NL2023283B1 - Method for removal of nitrite from an aqueous solution by electrochemical conversion and apparatus therefor - Google Patents
Method for removal of nitrite from an aqueous solution by electrochemical conversion and apparatus therefor Download PDFInfo
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Abstract
Described is a method for removal of nitrite from an aqueous solution by electrochemical conversion in an apparatus for performing electrochemical conversion, the apparatus comprising a first chamber (C1) for receiving a first electrolyte, the first chamber comprising one or more first electrodes (E1) arranged therein, a second chamber (C2) for receiving a second electrolyte, the second chamber comprising one or more second electrodes (E2) arranged therein, and one or more diaphragms (D1-D5) comprising calcium silicate, separating the first electrolyte in the first chamber from the second electrolyte in the second chamber, the method comprising the steps of: introducing a first electrolyte in the first chamber, and the aqueous solution in the second chamber, and applying a potential difference between the first and second electrodes of at least 1.23V, the second electrode or electrodes having the higher potential, thereby converting, in the second chamber, the nitrite present in the aqueous solution into nitrate. An apparatus for such method is described as well, where the diaphragm comprises calcium silicate.
Description
Method for removal of nitrite from an aqueous solution by electrochemical conversion and apparatus therefor
The invention relates to a method for removal of nitrite from an aqueous solution having a nitrite content of at least 100 mg/l N-NO2.
Nitrite is an undesired compound in waste/fertilizer streams, in particular in waste streams, originating from intensive livestock (of i.e. pigs, cattle, poultry), where high amounts of dung are produced. Said dung comprises high levels of inorganic nitrogen compounds, such as ammonium. Stables where such intensive livestock is placed, have a too high level of the odorous ammonia in the air, so that the air must be cleaned before being discharged to the environment. Such removal of ammonia usually takes place by so-called air scrubbers (e.g. biological and/or chemical and/or combinatory scrubbers where both chemical and biological removal is combined), wherein a liquid is brought in contact with the polluted air and as a result, the ammonia dissolves in the liquid. In chemical air washers, the liquid is processed with sulphuric acid, which reacts with the ammonia to form ammonium sulphate. Nevertheless, sulphuric acid is a potential treat for the environment. Process water from chemical air scrubbers has, on molecular basis, a sulphur to nitrogen ratio of about 3, and a pH ranging from 1.3 to 5.0. In biological air washers, bacteria are present that are responsible for oxidation of ammonia into nitrite followed by further oxidation of the nitrite to nitrate. The problem with the biological air scrubbers is that not all of the ammonia is converted to nitrate, resulting in the inorganic nitrogen compounds ammonia, nitrite and nitrate in the residual water. The nitrite level therein can be very high, of up to 12 or even 15 g/l NNO2 or higher, rendering the residual water to be treated as chemical waste. Said residual water can also be concentrated, e.g. by a reverse osmosis or evaporation method. Resulting in levels of 35, 50 g/l N-NO2 or higher. There is therefore high interest in removing the nitrite from the said residual water. Preferably the nitrite is converted into nitrate, converting the residual water from a chemical waste product to a valuable fertilizer. The term N-NO2 refers to the elementary nitrogen content in NO2,
i.e. only accounting for the weight of the nitrogen without the oxygen. Accordingly, 1 g/l NO2 would correspond to 0.30 g N-NO2.
It has now been found that high concentrations of nitrite can be converted in nitrate by electrochemical techniques, i.e. using an apparatus for electrochemical conversion, comprising
a. a first chamber for receiving a first electrolyte, the first chamber comprising one or more first electrodes arranged therein,
b. a second chamber for receiving a second electrolyte, the second chamber comprising one or more second electrodes arranged therein, and
c. one or more diaphragms, separating the first electrolyte in the first chamber from the second electrolyte in the second chamber, said one or more diaphragms comprising calcium silicate and being resistant to diffusion of the first and second electrolytes therethrough, while allowing passage of electrons, protons and/or hydroxyl ions through the diaphragm from the first chamber to the second chamber or vice versa when a potential difference is applied between the first and second electrodes.
Electrolysis is a very well-known process in which alkaline and PEM (proton exchange membrane) are the two main water electrolysis technologies. Two electrodes are positioned in a solution of an ion containing solution (electrolyte) and an electric current is allowed to be generated between the positive electrode (anode) and the negative electrode (cathode). The positively charged cations (such as protons) from the medium will migrate to the cathode where reduction of the said ions will take place. The negatively charged anions (such as hydroxide) from the medium will migrate to the cathode, where oxidation of said ions will take place. Such electrolysis is for example known in the production of hydrogen (and oxygen) from water, respectively. In order to improve the separation and purity, of the products that are produced at both sides, but also to improve the efficiency and safety, electrolysis apparatuses are often separated by a so-called diaphragm into two chambers, one chamber comprising the cathode, the other chamber the anode.
In the art, it is known to choose the diaphragm material such, that it has low resistance, good chemical and physical stability and high resistance to diffusion of electrolytes between compartments except for transport of the desired current carrying ion, see e.g. Vasudevan (2013) Res. J. Chem. Sci. vol 3(2), 1-3.
In the case of PEM electrolysis, the membrane allows protons to pass therethrough from one chamber to the other, whereas the passage of other ions, such as water and hydroxide, but also ions such as ammonia, nitrite, nitrate, sulphite and sulphate ions is not possible. Such proton exchange membranes are usually very thin (100 to 200 pm) and are gas tight. An example of such membrane is Nation®. PEM electrolysis is therefore costly and, because of the vulnerability thereof, is difficult to be used in large scale apparatuses.
Although the use of PEM membranes a diaphragm is possible, these membranes are very fragile and less suitable for large scale industrial processes where large volumes of electrolytes are subjected to electrochemical conversion. However, large scale electrolysis of is up to now commercially less feasible in view of operational costs, in particular in view of the electrical current to be applied and the costs for the diaphragms. Further, as the membranes are fragile and less pressure resistant, it is difficult to use these in larger formats with larger working volumes.
It was surprisingly found that calcium silicate plates, presently used as insulation and fire resistant material in the building industry and available in the form of bricks, conical and plate materials, has the required characteristics to be used as a diaphragm material in the method of the present invention, i.e. protons, electrons and hydroxide ions are allowed to pass, whereas passage of other ions is limited as much as possible. As the calcium silicate has a significant intrinsic rigidity, large surface can be provided from this material in a relatively cost-effective manner, allowing for large surface areas being available between the first and second chambers of the apparatus, resulting in a low ohmic resistance. Therefore, this high surface area results in lower voltages and less currency needed for the electrochemical conversion. This is particularly advantageous in large scale electrochemical conversion where large volumes of the aqueous solution are to be cleared from nitrite. Therefore, in the method of the present invention, an apparatus is used wherein the diaphragm comprises calcium silicate. An example of a suitable calcium silicate material is PROMASIL ® - 1100 from Promat GmbH, Germany.
In alkaline electrolysis, not only hydrogen, but also hydroxide and water are allowed to pass. In contrast to the PEM membranes, the diaphragm does not necessarily prevent the product gasses from cross-diffusing through it.
According to the invention, a method is provided for removal of nitrite from an aqueous solution having a nitrite content of at least 100 mg/l N-NO2, comprising the step of subjecting the said aqueous solution to an electrochemical conversion in an apparatus for performing electrochemical conversion as described above, wherein
i. a first electrolyte is introduced in the first chamber, allowing the first electrolyte being in contact with the first electrode, ii. the aqueous solution is introduced in the second chamber, allowing the aqueous solution being in contact with the second electrode, iii. a potential difference is applied between the first and second electrodes of at least 1.23V, the second electrode or electrodes having the higher potential, thereby converting, in the second chamber, the nitrite present in the aqueous solution into nitrate.
The aqueous solution, i.e. the second electrolyte, is introduced in the second chamber where the second electrode, that is positively charged i.e. the anode, is present. As discussed above, the said aqueous liquid preferably comprises the process liquid from the above-mentioned air scrubbers, in particular from biological air scrubbers, but can be any liquid having an increased nitrite level of at least 100 mg// N-NO2.By the application of a potential difference of at least 1.23V, the nitrite in aqueous solution can be converted, i.e. oxidized into nitrate. Surprisingly, it was observed that the ammonium present in the aqueous liquid was oxidized to a much lesser extent than expected. By the present method, all nitrite present in the aqueous medium can be oxidized to nitrate, whereas only about a quarter of the ammonium present in the aqueous medium is oxidized at the moment that all the nitrite is oxidized to nitrite. This is a great and surprising advantage, as ammonium is a valuable component in a fertilizing medium, so removal of ammonium by oxidation is less desired. Without the wish to be bound to any theory, it is believed that the oxidation of nitrite is mediated by the hydrolysis of water in the second chamber, where water molecules converted into oxygen radicals, which radicals would react with nitrite to form nitrate.
The first electrolyte can be any suitable electrolyte that is capable of conducting electrical current and can be identical to the aqueous solution as present in the second chamber, but can also be a salt solution, such as sodium chloride, or a hydroxide solution, such as sodium or potassium hydroxide. The skilled person will readily be able to choose a suitable electrolyte for the first chamber.
The aqueous solution has an elevated nitrite content of at least 100 mg/l NNO2. The method of the invention however is very suitable to remove nitrite, i.e. to convert nitrite into nitrate, from solutions having an elevated nitrite content, such as found in e.g. process water of biological air scrubbers or concentrates thereof. The nitrite content is therefore preferably at least 250 mg/l N-NO2, preferably at least 500 mg/ N-NO2I, more preferably at least 1 g/l N-NO2, more preferably at least 3.5 g/l N
NC>2, more preferably at least 5 g/l N-NO2, more preferably at least 10 g/l N-NO2, most preferably of at least 12 g/l N-NO2. However, any aqueous medium having an elevated nitrite content can be subjected to the method of the resent invention.
As according to the method nitrite has been observed to be preferentially oxidized into nitrite above the oxidation of ammonium, the aqueous medium has, in an attractive embodiment, an ammonium content of at least 500 mg/l N-NH3, preferably of at least 1 g/l N-NH3, more preferably of at least 2 g/l N-NH3, more preferably of at least 3.5 g/l N-NH3, most preferably of at least 5 g/l N-NH3. Such high levels of ammonium occur in process water of air scrubbers, in particular of biological air scrubbers, and by converting the nitrite therefrom into nitrate while leaving a significant portion of the ammonium in the solution, an attractive nitrate and ammonium rich solution can be obtained that can be used as fertilizer, or e.g. as nitrogen source for biological or chemical processes.
It is to be noted that common urban wastewater comprises up to 2 mg/l NNO2, and purification of urban wastewater by electrochemical conversion has never been contemplated and is not feasible. An electrochemical denitrification method for wastewater has been described (Sano et al. (2005) Mitsubishi Heavy Ind. Tech. Rev. Vol 42(4)), wherein the ammonium in the wastewater is intended to be converted into nitrogen gas. To this end, the wastewater is supplemented with chlorine ions to a concentration of 2 g/l, the chlorine ions being converted into CI2, that is converted into hypochlorite and hydrogen chloride. There is no diaphragm separating the apparatus into two separate chambers. In the single chamber, the ammonia reacts with the hypochlorite to mono- and dichloramine, that react into nitrogen gas. At the cathode, hydrogen is formed, and at the anode, chlorine ions are reduced to chlorine gas. However, the said denitrification method removes ammonium from the wastewater, while the conversion of nitrite to nitrate takes place to a much lesser extent. The present invention, however, intends to preferentially oxidize the nitrite into nitrate and to minimize loss of ammonium. Furthermore, the method of the present invention is preferably free of addition of any chlorine to the aqueous medium in the second chamber, and in the said chamber, chlorine does not participate in any relevant electrochemical reaction where a nitrogen compound is involved, nor is chlorine gas produced at the anode. The chlorine content of the aqueous solution of the present invention is preferably below 500 mg/l, more preferably below 250 mg/l, more preferably below 100 mg/ml, more preferably below 70 mg/ml, more preferably below mg/ml, more preferably below 40 mg/ml. In wastewater, the concentration of ammonium is about 84 mg/l N-NH3 and 1.9 mg/l N-NO3.
The aqueous solution may contain sulphur. In process water of chemical air scrubbers, the ratio between sulphur and nitrogen is usually about 3. As sulphur is a chemical pollutant, in the aqueous solution the sulphur content and the nitrogen content on molecular basis is preferably 1 or less, more preferably .5 or less, even more preferably 0.1 or less. Such ratio is e.g. found in process water of biological air scrubbers. The terms ‘sulphur’ and ‘nitrogen’ reflect the molecular sulphur and nitrogen present in the solution, respectively.
The pH of the aqueous solution, at least before step iii., is preferably between
5.5 and 8.5, more preferably between 6 and 8, most preferably between 6.5 and 7.5. Biological air scrubbers produce process water in these pH regions, and it has been shown that the nitrite content of such solutions can be reduced to 0. However, also more acid solutions can be subjected to the method of the present invention.
The aqueous solution is preferably process water of one or more biological air scrubbers or derived therefrom. The term ‘derived therefrom’ encompasses additional elaborations on the process water, such as dilution or concentration. Such process water usually has a nitrite content, ammonium content, chlorine content pH and sulphur to nitrogen content as described above.
In step iii., the potential difference is preferably at least 1.49 V, more preferably at least 3 V, even more preferably in the range of 5 - 70 V, more preferably in the range of 7 - 20V and most preferably in the range of 8 - 15 V. At 1,49V, water molecules are converted into protons and oxygen radicals are that are believed to react with nitrite to form nitrate. However, in order to have a more efficient process, the voltage is preferably increased to the above-mentioned values.
In an attractive embodiment, the pH of the aqueous solution in the second chamber is allowed to go to 2 - 3 during step iii. It was observed that during the electrochemical process, protons are formed in the aqueous solution, resulting in a pH drop. Without the wish to be bound to any explanation, it is believed that this pH drop may have a positive effect on the preferential oxidation of nitrite above ammonium. It is also possible to acidify the aqueous medium during or before step iii. to the envisaged range.
In order to allow the envisaged voltages to be applied without the need to incur high electrical currencies, the apparatus for performing the electrochemical conversion may be improved for improved performance of the method. To this end, the surface of the second electrode being in contact with the aqueous medium and the volume of the aqueous medium in the second chamber is chosen such, that the ratio between the said surface in m2 and the said volume in m3 is as high as possible, in particular 95:1 to 20:1, preferably 70:1 to 30:1, most preferably about 50:1. The term ‘about’ intends to mean to include a deviation in upper and lower direction of the envisaged value by 1%, preferably 2%, more preferably 3%, 5%, 7% or 10%. For example, when the aqueous medium in the second chamber has a volume of 600 litres, i.e.0.6 m3, the surface of the second electrode is preferably 12-42 m2, in particular about 30 m2.
A further adjustment to the apparatus for performing the method of the invention without consumption of excessive energy is to have the ratio between the surface of the one or more diaphragms being in contact with the aqueous solution in the second chamber in m2 and the volume of the aqueous solution in the second chamber in m3 to be as high as possible, in particular to be 1:0.1 to 1:0.5, preferably 1:0.2 - 1:0.4. This means that when the aqueous solution in the second chamber has a volume of 0.6 m3, the surface of the one or more diaphragms is preferably between
1.2 and 6 m2. The surface of the diaphragm can e.g. be increased by placing the first chamber in in the second chamber and designing a plurality of the walls defining the second chamber as diaphragms, i.e. resulting in the first chamber being surrounded by a plurality of diaphragms that are in contact with the first electrolyte of the second chamber. Possible architectures of such apparatus are explained below.
Attractively, the ratio between the volume of the first electrolyte in the first chamber and the volume of the aqueous solution in the second chamber is 1:1 to 1:10. The larger volume of the second chamber allows for increased oxidation of nitrite into nitrate, while limiting the voltage and current needed. For example, in case the aqueous solution in the second chamber has a volume of 600 litres, the first electrolyte in the first chamber preferably has a volume of 60 to 600 litres. Preferably, the above volume ratio is 1:3 to 1: 6.
In order to perform the method in an efficient way, the apparatus is designed such, that the one or more first electrodes are positioned in close vicinity of the surface of the one or more diaphragms being in contact with the first electrolyte. The term ‘close vicinity’ means that a first electrode is arranged such, that the distance (d1) between the said first electrode to the closest diaphragm is less than the distance (d2) between the said first electrode and the closest outer wall of the chamber. Preferably the distance d1 is less than 25% of d2, more preferable less than 10% of d2.
Preferably, the apparatus is designed such, that in use the one or more first electrodes while being in contact with the first electrolyte extend regularly along the one or more diaphragms in horizontal direction. This arrangement of the electrode or electrodes provides for a very efficient and evenly distributed electrical current over the diaphragm, resulting in improved efficiency of the method of the invention. The electrode can e.g. be designed as a conductive strip, extending over a portion, such as the middle portion of the diaphragm. The said strip can also be mounted on the diaphragm. The strip can also extend in vertical direction towards the lower and upper edges of the diaphragm, in order to improve the electrical current over a greater surface of the diaphragm.
The one or more first electrodes, while being in contact with the first electrolyte preferably extend regularly over the surface of the one or more diaphragms facing the first chamber. The electrodes can e.g. be designed as a conductive grid, extending over the surface of the diaphragms, facing to the first chamber, therewith optimizing the currency flow over the surface of the diaphragm.
The method can be performed very efficiently, when the one or more second electrodes comprise a stainless-steel surface, being in contact with the aqueous solution. Although other conductive material can be chosen as well, stainless steel has proven to be stable during the process, also at low pH. In contrast, nickel electrodes, that can also be used, are susceptible to corrosion and wear, resulting in metal deposit in the chamber. Other suitable materials can conveniently be chosen by the skilled person. High end materials known for this purpose, such as titanium, can be used as well, but from a cost perspective, stainless steel is preferred. The same is true for the one or more electrodes of the first chamber.
In another embodiment, the aqueous solution in the second chamber is subjected to ultrasonic treatment, that has been shown to improve the efficiency of the electrochemical conversion. If desired, the electrolyte in the first chamber can be subjected to ultrasonic treatment as well, e.g. when attractive compounds, such as e.g. hydrogen are produced therein.
In yet another embodiment, the aqueous solution in the second chamber is subjected to heating. The aqueous solution in the second chamber is preferably heated to above 50°C up to about 5 °C below the boiling point of the aqueous solution, i.e.
avoiding boiling. Such elevated temperature improves the efficiency of the electrochemical conversion as well. The aqueous solution is preferably heated to about 70°C - 90°C. If desired, the electrolyte in the first chamber can be subjected to heating as well, e.g. when one or more attractive compounds are to be produced in the first chamber as well.
In an attractive embodiment, the volume of the aqueous solution in the second chamber is 500 to 1500 litre, preferably 700 to 1100 litre. However, larger volumes are possible as well, since it was observed that the resistance in the system did not increase significantly.
In a preferred embodiment, the first electrolyte comprises a salt, such as NaCI, or a base, such a KOH. More preferably, the first electrolyte comprises 1-5 w/w% NaCI or KOH, in particular 1-3 w/w% NaCI or KOH.
The invention also relates to an apparatus for performing an electrochemical conversion, comprising:
a. a first chamber for receiving a first electrolyte, the first chamber comprising one or more first electrodes arranged therein,
b. a second chamber for receiving a second electrolyte, the second chamber comprising one or more second electrodes arranged therein, and
c. one or more diaphragms, separating, when present, the first electrolyte in the first chamber from, when present, the second electrolyte in the second chamber, said one or more diaphragms being resistant to diffusion of the first and second electrolytes therethrough, while allowing passage of electrons, protons and/or hydroxyl ions through the diaphragm from the first chamber to the second chamber or vice versa when a potential difference is applied between the first and second electrodes, the one or more diaphragms comprising calcium silicate.
As already discussed above, such apparatus is very well suitable for electrochemical conversion of nitrite into nitrate from a solution having elevated nitrite levels according to the method of the present invention. Because of the presence of diaphragm material of calcium silicate, large scale industrial electrochemical conversion is possible. Herein, large working volumes of e.g. inorganic nitrogen containing residual streams can be envisaged for the production of valuable reduced or oxidized compounds, next to hydrogen.
The apparatus is therefore in general suitable for producing an oxidation product from a constituent of a second electrolyte by electrochemical conversion, wherein the second electrode is designed as anode and the first electrode as cathode, where the constituent in the second electrolyte to be oxidized to the oxidation product in the second chamber and where the oxidation product is optionally collected from the second chamber. In particular, inorganic nitrogen compounds, e.g. from wastewater can be oxidized into nitrate, therewith depriving the waste water from compounds such as ammonium and nitrite. The method can therefore be very well understood as a method for removing undesired compounds from a medium by oxidizing these compounds into desired or less harmful compounds.
It is also possible to simultaneously produce reduction products from compounds, present in the first electrolysis medium from the first chamber.
The apparatus is also suitable for the preparation of a reduction product from a constituent from the second electrolyte by electrochemical conversion using, wherein the second electrode is designed as cathode and the first electrode as anode, comprising the step of allowing the constituent to be reduced to the reduction product in the second chamber and optionally collecting the reduction product from the second chamber. This arrangement is e.g. suitable for the production of ammonia and/or hydrogen from wastewater. When hydrogen is to be produced, the second electrolysis medium can also be water.
The term ‘inorganic nitrogen compounds’ as used herein, is intended to encompass at least ammonia, nitrite or nitrate or a combination of two or more thereof, unless otherwise indicated.
Accordingly, it is also possible to simultaneously produce, by the above method, oxidation products from compounds, present in the first electrolyte from the first chamber.
Preferably, the one or more diaphragms are substantially made of calcium silicate. Such diaphragm has over its full surface the exchange capacity and maximizes the surface between both chambers. However, it is also possible to provide for a diaphragm where only a portion thereof is calcium silicate. The calcium silicate should however extend from the surface of the wall facing towards the first chamber to the surface of the wall facing towards the second chamber in order to allow the envisaged passage of protons, electrons and hydroxyl ions over the diaphragm.
In an attractive embodiment, the diaphragm comprises a calcium silicate plate, and even more attractively, is substantially made of such a plate. The thickness is preferably as thin as possible, in order to provide for a diaphragm with the lowest possible ohmic resistance. However, a certain rigidity is needed to resist the forces exerted to the surface of the diaphragm by e.g. the first and second electrolytes, present in the first and second chamber, respectively. To this end, the thickness is preferably at least 1 cm, more preferably at least 2 cm, even more preferably at least
2.5 cm. It was found that at such thicknesses, the conductivity of the material still allows for attractively low voltage and currency for the envisaged electrochemical conversion to take place, while providing for a rigid wall, that resists large working volumes and pressures without breaking or leaking.
In a particularly attractive embodiment, the second chamber is defined by a plurality of diaphragms arranged within the first chamber. This means that the second chamber is located within the first chamber and limited by a plurality of diaphragms as described above, therewith providing for an increased exchange surface between the electrolyte present in the first chamber and the aqueous solution or second electrolyte in the second chamber. The term ‘electrolyte’ is intended to encompass any liquid medium that is capable of being subjected to electrochemical conversion, i.e. comprising compounds that are susceptible to oxidation or reduction as a result of an electric current. Such a medium can e.g. be a salt solution, but also water, e.g. from domestic, agricultural or industry origin that can be reduced to hydrogen and oxidized to oxygen.
The second chamber can be arranged within the first chamber, sharing a common bottom wall, upon which one round-shaped or oval, or three or more substantially perpendicularly extending separation walls can be mounted, that are connected to one another. Three separation walls will form a triangularly shaped second chamber, whereas four separation walls may provide for a substantially rectangularly shaped chamber. Said walls can comprise or constitute the diaphragm(s). Said walls are therefore preferably made of the calcium silicate as described above. In a very attractive embodiment, the second chamber is defined by a bottom wall and three or more side walls of calcium silicate, the bottom wall being arranged within the first chamber at a distance from the bottom wall of the first chamber. In such an arrangement, the bottom wall of the second chamber also allows for passage of protons electrons through the bottom wall thereof, again increasing the exchange surface, allowing for lower voltage and currencies to be used. In use, such design provides for a pressureless system, wherein a floating box, defining the second chamber, floats in the first electrolyte in the first chamber. While filling the second chamber with electrolyte, the system is levelled automatically and the box slowly sinks to the bottom, while the level of electrolyte in the first chamber inclines accordingly. However, it is also possible to provide for a bottom wall of an impermeable material, such as polyester or polypropylene. In that case, the bottom wall of the first chamber can lie on the bottom wall of the second chamber, so that the first chamber can be removed from the second chamber.
As described above, the one or more first electrodes of the apparatus are positioned in close vicinity of the surface of the one or more diaphragms in order to enable the presence of an envisaged potential difference between the first and second electrodes while limiting the necessary electrical current.
Preferably, as described above, the one or more first electrodes extend regularly along the one or more diaphragms in horizontal direction
Attractively, the one or more first electrodes extend regularly over the surface of the one or more diaphragms facing the first chamber, as described in more detail above.
Attractively, the first electrode comprises a conductive grid, arranged along the surface of the one or more diaphragms facing the first chamber. Such a grid allows for surface increase of the first electrode within the chamber, which is important in increasing the capacity of oxidation or reduction of compounds present in the first electrolysis medium in the said first chamber. The outer boundaries of the chamber are defined by the walls of the first chamber, not being the separation wall or separation walls.
Accordingly, the second electrode preferably comprises a plurality of conductive elements extending through the second chamber, therewith providing a surface increase of the second electrode within the second chamber. Such elements can e.g. be designed as rods or plates that are, in use, submersed in the second electrolyte medium.
The first and/or second electrode can be made of any suitable conductive material, such as a metal. However, not every metal is suitable for any electrochemical conversion. For example, cupper may be less suitable for use anode material as it can easily become oxidized, hampering the further electrochemical conversion process.
Although noble metals such as platinum and titanium are very suitable, stainless steel is preferred as this material is cost effective and has been shown to be an efficient material for electrochemical conversion without being significantly affected by oxidation, reduction or corrosion.
As explained above, in an attractive embodiment, the ratio between the surface of the second electrode in m2 and the volume of the second chamber for receiving the second electrolysis medium in m3 is 20:1 to 70:1, more preferably 30:1 to 60:1, most preferably about 50:1. The term ‘about’ is explained above. For example, when the chamber comprises 600 litres of second electrolysis medium, i.e.0.6 m3, the surface of the second electrode is preferably 12 - 42 m2, in particular about 30 m2. In particular when the second electrode comprises a plurality of conductive elements as described above; the surface of the second electrode is that of the of said conductive elements.
In particular, the ratio between the volume of the first chamber for receiving the first electrolysis medium and the volume of the second chamber for receiving the second electrolysis medium is 1:1.5 to 1:2.5. In this embodiment, the second chamber of the apparatus as disclosed herein is intended for the production of an envisaged compound by oxidation or reduction. The second chamber is filled with aqueous solution or the second electrolyte from which the envisaged compound is to be produced. When the said compound is produced by oxidation, the electrode of the second chamber is designed as anode. If the said compound is produced by reduction, the electrode of the second chamber is designed as cathode. The larger volume of the second chamber allows for increased production of the envisaged compound, while limiting the voltage and current needed. For example, in case the second chamber has a volume to receive 0.6 m3 of second electrolysis medium, the first chamber preferably has a volume to receive 0.24 to 0.40 m3 of second electrolysis medium.
In an attractive embodiment, the ratio between the surface of the one or more diaphragms facing the second chamber in m2 and the volume of the second chamber for receiving the second electrolysis medium in m3 is 1:0.1 to 1:0.5, preferably 1:1.5 to 1:10. Such a high ratio between the diaphragm and the volume of the second chamber can in particular be provided when the second chamber is located within the first chamber, preferably wherein the bottom wall of the second chamber is located at a distance of the bottom of the first chamber, therewith providing an diaphragm surface as explained above. This means that when the second chamber has a volume of 0.6 m3, the surface of the diaphragms facing the second chamber is preferably between 3 and 6 m2. E.g. when the second chamber has a volume to receive 0.6 m3 of the aqueous solution or second electrolyte and is a cube where five of the walls constitute diaphragms, the exchange surface is 3.5 m2, i.e. the above ratio being 1:0.17.
The volume of the second chamber for receiving the second electrolysis medium is preferably 500 to 1500 litre, more preferably 700 - 1100 litre. The first chamber can e.g. be provided by an intermediate bulk container (IBC) having a volume of 1 m3. Herein, a second chamber of 0.6m3 can be placed, defined by 5 calcium silicate walls having a thickness of 2.5 cm, the bottom wall of the second chamber being located at 10 cm from the bottom of the first chamber. The volume of the first chamber for receiving the first electrolysis medium is therewith about 0.3 m3. Such an arrangement has been shown to allow for nitrate production from wastewater by applying a voltage of 30V and a current of 11 A. It is also possible to provide for a first chamber having a square surface of 1.85 x 1.85 m and a height of e.g. 1 m. Herein, a first chamber can be arranged, made having a bottom plate of e.g. calcium silicate of 1.25 x 1.25 m and side walls, also of calcium silicate having a height of 70 cm. The first chamber can contain 1000 I (1m3) of the aqueous solution as described above or second electrolyte. If the first chamber is held on the bottom of the second chamber, only the side walls function as diaphragms, and the volume of the second electrolyte will be about 300 I when the liquid surfaces of both first and second electrolyte are at the same level. It is also possible to provide for a ‘floating’ arrangement, where the second chamber is kept at a distance of the bottom of the first chamber, therewith allowing the bottom of the second chamber to function as diaphragm. If the second chamber is e.g. held at a distance of 20 cm from the bottom of the first chamber, the volume of the first electrolyte in the first chamber will be about 575 I when the when the liquid surfaces of both first and second electrolyte are at the same level.
The apparatus of the invention may attractively comprise heating means in the first and/or second chamber, for heating the first and/or second electrolytes, respectively, in order to expedite the electrochemical conversion.
Similarly, the apparatus of the invention may comprise means for generating ultrasound waves in the first and/or second chamber that result in acceleration of the electrochemical conversion in the chambers.
The invention will now be further described by the following figures and examples, wherein figures 1A, 2A, 2C, 3A, 3C, 4A and 4C shows horizontal cross sections of different embodiments of the apparatus used in the method of the invention and figures 1B, 2B, 2D, 3B, 3D, 4B and 4D show side views of vertical cross sections of the same embodiments, respectively. For all figures, the same reference numbers will be used for the same features.
In figure 1A, a cross section of a basic apparatus 1 is depicted for performing the electrochemical conversion according to the invention viewed from above. The same apparatus is shown from the side in figure 1B. The apparatus 1A-B has an outer wall W and a separation diaphragm D1, defining a first chamber C1 and a second chamber C2. In the first chamber C1, an electrode E1 is positioned, and in the second chamber C2 an electrode E2. The first chamber C1 comprises electrolyte 1, whereas chamber C2 comprises the aqueous solution (electrolyte 2). The electrodes are designed as rods but can have any other suitable form or material (e.g. plated, stirring jackscrew). Heating of electrolytes in chamber C1 and C2 or before entering chamber C1 and C2 via heating elements can be performed for the desired purpose (not shown). Bonification of electrolytes in chamber C2 is possible via sonification elements (not shown).
In figure 2A, and 2C, cross sections of two embodiments of an apparatus of the invention is shown, viewed from above. The same apparatuses are shown from the side, as shown in figure 2B and 2D, respectively. Chamber C2 is arranged within a first chamber C1, the chamber C1 having an outer wall W. The separation diaphragm D1 is a continuous round diaphragm, defining chamber C2 therein. Electrode E2 is positioned in the centre of chamber C2. In chamber C1, electrodes E1 are positioned around diaphragm D1. The first chamber C1 comprises electrolyte 1, whereas chamber C2 comprises electrolyte 2. The electrodes can be designed as rods or any other suitable form or material (e.g. plated, stirring jackscrew). In the apparatus of figures 2A and 2B, the diaphragms D1-4 are attached directly to the bottom of chamber C1. However, the said bottom can also be made of an impermeable material such as polypropylene, and the said bottom can rest on the bottom of the first chamber. In the apparatus of figures 2C and 2D, the bottom diaphragm D5 of the chamber C2 defines the bottom thereof. Again, heating elements for heating the electrolytes in chamber C1 and C2 or before entering chamber C1 and C2 may be present (not shown).
Sonification elements for sonification of electrolyte 2 can be present in chamber 2 as well (not shown).
In figures 3A and 3C, cross sections of two other embodiments of an apparatus of the invention is shown, viewed from above, wherein the chamber C2 is arranged within chamber C1. The chamber C1 having an outer wall W. Chamber C2 is defined by three separator diaphragms D1-D3, defining a triangular shaped cross section of the chamber C2. Electrode E2 is positioned in the centre of chamber C2. Close to the outer surface of diaphragms D1-3, i.e. facing the first chamber, a grid shaped first electrode E1 is positioned in chamber 01. The same apparatuses are shown from the side, as shown in figures 3B and 3D, respectively. The first chamber C1 comprises electrolyte 1, whereas chamber C2 comprises electrolyte 2. The electrodes can be designed as rods or any other suitable form or material (e.g. plated, stirring jackscrew). In the apparatus of figures 3A and 3B, the diaphragms D1-3 are attached directly to the bottom of chamber C2. In the apparatus of figures 3C and 3D, the bottom diaphragm D5 of the chamber C2 defines the bottom thereof. However, the said bottom can also be made of an impermeable material such as polypropylene, and the said bottom can rest on the bottom of the first chamber. Again, heating elements for heating the electrolytes in chamber C1 and 02 or before entering chamber 01 and 02 may be present (not shown). Sonification elements for sonification of electrolyte 2 can be present in chamber 2 as well (not shown).
In figures 4A and 40, a cross section of two other embodiments of apparatuses of the invention are shown, viewed from above. Chamber 02 is again arranged within chamber 01. Chamber 02 is defined by 4 or more separation diaphragms, of which diaphragms D1-D4 are shown here, who define a rectangular or polygonal shape of chamber 02. Within chamber 02, 38 plates as electrodes 02 are positioned within chamber 02. Chamber 01 is limited by the outer wall W, defining the outer boundaries of chamber 01. Close to the surface of diaphragms D1-4 facing the first chamber, a grid shaped electrode E1 is positioned in chamber 01. The same apparatuses are shown from the side, as shown in figure 4B and 4D respectively. The first chamber 01 comprises electrolyte 1, whereas chamber 02 comprises electrolyte 2. Electrode E2 is shown to comprise plate shaped elements, extending through chamber 02. In the apparatus of figures 4B and 4D, the diaphragms D1-4 are attached directly to the bottom of chamber 01. In the apparatus of figures 40 and D, the bottom diaphragm D5 of the chamber 02 defines the bottom thereof. However, the said bottom can also be made of an impermeable material such as polypropylene, and the said bottom can rest on the bottom of the first chamber. Electrode E1, designed as a grid, extends over the outer surface of the diaphragms D1-4 facing the first chamber, along the circumference thereof. The electrodes can be designed as rods or any other suitable form or material (e.g. plated, stirring jackscrew). As explained above, electrolyte 2 can be residual water, or process water of biological air scrubbers, and electrode E2 can be designed as anode where nitrate is formed from the nitrite present in the residual water or process. Electrolyte 1 can be a salt or brine solution, such as a NaCI or KOH solution, i.e.3 wt.%. However, it is also possible for electrolyte 1 to be the same residual water as present in chamber C2. Heating of electrolytes in chamber C1 and C2 or before entering chamber C1 and C2 via heating elements (not shown) can be performed for this purpose. Sonification of electrolytes in chamber C2 is possible via sonification elements (not shown).
Example 1
In order to remove the nitrite in biological air scrubber water, a 22L system as depicted in figure 1A and 1B is made. In this basic setup, an ultrasonic cleaning apparatus (Multi-Clean Premium 550 from joke Technology GmbH) is used. This apparatus has next the option to heat via heating elements, also the possibility for ultra-sonification via ultrasonic elements (25Hz or 45Hz). Ultra-sonification (both, 25Hz and 45Hz) in C2 increases the conversion rate of nitrite to nitrate, however this feature is not used, due to energy consumption considerations. Used parameters (W, D1, E1, E2, Volumes, Temperature, Voltage, Amperage, Power) are shown in table 1.
The conversion rates are shown in figure 5.
Table 1: Setup parameters of apparatus for removing nitrite in 22L biological air scrubber water according figures 1A and 1B
| Electrolyte 1 | 3% NaCI solution |
| Volume electrolyte 1 (L) | 22 |
| Electrolyte 2 | Biological air scrubber water |
| Volume electrolyte 2 (L) | 22 |
| Temperature (°C) | 60-70 |
| Frequency sonification (kHz) | NA |
| Power sonification (Weft) | NA |
| Voltage (V) | 11.3 |
| Current (A) | 11.6 |
| Electrical Power (W) | 131.08 |
| Anode (E2) | Jackscrew; Stainless Steel (SAE grade 304) |
| Stirrer anode (rpm) | 104 |
| Surface area of anode | 19 plates; 1.0mm; 0.67 m2 |
| Specific power anode | 0.195 W/ cm2 |
| Cathode (E1) | Jackscrew; Stainless Steel (SAE grade 304) |
| Stirrer cathode (rpm) | 104 |
| Surface area of cathode | 19 plates; 0.67 m2 |
| Specific power anode | 0.195 W/ cm2 |
| Diaphragm surface area (D1; m2) | 0.116 |
| Diaphragm thickness (D1; mm) | 15 ((PROMASIL®- 1100 from Promat GmbH) |
| Diaphragm material (D1) | Calcium silicate |
| Wall material (W) | Stainless Steel (V2A grade) |
Example 2
In order to remove the nitrite in biological air scrubber water, a 300L system as depicted in figure 4C and 4D is made. In this setup, this apparatus has next to the option to heat via heating elements, also the possibility for ultra-sonification via ultrasonic elements (30Hz). Ultra-sonification in C2 increases the conversion rate of 15 nitrite to nitrate, however this feature is not used, due to energy consumption considerations. Used parameters (W, D1, E1, E2, Volumes, Temperature, Voltage, Amperage, Power) are shown in table 2. The conversion rates are shown in figure 6.
Table 2: Setup parameters of apparatus for removing nitrite in 300L biological air scrubber water according to 4C and 4D
| Electrolyte 1 | 3% NaCI solution |
| Volume electrolyte 1 (L) | 250 |
| Electrolyte 2 | Biological air scrubber water |
| Volume electrolyte 2 (L) | 300 |
| Temperature (°C) | 52-70 |
| Frequency sonification (kHz) | NA |
| Power sonification (Weft) | NA |
| Voltage (V) | 9.4 |
| Current (A) | 107 |
| Electrical Power (W) | 1005.8 |
| Anode (E2) | Plates fixed on threaded wire (Stainless Steel; SAE grade 304) |
| Stirrer anode (rpm) | NA |
| Surface area of anode | Plates; 0,5mm; 28.5 m2 |
| Specific power anode | 0.0035 W/ cm2 |
| Cathode (E1) | Wired grid; Stainless Steel (SAE grade 304) |
| Stirrer cathode (rpm) | NA |
| Surface area of cathode | NA |
| Specific power anode | 0.0035 W/ cm2 |
| Diaphragm surface area (D1; m2) | 2.6 |
| Diaphragm thickness (D1; mm) | 25 ((PROMASIL®- 1100 from Promat GmbH) |
| Diaphragm material (D1) | Calcium silicate |
| Wall material (W) | IBC (Intermediate Bulk Containers); High Density Polyethylene (HDPE) |
Example 3
In order to remove the nitrite in biological air scrubber water, a 600L system as depicted in figure 4C and 4D is made. In this setup, this apparatus has next to the 10 option to heat via heating elements, also the possibility for ultra-sonification via ultrasonic elements (30Hz). Ultra-sonification in 02 increases the conversion rate of nitrite to nitrate, however this feature is not used, due to energy consumption considerations. Used parameters (W, D1, E1, E2, Volumes, Temperature, Voltage, Amperage, Power) are shown in table 3. The conversion rates are shown in figure 7.
Table 3: Setup parameters of apparatus for removing nitrite in 600L biological air scrubber water according to 4C and 4D
| Electrolyte 1 | 3% NaCI solution |
| Volume electrolyte 1 (L) | 250 |
| Electrolyte 2 | Biological air scrubber water |
| Volume electrolyte 2 (L) | 600 |
| Temperature (°C) | 10-70 |
| Frequency sonification (kHz) | NA |
| Power sonification (Weft) | NA |
| Voltage (V) | 7.0 |
| Current (A) | 106 |
| Electrical Power (W) | 742 |
| Anode (E2) | Plates fixed on threaded wire (Stainless Steel; SAE grade 304) |
| Stirrer anode (rpm) | NA |
| Surface area of anode | Plates; 0,5mm; 28.5 m2 |
| Specific power anode | 0.0026 W/ cm2 |
| Cathode (E1) | Wired grid; Stainless Steel (SAE grade 304) |
| Stirrer cathode (rpm) | NA |
| Surface area of cathode | NA |
| Specific power anode | 0.0026 W/ cm2 |
| Diaphragm surface area (D1; m2) | 3.56 |
| Diaphragm thickness (D1; mm) | 25 ((PROMASIL®- 1100 from Promat GmbH) |
| Diaphragm material (D1) | Calcium silicate |
| Wall material (W) | IBC (Intermediate Bulk Containers); High Density Polyethylene (HDPE) |
Claims (15)
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| Application Number | Priority Date | Filing Date | Title |
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| NL2023283A NL2023283B1 (en) | 2019-06-09 | 2019-06-09 | Method for removal of nitrite from an aqueous solution by electrochemical conversion and apparatus therefor |
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| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| NL2023283A NL2023283B1 (en) | 2019-06-09 | 2019-06-09 | Method for removal of nitrite from an aqueous solution by electrochemical conversion and apparatus therefor |
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| NL2023283B1 true NL2023283B1 (en) | 2020-05-13 |
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