DE1206864B - Process for controlling chemical or electrochemical reactions using gas diffusion electrodes - Google Patents

Description

FEDERAL REPUBLIC OF GERMANY GERMAN W7TW PATENT OFFICE Int. Cl ..

EDITORIAL

BOIj

German class: 12 g -1/01

Number: 1206 864

File number: A 38960IV a / 12 g

Filing date: December 7, 1961

Opening day: December 16, 1965

The present invention relates to a method for reaction control of chemical or electrochemical Processes by controlling the composition of the exhaust gases by means of gas diffusion electrodes.

A gas diffusion electrode is a porous body that acts as a partition between two spaces one of which is the liquid electrolyte and the counter electrode, the other the Contains gas to be converted. The gas is under a certain overpressure, around the capillary pressure to counteract with which the electrolyte tries to penetrate into the pores of the gas diffusion electrode. In this way, a three-phase boundary electrode / electrolyte / gas, which is necessary for the electrochemical gas conversion, becomes created.

By changing the pore radius over the electrode cross section in such a way that the fine pores the electrolyte side and the coarse pores are facing the gas space, you can achieve that the three-phase boundary forms in the transition zone between the two layers. More details about the The construction and operation of gas diffusion electrodes can be found at E. Justi, M. Pilkuhn, W. Scheibe and A. Winsel, »Heavy-duty hydrogen diffusion electrodes for operation at Ambient temperature and low pressure «, Dep. D. Math.-Nat. Class d. Akad. Wiss. u. d. Lit., Mainz 8, 1959, Komm.-Verlag Steiner, Wiesbaden.

If a gas mixture is fed to a gas diffusion electrode, it contains the reaction gas as well Contains other gaseous constituents that stick to the electrode because of their indifferent and inert behavior are not implemented, there is a constant increase during the current draw the proportion of inert gas in the pores of the electrode; as a result, the electrode reaction is gradual practically comes to a standstill because the reaction gas, increasingly hindered by the inert gas cushion, diffuses to the three-phase boundary supplying current.

In order to prevent asphyxiation of gas diffusion electrodes, it has been proposed that in the Pores of the electrodes of a fuel element accumulated inert gases after the electrode potential has dropped to a specified minimum value by increasing the gas pressure periodically in the Blow out electrolyte space.

If, however, the gas mixture is allowed to flow through the gas space of the electrode in a suitable manner while the inert gas components are led away, this can be a steady stream for controlling chemical or chemical processes at a constant potential
electrochemical reactions by means of
Gas diffusion electrodes

Applicant:

Varta Aktiengesellschaft, Hagen (Westphalia);

Siemens-Schuckertwerke Aktiengesellschaft,

Berlin and Erlangen,

Erlangen, Werner-von-Siemens-Str. 50

Named as inventor:

Dr. August Winsel, Braunschweig

supply, the strength of which depends on the flow velocity ν of the gas. On the other hand is at constant flow velocity and constant end potential of the electrode current a unique Function of the reaction gas content.

This behavior of the diffusion electrodes can be used to measure the reaction gas content in Use gas mixtures.

In addition, it has now been found that this property can also use for the control of chemical and electrochemical reactions in which a reaction gas containing exhaust gas occurs, the content of reaction gas χ has a specific characteristic for the desired reaction sequence value. The prerequisite is that you control the course of the reaction by a control variable G, such as. B. pressure or temperature can influence such that the reaction gas content χ in the exhaust gas can be adjusted to a certain value.

For control purposes, the exhaust gas from the reactor is fed to a gas diffusion electrode as a control electrode; A very specific concentration χ of the reaction gas in the gas mixture supplied to the electrode is associated with each state, which is determined by the value triple electrode current /, electrode potential φ and flow velocity ν of the gas (at the gas inlet v _E or gas outlet v ^) in the gas space of the electrode. If you want to control a reaction so that the reaction gas χ in the exhaust gas of the reaction vessel has a predetermined value, you only need to change the controlled variable G so that the control electrode is determined by a value triple I _x , φ _χ and v _Ex or v _{Ax is} set, which corresponds to the desired concentration of reaction gas χ. You will be in the

509 758/422

generally hold two of the three variables I _x , φ _χ and V _x and only use the third variable, which is variable due to the respective proportion of the reaction gas χ, to control the reaction. In particular, it is advisable to set the flow rate of the gas at the outlet of the control electrode v _Ax and to use the electrode potential φ in galvanostatic operation or the current / control electrode for reaction control in potentiostatic operation.

In Fig. 1 shows three curves that show the dependence of the electrode current of an oxygen diffusion electrode as a control electrode of the oxygen content of the supplied gas mixture. The parameter is the electrode potential and the flow rate of the gas at the gas outlet:

Curve
No. potential
f [mVl Flow
speed
[Ncms / s]
^V Ax I.
II
III -450
-450
-1000 0.17
0.045
0.014

The reference potential is the rest potential of the diffusion electrode operated with the pure reaction gas.

The manner in which the control of the reaction according to the invention is carried out must be carried out on a case-by-case basis Decide case. Mostly one can chemical reactions by changing the temperature or the Influence the pressure in the reaction vessel. In addition, the reaction can also be controlled by changing the concentration ratios of the reactants in the reaction vessel; for gases e.g. B. right simply by changing the flow rate.

In Fig. FIG. 2 schematically shows the control of a reaction taking place in the container 2 by automatically changing the flow rate of the exhaust gas flowing out of the container 2. At 1 the reactants are introduced in 2, they leave it via the control valve 3 at 4. Part of the exhaust gas flows into the diffusion electrode 6 via the pipe 5 and leaves it at a constant speed at 5 '. The potential of 6 is kept constant with respect to the reference electrode 7, which allows the potential directly in front of the electrode 6 to be measured by the Luggin probe provided, by means of the potentiostat 11 connected via the lines 9 and 10. In this case, a current flows between 6 and the counter electrode 12 via the lines 13 and 14, as a result of which the electromagnet 15 controlling the valve 3 is excited. As a result, 3 is opened or closed to such an extent that the composition of the exhaust gas corresponds to the state characterized by the nominal value of the potentiostatic current I _x.

The method according to the invention is also particularly suitable for controlling the gas conversion in Fuel elements with diffusion electrodes for the electrochemical generation of energy from fuel gases and / or oxidizing gases are equipped.

In this case, all or part of the exhaust gas flowing continuously from one or more gas diffusion electrodes is fed to a further diffusion electrode as a control electrode. The flow rate of the exhaust gas is regulated manually or automatically in such a way that the state of the control electrode is described by a predetermined value triple I _x , φ _χ and v _Ax . Here, too, it is advisable to keep v _Ax and one of the other two variables I _x or <p _x constant so that the variable third variable can be used for control purposes. This procedure agrees with the arrangement according to FIG. 2 if the electrode current / is used for control and the container labeled 2 is viewed as the gas space of a diffusion electrode.

example 1

As a model for controlling a chemical reaction, coal combustion can be used in the event serve that one strives for a certain ratio of carbon monoxide to carbon dioxide. That The ratio of the two oxidation products is known to be greater than that if the residence time is sufficient Coal bed depends on the temperature, which in turn at constant heat conduction ratios of depends on the speed of the air supply.

As characteristic of the composition of the gas is to ensure a complete Oxygen conversion to view the carbon monoxide content. Carbon monoxide can be used at room temperature implemented on gas diffusion electrodes, which in particular contain copper, tungsten or molybdenum but not hydrogen. If in Fig. 2 the reaction space 2 contains the coal bed and at 1 the combustion air is supplied, at 5 a part of the exhaust gas that is transferred is carried out Valve 3 at 4 leaves the reactor, after a carbon dioxide scrubbing of the gas diffusion electrode 6. This sets a current that is characteristic of the composition of the gas. Will now be the The temperature in the reactor is too low. So you have to increase the flow rate of the air supplied and thus of the exhaust gas, like this that the temperature rises and the CO content increases again. The automatic regulation the gas velocity takes place in the manner shown.

If it is a question of controlling an exothermic reaction in the case shown, then must be catalytic reactions, which are endothermic, the reaction temperature due to external heat input be maintained. In such a case, instead of the speed, one becomes the reaction partner supplied regulate the heat supply using the control method according to the invention. So you can then supply an electrical booster heater with power when the gas diffusion electrode is too detects a low concentration of the characteristic component and its concentration when it is increased the reaction temperature increases, or vice versa.

Example 2

An oxyhydrogen battery, consisting of several hydrogen-oxygen cells, is arranged so that the hydrogen as well as the oxygen flows through the electrodes of the individual cells in series. as

Combustion gas becomes synthesis gas with the composition of ammonia, as an oxidizing gas air used. In order to make the best possible use of the reaction gases with perfect functioning of the electrodes To achieve, a part of the exhaust gases is a hydrogen and an oxygen diffusion electrode according to FIG. 2 supplied. In this case too, the reaction is controlled by means of the Exhaust gas speed, which is adapted to the load on the batteries so that the exhaust gas composition is constant. The control is carried out separately for the two reaction gases, so that one as Reactor 2, look at the oxygen electrodes on the one hand, and on the other hand, look at the hydrogen electrodes of the battery Has.

Claims (5)

Patent claims: 1. Process for controlling chemical or electrochemical reactions, in which at least a gaseous component to be converted electrochemically by gas diffusion electrodes is involved, the concentration of which in the exhaust gas by regulating the reaction sequence to one characteristic value can be set, characterized in that this Regulation is carried out continuously by a gas diffusion electrode used as a control electrode. 2. The method according to claim 1, characterized in that the adjustment and keeping constant the characteristic concentration value takes place by means of a controlled variable, which by the value triple flow velocity, electrode current strength and electrode voltage the gas diffusion electrode is controlled. 3. The method according to claims 1 and 2, characterized in that in each case two sizes of the value triple can be kept constant, while the controlled variable automatically functions as a function the third variable of the value triple is controlled in such a way that the concentration of the gaseous Component in the exhaust gas has reached the target value. 4. The method according to claims 1 to 3, characterized in that the controlled variable by the electrode voltage that is set in each case with the specified current intensity and Flow rate working control electrode is set. 5. The method according to claims 1 to 3, characterized in that the controlled variable by the amperage that is set in each case with the predetermined electrode voltage and predetermined flow rate working control electrode is regulated. Considered publications:
German publication No. 1018 946. 1 sheet of drawings 509 758/422 12.65 © Bundesdruckerei Berlin