Product prediction method of electric heating cracking furnace

Abstract

The invention discloses a product prediction method of an electric heating cracking furnace, a cracking product prediction model provided by the invention is completely consistent with the structure of the electric heating cracking furnace, a cracking reaction network is relatively perfect, and the cracking reaction can be described from the mechanism; and calculating to obtain the distribution of variables such as yield of each product in cracking reaction, cracking gas temperature and pressure along the tube length of the furnace tube.

Description

Technical Field

[0001] The invention belongs to the technical field of low-carbon olefin production, and in particular relates to a product prediction method for an electric heating cracking furnace. Background Art

[0002] The ethylene industry is the core of the petrochemical industry. The core part of the ethylene production unit is the cracking furnace. The radiation section of the cracking furnace is the main place for heat exchange. 70%-80% of the heat load of the whole furnace is borne by the radiation section. It is the most important part of the whole furnace. The traditional steam cracking furnace mainly hangs the cracking furnace tube in the center of the furnace, and the bottom and / or side walls of both sides of the furnace are respectively arranged with combustion nozzles. The liquid fuel is atomized and fully mixed with air and enters the steam cracking furnace through the combustion nozzle. The heat generated by the combustion is mainly transmitted to the cracking furnace tube in the form of radiation and convection, so that the temperature in the furnace tube reaches the steam cracking reaction temperature. Under normal circumstances, the geometric structure of the radiation section is relatively large, and due to the endothermic nature of the cracking reaction, a large amount of fuel combustion is required to provide heat. Therefore, a large flow of fuel gas is sprayed into the furnace through the furnace nozzle in the form of a high-speed jet, which has a significant impact on the flow of the flue gas in the furnace, thereby further affecting the mixing and combustion process of the fuel gas and changing the temperature distribution in the furnace. At the same time, the flue gas transfers heat to the cracking raw materials in the reaction tube, causing a complex reaction process. Due to the long period, high cost and limited measurement methods, experimental research on heat flux distribution is extremely difficult. Moreover, the simple mathematical models proposed by many domestic scholars cannot provide a deep understanding of the complex phenomena occurring in the cracking furnace.

[0003] When an alternating current is passed through the induction coil of the electric heating cracking furnace, an alternating magnetic field will be formed around the coil, and then a closed loop will be formed on the surface of the heating element to generate a layer of eddy current, so that the heating element itself will quickly generate heat, thereby heating the cracking furnace tube. As another external heating method, electric heating has the advantages of simple operation, uniform heating intensity on the reactor surface, long reactor operating life, and low carbon emissions compared to external combustion heating. ANSYS software has the advantages of multi-physical field coupling such as electromagnetic physical field, thermoelectric coupling field, and powerful numerical simulation functions. Through accurate calculations of electronic computers, it can obtain results close to reality and is widely used. Therefore, the electromagnetic induction coil is used to couple with the cracking furnace tube for simulation, and the yield of the cracking product is obtained through simulation calculation.

[0004] Patent CN103310123AU discloses a coupled modeling method for design verification and optimization of industrial ethylene steam cracking furnaces. Computational fluid dynamics is used to establish the flow, combustion, and heat transfer models of the furnace. The furnace tube is a one-dimensional plug flow, and free radical reaction kinetics is used. According to the PONA value and distillation range of the cracking raw material, its specific composition is obtained through relational database regression, and then the heat flux and tube wall temperature are used as boundary conditions to perform coupled calculations of the furnace tube until convergence. This technology mainly conducts modeling research on traditional steam cracking furnaces. By analyzing the data of the furnace flow field, temperature field, etc., it is found that the amount of heat absorbed by the cracking furnace tube in this heating method and the uniformity of heating are closely related to the arrangement of the burner. At the same time, the composition and measurement analysis of the flue gas are also very difficult.

[0005] Patent CN103440390A discloses a coupled simulation method for the radiation section of an industrial steam cracking furnace. In the furnace model, the k-ε turbulence model is used for flue gas flow, the PDF model is used for fuel gas combustion, the adiabatic model, P-1, DO model and DTRM model are used for flue gas radiation heat transfer, and the WSGGM model is used to calculate the flue gas radiation characteristics. In the furnace tube model, a molecular or free radical reaction kinetic model is used in the tube, and the furnace model is coupled with the furnace tube model to obtain the temperature, velocity, pressure and concentration distribution of the fluid in the cracking furnace under different radiation models. This technology mainly couples the furnace and the furnace tube for simulation, and the flue gas in the furnace has a distribution pattern of high temperature in the middle and lower parts and low temperature in the upper part. Therefore, due to the uneven temperature distribution in the furnace, the furnace tube is also heated unevenly in the furnace, and the received radiation heat is different, which will cause a decrease in ethylene yield.

[0006] Patent CN108052792A discloses an ethylene cracking furnace optimization model, including: obtaining the importance of substances in the Kumar model according to an improved web-level algorithm, realizing the screening and enrichment of important substances, and obtaining the lower limit of the decomposition reaction in the ethylene cracking reaction; obtaining the optimal range of the enrichment reaction according to the average relative error algorithm; obtaining the minimum structural parameter Sp value according to the sequential quadratic programming algorithm and the average relative error algorithm. This technology only studies the raw material cracking reaction process in the cracking furnace tube, but in fact the heat transfer mode of the cracking furnace tube and the temperature and flow rate of the cracking gas in the tube also have a great influence on its reaction. Summary of the invention

[0007] The purpose of the present invention is to provide a product prediction method for an electric heating cracking furnace, which comprehensively and systematically analyzes the complex processes such as material flow, heat transfer, mass transfer and cracking reaction in the electric heating cracking furnace tube and electromagnetic thermal coupling in the electromagnetic heating induction heating coil, and simultaneously analyzes the strong coupling effect between these complex processes. Based on the turbulent flow model of fluid mechanics, the cracking reaction kinetic model, the electromagnetic field mathematical model, the temperature field mathematical model and the heat transfer model, the transfer process in the cracking furnace tube and the cracking reaction process are coupled with the electromagnetic induction coil heating heat transfer process.

[0008] To achieve the above object, the present invention provides a product prediction method for an electric heating cracking furnace, characterized in that the cracking furnace comprises an electromagnetic induction coil and a cracking reaction furnace tube, an alternating magnetic field is generated by the electromagnetic induction coil, and the generated alternating magnetic field passes through the cracking reaction furnace tube, so that the material inside the cracking furnace tube generates eddy current under the action of the magnetic field and generates self-heating to heat the material in the cracking furnace, and an inorganic heat insulating material with heat insulating effect is arranged between the furnace tube and the electromagnetic coil, and the product prediction uses ANSYS software, and the method comprises the following steps:

[0009] Step 1, determining the design and operation data of the electric heating cracking furnace according to the cracking raw material properties, including the size of the cracking furnace tube, the cracking raw material feed amount, inlet temperature, cracking gas outlet temperature, outlet pressure, electromagnetic induction coil voltage and current;

[0010] Step 2, establishing an electromagnetic induction coil heating model, meshing the electromagnetic induction coil, applying loads and constraints, and calculating the heat flux of the electromagnetic induction coil along the outer wall of the cracking furnace tube;

[0011] Step 3, establishing a cracking furnace tube model, using molecular reaction or free radical reaction kinetics modeling, and calculating the tube wall temperature distribution along the tube length, the cracking gas temperature, pressure and concentration of each component along the tube length, as well as the outlet temperature, outlet pressure and the yield of each component at the outlet through the furnace tube model;

[0012] Step 4, energy coupling is performed on the electromagnetic induction coil heating model and the cracking furnace tube model, the actual industrial cracking furnace tube wall temperature measurement value is brought into the electromagnetic induction coil heating model as a boundary condition for temperature calculation, and then the heat flux distribution of the electromagnetic induction coil along the tube length direction obtained by the electromagnetic induction coil calculation is brought into the cracking furnace tube model as a boundary condition for calculation to obtain a new furnace tube outer wall temperature distribution, and this iteration is repeated until convergence.

[0013] In the product prediction method for an electric heating cracking furnace of the present invention, in step 2, sweeping is selected for grid division.

[0014] In the product prediction method of the electric heating cracking furnace described in the present invention, in step 2, the applied load and constraint conditions are the voltage and current on the coil.

[0015] In the product prediction method of the electric heating cracking furnace described in the present invention, in step 2, a sequential coupling method is used to calculate the electromagnetic induction and temperature field.

[0016] In the product prediction method of the electric heating cracking furnace described in the present invention, the electromagnetic induction coil heating model in step 2 includes an electromagnetic field mathematical model, a temperature field mathematical model and a heat transfer model.

[0017] In the product prediction method of the electric heating cracking furnace of the present invention, in step 3, the cracking tube is meshed by using hexahedral unit division.

[0018] In the product prediction method of the electric heating cracking furnace described in the present invention, the iterative convergence criterion in step 4 is whether the difference between the maximum values ​​of the outer wall temperature of the furnace tube obtained by two calculations is less than a preset threshold value. If it is less than the threshold value, it is converged.

[0019] Beneficial effects of the present invention:

[0020] The electric heating cracking furnace is based on the principle of electromagnetic induction. By passing an adjustable frequency current into the electromagnetic induction coil, an alternating magnetic field is generated inside the coil. The generated alternating magnetic field passes through the cracking reaction furnace tube. The internal material of the furnace tube is affected by the magnetic field to generate eddy currents and cause itself to heat up. At the same time, combined with the inorganic thermal insulation material with good thermal insulation effect between the furnace tube and the electromagnetic coil, the cracking device can have a higher heat transfer efficiency than the traditional external heating method. The electric heating cracking device has the advantages of faster hot start and low energy consumption. During the operation of the device, the temperature distribution of the cracking furnace tube is more uniform, which can effectively reduce the thermal damage of the furnace tube during operation and greatly extend the service life of the cracking furnace tube.

[0021] The cracking product prediction model provided by the present invention is completely consistent with the structure of the electric heating cracking furnace and the cracking reaction network is relatively complete. It can describe the cracking reaction from the mechanism and calculate the distribution of variables such as the yield of each cracking reaction product, cracking gas temperature, pressure, etc. along the length of the furnace tube. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is a schematic diagram of an electric heating cracking furnace of the present invention;

[0023] Figure 2 This is a schematic diagram of the grid division of the cracking furnace tube;

[0024] Figure 3 It is a flow chart of energy coupling between electromagnetic induction coil heating model and cracking furnace tube model;

[0025] Figure 4 It is a schematic diagram of the simulation of the coupling of electromagnetic field and temperature field in the process of establishing the electromagnetic induction coil heating model. DETAILED DESCRIPTION

[0026] The present invention is described in detail below by way of examples. It is necessary to point out that the following examples are only used to further illustrate the present invention and cannot be understood as limiting the scope of protection of the present invention. Those skilled in the art in this field can make some non-essential improvements and adjustments to the present invention based on the above content of the present invention.

[0027] A product prediction method for an electric heating cracking furnace, characterized in that the cracking furnace comprises an electromagnetic induction coil and a cracking reaction furnace tube, the electromagnetic induction coil generates an alternating magnetic field, the generated alternating magnetic field passes through the cracking reaction furnace tube, so that the material inside the cracking furnace tube generates eddy current under the action of the magnetic field and generates self-heating to heat the material in the cracking furnace, and an inorganic heat-insulating material with a heat-insulating effect is arranged between the furnace tube and the electromagnetic coil, the product prediction uses ANSYS software, and the method comprises the following steps:

[0028] Step 1, determining the design and operation data of the electric heating cracking furnace according to the cracking raw material properties, including the size of the cracking furnace tube, the cracking raw material feed amount, inlet temperature, cracking gas outlet temperature, outlet pressure, electromagnetic induction coil voltage and current;

[0029] Step 2, establishing an electromagnetic induction coil heating model, meshing the electromagnetic induction coil, applying loads and constraints, and calculating the heat flux of the electromagnetic induction coil along the outer wall of the cracking furnace tube;

[0030] Step 3, establishing a cracking furnace tube model, using molecular reaction or free radical reaction kinetics modeling, and calculating the tube wall temperature distribution along the tube length, the cracking gas temperature, pressure and concentration of each component along the tube length, as well as the outlet temperature, outlet pressure and the yield of each component at the outlet through the furnace tube model;

[0031] Step 4, energy coupling is performed on the electromagnetic induction coil heating model and the cracking furnace tube model, the actual industrial cracking furnace tube wall temperature measurement value is brought into the electromagnetic induction coil heating model as a boundary condition for temperature calculation, and then the heat flux distribution of the electromagnetic induction coil along the tube length direction obtained by the electromagnetic induction coil calculation is brought into the cracking furnace tube model as a boundary condition for calculation to obtain a new furnace tube outer wall temperature distribution, and this iteration is repeated until convergence.

[0032] In step 2, the electromagnetic induction coil heating model undergoes a complex electromagnetic-thermal coupling process. The modeling process of the electromagnetic induction coil model is as follows: (1) Establishing the electromagnetic field mathematical model, the temperature field mathematical model and the heat transfer model; (2) Meshing the electromagnetic induction coil and loading appropriate loads and constraints; (3) Establishing discrete equations in the solution domain, and treating the dependent variables at a limited number of locations in the calculation domain as basic unknowns through numerical methods, thereby establishing a set of algebraic equations about these unknowns, and then solving the algebraic equations to obtain these node values, while the values ​​at other locations in the calculation domain are determined based on the values ​​at the node positions. (4) Solving the discrete equations.

[0033] 1. Establish the electromagnetic induction heating coil model

[0034] 1.1. Basic assumptions

[0035] The cracking raw materials are heated by electromagnetic induction heating, and the model is simplified as follows: ① The skin effect of the winding coil wire is ignored, and the winding is regarded as a whole; ② The influence of the hysteresis characteristics of the metal structural material on the magnetic field is ignored.

[0036] 1.2. Establishment of mathematical model

[0037] 1.2.1 Mathematical model of electromagnetic field

[0038] According to electromagnetic theory, Maxwell's equations describing the basic characteristics of the electromagnetic field are:

[0039]

[0040]

[0041]

[0042]

[0043] Structural equation:

[0044] D=εE

[0045] J=σE+J

[0046] B=μ

[0047] Where: H is the magnetic field intensity vector (A / m); J is the conduction current vector (A / m 2 ); D is the electric displacement vector (C / m 2 ); E is the electric field intensity vector (V / m); B is the magnetic flux density vector (Wb / m 2 ), μ is magnetic permeability (H / m), σ is electrical conductivity (1 / Ω·m), ε is dielectric constant (F / m); is the Laplace operator; t is time (s).

[0048] In the process of electromagnetic induction heating, the internal heat source of the heated workpiece is generated by the eddy currents generated inside the workpiece. If you want to obtain the eddy current field, you need to obtain the magnetic potential generated by the electromagnetic induction coil in the workpiece, then you need to solve the Maxwell equations. The magnetic field intensity H and electromagnetic intensity E in the Maxwell equations used as the control equations are constantly changing, so even solving the simultaneous equations is difficult. Then you need to introduce an auxiliary variable to eliminate and reduce the equations. If you use Workbench to analyze the electromagnetic field and eddy current field, the auxiliary variable to be used is the compound magnetic potential. Introduction A is the vector magnetic potential,

[0049]

[0050] The essence of induction heating is to use the eddy current generated by electromagnetic induction in the conductor to achieve the purpose of heating. The induced eddy current is used as the internal heat source, and its intensity is

[0051] q=ρ|J| 2

[0052] Where: q is the intensity of the internal heat source (W / m 3 ), ρ is the material resistance (Ω·m), J is the induced current density (A / m 2 ).

[0053] 1.2.2 Temperature field mathematical model

[0054] According to the law of conservation of energy, the differential equation of the temperature field can be established. In the rectangular coordinate system, the equation is:

[0055]

[0056] Where: q sh is the internal heat intensity in the rectangular coordinate system (W / m 3 ); ρ is the material density (Kg / m 3 ), c is the specific heat capacity of the material (J / kg·℃), T is the temperature (℃), and K is its thermal conductivity (W / m·℃).

[0057] For isotropic materials, assuming that their physical properties (such as k, ρ, c, etc.) are constants, the thermal conductivity differential equation in the cylindrical coordinate system is:

[0058]

[0059] Where: T is temperature (℃); q is the intensity of the internal heat source (W / m 3); α is the angle of deviation from the x-axis or y-axis; k is the thermal conductivity of the isotropic material (W / ℃); ρ is the material density (Kg / m 3 ), c is the specific heat capacity of the material (J / kg·℃), t is the time (s), and T is the temperature (℃).

[0060] 1.2.3 Mathematical model of heat transfer

[0061] The thermal model can be controlled by the following transient heat conduction equation:

[0062]

[0063] Where: c(T) is the specific heat, which changes with temperature; T is the temperature vector; γ is the mass density; [λ(T)] is the thermal conductivity, which changes with temperature; w is the volume energy density vector of the heat source

[0064] The boundary conditions of the working surface are convection and radiation, as shown below

[0065]

[0066] Where: T a is the ambient temperature; h is the convective heat transfer coefficient; ε is the radiation coefficient; c s is the Stefan-Boltzman constant.

[0067] 1.3. Loads, boundary conditions and meshing

[0068] The boundary conditions for the simulation calculation of the electromagnetic field are that the magnetic potential at the outer edge of the far field (i.e., infinity) is zero, and a parallel boundary of magnetic lines of force is applied to the central axis of the cracking furnace; a uniform source current density (obtained by dividing the current intensity by the cross-sectional area of ​​the coil) load condition is applied to the induction coil part, which is also the excitation condition of the electromagnetic field. The radiation boundary condition is applied to the surface of the cracking furnace, because there is a heat insulation layer between the cracking furnace and the induction coil, so the surface radiation coefficient is very small.

[0069] The quality of meshing directly affects the accuracy of the calculation results and the efficiency of the calculation. Figure 2 In this embodiment, a sufficiently fine finite element grid is divided near the cracking furnace surface, and the inside is drawn sparsely, so as to ensure the calculation accuracy and reduce the number of units, saving calculation time. Specifically, regular hexahedral units are selected to divide the grid.

[0070] 1.4. Simulation of coupling between electromagnetic field and temperature field

[0071] The coupling calculation process is as follows Figure 4As shown in the figure, the following assumptions are made: (1) The electromagnetic induction coil model is assumed to be in thermal equilibrium, that is, a steady-state thermal analysis is performed; (2) The change of the electrical conductivity of the metal material with temperature is ignored, that is, only the unidirectional coupling of the electromagnetic field to the heat is considered.

[0072] 2. Establish cracking furnace tube model

[0073] 2.1 Reactor modeling

[0074] Since the cracking reaction occurs inside a thin tube with a large aspect ratio, and the residence time of the cracking raw material is very short, the flow rate is very high, and the tube is in a highly turbulent state, it can be considered that the radial mixing is very uniform, there is no concentration and temperature gradient, and the reaction tube can be simplified to a displaced plug flow model.

[0075] Steam thermal cracking is an endothermic reaction, and the heat required is provided by the electromagnetic induction coil. Therefore, the reaction tube is non-adiabatic and non-isothermal, and there is a pressure drop. The mass, momentum, energy conservation equations and component transport equations are needed to describe the cracking reaction in the tube.

[0076] (1) The mass conservation equation, also known as the continuity equation, is the mathematical expression of the law of mass conservation in the flow field. That is, the sum of the mass flowing through the control surface per unit time is equal to the mass increment in the control volume per unit time. Its mathematical expression is:

[0077]

[0078] In the formula, the subscript i can take values ​​of 1, 2, and 3 to represent three spatial coordinates; ρ is density; t is time; u i For speed.

[0079] (2) Momentum equation refers to the mathematical expression of the law of conservation of momentum in the flow field, that is, the sum of the resultant force acting on the control volume and the momentum flowing into the control volume through the control surface per unit time is equal to the increment of the momentum of the fluid in the control volume per unit time. It is described by the Navier-Stokes equation and its mathematical expression is:

[0080]

[0081] Where ρ is pressure, t is time, and u is i 、u j is the velocity component in the i and j directions; ρg i is the gravitational volume force in the i direction; τ ij is the stress tensor.

[0082] (3) Energy equation refers to the mathematical expression of the law of conservation of energy in the flow field, that is, the sum of the heat and work given to the control body by the outside world and the energy flowing into the control body from the control surface per unit time is equal to the sum of the fluid energy in the control body per unit time. The mathematical expression is:

[0083]

[0084] In the formula, C p is the specific heat of the fluid; T is the temperature; ρ is the density; u i 、u j is the velocity component in the i and j directions; Q is the heat generated by the internal heat source.

[0085] (4) Component equation

[0086]

[0087] Among them, R i is the net generation rate of the i-th component in the chemical reaction; S i is the net generation rate obtained by adding the diffusion term to the user-defined source term; Y i is the dependent variable; Represents divergence.

[0088] is the diffusion flux generated by the i-th component when there is a concentration gradient. The mass diffusion flux in laminar flow is: Where D i,m is the diffusion coefficient of the i-th component in the mixture; the mass diffusion flow rate in turbulence is: Among them Sc t is the turbulent Schmidt number, that is μ t is the turbulent viscosity, D t is the diffusion coefficient.

[0089] 2.2 Kinetic model

[0090] Naphtha undergoes thermal cracking reaction in the furnace tubes to produce trienes (ethylene, propylene, butadiene) and other products required by the industry. There are four main different models for simulating the cracking reaction process:

[0091] (1) The empirical model uses a multivariate regression method to associate the raw material property parameters and cracking operating conditions with the distribution of cracking products in the cracking reaction, thereby predicting the product yield. The empirical model is relatively simple and intuitive, and does not require the derivation of complex reaction mechanisms, but requires a large amount of experimental data for fitting regression and verification, and does not reveal the inherent kinetic laws of the cracking reaction. The empirical model can be used to analyze the influence of a few variables such as temperature on the yield of cracking furnace steam thermal cracking products, thereby optimizing the production operation of the cracking furnace. However, the empirical model can associate with fewer cracking products, usually targeting ethylene, and the model has a very limited scope of application, limited to specific raw materials, specific cracking furnaces, and a limited range of cracking furnace operating conditions. When extrapolated, the deviation is very large, which has obvious limitations.

[0092] (2) Lumped model. The so-called lumping is to combine a large number of single compounds in a complex reaction system into several virtual components based on the principle of similar kinetic characteristics. In the study of cracking reaction kinetics, each lumped component is considered as a virtual single component. The kinetic model of the simplified lumped reaction network is established by developing the reaction network of the virtual lumped components. Therefore, the lumped model is essentially a kind of empirical model. Since the kinetic properties of the components in a multi-component system are different, it is difficult to determine whether all the components can be described by a lumped kinetic model. In other words, if the kinetic properties of all the components in the system are very different, it is difficult to use a lumped model. In other words, the lumped model is difficult to adapt to the current situation of the variability of raw materials.

[0093] (3) The molecular model is a model for kinetic calculations based on the stoichiometric equation of the cleavage reaction. It does not consider the details of the free radical reaction, but only the apparent molecular reaction. It adopts a simplified method to handle complex cleavage reaction systems, which caters to the needs of industrial production and has been widely recognized by researchers.

[0094] Molecular reaction model: This model assumes that the cracking raw material is a single component. On this basis, the cracking reaction is simplified into a primary reaction and several secondary reactions, and the two reactions do not cross each other. The pre-calculated results of this model are very sensitive to the coefficient of the primary reaction, while the secondary reaction has no significant effect. Therefore, it is usually necessary to correlate the primary reaction coefficients of different oil products. Because this model involves the reaction process, it has stronger extrapolation and applicability than the empirical model, and the development cycle is relatively short. It is also a type of model that is widely used in industry.

[0095] (4) Free radical model: This model is based on free radical reactions. The reaction process includes chain initiation, H abstraction, free radical addition, free radical decomposition, free radical isomerization, chain termination and other processes. It has a detailed free radical reaction network, is highly versatile, and has very high accuracy and reliability.

[0096] Based on the above reasons, the present invention adopts a molecular model or a free radical model.

[0097] 3. Coupling simulation of electric heating coil and cracking furnace tube

[0098] Since the electromagnetic induction coil and the cracking furnace tube are described by different models, which cannot be realized simultaneously in ANSYS software, it is necessary to couple the two by iterative trial and error method. The iterative process is as follows: Figure 3 The specific steps are as follows:

[0099] (1) Establish an electromagnetic induction coil model and input the voltage, current, frequency and other conditions required for calculation;

[0100] (2) using the measured value of the actual industrial cracking furnace tube wall temperature as the initial temperature, setting the temperature as the boundary condition of the electromagnetic induction coil model, and calculating the electromagnetic induction coil model by solving the electromagnetic field and temperature field control equations. After the calculation converges, the heat flux distribution of the electromagnetic induction coil along the reaction tube direction is obtained;

[0101] (3) bringing the obtained heat flux of the electromagnetic induction coil into the cracking furnace tube model as a boundary condition for calculation, obtaining a new temperature distribution of the outer wall of the furnace tube, and obtaining the temperature distribution of the cracking furnace tube after the calculation converges;

[0102] (4) The temperature distribution of the cracking furnace tube wall is applied to the electromagnetic induction coil as a temperature boundary condition, and the calculated heat flux is used as the boundary condition of the cracking furnace tube for calculation.

[0103] Repeated iterative calculations are performed in this way, and the new temperature distribution of the outer wall of the furnace tube is compared with the previous temperature distribution of the outer wall of the tube. If the difference in wall temperature is small enough, the iterative calculation between the electromagnetic induction coil and the cracking furnace tube is considered to have converged. If the difference in temperature distribution is less than 5°C, the calculation is considered to have converged. Otherwise, it is necessary to continue iteration.

[0104] The furnace tube is set as mass inlet and pressure outlet. The calculation method uses SIMPLE algorithm, and other equations use upwind format with second-order accuracy.

[0105] Example 1

[0106] This embodiment uses liquefied petroleum gas as the cracking raw material, and establishes a model according to the above method. The electromagnetic coil group includes 30 turns of electromagnetic wire cracking furnace tube rings, the inner diameter of each turn of the electromagnetic coil is 15cm, the spacing between the electromagnetic coils is 1.2cm, the electromagnetic coil is a hollow copper tube with an inner diameter of 0.3cm, the inner diameter of the cracking reaction furnace tube is 5cm, the wall thickness is 0.4cm, and the material is Cr35Ni45 high temperature resistant chromium-nickel alloy. The length of the upper and lower ends of the cracking reaction furnace tube is 10cm longer than the upper and lower ends of the electromagnetic induction component. When the cracking furnace tube is meshed, the regular hexahedron unit is selected to divide the grid, with a total of 49299 grids and 93651 nodes. The electromagnetic induction coil selects sweeping for meshing, with a total of 15000 grids and 82739 nodes. The steam cracking inlet temperature is 850℃, the reaction residence time is 0.45s, the outlet pressure is 84kPa, and the cracking gas outlet temperature is 850℃. The electromagnetic induction coil voltage is 80v and the current is 3000A. The feed rate of liquefied petroleum gas is 1090 g / h, and the feed rate of water is 295 g / h. The product yields obtained by calculation are shown in Table 4.

[0107] Example 2

[0108] This embodiment uses mixed naphtha as the cracking raw material, and establishes a model according to the above method. The electromagnetic coil group includes 30 turns of electromagnetic wire cracking furnace tube rings, the inner diameter of each turn of the electromagnetic coil is 15cm, the spacing between the electromagnetic coils is 1.2cm, the electromagnetic coil is a hollow copper tube with an inner diameter of 0.3cm, the inner diameter of the cracking reaction furnace tube is 5cm, the wall thickness is 0.4cm, and the material is Cr35Ni45 high temperature resistant chromium-nickel alloy. The length of the upper and lower ends of the cracking reaction furnace tube is 10cm more than the upper and lower ends of the electromagnetic induction component. When the cracking furnace tube is meshed, the regular hexahedron unit is selected to divide the grid, with a total of 49299 grids and 93651 nodes. The electromagnetic induction coil selects sweeping for meshing, with a total of 15000 grids and 82739 nodes. The steam cracking reaction temperature is 840℃, the reaction residence time is 0.3s, and the outlet pressure is 90kPa. The electromagnetic induction coil voltage is 75v and the current is 2500A. The mixed naphtha feed rate is 1340 g / h, and the water feed rate is 506 g / h. The product yields obtained by calculation are shown in Table 4.

[0109] Example 3

[0110] This embodiment uses aromatic raffinate as cracking raw material, and establishes a model according to the above method. The electromagnetic coil group includes 30 turns of electromagnetic wire cracking furnace tube rings, the inner diameter of each turn of electromagnetic coil is 15cm, the spacing between electromagnetic coils is 1.2cm, the electromagnetic coil is a hollow copper tube with an inner diameter of 0.3cm, the inner diameter of the cracking reaction furnace tube is 5cm, the wall thickness is 0.4cm, and the material is Cr35Ni45 high temperature resistant chromium-nickel alloy. The length of the upper and lower ends of the cracking reaction furnace tube is 10cm more than the upper and lower ends of the electromagnetic induction component. When the cracking furnace tube is meshed, the regular hexahedron unit is selected to divide the grid, with a total of 49299 grids and 93651 nodes. The electromagnetic induction coil selects sweeping for meshing, with a total of 15000 grids and 82739 nodes. The steam cracking reaction temperature is 820℃, the reaction residence time is 0.2s, and the outlet pressure is 98kPa. The electromagnetic induction coil voltage is 70v and the current is 2300A. The aromatic raffinate feed rate was 1840 g / h, and the water feed rate was 850 g / h. The product yields obtained by calculation are shown in Table 4.

[0111] Comparative Example 1

[0112] The same electric heating steam cracking device as in Example 1 of the present invention was used to carry out steam cracking reaction on liquefied petroleum gas. During the experiment, the steam cracking reaction conditions used were the same as in Example 1. The yield of light olefins obtained by the reaction is shown in Table 4.

[0113] Comparative Example 2

[0114] Based on the electric heating steam cracking device used in Comparative Example 1, the steam cracking reaction of the mixed naphtha was evaluated. The steam cracking reaction conditions adopted were the same as those in Example 2. The yield of light olefins obtained by the reaction is shown in Table 4.

[0115] Comparative Example 3

[0116] Based on the electric heating steam cracking device used in Comparative Example 1, the steam cracking reaction of aromatic raffinate oil was evaluated. The steam cracking reaction conditions adopted were the same as those in Example 3. The yield of light olefins obtained by the reaction is shown in Table 4.

[0117] The compositions of the raw materials used in the examples and comparative examples are shown in Tables 1-3.

[0118] Table 1 Composition of liquefied petroleum gas

[0119]

[0120] Table 2 Mixed naphtha composition

[0121]

[0122] Table 3 Aromatic raffinate oil composition

[0123]

[0124] Table 4 Light olefin product yield

[0125]

[0126] The present invention is based on the actual electric heating cracking furnace for modeling, and the main product yield at the outlet is obtained by coupling simulation calculation of the electromagnetic induction coil and the cracking furnace, as shown in Table 4. The steam cracking reaction device provided by the present invention has a higher heat transfer efficiency than the traditional external heating method. The cracking device provided by the present invention has the advantages of faster hot start and lower energy consumption. The temperature distribution of the cracking furnace tube is more uniform during the operation of the device. It can be seen from Table 1 that the simulation calculation results of the electric heating cracking furnace model provided by the present invention are closer to the actual working conditions, which proves that the finite element numerical simulation method can be used to predict the products of the electric heating cracking furnace and provide a qualitative reference for the production process of the actual electric heating cracking furnace.

[0127] Of course, the present invention may have many other embodiments. Without departing from the spirit and essence of the present invention, technicians familiar with the field may make various corresponding changes and deformations based on the present invention, but these corresponding changes and deformations should all fall within the scope of protection of the claims of the present invention.

Claims

1. A method for predicting products of an electrically heated cracking furnace, characterized in that: The cracking furnace comprises an electromagnetic induction coil and a cracking reaction furnace tube. The electromagnetic induction coil generates an alternating magnetic field. The generated alternating magnetic field passes through the cracking reaction furnace tube, so that the material inside the cracking furnace tube generates eddy current under the action of the magnetic field and generates heat to heat the material in the cracking furnace. An inorganic heat insulating material with a heat insulating effect is arranged between the furnace tube and the electromagnetic coil. The product prediction uses ANSYS software. The method comprises the following steps: Step 1, determining the design and operation data of the electric heating cracking furnace according to the cracking raw material properties, including the size of the cracking furnace tube, the cracking raw material feed amount, inlet temperature, cracking gas outlet temperature, outlet pressure, electromagnetic induction coil voltage and current; Step 2, establishing an electromagnetic induction coil heating model, meshing the electromagnetic induction coil, applying loads and constraints, and calculating the heat flux of the electromagnetic induction coil along the outer wall of the cracking furnace tube; Step 3, establishing a cracking furnace tube model, using molecular reaction or free radical reaction kinetics modeling, and calculating the tube wall temperature distribution along the tube length, the cracking gas temperature, pressure and concentration of each component along the tube length, as well as the outlet temperature, outlet pressure and the yield of each component at the outlet through the furnace tube model; Step 4, energy coupling is performed on the electromagnetic induction coil heating model and the cracking furnace tube model, the actual industrial cracking furnace tube wall temperature measurement value is brought into the electromagnetic induction coil heating model as a boundary condition for temperature calculation, and then the heat flux distribution of the electromagnetic induction coil along the tube length direction obtained by the electromagnetic induction coil calculation is brought into the cracking furnace tube model as a boundary condition for calculation to obtain a new furnace tube outer wall temperature distribution, and this iteration is repeated until convergence.

2. The product prediction method of the electric heating cracking furnace according to claim 1, characterized in that: In step 2, select Sweep to perform meshing.

3. The product prediction method of the electric heating cracking furnace according to claim 1, characterized in that: In step 2, the loads and constraints applied are the voltage and current on the coil.

4. The product prediction method of the electric heating cracking furnace according to claim 1, characterized in that: In step 2, the electromagnetic induction and temperature fields are calculated using a sequential coupling method.

5. The product prediction method of the electric heating cracking furnace according to claim 1, characterized in that: The electromagnetic induction coil heating model in step 2 includes an electromagnetic field mathematical model, a temperature field mathematical model and a heat transfer model.

6. The product prediction method of the electric heating cracking furnace according to claim 1, characterized in that: In step 3, the cracking tube is meshed using hexahedral unit division.

7. The product prediction method of an electrically heated cracking furnace according to claim 1, characterized in that: The iterative convergence criterion in step 4 is whether the difference between the maximum values ​​of the outer wall temperature of the furnace tube obtained by two previous calculations is less than a preset threshold value. If it is less than the threshold value, it is converged.

Images