Modeling method of depressurization separation process of the mixture of high-pressure polyethylene and supercritical ethylene

doi:10.11949/0438-1157.20240859

Abstract

Abstract:

The depressurization separation process of the mixture of high-pressure polyethylene and supercritical ethylene involves complex two-phase flow and phase change mass transfer process between supercritical ethylene and polydisperse polyethylene. The existing models not only lack effective data verification, but also do not have the ability to accurately predict the molecular weight distribution of polydisperse polyethylene wax. Therefore, this paper first characterizes the molecular chain structure of high-pressure polyethylene and its polyethylene wax. Then, based on the separation mechanism, a modeling strategy for predicting the flow rate and molecular weight distribution of polyethylene wax at the gas phase outlet of the high-pressure separator is proposed. Finally, the effect of operating conditions on the yield of polyethylene wax is studied. The research methodology and results will not only provide theoretical guidance for optimizing the high-pressure polyethylene separation process and the targeted regulation of the wax content in the product, but also can be extended to the polymer separation process of other solution polymerization processes, which is of great significance.

Key words: high-pressure polyethylene, phase equilibria, depressurization separation, selective entrainment, model, two-phase flow

摘要：

超临界乙烯和高压聚乙烯混合物的减压分离过程涉及超临界乙烯与多分散性聚乙烯的复杂两相流动和相变传质过程，现有模型不仅缺少有效的数据验证，而且并不具备准确预测多分散性聚乙烯蜡分子量分布的能力。因此，首先对高压聚乙烯及其聚乙烯蜡的分子链结构进行表征分析，然后根据分离机理提出了一种预测高压分离器气相出口聚乙烯蜡流量及其分子量分布的建模策略，最后研究了操作条件对聚乙烯蜡产量的影响。研究方法和结果不仅可为高压聚乙烯分离过程的操作优化和产品含蜡量的定向调控提供理论指导，而且可以拓展到其他溶液法工艺的聚合物分离过程，具有重要的意义。

关键词: 高压聚乙烯, 相平衡, 减压分离, 选择性夹带, 模型, 两相流

CLC Number:

TQ 320.2

Xiaonan YOU, Xiaoqiang FAN, Yao YANG, Jingdai WANG, Yongrong YANG. Modeling method of depressurization separation process of the mixture of high-pressure polyethylene and supercritical ethylene[J]. CIESC Journal, 2025, 76(2): 695-706.

尤潇楠, 范小强, 杨遥, 王靖岱, 阳永荣. 超临界乙烯和高压聚乙烯混合物的减压分离过程建模方法[J]. 化工学报, 2025, 76(2): 695-706.

Figures/Tables 17

Fig.1 Schematic diagram of high-pressure circulation system

Fig.2 Molecular weight distribution of PE and PE-Wax

Table 1 Pure component parameters in the PC-SAFT equation of state［19］

参数	C₂H₄	LDPE
k_ij	-0.0568708
m	1.522416	0.04131746M
σ	3.4882223	3.4750711
ε/k_B	181.331214	267.179575

Fig.3 Cloud point curve of monodisperse polyethylene (T=221.16℃)

Fig.4 Schematic diagram of the supercritical ethylene-polyethylene separation mechanism (black represents the polyethylene phase, white represents the ethylene phase, gray represents the homogeneous phase)

Fig.5 Modeling flow chart

Table 2 Comparison of molecular weight distribution data for polyethylene product （PE-B） before and after data processing

数据	M_n	M_w	PDI
原始数据（PE-B）	15390	106031	6.9
46个伪组分	17419	121314	7.0
91个伪组分	16351	112950	6.9
226个伪组分	15746	108571	6.9

Fig.6 Schematic diagram of precipitation of monodisperse polyethylene (M=2000) during depressurization process

Table 3 Comparison of predictive capabilities of mathematical models

相关参数	Model A^[19]	Model B^[1]	Model C(this study Ⅰ)
形成机理	相平衡	夹带、传质	相平衡、气相剪切、减压溶解
流量信息	√	√	√
分子量分布	√	×	√

Fig.7 Comparison of model prediction effects under the operation conditions of PE-A

Fig.8 Comparison of model prediction effects under the operation conditions of PE-A

Fig.9 The effect of operating temperature on the critical gas velocity for shearing

Fig.10 The impact of operating temperature on the size distribution of droplets formed by shearing

Fig.11 The effect of operating temperature on critical droplet size and wax yield

Fig.12 The impact of operating pressure on the critical gas velocity for shearing

Fig.13 The effect of operating pressure on the size distribution of droplets formed by shearing

Fig.14 The impact of operating pressure on critical droplet size and wax yield

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