Journal of Textile Research ›› 2025, Vol. 46 ›› Issue (01): 16-24.doi: 10.13475/j.fzxb.20231201101

• Fiber Materials • Previous Articles     Next Articles

Process simulation of falling film liquid-state polymerization of polyester

CHEN Shichang1,2(), CAO Junhua1,2, CHEN Wenxing1   

  1. 1. National Engineering Laboratory for Textile Fiber Materials Processing Technology, Zhejiang Sci-Tech University, Hangzhou, Zhejiang 310018, China
    2. Zhejiang Provincial Innovation Center of Advanced Textile Technology, Shaoxing, Zhejiang 312000, China
  • Received:2023-12-08 Revised:2024-04-02 Online:2025-01-15 Published:2025-01-15

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Abstract:

Objective This research develops a comprehensive model for enhancing poly(ethylene terephthalate) (PET) production via liquid-state polymerization. By focusing on falling film flow and process simulation, the model aims to optimize the design of falling film reactors and refine the polymerization process, thereby improving the overall efficiency of industrial PET fiber production.

Method A mathematical model for continuous polymerization was developed, focusing on a six-reactor system including five reactors and an additional falling film reactor, based on a specific industrial setup. This model, calibrated with industrial data, defines the input parameters for the falling film reactor. It analyzes changes in key quality metrics like molecular weight and end-group content and explores the influences of temperature and pressure on these metrics within the reactor.

Results In this study, a mathematical model was constructed for a falling film liquid-phase polymerization reactor, tailored to simulate and analyze the distribution of component concentrations and polymer molecular weights within the reactor under varying operational conditions. The model demonstrated high accuracy, particularly under reaction conditions of 287 ℃ and 100 Pa, successfully yielding a product with an intrinsic viscosity (ηintr) value of 1.004 5 dL/g. The ηintr value at the product outlet showed only a 0.05% deviation, demonstrating the model's precision and its ability to reflect the reactor's production process accurately. The model meticulously details the axial distribution of components, showing a decrease in acid and hydroxyl end groups (Ea) and hydroxyl end group (Eg) along the reactor's length, while vinyl end groups (Ev) and degree of polymerization (DPN) increase almost linearly. Notably, the molecular flow rates of small molecules, ethylene glycol quality flow (QEG) and water quality flow (Qw), exhibit a nonlinear response along the reactor length, with an initially high evaporation rate that gradually slows as the reaction proceeds and the DPN increases. Furthermore, the model assesses how molecular weight (MWN) fluctuates with temperature changes, especially in the range of 270-300 ℃. The MWN initially rises with the temperature increase, then declines, particularly when the temperature exceeds 290 ℃, leading to a sharp increase in Ev concentration. At 300 ℃, the concentration of Ea surges dramatically to 40 mmol/kg. Concurrently, the contents of diethylene glycol (DEG) and DEG end groups (EDEG) decrease under these conditions. Under controlled conditions of 270-300 ℃ and 0.01-1.0 kPa, an increase in vacuum level results in a rise in MWN and a reduction in the production of Ea, EDEG, and DEG. This meticulous process optimization was evidenced by the enhanced MWN value of approximately 39 000 g/mol at the outlet, underscoring the model's effectiveness in optimizing PET production.

Conclusion This study established a mathematical model for a polyester falling film liquid-state polymerization reactor, integrating a liquid phase plug flow model, a fully mixed gas phase model, and coupling PET reaction kinetics with gas-liquid mass transfer and high-viscosity fluid dynamics to simulate the polycondensation process. This model reflects the concentration of components and molecular weight distribution inside the vertical falling film reactor, indicative of the degree and progression of high-viscosity molten polymerization, typically challenging to measure online in industrial production. Exploring the relationship between component and molecular weight distribution along the reactor axis could lead to the development of direct spinning processes for polyester products with different intrinsic viscosities. The model also analyzes the impact of reaction temperature and pressure on the molecular weight and end-group content during polymer melt polycondensation, suggesting that controlling reactor temperature (285-290 ℃) and pressure (0.1 kPa) with an input melt viscosity of 0.63 dL/g and carboxyl end group of 31 mmol/kg can produce high-viscosity polyester products with an ηintr value about 1.0 dL/g. Sensitivity analysis of the model to determine optimal operating parameters offers high industrial application value.

Key words: poly(ethylene terephthalate), liquid-state polymerization, reactor, polymerization, process simulation, polymerization process

CLC Number: 

  • TQ343.4

Fig.1

Continuous polymerization process model of polyester"

Tab.1

Comparison between model calculation and plant data of PET five kettle process"

反应釜类别 过程参数 ES/% τ/h MWN/(g·mol-1) [Ea]/(mmol·kg-1) [Eg]/ (mmol·kg-1)
酯化Ⅰ釜 目标值 92.0 2.0 720~980 — —
模拟值 92.9 1.8 917 731 1 255
酯化Ⅱ釜 目标值 96.0 2.0 1 000~1 200 — —
模拟值 95.0 1.9 1 203 520 1 062
预缩Ⅰ釜 目标值 98.5 1.0~2.0 2 800~4 000 — —
模拟值 99.5 1.9 4 632 127 301
预缩Ⅱ釜 目标值 99.4 1~1.5 10 000~12 000 — —
模拟值 99.7 0.9 10 890 55 127
终缩聚釜 目标值 99.9 1~1.2 19 200~21 000 28~32 —
模拟值 99.8 1.03 19 940 31 68

Tab.2

Outlet concentration in final polymerization process"

参数 符号 取值
反应温度/℃ T 282
数均分子量/(g·mol-1) MWN 19 940
特性黏度/(dL·g-1) ηintr 0.630
动力黏度/(Pa·s) μ 300
端羟基初始浓度/(mmol·kg-1) [Eg]0 68
端羧基初始浓度/(mmol·kg-1) [Ea]0 31
酯基初始浓度/(mmol·kg-1) [Z]0 5 200
乙烯基初始浓度/(mmol·kg-1) [Ev]0 0.048
端二甘醇基初始浓度/(mmol·kg-1) [EDEG]0 0.570
乙二醇初始浓度/(mmol·kg-1) [EG]0 0.025 7
水初始浓度/(mmol·kg-1) [W]0 0.016 2
二甘醇初始浓度/(mmol·kg-1) [DEG]0 0.012 27

Tab.3

Kinetic reactions for polymerization process"

序号 反应方程式
M1 Eg+Eg Z+EG
S2 Eg Ea+AA
S3 Ev+Eg Z+AA
S4 Eg+EG Ea+DEG
S5 Eg+EDEG Z+DEG
S6 Eg+Eg Ea+EDEG
S7 Ea+EG Eg+W
S8 Ea+Eg Z+W
S9 Z Ea+Ev

Fig.2

Schematic diagram of free falling liquid film flow"

Fig.3

Schematic of modeling process for vertical falling film reactor"

Tab.4

Simulation target values and simulation values of PET falling film polymerization reaction"

参数 目标值 模拟值
[Ea]/(mmol·kg-1) 20 16
[Eg]/(mmol·kg-1) — 33
[Ev]/(mmol·kg-1) — 0.11
[W]/(mmol·kg-1) 0 6.30×10-3
[EDEG]/(mmol·kg-1) — 0.28
[Z]/(mmol·kg-1) — 5.20×103
[EG]/(mmol·kg-1) 0 9.28×10-3
[DEG]/(mmol·kg-1) 0 2.90×10-3
τ/h 1.000 0 1.063 0
ηintr/(dL·g-1) 1.005 0 1.004 5

Fig.4

Distribution of components of liquid phase polyerization falling film kettle outside tube. (a) DPN and [Ea]; (b) [Eg] and [Ev]; (c) Qw and QEG"

Fig.5

MWN distribution along reactor axis at different temperatures"

Fig.6

Components concentration distribution of polymer along reactor axis at different temperatures"

Fig.7

MWN and components concentration distribution of polymer along reactor axis at different pressure"

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