CN103440390A - Coupling simulation method for radiation section of industrial steam cracking furnace - Google Patents

Abstract

The invention relates to a coupling simulation method for a radiation section of an industrial steam cracking furnace. In a hearth model, a k-epsilon turbulence model is adopted for the smoke flow; a PDF (probability distribution function) model is adopted for the burning of a fuel gas; a thermal insulation model, P-1, a DO (discrete ordinate) model and a DTRM (discrete transfer radiation model) are respectively adopted for the smoke radiation heat transfer; and the smoke radiation characteristics are calculated by use of a WSGGM (weighted-sum-of-gray-gases model). In a furnace tube model, a molecular or free radical reaction kinetic model is adopted for an in-tube process. The outer-wall temperature and heat flux of the furnace tube are used as the iteration variables for the coupling simulation of the hearth model and the furnace tube model. Thus, the temperature, speed, pressure and concentration distribution of the fluid in the cracking furnace and the tube wall temperature and heat flux distribution of the furnace tube can be obtained under different radiation models, the simulation result is compared with the industrial data, and the radiation model generating consistent results is determined as the radiation model of the cracking furnace. The method provided by the invention is widely applicable and can provide instruction to the optimization of the structure and technological parameters of different types of hydrocarbon steam cracking furnaces.

Description

The coupled simulation method of industrial steam pyrolysis furnace radiation section

Technical field

The present invention relates to a kind of coupled simulation method of industrial steam pyrolysis furnace radiation section, the model that utilizes the method to set up can be used for structure and the process parameter optimizing of industrial steam pyrolysis furnace.

Background technology

Tubular furnace has consequence at refinery and petrochemical plant.Tubular furnace is a kind of equipment of a large amount of power consumptions, is also a kind of commercial unit.Ethane cracking furnace is a kind of important tubular furnace, and it is the core of ethylene unit, is again big power consumer's (account for device total energy consumption 50%～60%), and pyrolysis furnace is improved, and will directly affect the operational economy of ethylene unit.Along with the development of petrochemical complex, the demand expanding day of market to ethene, ethane cracking furnace trend large scale development, this just needs modern thermal cracker have high-level efficiency, large production capacity and reliable, safety, environmental protection close friend and design cheaply.

The pyrolysis furnace radiation section is the main place of heat interchange, and 70～80% of full stove thermal load, undertaken by radiation section, and it is the most important position of full stove.Can say, the quality of a stove depends primarily on its radiation section performance.Existing pyrolysis furnace is mostly that boiler tube is suspended on burner hearth central authorities, the bottom of burner hearth both sides and/or sidewall are arranged respectively burner, fuel gas and air enter burner hearth by burner and burn, and liberated heat is mainly with radiation and to the streamed cracking stock passed in reaction tube.Generally, the geometry of radiation section is huger, and the heat absorptivity due to cracking reaction, need a large amount of fuel combustion heat supplies, therefore the fuel gas of larger flow sprays into burner hearth by burner in the high-speed jet mode, the mobile of flue gas in burner hearth made a significant impact, thereby further affect mixing and the combustion process of fuel gas, change the Temperature Distribution in burner hearth.Flue gas is passed to the cracking stock in reaction tube by heat simultaneously, makes it complicated course of reaction occurs, and vice versa.So, along the heat flux distribution of boiler tube length direction, be the tie of contact oil gas and fume side.Due to the restriction that is subject to long period, expensive and limited measurement means, to the experimental study of heat flux distribution, be extremely difficult.And the simple mathematical model that domestic many scholars propose can't carry out deep understanding to the various complicated phenomenons that occur in pyrolysis furnace.Therefore, numerical simulation is to optimize and design an effective tool of pyrolysis furnace.Yet, due to the shortage of experimental data, the checking of these simulations is quite time-consuming and difficult.

The raw material that the heat that the fuel combustion of radiation section burner hearth side discharges passes in boiler tube in the mode of convection current and radiation makes it complicated heat scission reaction occurs, and radiant heat transfer amount has wherein occupied 80%～90% of total heat transfer.Therefore, radiant heat transfer calculates the core that has just become the pyrolysis furnace heat balance, and the indoor radiant heat transfer rule of accurate description pipe external radiation is the key of pyrolysis furnace numerical simulation success or failure.In the pyrolysis furnace burner hearth, traditional radiation heat-transfer model has experienced zero-dimensional model to multidimensional model, then arrives the development of current Fluid Mechanics Computation (Computational Fluid Dynamics is called for short CFD) model.Rely on accurate and detailed analog information and the computing power improved constantly, the CFD technology becomes the important means of current pyrolysis furnace simulation gradually.In current ethane cracking furnace CFD simulation, domestic researcher has used a kind of radiation radiation heat-transfer model to be simulated.The external research for the impact of chamber flue gas temperature, concentration and radiative property relevant for more different radiation models, but with boiler tube, be not coupled, and result of calculation can not illustrate actual pyrolysis furnace ruuning situation.Because these researchs all are subject to radiation model separately or the restriction of computing method, can not promote the use of all tubular furnace, so, use the method for burner hearth and boiler tube coupled simulation, more different radiation models is for flow of flue gas, radiative property, along the thermoflux of boiler tube length direction with pipe surface temperature distribution, oil gas Temperature Distribution, gas concentration distribute and the impact of product yield etc., determine radiation model accurately, just become urgent need to solve the problem in large tubular stove numerical simulation.

Summary of the invention

The present invention has systematically analyzed flow of matter in industrial ethylene pyrolysis furnace reaction tube comprehensively, conduct heat, in mass transfer and cracking reaction and burner hearth, flow, conduct heat, the complex process such as mass transfer and combustion reaction, analyzed strong coupling between these complex processes simultaneously, based on hydromechanical turbulent flow model, radiation heat-transfer model, combustion model and cracking reaction kinetic model, burning diabatic process in transmission and cracking reaction process and burner hearth in the pyrolysis furnace reaction tube is coupled, designed the Coupled Numerical modeling method of cracking reaction in a kind of industrial ethylene pyrolysis furnace hearth combustion and boiler tube.In the burner hearth model, the burner hearth flow of flue gas adopts the Reynolds average model, and adopts the k-ε two-equation model sealing turbulent flow item wherein of standard; The mixability of based on fuel and air adopts non-premixed, and the burning of fuel gas adopts probability density function (Probability Density Function is called for short PDF) model; Burner hearth gas radiation heat transfer model adopts respectively insulating model, P-1, discrete coordinates (Discrete ordinate, be called for short DO) model and discrete propagate radiation model (Discrete Transfer Radiation Model, be called for short DTRM), and adopt ash gas weighted average model (Weighted-sum-of-gray-gases model is called for short WSGGM) to calculate the gas radiation characteristic.In the boiler tube model, the pipe internal procedure adopts molecular reaction or free radical cracking reaction model.The iteration variable of burner hearth model and boiler tube model coupled simulation is selected furnace tube outer wall temperature and thermoflux.Can obtain thus flue-gas temperature in the burner hearth under different radiation models, speed, concentration of component distribution, boiler tube inside and outside wall Temperature Distribution, in boiler tube heat flux distribution and pipe, pyrolysis gas temperature, speed, concentration of component distribute, and analog result and industrial data are compared, the radiation model that wherein produces consistent results is defined as to the radiation model of this ethane cracking furnace hearth combustion heat transfer system.This method adaptability is comparatively extensive, can be used for dissimilar hydrocarbons steam cracking furnace.

A kind of coupled simulation method of industrial steam pyrolysis furnace radiation section is characterized in that said method comprising the steps of:

Step 1: according to industrial hydrocarbon cracking stove actual conditions, determine pyrolysis furnace burner hearth to be simulated, boiler tube size and geometric detail, for burner hearth, boiler tube, carry out the grid division; And the starting condition of definite burner hearth fuel airshed, air mass flow, boiler tube raw material feed rate, temperature and pressure, determine the border conditions such as furnace outlet pressure, furnace wall heat dissipation coefficient and boiler tube top hole pressure.

Step 2: industrial hydrocarbons steam cracking furnace modeling is decomposed into burner hearth modeling and boiler tube modeling.

Step 2.1: the burner hearth modeling adopts the CFD method, and in burner hearth, flow of flue gas, combustion model adopt respectively standard k-ε turbulence model and the non-premix model of PDF.In burner hearth, radiation heat-transfer model adopts the modeling respectively of insulating model, P-1, DO and discrete propagate radiation model (DTRM), the burner hearth flue gas adopts ash gas weighted average model (Weighted-sum-of-gray-gases model is called for short WSGGM) to calculate its radiation characteristic.

Step 2.2: the boiler tube modeling adopts molecular reaction or free radical reaction Dynamic Modeling.

Step 3: have strong thermal coupling relation based on burner hearth and boiler tube, furnace tube outer wall temperature and boiler tube thermoflux are as burner hearth model and boiler tube model numerical value iterative variable.

Preferably, during described burner hearth grid is divided, burner region, the boiler tube district adopts the tetrahedron element grid division; Other zones of burner hearth adopt the hexahedral element grid division; During the boiler tube grid is divided, hexahedral element is used for the boiler tube tube wall is carried out to mesh refinement; The mixture unit is used for dividing the grid of boiler tube coupling part.The grid of burner hearth and boiler tube is divided as shown in Figure 1.The value of determining boiler tube and each physical quantity of furnace wall wall is 0, is considered as without slippage; Near wall, in viscous sublayer, adopt Standard law of wall to approach flowing and heat exchange of real process; Thermal boundary on the burner hearth wall is given the thermoflux boundary condition by thermal loss, according to pyrolysis furnace design code thermal loss, is total amount of heat 1%; Furnace wall surface temperature border adopts the temperature of factory's actual condition, the preliminary hypothesis of operating experience, utilizes self-defining function (UDF) to be assigned to tube wall.

Preferably, there are strong Energy Coupling relation in described burner hearth and boiler tube model, need first rule of thumb or the initial value that obtains the furnace tube outer wall Temperature Distribution is calculated in initialization, bring in the burner hearth model and calculated as boundary condition, then the tube wall heat flux distribution of burner hearth being calculated to acquisition is re-used as in boundary condition substitution boiler tube model to be calculated, and obtains new furnace tube outer wall Temperature Distribution.So iterate, until the maximum error value of twice tube wall temperature distribution is less than a predetermined threshold value, just think that iterative process restrains.

The invention provides a kind of coupling modeling method of industrial hydrocarbons steam cracking furnace, adopt Fluid Mechanics Computation to set up the flowing of burner hearth, burning, radiation model, the radiation model adopts insulating model, P-1, DO and DTRM model, boiler tube adopts molecular reaction or free radical reaction kinetic model, then using thermoflux and pipe surface temperature respectively as boundary condition, carry out the coupling of burner hearth boiler tube and calculate until convergence.By burner hearth velocity of flue gas, temperature and the CONCENTRATION DISTRIBUTION that different radiation models are obtained, coil outlet temperature, the boiler tube top hole pressure, the result such as products collection efficiency and industrial data compare, and determine best radiation model.This method adaptability is comparatively extensive, and can be used for provides guidance for dissimilar hydrocarbons steam cracking furnace structure and process parameter optimizing.

The accompanying drawing explanation

The grid that Fig. 1-1～Fig. 1-4 are ethane cracking furnace burner hearth and boiler tube is divided figure;

Figure is divided for the burner hearth grid in Fig. 1-1;

Fig. 1-2 is that the burner grid is divided partial enlarged drawing;

Figure is divided for the boiler tube grid in Fig. 1-3;

Fig. 1-4 are that boiler tube straight length grid is divided partial enlarged drawing;

Fig. 2 is the coupled simulation FB(flow block).

Embodiment

Modeled example below in conjunction with certain industrial ethylene pyrolysis furnace describes.

Referring to the objective for implementation that Fig. 1 is the present embodiment, is USC type tube cracking furnace, and this stove adopts the reducing boiler tube design of two journey branches, and it is configured as " U " type, and an outlet of 1 induction pipe connection, divide two misarrangements row vertically to be suspended on burner hearth central authorities; Burner hearth bottom is equipped with two row's base burning devices near side walls, is non-premix burner.What in the pyrolysis furnace burner hearth, occur is mainly burning and the reaction of fuel, and this is quality, the heat transfer reaction process of a complexity, and therefore, this just requires our this mobile chemical reaction process of accurate description.At first flowing in burner hearth be should describe, chemical reaction and radiation, convection heat transfer' heat-transfer by convection then on mobile basis, considered.Carefully dissect the burning of fuel and reaction physics, chemical process, the governing equation of its CFD simulation can be described by following mathematical model: 1. fluid flow model, and key is turbulence model; 2. the mass transfer model of each component, mainly consider the impact of combustion rate; 3. the heat TRANSFER MODEL, comprise convection heat transfer' heat-transfer by convection and radiant heat transfer, and wherein key is radiation heat-transfer model.Then by determining boundary condition and starting condition, dividing computing grid, set up discrete equation in solving territory.Then solve discrete equation, obtain the solution of this problem.

1. set up burner hearth CFD model

(1) flow model

In burner hearth, the flow of flue gas model adopts the standard k-ε two-equation model based on the Reynolds average equation to set up the mathematical model of sealing.The dissipative shock wave of continuity, momentum, tubulence energy, tubulence energy, energy and component transport equation are as shown in the formula expression:

$\frac{\∂\left(\mathrm{\ρ\φ}\right)}{\∂t}+\frac{\∂\left(\mathrm{\ρ}{U}_j\mathrm{\φ}\right)}{{\∂x}_j}=\frac{\∂}{{\∂x}_j}\left({\mathrm{\Γ}}_{\mathrm{\φ}}\frac{\∂\mathrm{\φ}}{{\∂x}_j}\right)+S_{\mathrm{\φ}}---\left(1\right)$

In formula, ρ is fluid density, kg/m ³; φ is dependent variable; U _jfor the speed component of j direction, m/s; x _jfor the coordinate of j direction, m; Γ _φfor the generalized diffusion process coefficient; S _φfor source item.

(2) combustion model

Pyrolysis furnace in this example only has Bottom Nozzle Used, and fuel gas and air enter respectively burner hearth, belongs to non-premix premixed combustion.Therefore, this example adopts the PDF model to describe the turbulent combustion process.

PDF model hypothesis chemical reaction rate is infinitely great and irreversible, that is, fuel gas and air can not coexist, and in the moment of contact, chemical reaction occurs immediately, and a step directly generates final product.Therefore, the transient heat chemical state of flue gas in burner hearth can mean with the mixing mark:

$f=\frac{Z_i-Z_{i,\mathrm{ox}}}{Z_{i,\mathrm{fuel}}-Z_{i,\mathrm{ox}}}---\left(2\right)$

In formula, Z _ithe element massfraction of-element i.Subscript ox means the value of oxidant stream porch, and fuel means the value of fuel flow porch.For the turbulent combustion process, molecular diffusion effect and the speed difference of each component can be ignored, so the component transport equation can be expressed as the conservation equation of average mixing mark:

$\frac{\∂}{\∂t}\left(\mathrm{\ρ}\stackrel{\‾}{f}\right)+\frac{\∂}{{\∂x}_i}\left(\mathrm{\ρ}{u}_i\stackrel{\‾}{f}\right)=\frac{\∂}{{\∂x}_i}\left(\frac{{\mathrm{\μ}}_i}{{\mathrm{\σ}}_t}\frac{\∂\stackrel{\‾}{f}}{{\∂x}_i}\right)+S_m---\left(3\right)$

In formula, S _mfor source item, can represent the quality transmission to gas phase of the liquid phase of mixing or solid phase, owing to being the homogeneous gas-phase burning in pyrolysis furnace, so this source item can be ignored.Except mixing mark, on average mix the mean square value of mark and also obey following conservation equation:

$\frac{\∂}{\∂t}\left(\mathrm{\ρ}\stackrel{\‾}{f^{\′2}}\right)+\frac{\∂}{{\∂x}_i}\left(\mathrm{\ρ}{u}_i\stackrel{\‾}{f^{\′2}}\right)=\frac{\∂}{{\∂x}_i}\left(\frac{{\mathrm{\μ}}_t}{{\mathrm{\σ}}_t}\frac{\∂\stackrel{\‾}{f^{\′2}}}{{\∂x}_i}\right)+C_g{\mathrm{\μ}}_t{\left(\frac{\∂f}{{\∂x}_i}\right)}^2-C_d\mathrm{\ρ}\frac{\mathrm{\ϵ}}{k}\stackrel{\‾}{f^{\′2}}---\left(4\right)$

In formula,

constant σ in above-mentioned two formulas _t, C _gand C _dbe respectively 0.85,2.86 and 2.0, by solving the conservation equation of average mixing mark and mean square value thereof, can obtain each constituent mass mark in burner hearth.

(3) radiation model

Due to the distribution of the leading thermoflux of radiant heat transfer in pyrolysis furnace radiation of burner hearth section along reaction tube, so need to set up a radiation model accurately.But for dissimilar tubular heater, how to determine correct radiation model from numerous radiation model (P-1, DO, Rosseland, surface emissivity heat exchange (S2S), DTRM), just become a primary study content of the present invention.When determine using which kind of radiation model, need the factor of considering to have: optical depth; Scattering and emission; The impact of particle; Translucent walls (within and without); The minute surface wall; The partial mirror wall; Non-gray radiation; Local heat source; There is no the radiant heat transfer in the closed cavity in the radiation medium situation.The present invention is in conjunction with the influence factor of radiation model, study the impact of different radiation Model on Crack solution stove chamber flue gas temperatures, concentration, speed and boiler tube thermoflux, pipe surface temperature, oil gas temperature, speed etc., determine radiation model accurately, thereby for determining that other types tubular heater radiation model provides theoretical foundation.In above radiation model, Rosseland is applicable to the situation that optical depth is greater than 3, and in the pyrolysis furnace burner hearth, the optical depth of flue gas generally all is less than 3, so this model is inapplicable; S2S is applicable to there is no the radiant heat transfer in the closed cavity in the radiation medium situation, the also situation of the gas radiation medium of inapplicable pyrolysis furnace.

For the medium with absorption, emission, scattering nature, in position , along direction

radiation transfer equation (RTE) be:

$\frac{\mathrm{dI}(\stackrel{\→}{r},\stackrel{\→}{s})}{\mathrm{ds}}+(a+{\mathrm{\σ}}_s)I(\stackrel{\→}{r},\stackrel{\→}{s})={\mathrm{an}}^2\frac{\mathrm{\σ}{T}^4}{\mathrm{\π}}+\frac{{\mathrm{\σ}}_s}{4\mathrm{\π}}{\∫}_0^{4\mathrm{\π}}I(\stackrel{\→}{r},\stackrel{{\→}^{\′}}{s})\mathrm{\Φ}(\stackrel{\→}{s},\stackrel{{\→}^{\′}}{s})d{\mathrm{\Ω}}^{\′}---\left(5\right)$

In formula,

for position vector,

for direction vector,

for the scattering direction, σ is Stefan-Boltzmann constant (5.672 * 10 ^-8w/m ²k ⁴), a and σ _sbe respectively absorption coefficient and the scattering coefficient of flue gas, unit is 1/m, and n is refractive index, and I is radiation intensity, and T is local temperature, and Φ is phase function, and Ω ' is solid angle.

1. insulating model

Insulating model is exactly the source item S at the solid area energy equation of equation (1) _φin do not comprise the radiation source item, do not consider the impact of radiant heat transfer in the CFD of burner hearth calculates.

2. P-1 model

P-1 radiation model is the simplest type in the P-N model.The starting point of P-N model is radiation intensity to be launched to become the spheric harmonic function of quadrature.If first four of only getting the orthogonal sphere hamonic function, for radiant heat flux q _r, can obtain following equation:

$q_r=\mathrm{\Γ}\▿G---\left(6\right)$

Wherein, $\mathrm{\Γ}=\frac{1}{(3(a+{\mathrm{\σ}}_s)-C{\mathrm{\σ}}_s)}---\left(7\right)$

G is incident radiation, and C is linear each diversity phase function coefficient.

The transport equation of G is:

$\▿(\mathrm{\Γ}\▿G)-\mathrm{aG}+4\mathrm{a\σ}{T}^4=S_G---\left(8\right)$

Wherein, S _gfor user-defined radiation source phase.

Merge equation (6) and (8), can obtain following equation:

$-\▿q_r=\mathrm{aG}-4\mathrm{a\σ}{T}^4---\left(9\right)$

expression formula can directly bring energy equation into, thereby obtain due to the caused calorie source of radiation.

3. DO model

The discrete coordinates model solution be the radiation transfer equation (RTE) sent from limited solid angle, each solid angle correspondence the fixed-direction under coordinate system (flute card)

the DO model is converted into equation (5) transport equation of the radiation intensity under space coordinates.How many (solid angle) directions are arranged

just solve how many (radiation intensity) transport equations.The method for solving of equation method for solving mobile with fluid and energy equation is identical.

The DO model is by vector radiation heat-transfer model on direction is regarded a field equation as, and its model equation is as follows:

$\▿\·\left(I(\stackrel{\→}{r},\stackrel{\→}{s})\stackrel{\→}{s}\right)+(a+{\mathrm{\σ}}_s)I(\stackrel{\→}{r},\stackrel{\→}{s})={\mathrm{an}}^2\frac{\mathrm{\σ}{T}^4}{\mathrm{\π}}+\frac{{\mathrm{\σ}}_s}{4\mathrm{\π}}\underset{0}{\overset{4\mathrm{\π}}{\∫}}I(\stackrel{\→}{r},\stackrel{{\→}^{\′}}{s})\mathrm{\Φ}(\stackrel{\→}{s},\stackrel{{\→}^{\′}}{s})d{\mathrm{\Ω}}^{\′}---\left(10\right)$

For gray-body radiation, the net radiation hot-fluid that leaves wall is:

$q_{\mathrm{out}}=(1-{\mathrm{\ϵ}}_w)q_{\mathrm{in}}+n^2{\mathrm{\ϵ}}_w\mathrm{\σ}{T}_w^4---\left(11\right)$

Wherein, ε _wgrey wall blackness, q _infor the incident radiation hot-fluid of wall, its formula is:

$q_{\mathrm{in}}={\∫}_{\stackrel{\→}{s}\·\stackrel{\→}{n}>0}^{4\mathrm{\π}}{I}_{\mathrm{in}}\stackrel{\→}{s}\·\stackrel{\→}{n}\mathrm{d\Ω}---\left(12\right)$

N is the refractive index near the wall medium, I _infor the radiation intensity of incident ray,

for the normal to a surface direction, Ω is the hemisphere solid angle.For all directions of leaving wall

the radiation intensity of wall is:

$I_0=\frac{q_{\mathrm{out}}}{\mathrm{\π}}---\left(13\right)$

So the source item of the energy equation of equation (1) equals thermoflux, it is the volumetric sources of solid interior, and its expression formula is

$S_h=\mathrm{\α}(\underset{4\mathrm{\π}}{\∫}I(r,\hat{s})\mathrm{d\Ω}-4\mathrm{\σ}{T}^4)---\left(14\right)$

4. DTRM model

The main hypothesis of DTRM radiation model replaces all radiation effects from radiating surface along certain solid angle with single (radiation) ray.The variation of radiation intensity (density of radiation), dI along the differential equation of its stroke ds is:

$\frac{\mathrm{dI}}{\mathrm{ds}}+\mathrm{aI}=\frac{\mathrm{a\σ}{T}^4}{\mathrm{\π}}---\left(15\right)$

In equation, the refraction coefficient of supposing gas is 1.In the DTRM model, to equation (15) from boundary surface,

Along radiation stroke integration.If along the ray stroke, a is constant, so, for I (s), has:

$I\left(s\right)=\frac{\mathrm{\σ}{T}^4}{\mathrm{\π}}(1-e^{-\mathrm{as}})+I_0{e}^{-\mathrm{as}}---\left(16\right)$

(4) radiation characteristic model

In the present invention, the fuel of pyrolysis furnace is gaseous fuel, burns very abundant, does not substantially have particle, thus scattering coefficient with absorption coefficient, compare, little a lot, do not consider scattering coefficient in therefore calculating.The calculating of radiation heat transfer working medium radiation characteristic is exactly mainly the absorption coefficient that calculates flue gas.For the calculating of smoke absorption coefficient, general diagram method commonly used, algebraic model, ash gas weighted average model, wide band model, arrowband model and line-line model etc. on engineering.With other models, compare, the WSGGM model has higher computational accuracy and efficiency, so it is used to describe the radiation characteristic of smoke mixture.WSGGM is between the complete ash gas model of oversimplification and considers the compromise model between each gas absorption band model fully.It is the blackness of real gas to be expressed as to the weighted sum of the blackness of several ash gas and a transparent gas, and therefore, for certain thickness gas-absorbing layer, its blackness is:

$\mathrm{\ϵ}=\underset{i=0}{\overset{I}{\mathrm{\Σ}}}{a}_{\mathrm{\ϵ},i}\left(T\right)(1-e^{-{\mathrm{\κ}}_i\mathrm{ps}})---\left(17\right)$

In formula, a _{ε, i}be the blackness weighting factor of the virtual ash gas of i kind, the blackness that the expression formula in the bracket of back is the virtual ash gas of i kind, κ _ibe the absorption coefficient 1/m of i kind ash gas, p is all absorption partial pressure summation Pa, and s is path m.

Weighting factor for transparent gas can calculate gained by following formula:

$a_{\mathrm{\ϵ},0}=1-\underset{i=1}{\overset{I}{\mathrm{\Σ}}}{a}_{\mathrm{\ϵ},i}---\left(18\right)$

Depend on a of temperature _{ε, i}can be by any approximation to function (matching), but generally adopt following form:

$a_{\mathrm{\ϵ},i}=\underset{j=1}{\overset{J}{\mathrm{\Σ}}}{b}_{\mathrm{\ϵ},i,j}{T}^{j-1}---\left(19\right)$

Wherein, b _{ε, i, j}for the polynomial coefficient about gas temperature.

The absorption coefficient of flue gas can obtain from following two formula:

When $s>{10}^{-4}m,a=-\frac{\ln (1-\mathrm{\ϵ})}{s}---\left(20\right)$

When $s\≤{10}^{-4}m,a=\underset{i=0}{\overset{I}{\mathrm{\Σ}}}{a}_{\mathrm{\ϵ},i}{\mathrm{\κ}}_{i}p---\left(21\right)$

2. set up the boiler tube model

In pipe, cracking reaction has experienced the developing stage of empirical model, molecular model and free radical reaction kinetic model.Empirical model directly carries out associatedly with yield of cracked product with several parameters (raw material/temperature/residence time etc.) of steam cracking reaction, can adopt regression fit or neural network model.The advantage of this model is simple, intuitive, and the construction cycle is short; But it is of limited application, can close co-product few, need a large amount of field data supports.Molecular model assumes single hydrocarbon by raw material, and cracking process is simplified to primary first-order equation and secondary reaction, supposes that both are without intersection, and different material forms only influential to the primary first-order equation stoichiometric coefficient, does not affect secondary reaction.Apply the cracking reaction kinetic model that this method is set up, have and calculate simple, highly versatile and the advantage such as very effective, so the method is comparatively extensive in industrial application.The free radical reaction kinetic model be take the Rice theory as basis, according to raw material, forms and the product distribution, presses free radical chain reactions mechanism description reaction process, asks for product by the differential equation group that solves all kinds of reactions and distributes.Because free radical mechanism has strictly characterized the course of cracking reaction, the free radical mechanism model of hydrocarbon cracking has very strong adaptability and very high precision and reliability.

The present invention adopts molecular reaction or free radical reaction model to describe the cracking reaction in boiler tube.

3. the establishment of industrial hydrocarbons steam cracking furnace coupled simulation method

For the burner hearth inner fluid flow, radiation, burning and boiler tube cracking reaction strong coupling relation, apply respectively different radiation models the industrial ethylene pyrolysis furnace carried out to burner hearth/boiler tube coupled simulation, the coupled simulation FB(flow block) as shown in Figure 2.

1. set up the burner hearth geometric model, required fuel gas and inlet temperature, pressure and the traffic mix of air, the conditions such as furnace wall thermal loss are calculated in input.

2. adopt experience or initialization to calculate the initial value that obtains the furnace tube outer wall Temperature Distribution, solve the flowing of flue gas in burner hearth, burning, radiation model, when the conservation equation residual error of these models reaches convergence (as quality and momentum conservation equation residual error are reduced to 10 ^-3), numerical convergence, obtain the heat flux distribution to boiler tube along the burner hearth of reaction tube direction.

3. take the heat flux distribution value as boundary condition, solve quality, energy and the momentum conservation equation of boiler tube inside, when the residual error of these equations reaches convergence, numerical convergence, obtain new tube wall temperature and distribute.

4. the previous tube wall temperature distribution with it that new tube wall temperature distributed compares, if the maximal value of Temperature Distribution differs by more than 1 ℃, the boundary condition just calculated new tube wall temperature as burner hearth, forwarding the 2nd step to recalculates, otherwise just think that coupling calculating restrains, and exports corresponding burner hearth boiler tube result of calculation.

In pyrolysis furnace, the governing equation of quality, momentum, energy, turbulent flow, chemical composition and conservation of radiance solves in order with controlling volumetric method.Nonlinear governing equation is discrete by the second upstreame scheme implicit expression, and linearization produces the system of equations of the dependent variable in each computing unit lattice.Adopt quality, momentum, energy and the component transport equation of semi-implicit method (SIMPLE) the Algorithm for Solving coupling of coupling pressure equation.

Model based on above foundation, can calculate each amount of corresponding output according to real-time input quantity.

Analyze gas radiation characteristic and the interior impact of flowing and reacting of flow, burning and pipe thereof that different radiation models calculate, analog result compares by industrial data, the radiation model that wherein produces consistent results is defined as to the radiation model of this ethane cracking furnace hearth combustion heat transfer system.

Only for the preferred embodiment of invention, not be used for limiting practical range of the present invention in sum.Be that all equivalences of doing according to the content of the present patent application the scope of the claims change and modify, all should be technology category of the present invention.

Claims (4)

1. the coupled simulation method of an industrial steam pyrolysis furnace radiation section, is characterized in that,

Step 1: determine pyrolysis furnace burner hearth to be simulated, boiler tube size and geometric detail, for burner hearth, boiler tube, carry out the grid division; And the initial value of definite burner hearth fuel airshed, air mass flow, boiler tube raw material feed rate, temperature and pressure, determine the boundary values such as furnace outlet pressure, furnace wall heat dissipation coefficient and boiler tube top hole pressure;

Step 2: burner hearth modeling and boiler tube modeling:

Step 2.1: the burner hearth modeling adopts the Fluid Mechanics Computation method, and in burner hearth, flow of flue gas, combustion model adopt respectively standard k-ε turbulence model and the non-premix model of probability density function; In burner hearth, radiation heat-transfer model adopts the modeling respectively of insulating model, P-1, DO and discrete propagate radiation model, and the burner hearth flue gas adopts the ash gas weighted average model to calculate its radiation characteristic;

Step 2.2: the boiler tube modeling adopts molecular reaction or free radical reaction Dynamic Modeling;

Step 3: based on burner hearth and boiler tube, have strong thermal coupling relation, furnace tube outer wall temperature and boiler tube thermoflux are as burner hearth model and boiler tube model numerical value iteration, restrain until burner hearth model and boiler tube model are coupled, and solve the variablees such as speed, temperature, pressure and concentration of fluid.

2. modeling method according to claim 1, is characterized in that, during described burner hearth grid is divided, and burner region, the boiler tube district adopts the tetrahedron element grid division; Other zones of burner hearth adopt the hexahedral element grid division; During the boiler tube grid is divided, hexahedral element is used for the boiler tube tube wall is carried out to mesh refinement; The mixture unit is used for dividing the grid of boiler tube coupling part.

3. modeling method according to claim 2, is characterized in that, the value of described boiler tube and each physical quantity of furnace wall wall is 0, is considered as without slippage; Near wall, in viscous sublayer, adopt Standard law of wall to approach flowing and heat exchange of real process; Thermal boundary on the burner hearth wall is given the thermoflux boundary condition by thermal loss, according to pyrolysis furnace design code thermal loss, is total amount of heat 1%; Furnace wall surface temperature border adopts self-defining function to be assigned to tube wall.

4. modeling method according to claim 1, is characterized in that, described coupling converges in iterative process, and the maximum error value of twice tube wall temperature distribution is less than a predetermined threshold value.

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