Numerical simulation method for whole process from plastic melting to pyrolysis

Abstract

The invention relates to a numerical simulation method for the whole process from plastic melting to pyrolysis. The method comprises the following steps: establishing a reaction model for plastic single particle melting and pyrolysis; establishing a flow, heat transfer and reaction calculation model based on the reaction model; establishing a geometric model and dividing grids according to the actual size of the reactor; calculating according to a calculation model and a geometric model of flow, heat transfer and reaction, and solving the whole process from melting to pyrolysis of the plastic single particles; and extracting, analyzing and calculating the obtained melting and pyrolysis process data of the single plastic particles to obtain a temperature field in the reactor, a melting rate and a change rule of product components. Compared with the prior art, the method has the advantages that heat transfer and reaction characteristics in the melting stage are considered, numerical prediction of pyrolysis is more accurate, optimization of key parameters in the whole process from melting to pyrolysis in the reactor is facilitated, and the like.

Description

Technical Field

[0001] The 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 express delivery, take-out, packaging and other industries, the consumption of plastics in daily life is increasing day by day, and will increase to about 590 million tons by 2050. Statistics show that the global recycling rate of plastic waste is only 9%, and the rest goes to landfills or incineration plants. Due to the continued potential harm of landfills to the atmosphere, soil and water, landfills are increasingly restricted, and the direct incineration of waste plastics will produce harmful gases and is also a waste of resources.

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

[0004] The difference between plastic pyrolysis and the pyrolysis process of general solid waste such as biomass and coal powder is that plastic undergoes a melting process from solid phase to liquid phase and a decomposition process from liquid phase to gas phase. In the pyrolysis process of domestic waste, the heat transfer characteristics of the melting process of plastic will affect the overall pyrolysis efficiency and have an important impact on the products and reactions. In order to achieve high heat transfer and avoid the blockage problem that is prone to occur in pilot and industrial-scale systems, the development of advanced reactor designs for pyrolysis of plastics needs to consider the impact of plastic melting and flow on the heat generation of 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 to pyrolysis of a single plastic particle.

[0005] After searching, the Chinese invention patent application publication number CN115935592A discloses a multi-component domestic waste pyrolysis numerical simulation method based on Mfix, which is used to simulate the pyrolysis numerical calculation of four typical domestic wastes: biomass, plastic, paper and fabric. The existing patent application has the problem of not simulating the melting process of plastic.

[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 whole 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, extracting and analyzing the single plastic particle melting pyrolysis process data calculated in step S4, and obtaining the variation 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 the characteristic parameters of the plastic granule 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] In the formula, 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 gravitational acceleration, 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, in K; Φ is the viscous dissipation; p is the pressure, in Pa; ρ is the density; is the speed, m / s;

[0031] The component transport equation is:

[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 a flow inlet boundary condition, the outlet as a pressure outlet boundary condition, and the reactor wall as a 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 during the 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 comprises:

[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, different 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 whole 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 whole process from the melting and pyrolysis of a single plastic particle is solved, and the variation law of the temperature field, melting rate and product composition in the reactor is obtained by analysis. Since the heat transfer and reaction characteristics in 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 whole 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 a single plastic particle under different geometric parameters, different plastic types, air flow temperature, velocity and other boundary conditions. The plastic melting process and the pyrolysis process can be coupled to comprehensively analyze the temperature field and component field under different working conditions. The data obtained are closer to the actual operation data and the results are more accurate. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 It is a flow chart of the numerical simulation method of the present invention;

[0046] Figure 2 It is a schematic diagram of the geometric model of the reactor of the present invention;

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

[0048] Figure 4 It is a schematic diagram of volume fraction of the melting process in the reactor of the present invention;

[0049] Figure 5 This is a schematic diagram of wax concentration distribution in the reactor of the present invention;

[0050] Figure 6 It is a 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 be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work 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 comprises the following steps:

[0054] Step S1, establishing a reaction model for 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) calculation and solution according to the flow, heat transfer and reaction model of step S2 and the geometric model of step S3;

[0058] Step S5: extracting the single plastic particle melting pyrolysis process data calculated in step S4 and comparing it with the experimental data obtained in step S1.

[0059] The reaction model for 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] Among them, the physical properties of plastic particles include density, specific heat capacity, thermal conductivity, and melting temperature. The pyrolysis adopts a multi-step reaction model, and the key reaction kinetic parameters of the pyrolysis and the distribution of pyrolysis products are obtained through analysis and summary.

[0064] The multi-step reaction model of plastic pyrolysis consists of five models 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 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 <![CDATA[k mpp =-k1-k2-k3]]> Wax <![CDATA[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 method (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 in.

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

[0075] The mass conservation equation:

[0076]

[0077] In the formula, 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 the calculation formula is as follows:

[0081]

[0082] Where p is pressure, in Pa; μ is dynamic viscosity, in Pa·s; g is 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;

[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, in K; Φ is the viscous dissipation; p is the pressure, in 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] The fluid volume model: the volume fraction of each fluid is tracked by each grid cell in the computational grid, the interface changes between the two phases in the computational domain are controlled and captured, and the variables and attribute values ​​in the computational domain are shared by the grid cells. The volume fraction transmission equation for tracking the gas-liquid interface with the volume fraction α is:

[0090]

[0091] In the formula, α is the fluid volume fraction, which indicates the proportion of a certain phase in a unit volume, and its value ranges from 0 to 1.

[0092]

[0093] Among them, the enthalpy pore technology: models the melting process, introduces the concept of liquid-solid mushy zone when describing the relationship between the two phases in the calculation domain, and the liquid fraction of 0 represents the solid phase, between 0-1 is considered to be the solid-liquid mushy zone, and 1 represents the liquid phase. The liquid phase fraction β represents the proportion of the liquid phase in the unit body.

[0094]

[0095] Material properties are values ​​associated with 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, according to the actual size of the experimental device, the geometric model is drawn using SpaceClaim or DesignModeler software, and the constructed model is meshed using Gambit or ICEM software, and the meshed grid uses a hexahedral grid.

[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-state solver is selected. In the iterative process, the pressure equation is discretized in the standard format, and the momentum equation is discretized in 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 Figure 3 to Figure 7 As shown, the key data such as temperature and pyrolysis products representing the melting pyrolysis state of plastic particles in the reactor are extracted. The pyrolysis products include wax, oil and non-condensable gas. According to the data obtained by experiments and numerical simulation calculations, the melting rate, temperature and product composition of different types of plastics, plastic particles of different shapes and sizes, and different air flow speeds and temperature conditions in the melting pyrolysis process are summarized, thereby optimizing the key parameters of the whole melting pyrolysis process in the reactor.

[0108] The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can easily think of various equivalent modifications or replacements within the technical scope disclosed by the present invention, and these modifications or replacements should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention shall be based on the protection scope of the claims.

Claims

1. A numerical simulation method for the whole 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 single plastic particle melting pyrolysis process data calculated in step S4, and obtaining the variation patterns of the temperature field, melting rate and product composition in the reactor.

2. The numerical simulation method for the whole 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 the characteristic parameters of the plastic granule 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 whole 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 whole process from plastic melting to pyrolysis according to claim 1 is 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 is composed of multiple irreversible first-order reactions.

5. The numerical simulation method for the whole process from plastic melting to pyrolysis according to claim 1, characterized in that: 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: In the formula, is the velocity, m / s; ρ is the density, kg / m 3 ; The momentum conservation equation: 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 gravitational acceleration, 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; The energy conservation equation: 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, in K; Φ is the viscous dissipation; p is the pressure, in Pa; ρ is the density; is the speed, m / s; The component transport equation is: 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.

6. The method for numerical simulation of the whole 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 algorithm of pressure is selected to calculate the coupling between pressure and velocity.

7. The method for numerical simulation of the whole 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 a flow inlet boundary condition, the outlet as a pressure outlet boundary condition, and the reactor wall as a no-slip adiabatic boundary condition.

8. The method for numerical simulation of the whole 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 in.

9. The method for numerical simulation of the whole process from plastic melting to pyrolysis according to claim 1, characterized in that: 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 during 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.

10. The method for numerical simulation of the whole process from plastic melting to pyrolysis according to claim 1, characterized in that: The step S5 comprises: 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, different air flow temperatures and air flow velocities are used.

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