Process for ammonia recovery with simultaneous hydrogen production

Abstract

The invention relates to a method for the recovery of ammonia and an associated use. In a method for the recovery of ammonia, an ammonium-containing solution (5) is electrochemically converted to ammonia in an electrochemical cell (1). The ammonia leaves the electrochemical cell (1) in a gaseous state (9).

Description

[0001] The invention relates to a method for the recovery of ammonia and an associated use.

[0002] It is desirable to recover ammonia from solutions such as urine or wastewater. Ammonia is a valuable raw material needed, for example, for the production of fertilizers, but also as a starting material for the production of a variety of other chemicals. Furthermore, removing ammonium from wastewater is desirable to protect ecosystems.

[0003] Techniques for recovering ammonia from wastewater are generally known. For example, the publication "Energy-Efficient Ammonia Recovery in an Up-Scaled Hydrogen Gas Recycling Electrochemical System" by P. Kuntke et al., published in ACS Sustainable Chem. Eng. 2018, 6, 7638-7644 (DOI: 10.1021 / acssuschemeng.8b00457), describes a system consisting of an electrochemical cell and a membrane reactor that can recover ammonia solution from artificial urine. However, this setup is technically complex.

[0004] The purpose of the invention is to recover ammonia from a solution in an improved manner.

[0005] The problem is solved by the method according to claim 1 and by the use according to the dependent claim. Advantageous embodiments are specified in the dependent claims.

[0006] To solve the problem, a process for the recovery of ammonia is used. Dissolved ammonium is electrochemically converted to ammonia in an electrochemical cell. In particular, hydrogen is also produced, preferably in the electrochemical cell. Specifically, the ammonia leaves the electrochemical cell in a gaseous state.

[0007] The process according to the invention enables the production of hydrogen and / or gaseous ammonia. In processes known from the prior art, no hydrogen is produced. Furthermore, it is always necessary to remove dissolved ammonia and process it further, for example, to convert it from solution into gaseous ammonia. In contrast, the process according to the invention is technically less complex. If a cathode or concentrate chamber is present in which dissolved ammonia collects, a continuous exchange of the liquid in this chamber is also necessary, since otherwise the diffusion through the membrane would be slowed down or prevented by the increasing ammonia concentration. This is not necessary according to the invention.

[0008] Ammonia recovery refers to the production of ammonia from a liquid containing ammonium, such as urine or wastewater.

[0009] An electrochemical reaction takes place, that is, a reaction under the influence of an electric current and / or an electric voltage. Specifically, an electric current and / or an electric voltage is applied between an anode and a cathode of the electrochemical cell. Typically, electrons are transported from the anode to the cathode.

[0010] In particular, no mass transport from the anode to the cathode and / or from the cathode to the anode is provided for or possible outside the electrochemical cell. Specifically, no flow path for mass transport from the anode to the cathode and / or from the cathode to the anode exists outside the electrochemical cell.

[0011] In particular, hydrogen is also produced, typically molecular and / or gaseous hydrogen. Hydrogen is a valuable chemical and a starting material for a wide variety of applications. Hydrogen is preferentially produced from the hydrogen atoms of dissolved ammonium (NH₄⁺). That is, for every ammonia molecule (NH₃), one hydrogen atom (H₂) is released, or for every two ammonia molecules (NH₃), one hydrogen molecule (H₂) is produced. The production of hydrogen is a key advantage here.

[0012] In addition to this hydrogen, hydrogen can be produced from water electrolysis.

[0013] The ammonia is preferably produced in gaseous form and / or leaves the cell in a gaseous state. Preferably, no ammonia solution is produced. While it is theoretically possible for small amounts of water to diffuse through the membrane due to osmotic pressure, and ammonia may be dissolved in the water, it is preferable that the majority of the ammonia is produced in gaseous form. This eliminates the need for a continuous exchange of a liquid to maintain the concentration gradient. Furthermore, no reaction or additional reactor is required to generate ammonia from a liquid solution for storage and / or further use. This reduces the technical complexity and costs.

[0014] Preferably, the solution is aqueous. For example, the solution may contain or be urine or wastewater.

[0015] In particular, the electrochemical cell is part of a stack which contains a large number of such electrochemical cells.

[0016] InIn one embodiment, a membrane of the electrochemical cell separates a liquid phase and a gas phase. The inventors have recognized that such a membrane enables the removal of gaseous ammonia in a particularly simple technical manner. Specifically, the membrane separates a chamber on the anode side, containing the ammonium-containing solution, from a chamber on the cathode side, containing the gas phase. The gas phase can comprise the produced ammonia and, optionally, a carrier gas. The membrane serves, in particular, as a separator for the spatial and electrical separation of the anode and cathode, or of the chamber on the anode side and the chamber on the cathode side, which is permeable only to ions. Specifically, the membrane allows the transport of ammonium and protons and / or is impermeable to substances such as ammonia and hydrogen.

[0017] In particular, an ionomer membrane is used. In one embodiment, the membrane comprises or is made from Nafion. Nafion is a perfluorinated copolymer with ionic properties, in which a sulfonic acid group acts as the ionic group, and can also be referred to as 2-[1-[Difluoro[(trifluorethenyl)oxy]methyl]-1,2,2,2-tetrafluoroethoxy]-1,1,2,2-tetrafluoroethanesulfonic acid. Such membranes have proven to be particularly advantageous.

[0018] In particular, no mass transport from the anode to the cathode and / or from the cathode to the anode is provided. Specifically, the electrochemical cell has no flow path for mass transport from the anode to the cathode and / or from the cathode to the anode. Nevertheless, ion transport occurs across the membrane, specifically only across the membrane.

[0019] In one embodiment, ammonium ions diffuse from the solution through the membrane of the electrochemical cell.

[0020] In one embodiment, ammonia is removed from the electrochemical cell through a porous transport layer. The porous transport layer can be arranged within the chamber or form the chamber itself, particularly on the cathode side.

[0021] In one embodiment, exactly one membrane is arranged between the anode and the cathode of the electrochemical cell. Therefore, there is no more than one membrane and / or no more than one separator.

[0022] Ion transport therefore only needs to occur through a membrane. The cell structure can thus be particularly simple. This allows for easy scaling and simple fabrication and use of stacks. In particular, the electrochemical cell comprises exactly one membrane. Specifically, the electrochemical cell comprises exactly two chambers, which are preferably separated by the membrane. The chambers can include porous transport layers or be designed as porous transport layers, as described below.

[0023] In one embodiment, the ammonium-containing solution is introduced into the electrochemical cell on the anode side. In another embodiment, ammonia is introduced out of the electrochemical cell on the cathode side.

[0024] In one embodiment, ammonia is carried out of the electrochemical cell by means of a carrier gas.

[0025] The carrier gas can be an inert gas and / or nitrogen. An inert gas is a gas that, under the conditions prevailing in the electrochemical cell, does not participate in chemical reactions or only to a negligible extent. For example, N₂ can be used.

[0026] A carrier gas transports the produced ammonia and any other products, such as hydrogen, away from the membrane, thus reducing the concentration at the membrane. This maintains the concentration gradient that drives diffusion through the cell. In this way, ammonia recovery can be carried out particularly efficiently.

[0027] In a further embodiment, water is split electrolytically. The applied electric current allows not only the production of ammonia from ammonium, but also the electrolytic splitting of water into oxygen and hydrogen. Thus, in addition to ammonia, an additional portion of the valuable raw material hydrogen is produced. Oxygen produced during electrolysis is primarily removed at the anode. Hydrogen is primarily removed at the cathode.

[0028] In particular, hydrogen leaves the electrochemical cell in a gaseous state.

[0029] In one embodiment, a catalyst is used on the anode side. In particular, the anode-side catalyst is applied directly to or coated onto the membrane. Iridium and / or iridium oxide, for example, can be used as the catalyst.

[0030] In one embodiment, a catalyst is used on the cathode side. In particular, the catalyst is applied directly to or coated onto the membrane. Platinum, for example, can be used as the catalyst.

[0031] Alternative catalyst materials for the anode side and / or the cathode side include gold, for example gold nanoparticles, high-entropy compounds such as high-entropy alloys, ruthenium and precious metal catalysts.

[0032] It is also possible, in principle, to provide the catalyst in another way, for example, by coating the porous transport layer with the respective catalyst. However, tests have shown that ammonia recovery is particularly advantageous when the catalyst is applied directly to or coated on the membrane. For example, the decalcification process is used for coating.

[0033] In one embodiment, a porous transport layer is provided for supplying the solution. In another embodiment, a porous transport layer is provided for removing the ammonia. A porous transport layer has an open or continuous pore structure that enables mass transport. The porous transport layer can provide a mechanically stable structure.

[0034] For example, a porous transport layer for supplying the solution is located on the anode-side catalyst. Similarly, a porous transport layer for removing the ammonia is located on the cathode-side catalyst.

[0035] The porous transport layer is located on the anode side, specifically between the anode and the membrane or catalyst. This porous transport layer on the anode side can comprise or be made of carbon paper or, preferably, titanium fleece. Titanium fleece coated with platinum, in particular, has proven to be durable and effective. The porous transport layer is located on the cathode side, specifically between the cathode and the membrane or catalyst. This porous transport layer on the cathode side can also comprise or be made of carbon paper.

[0036] In particular, the cathode is attached to the catalyst on the cathode side, typically on the outside. In particular, the anode is attached to the catalyst on the anode side, typically on the outside.

[0037] In another embodiment, the electrochemical cell is operated at a pressure higher than atmospheric pressure. It has been shown that increased pressure improves ammonia production and / or increases the yield.

[0038] In particular, a pressure boosting device is provided to adjust the increased pressure.

[0039] In a further embodiment, the electrochemical reaction takes place at a temperature of at least 30°C, in particular at least 40°C, preferably at least 50°C, in one embodiment at least 60°C or at least 70°C and / or at most 99°C, in particular at most 95°C, for example at most 90°C. In particular, at least one region of the electrochemical cell is heated to the aforementioned temperature. In particular, a heating device is provided to heat at least one region of the electrochemical cell to the temperature. Alternatively, the waste heat from the reaction can be used to reach the temperature, in particular in a stack with several electrochemical cells.

[0040] It has been shown that an increased temperature improves or accelerates the production of ammonia. Pressure and temperature are specifically chosen to ensure that the solution is in liquid form.

[0041] Alternatively, it is of course also possible to carry out the process at a temperature between 0°C and 30°C, for example at room temperature. The essential advantages of the invention can be achieved in the same way.

[0042] Another aspect of the invention is the use of an electrochemical cell for the electrochemical recovery of ammonia from an ammonium-containing solution. In particular, hydrogen is also generated in the electrochemical cell. Specifically, the ammonia leaves the electrochemical cell in a gaseous state. All features, properties, and advantages of the process described above also apply to this use, and vice versa.

[0043] Exemplary embodiments of the invention are explained in more detail below, also with reference to figures. Features of the exemplary embodiments can be combined individually or in multiples with the claimed subject matter, unless otherwise specified. The claimed scope of protection is not limited to the exemplary embodiments.

[0044] They show: Figure 1: a schematic representation of the process in an electrochemical cell, Figure 2: a setup for carrying out the process, and Figure 3: an exploded view of an electrochemical cell.

[0045] Figure 1Figure 1 shows an electrochemical cell 1 during the execution of the process. An anode 18 and a cathode 28 are present on the outside, which are subjected to an electric current and / or an electric voltage. In the example shown, an electric current with a defined current density, represented by the electrons e, is applied, and an electric voltage V is measured. A porous transport layer 14 borders the inside of the anode 18, to which the catalyst 12 is adjacent, preferably applied directly to the centrally located membrane 7. The catalyst 22 of the cathode side 20 is also preferably applied directly to the membrane 7. A cathode-side porous transport layer 24 and the aforementioned cathode 28 are adjacent to these.

[0046] An aqueous solution 5 containing ammonium is added via an inlet 32 ​​on the porous transport layer 14 on the anode side 10. The porous transport layer 14 thus conducts a liquid phase 3. The dissolved ammonium ions (NH4+) pass through the porous transport layer 14, past the catalyst 12, to the membrane 7 and are exchanged, in particular, for a proton (H+) ion.

[0047] This can also occur in the presence of other, especially dissolved, components such as the SO₄²⁻ shown. Water is electrochemically split, producing oxygen, protons, and electrons. Molecular oxygen (O₂) from the water electrolysis exits the porous transport layer 14 of the anode side 10 via the outlet 38, typically in gaseous form in a two-phase flow with a purified solution. The electrons (e⁻) travel via the electrical connection from the anode 18 to the cathode 28. The protons (H⁺) and the ammonium ions pass through the membrane 7. Two NH₄⁺ ions and two electrons combine to form two NH₃ molecules and two H₂ molecules. The resulting molecular hydrogen H₂ together with the resulting ammonia NH₃ is carried away in gaseous form via the porous transport layer 24 on the cathode side 20 and leaves the electrochemical cell 1 via the outlet 34. The porous transport layer 24 therefore conducts a gas phase 4.A carrier gas 30, such as N₂, can be introduced via inlet 36 to discharge the products 9 in the gaseous state. The gaseous products 9 are discharged via outlet 34 and supplied for use and / or storage.

[0048] The protons produced during water electrolysis can remain in the solution, interact with membrane 7 and / or diffuse through membrane 7 to form hydrogen.

[0049] Fig. 2Figure 1 shows a setup with an electrochemical cell 1 for carrying out the process. A solution 5 is fed into the electrochemical cell 1 via the anode-side inlet 32 ​​by means of a pump 44, for example a circulation pump. The pump can generate an overpressure to increase the throughput. The solution 5 comprises, for example, (NH₄)₂SO₄aq. The product(s) in the gaseous state 9 are discharged at the cathode-side outlet 34 and optionally subjected to gas purification 41, e.g. with H₂SO₄. Typically, NH₃ and H₂ are produced.

[0050] In the experimental setup shown here, the gas purification unit 41 can be used as a trap to convert gaseous ammonia to ammonium with sulfuric acid, since ammonium is non-volatile in liquid media and ammonia is gaseous under standard conditions. The amount of ammonia / ammonium is then determined, for example, using the indophenol method. Conversion to ammonium is only necessary for determination using the indophenol method and is purely optional. For other determination methods, such as gas chromatography, and / or if the ammonia is stored and / or reused, the gas purification unit 41 is not necessary. Alternatively, ammonia can be stored, for example, in gaseous form or—especially under pressure—in liquid form.

[0051] A carrier gas 30 can be introduced at the cathode-side inlet 36. This can be controlled or regulated via a flow control 40, for example, a mass flow controller, and / or optionally subjected to gas purification 41, e.g., with H₂SO₄, to remove impurities. The purified solution containing dissolved oxygen is discharged via the anode-side outlet 38. Oxygen (O₂) can be separated and / or discharged. Partial or complete recirculation can be provided, for example, by means of a three-way valve 43, to achieve a specific effluent quality. The purified solution flows to an outlet 42. Depending on requirements, the process can be optimized for the most thorough purification of the solution 5 or for the most efficient production of ammonia. Intermediate stages are also possible. Example of implementation

[0052] In one embodiment, an ammonium solution with 0.02 mol / l ammonium was prepared, e.g., by dissolving 132 mg of ammonium sulfate in 100 ml of deionized water. A flow rate of 20 ml / min was set using pump 44. The electrochemical cell 1 was heated to 80°C, in particular by means of heating cartridges. N₂ was supplied as a carrier gas 30 at a flow rate of 10 ml / min to the cathode side 20. The products were then purified 41 with 25 ml of 0.45 mol / l H₂SO₄ for absorption, with the aim of quantitatively detecting the ammonia using the indophenol method.

[0053] A current density of 0.8 A / cm² was applied to the cell for 30 minutes using a potentiostat. A voltage of approximately 2.3 V was measured. A recycling rate of 37.75 nmol / (cm²*s) was recorded. The same measurement was repeated two more times, with decreasing recycling rates observed, which can be attributed to a reduced ammonium concentration in the solution.

[0054] Using the indophenol method, a total amount of ammonia of 1080 µmol was detected. This means that 54% of the NH₄-N was recycled from the original solution. A residual amount of 453 µmol was detected in the remaining solution. Therefore, approximately 23% of the ammonium ions remained in the solution. The solution was thus purified to 77%. The reacted amount of ammonia also resulted in the production of 540 µmol of hydrogen.

[0055] A total charge of 3.46 × 10⁻³ C was converted. Based on the total charge of 0.8 C, this represents 0.445%. Thus, 99.545% of the electric current was used for water splitting, resulting in an additional hydrogen production of 4.13 µmol / (cm² / s).

[0056] Figure 3Figure 1 shows an exemplary structure of an electrochemical cell 1. Both the anode 18 and the cathode 28 each comprise outer end plates 46, adjacent bipolar plates 47, and adjacent seals 48, which centrally hold and seal the membrane electrode assembly 49 between them. The bipolar plates 48 contain pipes that lead to the outside through openings in the end plates 46 and to which connections for inlets and outlets can be attached. Inside, the pipes lead to a conductor structure 50, which adjoins the porous transport layer 14 or 24 and forms the inlets 32 and 36 as well as the outlets 34 and 38. Thus, a conductor structure can be present in addition to the porous transport layer.

[0057] The membrane electrode assembly 49 comprises the membrane 7, which includes the anode-side and cathode-side catalysts, as well as the porous transport layers 14 and 24.

[0058] In particular, at least a large proportion of the ammonia is produced in a gaseous state. Specifically, a large proportion of the ammonia leaves the electrochemical cell in a gaseous state. Ammonia is readily soluble in water. Therefore, as described above, it is possible that some of the ammonia dissolves in the water or aqueous solution within the cell. Nevertheless, the vast majority of the ammonia, typically more than 80% or more than 90%, or preferably more than 95%, is produced in a gaseous state and / or discharged from the cell.

[0059] In particular, at least a large proportion of the hydrogen is produced in a gaseous state. Specifically, at least a large proportion of the hydrogen leaves the electrochemical cell in a gaseous state. Hydrogen is poorly soluble in water. Specifically, the vast majority of the hydrogen, typically more than 95% or more than 98%, or preferably more than 99%, is produced and / or discharged from the cell in a gaseous state.

[0060] In particular, the process takes place in a single electrochemical cell. Specifically, gaseous ammonia is produced in the electrochemical cell. Specifically, the ammonia leaves the single electrochemical cell in a gaseous state. Specifically, no further unit and / or module is required to remove ammonia or gaseous ammonia from the electrochemical cell. Specifically, the produced ammonia is not converted into ammonium. Specifically, no ammonium leaves the electrochemical cell. Thus, only a single unit or cell is required to recover ammonia. A particularly simple and compact system is provided. Furthermore, the system is easily scalable.

[0061] In particular, hydrogen and ammonia are discharged together. Specifically, a mixture of hydrogen and ammonia leaves the electrochemical cell, preferably through a common outlet. In particular, a porous transport layer is provided to discharge hydrogen and ammonia together. In particular, a carrier gas is used to discharge ammonia and / or hydrogen. In particular, a carrier gas is used to discharge ammonia and hydrogen together. Reference symbol list

[0062] Electrochemical cell 1 Liquid phase 3 Gas phase 4 Solution 5 Water 6 membrane 7 Gaseous state 9 anode side 10 catalyst 12 Porous transport layer 14 chamber 16 anode 18 cathode side 20 catalyst 22 Porous transport layer 24 chamber 26 cathode 28 Carrier gas 30 Entrance 32 Exit 34 Entrance 36 Exit 38 Flow control 40 Gas purification 41 Sequence 42 Three-way valve 43 pump 44 End plate 46 Bipolar plate 47 seal 48 Membrane electrode assembly 49 Management structure 50

Claims

1. A method for recovering ammonia, wherein ammonium from an ammonium-containing solution (5) is electrochemically converted to ammonia in an electrochemical cell (1), wherein hydrogen is also produced in the electrochemical cell (1).

2. Method according to the preceding claim, wherein a membrane (7) of the electrochemical cell (1) separates a liquid phase (3) and a gas phase (4) from each other.

3. Method according to one of the preceding claims, wherein exactly one membrane (7) is arranged between an anode (18) and a cathode (28) of the electrochemical cell (1).

4. Method according to any of the preceding claims, wherein the electrochemical cell (1) has exactly two chambers (16, 26).

5. Method according to one of the preceding claims, wherein the ammonium-containing solution (5) is introduced into the electrochemical cell (1) on the anode side and / or ammonia is introduced out of the electrochemical cell (1) on the cathode side.

6. Method according to one of the preceding claims, wherein ammonia is carried out of the electrochemical cell (1) by means of a carrier gas (30).

7. Method according to one of the preceding claims, wherein the solution (5) is an aqueous solution (5) and water (6) is electrolytically split.

8. Method according to one of the preceding claims, wherein the oxygen produced is removed on the anode side and / or the hydrogen produced is removed on the cathode side.

9. Method according to any of the preceding claims, wherein ammonia and / or hydrogen leaves the electrochemical cell (1) in a gaseous state (9).

10. Method according to one of the preceding claims, wherein a catalyst (12) is used on the anode side, wherein the anode-side catalyst (12) is in particular applied directly to the membrane (7).

11. Method according to one of the preceding claims, wherein a catalyst (22) is used on the cathode side, wherein the cathode-side catalyst (22) is in particular applied directly to the membrane (7).

12. Method according to one of the preceding claims, wherein a porous transport layer (14) is provided for supplying the solution (5) and / or wherein a porous transport layer (24) is provided for removing the ammonia.

13. Method according to one of the preceding claims, wherein the electrochemical cell (1) is under a pressure increased compared to atmospheric pressure.

14. Method according to one of the preceding claims, wherein the electrochemical reaction takes place at a temperature of at least 30°C and / or at most 99°C, wherein the electrochemical cell (1) is in particular heated to the temperature.

15. Use of an electrochemical cell (1) for the electrochemical recovery of ammonia from an ammonium-containing solution (5), wherein hydrogen is also produced in the electrochemical cell (1).

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