A design method of a solar-driven porous media methane dry reforming reactor

Abstract

The application discloses a design method of a solar-driven porous medium methane dry reforming reactor, constructs a multi-physical field coupling model of the porous medium, deduces a design criterion formula of the methane reforming reactor based on the results of different pore size porosity models, finds the influence law of the porous medium structure on the solar energy conversion efficiency, and can screen out the porous skeleton parameter range for realizing high efficiency according to the criterion formula, thereby guiding the construction of the reactor.

Description

Technical Field

[0001] This invention belongs to the technical field of methane dry reforming reactors, specifically relating to a design method for a solar-driven porous media methane dry reforming reactor. Background Technology

[0002] Solar-driven methane reforming can reduce emissions of two greenhouse gases (CH4, CO2) while converting unstable solar energy into chemical energy for storage. Over the past few decades, dry methane reforming to produce syngas (H2, CO) has attracted considerable attention. Many studies have focused on the synthetic design of methane reforming catalysts to improve photothermal reaction rates and resist carbon buildup. However, reactor design has received less attention. Currently, methane reforming reactors can be broadly classified into two types: fixed-bed and porous framework reactors. The latter, due to its high surface area, good thermal conductivity, and long radiative transfer distance, has become the preferred choice for large-scale methane reforming to syngas production. Solar-driven porous framework methane reforming reactors involve the coupling of multiple physical fields, including radiative transfer, heat and mass transfer, and chemical reaction. The structural parameters (pore size, porosity) of the porous framework have a significant impact on this multi-physical field coupling, thus affecting the solar-to-chemical energy conversion efficiency. Therefore, designing the structural parameters of the porous framework is a key factor in improving the energy absorption efficiency of methane reforming.

[0003] Exhaustive methods are not very effective in guiding reactor design for the study of porous structures and are inefficient. Therefore, there is an urgent need for universal guidelines to guide the development of efficient solar cell absorption reactors. Summary of the Invention

[0004] The purpose of this section is to outline some aspects of the embodiments of the present invention and to briefly describe some preferred embodiments.

[0005] As one aspect of the present invention, the present invention provides a design method for a solar-driven porous medium methane dry reforming reactor, which is to simultaneously satisfy the conditions shown in formulas (1) to (3) to obtain the optimal pore size, porosity and dimensions of the porous medium methane dry reforming reactor;

[0006]

[0007] Among them, E app Let T be the activation energy of methane (J / mol), R be the universal gas constant (8.3145 J / mol·K), and T be the activation energy of methane. b The bulk gas temperature is (K), ΔH is the heat of reaction of methane reforming (J / mol), and r MDR The reforming reaction rate (mol / m 3 ·S), R Ωe h represents the feature size (m) of the skeleton unit. vVolumetric convective heat transfer coefficient (W / m) 3 ·K), S a The specific surface area (m²) of the skeleton unit -1 ), k me C is the external mass transfer coefficient (m / s). CH4，b The concentration of methane in the bulk gas stream (mol / m³) 3 ), where n is the reaction order, ΔT is the gas-solid temperature difference (K), and T S Let T be the solid temperature (K), Z be the position when ΔT is 0, and L be the length of the porous medium (m).

[0008] As a preferred embodiment of the design method of the solar-driven porous media methane dry reforming reactor of the present invention: the material of the porous media methane dry reforming reactor is a porous foam material, the components of which include silicon carbide, zirconium oxide, cerium oxide and nickel foam.

[0009] As a preferred embodiment of the design method for the solar-driven porous medium methane dry reforming reactor described in this invention: the solid boundary, including the heat flux density boundary condition, is: q = 400 KW / m 2 .

[0010] As a preferred embodiment of the design method of the solar-driven porous medium methane dry reforming reactor of the present invention: the reactants of the methane dry reforming reaction are methane and carbon dioxide, and the molar ratio of methane to carbon dioxide is 1:1.

[0011] As a preferred embodiment of the design method of the solar-driven porous media methane dry reforming reactor described in this invention, the porous media porosity of the solar-driven porous media methane dry reforming reactor is 0.7 to 0.95 (unit: 1).

[0012] As a preferred embodiment of the design method of the solar-driven porous media methane dry reforming reactor described in this invention, the pore size of the solar-driven porous media methane dry reforming reactor is 0.5 mm to 5 mm.

[0013] As a preferred embodiment of the design method for the solar-driven porous media methane dry reforming reactor described in this invention: the fluid boundary inlet velocity is v. inlet =0.03m / s.

[0014] As a preferred embodiment of the design method for the solar-driven porous media methane dry reforming reactor described in this invention: the porous media methane dry reforming reactor is cylindrical in shape, and its optimal length is: the length of the porous media methane dry reforming reactor when the temperature difference between the gas and fluid at the outlet of the porous media methane dry reforming reactor is 0.

[0015] The beneficial effects of this invention are as follows: This invention constructs a multi-physics coupling model of porous media, and derives the design criteria for a methane reforming reactor based on the results of models with different pore sizes and porosities. It also discovers the influence of porous media structure on solar energy conversion efficiency. Based on this criterion, the range of porous framework parameters that can achieve high efficiency can be selected, thereby guiding the construction of the reactor. Attached Figure Description

[0016] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. Wherein:

[0017] Figure 1 The solar energy conversion efficiency is calculated for different pore size-porosity combinations.

[0018] Figure 2 These are the temperature distribution curves of the fluid and solid along the central axis under the same optical thickness.

[0019] Figure 3 This represents the fluid-solid temperature distribution when the optical thickness is different but the pore size / porosity is the same.

[0020] Figure 4 This is a schematic diagram of the skeletal unit model of the supported catalyst.

[0021] Figure 5 The results are for the mass transfer criterion under different porosities and pore sizes.

[0022] Figure 6 The results are for the heat transfer criterion under different porosities and pore sizes.

[0023] Figure 7 A schematic diagram of a solar-driven porous medium methane reforming reaction model. Detailed Implementation

[0024] To make the above-mentioned objectives, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to specific examples.

[0025] This invention constructs a multi-physics coupling model of porous media, and derives a design criterion for a methane reforming reactor based on the results of models with different pore sizes and porosities. It also discovers the influence of porous media structure on solar energy conversion efficiency. Based on this criterion, the range of porous framework parameters that can achieve high efficiency can be selected, thereby guiding the construction of the reactor.

[0026] A solar-driven porous medium methane reforming reaction model was constructed using Fluent software UDF code, including radiative transfer, heat and mass transfer, chemical reactions, and gas flow within the porous medium. A schematic diagram of the physical model is shown below. Figure 7 As shown: an axisymmetric model with a length of 50mm and a radius of 40mm.

[0027] Continuity equation:

[0028]

[0029] Momentum conservation equation:

[0030]

[0031] Where φ is the porosity, d p Let ρ be the aperture (m), and u represent the velocity vector (m / s). f and μ f The density of the mixed fluid (kg / m³) 3 ) and dynamic viscosity (kg / m·s), P is pressure (Pa), and F is the resistance source term of the porous medium.

[0032] Fluid energy conservation equation:

[0033]

[0034] Where c pf T is the specific heat capacity of the fluid (J / mol·K). f Let λ be the fluid temperature (K). eff，f The effective thermal conductivity of the fluid (W / m·K)

[0035] Where S f For the fluid source term, it is the sum of convective heat transfer and chemical reaction endothermic heat transfer:

[0036] S f =h v (T s -T f )+S chem

[0037] h v Volumetric convective heat transfer coefficient (W / m) 3 ·K), T s T f For solid and fluid temperatures (K), S chem This is a chemical source term.

[0038] The volumetric convection heat transfer coefficient is determined according to the Wu model:

[0039]

[0040] For the energy equation in the solid term:

[0041]

[0042]

[0043] λ eff，s ε w T w , σ, T amb These represent the effective thermal conductivity of the solid (W / m·K), surface emissivity, solid surface temperature (K), and Stefan Boltzmann constant (W / m²). 2 ·K 4 The effective thermal conductivity of fluids and solids at ambient temperature (K) is expressed as follows:

[0044] λ eff，f =φλ f

[0045]

[0046] λ f , λ s Thermal conductivity of fluids and solids (W / m·K)

[0047] Radiative transfer equation for porous media:

[0048] Radiative heat transfer in porous media requires solving the following equations:

[0049]

[0050] The absorption coefficient, scattering coefficient, and extinction coefficient are expressed as:

[0051]

[0052]

[0053]

[0054] ε is the solid emissivity

[0055] To more accurately describe the radiation within porous media, the scattering phase function of porous media under diffuse reflection conditions is defined as:

[0056]

[0057] The radiation source term is represented as:

[0058]

[0059] Reaction model:

[0060] Considering the methane reforming reaction and the counter-current water gas side reaction, the reactions occurring in the system are as follows:

[0061]

[0062]

[0063] Using the Langmuir adsorption reaction model:

[0064]

[0065]

[0066]

[0067]

[0068]

[0069]

[0070]

[0071]

[0072] P CO2 P CH4 These are the partial pressures (atm) of carbon dioxide and methane, respectively.

[0073] The chemical reaction source term is:

[0074] S chem =R MDR ΔH MDR +R RWGS ΔH RWGS

[0075] Boundary conditions: Apply constant heat flux density boundary condition q = 400 KW / m to the solid boundary. 2 The fluid boundary is set with an inlet flow velocity of v. inlet =0.03 m / s, the reactants are a mixture of methane and carbon dioxide in a molar ratio of 1:1. The porosity of the porous media in the calculation model ranges from 0.7 to 0.95, with a calculation step size of 0.5, and the pore size ranges from 0.5 mm to 5 mm, with a step size of 0.5 mm.

[0076] Efficiency Results: With a porosity step size of 0.05 and a pore size step size of 0.5, the solar-to-chemical energy conversion efficiency was calculated for different combinations, based on the STF efficiency conversion formula:

[0077]

[0078] Where m CO，out , Δm CH4，out The values ​​are the mass flow rates (kg·s) of CO, H2, and CH4 produced at the outlet, respectively. The calculation results are as follows: Figure 1 As shown.

[0079] The exhaustive calculations show that energy efficiency increases with increasing porosity, reaching its maximum at a porosity of 0.95 for the same pore size. The optimal pore size is found in the range of 1-2 for the same porosity. We attempt to find a method for porosity-pore size combination design and derive a general criterion for selecting porosity and pore size. The structural parameters of porous media directly affect the radiation penetration depth and convective heat transfer coefficient, thus influencing the fluid-solid temperature distribution, and consequently the reaction rate and energy efficiency. Therefore, understanding the temperature distribution patterns under different parameters is essential.

[0080] Figure 2 The curves show the temperature distribution of the fluid and solid along the central axis at the same optical thickness: (a) optical thickness value of 7.5, (b) optical thickness value of 15, (c) optical thickness value of 30. From... Figure 2 As can be seen, with the same optical thickness, the higher the maximum fluid temperature, the higher the efficiency. Looking at the temperature distributions for the three optical thicknesses, the red temperature distribution in each figure shows the highest efficiency and the highest fluid temperature. A common characteristic is that the aperture cannot be less than 1 mm. For apertures less than 1 mm, the increased convective heat transfer coefficient significantly reduces the thermal non-equilibrium zone, causing the fluid-solid temperature curves to converge prematurely, resulting in an excessively low temperature in the thermal equilibrium zone and a decreased reaction rate. For apertures greater than 1 mm (such as 0.9-1 and 0.8-2 of optical thickness 15), although the blue temperature is slightly higher in the thermal equilibrium zone, the red fluid temperature is higher in the thermal non-equilibrium zone. Since the reaction rate in the thermal non-equilibrium zone is greater than that in the thermal equilibrium zone, the latter is more efficient than the former.

[0081] Figure 3 The fluid-solid temperature distribution is shown for different optical thicknesses but the same pore size / porosity. (a) Pore size is 5 mm. (b) Porosity is 0.95. (c) Pore size is 2 mm. For cases with different optical thicknesses but the same pore size or porosity, excessively small porosity or pore size will hinder the depth of radiation penetration, resulting in a small thermal non-equilibrium region and a reduced subsequent reaction rate. Excessively large pore size will lead to insufficient heat transfer, with heat exchange not completed before the outlet, thus reducing efficiency. Therefore, the optimal reaction temperature distribution is one where heat exchange is just completed at the outlet.

[0082] By comparing temperatures, we have obtained the optimal temperature distribution curve for achieving the highest efficiency. However, the external heat and mass transfer and reaction kinetics characteristics of porous media require quantitative expressions to describe the influence of porous media structure on efficiency, in order to screen out combinations that satisfy the criterion and thus determine the pore size and porosity that yield the highest efficiency. Therefore, starting from the basic macroscopic equations of reaction kinetics and energy balance equations, we derived the criterion for a catalyst-supported porous media methane reforming reactor.

[0083] 1. Derivation of the external heat transfer criterion:

[0084] Taking a single skeletal unit as the object of analysis, the macroscopic reaction rate equation for methane can be written in the following form:

[0085]

[0086] In the above formula, k0 is the pre-exponential factor, E is the activation energy of methane, and C CH4 This represents the methane concentration.

[0087] The function f(X) is expanded as a Taylor series at T0:

[0088] f(x)=f(x0)+f′(x0)(x-x0)+o(x-x0)

[0089] Ignoring infinitesimal terms and rearranging the reaction rate equation:

[0090]

[0091] in,

[0092] T0: Temperature of the main airflow component

[0093] k0: Pre-exponential factor.

[0094] Energy balance within the skeletal unit: the heat absorbed during dry methane reforming equals the heat transferred via convection.

[0095]

[0096] Substituting T-T0 from the reaction rate equation into:

[0097]

[0098] To ensure that the actual reaction rate does not deviate from the reaction rate at T0, a deviation of no more than 5% is considered acceptable.

[0099]

[0100] In the above formula:

[0101] V pThe volume of space occupied by a single skeleton unit (m3)

[0102] S e The surface area (m²) of a single skeleton unit is determined by the following relationship for the feature size of the skeleton unit:

[0103]

[0104] r MDR Methane reaction rate (mol / m³s)

[0105] h e : Convection heat transfer coefficient (W / m2 K) h v Volumetric convective heat transfer coefficient (W / m3 K), S a Specific surface area of ​​the skeleton (m⁻¹)

[0106] The following formula is derived for the external heat transfer criterion of the external frame:

[0107]

[0108] 2. Derivation of the criterion for external mass transfer:

[0109] The effective factor for external diffusion is defined as:

[0110]

[0111] On a single skeletal unit, the amount of airflow diffused outward to the outer surface is equal to the amount of surface reaction:

[0112]

[0113] k is the rate constant for the methane reaction, C CH4，b and C CH4，s denoted as methane concentration in the bulk gas stream and reactant concentration on the outer surface of the catalyst, and n is the reaction order.

[0114] Divide by

[0115]

[0116]

[0117]

[0118] Define the Damkohler criterion number:

[0119]

[0120] For a single skeleton unit:

[0121]

[0122]

[0123] To ensure an efficiency factor greater than 95%, external diffusion is assumed to be unrestricted. This is because the following relationship exists between the two:

[0124]

[0125] Therefore, when η e >0.95, Yes

[0126] Calculation of characteristic dimension (m):

[0127] External mass transfer coefficient (m / s):

[0128] Where D f The intrinsic diffusion rate of methane (m 2 / S)Shocf is the Sherwood number:

[0129] Sh OCF =1.0Re 0.47 Sc 1 / 3 Fg

[0130] The Reynolds number, Schmidt number, and geometric factor are calculated using the following formulas:

[0131]

[0132] The volume of porous foam is calculated using the following formula:

[0133]

[0134] The surface diameter of the porous framework is equal to the sum of the pore diameter and the support thickness.

[0135] d f =d p +t s

[0136] Foam relative density:

[0137]

[0138] Porosity: φ = 1 - ρ r Specific surface area: Surface area: S ga (m 2 ) = V OCF S ga Catalyst loading: Catalyst layer thickness:

[0139] In the above formula, d p For aperture (m); μ f ρ is the dynamic viscosity of the mixed gas (kg / m·s); u is the inlet velocity (m / s); ρ is the inlet velocity. f Density of the mixed gas (kg / m³) 3 ).

[0140] Evaluation of external heat and mass transfer: Based on the derived criteria, the mass and heat transfer criteria under different combinations are calculated to verify the applicability of the criteria.

[0141] Calculation of the mass transfer criterion: Figure 5 The results of the mass transfer criterion equations under different porosities and pore sizes are as follows. Figure 6 The results show the heat transfer criterion for different porosities and pore sizes. The mass transfer criterion results along the axial direction show that all combinations conform to the mass transfer criterion, with none exceeding the upper limit of 0.15. From the heat transfer results, pore sizes larger than 2 mm do not satisfy the heat transfer criterion for any porosity. This means that the endothermic reaction and convective heat transfer are mismatched, i.e., the heat transfer process cannot meet the endothermic reaction requirements, thus reducing efficiency. Table 1, arranged by optical thickness, shows the relationship between optical thickness and efficiency for different combinations. It can be seen that combinations that do not satisfy the heat transfer criterion have very low efficiency.

[0142] Table 1

[0143]

[0144]

[0145] From the perspective of optical thickness arrangements, combinations that do not meet heat transfer conditions have very low efficiency. Even combinations that meet heat transfer conditions, with large optical thicknesses, also show low efficiency. This is because with excessively thick optical thicknesses, the temperature reaches thermal equilibrium at the front end of the porous medium, and subsequent temperature decreases result in a very slow reaction rate, leading to reduced efficiency. Combining heat and mass transfer characteristics with temperature distribution, we propose a criterion for methane reforming in porous media:

[0146]

[0147] In the above formula, E app Let T be the activation energy of methane (J / mol), R be the universal gas constant (8.3145 J / mol·K), and T be the activation energy of methane. b The bulk gas temperature is (K), ΔH is the heat of reaction of methane reforming (J / mol), and r MDR The reforming reaction rate (mol / m 3 ·s), R Ωe h represents the feature size (m) of the skeleton unit. vVolumetric convective heat transfer coefficient (W / m) 3 ·K), S a The specific surface area (m²) of the skeleton unit -1 ), k me C is the external mass transfer coefficient (m / s). CH4，b The concentration of methane in the bulk gas stream (mol / m³) 3 ), where n is the reaction order, ΔT is the gas-solid temperature difference (K), and T S Let T be the solid temperature (K), Z be the position when ΔT is 0, and L be the length of the porous medium (m).

[0148] The highest efficiency is achieved when the pore size and porosity combination simultaneously meets the above criteria. Based on this criterion, foam design for high-efficiency driven methane reforming reactors can be guided to obtain the highest solar-to-chemical energy conversion efficiency.

[0149] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A design method for a solar-driven porous media methane dry reforming reactor, characterized in that: By simultaneously satisfying the conditions shown in equations (1) to (3), the optimal pore size, porosity, and dimensions of the porous medium methane dry reforming reactor can be obtained. ； Among them, E app For methane activation, the unit is J / mol; R is the universal gas constant, which is 8.3145 J / mol·K; T b ΔH is the bulk gas temperature, in K; ΔH is the heat of reaction for methane reforming, in J / mol; r MDR The reforming reaction rate is expressed in mol / m³. 3 ·s,;R Ωe h represents the feature size of the skeleton unit, in meters (m). v Volumetric convective heat transfer coefficient, in units of W / m 3 ·K;S a The specific surface area of ​​the skeleton unit, in m². -1 ;k me C is the external mass transfer coefficient, with units of m / s; CH4,b The concentration of methane in the bulk gas stream is expressed in mol / m³. 3 n is the reaction order; ΔT is the gas-solid temperature difference, in K; T S Z represents the solid temperature in Kelvin (K); Z represents the length when ΔT is 0 in meters (m); and L represents the length of the porous medium in meters (m).

2. The design method of the solar-driven porous media methane dry reforming reactor according to claim 1, characterized in that: The porous medium methane dry reforming reactor is made of a porous foam material, the components of which include silicon carbide, zirconium oxide, cerium oxide, and nickel foam.

3. The design method of the solar-driven porous media methane dry reforming reactor according to claim 1 or 2, characterized in that: The solid boundary condition, including the heat flux density boundary condition, is as follows: q =400 KW / m 2 .

4. The design method of the solar-driven porous media methane dry reforming reactor according to claim 1 or 2, characterized in that: The reactants of the methane dry reforming reaction are methane and carbon dioxide, with a molar ratio of methane to carbon dioxide of 1:

1.

5. The design method of the solar-driven porous media methane dry reforming reactor according to claim 1 or 2, characterized in that: The porous media porosity of the solar-driven porous media methane dry reforming reactor is 0.7~0.95, with units of 1.

6. The design method of the solar-driven porous media methane dry reforming reactor according to claim 1 or 2, characterized in that: The pore size of the solar-driven porous media methane dry reforming reactor is 0.5 mm to 5 mm.

7. The design method of the solar-driven porous media methane dry reforming reactor according to claim 1 or 2, characterized in that: The inlet velocity of the fluid boundary is V inlet =0.03 m / s .

8. The design method of the solar-driven porous media methane dry reforming reactor according to claim 1 or 2, characterized in that: The porous media methane dry reforming reactor is cylindrical in shape, and its length is the length of the porous media methane dry reforming reactor when the temperature difference between the gas and fluid at the outlet of the porous media methane dry reforming reactor is 0.

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