Method for the dehydrogenation of a liquid hydrogen carrier material

Abstract

Method for the dehydrogenation of a liquid hydrogen carrier material in the form of a metal hydride or a liquid organic hydrogen carrier (LOHC), comprising the following steps: a. The metal hydride or the LOHC is provided to a thin-film evaporator (1), wherein the LOHC is brought into contact with a catalyst which is added to the LOHC before it enters the thin-film evaporator (1), and / or wherein the LOHC comes into contact with a catalyst in the thin-film evaporator (1), b. In the thin-film evaporator (1), a film of the liquid hydrogen carrier material is mechanically generated on a cylindrical or conical heated wall (7) and held against the heated wall (7) by means of wiping elements (12, 13, 14) of a rotor (3), whereby the chemical bond of the hydrogen to the liquid hydrogen carrier material is broken by the supply of heat via the wall (7), the resulting hydrogen is withdrawn from the thin-film evaporator (1) and the remaining liquid hydrogen carrier material is collected.

Description

[0001] The invention relates to a method for dehydrating a liquid hydrogen carrier material according to the features of claim 1.

[0002] Progressive climate change necessitates a shift in energy supply to CO2-neutral sources, such as solar and wind power. Solar energy yields depend on the day-night cycle. In winter in the Northern Hemisphere, yields are significantly lower than in summer. For wind energy, periods of calm winds must be taken into account. It is desirable to store surplus electrical energy generated by solar or wind power plants and make it available for later use. Storing and buffering peak energy production requires the integration of large storage capacities.

[0003] Hydrogen is considered a promising material for the chemical storage of energy. When electricity is surplus, hydrogen can be produced in electrolyzers, transported, and, for example, converted back into electricity using fuel cells. Hydrogen has a high calorific value relative to its weight and produces no greenhouse gas emissions when burned. However, due to its low density, hydrogen is generally difficult to store. To achieve sufficiently high densities, hydrogen must either be compressed at sufficiently high pressures or liquefied at sufficiently low temperatures.

[0004] Due to the disadvantages of storing hydrogen as a pure substance, various novel storage methods for hydrogen have been developed. Chemical storage in liquids or suspensions offers advantages in terms of handling and cost-effectiveness. Hydrogen storage in the narrower sense includes chemical conversions of hydrogen, in which the hydrogen is recovered as such and not irreversibly converted into another fuel. Such storage methods include, for example, storage in liquid organic hydrogen carriers (LOHCs). LOHCs are organic compounds that can absorb and release hydrogen through chemical reactions. Storage using suspensions such as hydrides, in which the hydrogen is chemically bound to suspended metal particles, is also possible.These liquid hydrogen carrier materials allow large parts of the existing infrastructure for fossil fuels to be used.

[0005] Storing electrical energy using LOHC requires energy input, as the hydrogen must first be produced through electrolysis. To store the hydrogen, i.e., to hydrogenate the LOHC, the generated hydrogen is brought into contact with the storage medium and chemically bound to it. The LOHC reacts with the hydrogen at elevated pressure of approximately 30 to 50 bar and elevated temperature of approximately 150 to 200°C in the presence of a catalyst. The hydrogenation reaction is an exothermic reaction.

[0006] The hydrogen-charged LOHC is stable at ambient conditions and can be stored or transported. In a dehydrogenation apparatus, the hydrogen is then released. Dehydrogenation is an endothermic reaction at elevated temperatures above 150°C, which is carried out in the presence of a catalyst. Due to the elevated temperature, some of the LOHC also evaporates and must be separated from the hydrogen before it can be used, for example, in a fuel cell to generate electrical energy. The LOHC carrier medium can then be reused as a hydrogen storage medium.

[0007] Some metals, such as lithium, calcium, and magnesium, are also capable of storing hydrogen in the form of metal hydrides. For example, if magnesium dust is suspended in a liquid, a hydrogen storage medium is created that is easy and safe to handle, as well as cost-effective. Such a hydrogen storage medium can be a suspension of magnesium dust in mineral oil (https: / / carbon280.com). The material is non-toxic and does not react under ambient conditions. This makes storage and transport particularly easy. No catalyst is required for hydrogenation and dehydrogenation. The hydrogen released during dehydrogenation is particularly pure and can be used without further processing. Compared to LOHC, the energy requirement for hydrogenation and dehydrogenation is significantly lower. Heat must be supplied to release hydrogen from the suspension in order to break the chemical bond between the hydrogen and the metal hydrides.

[0008] In the past, various reactor concepts have been pursued for the release of hydrogen from liquid storage.

[0009] CN 114 772 548 A discloses a reactor for releasing hydrogen from a liquid oxygen hydrocarbon (LOHC). LOHC is introduced into the drum-shaped reactor via a tangential access point. The LOHC flows downwards along the wall. The walls are heated and coated with a catalyst. Hydrogen is drawn off at the top, and the remaining LOHC collects at the bottom. EP 3 281 912 A1 also discloses the release of hydrogen from a liquid oxygen hydrocarbon (LOHC) using a cylindrical separator into which the LOHC is introduced.

[0010] To provide the largest possible contact area between the loaded LOHC and the catalyst required for release, mesh structures and microstructured reactors can be used. The catalyst can be incorporated into the structure or applied as a layer to the surface of structured internals. Catalyst-coated structured plates can be inserted into a tube. Another design consists of stainless steel tubes filled with catalyst and heated externally with thermal oil. The hydrogen is separated, for example, after one pass through the reactor. The remaining loading is released and separated in further stages. (https: / / d-nb.info / 1227451172 / 34, Simonte, M.): Reactor concepts for the dehydrogenation of cyclic hydrocarbons for heat and hydrogen storage, dissertation at the KIT Faculty of Chemical Engineering and Process Engineering of the Karlsruhe Institute of Technology (KIT), Karlsruhe 2020).

[0011] In all gas-liquid system concepts, it has been shown that the transfer of the necessary process heat and the contact of the LOHC with the catalyst are of particular importance for the dehydrogenation performance of the reactors. If large quantities of hydrogen are released, parts of the apparatus are filled with hydrogen instead of LOHC, so that the available catalyst-coated surface is only partially available for the reaction. This can be an advantage of metal hydride suspensions, since no catalyst is required for hydrogen release. In summary, the following essential requirements for the reactors for hydrogen release can be derived: Effective heat transfer to the liquid hydrogen storage (LOHC or metal hydride suspension) is necessary to provide the required reaction heat. Furthermore, efficient removal of the hydrogen after release is essential.Furthermore, operation should also be possible with solid particles in liquid hydrogen carriers and also with overpressure operation to prevent evaporation of the hydrogen carrier.

[0012] The reactors used to date for the dehydrogenation of liquid hydrogen storage materials do not meet these requirements due to their inherent design. The invention therefore aims to provide a method for dehydrogenating a liquid hydrogen carrier material, namely an LOHC or a metal hydride in suspension, which avoids these disadvantages.

[0013] This problem is solved in a method with the features of claim 1.

[0014] Advantageous further developments of the invention are the subject of the dependent claims.

[0015] For the purposes of this invention, the term "liquid hydrogen carrier material" refers to LOHC and metal hydride suspensions (or simply metal hydride) that can be loaded with hydrogen, i.e., hydrogenated, and whose chemical bond to the hydrogen can be separated, i.e., dehydrogenated. The carrier materials can be reused, i.e., rehydrogenated after dehydrogenation. Dehydrogenation of LOHC requires the presence of a catalyst, whereas a catalyst is not required for the dehydrogenation of metal hydrides. The term "dehydrogenation" is used hereafter as assuming that the necessary boundary conditions are met in each case.

[0016] In the inventive process for dehydrogenating a liquid hydrogen carrier material, either a liquid organic hydrocarbon (LOHC) or a metal hybrid is used as the hydrogen carrier material. If an LOHC is used, a catalyst is added to the LOHC. The mixture of catalyst and LOHC is introduced into a thin-film evaporator. The thin-film evaporator serves as the reactor. It can also be referred to as a thin-film dehydrator, or, due to its high efficiency, as a high-performance thin-film dehydrator. The operating principle of thin-film evaporators is based on mechanically generating a film of a liquid mixture on a cylindrical or conical heated wall by means of a rotor. In this process, the chemical bond between the hydrogen and the liquid organic hydrogen carrier is broken in the presence of the catalyst by the application of heat through the wall. The hydrogen is then withdrawn from the thin-film evaporator.The remaining liquid organic support and the catalyst are collected. Alternatively or additionally, surfaces that come into contact with the LOHC within the thin-film evaporator are coated with a catalyst. In this case, the prior addition of the catalyst is optional or unnecessary. If a metal hydride is to be dehydrogenated, the presence of a catalyst can be omitted.

[0017] The inventive method enables efficient heat transfer into the hydrogen carrier to provide the desired heat of reaction. Simultaneously, the thin-film evaporator, functioning as a high-performance thin-film dehydrator, allows for efficient removal of the hydrogen after its release. Furthermore, the use of the thin-film evaporator allows operation even with solid particles in the liquid hydrogen carrier. Since the thin-film evaporator is operated in an encapsulated environment, overpressure operation is also possible to prevent evaporation of the hydrogen carrier.

[0018] In a vertical arrangement, i.e., with a vertical longitudinal axis, of the thin-film evaporator, the hydrogenated liquid hydrogen carrier is fed in at the top of the evaporator and distributed, for example, by a liquid distributor in a thin film on the heated wall. After passing through the liquid distributor, the thin film is intensively mixed by means of wiper elements. This mixing ensures very high heat flux densities, thus enabling high hydrogen release rates. Under these conditions, without the intervention of the wiper elements or wiper blades, a gas cushion would form under the liquid film, which would result in a significantly reduced heat transfer. The wiper elements, especially the wiper blades, of a rotor hold the film against the heated wall, so that no reduction in heat flux density occurs due to the insulating effect of a gas cushion.

[0019] This causes the liquid film to travel from top to bottom through the thin-film evaporator, where it is subjected to an intense heat flow. As it travels downwards through the thin-film evaporator, hydrogen is released and exits at the top.

[0020] For the flow of hydrogen in the thin-film evaporator, the rotor with the wiper elements must be designed to provide a sufficiently large free cross-section for the hydrogen gas flow. The partially or completely discharged liquid hydrogen carrier is extracted at the lower end of the thin-film evaporator.

[0021] The catalyst required for the release of hydrogen from the LOHC can be added to the liquid hydrogen storage in powder or particle form before being fed to the apparatus, or applied as a thin layer to the heated wall and / or to the wiper elements of the rotor.

[0022] Thin-film evaporator dehydrators can, in principle, be designed vertically or horizontally. The advantages of the invention also apply when the thin-film evaporator is arranged at an angle deviating from the vertical longitudinal axis or even horizontally.

[0023] The rotor's wiper elements can be implemented as rigid elements, movable flaps, or wipers subjected to a spring or restoring force. The design depends, among other things, on whether the feed mass flow, consisting of the liquid hydrogen carrier, contains particles, for example, due to a catalyst added to the LOHC. The number of wiper blades depends on the desired hydrogen release rate. The higher the number of wiper blades, the greater the heat flow and the higher the hydrogen release rate can be.

[0024] The reaction for the release of hydrogen takes place at elevated temperatures, e.g., in a temperature range of 250°C to 320°C. Higher temperatures are also possible. At these temperatures, at least some of the liquid hydrogen carrier also evaporates at normal pressure and must be removed from the hydrogen as an impurity before use. Thin-film evaporators, which are used as reactors according to the invention, also allow operation at overpressure, particularly in a pressure range of at least 1 bar overpressure. Preferably, the range is between 1 and 10 bar overpressure, so that hydrogen gas is released which, with small amounts of impurities, can be used without further treatment.

[0025] The use of thin-film evaporators as reactors for the dehydrogenation of LOHC according to the invention enables the remaining liquid organic hydrogen carrier material to be separated from the catalyst, allowing both the catalyst and the LOHC to be subjected to rehydrogenation. The catalyst is, in particular, a finely dispersed solid that is mixed with the LOHC, and the resulting suspension is introduced into the thin-film evaporator. Alternatively, the catalyst can be introduced into the thin-film evaporator separately from the LOHC and mixed with it there. If catalyst-coated areas are present within the thin-film evaporator, separation of the dehydrogenated LOHC from the catalyst is not necessary.

[0026] The invention is explained below with reference to a thin-film evaporator shown schematically in the drawing. The example relates to the dehydration of LOHC. The thin-film evaporator can be operated in the same way with a metal hydride without the prior addition of a catalyst.

[0027] The Fig. Figure 1 shows a cross-section of a thin-film evaporator 1 with a vertical orientation. The thin-film evaporator 1 has a housing 2, which is essentially cylindrical. A rotor 3 is located inside the housing 2, with an axis of rotation 4 that is longitudinally centered within the cylindrical housing. The rotor 3 is driven by a drive 5 at the upper end of the housing 4. The drive 5 comprises a motor and, optionally, a gearbox. The rotor 3 rotates in the direction of arrow P1.

[0028] In the region of the upper end of the rotor 3, a hydrogenated liquid organic hydrogen carrier, previously mixed with a catalyst, is introduced into the housing 2 in the direction of arrow P2. The liquid LOHC enters a liquid distributor 6 and is distributed as evenly as possible by the liquid distributor 6 onto an inner wall 7 of the housing 4. The inner wall 7 is heated radially from the outside. For this purpose, a heating medium 8 is guided in the direction of arrow P3 in a space between the inner wall 7 and an outer wall 9. The heating medium 8 is then discharged in the direction of arrow P4 at the lower end of the housing 2 from the space 10 between the inner wall 7 and the outer wall 9. The temperature of the inner wall 7 is in the range of 150 to 320°C. Higher temperatures can also be set to provide a sufficiently high heat flux density.This breaks the chemical bond between the hydrogen and the LOHC in the presence of the catalyst. Hydrogen gas (H₂) escapes from the LOHC and flows radially inwards from the inner wall 7, rising upwards. The hydrogen gas is drawn off upwards at outlet 11 at the head of the thin-film evaporator 1 in the direction of arrow P5. It can be used, for example, in a fuel cell to generate electrical energy.

[0029] The rotor 3 has several successive wiper elements 12, 13, 14 arranged longitudinally along the axis of rotation 4, which distribute a film of LOHC onto the heated wall 7. The dehydrated liquid organic hydrogen carrier collects in a funnel 15 at the base of the thin-film evaporator 1 and is drawn off downwards in the direction of arrow P6. The dehydrated LOHC can then be rehydrated. This is a closed cycle. Reference symbol: 1 Thin-film evaporator 2 cases of 1 3 Rotors in 1 4 axis of rotation of 3 5 Drive 6 liquid distributors 7 heated walls of 2 8 Heating medium 9 outer wall of 2 10. Space between 7 and 9 11 Exit at 2 12 wiper elements out of 3 13 wiper elements out of 3 14 wiper elements out of 3 15 funnels of 2 P1 Arrow / Direction of Rotation P2 Arrow / LOHC (hydrogenated) with catalyst P3 Arrow / Heating medium P4 Arrow / Heating medium P5 Arrow / Hydrogen P6 Arrow / LOHC dehydrated

Claims

[1] Method for the dehydrogenation of a liquid hydrogen carrier material in the form of a metal hydride or a liquid organic hydrogen carrier (LOHC), comprising the following steps: a. The metal hydride or the LOHC is provided to a thin-film evaporator (1), wherein the LOHC is brought into contact with a catalyst which is added to the LOHC before it enters the thin-film evaporator (1), and / or wherein the LOHC comes into contact with a catalyst in the thin-film evaporator (1), b. In the thin-film evaporator (1), a film of the liquid hydrogen carrier material is mechanically generated on a cylindrical or conical heated wall (7) and held against the heated wall (7) by means of wiping elements (12, 13, 14) of a rotor (3), whereby the chemical bond of the hydrogen to the liquid hydrogen carrier material is broken by the supply of heat via the wall (7), the resulting hydrogen is withdrawn from the thin-film evaporator (1) and the remaining liquid hydrogen carrier material is collected. [2] Method according to claim 1, characterized by , that the remaining liquid hydrogen carrier material is separated from the catalyst and subjected to further hydrogenation. [3] Method according to claim 1 or 2, characterized by , that the catalyst is mixed with the LOHC as a finely dispersed solid and the resulting suspension is introduced into the thin-film evaporator (1). [4] Method according to any one of claims 1 to 3, characterized by , that the LOHC and the catalyst are introduced separately from the LOHC into the thin-film evaporator (1) and mixed in the thin-film evaporator (1). [5] Method according to any one of claims 1 to 4, characterized by , that surfaces with which the LOHC in the thin-film evaporator (1) comes into contact are at least partially coated with a catalyst. [6] Method according to claim 5, characterized by , that the heated wall (7) is coated with the catalyst. [7] Method according to claim 5 or 6, characterized by , that the wiping elements (12, 13, 14) of the rotor (3) are at least partially coated with the catalyst. [8] Method according to any one of claims 1 to 7, characterized by, that the hydrogenated liquid hydrogen carrier material in the thin-film evaporator (1) is heated to a temperature above 150°C, in particular to a temperature in the range of 150°C to 400°C, preferably to a temperature in the range of 200°C to 300°C. [9] Method according to any one of claims 1 to 5, characterized by , that an operating pressure of 1 to 10 bar overpressure is set in the thin-film evaporator (1). [10] Method according to any one of claims 1 to 9, characterized by , that the LOHC is selected from the following group: toluene, N-ethylcarbazole, dibenzyltoluene and benzyltoluene.

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