Process simulation method for preparing meltable direct spinning polyamide 6 through falling film devolatilization reaction

Abstract

The invention relates to the field of chemical engineering, and discloses a process simulation method for preparing meltable direct spinning polyamide 6 through a falling film devolatilization reaction. According to the method, a falling film devolatilization reaction process model is established by adopting a numerical calculation and chemical process simulation software interaction method, and a material balance equation in the polyamide 6 falling film devolatilization process is subjected to decoupling calculation by utilizing physical equations such as reaction kinetics, mass transfer kinetics and hydrodynamics; basic physical quantity distribution (such as relative viscosity, molecular weight, end group concentration, monomer content and oligomer content) of a falling film reaction process is effectively and conveniently obtained, so that good guidance is provided for experimental research and industrial design.

Description

Technical Field

[0001] The invention relates to the field of chemical industry, and in particular to a process simulation method for preparing melt-spinnable polyamide 6 by falling film devolatilization reaction. Background Art

[0002] Polyamide fiber is the earliest industrialized synthetic fiber, and polyamide 6 (PA6) fiber is the leading variety among them. It has excellent wear resistance, elasticity, fatigue resistance, and easy dyeing. It is widely used in home textiles, clothing, carpets, and a variety of industrial textiles.

[0003] In industrial production, water is usually used as an initiator to carry out hydrolysis polymerization with caprolactam (CPL) monomer to prepare primary PA6 melt. The CPL ring-opening polymerization reaction rate is much greater than the self-condensation rate of aminocaproic acid, and the melt polymerization degree is related to the equilibrium water concentration. Although monofunctional acids are often used as end-capping agents, the CPL conversion rate can only reach 90%, and monomers and oligomers with a content of up to about 10% still remain in the melt polymerization system. The presence of a large amount of volatiles makes it impossible to carry out continuous melt processing, and can only be removed by long-term extraction with a large amount of hot water. Rapidly reducing the content of extractables, directly preparing high-quality polyamide 6 melt that can be directly processed by melt, and realizing melt direct spinning of polyamide 6 fibers are important technical problems that the industry needs to solve urgently.

[0004] At present, there are literature reports that low-temperature polymerization, anionic polymerization, introduction of other monomers for copolymerization, etc. are used to try to reduce the content of oligomers or volatile components in polyamide 6 to prepare polyamide 6 melt that can be melt-spun. These methods have a clearer reaction mechanism for the polymerization system, but usually do not consider heat transfer and mass transfer during large-scale application, which makes it impossible to achieve the desired effect due to many uncontrollable factors during large-scale application. Another way is to control the volatile content in polyamide 6 by adopting new devolatilization reaction equipment. This method has great reference significance for industrial production, but the understanding of the reaction law, the degree of polymerization and components and their changes in the reaction process are still a major challenge. Through process simulation, the process and result analysis of the falling film devolatilization reaction of polyamide 6 can be effectively carried out, which provides great convenience for exploring the effect of falling film devolatilization reaction.

[0005] The applicant did not find any similar reports through searching the prior art. Therefore, there is an urgent need for a method and technology that can effectively and conveniently analyze the material flow process and the distribution of basic physical quantities (such as number average molecular weight, relative viscosity, monomer content, oligomer content, and terminal carboxyl and terminal amine content) in the actual working conditions of falling film devolatilization, so as to understand the reaction law of the continuous polymerization-devolatilization production process and provide effective guidance for experimental research and industrial design. Therefore, it is urgent to propose a process simulation method for preparing low-volatile polyamide 6 melt by falling film devolatilization reaction. Summary of the invention

[0006] In order to solve the above technical problems, the present invention provides a process simulation method for preparing melt-spinnable polyamide 6 by falling film devolatilization reaction. The present invention adopts a method of interaction between numerical calculation and chemical process simulation software to establish a falling film devolatilization reaction process model, and uses physical equations such as reaction kinetics, mass transfer kinetics and fluid dynamics to decouple the material balance equation of the falling film devolatilization process of polyamide 6, effectively and conveniently obtain the distribution of basic physical quantities (such as relative viscosity, molecular weight, end group concentration, monomer content and oligomer content) of the falling film reaction process, thereby providing good guidance for experimental research and industrial design.

[0007] The specific technical solution of the present invention is: a process simulation method for preparing melt-spinnable polyamide 6 by falling film devolatilization reaction, which includes:

[0008] The polymerization reaction kinetic model, gas-liquid two-phase mass transfer and falling film fluid dynamics behavior of polyamide 6 were analyzed.

[0009] The reaction kinetics model, gas-liquid two-phase mass transfer model and fluid dynamics model were used to calculate the material balance of the entire polyamide 6 falling film devolatilization reaction process, and the boundary conditions and initial conditions were set. The material balance equation was solved by numerical calculation to establish the polyamide 6 falling film devolatilization reactor model.

[0010] The polymerization reaction kinetic model includes but is not limited to the hydrolysis ring-opening reaction and addition reaction of caprolactam monomer and the chain growth reaction mainly composed of the condensation polymerization reaction of aminocaproic acid;

[0011] The falling film devolatilization reactor is provided with a melt cavity, a film structure, a falling film support that can support the flow of the melt fluid, and a devolatilization port connected to the outside, and the devolatilization port is connected to a vacuum device. The polyamide 6 prepolymer / condensation polymer melt after hydrolysis and polymerization flows downward along the falling film support to complete the removal of volatiles and molecular chain growth reaction;

[0012] The whole process of falling film devolatilization reaction of polyamide 6 melt is constructed according to the reaction production process of polyamide 6. The index parameters of polyamide 6 melt with a certain degree of polymerization obtained after hydrolysis polymerization are taken as input values. The partial differential equations of reaction rate are calculated, and the performance parameters of the polymerization product and the content information of each characteristic component are output. The simulation results of the falling film devolatilization reaction process of polyamide 6 are analyzed.

[0013] Preferably, the relative viscosity of the polyamide 6 melt entering the falling film devolatilization reactor is 1.4 to 3.0, and the concentration of caprolactam monomer and its oligomer is 6 to 15% of the total mass of the melt.

[0014] Preferably, the relative viscosity of the polyamide 6 melt at the outlet of the falling film devolatilization reactor is 2.2-4.0, and the concentration of caprolactam monomer and its oligomer is 0.1-2% of the total mass of the melt.

[0015] Preferably, the polyamide 6 is polyamide 6 and its modified products obtained by copolymerization of caprolactam or caprolactam with other modified monomers; the melt-spinnable polyamide 6 (i.e., low-volatile polyamide 6) is polyamide 6 with a volatile component content of less than 2%, and the low-volatile polyamide 6 melt can be directly melt-spun, or added with fibers, or directly processed in a molding device to obtain a polyamide 6 product.

[0016] Preferably, the polymerization reaction kinetic model also includes but is not limited to side reactions such as ring-opening and addition reactions of oligomers and production of cyclic oligomers; and carrying out the polymerization reaction kinetic model analysis comprises the following steps:

[0017] (1) Determine the main reaction and side reaction in the polymerization process: Use the "chain segment analysis" method to simplify the reaction into a limited number of basic chain segment units, with the chain growth reaction as the leading factor, and determine the reaction equations for generating terminal amine groups, terminal carboxyl groups, oligomers, water and other components;

[0018] (2) Determination of reaction rate constant and equilibrium constant: Describe the effect of temperature on the reaction rate constant using the Arrhenius equation, specify the pre-exponential factor and activation energy in the equation, determine the reversible reaction equilibrium constant, and solve for the forward and reverse reaction rate values;

[0019] (3) Establishing reaction kinetic equations: Use the reaction kinetic model to analyze the reaction kinetic equations of each component, and compile and import the established equation group into an executable function script file in the numerical calculation software.

[0020] Preferably, the gas-liquid two-phase mass transfer analysis comprises the following steps:

[0021] (1) According to the component targets to be analyzed in the gas-liquid two-phase, select an appropriate physical property model and set relevant property values, including but not limited to relative molecular mass, critical temperature, pressure, compressibility, gas-liquid phase pressure and molar volume, polymer molecular weight and relative viscosity parameters, etc., and store the properties in a global data array;

[0022] (2) A simulation analysis method for the devolatilization process of small molecules in high-viscosity polymer melts. According to the gas-liquid phase equilibrium process, the volatile components in the system are determined, the gas-liquid equilibrium equation at the interface is constructed by calling the attribute groups of each component, and the corresponding settings are made in the numerical calculation software;

[0023] The volatile components are specifically: according to the significant influence of each volatile component on the reaction process, the volatile components in the gas phase include but are not limited to caprolactam and water, and the liquid phase includes but is not limited to caprolactam, water, and oligomers, and the phase equilibrium process parameters are set for the volatile components in the gas and liquid phases;

[0024] (3) Determine the equipment structure and process parameters of the falling film devolatilization process reactor that affect the gas-liquid mass transfer rate, introduce the mass transfer coefficient to calculate the mass transfer flux of the gas phase components, and iteratively calculate the mass transfer coefficient based on the gas phase mass transfer flux during the operation of the reactor in industry;

[0025] (4) Initialize the output property group of each component to complete the iterative solution calculation and obtain the mass transfer flux of the gas phase volatile components during the falling film devolatilization reaction of polyamide 6.

[0026] Preferably, the falling film fluid dynamics behavior analysis comprises the following steps:

[0027] (1) Determine the molecular diffusion coefficient during flow based on the motion of low-speed laminar polymers at very low Reynolds numbers;

[0028] (2) The thickness of the liquid film is much smaller than the axial dimension of the falling film tube, the liquid film flow is fully developed, the monomer diffusion obeys Fick's diffusion law, and the convection-diffusion equation is established;

[0029] (3) Relevant model parameters are set in MATLAB, including the material flow rate and the flow velocity of the micro-element in the free falling liquid film area.

[0030] Preferably, the construction of the polyamide 6 falling film devolatilization reactor model includes the following steps:

[0031] (1) The microelement mass balance equation of polymer volume in the falling film devolatilization reactor was established based on the polymerization reaction kinetics, gas-liquid two-phase mass transfer kinetics and falling film fluid dynamics;

[0032] (2) setting boundary conditions according to the operating parameters of the falling film devolatilization reactor in industry, selecting the discrete format, and using the numerical integration method (Runge-Kutta method) to solve the material balance equation;

[0033] (3) Define and initialize variables, constants, and parameters, use the software solver to solve the material balance equation, and output key process performance indicators for the gas and liquid phases.

[0034] Further preferably, the variables, constants and parameters include one or more of the number of components, the number of discrete points in the spatial direction, the number of ordinary differential equations to be solved, and the flow rate of the molten polymer; the key process performance indicators of the gas-liquid phase include one or more of the number average molecular weight, relative viscosity, monomer content, oligomer content and end group content.

[0035] Preferably, the whole process of constructing the polyamide 6 falling film devolatilization reaction comprises the following steps:

[0036] (1) Construction of the entire falling film devolatilization reaction process: Call the automatic control interface (Active X) of Aspen Plus to import the output data of the process as the model input value, run MATLAB to calculate the material balance equation, and obtain the changes of polyamide molecular weight, relative viscosity, monomer content, oligomer content, end group content and other variables with the reaction progress;

[0037] (2) The Active X interface specifically includes: creating a component model object of Aspen Plus, opening and running the model file according to the .bkp file path, setting the user interface visibility, and running in MATLAB to obtain and output the information of each component.

[0038] Preferably, the analysis of the simulation results of the falling film devolatilization reaction process of polyamide 6 comprises the following steps: importing the process simulation file into numerical calculation software for processing, outputting and displaying the acquired data as one or more of a data table, a graph or a curve, observing and analyzing the results of the simulation calculation, and understanding the distribution of performance indicators of each component in the falling film devolatilization process of polyamide 6.

[0039] Preferably, analyzing the simulation results of the falling film devolatilization reaction process of polyamide 6 further includes carrying out process simulation calculations of different falling film devolatilization reactor operating conditions, and analyzing the influence of the operating conditions on the falling film devolatilization reaction effect, specifically including the following steps:

[0040] In the numerical calculation software, the falling film devolatilization reactor unit module is iteratively calculated, and the influence of different operating parameters (such as feed relative viscosity, temperature, and pressure) on the reaction effect is systematically adjusted and analyzed;

[0041] Through multiple simulation runs, key running data (such as polymer molecular weight, relative viscosity, monomer concentration and oligomer concentration, end group content change, etc.) are collected and compared and analyzed.

[0042] Compared with the prior art, the beneficial effects of the present invention are:

[0043] (1) The principle of the present invention is that the material melt has high viscosity characteristics, and maintains the characteristics of plug flow motion during the wall-attached falling film flow, which can always maintain a large film-forming area and effectively improve the gas-liquid interface renewal rate. On the basis of fully understanding the mass transfer characteristics of polycondensation reactions and the fluid dynamics of high-viscosity materials, the falling film flow is used to enhance the polymer devolatilization, which is particularly suitable for the rapid devolatilization process of polyamide 6 with a high volatile content. The application results of this technology are obtained through process simulation.

[0044] (2) The present invention uses a method of coupling MATLAB and Aspen Plus to establish a polyamide 6 falling film devolatilization reaction process model, which fully considers the physical and chemical changes inside the reactor, overcomes the shortcomings of large investment and long period of experimental research, and can predict the distribution of component performance indicators in the falling film devolatilization reactor, thereby realizing computer reproduction of the falling film devolatilization reaction effect of polyamide 6.

[0045] (3) By implementing the present invention, the distribution of key component performance indicators that change with the reaction progress in the falling film devolatilization reactor during the falling film devolatilization process can be obtained, as well as the axial distribution of key component performance indicators that affect the falling film devolatilization process at different temperatures and pressures. The performance indicators (such as molecular weight, relative viscosity, monomer content, etc.) at various positions inside the reactor during the falling film devolatilization process can be effectively and conveniently obtained, thereby providing good technical guidance for actual production and industrial design.

[0046] (4) By implementing the present invention, the molecular weight change, monomer content change, oligomer content change and end group concentration change of the melt passing through the falling film devolatilization reactor can be effectively obtained, which intuitively reflects the distribution law of various indicators of the material in the falling film devolatilization reactor, so that products with good performance indicators can be obtained under optimal conditions.

[0047] (5) The present invention is applicable to the research and development of the performance index distribution and process conditions of the components outside the melt tube in the falling film devolatilization reactor during the falling film devolatilization process, providing a theoretical basis for actual industrial production, contributing to the breakthrough of the low-volatile polyamide 6 melt preparation technology, and providing a feasible modeling and numerical calculation simulation research method for the development and optimization of new technologies such as polyamide 6 direct spinning technology and polyamide 6 melt direct fiber processing technology, thereby saving research costs and having good application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1It is a schematic diagram of the flow of the falling liquid film of the melt attached to the wall in the falling film devolatilization reactor;

[0049] Figure 2 It is the workflow diagram of numerical calculation;

[0050] Figure 3 This is a process flow chart of polyamide 6 melt direct spinning;

[0051] Figure 4 This is the axial distribution diagram of the relative viscosity of polyamide 6 during the falling film devolatilization reaction;

[0052] Figure 5 It is the axial distribution diagram of the monomer and oligomer content during the falling film devolatilization reaction of polyamide 6;

[0053] Figure 6 This is the axial distribution diagram of the end group content during the falling film devolatilization reaction of polyamide 6;

[0054] Figure 7 The axial distribution diagram of molecular weight during falling film devolatilization reaction of polyamide 6 at different temperatures;

[0055] Figure 8 The axial distribution diagram of monomer content during the falling film devolatilization reaction of polyamide 6 at different temperatures;

[0056] Fig. 9 The axial distribution diagram of oligomer content during falling film devolatilization of polyamide 6 at different temperatures;

[0057] Fig.10 The axial distribution diagram of molecular weight during falling film devolatilization reaction of polyamide 6 under different pressures;

[0058] Fig.11 The axial distribution diagram of monomer content during the falling film devolatilization reaction of polyamide 6 under different pressures;

[0059] Fig.12 This is the axial distribution diagram of the oligomer content during the falling film devolatilization reaction of polyamide 6 under different pressures. DETAILED DESCRIPTION

[0060] In order to make the above-mentioned purposes, features and advantages of the present invention more obvious and easy to understand, the technical solution of the present invention is further described below through specific implementation methods. However, the implementation methods of the present invention are not limited to the scope of the embodiments.

[0061] Example 1

[0062] This example uses Aspen Plus and MATLAB to interact with each other to simulate the falling film devolatilization process of polyamide 6, mainly including: building a falling film devolatilization reactor unit module that couples reaction kinetics, gas-liquid two-phase mass transfer kinetics and falling film fluid dynamics in MATLAB, running Aspen Plus to output the results and link them to MATLAB to build the entire falling film devolatilization process, and analyzing the simulation results and process operating conditions after MATLAB is run.

[0063] First, determine the main and side reactions of the polymerization process: use the "chain segment analysis" method to simplify the reaction into a limited combination of basic chain segment units, with the condensation chain growth reaction as the leading factor, and determine the monomer, water, oligomer, terminal amine group (-NH 2 ), terminal carboxyl (-COOH) and other components. Use the Arrhenius equation to describe the effect of temperature on the reaction rate constant, determine the pre-exponential factor and activation energy in the equation, determine the reversible reaction equilibrium constant, and solve the forward and reverse reaction rate values. Use the second-order reaction kinetic model to analyze the reaction kinetic equations of each component, and compile the established equation group into the executable function script file of MATLAB;

[0064] In a reaction C+C→A+B in the polyamide 6 reaction system, where C is the reactant and A and B are the target products, the rate equation of the reaction is established using the second-order reaction kinetics, the reaction equilibrium constant is determined using the Arrhenius equation, and the reaction kinetics equation is compiled.

[0065] According to the component targets that need to be analyzed in the gas-liquid two-phase, select the appropriate physical property model and set the relevant property values, including relative molecular mass, critical temperature, pressure, compressibility, gas-liquid saturated vapor pressure, molar volume, polymer molecular weight and relative viscosity, and store the properties in the global data array; for the simulation analysis method of the small molecule devolatilization process of high-viscosity polymer melt, according to the gas-liquid phase equilibrium process, simplify the volatile components in the system, call each component property group to construct the gas-liquid equilibrium equation at the interface, and make corresponding settings in MATLAB.

[0066] The volatile components are specifically: according to the degree of influence of each volatile component on the reaction process, the volatile components in the gas phase will mainly be considered to include caprolactam monomer and water, and the liquid phase will include caprolactam, water and oligomers, and the phase equilibrium process parameters will be set for the volatile components in the gas and liquid phases.

[0067] Determine the vapor-liquid equilibrium equation at the interface, where the components involved are caprolactam and water. Calculate the saturated vapor pressure and molar volume of the components involved.

[0068] The effects of reactor equipment structure and process conditions (temperature, pressure) on the gas-liquid mass transfer rate during the falling film devolatilization reaction were determined, and the mass transfer coefficient was introduced to calculate the mass transfer flux of the gas phase components.

[0069] First, the mass flow equations of the components caprolactam and water are determined, and then the mass flow of the component caprolactam is determined.

[0070] The polymer flow behavior on the internal components of the falling film devolatilization reactor is analyzed. The polymer flow is assumed to be a low-speed laminar motion under extremely low Reynolds number. The liquid film thickness is much smaller than the axial dimension of the falling film tube. On the basis of sufficient flow development of the liquid film, the liquid film flow development process of the high-viscosity melt during the devolatilization process is analyzed.

[0071] See also Figure 1 The falling film devolatilization reactor in the present invention is installed vertically. Under the action of gravity and adhesion, the melt flows along the wall of the internal components of the reactor to form a liquid film. The liquid film forms a three-dimensional symmetrical flow pattern in the central axis of the internal components of the reactor. As the reaction proceeds, the viscosity of the polymer gradually increases. The present invention uses MATLAB software to establish the free falling liquid film flow process inside the falling film devolatilization reactor.

[0072] Construction of falling film devolatilization reactor model: The reaction kinetics model, gas-liquid two-phase mass transfer model and fluid dynamics model are substituted into the material balance equation to construct a group of partial differential equations. The input value of the falling film devolatilization reactor is used as the initial condition, and the first-order partial derivative of the final change of each component is 0, which is the boundary condition. The number of partial differential equations, time steps and spatial discrete points are set. The material balance equation is discretized using the finite difference method, and the discretized ordinary differential equations are solved using the ODE15 function programming in MATALB.

[0073] See also Figure 2 It is a MATLAB workflow diagram. The falling film reaction unit module solution stage can be carried out by importing the above modules into the material balance equation. It is necessary to set them one by one in MATLAB. The specific method is as follows: The above-mentioned objects, features and advantages of the present invention can be more obvious and easy to understand. The present invention is further described in detail below with reference to examples:

[0074] (1) Initially set the reactor length, flow rate, diffusion coefficient, number of spatial discrete points, spatial step, time span, and units in the calculation domain to be consistent with those in Aspen Plus; set the basic function solving method. In this embodiment, ODE15s is used to solve the equation.

[0075] (2) Setting the operating conditions: In the present invention, the operating pressure is 200 Pa, the temperature is 260° C., and the initial flow rate is set considering the influence of the melt viscosity.

[0076] (3) The model is set up. In order to more realistically predict the results of the falling film devolatilization reaction, the present invention selects a combination of reaction kinetics analysis, gas-liquid two-phase mass transfer analysis, and high-viscosity fluid dynamics behavior analysis to describe the changes of the polymer in the falling film devolatilization reactor.

[0077] (4) The present invention solves the material balance equation by calling the function files of each module in the .m script file.

[0078] The whole process of falling film devolatilization reaction was constructed: the automatic control interface (ActiveX) of Aspen Plus was called to import the output data of its process as the model input value, and MATLAB was run to calculate the material balance equation to obtain the changes of polyamide 6 molecular weight, relative viscosity, monomer content, oligomer content, end group content, etc. with the reaction progress.

[0079] This involves the interaction method between Aspen Plus and MATLAB software, that is, calling the automatic control interface (Active X) of Aspen Plus to import it into MATLAB, and then running the result. The node path of the called discharge stream is "Aspen.Application.Tree.FindNode('\Data\Streams\POLY\Output\MOLEFLOW\MIXED\CL')". This method well realizes the simplicity of industrial process calculation.

[0080] See also Figure 3 This is a new process for preparing polyamide fibers by melt direct spinning based on falling film devolatilization reaction. The present invention adopts a falling film devolatilization reactor to replace multiple systems such as cooling pelletizing, hot water extraction, drying, solid phase viscosity enhancement and extrusion melting to achieve direct spinning of polyamide 6 fibers. This method effectively simplifies the industrial production process and saves operating costs.

[0081] See also Figure 4 is the relative viscosity inside the polyamide 6 falling film devolatilization reactor (η r ), the relative viscosity at the reactor inlet is 2.35. According to the data of the simulation results of the present invention, it can be concluded that the relative viscosity of the reactor outlet material is 3.1, and its relative viscosity shows an increasing trend along the axial direction of the reactor.

[0082] See also Figure 5 is the monomer content (CPL) and oligomer content (C 0 ) content in the axial direction. According to the simulation results of the present invention, it can be concluded that as the reaction proceeds, the monomer content and the oligomer content gradually decrease along the axial direction, and the monomer content at the outlet is 0.23%, and the oligomer content is 0.115%.

[0083] See also Figure 6is the terminal group content ([NH 2 ], [COOH]). According to the data obtained from the simulation results of the present invention, it can be concluded that as the reaction proceeds, the terminal group content gradually decreases. The terminal amine group ([NH 2 ]) concentration is 21.06mmol / kg; terminal carboxyl group ([COOH]) concentration is 34.52mmol / kg.

[0084] See also Figure 7 The axial distribution diagram of the polymer molecular weight (Mn) in the membrane devolatilization reactor at different temperature drops. According to the data obtained from the simulation results of the present invention, it can be concluded that as the temperature gradually increases, the polymer molecular weight increases almost linearly in the axial direction. When the temperature exceeds 260°C, the effect of temperature on the polymer chain growth reaction is significantly weakened.

[0085] See also Figure 8 and Fig. 9 The monomer content (CPL) and oligomer content (C 0 ) content in the axial direction. According to the data obtained from the simulation results of the present invention, it can be concluded that with the increase of temperature, the monomer content and the oligomer content gradually decrease along the axial direction, and the higher the temperature, the less obvious this trend is.

[0086] See also Fig.10 The axial distribution diagram of the polymer molecular weight (Mn) in the membrane devolatilization reactor at different pressure drops. According to the data obtained from the simulation results of the present invention, it can be concluded that with the increase of vacuum degree, the polymer molecular weight increases almost linearly along the axial direction. When the pressure is 100Pa, the molecular weight reaches a maximum of 22860.4g / mol. When the reactor devolatilization pressure is further reduced to 50Pa, the molecular weight almost stops increasing, which indicates that the effect of pressure promoting the forward reaction has reached a maximum at this time.

[0087] See also Fig.11 and 12 is the monomer content (CPL) and oligomer content (C 0 ) content in the axial distribution diagram. According to the data obtained from the simulation results of the present invention, it can be concluded that a higher vacuum degree is conducive to the removal of small molecular substances. Therefore, with the increase of vacuum degree, the content of monomers and oligomers in the polyamide 6 melt is gradually reduced.

Claims

1. A process simulation method for preparing melt-spinnable polyamide 6 by falling film devolatilization reaction, characterized in that include: The polymerization reaction kinetic model analysis, gas-liquid two-phase mass transfer analysis and falling film fluid dynamics behavior analysis of polyamide 6 were carried out; The reaction kinetics model, gas-liquid two-phase mass transfer model and fluid dynamics model were used to calculate the material balance of the entire polyamide 6 falling film devolatilization reaction process, and the boundary conditions and initial conditions were set. The material balance equation was solved by numerical calculation to establish the polyamide 6 falling film devolatilization reactor model. The polymerization reaction kinetic model includes the hydrolysis ring-opening reaction and addition reaction of caprolactam monomer and the chain growth reaction mainly composed of the condensation polymerization reaction of aminocaproic acid; The falling film devolatilization reactor is provided with a melt cavity, a film structure, a falling film support that can support the flow of the melt fluid, and a devolatilization port connected to the outside. The polyamide 6 prepolymer / condensation polymer melt after hydrolysis and polymerization flows downward along the falling film support to complete the removal of volatiles and molecular chain growth reaction; The whole process of falling film devolatilization reaction of polyamide 6 melt was constructed according to the reaction production process of polyamide 6. The index parameters of polyamide 6 melt obtained after hydrolysis polymerization were taken as input values, and the partial differential equations of reaction rate were calculated. The performance parameters of the polymerization product and the content information of each characteristic component were output, and the simulation results of the falling film devolatilization reaction process of polyamide 6 were analyzed.

2. The process simulation method according to claim 1, characterized in that: The relative viscosity of the polyamide 6 melt entering the falling film devolatilization reactor is 1.4 to 3.0, and the concentration of caprolactam monomer and its oligomer is 6 to 15% of the total mass of the melt; The relative viscosity of the polyamide 6 melt at the outlet of the falling film devolatilization reactor is 2.2-4.0, and the concentration of caprolactam monomer and its oligomer is 0.1-2% of the total mass of the melt.

3. The process simulation method according to claim 1 or 2, characterized in that: The polyamide 6 is polyamide 6 and its modified product obtained by copolymerization of caprolactam or caprolactam and other modified monomers; The melt-spinnable polyamide 6 is a polyamide 6 with a volatile component content of less than 2%.

4. The process simulation method according to claim 1, characterized in that: The polymerization kinetic model also includes the ring-opening and addition reactions of oligomers and the production of cyclic oligomers; Carrying out the polymerization reaction kinetic model analysis comprises the following steps: (1) Determine the main reaction and side reaction in the polymerization process: Use the "chain segment analysis" method to simplify the reaction into a limited number of basic chain segment units, with the chain growth reaction as the leading factor, and determine the reaction equations for generating terminal amine groups, terminal carboxyl groups, oligomers, and water; (2) Determination of reaction rate constant and equilibrium constant: Describe the effect of temperature on the reaction rate constant using the Arrhenius equation, specify the pre-exponential factor and activation energy in the equation, determine the reversible reaction equilibrium constant, and solve for the forward and reverse reaction rate values; (3) Establishing reaction kinetic equations: Use the reaction kinetic model to analyze the reaction kinetic equations of each component, and compile and import the established equation group into an executable function script file in the numerical calculation software.

5. The process simulation method according to claim 1, characterized in that: The gas-liquid two-phase mass transfer analysis comprises the following steps: (1) According to the component targets to be analyzed in the gas-liquid phase, select the appropriate physical property model and set the relevant property values, including relative molecular mass, critical temperature, pressure, compressibility, gas-liquid phase pressure and molar volume, polymer molecular weight and relative viscosity parameters, and store the properties in the global data array; (2) A simulation analysis method for the devolatilization process of small molecules in high-viscosity polymer melts. According to the gas-liquid phase equilibrium process, the volatile components in the system are determined, the gas-liquid equilibrium equation at the interface is constructed by calling the attribute groups of each component, and the corresponding settings are made in the numerical calculation software; The volatile components are specifically: according to the significant influence of each volatile component on the reaction process, the volatile components in the gas phase include caprolactam and water, and the liquid phase includes caprolactam, water, and oligomers, and the phase equilibrium process parameters are set for the volatile components in the gas and liquid phases; (3) Determine the equipment structure and process parameters of the falling film devolatilization process reactor that affect the gas-liquid mass transfer rate, introduce the mass transfer coefficient to calculate the mass transfer flux of the gas phase components, and iteratively calculate the mass transfer coefficient based on the gas phase mass transfer flux during the operation of the reactor in industry; (4) Initializing the output attribute groups of the physical properties of each component to complete the iterative solution calculation and obtain the mass transfer flux of the gas phase volatile components during the falling film devolatilization reaction of polyamide 6.

6. The process simulation method according to claim 1, characterized in that: The falling film fluid dynamics behavior analysis includes the following steps: determining the molecular diffusion coefficient in the flow process according to the low-speed laminar polymer motion, establishing the convection-diffusion equation, and setting relevant model parameters in the numerical calculation software, including material flow rate, dynamic viscosity and flow velocity of micro-element in the free falling liquid film area.

7. The process simulation method according to claim 1, characterized in that: The construction of the polyamide 6 falling film devolatilization reactor model includes the following steps: (1) The volume microelement mass balance equation of the falling film devolatilization reactor in the downward flow direction was established based on the polymerization reaction kinetics, gas-liquid two-phase mass transfer kinetics and falling film fluid dynamics; (2) Setting boundary conditions according to the operating parameters of the falling film devolatilization reactor in industry, selecting the discrete format, and using the numerical integration method to solve the material balance equation; (3) Define and initialize variables, constants, and parameters, use the software solver to solve the material balance equation, and output key process performance indicators for the gas and liquid phases.

8. The process simulation method according to claim 1, characterized in that: The whole process of constructing the polyamide 6 falling film devolatilization reaction comprises the following steps: (1) calling the automatic control interface of the process simulation software to import the discharge data output from the preceding reaction process of the falling film devolatilization reactor as the model input value, running the numerical calculation software to calculate the material balance equation, and obtaining the data of the performance index changing with the reaction progress; (2) Create a component object model object for process simulation, open and run the model file, set the user interface visibility, and run it in the numerical calculation software to obtain the output information of each component.

9. The process simulation method according to claim 8, characterized in that: The performance indicators include one or more of the number average molecular weight, relative viscosity, monomer content, oligomer content, and terminal carboxyl group and terminal amine group content of the polymerization product.

10. The process simulation method according to claim 1, characterized in that: The analysis of the simulation results of the falling film devolatilization reaction process of polyamide 6 comprises the following steps: importing the process simulation file into the numerical calculation software for processing, outputting and displaying the acquired data as one or more of a data table, a graph or a curve, observing and analyzing the results of the simulation calculation, and understanding the distribution of the performance indexes of each component in the falling film devolatilization process of polyamide 6.

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