A numerical simulation method for the whole process of plastic melting and pyrolysis

Abstract

The present application relates to a kind of numerical simulation methods of plastic melting to pyrolysis whole process, which includes the following steps: establishing the reaction model of plastic single particle melting pyrolysis;Based on the calculation model of flow, heat transfer and reaction of reaction model;According to the actual size of reactor, establish geometric model and divide grid;According to the calculation model of flow, heat transfer and reaction and geometric model, calculation is carried out, and the whole process of plastic single particle melting to pyrolysis is solved;Extract and analyze the plastic single particle melting pyrolysis process data obtained by calculation, obtain the change law of temperature field, melting rate and product composition in reactor.Compared with prior art, the present application has the advantages of considering the heat transfer and reaction characteristics of melting stage, more accurate numerical prediction of pyrolysis, and being beneficial to the optimization of key parameters of melting to pyrolysis whole process in reactor.

Description

Technical Field

[0001] The present invention relates to the field of plastic resource processing, and in particular to a numerical simulation method for the entire process from plastic melting to pyrolysis. Background Art

[0002] With the development of industries like express delivery, food delivery, and packaging, daily plastic consumption is increasing, projected to reach approximately 590 million tons by 2050. Statistics show that the global plastic waste recycling rate is only 9%, with the remainder ending up in landfills or incinerators. Landfills are becoming increasingly restricted due to their potential harm to the atmosphere, soil, and water. Direct incineration of waste plastics produces harmful gases and is a waste of resources.

[0003] Plastic pyrolysis is the effective decomposition of large-molecule organic matter into small-molecule organic matter under high temperature, oxygen-free or low-oxygen environment. It is an effective resource utilization method. From the perspective of carbon neutrality and resource utilization, the high-value recycling of waste plastics has the dual attributes of energy and environmental protection.

[0004] The pyrolysis of plastics differs from the pyrolysis of general solid wastes such as biomass and pulverized coal in that plastics undergo a melting process from solid to liquid and a decomposition process from liquid to gas. During the pyrolysis of domestic waste, the heat transfer characteristics of the plastic melting process affect the overall pyrolysis efficiency and have a significant impact on the products and reactions. In order to achieve high heat transfer and avoid the blockage problems that are prone to occur in pilot and industrial-scale systems, the design of advanced reactors for pyrolysis of plastics needs to consider the impact of the plastic melting and flow on the heat generation during the pyrolysis process, thereby improving and increasing the energy efficiency of the pyrolysis process. At present, there has been no detailed study on the heat transfer characteristics of the entire process from melting of a single plastic particle to pyrolysis.

[0005] A search revealed Chinese invention patent application publication number CN115935592A, which discloses a multi-component domestic waste pyrolysis numerical simulation method based on Mfix. This method is used to simulate the pyrolysis of four typical types of domestic waste: biomass, plastics, paper, and fabric. However, this existing patent application fails to simulate the melting process of plastics.

[0006] How to achieve numerical simulation of the entire process from melting to pyrolysis of a single plastic particle in order to improve the energy efficiency of the pyrolysis process has become a technical problem that needs to be solved. Summary of the Invention

[0007] The purpose of the present invention is to overcome the defects of the above-mentioned prior art and provide a numerical simulation method for the entire process from plastic melting to pyrolysis.

[0008] The purpose of the present invention can be achieved by the following technical solutions:

[0009] According to one aspect of the present invention, a numerical simulation method for the entire process from plastic melting to pyrolysis is provided, the method comprising the following steps:

[0010] Step S1, establishing a reaction model for melting and pyrolysis of a single plastic particle;

[0011] Step S2: establishing a calculation model of flow, heat transfer and reaction based on the reaction model of step S1;

[0012] Step S3: Establishing a geometric model and dividing the grid according to the actual size of the reactor;

[0013] Step S4: performing calculations based on the flow, heat transfer, and reaction calculation models of step S2 and the geometric model of step S3 to solve the entire process from melting to pyrolysis of a single plastic particle;

[0014] Step S5: extract and analyze the data of the melting and pyrolysis process of the single plastic particle calculated in step S4 to obtain the changing patterns of the temperature field, melting rate and product composition in the reactor.

[0015] Preferably, the reaction model of the melting and pyrolysis of a single plastic particle in step S1 includes:

[0016] Measure characteristic parameters of the plastic pellet melting process;

[0017] The pyrolysis reaction rate of plastic particles is analyzed and summarized based on the measured characteristic parameters;

[0018] The distribution of plastic particle pyrolysis products is analyzed and summarized based on the measured characteristic parameters.

[0019] More preferably, the characteristic parameters of the plastic particle melting process include density, specific heat capacity, thermal conductivity and melting temperature.

[0020] Preferably, the reaction model is a multi-step reaction model, including models of plastic molecules, molten plastic, wax, oil and non-condensable gas, and the reaction model consists of multiple irreversible first-order reactions.

[0021] Preferably, the calculation model of flow, heat transfer and reaction includes mass conservation equation, momentum conservation equation, energy conservation equation and component transport equation;

[0022] The mass conservation equation:

[0023]

[0024] Where, is the velocity, m / s; ρ is the density, kg / m 3 ;

[0025] The momentum conservation equation:

[0026]

[0027] Among them, S is the momentum source term; F st is the surface tension term, p is the pressure, in Pa; μ is the dynamic viscosity, in Pa·s; g is the acceleration due to gravity, in m / s 2 ; I is the tensor; σ is the surface tension coefficient; is the normal interface unit vector; δ is the Dirac function on the interface, M is the melting shape constant, usually 100000; ω is a small value introduced to prevent β = 0, usually 0.001; β is the liquid fraction;

[0028] The energy conservation equation:

[0029]

[0030] Where h is the convective heat transfer coefficient, W / (K·m 2 ); k is thermal conductivity, W / (K·m); β is liquid fraction; L m is the heat of fusion, J / kg; T is the temperature, unit is K; Φ is the viscous dissipation; p is the pressure, unit is Pa; ρ is the density; is the speed, m / s;

[0031] The component transport equation:

[0032]

[0033] Where Y gn is the mass fraction of component n in the product; D gn is the diffusion coefficient of component n in the product; ρ gn is the density of component n in the product; is the heat source term of component n in the product; u is the velocity.

[0034] Preferably, the calculation model of the movement, heat transfer and reaction is solved by fluid dynamics, specifically: the fluid volume model and enthalpy pore technology are selected to calculate the heat transfer and flow of the plastic melting process, the k-ε model is selected to calculate the influence of gas on turbulence, and the implicit operator splitting algorithm of pressure is selected to calculate the coupling between pressure and velocity.

[0035] Preferably, the calculation model of the dynamics, heat transfer and reaction sets the gas inlet as the flow inlet boundary condition, the outlet as the pressure outlet boundary condition, and the reactor wall as the no-slip adiabatic boundary condition.

[0036] Preferably, the initial conditions of the calculation model of the dynamics, heat transfer and reaction are set as follows: plastic particles are placed in the center of the reactor, the initial temperature is set to 300K, and nitrogen is filled in.

[0037] Preferably, in step S4, boundary conditions of different plastic types, air flow temperatures, and velocities are set, geometric models of different particle sizes and shapes are established, and the pyrolysis process of plastic particles under different geometric parameters is calculated and solved;

[0038] The unsteady-state solver is selected for calculation and solution. During the iteration process, the pressure equation is discretized using the standard format, and the momentum equation is discretized using the second-order upwind format. The temperature field and component field of the plastic particle melting and pyrolysis process under different working conditions are obtained.

[0039] Preferably, the step S5 includes:

[0040] S501, extracting key data of temperature, melting rate, and product composition during the melting and pyrolysis process of plastic particles;

[0041] S502. Based on the data extracted in S501, analyze and summarize the changing patterns of the temperature field, melting rate and product composition in the reactor when different materials, air flow temperatures and air flow velocities are used.

[0042] Compared with the prior art, the present invention has the following beneficial effects:

[0043] 1) The numerical simulation method of the present invention involves the entire process from plastic melting to pyrolysis. By establishing a reaction model for the melting and pyrolysis of a single plastic particle, establishing a calculation model for flow, heat transfer and reaction, establishing the actual size of the reactor, establishing a geometric model and dividing the grid, the entire process from the melting and pyrolysis of a single plastic particle is solved, and the change pattern of the temperature field, melting rate and product composition in the reactor is obtained through analysis. Since the heat transfer and reaction characteristics of the melting stage are taken into account, the numerical prediction of pyrolysis is more accurate, thereby better realizing the optimization of key parameters of the entire process from melting to pyrolysis in the reactor.

[0044] 2) The present invention adopts a multi-step reaction model of plastic pyrolysis and selects CFD to solve the calculation model of flow, heat transfer and reaction. It solves and calculates the pyrolysis process of single plastic particles under different geometric parameters, different plastic types, airflow temperature, velocity and other boundary conditions. It can couple the plastic melting process and pyrolysis process to comprehensively analyze the temperature field and component field under different working conditions. The data obtained is closer to the actual operation data and the results are more accurate. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 Schematic diagram of the numerical simulation method of the present invention;

[0046] Figure 2 Schematic diagram of the geometric model of the reactor of the present invention;

[0047] Figure 3This is a schematic diagram of the axial temperature distribution in the reactor of the present invention;

[0048] Figure 4 Schematic diagram of volume fraction of the melting process in the reactor of the present invention;

[0049] Figure 5 Schematic diagram of wax concentration distribution in the reactor of the present invention;

[0050] Figure 6 Schematic diagram of oil concentration distribution in the reactor of the present invention;

[0051] Figure 7 Schematic diagram of gas concentration distribution in the reactor of the present invention. DETAILED DESCRIPTION

[0052] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts should fall within the scope of protection of the present invention.

[0053] This embodiment relates to a numerical simulation method for the entire process from plastic melting to pyrolysis, such as Figure 1 As shown, the method includes the following steps:

[0054] Step S1: establishing a reaction model for the melting and pyrolysis of a single plastic particle and measuring and obtaining experimental data;

[0055] Step S2: establishing flow, heat transfer and reaction models;

[0056] Step S3: Establishing a geometric model and dividing the grid according to the actual size of the reactor;

[0057] Step S4, performing computational fluid dynamics (CFD) calculations based on the flow, heat transfer, and reaction models of step S2 and the geometric model of step S3;

[0058] Step S5: extracting the data of the melting and pyrolysis process of the single plastic particle calculated in step S4 and performing comparative analysis with the data obtained from the experiment in step S1.

[0059] The reaction model for the melting and pyrolysis of a single plastic particle established in step S1 includes:

[0060] 1) Measure the physical properties of plastic particles;

[0061] 2) Summarize the pyrolysis reaction rates of plastic particles;

[0062] 3) Summarize the distribution of pyrolysis products of plastic particles;

[0063] The physical properties of plastic particles include density, specific heat capacity, thermal conductivity, and melting temperature. A multi-step reaction model is used for pyrolysis, and key kinetic parameters and product distribution are analyzed and summarized.

[0064] The multi-step plastic pyrolysis reaction model consists of a five-block model consisting of plastic molecules (PP), molten plastic (mPP), wax (Wax), oil (Oil), and non-condensable gas (Gas) to describe the pyrolysis of PP. The five-block model consists of six irreversible first-order reactions. The reaction rate equation uses the Arrhenius formula. The rates of each reaction are shown in Table 1, where R i represents the i-th reaction, A i represents the pre-exponential factor, E i represents the activation energy, R g represents the gas constant, T is the temperature, and i=1~6.

[0065] Table 1

[0066] reaction Reaction rate <![CDATA[mPP→Wax(R1)]]> <![CDATA[k1=A1exp(-E1 / (R g T))]]> <![CDATA[mPP→Gas(R2)]]> <![CDATA[k2=A2exp(-E2 / (R g T))]]> <![CDATA[mPP→Oil(R3)]]> <![CDATA[k3=A3exp(-E3 / (R g T))]]> <![CDATA[Wax→Gas(R4)]]> <![CDATA[k4=A4exp(-E4 / (R g T))]]> <![CDATA[Oil→Gas(R5)]]> <![CDATA[k5=A5exp(-E5 / (R g T))]]> <![CDATA[Wax→Oil(R6)]]> <![CDATA[k6=A6exp(-E6 / (R g T))]]>

[0067] The reaction rates of each species are shown in Table 2.

[0068] Table 2

[0069] species Reaction rate mPP k mpp = -k1-k2-k3 Wax k wax = k1 - k4 - k6 Oil <![CDATA[k oil =k3+k6-k5]]> Gas <![CDATA[k gas =k2+k4+k5]]>

[0070] Step S2: establishing a calculation model of flow, heat transfer and reaction;

[0071] 1) Select CFD for solution, select the fluid volume model and enthalpy pore technology to calculate the heat transfer and flow of the plastic melting process, select the k-ε model to calculate the effect of gas on turbulence, and select the pressure implicit operator splitting (PISO) algorithm to calculate the coupling between pressure and velocity.

[0072] 2) Boundary condition setting: Set the gas inlet to the flow inlet boundary condition, the outlet to the pressure outlet boundary condition, and the reactor wall to the no-slip adiabatic boundary condition.

[0073] 3) Initial condition setting: Plastic particles are placed in the center of the reactor, the initial temperature is set to 300K, and nitrogen is filled.

[0074] The plastic melting process is numerically simulated based on the selected fluid volume model and enthalpy pore technology. The calculation models of flow, heat transfer and reaction mainly include the mass conservation equation, momentum conservation equation, energy conservation equation and component transport equation.

[0075] The mass conservation equation:

[0076]

[0077] Where, is the velocity, m / s; ρ is the density, kg / m 3 ;

[0078] Momentum conservation equation:

[0079]

[0080] Where S is the momentum source term; F st is the surface tension term, and its calculation formula is as follows:

[0081]

[0082] Where p is pressure, unit is Pa; μ is dynamic viscosity, Pa·s; g is acceleration due to gravity, m / s 2 ; I is the tensor; σ is the surface tension coefficient; is the normal interface unit vector; δ is the Dirac function on the interface, M is the melting shape constant, usually 100000; ω is a small value introduced to prevent β = 0, usually 0.001; β is the liquid fraction;

[0083] Energy conservation equation:

[0084]

[0085] Where h is the convective heat transfer coefficient, W / (K·m 2 ); k is thermal conductivity, W / (K·m); β is liquid fraction; L m is the heat of fusion, J / kg; T is the temperature, unit is K; Φ is the viscous dissipation; p is the pressure, unit is Pa; ρ is the density; is the speed, m / s.

[0086] Species Transport equation:

[0087]

[0088] Where: Y gn is the mass fraction of component n in the product; D gn is the diffusion coefficient of component n in the product; ρ gn is the density of component n in the product; is the heat source term of component n in the product; u is the velocity.

[0089] Where the volume of fluid model: by calculating the volume fraction of each grid cell in the grid tracking each fluid, control and capture the interface between the two-phase changes in the calculation domain, and the variable, property values in the calculation domain by grid cells shared. With volume fraction α tracking gas-liquid interface volume fraction transport equation:

[0090]

[0091] In the formula, α is the volume fraction of fluid, indicating the proportion of a certain phase in a unit volume, the value range 0 to 1.

[0092]

[0093] Where the enthalpy porosity technique: modeling the melting process, in describing the two relevant when the calculation domain is introduced liquid-solid paste zone concept, liquid fraction of 0 represents the solid phase, between 0-1 is considered to be solid-liquid paste zone, for 1 represents the liquid phase. Liquid phase fraction β represents the proportion of liquid phase in the unit.

[0094]

[0095] Material properties are related to the value of each phase:

[0096] ρ = α pl ρ pl + α g ρ g ; μ = α pl μ pl + α g μ g

[0097] k = α pl k pl + α g k g ; c p = α pl c p，pl + α g c p,g

[0098] ρ pl = ρ s + β (ρ s - ρ l ); μ pl = μ pl,l

[0099] k pl = k s + β (k s - k l ); c p,pl = c p，s + β (c p，s - cp，l )

[0100] The subscripts pl, g, s, and l represent plastic phase, gas phase, solid plastic, and molten plastic, respectively.

[0101] S3. Build a geometric model and divide the grid according to the actual size of the reactor (experimental device)

[0102] like Figure 2 As shown, the geometric model is drawn using SpaceClaim or DesignModeler software according to the actual size of the experimental device, and the constructed model is meshed using Gambit or ICEM software, and the mesh is a hexahedral mesh.

[0103] S4. Perform CFD calculations based on flow, heat transfer, reaction models, and geometric models

[0104] In the CFD calculation and solution process, the unsteady solver is selected. In the iterative process, the pressure equation is discretized using the standard format, and the momentum equation is discretized using the second-order upwind format.

[0105] Set different boundary conditions such as plastic types, air flow temperature, and velocity for calculation, establish geometric models of different particle sizes and shapes, calculate the pyrolysis process of single plastic particles under different geometric parameters, and comprehensively analyze the temperature field and component field under different working conditions.

[0106] S5. Extract the pyrolysis value of the plastic particles calculated in step S4 and analyze it.

[0107] like Figures 3 to 7 As shown, key data representing the melt pyrolysis state of plastic particles within the reactor, such as temperature and pyrolysis products, including wax, oil, and non-condensable gases, are extracted. Based on experimental and numerical simulation data, the melting rate, temperature, and product composition of different plastic types, shapes, and sizes, as well as under varying airflow velocities and temperatures, are analyzed to optimize key parameters throughout the melt pyrolysis process within the reactor.

[0108] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in the present invention, and such modifications or substitutions are intended to be within the scope of protection of the present invention. Therefore, the scope of protection of the present invention shall be subject to the scope of protection of the claims.

Claims

1. A numerical simulation method for the entire process from plastic melting to pyrolysis, characterized in that: The method comprises the following steps: Step S1, establishing a reaction model for melting and pyrolysis of a single plastic particle; Step S2: establishing a calculation model of flow, heat transfer and reaction based on the reaction model of step S1; Step S3: establishing a geometric model and dividing the grid according to the actual size of the reactor; Step S4: performing calculations based on the flow, heat transfer, and reaction calculation models of step S2 and the geometric model of step S3 to solve the entire process from melting to pyrolysis of a single plastic particle; Step S5: extracting and analyzing the melting and pyrolysis process data of the single plastic particle calculated in step S4 to obtain the changing patterns of the temperature field, melting rate, and product composition in the reactor; The calculation model of flow, heat transfer and reaction includes mass conservation equation, momentum conservation equation, energy conservation equation and component transport equation; The mass conservation equation: Where, is the speed, m / s; is the density, kg / m 3 ; t is time, unit: s; The momentum conservation equation: Where S is the momentum source term; is the surface tension term, p is pressure, unit is Pa; is the dynamic viscosity, Pa·s; g is the acceleration due to gravity, m / s 2 ; I is a tensor; is the surface tension coefficient; is the normal interface unit vector; is the Dirac function on the interface, ; M is the melting shape constant, usually taken as 100000; To prevent = 0 and the small value introduced is usually 0.001; is the liquid fraction; The energy conservation equation: Where, h is the convective heat transfer coefficient, W / (K·m 2 ); k is thermal conductivity, W / (K·m); is the liquid fraction; is the heat of fusion, J / kg; T is the temperature, unit is K; is viscous dissipation; p is pressure, unit is Pa; is the density; is the speed, m / s; The component transport equation: Where, is the mass fraction of component n in the product; is the diffusion coefficient of component n in the product; is the density of component n in the product; is the heat source term of component n in the product; for speed; In step S4, boundary conditions of different plastic types, air flow temperatures, and velocities are set, geometric models of different particle sizes and shapes are established, and the pyrolysis process of plastic particles under different geometric parameters is calculated and solved; The unsteady-state solver is selected for calculation and solution. During the iteration process, the pressure equation is discretized using the standard format, and the momentum equation is discretized using the second-order upwind format. The temperature field and component field of the plastic particle melting and pyrolysis process under different working conditions are obtained.

2. The numerical simulation method for the entire process from plastic melting to pyrolysis according to claim 1, characterized in that: The reaction model of the melting and pyrolysis of a single plastic particle in step S1 includes: Measure characteristic parameters of the plastic pellet melting process; The pyrolysis reaction rate of plastic particles is analyzed and summarized based on the measured characteristic parameters; The distribution of plastic particle pyrolysis products is analyzed and summarized based on the measured characteristic parameters.

3. The numerical simulation method for the entire process from plastic melting to pyrolysis according to claim 2, characterized in that: The characteristic parameters of the plastic particle melting process include density, specific heat capacity, thermal conductivity and melting temperature.

4. The numerical simulation method for the entire process from plastic melting to pyrolysis according to claim 1, characterized in that: The reaction model is a multi-step reaction model, including models of plastic molecules, molten plastic, wax, oil and non-condensable gas, and the reaction model consists of multiple irreversible first-order reactions.

5. The numerical simulation method for the entire process from plastic melting to pyrolysis according to claim 1, characterized in that: The calculation model of the movement, heat transfer and reaction is solved by fluid dynamics, specifically: the fluid volume model and enthalpy pore technology are selected to calculate the heat transfer and flow of the plastic melting process, the k-ε model is selected to calculate the influence of gas on turbulence, and the implicit operator splitting method algorithm of pressure is selected to calculate the coupling between pressure and velocity.

6. The numerical simulation method for the entire process from plastic melting to pyrolysis according to claim 1, characterized in that: The calculation model of the dynamics, heat transfer and reaction sets the gas inlet as the flow inlet boundary condition, the outlet as the pressure outlet boundary condition, and the reactor wall as the no-slip adiabatic boundary condition.

7. The numerical simulation method for the entire process from plastic melting to pyrolysis according to claim 1, characterized in that: The initial conditions of the calculation model of the dynamics, heat transfer and reaction are set as follows: plastic particles are placed in the center of the reactor, the initial temperature is set to 300K, and nitrogen is filled.

8. The numerical simulation method for the entire process from plastic melting to pyrolysis according to claim 1, characterized in that: The step S5 includes: S501, extracting key data of temperature, melting rate, and product composition during the melting and pyrolysis process of plastic particles; S502. Based on the data extracted in S501, analyze and summarize the changing patterns of the temperature field, melting rate and product composition in the reactor when different materials, air flow temperatures and air flow velocities are used.

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