Method for simulating ammonia decomposition in catalytic channel based on COMSOL simulation software

Abstract

The invention belongs to the technical field of ammonia decomposition, and particularly relates to a method for simulating ammonia decomposition in a catalytic channel based on COMSOL simulation software. By utilizing simulation of software, part of orthogonal experiments and condition exploration can be reduced, the time and experiment cost can be reduced, researchers can clearly observe the reaction condition of an ammonia catalytic decomposition reaction area, the reaction performance and structural rationality of the catalyst can be evaluated through information such as concentration distribution of reaction substances, and the method has the advantages of simple operation and low cost. Researchers can analyze the ammonia catalytic decomposition process more conveniently, and powerful guidance is provided for experimental condition modification or structure optimization and the like of the catalyst in the next stage.

Description

Technical Field

[0001] The invention belongs to the technical field of ammonia decomposition, and in particular relates to a method for simulating ammonia decomposition in a catalytic channel based on COMSOL simulation software. Background Art

[0002] As a carbon-free clean energy carrier, hydrogen energy is considered to be one of the most promising alternatives to fossil fuels. Ammonia, as a zero-carbon substance, has the characteristics of high hydrogen content, easy liquefaction, and good safety. It is an excellent hydrogen carrier, so ammonia as a source of hydrogen production has great potential. Considering that ammonia is difficult to decompose to produce hydrogen under normal conditions, even though researchers have explored various catalysts with positive benefits for ammonia decomposition reactions and obtained many catalysts with excellent catalytic performance, research on ammonia catalytic decomposition is still ongoing.

[0003] In the traditional process, the research in the field of ammonia catalytic decomposition relies on a large number of experiments guided by the researchers' chemical intuition, and the performance exploration process of the prepared catalyst requires experimental measurements under various conditions. Both the material cost and the time cost in this process cannot be ignored. Moreover, it is difficult for researchers to always observe the detailed reaction conditions inside the catalytic part when the reaction occurs, which to a certain extent affects the researchers' optimization direction of the catalyst in subsequent materials and structures. Summary of the invention

[0004] In order to solve the problems existing in the prior art, the purpose of the present invention is to provide a method for simulating ammonia decomposition in a catalytic channel based on COMSOL simulation software. The method provided by the present invention can reduce some orthogonal experiments and condition explorations, reduce time and experimental costs, and enable researchers to clearly observe the reaction conditions in the ammonia catalytic decomposition reaction area. Through information such as the concentration distribution of the reactants, the reaction performance and structural rationality of the catalyst can be evaluated, allowing researchers to more conveniently analyze the ammonia catalytic decomposition process and provide powerful guidance for the modification of experimental conditions or structural optimization of the catalyst in the next stage.

[0005] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0006] The present invention provides a method for simulating ammonia decomposition in a catalytic channel based on COMSOL simulation software, comprising the following steps:

[0007] S1: Add the zero-dimensional reaction engineering module based on COMSOL simulation software to determine the kinetic parameters of each elementary reaction in the ammonia decomposition reaction;

[0008] S2: Based on the material properties of the catalytic channel and the internal fluid of ammonia decomposition in the actual state, determine the material properties of the catalytic channel and the internal fluid in the simulation state;

[0009] The internal fluid includes protective gas, ammonia gas and reaction products during ammonia decomposition;

[0010] S3: Based on the catalytic channel of ammonia decomposition under actual conditions, a three-dimensional structural model of the catalytic channel under simulation conditions is constructed, and then material properties are applied to the three-dimensional structural model of the catalytic channel to construct the physical field;

[0011] S4: input the dynamic parameters into the physical field and select the control equation;

[0012] S5: According to the reaction conditions of ammonia decomposition under the actual state, the initial conditions and boundary conditions of ammonia decomposition under the simulation state are determined, and the three-dimensional structure model is meshed to solve the control equation to obtain the material distribution of ammonia decomposition at different reaction times in the catalytic channel.

[0013] Preferably, the elementary reactions in the ammonia decomposition reaction process are R1 to R6:

[0014]

[0015] In R1 to R6, * represents a catalytic vacancy, and H*, N*, NH*, NH2*, and NH3* represent adsorbed substances;

[0016] 2* indicates 2 catalytic vacancies.

[0017] Preferably, the kinetic parameters include the forward and reverse reaction pre-exponential factors and the forward and reverse reaction activation energies of each elementary reaction, or the reaction equilibrium constant.

[0018] Preferably, the catalytic channel is composed of a channel wall and a catalytic coating attached to the channel wall; the material of the catalytic coating is a precious metal porous catalyst.

[0019] Preferably, the physical field includes a single physical field and multiple physical fields;

[0020] The single physics interface is chemistry, and the multiphysics interface is the coupling of the transport of diluted species in porous catalysts and the Brinkman equation.

[0021] Preferably, the material properties include the material properties of the wall of the catalytic channel and the catalytic coating, the geometric dimensions and positional relationship of the catalytic channel, and the fluid properties of the internal fluid.

[0022] Preferably, the protective gas is argon; and the reaction products are hydrogen and nitrogen.

[0023] Preferably, the control equation is solved by using a transient solver in the simulation software and setting a time step for solving.

[0024] Compared with the prior art, the method for simulating ammonia decomposition in a catalytic channel based on COMSOL simulation software proposed in the present invention has the following advantages: by utilizing software simulation, the present invention can reduce some orthogonal experiments and condition explorations, reduce time and experimental costs, and enable researchers to clearly observe the reaction conditions in the ammonia catalytic decomposition reaction area, and evaluate the reaction performance and structural rationality of the catalyst through information such as the concentration distribution of the reactants, so that researchers can more conveniently analyze the ammonia catalytic decomposition process and provide powerful guidance for the modification of experimental conditions or structural optimization of the catalyst in the next stage. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 A comparison chart of the equilibrium conversion rates at different temperatures between the kinetic parameter model disclosed in the embodiment of the present invention and the actual experiment;

[0026] Figure 2 It is a grid division diagram of the catalytic channel model disclosed in the embodiment of the present invention, wherein Figure 3 (a) is a three-dimensional view of the catalytic channel model. Figure 3 (b) is the mesh division diagram of the catalytic channel model;

[0027] Figure 3 is a distribution diagram of ammonia concentration at different time periods of the simulated ammonia decomposition process in the catalytic channel disclosed in the embodiment of the present invention, wherein Figure 3 (a), (b), and (c) are the distribution of ammonia concentration in the catalytic channel at reaction times t = 0.1 s, t = 0.5 s, and t = 2 s, respectively;

[0028] Figure 4 The concentration distribution of the diagonal cross section of the reaction products hydrogen and nitrogen when ammonia is close to stable decomposition in the simulated ammonia decomposition process in the catalytic channel disclosed in the embodiment of the present invention is shown, wherein Figure 4 (a) is the nitrogen concentration distribution, Figure 4 (b) is the hydrogen concentration distribution. DETAILED DESCRIPTION

[0029] The present invention provides a method for simulating ammonia decomposition in a catalytic channel based on COMSOL simulation software, comprising the following steps:

[0030] S1: Add the zero-dimensional reaction engineering module based on COMSOL simulation software to determine the kinetic parameters of each elementary reaction in the ammonia decomposition reaction;

[0031] S2: Based on the material properties of the catalytic channel and the internal fluid of ammonia decomposition in the actual state, determine the material properties of the catalytic channel and the internal fluid in the simulation state;

[0032] The internal fluid includes protective gas, ammonia gas and reaction products during ammonia decomposition;

[0033] S3: Based on the catalytic channel of ammonia decomposition under actual conditions, a three-dimensional structural model of the catalytic channel under simulation conditions is constructed, and then material properties are applied to the three-dimensional structural model of the catalytic channel to construct the physical field;

[0034] S4: input the dynamic parameters into the physical field and select the control equation;

[0035] S5: According to the reaction conditions of ammonia decomposition under the actual state, the initial conditions and boundary conditions of ammonia decomposition under the simulation state are determined, and the three-dimensional structure model is meshed to solve the control equation to obtain the material distribution of ammonia decomposition at different reaction times in the catalytic channel.

[0036] The present invention adds a zero-dimensional reaction engineering module based on COMSOL simulation software to determine the kinetic parameters of each elementary reaction in the ammonia decomposition reaction process.

[0037] As an embodiment, each elementary reaction in the ammonia decomposition reaction process is a microscopic kinetic model of ammonia decomposition, specifically the following six elementary reactions R1 to R6;

[0038]

[0039] In R1 to R6, * represents a catalytic vacancy, and H*, N*, NH*, NH2*, and NH3* represent adsorbed substances;

[0040] 2* indicates 2 catalytic vacancies.

[0041] As an embodiment, the kinetic parameters include the forward and reverse reaction pre-exponential factors and the forward and reverse reaction activation energies of each elementary reaction, or the reaction equilibrium constant.

[0042] As an implementation mode, the kinetic parameters are determined by: obtaining the equilibrium conversion rate of catalytic decomposition of ammonia by experiment, and then optimizing and deducing the kinetic parameters through the zero-dimensional reaction engineering module of COMSOL simulation software based on the equilibrium conversion rate of catalytic decomposition of ammonia.

[0043] The present invention determines the material properties of the catalytic channel and the internal fluid in the simulation state based on the material properties of the catalytic channel and the internal fluid in the actual state of ammonia decomposition.

[0044] As an embodiment, the internal fluid includes a protective gas, ammonia and reaction products when ammonia is decomposed; the reaction products include hydrogen and nitrogen.

[0045] As an embodiment, the catalytic channel is composed of a channel wall and a catalytic coating attached to the channel wall; the material of the catalytic coating is a noble metal porous catalyst. As an embodiment, the noble metal is Ru.

[0046] As an embodiment, the material properties include the material properties of the wall of the catalytic channel and the catalytic coating, the geometric dimensions and positional relationship of the catalytic channel, and the fluid properties of the internal fluid.

[0047] The present invention constructs a three-dimensional structural model of the catalytic channel under a simulation state based on the catalytic channel of ammonia decomposition under an actual state, and then applies material properties to the three-dimensional structural model of the catalytic channel to construct a physical field.

[0048] As an implementation mode, the physical field includes a single physical field and a multi-physical field; the interface of the single physical field is chemistry, and the interface of the multi-physical field is the coupling of dilute species transport in porous catalysts and the Brinkman equation.

[0049] As an implementation method, the single physical field interface is used to solve the chemical reaction process in the entire model solution.

[0050] As an embodiment, a multiphysics interface is used to solve the adsorption and heterogeneous reaction processes involving chemical substances when ammonia and the reaction products pass through the gaps of the porous medium.

[0051] The present invention inputs the kinetic parameters into the physical field and selects the control equation; according to the reaction conditions of ammonia decomposition under the actual state, the initial conditions and boundary conditions of ammonia decomposition under the simulation state are determined, and the three-dimensional structural model is meshed to solve the control equation to obtain the material distribution of ammonia decomposition at different reaction times in the catalytic channel.

[0052] As an implementation method, the control equation is solved by using a transient solver in the simulation software and setting a time step for solving.

[0053] The technical solutions provided by the present invention are described in detail below in conjunction with the embodiments, but they should not be construed as limiting the protection scope of the present invention.

[0054] Example 1

[0055] S1: Add the zero-dimensional reaction engineering module based on COMSOL simulation software to determine the kinetic parameters of each elementary reaction in the ammonia decomposition reaction.

[0056] Specifically, the kinetic parameters described in this embodiment are obtained by the equilibrium conversion rate of ammonia catalytic decomposition of the catalyst (Ru) at five corresponding temperatures obtained in advance experiments, and then the kinetic parameters are optimized and deduced through the zero-dimensional reaction engineering module of COMSOL simulation software based on the equilibrium conversion rate of ammonia catalytic decomposition.

[0057] The kinetic parameters are shown in Table 1, where A if , A ibThey represent the forward reaction pre-exponential factor and reverse reaction pre-exponential factor of each elementary reaction (in 1 / s), E aif , E aib They represent the forward reaction activation energy and reverse reaction activation energy of each elementary reaction (in kcal / mol), k eqi Represents the reaction equilibrium constant of each elementary reaction (unit is 1), i=1,2,3,4,5,6.

[0058] The matching degree between the reaction model corresponding to the kinetic parameters in Table 1 and the equilibrium conversion rate of the actual experiment at different temperatures is as follows: Figure 1 As shown, it shows that the parameters are reasonable and the kinetic parameters of each elementary reaction in the ammonia decomposition reaction have been determined.

[0059] Table 1 Kinetic parameters of each elementary reaction of ammonia decomposition

[0060]

[0061]

[0062] S2: Based on the material properties of the catalytic channel and the internal fluid of ammonia decomposition in the actual state, determine the material properties of the catalytic channel and the internal fluid in the simulation state; the internal fluid includes the protective gas, ammonia gas and reaction products during ammonia decomposition.

[0063] Specifically, the material properties include: the cross section of the catalytic channel is a square with a side length of 1.22mm and a channel length of 30mm. The catalytic coating is attached to the channel wall with a thickness of 0.04545mm. Considering the square central symmetric structure of the channel, a 1 / 4 channel is considered to simplify the simulation calculation. For the catalytic layer, in order to simulate the actual structure as much as possible, the catalytic layer at the intersection of the square edges is thickened with a rounded shape, and the arc radius is 0.18182mm. The position relationship is as follows: Figure 2 As shown, Figure 2 is the modeling diagram of 1 / 4 catalytic channel, where Figure 2 (a) is the three-dimensional view of the model. Figure 2 (b) is the model mesh division diagram;

[0064] Among them, the catalytic coating material is the precious metal Ru, and the internal fluid includes a protective gas, reactants and reaction products; the protective gas is argon, the reactant is ammonia, and the reaction products are hydrogen and nitrogen. The parameters related to the type of material or fluid are called through the material library in the software according to the above-mentioned material type. The material property parameters of the catalytic coating are listed in Table 2. Unless otherwise specified, the remaining relevant parameters adopt the default values ​​in the software, such as the density of the protective gas argon, and the dynamic viscosity parameters are expressed as functions of the relevant temperature by default. The default gas_3(T) represents density, eta(T) represents dynamic viscosity, and the same applies to other gases. The reaction temperature in the catalytic channel is set to 595K, and the pressure is set to normal pressure and maintained at normal pressure.

[0065] Table 2 Material properties of catalytic coating

[0066]

[0067]

[0068] S3: Based on the catalytic channel of ammonia decomposition under actual conditions, a three-dimensional structural model of the catalytic channel under simulation conditions is constructed, and then material properties are applied to the three-dimensional structural model of the catalytic channel to construct a physical field.

[0069] S4: input the dynamic parameters into the physical field and select the control equation;

[0070] Specifically, the physical fields are single physical fields and multi-physical fields, the interface of the single physical field is chemistry; the interface of the multi-physical field is the coupling of the dilute species transport in the porous catalyst and the Brinkman equation (the dilute species transport in the porous catalyst is used as the species transport interface, and the Brinkman equation interface is used as the fluid flow interface to couple the above two interfaces).

[0071] The kinetic parameters of the six elementary reactions R1 to R6 are added to the chemical interface component, the protective gas argon is added, and the type is set to solvent; the mixture property type is set to dilute substances; and the dilute substance transport in the substance-coupled porous catalyst solved in substance matching.

[0072] The catalytic coating is set as a porous catalyst, and the effective diffusion coefficient of the internal material is calculated using the Bruggeman approximation. After the approximate calculation, the diffusion coefficient of each material is taken as 1×10 -5 m 2 / s. The gas flow is assumed to be stable, laminar and weakly compressible. Since the reaction itself has little effect on the system temperature in the entire reaction system, it is assumed that the system is constant and unaffected.

[0073] Specifically, the control equations are given by the computational model built into the COMSOL simulation software after the components are set up, where the mass transfer equation is expressed as:

[0074]

[0075] In this equation, represents the vector differential operator, u represents the velocity vector (in m / s), C i Indicates concentration (in mol / m 3 ), D i Table D i The diffusion coefficient (unit: m 2 / s), R i Indicates the reaction rate of a substance (in mol / (m 3 ·s)).

[0076] The substance adsorption equation is expressed as:

[0077]

[0078] In this equation, for substance i, C P,e,i represents the equilibrium amount of adsorption on the catalyst (in mol / kg), K L,i represents the Langmuir constant (in m 3 / mol), C Pmax,i Indicates the maximum adsorption capacity (in mol / kg), C i Indicates the molar concentration of a substance in the gas phase (in mol / m 3 ).

[0079] S5: According to the reaction conditions of ammonia decomposition under the actual state, the initial conditions and boundary conditions of ammonia decomposition under the simulation state are determined, and the three-dimensional structure model is meshed to solve the control equation to obtain the material distribution of ammonia decomposition at different reaction times in the catalytic channel.

[0080] The boundary conditions and initial conditions include: ammonia flows into the catalytic channel at the inlet, the velocity profile is flat, and the flow rate is set to 20 mL / min. The boundary conditions at the outlet are set to atmospheric pressure, and the normal gradients of the velocity and species mass fraction (relative to the flow direction) are set to zero (Neumann boundary conditions), no-slip boundary conditions are applied to the entire channel wall, and symmetric boundary conditions are applied to the symmetry plane.

[0081] The control equations are discretized using non-uniform adaptive grids, and the three-dimensional structure of the catalytic channel is gridded. Since the catalytic decomposition of ammonia begins to react and generate products at the inlet, the longitudinal grid sweep is set to an exponential change, which provides higher accuracy for the gas reaction simulation solution at the inlet section. The specific grid division is as follows: Figure 2 (b) As shown in the figure, during the entire simulation calculation process, the grid sequence of 53,270 free triangle grid units provides reasonable calculation accuracy.

[0082] The control equation is solved using the transient solver in the simulation software. The time step is set to solve the problem, and the material distribution in the catalytic channel at different reaction times is obtained to analyze the ammonia catalytic decomposition reaction. The output step is set to 0.05 seconds per step, and the ammonia catalytic decomposition in the catalytic channel when the reaction lasts for 2 seconds is analyzed. Specifically, the 3D model of ammonia concentration at different time periods based on COMSOL simulation software shows the following: Figure 3 As shown in (a~c), the ammonia concentration distribution at three time nodes of reaction time t=0.1s, 0.5s, and 2s was intercepted and analyzed. Among them, the ammonia concentration distribution was close to steady state at t=2s, and it was found that the ammonia flow ended the reaction at the front end of the channel in the steady state. This is related to the low gas supply of 20mL / min for the entire catalytic bed in the channel simulation and the high catalytic activity shown by the kinetic parameters of the catalyst.

[0083] from Figure 3 (a) It can be clearly seen that ammonia reacts rapidly after it enters through the inlet. Figure 3 Middle (a) ～ It can be seen from (c) that the ammonia concentration decreases rapidly along the flow direction of the fluid, which is related to the high specific surface area and high adsorption rate of the catalyst and the high diffusion coefficient inside the catalyst. Figure 3 Middle (a) ～ (c) It is found that the flow shape of ammonia is almost invisible in the free flow area in the channel. This is because the high diffusion coefficient of ammonia at higher temperatures and the slower channel gas flow rate make the concentration of ammonia almost unchanged in the plane perpendicular to the flow direction.

[0084] When ammonia is close to stable decomposition at t = 2s, the corresponding product hydrogen and nitrogen diagonal cross-section concentration distribution is shown in Figure 4As shown in (ab), the concentration distributions of the two are similar but the concentration values ​​are different. The concentrations of both begin to rise after a certain distance from the inlet, which is because the inlet is the ammonia reaction section. The images of the two show complementary characteristics to the ammonia concentration distribution, indicating that ammonia generates corresponding gaseous products very quickly after decomposition, which is consistent with the actual experimental situation. At the same time, the concentration changes of the two gaseous products along the plane perpendicular to the axis are consistent with ammonia, which is also related to the high diffusion coefficient and low flow rate of hydrogen and nitrogen at high temperature. The above analysis further confirms the feasibility of the model.

[0085] Although the above embodiment describes the present invention in detail, it is only a part of the embodiments of the present invention, not all of the embodiments. Other embodiments can be obtained based on this embodiment without creativity, and these embodiments all fall within the protection scope of the present invention.

Claims

1. A method for simulating ammonia decomposition in a catalytic channel based on COMSOL simulation software, characterized in that: The following steps are involved: S1: Add the zero-dimensional reaction engineering module based on COMSOL simulation software to determine the kinetic parameters of each elementary reaction in the ammonia decomposition reaction; S2: Based on the material properties of the catalytic channel and the internal fluid of ammonia decomposition in the actual state, determine the material properties of the catalytic channel and the internal fluid in the simulation state; The internal fluid includes protective gas, ammonia gas and reaction products during ammonia decomposition; S3: Based on the catalytic channel of ammonia decomposition under actual conditions, a three-dimensional structural model of the catalytic channel under simulation conditions is constructed, and then material properties are applied to the three-dimensional structural model of the catalytic channel to construct the physical field; S4: input the dynamic parameters into the physical field and select the control equation; S5: According to the reaction conditions of ammonia decomposition under the actual state, the initial conditions and boundary conditions of ammonia decomposition under the simulation state are determined, and the three-dimensional structure model is meshed to solve the control equation to obtain the material distribution of ammonia decomposition at different reaction times in the catalytic channel.

2. The method according to claim 1, characterized in that The elementary reactions in the ammonia decomposition reaction are R1 to R6: In R1 to R6, * represents a catalytic vacancy, and H*, N*, NH*, NH2*, and NH3* represent adsorbed substances; 2* indicates 2 catalytic vacancies.

3. The method according to claim 1, characterized in that The kinetic parameters include the forward and reverse reaction pre-exponential factors and the forward and reverse reaction activation energies of each elementary reaction, or the reaction equilibrium constant.

4. The method according to claim 1, characterized in that The catalytic channel is composed of a channel wall and a catalytic coating attached to the channel wall; the material of the catalytic coating is a precious metal porous catalyst.

5. The method according to claim 1, characterized in that The physical field includes a single physical field and a multi-physical field; The single physics interface is chemistry, and the multiphysics interface is the coupling of the transport of diluted species in porous catalysts and the Brinkman equation.

6. The method according to claim 4, characterized in that The material properties include the material properties of the wall surface of the catalytic channel and the catalytic coating, the geometric dimensions and positional relationship of the catalytic channel, and the fluid properties of the internal fluid.

7. The method according to claim 1, characterized in that The protective gas is argon; the reaction products are hydrogen and nitrogen.

8. The method according to claim 1, characterized in that The control equation is solved by using a transient solver in the simulation software and setting a time step to solve the equation.

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