EP0252172A1 - Electrolysis process - Google Patents

Abstract

An electroltytic process is described by which in order to increase the depletion factory, the material coefficient of the electrolyte passing through an electrolytic cell is increased along the direction of the electrolytic flow.

Description

The invention relates to an electrolysis process, wherein an electrolyte is passed through an electrolysis cell and the mass transfer coefficient is increased by introducing mechanical energy.

In order to achieve electrochemical conversions, for example in aqueous solutions, a wide variety of designs of electrochemical cells are used, for example in the case of smaller concentrations, cells with a fixed bed of graphite granules, metal wool or metal foam or stacks of expanded metals. At larger concentrations one uses in general plate cells. It is also known that the mode of action of a cell, in particular a plate cell, can be improved by increasing the stream transport coefficient. This is achieved, for example, by circulating the medium to be treated at high speed through the cell and then passing the treated liquid in batches or by metering a small volume flow into the system in front of the cell and branching off a corresponding small volume flow after the cell. Other methods of increasing the mass transfer coefficient are mechanical stirring or the introduction of gas into the cell. The rising gas bubbles also increase the mass transfer coefficient. The electrode is also often moved in order to obtain a higher relative movement of the electrolyte and the electrode. This can be done by vibrating the electrode, by swirling a fixed bed or by rotating electrodes. It is also known to mechanically limit the boundary layer in a plate cell Disrupt and break up movement of particles or other bodies and thus increase the mass transfer coefficient.

The use of ultrasound is also known. It is also known that this last method not only has the advantage of the increased mass transfer coefficient, but additionally reduces the gas bubble coverage of the electrode surface in that the gas bubbles are detached better from the surfaces.

A good overview of the pending problems with proposed solutions based on various principles of movement is described in an article that appeared in the journal "Neue Hütte", September 1982, pages 317 - 322. Another article, which appeared in the magazine "Erzmetall", 1974, pages 107-114, describes a detailed solution. An electrolysis process is described here in which the electrolyte is passed through the electrolysis cell. The invention is based on this prior art.

Reference should also be made to a summary article, published in "Quarterly Reviews" 7 (1953), pages 84-101.

In the first two publications mentioned, the mass transfer coefficient is increased by introducing mechanical energy, but in the prior art the entire electrolytic cell has the mechanical vibrations or other means to increase the Mass transfer coefficients exposed, without doing a differentiation according to the electrolyte flow in the electrolytic cell.

With fixed bed cells and also with plate cells, there is the problem regardless of the mass transfer coefficient that the cathodic and anodic overvoltage from the cell inlet to the cell outlet do not remain constant. (We now refer to an electrochemical cell as a unit in which the cathode and anode are each made of one piece and are not segmented in the direction of the electrolyte flow, so that different potentials can be set.)

The shift in the cathodic and anodic overvoltage stems from the fact that the cell voltage is the same at the cell inlet and cell outlet, but the current density is generally much lower at the cell outlet. If the resistance of the treated solution from cell inlet to cell outlet remains essentially the same as is generally the case, the proportion of the ohmic voltage drop also changes with the decreasing current density. Because the cell voltage is constant, the overvoltages must necessarily increase. This is described by the equation
Cell voltage = cathodic overvoltage + anodic overvoltage + local cell resistance x local current

The change in the electrode overvoltages can be very annoying because it leads to the cell outlet in one Comes in the area where undesirable side reactions take place at the electrode. In most cases this is, for example, the production of hydrogen at the cathode.

It is known that when using fixed bed cells this problem can be alleviated by offering a larger fixed bed volume at the cell outlet or by increasing the packing density in another way, e.g. through smaller granulation of the granulate used or a stronger compression of a filling with metal wool or metal foam. In this way, the local current flowing there can then be increased, and the contribution of the ohmic voltage drop becomes somewhat larger again. (See e.g. DE-PS 26 22 497 or 35 32 573.)

The invention is therefore based on the object of proposing an electrolysis process of the type mentioned at the outset, in which the current density at the cell outlet is appreciably increased, without the occurrence of undesirable side reactions.

To achieve this object, the invention is characterized in that the increase in the mass transfer coefficient increases substantially along the direction of the electrolyte flow.

The idea on which the invention is based is therefore to increase the current density at the cell outlet by increasing the mass transfer coefficient there in comparison to the cell outlet, for example by exposing the electrolyte to pressure waves, the intensity of which on Cell outlet is stronger than at the cell inlet. It is evident that in a liquid-filled, open-topped container, the housing of which is caused to vibrate, for example by impacts, the amplitude of the vibration is greater in the upper region than in the lower region, in which the side plates are held together by a base. For example, if an electrochemical cell consists of a rectangular box in which the electrodes are suspended as plates, and if this box is vibrated from the outside by vibrators, the amplitude of the vibration in the upper area of the box is greater than in the lower area. If one now places the cell inlet downwards in the box and the cell outlet upwards, the stream transport coefficient along the direction of the electrolyte flow can be influenced in this way, and this essentially increases. The gas bubbles at the cell outlet can then be reduced or avoided.

It is now possible to achieve a larger depletion factor in a single cell without gas developing at the electrodes. The undesirable side reactions can then be avoided. In many reactions, it can be assumed that the electrochemical conversion almost completely comes to a standstill when gas bubbles begin to cover the electrode surface.

The achievable depletion ratio (= inlet concentration by outlet concentration) is then essentially determined by the concentration up to which the development of gas bubbles in a cell Can avoid side reaction. With the given method one can achieve a considerable advantage, as will be made clear on the following calculation.

The gas bubble development therefore occurs when the difference in the current density at the cell inlet and cell outlet exceeds a certain value G. This current density is proportional to the product of mass transfer coefficient and concentration. If we designate the values for the cell inlet with the index 0, the values for the cell outlet with the index 1, the mass transfer coefficient with K and the concentration with C, then the following inequality must be satisfied so that no undesirable side reaction occurs.
G> K 0 x C 0 - K 1 x C 1

If one can now achieve that the ratio of K 0: K 1 = 1: 2, the possible C 1, the outlet concentration, will obviously be half as large as in the case where K 0 = K 1. That means you can halve the achievable final concentration again. The change in the mass transfer coefficient can easily be much larger and a correspondingly greater reduction in the outlet concentration is then possible.

• Fig. 1 - schematisch in einer Ansicht eine Elektro­lysezelle zur Erläuterung des Prinzips des erfindungsgemäßen Elektrolyseverfahres;
• Fig. 2 - ein Diagramm, wobei als Beispiel über der Länge der Elektrolysezelle in Richtung des Elektrolytflusses (entsprechend der Höhe der Elektrolysezelle in Fig. 1) die Amplitude der hierbei angewendeten Druckwellen aufge­tragen ist.

The invention is explained in more detail below on the basis of an exemplary embodiment from which further important features result. It shows:

• Fig. 1 - schematically in a view an electrolysis cell to explain the principle of the electrolysis process according to the invention;
• 2 shows a diagram, the amplitude of the pressure waves used being plotted as an example over the length of the electrolytic cell in the direction of the electrolyte flow (corresponding to the height of the electrolytic cell in FIG. 1).

1 shows a vessel 1 of an electrolysis cell, in which an electrolyte 2 is located. An anode 3, which is surrounded by a diaphragm 4, and several cathodes 5 are immersed in the electrolyte. The electrolyte is continuously introduced into the vessel 1 via a side inlet 8 in the direction of arrow 9. It leaves the vessel via an overflow at the upper edge of the vessel or via holes provided there or the like. A distributor pipe, not shown and laid over the bottom of the vessel, ensures a uniform distribution of the electrolyte, the direction of flow of which is indicated in the cell by the arrows 10 .

On the left in FIG. 1, to explain the principle of the method according to the invention, it is indicated that several whorls 11, 12, 13, 14, 15 are located one behind the other in the direction of the electrolyte flow 10. The lowermost whisk 11, which is therefore in the vicinity of the electrolyte inlet, is driven at a low speed, while the uppermost whisk 15, which is in the vicinity of the outlet, is driven at the highest speed. The intermediate whorls 12, 13, 14 are driven at a medium speed such that the swirling of the electrolyte caused by the whorls increases in the direction of the electrolyte flow 10.

This representation is only intended to illustrate the principle of the method according to the invention; in practice, the resulting increase in mass transfer coefficient will be carried out in another way, preferably by a vibrator 6. This is attached to the wall of the vessel 1, preferably in the upper region of the wall, so that the sound waves emanating from it have their greatest amplitude in the outlet region of the vessel. One could also fasten several of the vibrators 6 one above the other on the wall of the vessel, in which case the top vibrator is then subjected to a greater amplitude than the bottom one of the vibrators.

Experiments have shown that the excitation of at least one of the walls of the vessel 1 with these vibrators or with at least one of the vibrators is sufficient for the desired effect. It is therefore not necessary to vibrate the entire electrolysis vessel, as was the case in the prior art, quite apart from the fact that this method cannot influence the mass transfer coefficient in the direction of the electrolyte flow.

Other solutions have already been pointed out in the claims, for example that electrodes 3 and / or 5 can also be excited to vibrate, etc. It is common to all principles that the vibration energy introduced into the bath is greater in the area of the outlet than in the area of the inlet.

Fig. 1 also shows that the vibrator 6 emits vibrations in the direction of arrow 7, which are essentially perpendicular to the plane of the plates 3, 5.

2 shows a diagram as a further explanation, the direction of the electrolyte flow 10 being indicated as the abscissa x. The ordinates y are the amplitudes of the mechanical vibrations with which the electrolysis cell is subjected. It also follows from this that a smaller amplitude acts on the electrolyte at the inlet 16 of the electrolytic cell than at the outlet 17. This is shown by the curve 18.

Claims (8)

1. electrolysis process, wherein an electrolyte (2) is passed through an electrolysis cell (1) and the mass transfer coefficient is increased by introducing mechanical energy,
characterized by
that the increase in the mass transfer coefficient along the direction (10) of the electrolyte flow increases substantially. 2. The method according to claim 1,
characterized by
that the increase in mass transfer coefficient is achieved by a relative movement of electrodes and electrolyte. 3. The method according to claim 1,
characterized by
that the increase in the mass transfer coefficient is achieved in that the electrolyte is exposed to pressure waves which propagate essentially perpendicular to the electrode surfaces running parallel to one another. 4. The method according to claim 3,
characterized by
that the difference in mass transfer coefficient along the direction of the electrolyte flow is achieved by changing the amplitude of the pressure wave. 5. The method according to claim 4,
characterized by
that the amplitude of the pressure wave in the area of the cell outlet is at least a factor of 4 greater than in the area of the cell inlet. 6. The method according to claim 4 or 5,
characterized by
that the pressure wave is generated by one or more vibrators which are attached to the housing of the electrochemical cell. 7. The method according to claim 6,
characterized by
that the change in the amplitude of the pressure wave along the direction of the electrolyte flow is achieved by the arrangement of the vibrators and / or the structural design of the cell housing. 8. The method according to any one of the preceding claims,
characterized by
that the desired electrochemical conversion takes place on a fixed bed electrode.

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