CN103150433B - The Coupled Numerical modeling method of cracking reaction in industry dichloroethane cracking furnace hearth combustion and boiler tube - Google Patents

Abstract

The invention provides the Coupled Numerical modeling method of cracking reaction in a kind of industrial dichloroethane cracking furnace hearth combustion and boiler tube, dichloroethane cracking furnace is divided into hearth model and furnace tube model by the method in modeling, and respectively to burner hearth boiler tube grid division.The tube wall temperature that burner hearth provides using furnace tube model is as boundary condition, and utilize combustion model, flow model, heat transfer model calculates chamber flue gas temperature, speed, the important burner hearth parameter distribution such as concentration of component; The boiler tube thermoflux that boiler tube then calculates with burner hearth is for boundary condition, utilize cracking reaction model in pipe, consider the mass conservation in pipe, momentum conservation, energy conservation relation calculates the Process Gas temperature along pipe range direction, pressure, CONCENTRATION DISTRIBUTION, thus under favourable analysis current operational conditions, ethylene dichloride cracking conversion ratio, selectivity, the principal economic indicators such as unit consumption, are conducive to guide field process optimization.And this modeling method is applicable to all kinds of high-temperature cracking furnace, there is adaptability widely.

Description

Coupling numerical modeling method for combustion of industrial dichloroethane cracking furnace hearth and cracking reaction in furnace tube

Technical Field

The invention relates to a process device modeling method, in particular to a coupling numerical modeling method for combustion of a hearth of an industrial dichloroethane cracking furnace and cracking reaction in the furnace tube.

Background

The dichloroethane cracking furnace is the core unit of the chloroethylene production device and the energy consumption is large, the benefit of the whole chloroethylene device is closely related to the design and the operation level of the cracking furnace, and the key for improving the economic benefit of the chloroethylene production device lies in high-level design and how to optimize the operation condition of the cracking furnace. At present, most of dichloroethane cracking technologies and devices in China are introduced from foreign countries, and the introduction of advanced and mature chloroethylene devices provides a higher starting point for the development of chloroethylene industry in China. However, the operation level of dichloroethane cracking in China generally falls behind the world advanced level, and the dichloroethane cracking conversion rate is low, the selectivity is low, and the unit consumption is high. The secrecy of the patent trader on the key technology makes the mastering of the dichloroethane cracking process mechanism in China not deep enough, and the technical level is difficult to make a substantial breakthrough. The understanding of the fluid flow, heat transfer, mass transfer and chemical reaction of substances inside the cracking furnace is not clear enough in China, and the theory support is insufficient, so that the foreign technology is mainly simulated during the modification of the cracking furnace and the design of the domestic cracking furnace, no theoretical basis exists, the modification and the design are improper, or when the cracking raw materials and the operating conditions are changed, the operating parameters can only be determined by experience, certain blindness is brought to the design and the operation, and the potential of the device is not fully exerted. Therefore, the foreign complete technology is introduced and simulated and modified without paying attention to the innovation from the fundamental and basic technology, so that the vinyl chloride cracking production technology in China always lags behind the world leading level and lacks competitiveness internationally.

In order to comprehensively master the operation mechanism of the dichloroethane cracking furnace, master the heat coupling relationship between the furnace chamber and the furnace tube, and know the key parameters which have important influences on the operation cycle of the dichloroethane cracking furnace, the dichloroethane cracking conversion rate, the selectivity, the unit consumption and other important performance indexes, the principle modeling of the dichloroethane cracking furnace is particularly important. The research and development of the prior mathematical model of the dichloroethane cracking furnace focuses on the description of the cracking reaction kinetics, does not take the mutual influence among the cracking reaction, the flow and the heat transfer into consideration, and greatly simplifies the fluid flow and the heat transfer process in the reaction tube. In the simulation research of the hearth, the radiation heat transfer process is mainly simulated, the radiation heat transfer process in the hearth is calculated by adopting simplified methods such as a Roeby-Emames method, a Belokang method, a regional method and the like, the fuel combustion mechanism process is not simulated, the composition and the temperature of the flue gas are estimated by simply utilizing the heat release rate of the fuel, and the influence of the combustion and the flue gas flowing process on the heat transfer is ignored.

With the great increase of computer computing capability, complex and time-consuming Computational Fluid Dynamics (CFD) has become an important method for solving various fields related to fluid flow, such as mechanical manufacturing, chemical engineering, and the like. CFD is a branch of hydrodynamics, and a detailed numerical simulation method is used for solving a nonlinear partial differential equation instead of an analytic method, so that the problem that many theoretical hydrodynamics cannot solve is solved. The non-linear momentum, heat, mass and component conservation equations of the flow, heat transfer, mass transfer and reaction processes in most engineering problems can be discretized by CFD, original continuous physical quantity fields (velocity field, temperature field, concentration field and the like) in space coordinates are replaced by variable value sets on a plurality of discrete points, an algebraic equation set about the relation between field variables on the discrete points is established, the discrete equation set is closed under the known boundary conditions, numerical solution is carried out to obtain approximate solutions of the physical quantity fields, and the distribution of each physical quantity (such as velocity, temperature, concentration and the like) in the whole research system is given. Details of the exact flow, heat transfer, mass transfer, and reaction processes. The CFD in a narrow sense only studies the fluid flow phenomenon, but with the development of other research fields (such as combustion, radiation, chemical reaction, etc.), the longer the antenna of the CFD is, the wider the coverage area is. As in the chemical industry, reactor simulation may be performed by combining a reaction model describing a chemical reaction with a flow model; effectively simulating the reaction and flow conditions in equipment such as a reactor. Thus, in theory, the CFD method works effectively whenever there is fluid flow. Therefore, the CFD technology is introduced into the mechanism modeling of the dichloroethane cracking furnace, which is helpful for more clearly understanding important information such as heat coupling, flow field distribution and the like of the dichloroethane cracking furnace tube. The method further provides powerful theory and data support for the design and the modification of the ethylene cracking furnace, the optimization operation and the development of a new technology, thereby providing technical support for the localization and old modification of the ethylene cracking furnace.

Disclosure of Invention

In order to solve the defects of the existing model, the invention comprehensively and systematically analyzes the complex processes of substance flow, heat transfer, mass transfer and cracking reaction in the reaction tube of the dichloroethane cracking furnace, the flow, heat transfer, mass transfer, combustion reaction and the like in the hearth, and simultaneously analyzes the strong coupling action among the complex processes, and the coupling numerical modeling method for combustion of the hearth of the industrial dichloroethane cracking furnace and the cracking reaction in the furnace tube is designed on the basis of a turbulent flow model, a radiation heat transfer model, a combustion model and a cracking reaction kinetic model of hydrodynamics to couple the processes of the transfer and cracking reaction in the reaction tube of the cracking furnace and the combustion heat transfer process in the hearth.

The model of the invention consists of a hearth model and a furnace tube model, wherein: in the hearth model, a full premixing mode is adopted according to the mixing degree of fuel and air; the fuel gas is combusted by adopting a simplified one-stage series combustion model; the combustion chemical reaction adopts a turbulence-chemical reaction interaction model-vortex dissipation model; the furnace hearth smoke flow adopts a Reynolds average model, and a standard k-double-stroke model is adopted to seal a turbulence term in the furnace hearth smoke flow; the furnace flue gas radiation heat transfer model adopts a discrete coordinate model, and adopts a multi-ash gas weighting model to calculate the flue gas radiation characteristic. In the furnace tube model, simplified ethylene dichloride one-stage series connection cracking reaction is adopted; the process gas flow model is consistent with the flue gas flow model. The iterative variables of the coupling simulation of the hearth model and the furnace tube model select the temperature and the heat flux of the outer wall of the furnace tube. Therefore, the distribution of the temperature, the speed and the component concentration of the flue gas in the hearth, the distribution of the temperature of the inner wall and the outer wall of the furnace tube, the distribution of the heat flux of the furnace tube and the distribution of the temperature, the speed and the component concentration of the cracking gas in the furnace tube can be realized, so that the internal characteristics of the dichloroethane cracking furnace can be known more accurately, and theoretical support is provided for the operation optimization, the process transformation, the new process design and the like of the cracking furnace.

A coupling numerical modeling method for a hearth and a furnace tube of an industrial dichloroethane cracking furnace comprises the following steps:

step 1: determining the furnace tube size of the furnace chamber of the dichloroethane cracking furnace to be simulated, and aiming at the furnace chamber and the furnace

Meshing the tubes; determining boundary conditions according to process parameters, including: combustion of side wall burners

Gas inlet flow, air inlet flow, gas flow and temperature at inlet of reaction tube, and furnace wall heat

Loss coefficient, hearth flue gas outlet pressure and furnace tube cracking gas outlet pressure;

step 2: establishing a hearth model:

step 2.1: the flue gas flow model in the hearth adopts the standard k based on the Reynolds average equation

Establishing a closed model by using a bi-equation model;

step 2.2: the fuel gas in the hearth adopts a one-stage series combustion reaction model and flows during combustion

The model adopts a finite velocity/vortex dissipation model;

step 2.3: the radiation heat transfer model in the hearth adopts a discrete coordinate model, and the hearth flue gas adopts

Calculating the radiation characteristic of the multi-ash gas weighted model;

and step 3: establishing a furnace tube model:

step 3.1: the ethylene dichloride cracking reaction in the furnace tube adopts a first-stage series reaction model, and the cracking reaction is carried out

The reaction kinetics conform to the Allen-Raeus formula;

step 3.2, determining gas density, heat capacity, viscosity, heat conductivity coefficient and expansion coefficient in the furnace tube model

Parameters of a dispersion coefficient calculation formula;

and 4, step 4: based on the existence of serious heat coupling relationship between the furnace chamber and the furnace tube and the acquisition in the step 1

Initial and boundary conditions of (1), furnace tube outer wall temperature and furnace tube heat flux as furnace chamber model

Iterative coupling variables with each other when the furnace tube model numerical value is solved, and the hearth and the furnace tube model are carried out

Until the model converges, the obtained model relates to all parameter values.

Further, in the step 1, grid division is performed on the hearth and the furnace tube, and a burner area and a furnace tube area in the hearth adopt tetrahedral units to divide grids; other areas of the hearth adopt hexahedral units for dividing grids; the wall surface of a straight pipe of the furnace tube in the furnace tube model adopts hexahedral units to divide grids; the bent pipe adopts a mixture unit to divide grids.

Further, in the step 2 and the step 3, the furnace tube and the wall surface of the hearth wall are regarded as non-slip boundaries; in the viscous bottom layer near the wall surface, a standard wall surface function is adopted to approximate the flow and heat exchange in the actual process; the heat boundary on the hearth wall gives a heat flux boundary condition through heat loss; the boundary of the wall surface of the furnace tube is assigned to the tube wall by a self-defined function, and in the model of the furnace tube, the self-defined function of the heat flux of the outer wall of the furnace tube is defined as Q (x) = a₁+b₁x+c₁x²+d₁x³+e₁x⁴+f₁x⁵In the furnace chamber model, the furnace tube outer wall temperature custom function is defined as T (x) = a₂+b₂x+c₂x²+d₂x³+e₂x⁴+f₂x⁵Wherein a is₁、b₁、c₁、d₁、e₁、f₁、a₂、b₂、c₂、d₂、e₂、f₂X is a coordinate along the radial direction of the furnace tube as a parameter to be fitted; q is heat flux, and T is furnace tube outer wall temperature;

further, the first-stage series combustion model in the step 2.2 is as follows:

using a finite rate model, the chemical source term is calculated using the Arrhenius formula:

$R_i=M_{w,i}\underset{r=1}{\overset{N_R}{\mathrm{\Σ}}}{\hat{R}}_{i,r}$

M_w,iis the molecular weight of component i, N_RFor the number of equations, R_iThe net production rate of component i caused by the chemical reaction,the production/decomposition rate of the ith substance in the r reaction is expressed as:

${\hat{R}}_{i,r}=\mathrm{\Γ}(v_{i,r}^{\′\′}-v_{i,r}^{\′})\left(k_{f,r}\underset{j=1}{\overset{N}{\mathrm{\Π}}}{\left[C_{j,r}\right]}^{({\mathrm{\η}}_{j,r}^{\′}+{\mathrm{\η}}_{j,r}^{\′\′})}\right)$

wherein the net effect of the third object on the reaction rate, v'_i,r，ν″_i,rIs the stoichiometric coefficient, k_f,rForward reaction rate constant, C_j,rIs molar concentration, η'_j,r，η″_j,rIs the rate index, N is the total number of substances participating in the reaction;

using a vortex dissipation model, the rate of production R of a substance k in a reaction R_i,kThe smaller of the two terms in the following formula:

$R_{i,k}=\min [v_{i,k}^{\′}{M}_{w,i}\mathrm{A\ρ}\frac{\mathrm{\ϵ}}{k}\min \left(\frac{Y_R}{v_{R,k}^{\′}{M}_{w,R}}\right),v_{i,k}^{\′}{M}_{w,i}\mathrm{AB\ρ}\frac{\mathrm{\ϵ}}{k}\frac{{\mathrm{\Σ}}_P{Y}_P}{{\mathrm{\Σ}}_j^N{v}_{j,k}^{\′\′}{M}_{w,j}}$

v 'therein'_i,k，ν′_R,k，ν″_j,kIs a stoichiometric coefficient, M_w,i，M_w,R，M_w,jMolecular weight, A, B are empirical coefficients, ρ gas density,is the Lonned-Jones potential energy parameter, Y_R，Y_PIs the mass fraction of the product.

Further, in step 2.1, a closed mathematical model is established based on a standard k-two-equation model of a Reynolds average Navier-Stokes equation, and mass, momentum, turbulence energy, dissipation rate of the turbulence energy, energy and component transport equations are expressed as follows:

continuity equation: $\frac{\∂}{{\∂x}_i}\left({\mathrm{\ρU}}_i\right)=0$

the momentum equation: $\frac{\∂}{{\∂x}_j}\left({\mathrm{\ρU}}_i{U}_j\right)=\frac{{\∂p}_{\mathrm{eff}}}{{\∂x}_i}+\frac{\∂}{{\∂x}_j}\left[{\mathrm{\μ}}_{\mathrm{eff}}(\frac{{\∂U}_i}{{\∂x}_j}+\frac{{\∂U}_j}{{\∂x}_i}-\frac{2}{3}{\mathrm{\δ}}_{\mathrm{ij}}\frac{{\∂U}_l}{{\∂x}_l})\right]$

k-equation: $\frac{\∂}{{\∂x}_i}\left(\mathrm{\ρ}{\mathrm{kU}}_i\right)=\frac{\∂}{{\∂x}_i}\left[(\mathrm{\μ}+\frac{{\mathrm{\μ}}_t}{{\mathrm{\σ}}_k})\frac{\∂k}{{\∂x}_j}\right]+G_k-\mathrm{\ρ\ϵ}$

-the equation: $\frac{\∂}{{\∂x}_i}\left(\mathrm{\ρ\ϵ}{U}_i\right)=\frac{\∂}{{\∂x}_j}\left[(\mathrm{\μ}+\frac{{\mathrm{\μ}}_t}{{\mathrm{\σ}}_{\mathrm{\ϵ}}})\frac{\∂\mathrm{\ϵ}}{{\∂x}_j}\right]+\frac{\mathrm{\ϵ}}{k}(C_{1\mathrm{\ϵ}}{G}_k-C_{2\mathrm{\ϵ}}\mathrm{\ρ\ϵ})$

energy equation:

$\frac{\∂}{{\∂x}_i}\left[U_i(\mathrm{\ρE}+p)\right]=\frac{\∂}{{\∂x}_j}(k_{\mathrm{eff}}\frac{\∂T}{{\∂x}_j}-\underset{j}{\mathrm{\Σ}}{h}_j{\stackrel{\→}{J}}_j+U_i{\mathrm{\μ}}_{\mathrm{eff}}[(\frac{{\∂U}_j}{{\∂x}_i}+\frac{{\∂U}_i}{{\∂x}_j})-\frac{2}{3}\frac{{\∂U}_l}{{\∂x}_l}{\mathrm{\δ}}_{\mathrm{ij}}\left]\right)+S_h$

component transport equation: $\frac{{\∂\mathrm{\ρU}}_j{Y}_i}{{\∂x}_j}=\frac{\∂}{{\∂x}_j}\left[({\mathrm{\ρD}}_{i,m}+\frac{{\mathrm{\μ}}_t}{{\mathrm{Sc}}_t})\frac{{\∂Y}_i}{{\∂x}_j}\right]+R_i$

wherein U is_i,U_j,U_lIs the velocity component in the i, j, k direction, x_i,x_j,x_lIs the coordinate in the i, j, k direction, p is the gas density, p_effEffective pressure, mu_effIn order to be of an effective viscosity,_ijis a function of kronecker, k is the kinetic energy of turbulence, μ is the viscosity of the gas molecule, μ_tFor turbulent viscosity, G_kFor the generation term of turbulent kinetic energy, for the dissipation ratio of turbulent kinetic energy, S_hAs a source term in the energy equation, C_μ,C₁,C₂,σ_k，σ Is a standard k-model parameter, E is the total energy per unit mass, p is the pressure, k_effThe conductivity of the water is measured by the conductivity meter,is the diffusion flux of the component, h_jIs the enthalpy of component j, Y_jIs the mass fraction of component j, D_i,mIs the mass diffusion coefficient of component i, Sc, in the mixture_tIs the turbulent Schmitt number, R_iIs the net production rate of component i caused by the chemical reaction.

Further, in step 2.3, the radiation heat transfer model adopts a discrete coordinate model, and the mathematical expression of the model is as follows:

$\▿\·\left(I(\stackrel{\→}{r},\stackrel{\→}{s})\stackrel{\→}{s}\right)+(\mathrm{\α}+{\mathrm{\σ}}_s)I(\stackrel{\→}{r},\stackrel{\→}{s})={\mathrm{\αn}}^2\frac{{\mathrm{\σT}}^4}{\mathrm{\π}}+\frac{{\mathrm{\σ}}_s}{4\mathrm{\π}}{\∫}_0^{4\mathrm{\π}}I(\stackrel{\→}{r},{\stackrel{\→}{s}}^{\′})\mathrm{\Φ}(\stackrel{\→}{s}\·{\stackrel{\→}{s}}^{\′})d{\mathrm{\Ω}}^{\′};$

wherein I is the intensity of the radiation,in the form of a position vector, the position vector,is a direction vector, α is an absorption coefficient, σ_sThe scattering coefficient is n, the refractive index is n, the sigma is a Stefan-Boltzmann constant, T is the flue gas temperature phi which is a phase function, and omega' is a solid angle;

the radiation characteristic of the flue gas in the hearth is calculated by adopting a multi-ash-gas weighted model, and the model approximately processes the blackness of the real gas into the weighted sum of the blackness of a plurality of ash gases:

$\mathrm{\ϵ}=\underset{i=0}{\overset{I}{\mathrm{\Σ}}}{\mathrm{\α}}_{\mathrm{\ϵ},i}\left(T\right)(1-e^{-k_i\mathrm{ps}})$

α therein_,iThe emissivity weighting factor of the ith kind of virtual ash gas, the total absorption coefficient of the flue gas can be expressed as:

when s is>10^-4m, $\mathrm{\α}=-\frac{\ln (1-\mathrm{\ϵ})}{s};$ When s is less than or equal to 10^-4m， $\mathrm{\α}=\underset{i=0}{\overset{I}{\mathrm{\Σ}}}{\mathrm{\α}}_{\mathrm{\ϵ},i}{k}_{i}p.$

Further, step 3.1 first-stage series cracking reaction of dichloroethane cracking gas

The kinetics of the method accord with an Allen-Raeus formula, and the chemical reaction rate is represented by the following formula:

${\hat{R}}_{i,r}=\mathrm{\Γ}(v_{i,r}^{\′\′}-v_{i,r}^{\′})\left(k_{f,r}\underset{j=1}{\overset{N}{\mathrm{\Π}}}\right[C_{j,r}]{\mathrm{\η}}_{j,r}^{\′}-k_{b,r}\underset{j=1}{\overset{N}{\mathrm{\Π}}}[C_{j,r}\left]{\mathrm{\η}}_{j,r}^{\′\′}\right)$

whereinIs the production/decomposition rate of the ith substance in the r-th reaction, v'_i,r，ν″_i′rIs composed of

Stoichiometric coefficient, η'_j,r，η″_j,rIs a reaction rate index, k_f,rIs a constant of the rate of the forward reaction,

k_b,ras a reverse reaction rate constant, C_j,rIs the molar concentration of component j.

Further, the furnace model convergence condition is that the furnace model obtains a group of new heat fluxes on the tube wall, and the comparison of the new heat fluxes on the tube wall obtained by the furnace model and the heat fluxes on the tube wall obtained by the previous calculation reaches preset precision.

Further, the furnace tube model convergence condition is that the furnace tube model obtains a group of new tube wall temperatures, and the tube wall temperatures are compared with the furnace tube outer wall temperatures obtained through previous calculation to reach preset precision.

The invention provides a coupling numerical modeling method for combustion of a hearth of an industrial dichloroethane cracking furnace and cracking reaction in a furnace tube. The furnace chamber takes the temperature of the outer wall of the tube given by the furnace tube model as a boundary condition, and the combustion model, the flow model and the heat transfer model are utilized to calculate important furnace chamber parameter distribution such as the temperature, the speed, the component concentration and the like of the furnace chamber flue gas; the furnace tube takes the heat flux of the furnace tube calculated by the hearth as a boundary condition, and utilizes an in-tube cracking reaction model to calculate the temperature, pressure and concentration distribution of process gas along the length direction of the tube by considering the relationship of mass conservation, momentum conservation and energy conservation in the tube, so that the important economic indexes such as the dichloroethane cracking conversion rate, selectivity, unit consumption and the like under the current operating condition can be analyzed, and the optimization of the field process can be guided. The modeling method is suitable for various pyrolysis furnaces and has wide adaptability.

Drawings

FIG. 1 is a grid section of a dichloroethane cracking furnace.

FIG. 2 is a block diagram of a dichloroethane cracking furnace hearth tube coupling iterative solution.

FIG. 3 hearth smoke flow velocity field distribution diagram (y =0.0485m)

FIG. 4 hearth gas temperature field distribution diagram (y =0.0485m)

FIG. 5 furnace flue gas O2 concentration distribution (y =0.0485m)

FIG. 6 distribution diagram of CO concentration in furnace flue gas (y =0.0485m)

FIG. 7 distribution diagram of CO2 concentration in furnace flue gas (y =0.0485m)

FIG. 8 is a distribution diagram of heat flux of the furnace tube along the length direction of the furnace tube

FIG. 9 is a distribution diagram of cracked gas temperature in the furnace tube along the length direction of the furnace tube

FIG. 10 is a distribution diagram of the flow velocity of cracked gas in the furnace tube along the length direction of the furnace tube

FIG. 11 is a distribution diagram of cracked gas pressure distribution in the furnace tube along the length direction of the furnace tube

FIG. 12 is a distribution diagram of the molar flow of cracked gas in the furnace tube along the length direction of the furnace tube

Detailed Description

The complete dichloroethane cracking furnace comprises a convection section and a radiation section, wherein the convection section is mainly used for preheating and gasifying liquid dichloroethane and sending the liquid dichloroethane into the radiation section, the radiation section further utilizes high-temperature flue gas released by fuel combustion to heat the dichloroethane, the dichloroethane is subjected to cracking reaction rapidly, and chloroethylene and byproducts form cracked gas. The invention therefore primarily concerns the radiant section with cracking reactions and assumes a constant temperature of the dichloroethane steam entering the radiant section. Although in the cracking furnace, heat transfer closely couples the various physical and chemical processes occurring in the furnace and the furnace tubes, they are relatively independent from the subject. Therefore, from the modeling point of view, a furnace mathematical model and a furnace tube mathematical model can be respectively established. The interface of the two spaces of the hearth and the furnace tube is the outer wall of the furnace tube, and the hearth model and the furnace tube model can be used for connecting the two models together through the temperature of heat flux on the outer wall of the tube to form a complete mathematical model of the radiation section of the cracking furnace.

Mathematical model of furnace

In the hearth of the dichloroethane cracking furnace, fuel and air are sprayed out from each row of the side wall and are rapidly combusted, namely, the flow of fluid with chemical reaction is technically determined. By this definition, combustion is a complex process of transfer and reaction, so that the distribution of the various physical quantities within the furnace in the furnace space is clearly shown, as long as the flowing chemical reaction is accurately described. With the CFD method, the furnace can first be described with respect to the flow within the furnace, and then take into account chemical reactions and radiative and convective heat transfer on a flow basis. However, in fact, these two factors are not strictly sequential, but cause and affect each other: the fluid flow affects the concentration profile of the components and thus the chemical reaction, which in turn affects the heat transfer, which in turn affects the flow, which processes are interdependent and inseparably coupled together. Therefore, the establishment of the mathematical model of the furnace should allow the organism to show the correlation of these. The physical and chemical process of combustion is carefully analyzed and decomposed, and each sub-process after decomposition can be described by using several mathematical models: (1) fluid flow models, the key being turbulence models; (2) the mass transfer model of each component mainly considers the influence of the combustion chemical reaction rate; (3) the heat transfer model comprises convection heat transfer and radiation heat transfer, wherein the key is the radiation heat transfer model.

(1) Fluid flow model

In the hearth of the cracking furnace, the flow of the flue gas is assumed to be embodied in a turbulent flow mode, so that the flow of the flue gas is described in a Reynolds average conservation equation assumed by a turbulent transport term, so that a simultaneous equation set is closed. Because the burner nozzle installed in the hearth of the cracking furnace sprays fuel at a high speed, the fuel is combusted in the hearth to release heat, and the flow velocity of formed flue gas is very high, the flow in the hearth is basically in a turbulent flow state, and the flow rule is relatively complex. The existing turbulence modes which have relatively influence comprise a zero equation model, a single-pass model, a double-pass model, an algebraic stress model, a Reynolds stress model and other multi-pass models. Of these models, the two-pass model is more commonly applied. The results of the previous researches show that the algebraic stress model and the Reynolds stress model are optimal but are most difficult to realize, the requirements on the performance of the computer are high, and the k-mode is a two-way model and has the advantages of wide application range, high precision and relatively easy solution. Therefore, the method selects a standard k-double equation model based on the Reynolds average Navier-Stokes (RANS) equation to establish a closed mathematical model. Mass, momentum, turbulent kinetic energy, dissipation ratio of turbulent kinetic energy, energy and component transport equations are represented as follows:

continuity equation: $\frac{\∂}{{\∂x}_i}\left({\mathrm{\ρU}}_i\right)=0$

the momentum equation: $\frac{\∂}{{\∂x}_j}\left({\mathrm{\ρU}}_i{U}_j\right)=\frac{{\∂p}_{\mathrm{eff}}}{{\∂x}_i}+\frac{\∂}{{\∂x}_j}\left[{\mathrm{\μ}}_{\mathrm{eff}}(\frac{{\∂U}_i}{{\∂x}_j}+\frac{{\∂U}_j}{{\∂x}_i}-\frac{2}{3}{\mathrm{\δ}}_{\mathrm{ij}}\frac{{\∂U}_l}{{\∂x}_l})\right]$

k-equation: $\frac{\∂}{{\∂x}_i}\left(\mathrm{\ρ}{\mathrm{kU}}_i\right)=\frac{\∂}{{\∂x}_j}\left[(\mathrm{\μ}+\frac{{\mathrm{\μ}}_t}{{\mathrm{\σ}}_k})\frac{\∂k}{{\∂x}_j}\right]+G_k-\mathrm{\ρ\ϵ}$

-the equation: $\frac{\∂}{{\∂x}_i}\left(\mathrm{\ρ\ϵ}{U}_i\right)=\frac{\∂}{{\∂x}_j}\left[(\mathrm{\μ}+\frac{{\mathrm{\μ}}_t}{{\mathrm{\σ}}_{\mathrm{\ϵ}}})\frac{\∂\mathrm{\ϵ}}{{\∂x}_j}\right]+\frac{\mathrm{\ϵ}}{k}(C_{1\mathrm{\ϵ}}{G}_k-C_{2\mathrm{\ϵ}}\mathrm{\ρ\ϵ})$

energy equation: $\frac{\∂}{{\∂x}_i}\left[U_i(\mathrm{\ρE}+p)\right]=\frac{\∂}{{\∂x}_j}(k_{\mathrm{eff}}\frac{\∂T}{{\∂x}_j}-\underset{j}{\mathrm{\Σ}}{h}_j{\stackrel{\→}{J}}_j+U_i{\mathrm{\μ}}_{\mathrm{eff}}[(\frac{{\∂U}_j}{{\∂x}_i}+\frac{{\∂U}_i}{{\∂x}_j})-\frac{2}{3}\frac{{\∂U}_l}{{\∂x}_l}{\mathrm{\δ}}_{\mathrm{ij}}\left]\right)+S_h$

component transport equation: $\frac{{\∂\mathrm{\ρU}}_j{Y}_i}{{\∂x}_j}=\frac{\∂}{{\∂x}_j}\left[({\mathrm{\ρD}}_{i,m}+\frac{{\mathrm{\μ}}_t}{{\mathrm{Sc}}_t})\frac{{\∂Y}_i}{{\∂x}_j}\right]+R_i$

wherein $p_{\mathrm{eff}}=p+\frac{2}{3}\mathrm{\ρk},$ μ_eff=μ+μ_t， ${\mathrm{\μ}}_t={\mathrm{\ρC}}_{\mathrm{\μ}}\frac{k^2}{\mathrm{\ϵ}},$ ${\mathrm{\δ}}_{\mathrm{ij}}=\left\{\begin{array}{c}1(i=j)\\ 0(i\≠j)\end{array}\right.,$ $G_k={\mathrm{\μ}}_t[(\frac{{\∂\mathrm{\μ}}_i}{{\∂x}_j}+\frac{{\∂\mathrm{\μ}}_j}{{\∂x}_i})-\frac{2}{3}\frac{{\∂\mathrm{\μ}}_l}{{\∂x}_l}{\mathrm{\δ}}_{\mathrm{ij}}]\frac{{\∂u}_i}{{\∂x}_j},$ k_eff=k+k_t， $E=h-\frac{p}{\mathrm{\ρ}}+\frac{U^2}{2}$ $h=\underset{j}{\mathrm{\Σ}}{Y}_j{h}_j,$ $h_j={\∫}_{T_{\mathrm{ref}}}^T{c}_{p,j}\mathrm{dT},$ The standard k-model parameter value is C_μ=0.09，σ_k=1.0，σ =1.3，C₁=1.44And C₂= 1.92. For the calculation of the enthalpy of the components, T_refSet to 298.15K.

(2) Combustion model

The industrial dichloroethane cracking furnaces established at present all adopt a side wall burner heat supply combustion mode. The invention is a dichloroethane cracking furnace of a three-well device, and also adopts side wall burners for heat supply, wherein 136 premixed burners are respectively arranged on two sides of the furnace wall of the cracking furnace, namely, fuel and air are premixed and then sprayed out for combustion.

The fuel and the air are sprayed out from the nozzle to form high-speed jet flow and react quickly, and the invention describes the combustion of the wall burner by adopting a turbulence-chemical reaction interaction model, namely a vortex dissipation model. In the finite rate model, the chemical source term is calculated using the Arrhenius formula, and the chemical reaction net source term for chemical i is calculated by N with its participation_RThe sum of the Arrhenius reaction sources for each chemical reaction was calculated to yield:

$R_i=M_{w,i}\underset{r=1}{\overset{N_R}{\mathrm{\Σ}}}{\hat{R}}_{i,r}$

wherein M is_w,iIs the molecular weight of the i-th species,the production/decomposition rate of the ith substance in the r reaction is expressed as:

${\hat{R}}_{i,r}=\mathrm{\Γ}(v_{i,r}^{\′\′}-v_{i,r}^{\′})\left(k_{f,r}\underset{j=1}{\overset{N}{\mathrm{\Π}}}{\left[C_{j,r}\right]}^{({\mathrm{\η}}_{j,r}^{\′}+{\mathrm{\η}}_{j,r}^{\′\′})}\right)$

production rate R of substance k in reaction R in vortex dissipation model_i,kGiven by the smaller of the two expressions:

$R_{i,k}=v_{i,k}^{\′}{M}_{w,i}\mathrm{A\ρ}\frac{\mathrm{\ϵ}}{k}\min \left(\frac{Y_R}{v_{R,k}^{\′}{M}_{w,R}}\right)$

$R_{i,k}=v_{i,k}^{\′}{M}_{w,i}\mathrm{AB\ρ}\frac{\mathrm{\ϵ}}{k}\frac{{\mathrm{\Σ}}_P{Y}_P}{{\mathrm{\Σ}}_j^N{v}_{j,k}^{\′\′}{M}_{w,j}}$

in the model, the fuel gas of the dichloroethane cracking furnace of the device adopts Liquefied Petroleum Gas (LPG), and the main component of the LPG comprises C₃H₈/C₄H₆/C₄H₈/C₄H₁₀. The invention adopts a one-stage series combustion reaction model.

C₃H₈+3.5O₂→3CO+4H₂O

C₄H₆+3.5O₂→4CO+3H₂O

C₄H₈+4O₂→4CO+4H₂O

C₄H₁₀+4.5O₂→4CO+5H₂O

CO+0.5O₂→CO₂

(3) Radiation model

Since the discrete coordinate model can be applied to any optical depth medium, not only can the radiative heat exchange between gas and particles be calculated, but also the non-gray body radiation can be calculated by using the gray belt model, the influence of scattering is considered, and the specular reflection and the radiation in the semitransparent medium are allowed to occur, so that the method is particularly suitable for the problem of having a local heat source. In addition, the discrete coordinate algorithm is simple, reliable and small in calculation amount, so that the method is widely applied. The radiation heat transfer model of this patent uses the discrete coordinate model (discreteOrdinates), solves the radiation propagation equation that the discrete solid angle of finite quantity sent out, and the number of radiation propagation equation is the same with the moderate amount number of direction in the space coordinate system, and its mathematical expression is:

$\▿\·\left(I(\stackrel{\→}{r},\stackrel{\→}{s})\stackrel{\→}{s}\right)+(\mathrm{\α}+{\mathrm{\σ}}_s)I(\stackrel{\→}{r},\stackrel{\→}{s})={\mathrm{\αn}}^2\frac{{\mathrm{\σT}}^4}{\mathrm{\π}}+\frac{{\mathrm{\σ}}_s}{4\mathrm{\π}}{\∫}_0^{4\mathrm{\π}}I(\stackrel{\→}{r},{\stackrel{\→}{s}}^{\′})\mathrm{\Φ}(\stackrel{\→}{s}\·{\stackrel{\→}{s}}^{\′})d{\mathrm{\Ω}}^{\′}$

the furnace chamber is filled with a large amount of flue gas (especially CO, CO)₂And H₂O) to participate in radiant heat transfer. Among these fumes, CO₂And H₂O has a different absorption bandwidth. The simulation uses a multiple ash gas weighting model (WSGGM) to calculate the radiation characteristics of the flue gas. The model divides the blackness of real gas into a weighted sum of several blackness of grey gas, and the expression is as follows:

$\mathrm{\ϵ}=\underset{i=0}{\overset{I}{\mathrm{\Σ}}}{\mathrm{\α}}_{\mathrm{\ϵ},i}\left(T\right)(1-e^{-k_i\mathrm{ps}})$

for the open area, due to its higher spectral absorptance, the absorption coefficient of the i =0 component is set to 0, weighted by the absorption coefficient:

${\mathrm{\α}}_{\mathrm{\ϵ},0}=1-\underset{i=1}{\overset{I}{\mathrm{\Σ}}}{\mathrm{\α}}_{\mathrm{\ϵ},i}$

temperature dependent a_,iCan be approximated (fitted) by any function, but generally takes the form:

${\mathrm{\α}}_{\mathrm{\ϵ},i}=\underset{j=1}{\overset{J}{\mathrm{\Σ}}}{b}_{\mathrm{\ϵ},i,j}{T}^{j-1}$

the total absorption coefficient of the mixed gas is calculated by the following formula:

when s is>10^-4m， $\mathrm{\α}=-\frac{\ln (1-\mathrm{\ϵ})}{s};$ When s is less than or equal to 10^-4m， $\mathrm{\α}=\underset{i=0}{\overset{I}{\mathrm{\Σ}}}{\mathrm{\α}}_{\mathrm{\ϵ},i}{k}_{i}p.$

Furnace tube mathematical model

In the reaction tube, the cracking raw material is subjected to complex cracking reaction while being subjected to fluid flow, heat transfer and mass transfer. In terms of transfer process, the furnace tube and the hearth are both in fluid flow with chemical reaction, and the only difference is that one radiation heat transfer process is omitted in the tube. From the chemical reaction point of view, although the cracking reaction mechanism of the raw materials in the tube is different from the combustion reaction mechanism in the furnace, the equations to be solved by the two are similar, and the component equations are solved. Therefore, the equations involved in each model in the tube are substantially the same as those in the furnace, but one less radiant heat transfer equation is involved.

(1) Kinetic model of cracking reaction

For the dichloroethane cracking reaction procedure, a one-stage series reaction model was used:

(2) mathematical model for transfer reaction in pipe

When the first-stage series reaction model is used for calculating the distribution of components in the pipe, the CFD needs to solve the component process, and most importantly, a source term in an equation is determined, and the source term reflects the influence of chemical reaction. The kinetics of the cleavage reaction follows the Alnerius equation, and therefore the chemical reaction rate is represented by the formula:

${\hat{R}}_{i,r}=\mathrm{\Γ}(v_{i,r}^{\′\′}-v_{i,r}^{\′})\left(k_{f,r}\underset{j=1}{\overset{N}{\mathrm{\Π}}}\right[C_{j,r}]{\mathrm{\η}}_{j,r}^{\′}-k_{b,r}\underset{j=1}{\overset{N}{\mathrm{\Π}}}[C_{j,r}\left]{\mathrm{\η}}_{j,r}^{\′\′}\right)$

in CFD simulation, the formula is also called finite rate model (FRC). In addition to these 4 component equations, the fluid flow equation and the energy equation are also solved in the calculation of the transfer reaction process in the tube, and the form of the equations is the same as that of the equations in the furnace, so that the detailed description is omitted, and the total of 11 equations is obtained. Finally, to make these partial differential equations definite, the corresponding boundary conditions must be given and the entire system of equations must be closed.

(3) Properties of matter

The physical properties of the gas such as density, heat capacity, viscosity, thermal conductivity and diffusion coefficient in the mathematical model of the reaction tube are calculated by the following equations.

The process gas in the reaction tube is at high temperature and low pressure, and the density of the mixed gas is calculated by an ideal gas state equation.

$\mathrm{\ρ}=\frac{p}{\mathrm{RT\Σ}\frac{Y_i}{M_i}}$

The one-component viscosity is calculated by kinetic theory:wherein,

the viscosity of the cracking mixed gas is obtained by a single-component viscosity formula weighting technology:

wherein,

single component gas heat capacity: c_pi=A_i+B_iT+C_iT²+D_iT³And then the heat capacity of the mixed gas is as follows:

in the formula, A_i、B_i、C_i、D_iThe coefficient was calculated for the heat capacity of component i.

The single-component thermal conductivity coefficient is calculated by a kinetic theory:then mix

Gas thermal conductivity coefficient:

the diffusion coefficient between the two components was calculated by kinetic theory:wherein, ${\mathrm{\Ω}}_D={\mathrm{\Ω}}_D\left(\frac{T}{{(\mathrm{\ϵ}/k)}_{\mathrm{ij}}}\right),$ ${(\mathrm{\ϵ}/k)}_{\mathrm{ij}}=\sqrt{{(\mathrm{\ϵ}/k)}_i{(\mathrm{\ϵ}/k)}_j},$ ${\mathrm{\σ}}_{\mathrm{ij}}=\frac{1}{2}({\mathrm{\σ}}_i+{\mathrm{\σ}}_j).$ the diffusion coefficient of the mixed gas can thus be obtained:

dichloroethane furnace chamber and furnace tube coupling simulation

Mesh partitioning

Due to the symmetry of the dichloroethane cracking furnace, only half of the dichloroethane cracking furnace in the width direction was used for the simulation in order to reduce the amount of CFD calculation. Grid division of a hearth: the tetrahedral unit is used for dividing grids of the burner area and the furnace tube area; the hexahedral unit is used for dividing grids of other areas of the hearth. Grid division of the furnace tube: the hexahedral unit is used for dividing grids on the wall surface of the furnace tube; the mixing body unit is used for dividing grids of the furnace tube connecting part. The gridding of the hearth and the furnace tube is divided as shown in figure 1:

boundary condition

(1) Entry boundary conditions: and determining the gas inlet flow, the air inlet flow and the inlet gas flow and the temperature of the reaction tube of the side wall burner according to the process parameters.

(2) Wall surface boundary conditions: adopting a non-slip hypothesis for both the reaction tube and the wall surface of the furnace wall, namely that the values of all physical quantities on the wall surface are 0; the flow and heat exchange in the viscous bottom layer near the wall surface are approximately treated by adopting a standard wall surface function; the heat boundary on the hearth wall gives a heat flux boundary condition through heat loss; the furnace tube wall temperature boundary adopts the temperature preliminarily assumed by the actual working condition and the operation experience of a factory, and is assigned to the tube wall, and particularly can be assigned through a user-defined function (UDF), wherein the UDF function is defined as: q (x) = a₁+b₁x+c₁x²+d₁x³+e₁x⁴+f₁x⁵In the furnace chamber model, the furnace tube outer wall temperature UDF is defined as t (x) = a₂+b₂x+c₂x²+d₂x³+e₂x⁴+f₂x⁵；。

(3) Exit boundary conditions: and determining the outlet pressure of the furnace flue gas of the cracking furnace and the outlet pressure of the cracking gas of the furnace tube according to the process conditions.

Coupled simulation solving

The mathematical models of the transfer reaction in the furnace and the tube are respectively established by a computational fluid dynamics platform ansys14.0 (fluent). However, we say that the two sets of mathematical models do not exist in isolation, but are coupled together in a mutual relationship, and the "ligament" of the relationship is the temperature and the heat flux of the pipe wall, so that the two mathematical models must be coupled through a thermal boundary to calculate a final result, and a specific algorithm is as follows:

(1) the invention selects a group of actually measured pipe wall temperatures as initial values, and gives the initial values to the pipe wall boundaries in the hearth model, thereby calculating the combustion and flow processes of fuel gas in the hearth and obtaining a group of heat flux data for verifying the distribution of the furnace pipes in the length direction. The set of heat flux data is compared to previously calculated heat flux data, if the difference between the two reaches a specified accuracy.

(2) And (3) endowing the heat flux obtained by calculation in the step (1) to the pipe wall boundary of the built in-pipe model, and obtaining a group of new pipe wall temperatures by calculating the heat transfer process and the in-pipe cracking process of the furnace pipe. (3) And (3) comparing the new group of tube wall temperatures obtained by calculation in the step (2) with the tube wall temperature obtained by previous calculation, stopping calculation if the difference value of the two temperatures reaches the specified precision (the precision is less than 1 ℃), and otherwise, repeating the steps (1) and (2) and continuing iteration until the calculation precision is met.

The dichloroethane cracking furnace hearth combustion heat transfer model and the in-pipe transfer reaction model coupling solving process is shown in figure 2.

Analysis of results

Determining boundary conditions, closing the degree of freedom of CFD calculation, and obtaining the detailed distribution condition of important variables in the hearth and the furnace tube through CFD simulation. Fig. 3 to 7 show the furnace flue gas velocity field, temperature field, CO, O2, CO2 concentration field distribution on a tangential plane in the furnace width direction y =0.0485m, respectively. FIGS. 8 to 12 show the distribution curves of the heat flux of the furnace tube, the temperature of the cracking gas in the tube, the flow rate of the cracking gas in the tube, the pressure of the cracking gas in the tube, and the composition of each component of the cracking gas in the tube along the length direction of the furnace tube, respectively. As can be seen from the data analysis in the table 1, the error between the key data provided by the CFD method simulation and the field measured data is small, so that the modeling of the dichloroethane cracking furnace by using the CFD method is very accurate.

TABLE 1CFD modeling and one-dimensional modeling method key data comparison table

Appendix symbol table

A empirical constant

B empirical constant

C_j,rMolar concentration of component j in reaction r, kgmol/m³

C_piHeat capacity of a single component i, J/kg/K

D_i,mMass diffusion coefficient of component i, m, in the mixture²/s

D_ijBinary mass diffusion coefficient of component i in j, m²/s

D_tEffective mass diffusion coefficient, m, due to turbulence²/s

Total energy per unit mass, J/kg

G_kGeneration term of turbulent kinetic energy, J/m³/s

h shows enthalpy, J/kg

h_jEnthalpy of component J, J/kg

I intensity of radiation, J/m²/s

Diffusion flux of Components, kg/m²/s

k kinetic energy of turbulence, m²/s^-2

k_effEffective conductivity, W/m/K

k_f,rForward rate constant of reaction r, 1/s

k_b,rInverse rate constant of reaction r, 1/s

k_iAbsorption coefficient of i-th ash gas, 1/m

k_tTurbulent Heat transfer coefficient, W/m/K

M_w,iMolecular weight, g/mol, of component i

n refractive index

Number of chemical species in N System

p pressure and the sum of the partial pressures of all the absorption gases, Pa

p_effEffective pressure, Pa

Position vector

R ideal gas constant, R =8.314J/mol/K

R_iNet production of component i by chemical reaction, gmol/m³/s

R_jReaction rate of j participation, kmol/m/s

Direction vector

Scattering direction vector

s path length

Sc_tTurbulent Schmidt number

S_hSource term in energy equation, J/m³/s

T local temperature and oil gas temperature, K

U_i,U_j,U_lVelocity component in i, j, k direction, m/s

x_i,x_j,x_lCoordinates in the i, j, k directions, m

X_iMole fraction of component i

Y_jMass fraction of component j

Y_PMass fraction of product P

Y_RMass fraction of reactant R

α absorption coefficient, 1/m and dependence on p_tConversion factor of a unit

α_,iEmissivity weight factor of ith virtual gray gas

_ijKronecker function

Dissipation ratio of turbulent kinetic energy, m²/s³And emissivity

K Lonnerd-Jones potential energy parameter, K

Viscosity of the gas molecules, kg/m/s

μ_effEffective viscosity, kg/m/s

μ_tTurbulent viscosity, kg/m/s

Rho gas density, kg/m³

Net effect of third object on reaction rate

ν′_i,rStoichiometric coefficient of reactant i in reaction r

ν″_i，rStoichiometric coefficient of product i in reaction r

η′_j,rVelocity index of reactant j in reaction r

η″_j,rVelocity index of product j in reaction r

σ Stefan-Boltzmann constant, σ =5.672 × 10^-8W/m²K⁴

σ_iComponent i Lonneder-Jones collision diameter, angstrom

σ_SScattering coefficient, 1/m

Phi phase function

Omega solid angle

Ω_DIntegral of diffusion collision

Ω_μiNumber of viscosity impact integrals of component i

The foregoing is by way of example only, and not limiting. It is intended that all equivalent modifications or variations without departing from the spirit and scope of the present invention shall be included in the appended claims.

Claims (7)

1. A coupled numerical modeling method for combustion of a hearth of an industrial dichloroethane cracking furnace and cracking reaction in the furnace tube is characterized by comprising the following steps:

step 1: determining the size of a furnace tube of a hearth of the dichloroethane cracking furnace to be simulated, and performing grid division on the hearth and the furnace tube; determining boundary conditions according to process parameters, including: the gas inlet flow and the air inlet flow of the side wall burner, the inlet gas flow and the temperature of the reaction tube, the heat loss coefficient of the furnace wall, the outlet pressure of the hearth flue gas and the outlet pressure of the furnace tube cracking gas;

the mesh division is carried out on the hearth and the furnace tube, wherein a burner area and a furnace tube area in the hearth adopt tetrahedral units for dividing meshes; other areas of the hearth adopt hexahedral units for dividing grids; the wall surface of a straight pipe of the furnace tube in the furnace tube model adopts hexahedral units to divide grids; the bent pipe adopts a mixture unit to divide grids;

step 2: establishing a hearth model:

step 2.1: a flue gas flow model in the hearth adopts a standard k-double-stroke model based on a Reynolds average equation to establish a closed model;

step 2.2: a first-stage series combustion reaction model is adopted for fuel gas in a hearth, and a finite rate/vortex dissipation model is adopted for a flow model during combustion;

step 2.3: the radiation heat transfer model in the hearth adopts a discrete coordinate model, and the hearth flue gas adopts a multi-ash gas weighting model to calculate the radiation characteristic of the hearth flue gas;

and step 3: establishing a furnace tube model:

step 3.1: the ethylene dichloride cracking reaction in the furnace tube adopts a first-stage series reaction model, and the cracking reaction kinetics conforms to an Allen-Raeus formula;

step 3.2, determining parameters of a gas density, heat capacity, viscosity, heat conductivity coefficient and diffusion coefficient calculation formula in the furnace tube model;

and 4, step 4: based on the serious heat coupling relationship between the hearth and the furnace tube and the initial condition and the boundary condition obtained in the step 1, the outer wall temperature and the heat flux of the furnace tube are used as mutual iterative coupling variables when numerical values of the hearth model and the furnace tube model are solved, and the hearth model and the furnace tube model are circularly iterated until the model converges to obtain the model related to each parameter;

in the step 2 and the step 3, the furnace tube and the wall surface of the hearth wall are regarded as non-slip boundaries; in the viscous bottom layer near the wall surface, a standard wall surface function is adopted to approximate the flow and heat exchange in the actual process; the heat boundary on the hearth wall gives a heat flux boundary condition through heat loss; the boundary of the wall surface of the furnace tube is assigned to the wall surface by a self-defined function, and in the model of the furnace tube, the self-defined function of the heat flux of the outer wall of the furnace tube is defined as Q (x) a₁+b₁x+c₁x²+d₁x³+e₁x⁴+f₁x⁵In the furnace chamber model, the furnace tube outer wall temperature customized function is defined as T (x) a₂+b₂x+c₂x²+d₂x³+e₂x⁴+f₂x⁵Wherein a is₁、b₁、c₁、d₁、e₁、f₁、a₂、b₂、c₂、d₂、e₂、f₂X is a coordinate along the radial direction of the furnace tube as a parameter to be fitted; q is the heat flux, and T is the furnace tube outer wall temperature.

2. The coupled numerical modeling method for furnace chamber combustion and in-furnace cracking reaction of an industrial dichloroethane cracking furnace according to claim 1, characterized in that the first-stage series combustion model in step 2.2 is:

$C_x{H}_y+(\frac{x}{2}+\frac{y}{4})O_2=xCO+\frac{y}{2}{H}_{2}O$

$CO+\frac{1}{2}{O}_2={\mathrm{CO}}_2;$

using a finite rate model, the chemical source term is calculated using the Arrhenius formula:

$R_i=M_{w,i}\underset{r=1}{\overset{N_R}{\Σ}}{\hat{R}}_{i,r}$

M_w,iis the molecular weight of component i, N_RFor the number of equations, R_iThe net production rate of component i caused by the chemical reaction,the production/decomposition rate of the ith substance in the r reaction is expressed as:

${\hat{R}}_{i,r}=\mathrm{\Γ}({\mathrm{\ν}}_{i,r}^{\′\′}-{\mathrm{\ν}}_{i,r}^{\′})\left(k_{f,r}\underset{j=1}{\overset{N}{\Π}}{\[C_{j,r}\]}^{({\mathrm{\η}}_{j,r}^{\′}+{\mathrm{\η}}_{j,r}^{\′\′})}\right)$

wherein the net effect of the third object on the reaction rate, v'_i,r，ν″_i,rIs the stoichiometric coefficient, k_f,rForward reaction rate constant, C_j,rIs molar concentration, η'_j,r，η″_j,rIs the rate index, N is the total number of substances participating in the reaction;

using a vortex dissipation model, the rate of production R of a substance k in a reaction R_i,kThe smaller of the two terms in the following formula:

$R_{i,k}=min\[{\mathrm{\ν}}_{i,k}^{\′}{M}_{w,i}A\mathrm{\ρ}\frac{\mathrm{\ϵ}}{k}min\left(\frac{Y_R}{{\mathrm{\ν}}_{R,k}^{\′}{M}_{w,R}}\right),{\mathrm{\ν}}_{i,k}^{\′}{M}_{w,i}AB\mathrm{\ρ}\frac{\mathrm{\ϵ}}{k}\frac{{\mathrm{\Σ}}_P{Y}_P}{{\mathrm{\Σ}}_j^N{\mathrm{\ν}}_{j,k}^{\′\′}{M}_{w,j}}\]$

v 'therein'_i,k，ν′_R,k，ν″_j,kIs a stoichiometric coefficient, M_w,i，M_w,R，M_w,jMolecular weight, A, B are empirical coefficients, ρ gas density,is the Lonned-Jones potential energy parameter, Y_R，Y_PIs the mass fraction of the product.

3. The method for modeling the coupling value of the furnace chamber combustion and the in-furnace cracking reaction of the industrial dichloroethane cracking furnace according to claim 1, wherein a closed mathematical model is established based on a standard k-two-equation model of the Reynolds average Navier-Stokes equation in step 2.1, and the mass, momentum, turbulence energy, dissipation ratio of the turbulence energy, energy and component transport equations are expressed as follows:

continuity equation: $\frac{\∂}{\∂x_i}\left({\mathrm{\ρU}}_i\right)=0$

the momentum equation: $\frac{\∂}{\∂x_j}\left({\mathrm{\ρU}}_i{U}_j\right)=\frac{\∂p_{eff}}{\∂x_i}+\frac{\∂}{\∂x_j}\[{\mathrm{\μ}}_{eff}(\frac{\∂U_i}{\∂x_j}+\frac{\∂U_j}{\∂x_i}-\frac{2}{3}{\mathrm{\δ}}_{ij}\frac{\∂U_l}{\∂x_l})\]$

k-equation: $\frac{\∂}{\∂x_i}\left({\mathrm{\ρkU}}_i\right)=\frac{\∂}{\∂x_j}\[(\mathrm{\μ}+\frac{{\mathrm{\μ}}_t}{{\mathrm{\σ}}_k})\frac{\∂k}{\∂x_j}\]+G_k-\mathrm{\ρ}\mathrm{\ϵ}$

-the equation: $\frac{\∂}{\∂x_i}\left({\mathrm{\ρ\ϵU}}_i\right)=\frac{\∂}{\∂x_j}\[(\mathrm{\μ}+\frac{{\mathrm{\μ}}_t}{{\mathrm{\σ}}_{\mathrm{\ϵ}}})\frac{\∂\mathrm{\ϵ}}{\∂x_j}\]+\frac{\mathrm{\ϵ}}{k}(C_{1\mathrm{\ϵ}}{G}_k-C_{2\mathrm{\ϵ}}\mathrm{\ρ}\mathrm{\ϵ})$

energy equation:

$\frac{\∂}{\∂x_i}\[U_i\left(\mathrm{\ρ}E+p\right)\]=\frac{\∂}{\∂x_j}\left(k_{eff}\frac{\∂T}{\∂x_j}-\underset{j}{\Σ}{h}_j{\stackrel{\→}{J}}_j+U_i{\mathrm{\μ}}_{eff}\[\left(\frac{\∂U_j}{\∂x_i}+\frac{\∂U_i}{\∂x_j}\right)-\frac{2}{3}\frac{\∂U_l}{\∂x_l}{\mathrm{\δ}}_{ij}\]\right)+S_h$

component transport equation: $\frac{\∂{\mathrm{\ρU}}_j{Y}_i}{\∂x_j}=\frac{\∂}{\∂x_j}\[({\mathrm{\ρD}}_{i,m}+\frac{{\mathrm{\μ}}_t}{{\mathrm{Sc}}_t})\frac{\∂Y_i}{\∂x_j}\]+R_i$

wherein U is_i,U_j,U_lIs the velocity component in the i, j, k direction, x_i,x_j,x_lIs the coordinate in the i, j, k direction, p is the gas density, p_effEffective pressure, mu_effIn order to be of an effective viscosity,_ijis a function of kronecker, k is the kinetic energy of turbulence, μ is the viscosity of the gas molecule, μ_tFor turbulent viscosity, G_kFor the generation term of turbulent kinetic energy, for the dissipation ratio of turbulent kinetic energy, S_hAs a source term in the energy equation, C_μ,C₁,C₂,σ_k，σ Is a standard k-model parameter, E is the total energy per unit mass, p is the pressure, k_effThe conductivity of the water is measured by the conductivity meter,is the diffusion flux of the component, h_jIs the enthalpy of component j, Y_jIs the mass fraction of component j, D_i,mIs the mass diffusion coefficient of component i, Sc, in the mixture_tIs the turbulent Schmitt number, R_iIs the net production rate of component i caused by the chemical reaction.

4. The coupled numerical modeling method for combustion of the furnace chamber of the industrial dichloroethane cracking furnace and the cracking reaction in the furnace tube as recited in claim 1, characterized in that the radiation heat transfer model in step 2.3 adopts a discrete coordinate model, and the mathematical expression of the model is as follows:

$\▿\·\left(I\right(\stackrel{\→}{r},\stackrel{\→}{s}\left)\stackrel{\→}{s}\right)+(\mathrm{\α}+{\mathrm{\σ}}_s)I(\stackrel{\→}{r},\stackrel{\→}{s})={\mathrm{\αn}}^2\frac{{\mathrm{\σT}}^4}{\mathrm{\π}}+\frac{{\mathrm{\σ}}_s}{4\mathrm{\π}}{\∫}_0^{4\mathrm{\π}}I(\stackrel{\→}{r},{\stackrel{\→}{s}}^{\′})\mathrm{\Φ}(\stackrel{\→}{s}\·{\stackrel{\→}{s}}^{\′}){\mathrm{d\Ω}}^{\′};$

wherein I is the intensity of radiationThe degree of the magnetic field is measured,in the form of a position vector, the position vector,is a direction vector, α is an absorption coefficient, σ_sThe scattering coefficient is n, the refractive index is n, the sigma is a Stefan-Boltzmann constant, T is the flue gas temperature phi which is a phase function, and omega' is a solid angle;

the radiation characteristic of the flue gas in the hearth is calculated by adopting a multi-ash-gas weighted model, and the model approximately processes the blackness of the real gas into the weighted sum of the blackness of a plurality of ash gases:

$\mathrm{\ϵ}=\underset{i=0}{\overset{I}{\Σ}}{\mathrm{\α}}_{\mathrm{\ϵ},i}\left(T\right)(1-e^{-k_{i}ps})$

α therein_,iThe emissivity weighting factor of the ith kind of virtual ash gas, the total absorption coefficient of the flue gas can be expressed as:

when s > 10^-4m, $\mathrm{\α}=-\frac{ln(1-\mathrm{\ϵ})}{s};$ When s is less than or equal to 10^-4m， $\mathrm{\α}=\underset{i=0}{\overset{I}{\Σ}}{\mathrm{\α}}_{\mathrm{\ϵ},i}{k}_{i}p.$

5. The coupled numerical modeling method for combustion of industrial dichloroethane cracking furnace and cracking reaction in furnace according to claim 1, characterized in that the step 3.1 of first-stage series cracking reaction of dichloroethane cracking gas is

The kinetics of the method accord with an Allen-Raeus formula, and the chemical reaction rate is represented by the following formula:

${\hat{R}}_{i,r}=\mathrm{\Γ}({\mathrm{\ν}}_{i,r}^{\′\′}-{\mathrm{\ν}}_{i,r}^{\′})(k_{f,r}\underset{j=1}{\overset{N}{\Π}}\[C_{j,r}\]{\mathrm{\η}}_{j,r}^{\′}-k_{b,r}\underset{j=1}{\overset{N}{\Π}}\[C_{j,r}\]{\mathrm{\η}}_{j,r}^{\′\′})$

whereinIs the production/decomposition rate of the ith substance in the r-th reaction, v'_i,r，ν″_i,rIs stoichiometric coefficient, η'_j,r，η″_j,rIs a reaction rate index, k_f,rIs a forward reaction rate constant, k_b,rAs a reverse reaction rate constant, C_j,rIs the molar concentration of component j.

6. The coupled numerical modeling method for furnace combustion and in-furnace cracking reactions of an industrial dichloroethane cracking furnace according to claim 1, characterized in that the furnace model convergence condition is that the heat flux on the tube wall obtained by the furnace model for a new group of tube walls reaches a preset accuracy compared with the heat flux on the tube wall obtained by the previous calculation.

7. The coupled numerical modeling method for furnace chamber combustion and in-furnace cracking reactions of an industrial dichloroethane cracking furnace according to claim 1, characterized in that the furnace model convergence condition is that a new set of wall temperatures obtained by the furnace model reaches a preset accuracy compared with the previously calculated furnace wall temperature.