A process simulation method for preparing melt-spinnable polyamide 6 by falling film devolatilization reaction

Abstract

The present invention relates to the field of chemical engineering and discloses a process simulation method for preparing melt-spinnable polyamide 6 by a falling film devolatilization reaction. The present invention utilizes an interactive method of numerical calculation and chemical process simulation software to establish a falling film devolatilization reaction process model. Physical equations such as reaction kinetics, mass transfer dynamics, and fluid dynamics are utilized to perform decoupling calculations on the material balance equation for the falling film devolatilization process of polyamide 6. This effectively and conveniently obtains the distribution of basic physical quantities (such as relative viscosity, molecular weight, end group concentration, monomer content, and oligomer content) during the falling film reaction process, thereby providing excellent guidance for experimental research and industrial design.

Description

Technical Field

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

[0002] Polyamide fiber is the earliest industrialized synthetic fiber, and polyamide 6 (PA6) fiber is the dominant variety. It has excellent wear resistance, elasticity, fatigue resistance, and easy dyeing. It is widely used in home textiles, clothing, carpets, and various 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 continuous melt processing impossible and can only be removed through 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 urgently needs to solve.

[0004] Currently, literature reports have attempted to reduce the content of oligomers or volatile components in polyamide 6 by using low-temperature polymerization, anionic polymerization, and the introduction of other monomers for copolymerization, in order to prepare a polyamide 6 melt suitable for melt spinning. These methods provide a relatively clear understanding of the reaction mechanism of the polymerization system, but generally fail to consider heat and mass transfer during large-scale application, resulting in many uncontrollable factors that prevent the desired effect from being achieved during large-scale application. Another approach is to control the volatile content in polyamide 6 by using new devolatilization reaction equipment. This method has great reference value for industrial production, but the understanding of the reaction laws and the analysis of the polymerization degree and components and their changes during the reaction process remain major challenges. Process simulation can effectively analyze the process and results of the polyamide 6 falling film devolatilization reaction, greatly facilitating the exploration of the effect of the falling film devolatilization reaction.

[0005] The applicant has not found any similar reports through a search of 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 the content of terminal carboxyl and terminal amine groups) during the reaction process under actual falling film devolatilization conditions. This can help understand the reaction patterns 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 melts by falling film devolatilization reaction. Summary of the Invention

[0006] To address the above-mentioned technical problems, the present invention provides a process simulation method for preparing melt-spinnable polyamide 6 by a falling film devolatilization reaction. This method utilizes an interactive method of numerical calculation and chemical process simulation software to establish a falling film devolatilization process model. By utilizing physical equations such as reaction kinetics, mass transfer dynamics, and fluid dynamics, the material balance equation for the falling film devolatilization process of polyamide 6 is decoupled and calculated. This method effectively and conveniently obtains the distribution of basic physical quantities (such as relative viscosity, molecular weight, end group concentration, monomer content, and oligomer content) during the falling film reaction, providing excellent 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 analysis, gas-liquid two-phase mass transfer analysis and falling film fluid dynamics behavior analysis of polyamide 6 were carried out.

[0009] The material balance of the entire polyamide 6 falling film devolatilization reaction process was calculated using a reaction kinetics model, a gas-liquid two-phase mass transfer model, and a fluid dynamics model. Boundary conditions and initial conditions were set, and the material balance equation was solved using numerical calculations to establish a 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 including 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 member capable of supporting the flow of the melt fluid, and a devolatilization port connected to the outside. 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 member, completing the removal of volatiles and molecular chain growth reaction.

[0012] The entire process of the falling film devolatilization reaction of polyamide 6 melt was constructed based on the polyamide 6 reaction production process. The index parameters of the polyamide 6 melt with a certain degree of polymerization obtained after hydrolysis polymerization were used as input values. The partial differential equations for the reaction rate were calculated, and the performance parameters of the polymerization product and the content information of each characteristic component were output. The simulation results of the falling film devolatilization reaction process of polyamide 6 were 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 oligomer thereof 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 oligomer thereof 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%. The low-volatile polyamide 6 melt can be directly melt-spun, or added to fibers, or directly processed into molding equipment to obtain a polyamide 6 product.

[0016] Preferably, the polymerization reaction kinetic model further 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 reactions during the polymerization reaction: 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 dominant factor, and determine the reaction equations for generating terminal amino 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 rates;

[0019] (3) Establishment of reaction kinetic equations: Use the reaction kinetic model to analyze the reaction kinetic equations of each component, and compile 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 target components to be analyzed in the gas-liquid 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 highly viscous polymer melts is used. 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: based on 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 volatile components in the liquid phase include but are 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 reactor equipment structure and process parameters that affect the gas-liquid mass transfer rate during the falling film devolatilization reaction process, 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 attribute 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 low-speed laminar polymer motion 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) Set relevant model parameters in MATLAB, including material flow rate and flow velocity of micro-elements 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 the 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) Boundary conditions were set based on the operating parameters of the falling film devolatilization reactor in industry, a discrete format was selected, and the material balance equation was solved using the numerical integration method (Runge-Kutta method);

[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 construction of the polyamide 6 falling film devolatilization reaction process includes the following steps:

[0036] (1) Construction of the entire falling film devolatilization reaction process: The automatic control interface (Active X) of Aspen Plus was called to import the output data of the process as the model input value, and MATLAB was run to calculate the material balance equation to 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 in 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 information of each component.

[0038] Preferably, the analysis of the simulation results of the falling film devolatilization reaction process of polyamide 6 includes 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 performing 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 comprising the following steps:

[0040] In numerical calculation software, iterative cycle calculations were performed on the falling film devolatilization reactor unit module, systematically adjusting and analyzing the effects of different operating parameters (such as feed relative viscosity, temperature, and pressure) on the reaction performance;

[0041] Through multiple simulation runs, key data of the run (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 present invention has the following beneficial effects:

[0043] (1) The principle of the present invention is based on the high viscosity of the material melt. When the material melt is subjected to the wall-attached falling film flow, the material melt maintains the characteristics of plug flow motion, 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 the polycondensation reaction and the fluid dynamics of high-viscosity materials, the falling film flow is used to enhance the polymer devolatilization. The invention 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 the method of coupling MATLAB and Aspen Plus to establish a model of the falling film devolatilization reaction process of polyamide 6, fully considering the physical and chemical changes inside the reactor, overcoming the shortcomings of large investment and long cycle of experimental research, and can predict the distribution of component performance indicators in the falling film devolatilization reactor, 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 obtained effectively and conveniently, 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 material indicators 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 distribution of performance indicators and process conditions of 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 a breakthrough in the preparation technology of low-volatile polyamide 6 melt, 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 melt-attached falling film flow in the falling film devolatilization reactor;

[0049] Figure 2 This is a flowchart of the numerical calculation workflow;

[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 The axial distribution diagram of 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 the 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] Figure 9 The axial distribution diagram of oligomer content during the falling film devolatilization reaction of polyamide 6 at different temperatures;

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

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

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

[0060] To make the above-mentioned objects, features and advantages of the present invention more clearly understood, the technical solutions of the present invention are further described below through specific embodiments. However, the embodiments of the present invention are not limited to the scope of the embodiments.

[0061] Example 1

[0062] This example simulates the falling-film devolatilization process of polyamide 6 using the interactive interface between Aspen Plus and MATLAB. The simulation primarily involves building a falling-film devolatilization reactor unit module in MATLAB that couples reaction kinetics, gas-liquid two-phase mass transfer kinetics, and falling-film fluid dynamics. After running Aspen Plus, the output is linked to MATLAB to construct the entire falling-film devolatilization process. Finally, the simulation results and process operating conditions are analyzed after running MATLAB.

[0063] First, determine the main and side reactions of the polymerization process: use the "chain link analysis" method to simplify the reaction into a limited combination of basic chain link units, with the condensation chain growth reaction as the dominant factor, and determine the reaction kinetic equations of components such as monomers, water, oligomers, terminal amine groups (-NH2), and terminal carboxyl groups (-COOH). 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 an executable function script file in MATLAB;

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

[0065] Based on the target components to be analyzed in the gas-liquid two-phase structure, an appropriate physical property model is selected and relevant property values ​​are set, including parameters such as relative molecular mass, critical temperature, pressure, compressibility, saturated vapor pressure of the gas-liquid phase, molar volume, polymer molecular weight, and relative viscosity, and the properties are stored in a global data array. For the simulation analysis method of the small molecule devolatilization process of high-viscosity polymer melts, the volatile components in the system are simplified according to the gas-liquid phase equilibrium process, and the property groups of each component are called to construct the gas-liquid equilibrium equation at the interface, and the corresponding settings are made in MATLAB.

[0066] The volatile components are specifically: according to the degree of influence of each volatile component on the reaction process, it will be mainly considered that the volatile components in the gas phase include caprolactam monomer 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.

[0067] Determine the vapor-liquid equilibrium equation at the interface for 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 rate 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 scale 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 set of partial differential equations. The input value of the falling film devolatilization reactor is used as the initial condition, and the final first-order partial derivative of the change of each component is 0, which is the boundary condition. The number of partial differential equations, time step and number of spatial discrete points are set. The material balance equation is discretized using the finite difference method, and the discretized ordinary differential equation is solved using the ODE15 function programming in MATALB.

[0073] See also Figure 2 The above-mentioned objects, features and advantages of the present invention can be more clearly understood. The present invention will be further described in detail with reference to examples below:

[0074] (1) Initially set the reactor length, flow rate, diffusion coefficient, number of spatial discrete points, spatial step, time span, and units within the calculation domain to be consistent with those in Aspen Plus; set the basic function solution 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 uses a combination of reaction kinetics analysis, gas-liquid two-phase mass transfer analysis, and high-viscosity fluid dynamics behavior analysis to describe the changes in 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 entire falling film devolatilization reaction process 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 in the molecular weight, relative viscosity, monomer content, oligomer content, end group content, etc. of polyamide 6 as the reaction progressed.

[0079] This involves interacting with Aspen Plus and MATLAB software. This involves importing data from Aspen Plus's automation interface (Active X) into MATLAB and then running the results. The node path for the output stream is "Aspen.Application.Tree.FindNode('\Data\Streams\POLY\Output\MOLEFLOW\MIXED\CL')." This method effectively achieves simplicity in industrial process calculations.

[0080] See also Figure 3 This novel process for producing polyamide fibers through melt-spinning using a falling film devolatilization reaction replaces multiple processes, including cooling and 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 reduces 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 Figure 3 shows the axial distribution of monomer content (CPL) and oligomer (CO) content in a falling-film devolatilization reactor for polyamide 6. Simulation results show that the monomer and oligomer contents gradually decrease along the axial direction as the reaction proceeds, reaching a monomer content of 0.23% and an oligomer content of 0.115% at the outlet.

[0083] See also Figure 6The axial distribution of the end group content ([NH2], [COOH]) in the falling-film devolatilization reactor for polyamide 6 is shown. Data derived from simulations of the present invention indicate that the end group content gradually decreases as the reaction proceeds. The concentration of terminal amine groups ([NH2]) at the reactor outlet is 21.06 mmol / kg; the concentration of terminal carboxyl groups ([COOH]) is 34.52 mmol / kg.

[0084] See also Figure 7 Figure 3 is an axial distribution diagram of the polymer molecular weight (Mn) in the membrane devolatilization reactor at different temperatures. 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 along 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 Figure 9 The axial distribution diagram of the monomer content (CPL) and oligomer (C0) content in the membrane devolatilization reactor at different temperatures can be concluded from the data obtained from the simulation results of the present invention that as the temperature increases, the monomer content and oligomer content gradually decrease along the axial direction, and the higher the temperature, the less obvious this trend is.

[0086] See also Figure 10 Figure 3 is an 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 100 Pa, the molecular weight reaches a maximum of 22860.4 g / mol. When the reactor devolatilization pressure is further reduced to 50 Pa, the molecular weight almost stops increasing, indicating that the effect of pressure in promoting the forward reaction has reached a maximum at this time.

[0087] See also Figure 11 and 12 Figure 1 shows the axial distribution of monomer content (CPL) and oligomer (CO) content within a membrane devolatilization reactor at different pressure drops. Simulation data from the present invention indicate that higher vacuum levels favor the removal of small molecules. Therefore, as the vacuum level increases, the monomer and oligomer content in the polyamide 6 melt decreases.

Claims

1. A process simulation method for preparing melt-spinnable polyamide 6 by falling film devolatilization reaction, characterized in that include: Conducted polymerization reaction kinetic model analysis, gas-liquid two-phase mass transfer analysis, and falling film fluid dynamics analysis on polyamide 6; The material balance of the entire polyamide 6 falling film devolatilization reaction process was calculated using a reaction kinetics model, a gas-liquid two-phase mass transfer model, and a fluid dynamics model. Boundary conditions and initial conditions were set, and the material balance equation was solved using numerical calculations to establish a 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 of aminocaproic acid condensation polymerization reaction; The falling film devolatilization reactor is provided with a melt cavity, a film structure, a falling film support member capable of supporting the flow of the melt fluid, and a devolatilization port connected to the outside world. The polyamide 6 prepolymer / condensation polymer melt after hydrolysis and polymerization flows downward along the falling film support member while completing the removal of volatiles and molecular chain growth reaction. The entire process of the falling film devolatilization reaction of polyamide 6 melt was constructed based on the polyamide 6 reaction production process. The index parameters of the polyamide 6 melt obtained after hydrolysis polymerization were used as input values. The partial differential equations for the reaction rate were calculated, and the performance parameters of the polymerization product and the content information of each characteristic component were output. The simulation results of the falling film devolatilization reaction process of polyamide 6 were analyzed.

2. The process simulation method according to claim 1, wherein: 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 oligomers 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, wherein: 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 includes 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 main factor, and determine the reaction equations for the generation of terminal amino 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 rates; (3) Establishment of reaction kinetic equations: Use the reaction kinetic model to analyze the reaction kinetic equations of each component, and compile the established equation group into an executable function script file in the numerical calculation software.

5. The process simulation method according to claim 1, wherein: The gas-liquid two-phase mass transfer analysis comprises the following steps: (1) According to the target components 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 highly viscous polymer melts is used. 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: based on 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 reactor equipment structure and process parameters that affect the gas-liquid mass transfer rate during the falling film devolatilization reaction process, 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) Initialize the output attribute 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.

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

7. The process simulation method according to claim 1, wherein: Constructing the polyamide 6 falling film devolatilization reactor model comprises the following steps: (1) Based on the polymerization reaction kinetics, gas-liquid two-phase mass transfer kinetics, and falling film fluid dynamics, a volume microelement mass balance equation for the downward flow direction of the falling film devolatilization reactor was established; (2) Setting boundary conditions based on the operating parameters of the falling film devolatilization reactor in industry, selecting a discrete format, and using numerical integration methods 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, wherein: 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 output data of 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 process; (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, wherein: 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, wherein: The analysis of the simulation results of the falling film devolatilization reaction process of polyamide 6 includes 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.

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