell,

Fig. 3

a third variant of the cell,

Fig. 4

a horizontal section along the line IV-IV in Fig. 3,

Fig. 5

the horizontal section through a cell similar to Fig. 3,

Fig. 6

a cell with bipolar electrodes and

Fig. 7

a cell with a gas diffusion electrode.

According to Figure 1, the electrochemical cell is in a liquid and gas tight housing (1) and has a Anode (2) and a cathode (3). The two electrodes are with an external DC voltage source, not shown connected. There is a in the lower area of the cell Liquid sump (4), the liquid surface through a dashed line (5) is indicated. The liquid serves as an electrolyte, it is partly circulated and for this purpose through the line (7), the pump (8), the Return line (9) and the distributor (10) returned and from sprayed on top of the electrodes. Part of the liquid is withdrawn as a product through line (12) and leads Circulate fresh liquid through line (13). On Gas or a gas mixture is brought up in line (15) and first let it enter the swamp (4) before it flows upward between the sprayed electrodes, the desired reaction takes place. Exhaust gas is removed from the Housing (1) through the line (11). Depending on the type of Implementation of this gas can also be viewed as a product become.

As can be seen in Figure 1, only the lower part is located the electrodes (2) and (3) in the electrolyte sump (4), whereby by a current can flow through this sump between the electrodes. At least 20% of the total surface of the electrodes is located above the sump (4) Surfaces through the outgoing from the distributor (10) Liquid droplets are at least partially wetted. At the same time, the gas coming from line (15) or Gas mixture up along the wetted Electrode surfaces. Usually 20 to 95% of the entire surface of the electrodes above the sump (4) are located.

In the cell of FIG. 2, one of the electrodes is in it Case the cathode (3a), as a liquid and gas permeable Bulk or pack formed, the elements have electrically conductive contact with each other. The anode (2) is formed by a horizontal plate that extends completely in the swamp (4). The surface (5) of the Bottom extends to the lower area of the cathode (3a). The Gas is supplied through line (15) and the other parts the arrangement they already have together with FIG. 1 explained meaning.

In the cell of Figure 3, the anode is replaced by several vertical, parallel plates (2a) formed, the lower Area into the liquid sump (4). The cathode (3) is a horizontal plate located in the sump (4) educated. The remaining parts of the arrangement in FIG. 3 have already been explained together with FIG Circulation pump (8) was shown in Figure 3 for simplification omitted. In the horizontal section of Figure 4 are vertical anode plates (2a) can also be seen. In contrast to FIGS. 3 and 4, the anode can also be used as form several concentric cylinders (2b), the bottom and are open at the top and are partially in the electrolyte sump. in the Such a cell can otherwise correspond to FIG. 3 be trained. Deviating from the illustration in FIG. 3 up to 6 the electrical positive pole can be drawn to the anode and the negative pole is placed on the drawn cathode, without otherwise changing the cell.

Figure 6 shows a cell with bipolar electrodes, which as parallel, vertical plates can be formed and stand between the end anode (2) and the end cathode (3). Deviating from this, bipolar electrodes can also be used as concentric cylinder be formed. The remaining parts of the Arrangements according to FIG. 6 have already been carried out together with FIG. 1 explained.

According to Figure 7 there is one in the housing (1) Gas diffusion cathode (3b), to which a liquid-free Gas space (17) belongs. The gas is through line (15a) fed and withdrawn through line (15b). In the gas room (17) some of the gas comes through the porous structure of the Gas diffusion cathode (3b) in contact with the electrolyte which is in the sump (4) and from the distributor (10) is sprayed. Inside the structure of the cathode (3b) comes it so to the contact between gas and liquid without disruptive amounts of gas or liquid the cathode structure penetrate completely. In a manner known per se, the Gas diffusion cathode (3b) from a metal network and one attached to it attached carbon cloth. Are advantageous the fibers of the carbon cloth at least partially hydrophobized, as is also known.

Example 1:

An arrangement corresponding to FIG. 3 is used in the laboratory worked, but the cathode (3) described there is now to the anode. The anode is horizontal and in the sump (5) arranged fully submerged, it consists of a Circular disc made of expanded titanium, activated with platinum is, the diameter is 100 mm and the thickness is 1 mm. The The cathode is replaced by 8 parallel, vertical plates of 90 mm Height and 50 mm width formed, which is a distance of 4 mm have and which are electrically connected to each other. The cathode plates are made of expanded titanium, which with Platinum is activated. The cathode plates dip 20 mm into the Electrolyte sump. The container (1) is made of glass.

The cathode plates are sprinkled with an aqueous solution from above, which contains 5 g NaOH and 6.3 g Na ₂ SO ₃ per liter and has a temperature of 50 ° C. Air is supplied through line (15) and the exhaust air from line (11) is partly returned to line (15). The amount of the recycle gas is 450 Nl / h, fresh air is added to the recycle gas in an amount of 100 Nl / h. The gas is fed into the sump (4) 10 mm below the liquid level (5), the amount of liquid in circulation is 4 1, the aim of the process is the oxidation of sulfite ions to sulfate ions.

A first test was carried out with a current of 1A, a second test was carried out without current as a control, and finally a third test was carried out without electrodes. After a test period of 2 hours each, the following fractions of the sulfite ions were oxidized to sulfate ions: 1. attempt 66% by weight 2nd attempt 17% by weight 3rd attempt 5% by weight

Example 2:

One works in the laboratory with an apparatus according to FIG. 2, whereby the anode (2) through a circular graphite fleece of 100 mm Diameter and a thickness of 25 mm is formed extends horizontally in the sump (4) of the glass container (1). The The cathode (3a) is replaced by four Graphite fleece layers with a total height of 100 mm, being at the lower and upper ends of the cathode Stabilizing a polypropylene network is located. The cathode (3a) is immersed 20 mm deep in the sump (4), the diameter the cathode is just like the inside diameter of the Container (1) 120 mm.

An aqueous solution of 4.2 g / l NaOH and 8.5 g / l Na ₂ SO ₃ is kept in circulation in an amount of 4 l and sprayed onto the cathode (3a), the gassing takes place as in example 1. Likewise As in Example 1, three different tests, each lasting 2 hours, are carried out and the following results are found with regard to the proportion of sulfite ions oxidized to sulfate ions: 1. Trial (current 1A) 47% by weight 2nd Attempt (currentless) 9% by weight 3rd Trial (without electrodes) 3% by weight

Claims (6)

1. A process for reacting a gas or gas mixture in the presence of an ion-conducting liquid in an electrochemical cell having at least two electrodes, namely at least one anode and at least one cathode, wherein an externally-applied electrical d.c. voltage acts between the cathode and the anode and a direct current flows through the ion-conducting liquid, characterised in that in the lower region of the cell there is a sump of the ion-conducting liquid, into which the electrodes dip, that at least 20% of the entire surface of at least one of the electrodes is located outside the sump in an upper region through which the gas or gas mixture flows, and that the upper region is sprinkled or sprayed with the ion-conducting liquid, the electrode surface being at least partially wetted, while the gas or gas mixture flows along the electrode surfaces.
2. A process according to Claim 1, characterised in that the gas or gas mixture is oxidised in contact with the liquid and the electrodes.
3. A process according to Claim 1, characterised in that the gas or gas mixture is reduced in contact with the liquid and the electrodes.
4. A process according to Claim 1 or one of the following claims, characterised in that 20 to 95% of the entire surface of at least one of the electrodes is located above the sump consisting of the ion-conducting liquid.
5. A process according to Claim 1 or one of the following claims, characterised in that the surface of at least part of the electrodes is formed to be catalytically effective.
6. A process according to Claim 1 or one of the following claims, characterised in that at least one electrode is in the form of a gas diffusion electrode, the gas diffusion electrode being in contact firstly with a gas chamber through which gas or gas mixture flows and the other side of the electrode being sprinkled or sprayed with the liquid.

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