A numerical simulation method for gas-solid two-phase reaction flow in a moving bed or packed bed

Abstract

The present invention relates to a numerical simulation method for moving bed or packed bed type gas-solid two-phase reaction flow, comprising the following steps: establishing two calculation examples for the gas-phase and solid-phase flows, respectively, setting boundary conditions and initialization conditions, using a porous medium model to describe the gas-phase flow in the gas-phase calculation example and creating additional variables to characterize the solid-phase parameters, using a viscous flow model to describe the solid-phase flow in the solid-phase calculation example and creating additional variables to characterize the gas-phase parameters, achieving gas-solid coupling by continuously iteratively updating the additional variables; creating a temporary database; reading grid node information and corresponding data; setting the output / input file format; setting the coupling frequency and determining the frequency of gas-solid two-phase data exchange; updating the corresponding additional variables; setting termination judgment and convergence criteria; starting the calculation program; running the program; and outputting the results. The present invention improves the simulation applicability and prediction accuracy of moving bed / packed bed type gas-solid reaction two-phase flows.

Description

Technical Field

[0001] The present invention relates to the technical field of numerical simulation of multiphase reaction flow fluid mechanics, and more particularly to a numerical simulation method for moving bed or packed bed type gas-solid two-phase reaction flow. Background Art

[0002] Gas-solid and even multiphase reaction flows are widely used in metallurgy, chemical industry, petroleum, energy and other fields. The design, scale-up, control and parameter optimization of these process units require a deep understanding of the fluid flow, transfer phenomena and reactions in the processes involved. For some complex systems, such as blast furnaces, vertical furnaces and other multiphase reactors, it is often difficult to obtain their internal thermochemical state and fluid flow conditions through traditional experimental methods, which greatly restricts the feasibility of industrial design and parameter optimization. In order to get rid of this constraint, more and more scientific researchers have turned their attention to numerical simulation technology. In recent years, with the development of computer technology and the progress of basic theoretical research, numerical simulation technology has been increasingly used in the field of engineering technology, and its applicability and accuracy have been widely recognized.

[0003] Computational fluid dynamics (CFD) is a key branch of numerical simulation technology. Its fundamental principle is to solve the governing equations (i.e., conservation equations) of the processes involved. Currently, CFD methods are relatively mature and widely available. Commercial software packages such as ANSYS Fluent and ANSYS CFX, as well as open-source software such as OpenFoam, enjoy a large following and widespread application. These software packages have successfully simulated single-phase and multiphase flows, as well as interphase thermal and mass coupling. Their reliability is unquestionable, and they generally include a variety of built-in multiphase flow models for users to choose from, such as TFM, VOF, and porous media models. However, due to their limited versatility, they still lack effective modeling tools for complex multiphase reaction flow systems, such as the aforementioned moving-bed / packed-bed furnaces like blast furnaces and shaft furnaces. Taking a shaft furnace as an example, the gas phase is pumped from the bottom of the reactor into the top and out, while the solid phase flows from the top and out from the bottom. The gas and solid phases flow in opposite directions, accompanied by intense interphase mass, heat, and momentum exchange. Existing CFD software frameworks make it difficult to systematically and comprehensively simulate the aforementioned gas-solid reverse reaction flow, particularly the solid-phase flow. The current common approach treats the solid phase as a porous medium, accounting for heat, mass, and momentum transfer between the gas and solid phases without addressing the solid-phase flow. Consequently, it fails to provide information of practical engineering significance, such as charge trajectory, direct prediction of yield, and accurate gas-solid distribution within the furnace.

[0004] Therefore, the existing technology needs to be improved. Summary of the Invention

[0005] In view of the shortcomings of the existing technology, the present invention proposes a numerical simulation calculation method for moving bed or packed bed type gas-solid reaction two-phase flow.

[0006] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0007] According to aspects of the present invention, the present invention provides a method for numerical simulation of gas-solid two-phase reaction flow in a moving bed or packed bed, comprising the following steps:

[0008] 1) For the gas-solid two-phase flow, a gas-phase example and a solid-phase example are established to solve the gas-solid two-phase flow respectively, and the boundary conditions and initialization conditions are set respectively. The gas-phase example uses a porous medium model to describe the gas-phase flow and creates additional variables to store solid-phase related information. The solid-phase example uses a viscous flow model to describe the solid phase and creates additional variables to store gas-phase related information. Gas-solid coupling is achieved by continuously iteratively updating the additional variables in the gas-phase example and the additional variables in the solid-phase example.

[0009] 2) Create a temporary database, ensure that the gas phase example and the solid phase example are in the same working directory, and create an additional separate folder in the current working directory to store intermediate data and coupling information;

[0010] 3) Read the grid node information, obtain the parameter information in the calculation process by accessing the memory, and select the variables that need data coupling according to the simulation object and read the corresponding data;

[0011] 4) Set the output / input file format and determine the storage accuracy and format of each variable data;

[0012] 5) Set the coupling frequency and determine the frequency of gas-solid two-phase data exchange, that is, the number of iterations after which the corresponding data is output or read in;

[0013] 6) During the calculation process, the corresponding information is read by accessing the temporary database, and the additional variables in the gas phase case and the additional variables in the solid phase case are updated;

[0014] 7) Set termination judgment and convergence basis;

[0015] 8) Start the calculation program and start the gas phase calculation example and solid phase calculation example in the order of gas-solid data coupling;

[0016] 9) The program runs. During the program running, the gas phase example and the solid phase example continuously exchange information until the calculation converges or automatically terminates;

[0017] 10) Output the results.

[0018] In one embodiment of the present invention, in step 1), the boundary conditions and initialization conditions include: inlet flow rate, temperature, composition; outlet type, outlet pressure; wall temperature, heat exchange coefficient, wall roughness, etc.

[0019] In one embodiment of the present invention, additional variables in the gas phase calculation example include: solid phase temperature, solid phase velocity, porosity, and component concentration; additional variables in the solid phase calculation example include: gas phase temperature, gas phase velocity, heat transfer coefficient, component concentration, and reaction rate.

[0020] In one embodiment of the present invention, in step 3), the parameter information includes flow rate, pressure, temperature, and additional variables in the gas phase calculation example and the solid phase calculation example newly created in step 1); the corresponding data is stored in array form.

[0021] In one embodiment of the present invention, in step 4), each variable data is stored in a text format and the data precision is retained to a certain number of decimal places depending on the accuracy requirement.

[0022] In one embodiment of the present invention, in step 6), solid phase information is read for gas phase calculation examples, and gas phase information is read for solid phase calculation examples. The information to be read depends on the simulation object.

[0023] In one embodiment of the present invention, the additional variables in the gas phase calculation example and the additional variables in the solid phase calculation example are updated respectively by importing the read data into the computer memory to replace the original data array.

[0024] In one embodiment of the present invention, the additional variables in the gas phase case and the additional variables in the solid phase case are not solved during the calculation process and are only updated through data coupling.

[0025] In one embodiment of the present invention, in step 7), the termination criterion is whether the calculated residual is less than a preset threshold or a maximum number of iteration steps.

[0026] In one embodiment of the present invention, steps 2) to 6) are implemented through a user-defined program.

[0027] By adopting the above technical solution, the present invention has the following advantages compared with the prior art:

[0028] The present invention proposes a gas-solid flow coupling method. By separately modeling the gas-solid two-phase flow and using a coupling method to consider the momentum, energy, and mass transfer processes between the phases, a complete consideration of the gas-solid two-phase flow is achieved, thereby improving the simulation applicability and prediction accuracy of existing CFD commercial software for moving bed / packed bed gas-solid reaction two-phase flow. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1A flow chart of a numerical simulation method for a moving bed or packed bed gas-solid reaction two-phase flow provided by the present invention is shown;

[0030] Figure 2 Shown Figure 1 Detailed flowchart of the main steps;

[0031] Figure 3 (a) and (b) respectively show schematic diagrams of the vertical furnace structure and the grid division of its calculation model in Example 1 of the present invention;

[0032] Figure 4 (a) and (b) are schematic diagrams showing the gas-solid two-phase flow and the gas-solid temperature field distribution in the shaft furnace under typical working conditions in Example 1 of the present invention, respectively;

[0033] Figure 5 A schematic diagram showing the distribution of pellet components in a shaft furnace under typical working conditions in Example 1 of the present invention is shown;

[0034] Figure 6 A schematic diagram showing the gas component distribution in a shaft furnace under typical operating conditions in Example 1 of the present invention is shown;

[0035] Figure 7 A schematic diagram of the multiphase flow and temperature field distribution inside the blast furnace in Example 2 of the present invention is shown, wherein (a) represents the gas phase, (b) represents the solid phase, and (c) represents the liquid phase. DETAILED DESCRIPTION

[0036] It should be understood that the embodiments of the present invention shown in the exemplary embodiments are merely illustrative. Although only a few embodiments have been described in detail in the present invention, it is readily apparent to those skilled in the art that various modifications are feasible without departing substantially from the teachings of the subject matter of the present invention. Accordingly, all such modifications should be included within the scope of the present invention. Without departing from the gist of the present invention, other replacements, modifications, variations, and deletions may be made to the design, operating conditions, and parameters of the following exemplary embodiments.

[0037] like Figure 1 As shown, the present invention provides a numerical simulation method for gas-solid two-phase reaction flow in a moving bed or packed bed, comprising the following steps:

[0038] S101: For gas-solid two-phase flow, a gas-phase example and a solid-phase example are established to solve the gas-solid two-phase flow respectively, and boundary conditions and initialization conditions are set respectively. The gas-phase example adopts a porous medium model to describe the gas-phase flow and creates additional variables to store solid-phase related information. The solid-phase example adopts a viscous flow model to describe the solid phase and creates additional variables to store gas-phase related information. Gas-solid coupling is achieved by continuously iteratively updating the additional variables in the gas-phase example and the additional variables in the solid-phase example.

[0039] S102: Create a temporary database, ensure that the gas phase and solid phase examples are in the same working directory, and create a separate folder in the current working directory to store intermediate data and coupling information;

[0040] S103: Reading grid node information, obtaining parameter information during the calculation process by accessing the memory, and selecting variables that require data coupling based on the simulation object, and reading the corresponding data;

[0041] S104: Setting the output / input file format and determining the data storage accuracy and format of each variable;

[0042] S105: Setting the coupling frequency and determining the frequency of gas-solid two-phase data exchange, that is, the number of iteration steps after which the corresponding data is output or read;

[0043] S106: During the calculation process, corresponding information is read by accessing a temporary database, and additional variables in the gas phase calculation example and additional variables in the solid phase calculation example are updated;

[0044] S107: Setting termination judgment and convergence basis;

[0045] S108: Start the calculation program, and start the gas phase calculation example and the solid phase calculation example in the order of gas-solid data coupling;

[0046] S109: The program runs. During the program running, the gas phase example and the solid phase example continuously exchange information until the calculation converges or automatically terminates.

[0047] S110: Output the result.

[0048] The present invention achieves a complete consideration of gas-solid two-phase flow by separately modeling the gas-solid two-phase flow and using a coupling method to consider the momentum, energy, and mass transfer processes between the phases, thereby improving the simulation applicability and prediction accuracy of existing CFD commercial software for moving bed / packed bed gas-solid reaction two-phase flow.

[0049] In the above method, in step S101, the boundary conditions and initialization conditions include: inlet flow rate, temperature, composition; outlet type, outlet pressure; wall temperature, heat exchange coefficient, wall roughness, etc.

[0050] In the above method, in step S101, the additional variables in the gas phase calculation example include: solid phase temperature, solid phase velocity, porosity and component concentration; the additional variables in the solid phase calculation example include: gas phase temperature, gas phase velocity, heat transfer coefficient, component concentration and reaction rate.

[0051] In the above method, in step S103, the parameter information includes flow rate, pressure, temperature, and additional variables in the gas phase calculation example and the solid phase calculation example newly created in step S101; the corresponding data is stored in array form.

[0052] In the above method, in step S104 , each variable data is stored in a text format and the data precision is retained to a certain number of decimal places according to the precision requirement, for example, to four decimal places.

[0053] In the above method, in step S106, solid phase information is read for a gas phase calculation example, and gas phase information is read for a solid phase calculation example. The information to be read depends on the simulation object.

[0054] In the above method, in step S106, the additional variables in the gas phase calculation example and the additional variables in the solid phase calculation example are updated respectively by importing the read data into the computer memory to replace the original data array.

[0055] In the above method, in step S106 , the additional variables in the gas phase case and the additional variables in the solid phase case are not solved during the calculation process and are only updated through data coupling.

[0056] In the above method, in step S107, the termination criterion is whether the calculated residual is less than a preset threshold or the maximum number of iteration steps.

[0057] In the above method, steps S102 to S106 are implemented through a user-defined program.

[0058] The above technical solutions of the present invention are described in detail below through specific embodiments.

[0059] In the present invention, computational fluid dynamics (CFD) technology is used to couple and solve the packed bed / moving bed type gas-solid flow and its heat and mass transfer phenomena. Both gas and solid phases are regarded as continuous phases and modeled based on the Euler framework. The momentum, energy and mass exchange processes between the gas and solid phases are considered by introducing source terms in the control equations. A CFD commercial software package (ANSYS CFX or other solvers) is used to solve the gas-solid two-phase control equations. In the solution process, the gas-solid two-phase flow is solved separately using two separate examples, and the heat and mass transfer between the gas and solid phases is realized through an external database. The two examples read and write the information necessary for the coupled calculation in the external database in the form of text (specifically, Figure 2 ), and then realize the coupled solution.

[0060] In packed bed / moving bed reactors, the gas and solid phases flow in countercurrents. The bottom of the bed serves as both the gas and solid phase inlet, while the top of the bed serves as both the gas and solid phase inlet. This method solves the gas and solid phases separately, avoiding the inability to set two sets of boundary conditions for the same boundary in a single calculation. Furthermore, because the gas and solid phase solutions are based on established commercial software packages, the model development effort and time cost are both low, making it scalable.

[0061] Specifically, taking the ANSYS CFX solver as an example, Figure 1-2 As shown, the method of the present invention can be roughly divided into the following steps:

[0062] In step S101, a separate case is first established for gas-solid two-phase flow to solve the gas-solid two-phase flow separately. The gas phase flow is recorded as the G case and saved as a G.def file, and the solid phase flow is recorded as the S case and saved as an S.def file. The initialization conditions and boundary conditions are set separately, including inlet flow rate, temperature, composition; outlet type, outlet pressure; wall temperature, heat exchange coefficient, roughness, etc. The gas phase G case uses a porous media model to describe the gas phase flow. At the same time, several sets of additional variables (scalar or vector) are newly created to store solid phase related information. Depending on the simulation object, these additional variables may include but are not limited to solid phase temperature (Ts, scalar), solid phase velocity (Us, vector), porosity (epsilon, scalar), component concentration (mf, scalar), etc., and these additional variables are not solved separately, but updated through coupling. Similarly, a similar approach is used for solid-phase simulations. Depending on the simulation object, additional variables can include, but are not limited to, gas temperature (Tg, scalar), gas velocity (Ug, vector), heat transfer coefficient (HTC, scalar), component concentration (mf, scalar), reaction rate (R, scalar), etc. Gas-solid coupling can be achieved by continuously iteratively updating the additional variables in the two simulations.

[0063] In step S102, a temporary database is created. Ensure that the two examples G and S are in the same working directory, and create a separate folder in the current working directory to store intermediate data and coupling information.

[0064] In step S103, grid node information is read. Parameter information from the calculation process is obtained by accessing memory. This information is typically stored at the grid nodes, such as flow rate, pressure, temperature, and the additional variables created in step 1. Depending on the simulation object, the variables requiring data coupling are selected and the corresponding data is read. This data is typically stored in array form.

[0065] In step S104, the output / input file format is set, and the storage precision and storage format of each variable data are determined. In the present invention, the data is stored in text format, and the data precision is generally retained to four decimal places.

[0066] In step S105, the coupling frequency is set to determine the frequency of gas-solid two-phase data exchange, that is, the number of iteration steps after which the corresponding data is output or read.

[0067] In step S106, additional variables are updated. During the calculation process, corresponding information is retrieved from a temporary database. For the G case, solid phase information is retrieved, while for the S case, gas phase information is retrieved. The required information depends on the simulation object. The retrieved data is imported into computer memory, replacing the original data array, thus updating the additional variables. It is important to note that additional variables are not solved during the calculation process and can only be updated through data coupling.

[0068] In step S107, the termination judgment and convergence criteria are set. Generally, the calculation residual is less than a certain value (for example, 1e-5) or the maximum number of iteration steps is used as the termination criterion.

[0069] In step S108, the calculation program is started, and the gas-solid calculation examples are started in sequence according to the gas-solid data coupling order.

[0070] In step S109, the program runs. During the program running, the two calculation examples G and S will continuously exchange information until the calculation converges or automatically terminates.

[0071] In step S110 , the result is output.

[0072] It should be noted that steps S102-S106 need to be implemented through a user-defined program.

[0073] The present invention can be specifically implemented in the following ways:

[0074] The gas and solid phases are modeled separately and the corresponding solver executable calculation files are set, and a temporary database is established in the working directories of the two. The G example is responsible for solving the gas phase flow, while the S example is responsible for solving the solid phase flow. The two examples realize data interaction by calling user subroutines, and then couple the two-phase flow and interphase heat and mass exchange in the furnace. Specifically, the G example will output gas phase temperature, velocity, reaction distribution and other information at a certain calculation interval during the solution process, and write it into the temporary database in the form of a text file, and the above information is read into the S example through the user subroutine to solve the solid phase temperature and component distribution. The S example feeds back solid phase temperature, velocity, solid phase component and other information to the temporary database in the same way. By constantly exchanging data information, the coupled solution of gas-solid two-phase reaction flow is achieved.

[0075] Example 1

[0076] Numerical Simulation of Gas-Based Shaft Furnace for Ironmaking

[0077] The above method is used to simulate the vertical moving bed gas-solid reaction flow, which is widely used in metallurgical processes. The vertical furnace structure and its calculation model grid division are as follows: Figure 3 As shown in (a) and (b), iron ore is added from the furnace top and flows out from the bottom, while high-temperature reducing gas is pumped in from the bottom. The iron ore flows out at a controlled rate under the action of a spiral feeder at the bottom, driving the bed downward. The downward feed line at the top is continuously replenished with fresh ore. As the ore moves downward, it comes into contact with the rising reducing gas, gradually heating and reducing it to produce metallic iron.

[0078] During the calculations, both the gas and solid phases are treated as continuous media. The gas phase includes components such as CO, CO2, H2O, H2, CH4, and N2, while the solid phase is assumed to be a mixture of pellets and slag-forming agents, consisting of components such as Fe2O3, Fe3O4, FeO, and Fe. The gas-solid flow is described by two separate sets of mass, momentum, and energy conservation equations, which are solved for their components using scalar conservation equations. The residence time distribution of the pellets within the furnace is obtained by solving the residence time of the solid flow. Pellet size is tracked using separate scalar equations, allowing direct solution for the porosity at each location in the bed, thus enabling more accurate prediction of the gas flow distribution. The gas flow resistance is estimated using a modified Ergun equation, and the gas-solid heat exchange is described using the Ranz-Marshall empirical formula. The staged reduction reaction of the ore (Fe2O3 → Fe3O4 → FeO → Fe) is described using a "three-interface" unreacted core model.

[0079] In the above embodiment 1, Figure 4 As shown in (a) and (b), the gas-solid two-phase flow and temperature distribution in the shaft furnace under typical working conditions are shown respectively; Figure 5 As shown in Figure 2, the distribution of pellet components inside the shaft furnace under typical working conditions is shown; Figure 6 As shown, the gas component distribution in the experimental shaft furnace under typical working conditions is displayed.

[0080] Example 2

[0081] Numerical Simulation of Ironmaking Blast Furnace

[0082] The smelting process in a blast furnace is similar to that of a shaft furnace, but more complex. In addition to iron ore, coke is added to the furnace top as a solid fuel. However, the size and permeability of coke differ significantly from those of ore, necessitating more precise predictions of solid-phase flow to account for the complex spatial distribution of airflow within the furnace and the resulting changes in macroscopic performance parameters. Furthermore, in the lower half of the blast furnace, iron ore melts at high temperatures and collects in the form of droplets or trickles at the bottom. Consequently, numerical simulations of blast furnaces must consider at least three phases: gas, solid, and liquid.

[0083] The blast furnace smelting process was numerically simulated using a method similar to that in Example 1. The solid flow, bed porosity prediction, and liquid flow simulation methods are not related to the calculation method described in the present invention and are not separately described here.

[0084] In the above embodiment 2, Figure 7 As shown in the figure, the complex multiphase flow and temperature field distribution inside the blast furnace under typical conditions are demonstrated, where (a) represents the gas phase, (b) represents the solid phase, and (c) represents the liquid phase.

[0085] Therefore, the present invention achieves a complete consideration of gas-solid two-phase flow by separately modeling the gas-solid two-phase flow and using a coupling method to consider the momentum, energy, and mass transfer processes between phases, thereby improving the simulation applicability and prediction accuracy of existing CFD commercial software for moving bed / packed bed gas-solid reaction two-phase flow.

[0086] The above description is only a preferred embodiment of the present invention and is not intended to limit the scope of implementation of the present invention. If the present invention is modified or replaced with equivalents without departing from the spirit and scope of the present invention, it should be included in the scope of protection of the claims of the present invention.

Claims

1. A numerical simulation method for gas-solid two-phase reaction flow in a moving bed or packed bed, characterized in that: The following steps are involved: 1) For gas-solid two-phase flow, a gas-phase example and a solid-phase example are established to solve the gas-solid two-phase flow respectively, and the boundary conditions and initialization conditions are set respectively. The gas-phase example uses a porous medium model to describe the gas-phase flow and creates additional variables to store solid-phase related information. The solid-phase example uses a viscous flow model to describe the solid phase and creates additional variables to store gas-phase related information. Gas-solid coupling is achieved by continuously iteratively updating the additional variables in the gas-phase example and the additional variables in the solid-phase example. 2) Create a temporary database, ensure that the gas phase example and the solid phase example are in the same working directory, and create a separate folder in the current working directory to store intermediate data and coupling information; 3) Read the grid node information, obtain the parameter information during the calculation process by accessing the memory, and select the variables that need data coupling according to the simulation object and read the corresponding data; 4) Set the output / input file format and determine the storage accuracy and format of each variable data; 5) Set the coupling frequency and determine the frequency of gas-solid two-phase data exchange, that is, the number of iterations after which the corresponding data is output or read in; 6) During the calculation process, corresponding information is read by accessing a temporary database, and additional variables in the gas phase case and the solid phase case are updated. For the gas phase case, solid phase information is read, and for the solid phase case, gas phase information is read. The information to be read depends on the simulation object. The additional variables in the gas phase case and the additional variables in the solid phase case are updated by importing the read data into the computer memory to replace the original data array. The additional variables in the gas phase case and the additional variables in the solid phase case are not solved during the calculation process and are only updated through data coupling; 7) Set termination judgment and convergence basis; 8) Start the calculation program and start the gas phase example and solid phase example in the order of gas-solid data coupling; 9) The program runs. During the program running, the gas phase example and the solid phase example continuously exchange information until the calculation converges or automatically terminates; 10) Output the results.

2. The moving bed or packed bed gas-solid two-phase reaction flow numerical simulation method according to claim 1, characterized in that: In step 1), the boundary conditions and initialization conditions include: inlet flow rate, temperature, composition; outlet type, outlet pressure; wall temperature, heat exchange coefficient, and wall roughness.

3. The method for numerical simulation of gas-solid two-phase reaction flow in a moving bed or packed bed according to claim 2, characterized in that: In gas phase cases, additional variables that are considered as simulation objects include: solid phase temperature, solid phase velocity, porosity, and component concentration; in solid phase cases, additional variables that are considered as simulation objects include: gas phase temperature, gas phase velocity, heat transfer coefficient, component concentration, and reaction rate.

4. The method for numerical simulation of gas-solid two-phase reaction flow in a moving bed or packed bed according to claim 1, characterized in that: In step 3), the parameter information includes flow rate, pressure, temperature, and additional variables in the gas phase calculation example and the solid phase calculation example newly created in step 1); the corresponding data are stored in array form.

5. The moving bed or packed bed gas-solid two-phase reaction flow numerical simulation method according to claim 1, characterized in that: In step 4), each variable data is stored in a text format and the data precision is retained to a certain number of decimal places depending on the accuracy requirement.

6. The method for numerical simulation of moving bed or packed bed gas-solid two-phase reaction flow according to claim 1, wherein in step 7), the termination criterion is that the calculation residual is less than a preset threshold or a maximum number of iteration steps.

7. The method for numerical simulation of gas-solid two-phase reaction flow in a moving bed or packed bed according to claim 1, characterized in that: Steps 2) to 6) are implemented through a user-defined program.

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