EP0020890B1 - Process for dechlorinating the anolyte of an alkali chloride electrolysis cell - Google Patents

Description

The invention relates to a process for the extensive removal of chlorine from the anolyte of an alkali metal chloride electrolysis which leaves hot and saturated with chlorine a pressure electrolysis carried out at a pressure of more than 7 bar.

In the industrial processes for dechlorinating the anolyte of electrolysis cells, which operate under normal pressure, the anolyte is dechlorinated by relaxing it in a container kept under vacuum. During this spontaneous expansion, the dissolved chlorine evaporates, so that a dechlorinated anolyte remains in the vacuum container. The chlorine-containing vapors formed during the evaporation are cooled, the resulting chlorine-containing condensate is pumped back into the anolyte and the portion which is not condensed during the cooling, essentially consisting of chlorine and water vapor, is brought back to normal pressure and then dried. Complete dechlorination of the anolyte is, however, only guaranteed if the vacuum is chosen so low at a given temperature of the anolyte that the anolyte comes to a boil during the expansion. In practice, however, the relaxation often takes place in a simple container, so that due to the mixing effect there is no complete dechlorination. The residual dechlorination is then carried out in such a way that the remaining chlorine is blown out by means of air and the air laden with chlorine and water vapor is cooled, and is introduced into the chlorine destruction system by means of a blower.

The disadvantages of this electrolysis process, which is operated in many variants under normal pressure, are obvious: Since the temperature increases together with the chlorine, disproportionately increasing amounts of water vapor are removed from the cells, which then have to be removed from the chlorine stream by cooling and drying Temperature of the anolyte to max. limited to about 85 ° C. If the temperature is lower, then in order for the anolyte to boil, it must be released into a correspondingly higher vacuum. However, this increases the volume of the vapor, which requires larger apparatus and line cross sections. In particular, the chlorine compressor must be designed for a large intake volume and for higher performance. It must be taken into account here that the parts of the apparatus which come into contact with moist chlorine must be made of expensive special materials because of the risk of corrosion. In addition, the energy consumption in the electrolysis cell increases as the anolyte temperature falls.

The above-mentioned residual dechlorination of the anolyte by blowing in air has the disadvantage that the chlorine-laden air has to be dechlorinated in the chlorine destruction system, which leads to a large amount of hypochlorite.

The extraction of chlorine and salt-free condensate is only possible to a limited extent in the electrolyses operated under normal pressure, since the temperature of the anolyte is only slightly reduced during the expansion with the usual available vacuum systems. A larger part of the heat content of the anolyte can only be used for the evaporation of water if the applied vacuum is significantly improved. However, this means higher technical effort for vacuum generation and also leads to an increase in the vapor volume. The condensate from the vapors also contains chlorine and, in order to be able to continue to be used, would have to be dechlorinated with the help of a second relaxation after warming up or by stripping. However, this is associated with a disproportionately large effort.

Some of these disadvantages can be remedied by carrying out the electrolysis under pressure, since this allows higher anolyte temperatures to be achieved. So it is z. 8. From DE-OS 2 729 589 it is already known to carry out the electrolysis using a cation exchange membrane and at a pressure of 1 -5 ata. Advantages are stated that the cell voltage can be reduced and that the cell temperature can be increased without increasing the cell voltage. Furthermore, when using a cation exchange membrane, the electrolysis can be carried out at a high current density without the membrane being damaged. Furthermore, for the liquefaction of the chlorine, the drive energy required for compression can be reduced or completely saved. The Joule heat of the anolyte generated in the electrolysis cell can be used as a heat source for the concentration of the alkali metal hydroxide.

DE-OS 2729589, however, warns against using pressures of 7 bar or more, since there is otherwise the risk that the membrane cells of the cation exchange membrane will no longer be able to withstand the high operating pressure. According to the information in the cited patent application, the hot chlorine produced is cooled by direct heat exchange with cold alkali chloride solution and cold water. The dissolved chlorine must finally be removed from the water by vacuum treatment. Since the working pressure of the electrolysis is below the condensing pressure of chlorine at room temperature, liquefaction is only possible with the help of a compressor or by using cooling units.

The task was therefore to develop an economical process for processing the products, that arise in the anode compartment of an alkali chloride electrolysis cell. The electrical heat loss should be used as widely as possible and the liquefaction of the chlorine should be particularly easy.

A process has now been found for dechlorinating and cooling the anolyte of an alkali chloride electrolysis cell by means of lowering the pressure, which is characterized in that the electrolysis is operated under a pressure of at least 8 bar in the anode compartment, and the products flowing out of the anode compartment are mechanically operated in a separator separated into anolyte and gases formed, the separated anolyte is released at a temperature which is above the boiling point of the anolyte at atmospheric pressure in a strip column to a pressure which is between atmospheric pressure and 2 bar, with the proviso that under these conditions the Anolyte boils, and then the anolyte freed from chlorine by the expansion is separated from the gas phase formed in the stripping column.

Pressure in the anode compartment of 8-20 bar, in particular 8-12 bar, are preferred. At pressures above about 50 bar, investment and operating costs rise sharply.

The boiling point of the anolyte during relaxation naturally depends somewhat on the current barometer reading ("atmospheric pressure"). At the soye concentrations of the used anolyte which are usually used in alkali metal chloride electrolysis, however, feed temperatures into the stripping column of at least 103 ° C., preferably at least 105 ° C., in particular at least 110 ° C., are sufficient to bring the used anolyte to a boil by depressurization . The feed temperature is preferably max. 140 ° C, especially max. 130 ° C.

When the pressure is released, the dissolved chlorine and water evaporate. At the same time, the anolyte cools down.

Insofar as membrane cells are used for the alkali chloride electrolysis, the problem of the mechanical resistance of the cation exchange membrane mentioned in DE-OS 2 729 589 can be solved even at working pressures of over 8 bar. For example, the membrane can be pressed directly onto an electrode, but preferably the anode. This electrode is then preferably designed openwork, for. B. made of expanded metal. In this way it is achieved that the membrane is supported by the electrode surface, but the circulation of the electrolyte is still sufficient.

It can also be achieved with the aid of an automatic pressure control known per se that the pressure difference between the cathode and anode compartments does not exceed a certain size and additional valves for the removal of chlorine or anolyte, or hydrogen or catholyte, if necessary, are opened.

This pressure difference should be a maximum of 5 bar, better a maximum of 3 bar, even better a maximum of 1 bar, even better a maximum of 0.5 bar, preferably a maximum of 0.1 bar. So that the membrane is pressed against the electrode, the pressure difference should, however, be at least 5 mbar, preferably at least 10 mbar.

The same materials that are used for the construction of normal pressure electrolysis cells can be used in the manufacture of the electrolysis cell, which works at a pressure of over 8 bar, for example titanium for the inside of the anode compartment and steel for the inside of the cathode compartment.

A pressure electrolysis cell, which is particularly well suited for working pressures of at least 8 bar, is the subject of a parallel application with the same priority (EP-AI-0 020 887) by the applicant ("electrolysis apparatus"). It is briefly described in Example 2 (with the associated FIGS. 1 and 2a, 2b).

It is not absolutely necessary to feed the entire amount of anolyte which has been freed of chlorine in the separator into the stripping column. Part of the brine dechlorinated in the separator can also be pumped back into the anode compartment directly or via a cooler, for example in order to increase the internal brine flow and to improve the dissipation of the heat loss from the cells.

The stripping column will generally be designed as a standing cylindrical container, which can contain various internals (e.g. trays or packing layers). However, the strip column can also be designed as a horizontal container. It is only important that no backmixing can take place between the incoming and the outgoing brine and that the brine has sufficient evaporation area available. The evaporation area and the dwell time of the brine in the stripping column must be such that the majority of the chlorine in the column is removed. It is advantageous, but not necessary, to attach a droplet separator to the top of the column in order to retain entrained liquid constituents.

If the temperature at which the anolyte leaves the anode compartment is below the boiling point at atmospheric pressure, it must be heated before it is fed into the stripping column.

To improve the dechlorination, steam can also be blown into the stripping column from below. Installations (e.g. floors or packing) are advantageous for improving the gas exchange between boiling anolyte and steam.

In principle, it is also possible to operate the strip column under reduced pressure, for. B. when the temperature of the dechlorinated anolyte is still below the boiling point at atmospheric pressure. However, the technical effort to create the vacuum and to treat the large gas volumes that arise is considerably.

It is therefore better to operate the electrolysis in such a way that the anolyte already leaving the anode compartment has a temperature which is above the boiling point at atmospheric pressure. The temperature of the anolyte in the cell is preferably at least 90 ° C., preferably 105-140 ° C., in particular 110-130 ° C.

Working pressures of max. 1.5, especially of max. 1.1 bar, preferred.

When the anolyte is boiled in the stripping column, a gas is formed which mainly consists of chlorine and water vapor. In order to simplify the further working up of this gas stream, it is advantageous to condense the main amount of water by cooling. This creates a chlorine-containing condensate, which can be pumped back into the anode compartment of the electrolytic cell, for example by adding it to the brine. The condensation of the water vapor advantageously takes place on cold surfaces, i.e. H. through indirect cooling.

The further work-up preferably consists in adding cold (i.e. colder than the temperature of the gas phase) liquid-aqueous phase to the top of the stripping column and thus removing the main part of the remaining water vapor from the gas phase.

For example, cold catholyte under reduced pressure can be used as cooling medium, which can be obtained from hot catholyte by relaxation and subsequent vacuum treatment. While the water vapor is partially condensed and the chlorine is cooled, the catholyte boils. In this way, the heat of condensation of the water vapor can be used to evaporate the catholyte.

The chlorine-containing condensate obtained can be used, among other things, to sprinkle the internals of the stripping column (packing, trays) from above and thus keep them moist. In this way, the salt mist that occurs when the hot anolyte relaxes is better retained. ,

But you can also from the condensate by blowing in inert gases, for. B. of air, remove the majority of chlorine. However, because of the small amounts of condensate and the additional equipment, this variant is not advantageous for small systems.

The parts that are not liquefied during the condensation (chlorine, water vapor) can be compressed and e.g. B. can be returned to the separator.

The gas phase formed in the stripping column does not have to be freed from the main amount of water by condensation. You can . also feed directly to a neutralization column, in which hypochlorite is produced, or - in the case of smaller plants - to chlorine destruction.

The anolyte largely freed of chlorine in the stripping column can be introduced into a vacuum container and further expanded there. The vapors obtained can be condensed by further cooling. Cooling takes place as soon as the anolyte is let down in the vacuum container. The degree of cooling depends on the level of the vacuum.

The vacuum container can be carried out lying or standing. It is essential that there is a sufficiently large evaporation surface and that backmixing between fresh, warm and cooled brine is avoided.

The chlorine- and salt-free condensate that occurs when condensing the vapors of the vacuum container can be used for many purposes. If the alkali metal chloride electrolysis is operated according to the membrane cell process, it is advantageous to add the chlorine- and salt-free condensate to the catholyte of the membrane cell, for example by introducing it directly into the cathode compartment. The condensate can also be added at the salt dissolver. In both cases, the amount of soft water to be procured is reduced.

If chlorine- and salt-free condensate can be dispensed with, i.e. H. if sufficient salt-free water is available, the second relaxation in the vacuum container is unnecessary.

The latent heat of evaporation which is released during the condensation of the vapors which arise during the expansion in the vacuum container can also be used for the evaporation of the catholyte.

It has been found that by the measures according to the invention, in particular by increasing the anolyte temperature in the cell, in a very economical manner, i. H. with a very low electrical and thermal energy input to a chlorine stream that can be easily liquefied. This liquefaction works without compression work, only by water cooling, without the use of additional cold. Since liquefied chlorine contains very little water in solution at room temperature, the effort for drying the chlorine is also low. The method according to the invention proves to be particularly advantageous in connection with membrane cell electrolysis.

When cells are started up, the anolyte, which leaves the cell at a pressure of at least 8 bar, will generally not yet have reached the boiling point at atmospheric pressure. In this case, the anolyte can be heated, for example, in a heat exchanger or the expansion of the anolyte in the stripping column can be supported by adding steam. This process for dechlorinating the anolyte of the alkali metal chloride electrolysis by lowering the pressure is characterized in that the electrolysis is carried out under a pressure of at least 8 bar Anode compartment is operated, the products flowing from the anode compartment of the electrolytic cell are mechanically separated into anolyte and gases formed in a separator, the separated anolyte is depressurized to a pressure in a strip column at a temperature below the boiling point of the anolyte at atmospheric pressure , which is between atmospheric pressure and 2 bar, the anolyte is treated in countercurrent with steam in the strip column until it boils and the anolyte freed of chlorine by the relaxation and steam treatment is separated from the resulting gas phase. Introducing steam into the strip column causes some dilution of the anolyte. However, this measure may be desirable because water is removed from the anolyte in a membrane electrolysis cell.

A special embodiment of the method according to the invention can be seen in the flow diagram of FIG. (3). The combination of apparatuses shown there is only of exemplary importance, so that, in individual cases, a different circuit and a different embodiment of apparatuses is entirely possible, depending on the circumstances.

The pressure electrolysis cell (4) is divided into anode space (79) with anode (12) and cathode space (89) with cathode (16) by a membrane (14). Fermented brine is pressed into the anode compartment (79) through line (21A). A mixture of H ₂ and catholyte is removed from the cathode compartment (89) through line (21C).

The mixture of depleted brine, chlorine and water vapor coming from the anode compartment (79), which has a temperature of e.g. 110 ° C, is introduced via the line (21D) into the separator {50) with the drip layer (51). This separates the liquid from the vaporous parts. The chlorine-water vapor mixture, which still has a low content of oxygen and inert gases, passes through the drip layer (51) and, under electrolysis pressure, passes through line (52) for further processing, for example drying and liquefaction. The relaxed anolyte (53) obtained in (50) (saturated with chlorine in accordance with pressure and temperature) is drawn off from the separator (50) and opens via the line (54) and the expansion valve (55) in the strip column (56) relaxed a lower pressure (here: atmospheric pressure). This causes the anolyte to boil. In this way, it is completely dechlorinated in the strip column.

The expulsion of the chlorine in (56) can be supported by water vapor, which is supplied via line (57). A particularly good contact between the relaxed anolyte and water vapor is achieved through the packing layer (58). This addition of water vapor - as stated above - is particularly useful if the anolyte temperature has not yet reached the boiling point when starting up a system. The upper layer of the body (59) frees the chlorine / water vapor mixture from brine droplets. The chlorine-water vapor mixture leaves the column (56) via line (60). Part of the steam is deposited in the condenser (61) and the condensate (62) is collected in the collecting vessel (63). A cooling medium (for example cooling water or expanded catholyte which has been further cooled by vacuum evaporation) is introduced through line (64) and leaves the condenser warmed up through line (65).

This chlorine-containing condensate is returned to the electrolysis via line (66), pump (67) and line (68), some of which can be fed to the strip column (56) via line (69). This can ensure that the packed bed (59) of the strip column (56) remains moist and the retention of brine droplets is improved.

The chlorine-water vapor mixture which is not condensed in (63) is passed via line (70), into which the compressor (71) is inserted, into the separator (50). Other parts can be directed via line (72) for hypochlorite production or a liquefaction plant for chlorine.

The brine completely dechlorinated in the strip column (56) is drawn off via line (73) and expanded into the vacuum container (75) via the expansion valve (74). The level of the vacuum in the container (75) depends on the temperature at which the brine (76) concentrated there should leave the container (75), or on the amount of chlorine- and acid-free condensate which is present when the brine is concentrated should be won. The brine cooled in the container (75) leaves it via the line (77). It is pumped back with the help of the pump (78) into the salt dissolver and the brine cleaning (not shown) and finally into the anode compartment (79). The water vapor developed in the container (75) is freed of entrained brine droplets in the drip layer (80) and led via the line (81) to the condenser (82), where water vapor condenses. The condenser (82) can be acted upon via the line (83) with cooling water which, when heated, leaves the condenser again via the line (84); However, it is also possible to use at least part of the large amount of heat obtained for the catholyte evaporation, ie for cooling in (82) lye as a coolant. The condensate generated in (82) is conducted via line (85) to the condensate tank (86) and collected there. Via the line (92) into which the pump (88) is inserted, the condensate (87) can be fed into the line (21B), through which the circulating catholyte is returned to the cathode chamber (89). In this way, the concentration of the catholyte can be kept constant. The condensate (87) can also be fed to the salt dissolver (not shown). By using the vacuum pump (90) via line (91) the condensate container (86) is connected, the vacuum is generated in the condensate container (86) and in the container (75).

example 1

At a selected cell pressure of 10 bar, a cell temperature of 115 ° C, a planned chlorine production of 170,000 tpy, an assumed depletion of the brine from 260 kg to 220 kg NaCI / t brine, a brine run of 825 t / h is calculated, a chlorine production of 20 t / h and a salt consumption of 33 t NaCl / hour. The anolyte, which still leaves the separator at cell temperature, contains 1.2 to 1.6 t / h. Chlorine dissolved; this corresponds to approx. 6 to 8% of the amount of chlorine produced. After condensation of the vapors from the strip column, the 1.2 to 1.6 t / h mentioned remain in the gas phase. Chlorine together with about 0.035 t / h Steam. The condensate of the vapors of the strip column (e.g. 0.5 t / h) contains only a little chlorine dissolved and can be pumped into the salt dissolving station. The brine itself leaves at boiling temperature, i.e. at approx. 107 ° C, the strip column. If a pressure of 400 mbar is maintained when the strip column is expanded into the vacuum container, the dechlorinated brine cools down to about 83'C by evaporation. Here 29 t / h Steam released; if the pressure in the vacuum container is only 520 mbar, the brine only cools down to 90 ° C and 20 t / h evaporate. Steam. The amount of heat generated in the condensation of the vapors is sufficient to evaporate the cell solution, for example from 25% by weight to 50% by weight. In this respect, the use of external steam for the concentration is made unnecessary.

Example 2

• Fig. 1 zeigt eine Ansicht des Elektrolyseapparates teilweise geschnitten.
• Fig.2a zeigt eine Aufsicht a_L ,e druckaufnehmenden Organe des Elektrolyseapparates.
• Fig. 2b die Ansicht Ilb-Ilb der Fig. 2a.

The electrolysis apparatus for the production of chlorine from aqueous alkali chloride solution, which is resistant to pressures of more than 10 bar, has at least one electrolysis cell, the anode and cathode of which are separated from one another by a partition in a housing made of two half-shells, the housing having devices for supplying the electrolysis starting materials and is provided for discharging the electrolysis products, and the partition wall is clamped between the edges of the half-shells by means of sealing elements and is held between power transmission elements made of electrically non-conductive material and extending as far as the electrodes. This electrolysis apparatus is characterized in that the electrodes are supported flatly against one another via spacers which are fastened to half-shells with a substantially circular cross-section and are mechanically and electrically conductively connected via their edges to the half-shells, and the terminal half-shells of the Electrolysis apparatus are supported by pressure-absorbing organs.

• Fig. 1 shows a view of the electrolysis apparatus partly in section.
• 2a shows a top view a _L , e of the pressure-absorbing organs of the electrolysis apparatus.
• Fig. 2b the view Ilb-Ilb of Fig. 2a.

The electrolysis apparatus has at least one electrolysis cell 4. Each individual electrolytic cell 4 essentially consists of the two flange parts 1 and 2, between which the membrane 14 is sealed, and which are held together with the screws 6. The flange parts 1 and 2 are electrically insulated from each other, e.g. B. by means of insulating sleeves 3. In the flanges 1. and 2, the half-shells 9 and 11 are inserted, which line the flanges 1 and 2 from the inside and are pulled with their brims over the sealing surfaces of the flanges 1 and 2. The sealing rings 13 and 15 provide a seal against the membrane 14. The anode 12 and the cathode 16 are secured to the half-shells 9 and 11. The bottoms of the half-shells 9 and 11 of adjacent cells press against one another under the internal pressure of the cells; they can be separated from one another by a film 10 (plastic or metal). Surrounding beads in the half-shells 9 and 11 cause a membrane-like behavior (not shown). The spacers 17 and 18 (electrically conductive bolts), which are used for power supply and power transmission, have on their front side inside the cell power transmission elements 19 and 20, for. B. discs of insulating material, between which the membrane 14 is clamped. The anode 12 and the cathode 16 are fastened to the spacers 17 and 18, respectively. The anolyte and the catholyte are supplied and discharged via lines 21 which are guided radially through the flanges 1 and 2.

The terminal half-shells of the electrolysis apparatus are supported by pressure-absorbing organs. The organs consist of the two plates 7 and the tie rods 8. Instead of the tie rods, the two plates 7 can be connected to hydraulic devices (not shown). The outward-pointing half-shell 9 or 11 of the last cell 4 is supported against the internal pressure of the cell by the plate 7, which may snap into the flange 2 or 1 with a spring 22. The two end plates 7 are pulled together via the tie rods 8, so that the liquid pressure on the half-shells is compensated for via the tie rods. They rest on foot elements 5. In the plates 7 there are threaded bolts 23 which exert pressure on the spacers 17 and 18 when screwed in. The threaded bolts 23 are connected to the power supply lines 24 by means of appropriate devices 25. The supply cables (not shown) are connected to these power supply lines 24 sen. Before starting up the electrolysis apparatus, the individual electrolysis cells 4 are pressed together with the pressure-absorbing member and then the threaded bolts 23 are tightened, so that the electrical contacts are made through the spacers 17 and 18 through all cells. The individual electrolytic cells have an essentially circular cross section, ie the cross section in the electrode plane is circular, elliptical, oval or the like.

Claims (18)

1. Process for the dechlorination and cooling of the anolyte of an alkali metal chloride electrolysis by pressure release, which comprises effecting the electrolysis under a pressure of at least 8 bars in the anode space, separating the products leaving the anode space of the electrolytic cell in anolyte and resulting gases mechanically by means of a separator, depressurizing the separated anolyte with a temperature above the boiling point of the anolyte at atmospheric pressure in a stripping column to a pressure between atmospheric pressure and 2 bars, with the proviso that under these conditions the anolyte is brought to the boil, and separating subsequently the anolyte freed from chlorine by said pressure release from the gaseous phase having been formed in the stripping column.

2. Process as claimed in claim 1, wherein the stripping column contains built-in elements of a large surface.

3. Process as claimed in claim 1, which comprises further depressurizing the brine into a vacuum vessel, after it has left the stripping column, and condensing the resulting vapors by cooling.

4. Process as claimed in claim 1, which comprises effecting the electrolysis at a pressure of from 8 to 20 bars.

5. Process as claimed in claim 1, which comprises reducing the pressure in the stripping column to a maximum of 1.5, preferably a maximum of 1.1 bars.

6. Process as claimed in claim 1, which comprises blowing steam into the stripping column from below, in order to facilitate the dechlorination of the anolyte.

7. Process as claimed in claim 1, which comprises effecting the electrolysis in a way that the anolyte reaches a temperature of at least 90° C.

8 Process as claimed in claim 7, wherein the temperature of the anolyte is in the range of from 105 to 140° C, preferably from 110 to 130° C.

9. Process as claimed in claim 1, which comprises condensing the main amount of water by cooling from the gaseous phase having been formed in the stripping column.

10. Process as claimed in claim 9, which comprises compressing the gaseous phase not having condensed when cooling with water and consisting essentially of chlorine and steam, and recirculating said phase into the separator.

11. Process as claimed in claim 9, which comprises irrigating the stripping column with part of the chlorinecontaining condensate.

12. Process as claimed in claim 1, which comprises effecting the alkali metal chloride electrolysis according to the membrane cell process.

13. Process as claimed in claims 3 and 12, which comprises adding the condensate being free from chlorine and salt and resulting from the condensation of the vapors of the vacuum vessel to the catholyte of the membrane cell.

14. Process as claimed in claim 12, which comprises choosing the pressure in the anode space and the cathode space of the electrolytic cell such that the pressure difference is 5 bars at a maximum.

15. Process as claimed in claim 12, which comprises maintaining a higher pressure in the cathode space than in the anode space and pressing the membrane on the anode.

16. Process as claimed in claim 15, which comprises using an anode of expanded metal.

17. Process as claimed in claim 3 or 9, which comprises using the heat generated in the condensation of the steam or the vapors for the evaporation of the catholyte.

18. Process for the dechlorination of the anolyte of the alkali metal chloride electrolysis by a decrease of pressure, which comprises effecting the electrolysis under a pressure of at least 8 bars in the anode space, separating the products leaving the anode space of the electrolytic cell in anolyte and resulting gases mechanically by means of a separator, depressurizing the separated anolyte with a temperature below the boiling point of the anolyte at atmospheric pressure in a stripping column to a pressure between atmospheric pressure and 2 bars, treating in the stripping column the anolyte in the countercurrent with steam, until it reaches boiling point, and separating the anolyte freed from chlorine by the pressure release and the steam treatment from the gaseous phase having been formed.

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