Structure optimization method for high-pressure LDPE tubular reactor

doi:10.11949/0438-1157.20240420

Abstract

Abstract:

High-pressure tubular reactors are crucial equipment for the production of LDPE (low-density polyethylene) and EVA (ethylene-vinyl acetate copolymer) and other products. Due to the harsh conditions of high-pressure polymerization reactions, the design of high-pressure tubular reactors must not only meet the process requirements of polymerization but also the mechanical strength and fatigue life requirements of the equipment. Taking the high-pressure LDPE tubular reactor as the research object, a two-step optimization strategy for the structure of tubular reactors is proposed. Firstly, the required pipe diameter ratio of the reactor is determined according to the ultra-high pressure vessel standards, and the influence of the initiator formulation on the heating rate of the tubular reactor is calculated based on the detailed model of the tubular reactor. Secondly, the total annual cost per unit conversion is used as the objective function, and genetic algorithms are employed to optimize the structural parameters such as the pipe diameter and length of the partition. The structure optimization method proposed in this paper is highly accurate and will provide theoretical guidance for the optimal design and technical transformation of high-pressure LDPE tubular reactors.

Key words: high-pressure, polymerization, tubular reactor, structural design, optimal design, reaction engineering

摘要：

高压管式反应器是生产低密度聚乙烯（LDPE）和乙烯-醋酸乙烯酯共聚物（EVA）等产品的重要装备。高压聚合反应条件苛刻，高压管式反应器的设计不仅要满足聚合反应工艺要求，还要满足装备的机械强度和疲劳寿命要求。以高压法LDPE管式反应器为研究对象提出了管式反应器结构的两步法优化策略，首先根据超高压容器标准确定反应器的管径比要求，并基于管式反应器的详细模型计算引发剂配方对管式反应器升温速率的影响规律，然后以单位转化率下的年度总费用作为目标函数，采用遗传算法优化得到反应器的管径、分区长度等结构参数。所提出的管式反应器的结构优化方法具有较高的准确性，可为高压法LDPE管式反应器的优化设计和技术改造提供理论指导。

关键词: 高压, 聚合, 管式反应器, 结构设计, 优化设计, 反应工程

CLC Number:

TQ 320.2

Wenjie CONG, Jiawen HUANG, Xiaoqiang FAN, Yao YANG, Jingdai WANG, Yongrong YANG. Structure optimization method for high-pressure LDPE tubular reactor[J]. CIESC Journal, 2024, 75(10): 3557-3567.

从文杰, 黄嘉雯, 范小强, 杨遥, 王靖岱, 阳永荣. 高压法LDPE管式反应器的结构优化方法[J]. 化工学报, 2024, 75(10): 3557-3567.

Figures/Tables 15

Fig.1 Structure of a typical LDPE tubular reactor

Fig.2 Variation of properties of mixed feedstock in LDPE tubular reactor: (a) density; (b) viscosity; (c) specific heat at constant pressure; (d) thermal conductivity

Fig.3 Variation of fouling thermal resistance along reactor length

Fig.4 Effect of initiator’s formula on reactor temperature profile

Table 1 Effect of initiator’s formula on ethylene conversion and consumption of initiator

参数	DTBP∶TBPB∶TBPEH(摩尔比)
参数	1∶1∶1	1∶2∶2	1∶2∶4
引发剂用量指数	1.000	1.327	1.762
乙烯摩尔转化率	0.30	0.30	0.30

Table 2 Main parameters and values in the objective function

参数	数值
工业用电费用p_e /(CNY/(kW·h))	0.95
反应器运行时间t/h	7200
反应管费用系数C_r	3
反应管密度ρ_r /(kg/m³)	7900
压缩机进口压力p₁，出口压力p₂/bar	300，3000

Fig.5 Optimization process of tubular reactor structure

Table 3 Elementary reactions of free radical polymerization［31］

基元反应	化学方程式
引发剂分解（链引发）	$I \overset{k_{d}}{\to} 2 R^{0}$
链增长	$R_{x} + M \overset{k_{p}}{\to} R_{x + 1}$
耦合终止	$R_{x} + R_{y} \overset{k_{p}}{\to} D_{x + y}$
歧化终止	$R_{x} + R_{y} \overset{k_{p}}{\to} D_{x} + D_{y}$
链转移至单体	$R_{x} + M \overset{k_{t r m}}{\to} D_{x} + R_{1}$
链转移至聚合物	$R_{x} + D_{y} \overset{k_{t r p}}{\to} D_{x} + R_{y}$
链转移至链转移剂	$R_{x} + C T A \overset{k_{t r a}}{\to} D_{x} + R_{1}$
β裂解	$R_{x} + D_{y} \overset{k_{β}}{\to} D_{x} + R_{z} + D_{y - z}^{=}$
链间转移	$R_{x} \overset{k_{b b}}{\to} R_{x}^{'}$

Table 3 Elementary reactions of free radical polymerization［31］

基元反应	化学方程式
引发剂分解（链引发）	$I \overset{k_{d}}{\to} 2 R^{0}$
链增长	$R_{x} + M \overset{k_{p}}{\to} R_{x + 1}$
耦合终止	$R_{x} + R_{y} \overset{k_{p}}{\to} D_{x + y}$
歧化终止	$R_{x} + R_{y} \overset{k_{p}}{\to} D_{x} + D_{y}$
链转移至单体	$R_{x} + M \overset{k_{t r m}}{\to} D_{x} + R_{1}$
链转移至聚合物	$R_{x} + D_{y} \overset{k_{t r p}}{\to} D_{x} + R_{y}$
链转移至链转移剂	$R_{x} + C T A \overset{k_{t r a}}{\to} D_{x} + R_{1}$
β裂解	$R_{x} + D_{y} \overset{k_{β}}{\to} D_{x} + R_{z} + D_{y - z}^{=}$
链间转移	$R_{x} \overset{k_{b b}}{\to} R_{x}^{'}$

Table 4 Kinetic rate constants for initiator decomposition and ethylene homopolymerization［31］

基元反应	速率常数	k₀/(L/(mol·s))	(E_a/R)/K	V_a/(cm³/mol)
DTBP分解	k_DTBP	2.00×10¹⁶	19846	10
TBPB分解	k_TBPB	2.23×10¹⁶	18233	0
TBPEH分解	k_TBPEH	1.63×10¹⁴	15167	4.9
链增长	k_p	1.25×10⁸	4061	-19.7
链终止	k_tc= k_td	1.25×10⁹	503	13
链转移至单体	k_tm	1.25×10⁵	4061	-19.7
链转移至聚合物	k_tp	4.38×10⁸	6606	4.4
链转移至链转移剂	k_tra	2.62×10⁷	5973	-19.5
β裂解	k_β	1.292×10⁷ s^-1	5671	-16.8
链间转移	k_bb	7.8×10⁸ s^-1	5319	-15.9

Table 5 Physical and reactor parameters

参数	数值
进料温度T_in /℃	170~180
温度上升速率ΔT_up /(℃/m)	0.5~2.0
冷却水温度t_in /℃	160~170
进料压力p_in /MPa	200~300
反应器内径d_i /m	0.056
物料密度ρ_m /(kg/m³)	510
物料比热容c_p_,m /(kJ/(kg·K))	2.88
物料热导率k_m/(W/(m·K))	0.19
物料黏度μ_m	式（1）
反应器段数n	4

Fig.6 Industrial and predicted reactor temperature profile

Fig.7 Effect of reactor length on ethylene conversion, reaction zone and cooling zone length

Fig.8 Effect of ethylene conversion on reactor length, reaction zone and cooling zone length

Fig.9 (a) Variation of conversion rate with heat transfer coefficient; (b) Variation of TAC x with heat transfer coefficient; (c) Variation of conversion rate with flow rate; (d) Variation of TAC x with flow rate

Table 6 Comparison between design values and industrial values of tubular reactor structures

项目

转化率/%

总长度/m

第一反应区

长度/m

第一冷却区

长度/m

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