WO2016159763A1

WO2016159763A1 - Method and device for treating an effluent stream from one or more electrolytic cells

Info

Publication number: WO2016159763A1
Application number: PCT/NL2016/050209
Authority: WO
Inventors: Hendrik Martin Zilvold
Original assignee: Van Den Heuvel Watertechnologie B.V.
Priority date: 2015-03-27
Filing date: 2016-03-25
Publication date: 2016-10-06
Also published as: WO2016159763A9; NL2014542A; NL2014542B1

Abstract

The present invention relates to a method for treating an effluent stream from one or more electrolytic cells, in which electrolytic cells production of a halogen takes place by electrolysis of at least one water-dissolved halide compound. The present invention further relates to a device for treating an effluent stream from one or more electrolytic cells, in which electrolytic cells production of a halogen takes place by electrolysis of at least one water-dissolved halide compound, which device comprises the necessary pumps, valves and piping.

Description

Title: Method and device for treating an effluent stream from one or more electrolytic cells

Description

The present invention relates to a method for treating an effluent stream from one or more electrolytic cells, in which electrolytic cells production of a halogen takes place by electrolysis of at least one water-dissolved halide compound. The present invention further relates to a device for treating an effluent stream from one or more electrolytic cells, in which electrolytic cells production of halogen compounds takes place by electrolysis of at least one water-dissolved halide compound, which device comprises the necessary pumps, valves and piping.

European publication EP 0 1 10 033 discloses a method for refining a brine solution, using an effluent stream drawn from the anode compartment. Said effluent stream is led to an anolyte recirculation vessel and from said recirculation vessel to a dechlorinator, whereupon the thus dechlorinated stream is mixed with concentrated brine, after which the thus treated brine is returned to the anode compartment.

US patent US 4,481 ,088 discloses the formation of an effluent stream from an anode compartment, which effluent stream is subsequently dechlorinated in a dechlorinator, after which the stream thus formed is returned to the anode compartment as anolyte liquor in a number of intermediate steps.

Also the method disclosed in US publication US 2009/0026084 shows that an effluent stream is drawn from the anode compartment, which effluent stream is subsequently dechlorinated, whereupon a number of subsequent steps are carried out and subsequently the thus treated effluent stream is returned to the anode compartment.

US patent US 4, 169,773 discloses an electrolysis process wherein anolyte is recirculated and resaturated before being returned to the anolyte compartment.

US patent US 4,839,003 discloses an electrolysis method wherein magnesium and calcium are removed from a common salt solution before electrolysis takes place.

Devices for carrying out electrolysis of a halide compound in an electrolytic cell are known per se from European patents EP 1 298 231 , EP 0 427 340 and EP 0 958 407 in the name of the present inventor. From EP 1 298 232 there is known a device in which several electrolytic cells are electrically connected in series, which electrolytic cells each comprise a cell element provided with underlying supply pipes for supplying electrolyte and, near the upper side thereof, collecting- discharge pipes for electrolyte and the gases formed during the electrolytic process, a cathode compartment including a cathode and an anode compartment including an anode and a diaphragm or semi-permeable membrane, in which the electrolytic cells have been pressed together between two end plates with a certain bias, so that each anode compartment and each cathode compartment is configured as one unit together with the supply pipes and the collecting-discharge pipes. Other types of electrolytic cells are disclosed in, for example, US patent No. 5,064,514, US patent No. 5,082,543, International publication WO 98/32900 and German Offenlegungsschrift 199 10 639.

The above-mentioned electrolytic cells are mainly used in the preparation, using an electrolytic process, of chlorine gas intended for chlorinating water, for example swimming pool water, drinking-water or waste water. If electrolytic apparatus is used for, for example, swimming pool water, drinking-water or waste water, such apparatus will be operated at varying capacities, because the chlorine demand of such processes varies. Consequently, other electrolytic process parameters will vary as well.

A few of the above-mentioned documents show one or more process steps, viz i) vacuum dechlorination, wherein first the pH must be reduced using hydrochloric acid so as to obtain gaseous chlorine (not in HCIO form) as much as possible, whereupon the pH must be increased again, which process requires the use of an expensive, because of the corrosive properties of chlorine, pump that can draw in chlorine gas, ii) removal of calcium and magnesium through the addition of chemicals and subsequent filtering, iii) concentration of brine and/or NaOH using steam, iv) removal of chlorate. Such process steps usually add to the cost increasing and are complex to execute. In a swimming pool, for example, there is no operator who monitors the process, whilst many swimming pools do not have technical staff present on a continuous basis. Moreover, the use of additional chemicals can be regarded as problematic, in which regard the storage and transport thereof and the use of professional staff must be considered. Usually NaCI (common salt) is used as the starting material for the production of chlorine by means of such electrolytic cells and apparatus. In practice this means that a saturated brine solution, for example in a concentration of about 300 g/l, is for example to the anode compartment (+pole). It is desirable in that regard, mainly for process reasons, that said concentration in the anode compartment does not decrease any further. Because of this condition, a so-called spent anolyte stream still containing a significant brine concentration streams from the anode compartment. The present inventor has thus found that the realised salt efficiency is not 100%.

If an electrolytic process is used for treating swimming pool water, drinking-water or waste water, for example, the spent anolyte stream is usually discharged into the sewage system in practice, and valuable starting materials are thus lost. In addition to that, the present inventor has found that the spent anolyte stream must be considered to be an acidic stream. Chemical analysis of the spent anolyte stream has shown that it also contains residual amounts of chlorine. Furthermore, the spent anolyte stream has a corrosive nature and this solution smells of chlorine.

It is an object of the present invention to provide a method for treating the spent anolyte stream in such a manner that residual streams formed during the production of a halogen in electrolytic cells can be usefully reused.

More in particular it is an object of the present invention to provide a method for increasing the salt efficiency, in particular in a process in which production of a halogen takes place in one or more electrolytic cells by electrolysis of at least one water-dissolved halide compound.

Another object of the present invention is to provide a method for treatment of the spent anolyte stream from one or more electrolytic cells, wherein the use of large amounts of additional chemicals is minimised.

Yet another object of the present invention is to provide a method for treating the spent anolyte stream from one or more electrolytic cells, realising a process by which a precise and efficient control and measurement of the pH and chlorine content can be achieved in spite of the low volume stream of the spent anolyte stream.

The present invention thus relates to a method for treating an effluent stream from one or more electrolytic cells, in which electrolytic cells production of a halogen takes place by electrolysis of at least one water-dissolved halide compound, wherein the method comprises the following steps:

i) drawing an effluent stream from the anode compartment of the above-mentioned one or more electrolytic cells,

ii) mixing the effluent stream obtained in step i) with an aqueous stream,

iii) subjecting the mixed stream obtained in step ii) to the treatment for removing halogen compounds,

iv) supplying the stream obtained in step iii), from which halogen compounds have been removed, to the mixed stream obtained in step ii), in such a manner that a circuit is formed in which the treated stream, from which halogen compounds have been removed, is re-subjected to a treatment for removing halogen compounds after being mixed with the aforesaid effluent stream.

Using the above method, one or more of the objectives are achieved. In particular, by using a treatment for removing halogen compounds, a residual stream which is less corrosive and which causes less odour nuisance is obtained. It will be understood that various effluent streams from various electrolytic cells can be combined, and that the effluent stream thus combined can be further processed according to the present method. In the embodiment in which a brine solution (common salt) is used, components to be removed are for example CIO^" and HCIO and derivatives thereof. The term "remove" is to be understood to include the conversion of components into wanted components, in which connection in particular CIO^" ions are mentioned in the embodiment in which a brine solution (common salt) is used. Such CIO^" ions can in that case be usefully used in the production of halogens for being re-subjected to the electrolysis step in one or more electrolytic cells. Thus an increased salt efficiency is realised. In the present method mention can be made of an extra inner circuit for the step of removing halogen compounds, in particular the dechlorination step. The present inventor has found that due to the "mixing" that takes place in the present method, the effect of slightly too much/too little pH correction is smoothed out because of a large volume stream and a larger volume, viz water, in the circuit. The same applies as regards a possible addition of a dose of hydrogen peroxide, because such a dose will never be absolutely precise, in particular in the situation of a varying chlorine production, as can be the case in swimming pools. Because the vessel in which said mixing takes place is an open vessel, the vessel is not pressurised, which is desirable from the viewpoint of cost. Furthermore it is possible in this way to realise a pressureless connection of the supply line of effluent stream in a simple manner. The term "halogen compounds" is to be understood to include chemical compounds comprising one or more chlorine, fluorine, bromine or iodine atoms. In practice a specific halogen compound, viz common salt (NaCI) is generally used in electrolytic cells. All compounds derived from CI, whether or not in combination with O and H atoms, possibly with traces of other halogens, as mentioned before, are regarded as halogen compounds in the present application.

From the viewpoint of process control it is desirable that the rate of the stream that is being subjected to a treatment for removing halogen compound components is greater than or equal to the rate of the aforesaid effluent stream drawn from the anode compartment of said one or more electrolytic cells. As according to the present invention the rate of flow in the inner circuit is many times greater than the rate of flow of the aforesaid effluent stream, the occurrence of measuring, mixing and homogeneity problems will be minimized.

In a special embodiment it is preferable if the rate of flow in the aforesaid circuit is at least 10 times, preferably at least 50 times, especially preferably at least 100 times, in particular 500 times, greater than the rate of flow of the aforesaid effluent stream drawn from the anode compartment of said one or more electrolytic cells. The use of such settings will lead to the minimisation of the aforesaid measuring, mixing and homogeneity problems.

Suitable treatments for removing halogen include one or more treatments selected from the group of using a catalyst, an adsorption medium, an absorption medium and a halogen compound-neutralising medium. Such treatments can be used as separate treatments, but in a special embodiment it is also possible to use a combination of two or more treatments.

One embodiment of the treatment for removing halogen compounds comprises a step of contacting with active carbon. The active carbon may be present in a column or reactor, to which column or reactor the aqueous stream to be treated is fed. The percentage of halogen compounds in the stream exiting from the aforesaid column or reactor will have decreased in comparison with the stream being supplied. Such columns or reactors are saturated after a certain process time and must subsequently be regenerated. Another embodiment of the treatment for removing halogen compounds comprises a step of adding H₂0₂. The addition of hydrogen peroxide has a neutralising effect on the halogen compounds that are present in the aqueous stream to be treated.

In a special embodiment it is desirable that an aqueous basic stream is supplied to the aforesaid circuit. Such an aqueous basic stream in particular functions to increase the pH of the aqueous stream, in particular if the treatment for removing halogen compounds comprises a medium or a process component that is not capable of resisting aqueous streams with a low pH.

A suitable aqueous basic stream is for example an aqueous stream from the aforesaid one or more electrolytic cells, by means of which a useful integration is thus effected between the electrolytic cells on the one hand and the aforesaid circuit on the other hand.

According to a special embodiment, it is desirable that the addition of said aqueous basic stream takes place on the basis of measurement of the pH value of a stream in the aforesaid circuit, in particular the measurement of a stream before or after the aforesaid treatment for removing halogen compounds has taken place.

Said aqueous basic stream is preferably added to the mixed stream obtained in step ii), so that in this way intensive mixing of the aforesaid streams takes place.

Preferably, a purge stream is drawn from said circuit in order to prevent accumulation of unwanted components and enable the discharge of a residual stream into the environment.

In view of the pH and the concentration of components that cause odour nuisance it is preferable if the transport of said effluent stream drawn from one or more electrolytic cells to the location where step ii) is carried out takes place due to the natural slope. Generally an underpressure prevails in the anode compartment of the electrolytic cell, in particular in order to prevent halogen compounds from undesirably escaping into the environment, so that the natural slope can effect the intended transport. In such an embodiment, the use of pumps, for example, is less desirable.

In a preferred embodiment, a stream drawn from the circuit is added to the aforesaid water-dissolved halide compound before said halide compound is subjected to an electrolytic process. The stream thus drawn in particular contains components that can be subjected to an electrolytic process, by means of which components the so-called salt efficiency of the electrolytic cell can be increased.

To prevent the development of harmful and undesirable gases it is preferable if the transport of said effluent stream drawn from one or more electrolytic cells to the location where step ii) is carried out takes place without contact with the ambient air.

From the viewpoint of process control, quality control and worker safety it is preferable if the performance of said treatment for removing halogen compounds is measured by monitoring the thus treated stream, for example using a redox sensor.

The present invention further relates to a device for treating an effluent stream from one or more electrolytic cells, in which electrolytic cells production of a halogen takes place by electrolysis of at least one water-dissolved halide compound, which device comprises the necessary pumps, valves and piping, wherein said device comprises a pipe for drawing an effluent stream from the anode compartment of said one or more electrolytic cells, which pipe leads to a mixing element, wherein a pipe connects the aforesaid mixing vessel to an element for carrying out a treatment for removing halogen compounds, wherein the discharge pipe of said element is connected to said mixing vessel so as to form a circuit.

It is desirable that the circuit in said device includes a purge pipe, that it includes a pipe that is connected to a container in which the halide compound to be electrolysed is present, and that said circuit is provided with a supply pipe for supplying an aqueous basic stream, in particular that said supply pipe comes from one or more electrolytic cells.

For reasons of process control and process safety it is preferable if one or more sensors are present near said device, which sensors monitor the concentration of halogen compounds in the ambient air.

The present invention is used in particular in environments in which the presence of halogens is desirable, for example as a disinfectant for swimming pools or drinking-water. Examples of halides in particular include chlorides or compounds derived therefrom.

The present invention will now be explained with reference to a drawing, which drawing must not be construed as being limitative, however. Figure 1 is a schematic view of an embodiment of the present invention.

Figure 2 is a schematic view of a special embodiment of the present invention.

Figure 1 shows a schematic view of a special embodiment of the present apparatus 1 , in which an electrolytic cell 2 consisting of two separate compartments 3 and 4 separated from each other by a permeable membrane 5 is used for forming chlorine on the basis of common salt. The use of common salt as a starting material for the electrolytic process is merely mentioned by way of example; other halides, or combinations and mixtures of halides, for example potassium chloride, can also be used. Thus, a brine solution is supplied from the container 13, via a pipe 14, to an anode compartment 3 in which an anode 6 is present. As a result of a chemical reaction, NaCI is converted into chlorine gas that is supplied, via the pipe 9, to a swimming pool, for example, using a venturi pump 10 and a venturi 1 1. In the cathode compartment 4, in which a cathode 7 is present, hydrogen is formed, which hydrogen is discharged via a pipe 8. Furthermore, a lye solution is formed in the cathode compartment 4, which lye thus formed is discharged to a container 15 via a pipe 16. The electrolytic cell 2 is also provided with an aqueous stream 37, which is supplied via the container 36. In the anode compartment 3, so-called spent brine or effluent 12 is supplied to the container 19 in the anode compartment 3, which effluent stream 12 has a low pH and which also comprises chlorine components. In the container 19, in which an amount of water is already present, mixing thus takes place between the effluent stream 12 and the aqueous stream that is present in the container 19. The mixed stream is drawn from the container 19 and supplied, for example via filter 20 and pump 21 , to element 23 for carrying out the treatment for removing halogen compounds. The treatment for removing halogen compounds can result in the formation of a stream 24 in which the percentage of halogen compounds has been reduced to a desired level. Said desired level can be such that a small residual amount of halogen compounds can still be detected in the stream 24. The stream 22 is thus treated in such a manner in element 23 that the halogen compounds present in the stream 22 are moved to a significant extent, wherein a thus treated stream 24 is formed, which may be returned to the container 19 via filter 25, sensors 26-29, valve 30 and valve 32. The circuit formed in this manner thus comprises the aqueous streams indicated at 22, 24, 31 and 34. Examples of sensors 26-29 include sensors for measuring the pH, sensors for measuring the rate of flow, sensors for measuring the chlorine content, in particular the percentage of freely available chlorine. Furthermore it is desirable that one or more sensors that monitor the concentration of halide compound in the ambient air be present in the space in which the present device is disposed. When a specified limiting value is exceeded, the operation of the device is stopped, for example. The element 23 may comprise a container with active carbon, for example, wherein the percentage of freely available chlorine that is present in the stream 22 after passage through the element 23 will have decreased, viz the stream 24. The element 23 may also comprise a hydrogen peroxide metering device, for example, in which case one of the sensors 26-29 measures the percentage of freely available chlorine in the stream 24, and is said percentage of freely available chlorine is too high, hydrogen peroxide will be added, as a result of which the percentage of freely available chlorine will decrease to the desired value. In the figures, the sensors 26-29 are disposed downstream of the element 23, but (some of) the sensors 26-29 may also be disposed at an upstream position.

The valve 30 may be set so that part of the stream 24 is returned to the container 13 via a pipe 35. The valve 30 may be set so that part of the stream 24 is returned to the container 19 via streams 31 and 34. The valve 32 may be set so that part of the stream 31 is drawn from the aforesaid circuit as a purge stream 33.

In figure 1 it can also be discerned that formation of an aqueous basic stream takes place in the cathode compartment 4, wherein an aqueous basic stream is drawn from the container 15 via a pipe 17 and subsequently supplied to a container 19. The container 19 is also provided with a level sensor 18, by means of which the level in the container 19 is measured. If the level in the container 19 falls below or rises above a predetermined value, the sensor 18 will respond to this and the level in the holder 19 will be restored to the desired value, for example in that an aqueous stream (not shown) is supplied to or drawn from the container 19.

The present inventor has carried out experiments using the setup shown in figure 1 , setting the flow of the effluent stream 12 at a value of, for example, 3 l/h. The speed at which the pump 21 in the aforesaid circuit was controlled was such that the velocity of the aqueous stream, indicated at 22, was about 1000 l/h. In such an embodiment it is possible to set the valve 30 such that the flow through the pipe 35 is for example 2.7 l/h, in which case the valve 32 will be set so that the volume flow, discharged as a purge stream 33, is for example 0.3 l/h. Said values are only mentioned by way of illustration and are in no case to be construed as limitative. The presence of filters and pumps as shown in the figure are not to be construed as limitative.

The pipe exiting from the anode compartment 3, which extends to the container 19, opens into the container 19 in such a manner that contact with the outside air is minimized. Furthermore, an underpressure is created in the anode compartment 3 during operation, so that the extent to which halogen components can escape as a gas is minimised as well. For the transport of the effluent stream 12 from the electrolytic cell 2 to the container 19 it is desirable that the electrolytic cell 2 is disposed at a higher level than the container 19, so that the transport of the effluent stream 12 can take place due to the natural slope.

The numerals used in figure 2 are derived from figure 1 . The device 1 , in which an electrolytic cell 2 consisting of two separate compartments 3 and 4 separated from each other by a permeable membrane 5 is used for forming chlorine on the basis of common salt, as shown in figure 1 , is different from the device 55 shown in figure 2 as regards the "inner circuit". The inner circuit in the device 1 is made up of items 19-21 -22-23-24-25-26-27-28-29-30-31-32-34, whereas the inner circuit in the device 55 is made up of items 19-21 -22-23-24-25-26-27-28-29-31. According to the device 55 shown in figure 2, an aqueous stream is drawn from the container 19, in which container mixing of streams 31 , 12 and 17 takes place, which aqueous stream is supplied to the element 23. After being treated, the stream 24 thus obtained is returned to the container 19 as stream 31 . The pump 21 is driven at a high speed, such that the rate of flow in the aforesaid inner circuit is significantly higher than the rate of flow of the stream 12 exiting from the electrolytic cell 2, which stream 12 is directed to the container 19. In the device 55, as shown in figure 2, a stream 51 is drawn from the container 19 and returned to the container 13. The stream 50 can be regarded as a purge stream. The rate of flow in the inner circuit is preferably many times higher than the rate of flow of the effluent stream exiting from the electrolytic cell(s).

The present invention is in particular suitable for so-called in situ electrolysis apparatus with a maximum chlorine production in the 0.05 - 100 kg/h range, in particular 0.2 - 10 kg/h. For such electrolysis apparatus the following process windows apply: maximum brine flow l/h ranging from 0.625 - 1250, in particular 4 - 80 l/h, maximum anolyte flow: 0.625 - 1250, in particular 4 - 80 l/h, maximum purge l/h ranging from: 0.0625 - 125, in particular 0.4 - 8 l/h, circulation across tank (19) m³/h: ranging from 0.1875 - 375, in particular 1 - 30 m³/h.

The above descriptions of the figures are purely meant by way of illustration and are not to be construed as limiting the scope of the present invention, which scope is defined in the appended claims.

Claims

1 . A method for treating an effluent stream from one or more electrolytic cells, in which electrolytic cells production of a halogen takes place by electrolysis of at least one water-dissolved halide compound, characterised in that the method comprises the following steps:

ii) mixing the effluent stream obtained in step i) with an aqueous stream,

2. A method according to claim 1 , characterised in that the rate of the stream that is being subjected to a treatment for removing halogen compounds is greater than or equal to the rate of the aforesaid effluent stream drawn from the anode compartment of said one or more electrolytic cells, in particular that the former rate of flow is at least 10 times, preferably at least 50 times, especially preferably at least 100 times, in particular 500 times, greater than the rate of flow of the aforesaid effluent stream drawn from the anode compartment of said one or more electrolytic cells.

3. A method according to any one or more of the preceding claims, characterised in that the treatment for removing halogen compounds includes one or more treatments selected from the group of using a catalyst, an adsorption medium, an absorption medium and a halogen compound-neutralising medium.

4. A method according to claim 3, characterised in that the treatment for removing halogen compounds comprises a step of contacting with active carbon.

5. A method according to claim 3, characterised in that the treatment for removing halogen compounds comprises a step of adding H₂0₂.

6. A method according to any one or more of the preceding claims, characterised in that an aqueous basic stream is supplied to the aforesaid circuit.

7. A method according to claim 6, characterised in that said aqueous basic stream is obtained from the aforesaid one or more electrolytic cells.

8. A method according to one or both of claims 6 - 7, characterised in that the addition of said aqueous basic stream takes place on the basis of measurement of the pH value of a stream in the aforesaid circuit, in particular the measurement of a stream before or after the aforesaid treatment for removing halogen compounds has taken place.

9. A method according to one or more of claims 6 - 8, characterised in that said aqueous basic stream is added to the mixed stream obtained in step ii).

10. A method according to any one or more of the preceding claims, characterised in that a purge stream is drawn from the aforesaid circuit.

1 1 . A method according to any one or more of the preceding claims, characterised in that the transport of said effluent stream drawn from one or more electrolytic cells to the location where step ii) is carried out takes place due to the natural slope.

12. A method according to any one or more of the preceding claims, characterised in that a stream drawn from the circuit is added to the aforesaid water- dissolved halide compound before said halide compound is subjected to an electrolytic process.

13. A method according to any one or more of the preceding claims, characterised in that the transport of said effluent stream drawn from one or more electrolytic cells to the location where step ii) is carried out takes place without contact with the ambient air.

14. A method according to any one or more of the preceding claims, characterised in that the performance of said treatment for removing halogen compounds is measured by monitoring the thus treated stream.

15. A method according to claim 14, characterised in that said monitoring takes place by using a redox sensor.

16. A device for treating an effluent stream from one or more electrolytic cells, in which electrolytic cells production of a halogen takes place by electrolysis of at least one water-dissolved halide compound, which device comprises the necessary pumps, valves and piping, characterised in that said device comprises a pipe for drawing an effluent stream from the anode compartment of said one or more electrolytic cells, which pipe leads to a mixing vessel, wherein a pipe connects the aforesaid mixing vessel to an element for carrying out a treatment for removing halogen compounds, wherein the discharge pipe of said element is connected to said mixing vessel so as to form a circuit.

17. A device according to claim 16, characterised in that said circuit includes a purge pipe.

18. A device according to one or both of claims 16 - 17, characterised in that said circuit includes a pipe that is connected to a container in which the halide compound to be electrolysed is present.

19. A device according to one or more of claims 16 - 18, characterised in that said circuit is provided with a supply pipe for supplying an aqueous basic stream.

20. A device according to claim 19, characterised in that said supply pipe comes from one or more electrolytic cells.

21 . A device according to one or more of claims 16 - 20, characterised in that one or more sensors are present near said device, which sensors monitor the concentration of halogen compounds in the ambient air.

14

pipe for drawing an effluent stream from the anode compartment of said one or more electrolytic cells, which pipe leads to a mixing vessel, wherein a pipe connects the aforesaid mixing vessel to an element for carrying out a treatment for removing halogen compounds, wherein the discharge pipe of said element is connected to said mixing vessel so as to form a circuit.