A quick calculation method for the production capacity of a wiped film evaporator for producing lyocell fibers
By simplifying the physical model and improving the Lee phase change model, and combining the VOF and SST k-models, the accuracy and efficiency issues of capacity calculation for scraped film evaporators were resolved, enabling rapid and accurate capacity prediction.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- DONGHUA UNIV
- Filing Date
- 2026-05-07
- Publication Date
- 2026-06-05
AI Technical Summary
Existing methods for calculating the capacity of scraped film evaporators lack accuracy, and the adjustment coefficient of the Lee phase change model is difficult to determine, resulting in inaccurate simulation results and long calculation cycles, which cannot quickly and effectively guide production.
A simplified physical model and an improved Lee phase transition model are adopted. The phase transition adjustment coefficient is replaced by the interface temperature difference. CFD simulation calculations are performed by combining the VOF multiphase flow model and the SST k-model, which eliminates the manual parameter tuning step and improves the calculation efficiency.
It achieves rapid and accurate calculation of the production capacity of scraped film evaporators, reduces the number of simulation iterations, and reduces the error to less than 7%, providing a fast calculation tool for production.
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Figure CN122154564A_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of lyocell fiber production technology and relates to a method for rapidly calculating the production capacity of a scraped film evaporator for producing lyocell fibers. Background Technology
[0002] The scraped-film evaporator is the core equipment in the production of lyocell fibers. It efficiently removes water from NMMO aqueous solutions, dissolving the cellulose to obtain the spinning solution. In this process, the evaporation rate is a key indicator of the equipment's production capacity. Numerical simulation of the internal evaporation process of the scraped-film evaporator is crucial for accurately and quickly calculating production capacity during the equipment design and manufacturing process, as it serves as a convenient tool for further guiding production.
[0003] Numerous scholars have proposed various empirical formulas to evaluate the evaporation capacity of evaporators. However, due to the differences in operating conditions across various production sectors, these formulas lack universality and are difficult to directly apply. Subsequently, experimental platforms have gradually become an important means of studying evaporation performance, capable of revealing the impact of various parameter changes on evaporation effects more accurately. However, this method is usually only applicable to research and cannot directly calculate engineering indicators such as annual production capacity. Furthermore, scraped film evaporators typically have an external heating jacket, and the opaque structure makes it difficult to directly observe the internal operating status, hindering in-depth understanding of the evaporation mechanism. At the same time, the construction of experimental platforms often consumes significant resources, limiting their guidance for parameter optimization in practical engineering.
[0004] With the development of computer technology, the accuracy and operability of numerical simulation methods have been significantly improved, and they have now become an important tool widely used by researchers in related fields. Some researchers have directly modeled and calculated scraped film evaporators, and the reliability of their calculation results has been improved. Although simulations that reproduce the actual model can calculate the annual production capacity, the large size of the evaporator requires a large number of meshes, and the quality is difficult to guarantee. The modeling is also quite complex, and the calculation cycle is very long.
[0005] In the process of CFD simulation calculation of the capacity of a scraped film evaporator, the modeling usually requires the introduction of the Lee phase change mass transfer model to calculate the evaporation rate. The accuracy of this model is directly related to whether the capacity, internal temperature field, dryness distribution, gas-liquid phase structure and overall heat transfer process can be reliably simulated, and therefore it is a key factor affecting the credibility of the simulation results.
[0006] In traditional simulations, the adjustment coefficients of the Lee phase change model are often determined based on experimental data or researcher experience. This trial-and-error approach to coefficient adjustment is not only time-consuming and labor-intensive, but also, when developing new scraped film evaporator structures, the lack of corresponding experimental data makes it difficult to predetermine suitable coefficient values, leading to inaccurate prediction of equipment capacity and becoming a major bottleneck for achieving efficient simulation and design optimization. The Lee phase change model uses the difference between the interface temperature and the saturation temperature as the driving force for phase change. Due to its clear theory and ease of coupling with multiphase flow models such as VOF and Euler, it is widely used for simulating boiling, evaporation, and condensation processes. However, the most prominent problem in practical applications of this model is the significant uncertainty in the value of its adjustment coefficients. As a regulator of the mass transfer rate, the adjustment coefficient mainly drives the phase change process by controlling the temperature deviation (i.e., the deviation between the interface temperature and the saturation temperature). Although some studies suggest setting its default value to 0.1 and fine-tuning it based on specific conditions, due to differences in fluid properties and operating conditions, the reasonable range of values for this coefficient can span several orders of magnitude (e.g., from 0.1 to 10). 7 This presents a significant challenge to the practical application of the model.
[0007] The main reasons why the adjustment coefficient in the Lee phase transition model is difficult to determine are: (1) Unknown key parameters: The model involves key parameters such as bubble diameter and adjustment coefficient, which are usually difficult to obtain accurately before actual calculation; (2) Overly idealized physical assumptions: The theoretical expression of the Lee model is based on several idealized assumptions, such as flat interphase interfaces and dispersed phase systems with equal diameters. These assumptions are difficult to fully hold in reality, resulting in deviations between theoretical and actual values; (3) Extremely wide range of values: The range of values for the adjustment coefficient is very wide in different studies, which indicates that it lacks a unified standard for determination and needs to be fine-tuned according to specific problems; it needs to rely on experimental data for calibration; Due to the above reasons, the adjustment coefficient usually cannot be directly calculated theoretically and must be repeatedly fine-tuned through experimental data to match actual physical phenomena. This process is both time-consuming and lacks universality. Literature review shows that the phase transition adjustment coefficient in the Lee model is almost always assumed to be a constant value, which empirically depends on experimental data or numerical experiments. More methods need to be proposed to solve this weakness, which will help make numerical analysis more reasonable and effective.
[0008] CN115563895A discloses a simulation method for visualizing flow boiling using a VOF visualization model. A mesh model is established using ICEM software, and a physical model for flow boiling visualization is built using ANSYS-Fluent software. The model includes a VOF model, a Lee phase transition model, and a CSF (continuum surface force) surface tension model. The Lee phase transition model is imported into the UDF (User-Defined Function) interface, and simulation calculations are performed after completing the simulation settings. Finally, the model's validity is verified based on experimental results. This invention achieves visualization of flow boiling, providing a simulation method for in-depth research on flow boiling. The invention sets the phase transition intensity control factor of the Lee phase transition model to 0.1, but does not explain the reason or method for setting this value.
[0009] CN120068581A discloses a multi-objective optimization analysis method and system for visualizing flash boiling waste heat recovery. This invention integrates the VOF model and an improved Lee model to more accurately describe the flash process, featuring high heat recovery efficiency and high water resource recovery rate. However, the invention does not mention how the phase change adjustment coefficient in the Lee model used is determined.
[0010] The literature (Analysis of the value of mass transfer coefficient in Lee's phase change mass transfer equation [J]. Journal of Harbin Institute of Technology, 2014, 46(12):15~19.) addresses the problem that the phase change adjustment coefficient in Lee's phase change mass transfer equation mainly relies on empirical values. This paper proposes a method to determine the phase change adjustment coefficient, providing two indicators for evaluating the rationality of the mass transfer coefficient: latent heat share (the proportion of latent heat transfer to total heat transfer) and saturation temperature difference (the difference between fluid temperature and saturation temperature). A steady-state analysis model is established, and theoretical expressions for latent heat share and saturation temperature difference are derived. Based on these, the influence of the phase change adjustment coefficient on the simulation results is analyzed. The theoretical solutions under specific conditions verify the correctness of the analysis results. The analysis results show that the larger the phase change adjustment coefficient, the higher the calculation accuracy. Based on the analysis results, the reasons for the large differences in the values of this coefficient in different literatures are explained, and a general method for determining the coefficient is given. Finally, a reasonable range of values for the mass transfer coefficient in common operating condition simulations is recommended. This paper presents a method for determining the phase change mass transfer coefficient, which narrows the range of values and reduces the time spent exploring the phase change adjustment coefficient, but it still fails to meet the requirement of accuracy.
[0011] CN118504365A discloses a method for estimating the annual production capacity of a scraped film evaporator when processing new materials. This invention simplifies the physical model, reduces the number of computational grids, and reduces the amount of computation. However, the evaporation coefficient used needs to be obtained by processing historical material data, which is time-consuming.
[0012] CN109887551B discloses a modeling method for mass transfer control under pure pneumatic operation conditions in MIHA (Mild Hybrid Reactor). By analyzing the bubble generation process under pure pneumatic conditions, an energy conversion model is established within a bubble breaker. Based on the energy conversion model within the bubble breaker and liquid circulation, the liquid flow rate is calculated, and the energy dissipation rate and bubble size in the intense gas-liquid mixing zone are obtained, ultimately leading to a mass transfer calculation model. This invention establishes a mass transfer control model for MIHA under pure pneumatic operation conditions, comprehensively reflecting the influence of reactor structure, system properties, operating parameters, and input energy on mass transfer. It can provide guidance for reactor design and MIHA reaction system design, guiding the design of efficient reactor structures and reaction systems. This invention obtains a rate and conversion rate model through bubble generation → energy conversion → mass transfer → reaction, and predicts reaction rate and conversion rate. However, some key parameters rely on empirical determination, such as bubble diameter estimated empirically and constants directly specified for physical property parameters. The model established by this invention has strong adjustability but weak predictive ability, failing to supplement a modeling approach with strong generalizability.
[0013] CN107346378B discloses a modeling method for controlling the mass transfer rate in a micro-interface enhanced reactor. Through rigorous theoretical derivation, it establishes calculation models for both the gas-side and liquid-side mass transfer coefficients. The mass transfer rate structure-performance control model constructed using this invention can intuitively show the relationship between the mass transfer rate and bubble size, laying a theoretical foundation for the study of micro-interface systems. It also allows for the maximization of energy and material efficiency in the reaction process by adjusting structural and operational parameters, or the design of highly efficient reactor structures given reaction targets and energy and material consumption. However, this model is based on assumptions such as uniform flow and ideal bubble distribution, which deviates significantly from actual complex multiphase flows. Furthermore, the model's calculation chain is long, parameter dependencies are strong, and errors are easily amplified step by step, leading to insufficient reliability of the prediction results.
[0014] Therefore, it is of great significance to study a rapid calculation method for the production capacity of a scraped film evaporator for producing lyocell fibers in order to solve the problems existing in the prior art. Summary of the Invention
[0015] The purpose of this invention is to solve the problems existing in the prior art and provide a method for rapidly calculating the production capacity of a scraped film evaporator for producing lyocell fibers.
[0016] To achieve the above objectives, the technical solution adopted by the present invention is as follows:
[0017] A rapid calculation method for the production capacity (evaporation rate) of a scraped film evaporator for producing lyocell fibers is proposed. A simplified physical model of the scraped film section is created using Fluent meshing software and imported into the CFD simulation software (Fluent). Material property parameters and operating conditions are set in the CFD simulation software. Based on these parameters, the appropriate mathematical model is determined to be the VOF multiphase flow model (existing technologies include various multiphase flow models; this invention selects the most suitable VOF model based on the operating conditions. When two-phase fluid flows within the scraped film evaporator, the VOF model can effectively monitor the volume fraction of each fluid in the two phases). SST k- The model (existing viscosity models also include various types; this invention also selects the most suitable SST k- based on the working conditions) This model considers the transport process of turbulent shear stress, which broadens its applicability compared to previous models, especially when heat transfer and shear flow at the wall are important. Therefore, to better capture the evaporative heat transfer of the near-wall liquid film, this invention employs SST k- The model) and the improved Lee phase transition model were used. Finally, the boundary conditions were set according to the actual working conditions and material properties, and the evaporation rate of the scraped film evaporator was obtained by simulation calculation.
[0018] The formula for the improved Lee phase transition model is as follows:
[0019] ;
[0020] Where s is the mass source phase, This represents the volume fraction of the liquid. Let be the thermal conductivity of the liquid film. The wall temperature, The saturation temperature This is the difference between the gas-liquid interface temperature and the fluid saturation temperature. Latent heat of vaporization For liquid film thickness, The fluid temperature.
[0021] As a preferred technical solution:
[0022] The simplified physical model determination process for the scraped film evaporator used in the production of lyocell fibers, as described above, is as follows:
[0023] (a) Obtain the overall model of the scraped film section of the scraped film evaporator, and simplify and shorten the overall model in the axial direction to obtain an axially simplified model with a total height of h; the value of h ranges from 20% to 90% of the overall model height;
[0024] A scraped film evaporator can generally be divided into the following sections along its axial direction: an inlet development section, a main stable section, and an outlet disturbance section. This invention focuses on the study of evaporation rate, which is related to the cumulative change in height and cannot be arbitrarily shortened. The simplified physical model should retain a sufficient length of the inlet development section while extracting a representative stable section in the middle. In other words, the simplified height must at least satisfy the condition that the area under study has escaped the end effect of the inlet development section.
[0025] Engineering experience generally suggests that the proportions of the inlet development section, the main stabilization section, and the outlet disturbance section in the equipment height are 10%~20%, 60%~80%, and 5%~10%, respectively. Therefore, the height of the simplified physical model should be at least 20% of the total equipment height and at most 90%, meaning the height of the simplified physical model is in the range of 20%~90%.
[0026] (b) In the simplified axial model with a total height of h, a certain number of radial feature sections A are cut out, and the feature sections from top to bottom are A1, A2, A3...;
[0027] (c) Statistically calculate the evaporation rate at each characteristic section and fit the data to derive the corresponding relationship between the axial position and the evaporation rate at each radial interface:
[0028] ;
[0029] Where L is the axial position coordinate. This represents the evaporation rate on the corresponding radial cross section.
[0030] (d) Substitute the axial position coordinates of the outlet section into the relationship in (c) to obtain the evaporation rate of the outlet section. At the same time, compare it with the evaporation rate of the outlet section under the overall model. If the error is less than 20%, then the axial simplified model with a total height of h obtained in (a) is the simplified physical model of the scraping film section.
[0031] The method for rapidly calculating the production capacity of a scraped film evaporator for producing lyocell fibers, as described above, includes material properties such as specific heat capacity, standard enthalpy, viscosity, density, thermal conductivity, and saturation temperature, and operating parameters such as rotational speed, feed rate, and feed temperature.
[0032] The above-described method for rapid calculation of the production capacity of a scraped film evaporator for producing lyocell fibers involves compiling an improved Lee phase change model into a UDF program and then importing it into CFD simulation software via the UDF interface.
[0033] The above describes a rapid calculation method for the capacity of a scraped-film evaporator for producing lyocell fibers, with the following boundary conditions: Where x represents the coordinate along the thickness direction of the liquid film. Indicates the velocity of the liquid.
[0034] The above-described method for rapid calculation of the capacity of a scraped-film evaporator for producing lyocell fibers yields the following simulation calculation: Evaporation rate V2 of the scraped-film evaporator is:
[0035] ;
[0036] in, The inlet mass flow rate in the simulation. The outlet mass flow rate in the simulation.
[0037] The method described above for rapidly calculating the capacity of a scraped film evaporator for producing lyocell fibers shows that the error between the simulated evaporation capacity V2 and the theoretical evaporation capacity V1 is less than 7%.
[0038] The method described above for rapidly calculating the production capacity of a scraped film evaporator for producing lyocell fibers uses the interface temperature difference as a controllable parameter to replace the phase change adjustment coefficient in the traditional Lee model, omitting the manual parameter adjustment step, and the number of simulation iterations does not exceed 24,000 steps. In contrast, the traditional Lee model requires a large amount of manual parameter adjustment time and the number of simulation iterations will exceed 30,000 steps.
[0039] The principle behind the original Lee model for calculating evaporation:
[0040] The original Lee model incorporates the mass and energy transfer of the phase transition into the mass transfer and energy equations in the form of source terms:
[0041] (1);
[0042] Energy equation:
[0043] (2);
[0044] Where S is the mass source term of the original Lee model (phase change mass rate per unit volume). This is the phase transition adjustment coefficient. For fluid density, For fluid viscosity, The effective thermal conductivity of the turbulent fluid. For the velocity of the liquid; the first term on the right side of the equation. For sensible heat transfer, For latent heat exchange;
[0045] From the energy equation, the heat Q required for evaporation is:
[0046] Q= (3);
[0047] Q = U × A × ΔT; (4);
[0048] S=U×A×ΔT / (5);
[0049] in, U is the phase change mass rate per unit volume, A is the heat transfer area, and ΔT is the heat transfer temperature difference.
[0050] Due to differences in fluid properties and operating conditions, during the simulation process... The reasonable range of values for this coefficient can span multiple orders of magnitude (e.g., from 0.1 to 10). 7 This makes adjusting to a reasonable coefficient time-consuming and laborious.
[0051] Regarding the phase change adjustment coefficient To address the difficulty in determining the exact temperature, this invention proposes introducing an interfacial temperature difference (the difference between the gas-liquid interface temperature and the fluid saturation temperature). Alternative phase change adjustment coefficient The method used to optimize the original Lee model is as follows:
[0052] The establishment of the improved model mainly consists of three steps: 1. Liquid film flow analysis: Based on the momentum balance relationship of the liquid film micro-element, the velocity distribution inside the liquid film is derived, and the mass flow rate along the wall direction is further obtained; 2. Liquid film heat transfer analysis: Through energy conservation, the increase in liquid film mass due to condensation phase change is linked to the latent heat carried away by the wall, thereby establishing the relationship between liquid film thickness and heat transfer process; 3. The phase change mass source term of the Lee model is introduced into the energy conservation equation, and combined with the aforementioned liquid film flow and heat transfer analysis results, the new phase change adjustment coefficient proposed in this invention is obtained.
[0053] Specifically, the first step is liquid film flow analysis, which obtains the velocity distribution and mass flow rate from the liquid film momentum balance;
[0054] For the liquid within the controlled volume in a scraped film evaporator, the momentum conservation equation is:
[0055] (6);
[0056] in: It refers to the viscosity of the liquid; It is the velocity of the liquid; It is the density of the liquid; It is gravitational acceleration; It's pressure;
[0057] In most cases, it is assumed that: Then the above formula simplifies to:
[0058] (7);
[0059] The boundary conditions are: (8);
[0060] Derivation of liquid film thickness:
[0061] Integrating the above momentum conservation equation (7) twice yields:
[0062] (9);
[0063] Apply boundary conditions:
[0064] 1. When x=0, =0, then =0;
[0065] 2. When hour, ,but:
[0066] (10);
[0067] Therefore, the velocity distribution within the liquid film along the y-direction is as follows:
[0068] (11);
[0069] Mass flow rate per unit thickness at position y. for:
[0070] (12);
[0071] Integrating equation (12) yields:
[0072] (13);
[0073] Therefore, the mass flow rate per unit thickness is: .
[0074] The second step: liquid film heat transfer analysis, which uses energy balance to link the "mass increase due to phase change" with the "latent heat carried away by the wall".
[0075] The relationship between liquid film thickness and heat transfer: the wall surface transfers latent heat to the liquid film. Assuming that heat convection in the liquid film is negligible and heat is transferred only through conduction, according to the law of conservation of energy, the mass evaporated per unit length is equal to the latent heat of phase change absorbed.
[0076] (14);
[0077] The left side of equation (14) represents the mass of evaporation, and the right side represents the latent heat of phase change. Wherein: It is the thermal conductivity of the liquid film; It is the interface temperature; It is the wall temperature.
[0078] Mass per unit width: Substituting into equation (14), we get:
[0079] liquid film thickness :
[0080] (15);
[0081] Step 3: Write the source terms of the Lee model into the energy conservation equation and solve for the new phase transition adjustment coefficient obtained in this invention;
[0082] According to the original Lee model, the quality source term at the interface is: (16);
[0083] in: It is the phase transition adjustment coefficient; It is the volume fraction of the liquid; It is the density of the liquid; It is the saturation temperature; It refers to the fluid temperature.
[0084] According to the law of conservation of energy and Fourier's law:
[0085] (17);
[0086] Integrating equation (17) in the x-direction yields a new phase transition adjustment coefficient:
[0087] The left side of equation (17) represents the latent heat source in the Lee model, and the right side represents Fourier conduction. Both sides of equation (17) By directly canceling out the integral over the x-direction (which is the film thickness direction), we essentially incorporate the thickness of the interface elements in the normal direction into equation (17), leaving only the result. The function.
[0088] (18);
[0089] in, It is the difference between the gas-liquid interface temperature and the fluid saturation temperature.
[0090] By incorporating the new phase transition adjustment coefficient into the original Lee model, we obtain the improved Lee phase transition model of this invention:
[0091] (19).
[0092] Beneficial effects:
[0093] This invention provides a rapid method for calculating the production capacity of a scraped film evaporator for producing lyocell fibers. It uses the interface temperature difference as a controllable parameter to replace the phase change adjustment coefficient in the traditional Lee model, omitting the manual parameter adjustment step. The improved Lee phase change model and simplified physical model adopted provide a fast and accurate method for calculating the production capacity of the scraped film evaporator. Attached Figure Description
[0094] Figure 1 A schematic diagram of the process of establishing the improved Lee phase change model; where (a) is a schematic diagram of the liquid film inside the scraped film evaporator, (b) is a schematic diagram of the liquid film force balance, and (c) is a schematic diagram of heat transfer inside the liquid film;
[0095] Figure 2 For simulation calculation of physical models;
[0096] Figure 3 A comparison of pressure and temperature in the evaporation environment near the liquid film for the original model (original Lee model) and the improved model (improved Lee phase transition model);
[0097] Figure 4 A comparison diagram of the pressure ratios of the evaporation environment near the liquid film in the original model and the improved model;
[0098] Figure 5 A comparison diagram of the material temperature (i.e., the material temperature near the wall) and the saturation temperature in the axial direction of the original model and the improved model;
[0099] Figure 6 This relates the axial position to the evaporation rate.
[0100] Figure 7 To compare the simulated evaporation rate with the theoretical evaporation rate;
[0101] Figure 8 A comparison of the simulation iteration steps between the original model and the improved model. Detailed Implementation
[0102] The present invention will be further described below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.
[0103] A rapid calculation method for the production capacity of a scraped-film evaporator for producing lyocell fibers, the specific steps of which are as follows:
[0104] (1) The process of determining the simplified physical model of the scraping section is as follows:
[0105] (a) such as Figure 2As shown, the overall model of the scraped film section of the scraped film evaporator is obtained, and the overall model is simplified and shortened in the axial direction to obtain an axially simplified model with a total height of h.
[0106] (b) In the simplified axial model with a total height of h, a certain number of radial feature sections A are cut out, and the feature sections from top to bottom are A1, A2, A3...;
[0107] (c) Statistically calculate the evaporation rate at each characteristic section and fit the data to derive the corresponding relationship between the axial position and the evaporation rate at each radial interface:
[0108] ;
[0109] Where L is the axial position coordinate. This represents the evaporation rate on the corresponding radial cross section.
[0110] (d) Substitute the axial position coordinates of the outlet section into the relationship in (c) to obtain the evaporation rate of the outlet section. At the same time, compare it with the evaporation rate of the outlet section under the overall model. If the error is less than 20%, then the axial simplified model with a total height of h obtained in (a) is the simplified physical model of the scraping film section.
[0111] (2) The simplified physical model of the scraping section is established by using fluent meshing software to create a mesh model and then imported into the CFD simulation software. Material property parameters and working condition parameters are set in the CFD simulation software.
[0112] Material properties include: specific heat capacity, standard enthalpy, viscosity, density, thermal conductivity, and saturation temperature. Operating parameters include: rotational speed, feed rate, and feed temperature.
[0113] (3) Based on the material properties and operating parameters, the mathematical model used is determined to be the VOF multiphase flow model and the SST k- model. The model and the improved Lee phase transition model are compiled into a UDF program and then imported into the CFD simulation software through the UDF interface;
[0114] The formula for the improved Lee phase transition model is as follows:
[0115] ;
[0116] Where s is the mass source phase, This represents the volume fraction of the liquid. Let be the thermal conductivity of the liquid film. The wall temperature, The saturation temperature This is the difference between the gas-liquid interface temperature and the fluid saturation temperature. Latent heat of vaporization For liquid film thickness, For fluid temperature;
[0117] Figure 1 A physical schematic diagram of the improved Lee phase transition model is given. (a) shows the overall schematic diagram of the liquid film on the wall. (a) is the wall temperature, T is the fluid temperature, Ti is the gas-liquid interface temperature, x is perpendicular to the wall and points from the wall towards the vapor zone, and y is downward along the wall, which is also the direction of liquid film development; (b) is a schematic diagram of the flow analysis of the liquid film, in which a small control volume dx×dy is taken from the liquid film to analyze the forces acting on the liquid film, where Gravity and viscous shear Pressure , (c) is an analysis of the heat transfer process within the liquid film, showing the absorption of latent heat at the left interface. Heat is conducted to the liquid film.
[0118] Boundary conditions are Where x represents the coordinate along the thickness direction of the liquid film. Indicates the velocity of the liquid;
[0119] (4) Set the boundary conditions according to the actual working conditions and material properties, and perform simulation calculations to obtain the evaporation capacity V2 of the scraped film evaporator;
[0120] The evaporation rate V2 of the scraped film evaporator obtained from simulation calculation is:
[0121] ;
[0122] in, The inlet mass flow rate in the simulation. The outlet mass flow rate in the simulation;
[0123] (5) Calculation of theoretical evaporation (the theoretical evaporation is the amount of evaporation required to achieve the corresponding production capacity).
[0124] The exported NMMO content all exceeded 75%, which can effectively dissolve cellulose and meet the quality requirements of the spinning solution. The theoretical evaporation rate was calculated as follows:
[0125] ;
[0126] ;
[0127] ;
[0128] in, This represents the import quality flow rate in actual production. Export quality flow rate in actual production; The percentage of imported NMMO by weight. The proportion of exported NMMO quality, and ≥75%;
[0129] At this theoretical evaporation rate, the corresponding production capacity can be achieved at this feed rate, the cellulose can be basically completely dissolved, and the spinning solution meets the quality requirements.
[0130] The number of simulation iterations does not exceed 24,000 steps; the error between the evaporation capacity V2 obtained from the simulation calculation and the theoretical evaporation capacity V1 is less than 7%.
[0131] The following specific embodiments illustrate a rapid calculation method for the capacity of a scraped-film evaporator for producing Lyocell fibers, using a production of 50,000 tons / year of Lyocell spinning solution as an example:
[0132] (1) The process data are as follows:
[0133] Imported components: cellulose / NMMO / water (11.7wt% / 67.19wt% / 21.11wt%); Exported components: cellulose / NMMO / water (12.90wt% / 76.21wt% / 10.89wt%); Zero-shear viscosity: 4000 Pa·s; Feed rate: 2.127 g / s; Liquid density: 1156 kg / m³ 3 Rotational speed: 90 rpm; Feed temperature: 87℃; Wall temperature: 126℃; Specific heat capacity: 2500 J / (kg·℃); Thermal conductivity: 0.5 W / (m·℃); Standard enthalpy: 75.4 KJ / mol; Saturation temperature variation with axial position: L is the axial position coordinate.
[0134] (2) Model validation standard one:
[0135] The evaporation environment pressure ratio refers to the ratio of the actual vapor pressure to the saturation pressure corresponding to the temperature. This parameter reflects the driving force and limiting conditions of the evaporation system, and its value directly affects the evaporation rate, phase change behavior, and energy consumption. For the improvement to be effective, the evaporation environment pressure ratio of the improved model should be basically consistent with that of the original model. Figures 3-4 The data shown are the pressure, temperature, and evaporation environment pressure ratio under the two models. It can be seen that the evaporation environment pressure ratio differs by 0.2% between the two models. The optimization of the mass transfer model did not significantly change the mass transfer behavior near the liquid film, indicating the reliability of the improved Lee phase change model.
[0136] (3) Model Validation Standard Two:
[0137] The Lee phase transition model primarily drives the phase transition process by controlling the temperature deviation (i.e., the deviation between the interface temperature and the saturation temperature). During the boiling evaporation process of the material in the scraped film evaporator, the fluid temperatures of the two phases should be close to the saturation temperature, that is, the saturation temperature difference ΔT (the difference between the fluid temperature and the saturation temperature) should be close to 0. This invention uses the saturation temperature difference as an indicator to verify the accuracy of the improved Lee phase transition model. Only when the saturation temperature difference ΔT is close to 0 can it be said that the improved model is basically effective.
[0138] In an evaporator, phase transformation is the dominant process. Under this mechanism, the phase interface temperature should approach the saturation temperature, meaning the fluid temperature and the saturation temperature should approach zero. The preset interface temperature difference is... The simulation was performed and compared with the original model. Figure 5 In the middle (a), the material temperature and saturation temperature in the axial direction are shown in the original Lee model. Figure 5 (b) shows the material temperature and saturation temperature in the axial direction under the improved Lee phase change model. The results of the improved Lee phase change model show that the fluid temperature is closer to the saturation temperature, and the simulation results are closer to the actual heat exchange conditions.
[0139] (4) Model Validation Standard Three:
[0140] The improved Lee phase change model can only be considered correct if the evaporation rate at the outlet section conforms to the following corresponding relationship.
[0141] ;
[0142] Simulation calculations were performed on a 1m model, and 10 characteristic sections were taken from 0 to 1m along the axial height of the model. The corresponding relationship between the axial position and the evaporation amount at each radial interface was fitted, as shown in the following formula. Figure 6 As shown, the evaporation at the outlet section 1m differs from the evaporation at 1m in the formula by 3.1%. This simplification method in the axial direction is verified to be effective. The 1m model can be further simplified in axial height (simplified to 0.7m), that is, the overall model height H is 1m, and the total height h of the axially simplified physical model is 0.7m, thereby reducing the simulation calculation time.
[0143] Figure 7 The simulation results based on this invention are presented, and the comparison between the calculated evaporation and the theoretical evaporation shows that the error is 6.8%. This result verifies that the proposed model has excellent accuracy in calculating evaporation (production capacity). Figure 8To compare the simulation iteration steps of the two models using a simplified model, the improved Lee model requires no more than 24,000 simulation iteration steps, a reduction of 25%. By improving the Lee phase change model and simplifying the physical model, not only is the tedious process of repeatedly adjusting the phase change adjustment coefficient in traditional methods avoided, but a fast and accurate calculation method can also be provided for the capacity prediction of scraped film evaporators.
Claims
1. A method for rapidly calculating the production capacity of a scraped-film evaporator for producing lyocell fibers, characterized in that: A simplified physical model of the scraping section was created using Fluent Meshing software to build a mesh model, which was then imported into CFD simulation software. Material properties and operating parameters were set in the CFD simulation software. Based on these parameters, the mathematical model used was determined to be the VOF multiphase flow model and the SST k-model. The model and the improved Lee phase transition model were used. Finally, the boundary conditions were set according to the actual working conditions and material properties, and the evaporation rate of the scraped film evaporator was obtained through simulation calculation. The formula for the improved Lee phase transition model is as follows: ； Where s is the mass source phase, This represents the volume fraction of the liquid. Let be the thermal conductivity of the liquid film. The wall temperature, The saturation temperature This is the difference between the gas-liquid interface temperature and the fluid saturation temperature. Latent heat of vaporization For liquid film thickness, The fluid temperature.
2. The method for rapid calculation of the production capacity of a scraped-film evaporator for producing lyocell fibers according to claim 1, characterized in that, The simplified physical model of the scraping section is determined as follows: (a) Obtain the overall model of the scraped film section of the scraped film evaporator, and simplify and shorten the overall model in the axial direction to obtain an axially simplified model with a total height of h; the value of h ranges from 20% to 90% of the overall model height; (b) In the simplified axial model with a total height of h, a certain number of radial feature sections A are cut out, and the feature sections from top to bottom are A1, A2, A3...; (c) Statistically calculate the evaporation rate at each characteristic section and fit the data to derive the corresponding relationship between the axial position and the evaporation rate at each radial interface: ； Where L is the axial position coordinate. This represents the evaporation rate on the corresponding radial cross section. (d) Substitute the axial position coordinates of the outlet section into the relationship in (c) to obtain the evaporation rate of the outlet section. At the same time, compare it with the evaporation rate of the outlet section under the overall model. If the error is less than 20%, then the axial simplified model with a total height of h obtained in (a) is the simplified physical model of the scraping film section.
3. The method for rapid calculation of the production capacity of a scraped-film evaporator for producing lyocell fibers according to claim 1, characterized in that, Material properties include: specific heat capacity, standard enthalpy, viscosity, density, thermal conductivity, and saturation temperature. Operating parameters include: rotational speed, feed rate, and feed temperature.
4. The method for rapid calculation of the production capacity of a scraped-film evaporator for producing lyocell fibers according to claim 1, characterized in that, After the improved Lee phase transition model is compiled into a UDF program, it is imported into the CFD simulation software through the UDF interface.
5. The method for rapid calculation of the production capacity of a scraped-film evaporator for producing lyocell fibers according to claim 1, characterized in that, Boundary conditions are Where x represents the coordinate along the thickness direction of the liquid film. Indicates the velocity of the liquid.
6. The method for rapid calculation of the production capacity of a scraped-film evaporator for producing lyocell fibers according to claim 1, characterized in that, The evaporation rate V2 of the scraped film evaporator obtained from simulation calculation is: ； in, The inlet mass flow rate in the simulation. The outlet mass flow rate in the simulation.
7. The method for rapid calculation of the production capacity of a scraped-film evaporator for producing lyocell fibers according to claim 6, characterized in that, The error between the evaporation capacity V2 obtained from the simulation calculation and the theoretical evaporation capacity V1 is less than 7%.
8. The method for rapid calculation of the production capacity of a scraped-film evaporator for producing lyocell fibers according to claim 1, characterized in that, The number of simulation iterations does not exceed 24,000.