EP0234256B1 - Process for carrying out hcl membrane electrolysis - Google Patents

Description

The present invention relates to an improved method for performing HCI membrane electrolysis.

According to the prior art, when carrying out the HCI electrolysis, a 20 to 26% strength aqueous hydrochloric acid is fed to the individual cells divided by a diaphragm and combined to form an electrolyzer of 30-45 units from two separate circuits for catholyte or anolyte acid, wherein the anolyte acid flows through all the anode compartments in parallel and at the same time the catholyte acid flows through all the cathode compartments in parallel. 30% hydrochloric acid is fed into both circuits to strengthen the depleted acid (Winnacker Küchler: Chemical Technology, Volume 2, 4th edition 1982, p. 443 ff).

A disadvantage of this diaphragm method is that due to the permeability of the diaphragm for the electrolytes, an exchange of fluid inside the cells that cannot be controlled from the outside takes place. For example, the chlorine dissolved in the anolyte acid is partially expelled there after passing through the catholyte side and thus contributes to the contamination of the hydrogen. Another part is reduced cathodically, which leads to a reduction in the product current efficiency. Partial mixing of the chlorine and hydrogen gases produced in the electrolysis can also occur through the diaphragm.

These disadvantages can be avoided by using an ion exchange membrane instead of the diaphragm.

In DE-A 2 844 499, for example, it has been proposed to use an ion exchange membrane coated on both sides with electrocatalysts and to supply or remove the current via "collector electrodes".

This process, described as a "solid polymer electrolyte" (SPE) system, has the advantage that only one anolyte circuit is required, since the protons discharged on the cathode side migrate from the anode side through the membrane, so theoretically no "depletion" in the catholyte Ion occurs. In practice, however, water of hydration passes over to the cathode side, which must be removed.

A disadvantage of this method, however, is that the current transfer from the collector electrodes to the "working electrodes" lies within the electrolyte and thus defined current transfers are very difficult to set and cannot be controlled from the outside.

Another disadvantage of DE-A 2 844 499 can be seen in the fact that the metal sieves proposed as collector electrodes are only very short in technical application on the anode side and on the cathode side only under cathodic protection. Corresponding sieves made of graphite are very expensive and mechanically not very stable given the size of technical electrolysers.

Reduced oxides from the group of noble metals, which are more or less mixed with graphite, are specified as electrocatalysts. However, these systems are much less stable than graphite under operating conditions. But if, for reasons of durability, graphite has to be used for the collector electrodes and also for the electrocatalytic layers on the membranes, the solid graphite electrodes of the conventional type are no longer a significant disadvantage. In addition, the recurring difficulties in the adhesion of the electrocatalyst to the membrane are avoided .

However, if the electrodes are separated from the membrane, a conductive electrolyte is also required on the cathode side. The conductivity maximum of aqueous hydrochloric acid is known to be at a concentration between 17 and 22% by weight of HCl. Since the concentration of the HCI solution in the cathode compartment decreases as a result of the transport of hydrated water through the membrane, the HCI solution must be renewed.

The object of the present invention is to provide a method for carrying out the HCl electrolysis which does not have the disadvantages of the methods described. This method is intended to combine the advantage of the SPE cell with only one electrolyte circuit with the advantages of an electrolysis cell with a finite electrode / membrane spacing.

This object is achieved in a simple manner by a method for carrying out the HCl membrane electrolysis, in which at least part of the electrolyte leaving the cathode space is fed directly into the inlet of the anode chamber.

The concentrated hydrochloric acid to be electrolyzed is first fed into the catholyte chamber of a cell divided by a cation exchange membrane. The escaping acid, which is diluted by the hydrate water transported with the protons to the cathode side, is then introduced into the anolyte compartment of the cell. In the method according to the invention, the water of hydrate penetrating the membrane is conducted in a short circle, the volume flow of the acid to be returned to the HCl absorption is reduced by this proportion.

• Fig. 1 dient zur Verdeutlichung des erfindungsgemäßen Verfahrens. Die Menge der bei A (Fig. 1) zugeführten konzentrierten Salzsäure wird so geregelt, daß am Ablauf B die entsprechende Menge verarmter HCI abgeführt wird. Für den Fall, daß die Konzentration bei A 30 %, die bei B 18 % beträgt, stellt sich bei einer spezifischen Belastung von 3 KA/m2 in der Katholytkammer eine mittlere Konzentration von ca. 24 % ein.
• Fig. 2 zeigt ein mögliches erfindungsgemäßes Schaltschema für eine Vielzahl von Einzelzellen, wie sie z.B. in einem aus 30-45 Einzelelementen zusammengesetzten Elektrolyseur vorhanden sind.

The method according to the invention is particularly advantageously carried out in such a way that part of the electrolyte leaving the electrolyte spaces is in each case returned to the same electrolyte space.

• 1 serves to illustrate the method according to the invention. The amount of concentrated hydrochloric acid supplied at A (FIG. 1) is regulated so that the corresponding amount of depleted HCl is removed at outlet B. In the event that the concentration at A is 30% and that at B is 18%, an average concentration of approx. 24% is established at a specific load of 3 KA / m2 in the catholyte chamber.
• 2 shows a possible circuit diagram according to the invention for a large number of individual cells, such as those found in an electrolyzer composed of 30-45 individual elements.

The present invention is explained by way of example below, without any limitation of the invention being thereby seen.

example

1 shows a cell (1) according to the invention. An ion exchange membrane (4) of the Du Pont Nafion NX 430 type divides the cell into a cathode compartment (2) and an anode compartment (3). At point A, 0.2 l / h of 30% hydrochloric acid are fed into the catholyte circuit (5) of the cell, the mixture enters the cell from below. A corresponding amount of 21% HCI overflows through line (6) to the anolyte side and is fed into the anolyte circuit (7). The impoverished acid finally leaves the cell at around 18%. At point B, the corresponding amount of HCI is removed from the anolyte cycle so that its volume remains constant. With a specific load of 30 Aldm2 and an electrolyte temperature of about 80 _° C, a cell voltage of 1.9 to 2.1 volts is set, depending on the distance between the electrodes. The chlorine and hydrogen gases formed during electrolysis leave the cell together with the electrolyte; the gas lift effect means that more electrolyte leaves the cell than fresh acid has to be added according to the current strength. This excess amount is immediately fed back to the cell inlet through the corresponding circuit lines (5) and (7). The chlorine and hydrogen products are separated from the corresponding electrolytes in so-called gas separators at points C and D.

Claims (2)

1. Process for perfoming HCI membrane electrolysis, characterised in that the aqueous solution of HCI to be electrolysed is fed into the cathode space and that at least one part of the electrolyte leaving the cathode space is fed directly into the inlet of the anode chamber.

2. Process according to claim 1, characterised in that part of the electrolytes leaving the electrolyte spaces are returned into the same electrolyte space.

Images