Method for nitrogen fixation in a plasma reactor

Abstract

The invention relates to a method for the fixation of nitrogen, in particular ammonia synthesis, in a plasma reactor, wherein the method comprises the following steps: a) providing a synthesis gas formed from one or more gaseous reactants for the synthesis of a synthesis product, wherein in the case of two or more gaseous reactants, the mixing ratio of the molar proportions of the reactants is determined as follows: a.1) determining all atomic types in the synthesis product; a.2) calculating the inverse of the sum of the effective cross sections for ionization and excitation of the respective atomic types by electron impact; a.3) multiplying the inverse of the sum of the effective cross section values of the atomic types by the number of atoms of the respective atomic types in the synthesis product; a.4) determining the mixing ratio of the reactants, wherein the number of atoms of the atomic types in the reactants approximately corresponds to the ratio of the product of the inverses of the sum of the effective cross section values of the respective atomic types; b) providing a process gas by mixing a gaseous catalyst with the synthesis gas, wherein the molar proportion of the catalyst in the process gas is not greater than 33%; c) introducing the process gas into a plasma reactor for the synthesis of the synthesis product from the reactants; d) isolating the synthesis product; e) recycling the mixture of excess or residual reactants and gaseous catalyst, as well as new reactants, to obtain a process gas having a mixing ratio according to steps a) and b); f) repeating steps c) to f).

Description

Technical Field

[0001] This invention relates to a method for nitrogen fixation, particularly ammonia synthesis, in a plasma reactor. Background Technology

[0002] The so-called Haber-Bosch process remains the most commonly used method for ammonia synthesis worldwide. The core step of this process, the synthesis of ammonia from atmospheric nitrogen and hydrogen, is carried out on a solid catalyst at pressures of approximately 150 to 350 bar and temperatures of approximately 400 to 500 °C. This process emits approximately 420 million tons of CO2 annually worldwide. In addition, other processes used for nitrogen fixation (such as nitric acid preparation) also generate additional emissions. A significant portion of these emissions is due to heating the process gases hydrogen and nitrogen to over 450 °C and compressing them to up to 20 MPa (Shi, Run, et al. “The Journey toward Low Temperature, Low Pressure Catalytic Nitrogen Fixation.” Advanced Energy Materials 10.19 (2020):2000659). High temperatures and high pressures are designed for the use of solid catalysts (such as Ru, Mo, or Fe-based catalysts). These catalysts are necessary to obtain significant yields of the relevant nitrogen compounds. However, from a thermodynamic perspective, low temperature and high pressure are ideal for the optimal reaction pathway.

[0003] Solid catalysts have another serious drawback: they can only be arranged in two dimensions within the reactor, meaning that the necessary chemical reactions can only occur on the catalyst surface, which significantly reduces the number of available reaction sites.

[0004] Based on current technology, some of the high-temperature issues can be addressed through electrochemical and plasma-supported synthesis processes, since these processes do not require high temperatures.

[0005] This is typically achieved using the aforementioned solid catalysts to achieve significant yields.

[0006] The preparation of ammonia via low-temperature plasma is described, for example, in US4877589, CN111362278, or WO2011104386. Solid-state catalysts are used to obtain considerable ammonia yields.

[0007] What distinguishes other existing technologies on the subject of ammonia synthesis and nitrogen fixation is that significant yields from a mixture of hydrogen and nitrogen can only be obtained by using solid catalysts.

[0008] JP2017164736 A describes a plasma reactor and method for synthesizing ammonia in the presence of a catalyst at temperatures ranging from 25 to 500 °C and pressures ranging from 101 to 1000 kPa. Specifically, aluminum, titanium, iron, palladium, nickel, copper, zinc, silver, platinum, and gold are mentioned as catalysts. JP2017164736 A investigated the effectiveness of the Penning effect in ammonia synthesis by adding helium, argon, or hydrogen to the feed gases (hydrogen and nitrogen). The ammonia yield can increase with increasing amounts of rare gases in the reaction mixture, with a value of 75% helium explicitly mentioned. No significant increase in yield was observed at 50% or lower amounts.

[0009] WO2020115473 A1 describes a plasma reactor and process for nitrogen fixation to prepare oxynitrides (particularly NO and NO2). The process is carried out in the presence of a catalyst at pressures ranging from atmospheric pressure to 4 bar. Only a temperature reduction to below 1000 K after the reactor is specified. The catalysts mentioned are iron, nickel, zeolites, transition metal oxides, platinum, and rhenium. Rare gases (Ar, Ne, He) and carbon dioxide are added to reduce the ionization potential of the plasma. The quantities are not specified. Furthermore, there is no correlation between the yields of the inert gases and the nitrogen oxides.

[0010] Hessel, V. et al. "Energy, catalyst and reactor considerations for (near)-industrial plasma processing and learning for nitrogen-fixationreactions" Catalysis today 2013, 211, 9-28,<doi:10.1016 / j.cattod.2013.04.005> This provides an overview of the current state of research and industrial applications of plasma-assisted reactions. It also covers nitrogen fixation, and particularly ammonia preparation, in which solid-state catalysts are consistently used.

[0011] Hessel, V. et al. (“Industrial applications of plasma, microwave and ultrasound techniques: Nitrogen-fixation and hydrogenation reactions” Chemical Engineering and Processing: Process Intensification 2013, 71, 19-30,<doi:10.1016 / j.cep.2013.02.002> The industrial applicability of plasma-assisted nitrogen fixation related to nitric oxide preparation using catalysts and plasmas that sometimes contain rare gases was investigated.

[0012] EP3162435 A1 describes the preparation of ammonia at room temperature in a plasma reactor using an interfacial reaction at the phase interface between nitrogen and water. The use of a catalyst and the addition of rare gases are not described.

[0013] WO2019183646 A1 describes the electrochemical preparation of ammonia from nitrogen-containing plasma and water at 25°C and atmospheric pressure without a catalyst. In a comparative test, argon was used in the plasma and nitrogen was introduced, but this reduced the ammonia yield.

[0014] Nakajima, J. et al. (“Synthesis of ammonia using microwave discharge atatmospheric pressure”, Thin Solid Films, Volume 516, Issue 13, 2008, pp. 4446-51, doi.org / 10.1016 / j.tsf.2007.10.053) described the synthesis of nitrogen and hydrogen from atmospheric microwave plasma. These reactions can facilitate the addition of Ar to the plasma gas, thereby promoting the synthesis of active nitrogen species, particularly N and N₂. + The formation of N2 in the plasma leads to the expectation that ammonia production will increase with increasing Ar flow rate. However, when the Ar flow rate exceeds 10 L / min, the ammonia production rate begins to decrease as the absolute amount of N2 in the plasma gas decreases.

[0015] References:

[0016] Summary of the Invention

[0017] One object of the present invention is to provide a nitrogen fixation method optimized for high yield and low energy input.

[0018] The nitrogen fixation method in a plasma reactor includes the following steps: (a) providing a synthesis gas of one or more gaseous reactants for the synthesis of the product, wherein, in the case of two or more gaseous reactants, the mixing ratio of the molar proportions of the reactants is determined as follows: (a.1) determining all atomic types in the product; (a.2) calculating the reciprocal of the total effective cross-section of the ionization and excitation of atoms by electron collisions of the respective atomic types (determined at approximately 5 eV); (a.3) multiplying the reciprocal of the total effective cross-section value of the atomic types by the number of atoms of the atomic types in the product (the weighted reciprocal of the total effective cross-section of the atomic types); (a.4) determining the mixing ratio of the reactants, wherein the number of atomic types in the reactants approximately corresponds to the ratio of the product of the reciprocals or the weighted reciprocals of the total effective cross-section values ​​of the respective atomic types;

[0019] (b) Providing process gas by mixing gaseous catalyst with synthesis gas, wherein the molar proportion of catalyst in the process gas is not greater than 33%;

[0020] (c) Introducing process gases into a plasma reactor to synthesize the synthesis products from the reactants;

[0021] (d) Separate the synthesized product;

[0022] (e) Recycle excess or residual reactants and gaseous catalyst, as well as a mixture of new reactants, to obtain a process gas having the mixing ratios according to steps a) and b);

[0023] (f) Repeat steps (c) to (f).

[0024] The increased efficiency of chemical reactions is achieved by the fact that the gases mentioned are relatively easy to ionize, thus providing a high electron density with relatively low energy input, and then dissociating, ionizing or chemically exciting the molecules of the starting gas (O2, N2, H2, etc.) (most importantly by exciting the rotational and vibrational degrees of freedom of the starting molecules) to increase the chemical potential and allow the reaction to occur faster with lower energy input and lower temperature.

[0025] Another advantage of the described process is based on the fact that the atoms / molecules of the gaseous catalyst are uniformly distributed across the reactor volume, thus providing three-dimensional scaling of the reaction sites, which significantly improves efficiency.

[0026] This invention is also based on the understanding that when using a gaseous catalyst, calculating the mixing ratio by the total effective cross-section of each atomic type leads to better yields with lower energy input. This unique mixing ratio of reactants in conjunction with the use of a gaseous catalyst can be explained by the fact that, when using such a catalyst, the chemical process is initiated by collisions between process gas particles and free electrons provided by the catalyst gas. For this reason, the effective cross-section of inelastic and elastic collisions between electrons and process gas particles needs to be considered when determining the optimal gas mixture. However, the effective cross-section of nitrogen is approximately three times that of hydrogen.

[0027] For the total effective cross section, the following values ​​can be assumed:

[0028]

[0029] For example, to prepare the synthesis product ammonia (NH3) from nitrogen (N2) and hydrogen (H2), the mixing ratio of the reactants (N2 and H2) in the synthesis gases can be calculated as follows. The atomic types of NH3 are N and H (step a.1). The total effective cross section (wq) of N and H [in 10^-20 m] 2 The values ​​are wqN=14 and wqH=5, respectively. The reciprocals (1 / wq) are 1 / wqN=0.07 and 1 / wqH=0.2, respectively. The sum of the reciprocals (1 / wq) of all atoms of the respective atomic types (N and H) in the synthesized product is 1 x 1 / wqN=0.07 and 3 x 1 / wqH=0.6 (step a.3). This results in an optimal mixing ratio of N2:H2 of 1:8.4 or an approximate mixing ratio of 1:9 (step a.4).

[0030] According to the present invention, for the optimal yield of ammonia synthesis, the optimal mixing ratio of reactants N2 and H2 is N2 / H2 ≈ 1 / 9. This contrasts with the mixing ratio (N2 / H2 = 1 / 3) in the conventional Haber-Bosch process. Therefore, compared to the conventional Haber-Bosch process, this process can be operated with a gas mixture containing lower nitrogen content, which can also lead to more efficient fixation.

[0031] At the end of the process cycle, the respective reactive nitrogen compounds (i.e., the desired products of the reaction, such as NH3) are discharged, and the non-reactive reactants and gaseous catalyst are returned to the reactor. The percentage of recycled gaseous reactants remains approximately constant. Therefore, this process is also distinguished by the fact that the catalyst continuously leaves the reactor and is then transferred back into the reactor chamber. Only fresh reactants are added to restore the initial ratio between the syngas and the catalyst.

[0032] In addition to ammonia synthesis, this process can also be used to fix nitrogen by synthesizing nitrogen oxides (NOx), hydrogen cyanide (HCN), nitric acid (HNO3), and urea (CH4N2O).

[0033] In some embodiments, the gaseous reactants may be selected from the group consisting of hydrogen (H2), nitrogen (N2), oxygen (O2), and methane (CH4), as well as other gaseous hydrocarbons. Reactants such as peroxyacetyl nitrate (PAN; CH3C(O)OONO2), peroxypropionyl nitrate (PPN; C2H5C(O)OONO2), peroxybenzoyl nitrate (PBzN; C6H5C(O)OONO2), peroxyacryloyl nitrate (APAN; CH2CHC(O)OONO2), peroxyisobutyryl nitrate (PiBN; (CH3)2CHC(O)OONO2), or peroxymethacryloyl nitrate (MPAN; CH2C(CH3)C(O)OONO2) may also be used, and may even be the sole reactant.

[0034] In some embodiments, the gaseous catalyst may be a gas that is not physically or chemically combined with the reaction products of the nitrogen fixation chemical reaction. For example, the gaseous catalyst may be a rare gas, preferably argon, helium, neon, xenon, and radon.

[0035] In some embodiments, rare gases may be added in the following preferred molar ratios:

[0036]

[0037] In some embodiments, the synthesis can be carried out at atmospheric pressure or higher, preferably at least 2 bar, more preferably at least 5 bar.

[0038] In some embodiments, the synthesis can be carried out at a temperature of up to 100°C, preferably 25°C or lower. The synthesis can also be carried out at a temperature below 0°C.

[0039] In some embodiments, the process can be carried out using a different solid catalyst. However, the present invention uses a gaseous catalyst in the electrochemical synthesis process for nitrogen fixation, thereby eliminating the need for a solid catalyst. Therefore, it is preferred not to use a solid catalyst.

[0040] In some embodiments, plasma can be generated by direct current discharge, high-frequency discharge, laser ionization, radioactive radiation, pulsed direct current discharge, or a combination thereof.

[0041] In high-frequency discharge scenarios, process gases can be excited using alternating electromagnetic fields (e.g., radio frequency or microwave discharges). Typical frequency ranges here are from 1 kHz to 100 GHz. A duty cycle between 0 and 100% can be used for pulsed DC discharges. Pulse durations can range from 1 millisecond to 1 femtosecond. The amplitude of the alternating electromagnetic field can range from 100 V to 100 MV.

[0042] In some embodiments, the plasma can be in thermal equilibrium, wherein the average electron temperature is equal to the average ion and neutral particle temperature; preferably, it can have at least a slight thermal imbalance, wherein the average electron temperature is one order of magnitude higher than the average ion and neutral particle temperature; more preferably, it can have an extreme thermal imbalance, wherein the average electron temperature is several orders of magnitude higher than the average ion and neutral particle temperature.

[0043] Another advantage compared to existing methods is the complete elimination of CO2 emissions generated in such methods. For example, in the conventional Haber-Bosch process, fossil fuels (such as gas or coal) are directly integrated into the process flow. On the other hand, in electrochemical methods, the entire synthesis can be carried out using renewable energy sources. Attached Figure Description

[0044] The invention will now be explained in more detail with reference to the accompanying drawings and examples, wherein:

[0045] Figure 1 A diagram illustrating the nitrogen fixation method is shown. Detailed Implementation

[0046] Figure 1 A diagram illustrating the nitrogen fixation process is shown. In the first step 1, a synthesis gas is provided by several reactants E1 and E2. In the second step 2, the synthesis gas and a gaseous catalyst K are combined to form a process gas with a defined reactant-to-catalyst ratio, and this gas is finally fed into a plasma reactor 3. The synthesis product S is synthesized in the plasma reactor and then separated from the residual process gas R in step 4. The residual process gas R, containing a reduced amount of reactants, is recycled and enriched with fresh synthesis gas, thereby again providing a process gas with a defined reactant-to-catalyst ratio.

[0047] The mixing ratio of the reactants was determined as follows:

[0048] a.1) Determine all atom types of the synthesized product;

[0049] a.2) Calculate the reciprocal of the total effective cross section of ionization and excitation of atoms by electron collisions of their respective atom types (at approximately 5 eV);

[0050] a.3) Multiply the reciprocal of the total effective cross section value of the atomic type by the number of atoms of each atomic type in the synthesized product (weighted reciprocal of the total effective cross section of the atomic type).

[0051] a.4) The mixing ratio of reactants is determined by making the number of atomic types of the reactants approximately correspond to the inverse product or weighted ratio of the total effective cross-sectional values ​​of their respective atomic types.

[0052] Example 1 - Synthesis of ammonia (NH3) from N2 and H2

[0053] Step a.1: The synthesized product ammonia NH3 is composed of atoms of type N and H.

[0054] Step a.2: Total effective cross section (wq) of N and H [in 10^-20 m] 2 The values ​​of N and H are wqN = 14 and wqH = 5, respectively. The reciprocals (1 / wq) are 1 / wqN = 0.07 and 1 / wqH = 0.2, respectively. The sum of the reciprocals (1 / wq) of all atoms of the respective atomic types (N and H) in the synthesized product is 1 x 1 / wqN = 0.07 and 3 x 1 / wqH = 0.6 (step a.3). This results in an optimal mixing ratio of N2:H2 of 1:8.4 or an approximate mixing ratio of 1:9 (step a.4).

[0055]

[0056] Example 2-NO

[0057]

[0058] Example 3-NO2

[0059]

[0060] Example 4-NO3

[0061]

[0062] Example 5 - HNO3

[0063]

[0064] List of reference numerals

[0065]

Claims

1. A method for nitrogen fixation in a plasma reactor, wherein the method comprises the following steps: a) A synthesis gas formed from multiple gaseous reactants is provided for the synthesis of the synthesis product, wherein the mixing ratio of the molar proportions of the reactants is determined as follows: a.1) Determine all atom types in the synthesized product; a.2) Calculate the reciprocal of the total effective cross section of atomic ionization and excitation by electron collisions on each atomic type; a.3) Multiply the reciprocal of the total effective cross-sectional value of the atomic type by the number of atoms of each atomic type in the synthetic product; a.4) Determine the mixing ratio of the reactants, wherein the number of atoms of a certain atomic type in the reactants corresponds to the ratio of the inverse product of the total effective cross-sectional values ​​of the corresponding atomic type; b) The process gas is provided by mixing a gaseous catalyst with a synthesis gas, wherein the molar proportion of the catalyst in the process gas is no more than 33%; c) Introducing process gases into a plasma reactor to synthesize the synthesis products from the reactants; d) Separate the synthesized product; e) Recycle excess or residual reactants and gaseous catalyst, as well as a mixture of new reactants, to obtain a process gas having a mixing ratio according to steps a) and b); f) Repeat step c) up to f).

2. The method according to claim 1, characterized in that, The gaseous reactant is nitrogen (N2) and at least one selected from the group consisting of hydrogen (H2), oxygen (O2) and methane (CH4).

3. The method according to claim 1 or 2, characterized in that, The gaseous catalyst is a rare gas.

4. The method according to claim 3, characterized in that, The rare gases are argon, helium, neon, xenon, and radon.

5. The method according to claim 4, characterized in that, The molar ratios of the rare gases argon, helium, neon, xenon, and radon are as follows: Argon 3-13% Helium 6-25% Neon gas 10-25% Xenon 1-13% Radon content: 1-13%.

6. The method according to claim 1 or 2, characterized in that, The synthesis is carried out at atmospheric pressure or higher.

7. The method according to claim 6, characterized in that, The synthesis was carried out under a pressure of at least 2 bar.

8. The method according to claim 6, characterized in that, The synthesis was carried out under a pressure of at least 5 bar.

9. The method according to claim 1 or 2, characterized in that, The synthesis was carried out at a temperature of up to 100°C.

10. The method according to claim 9, characterized in that, The synthesis was carried out at 25°C or lower.

11. The method according to claim 1 or 2, characterized in that, The synthesis was carried out at a temperature below 0°C.

12. The method according to claim 1 or 2, characterized in that, No solid catalyst is used.

13. The method according to claim 1 or 2, characterized in that, Plasma is generated through direct current discharge, high-frequency discharge, laser ionization, radioactive radiation, pulsed direct current discharge, or a combination thereof.

14. The method according to claim 1 or 2, characterized in that, The plasma is in thermal equilibrium, where the average electron temperature is equal to the average ion and neutral particle temperature.

15. The method according to claim 1 or 2, characterized in that, The plasma is in a slight thermal imbalance, which means that the average electron temperature is an order of magnitude higher than the average ion and neutral particle temperature.

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