摘要
大型预氧化装备是PAN基碳纤维工业化生产的关键设备之一,其主要技术指标对提高碳纤维产量和性能至关重要。如何缩短预氧化升温时间,提高温度均匀性和控温精度,降低预氧化装备能耗,是国产大型预氧化装备需要解决的关键技术。
本文借助于计算机仿真与模拟技术,对大型预氧化装备的温度控制特性进行了系统研究,主要包括：
(1)温度控制算法研究及应用
建立了大型预氧化装备的温控系统模型,通过实验获得了大型预氧化装备的阶跃响应曲线,利用辅助变量最小二乘法进行了模型参数的辨识。
研究了用于大型预氧化装备的模糊自适应PID控制算法和基于BP神经网络的自适应PID控制算法并利用MATLAB进行仿真研究。仿真结果表明,与常规PID相比,两种算法都具有控制精度高、超调量小、抗干扰能力强等优点,其中基于BP神经网络的自适应PID控制在超调量及抗干扰能力方面优于模糊自适应PID控制,但BP神经网络算法复杂,其响应时间比模糊自适应PID控制稍长。
针对模糊控制存在量化误差的缺陷,采用线性插值的方法,对模糊规则进行细化,推导了相应的计算公式,并在PLC上编程实现,消除了量化误差,显著改善了系统性能。
(2)流场与温度场模拟研究
以计算流体动力学和传热学的基本控制方程为基础,建立了大型预氧化装备炉内气体流动与传热的数学模型；利用有限体积法,对流动与传热的控制方程进行离散化处理,得到了瞬态流动与传热的离散方程；根据SIMPLE算法推导了速度与压力修正值的计算公式。
通过分析大型预氧化装备的结构特点,合理简化,采用二维模型研究大型预氧化装备的流场和温度场,利用GAMBIT建立了不同条件下的模型并划分网格,采用Fluent进行了模拟。
根据模拟结果,针对大型预氧化装备的结构、导风罩旋转角度以及PAN原丝在炉膛中的分布提出了一系列优化建议。
在改进结构设计、优化导风罩旋转角度的基础上进行了温度场模拟,结果表明,改进和优化后的大型预氧化装备空载时具有较好的温度均匀性。
(3)控制系统设计与软件开发
根据本文研究成果,设计了一套具有高性能、高可靠性、高性价比和良好扩展能力的控制系统方案,并成应用于年产800吨PAN基碳纤维生产线的研制开发。
开发了大型预氧化装备温度控制系统的PLC软件,利用多项式拟合加外部补偿的方式实现了0.02℃的高精度温度测量；采用模糊自适应PID控制和分段温度控制策略确保升温速率和控温精度；利用基于RS485总线的USS通讯实现了对3个子网共72台变频器的实时控制,通过合理的参数设置和软件优化,实时性完全满足生产要求；设计了一种智能预氧化装备升温调度算法并编程实现,该算法对大型预氧化装备的升温过程采用分段升温、分时控制,智能调度整条生产线的可用功率,在满足控温精度的条件下,可降低35%的变压器容量。
(4)对大型预氧化装备的送风系统进行了优化,并测试了两种大型预氧化装备优化后的控温精度,控温点精度±0.8℃；多热电偶立体分布测温和预氧丝氧含量测试数据表明优化后的大型预氧化装备具有较好的温度均匀性,炉膛内各测温点的最大偏差为2.6℃。
Large-scale pre-oxidation equipment is one of the key equipments for the production of PAN based carbon fiber. Its major technical parameters are essential to improve the production and performance of carbon fiber. How to shorten the pre-oxidation heating-up time, improve temperature uniformity and control accuracy, and reduce the energy consumption, is the key technology to be solved for domestic large-scale pre-oxidation equipment.
With the help of computer simulation, a systematic study was conducted on the temperature control characteristics of large-scale pre-oxidation equipment. It includes:
(1) Research and application of temperature control algorithm
A temperature control system model of large-scale pre-oxidation equipment was established and a step response curve was acquired by experiments. Finally the model parameters were identified using the auxiliary variable least squares method.
Fuzzy adaptive PID algorithm and BP-based adaptive PID algorithm were studied and simulated in MATLAB. The simulation results show that, compared with conventional PID, both algorithms have the advantages of high precision, small overshoot, and strong anti-interference ability. The BP-based adaptive PID algorithm is better than fuzzy adaptive PID in the aspect of overshoot and anti-jamming, but its response time is a little longer than fuzzy adaptive PID due to the complexity of BP algorithm.
Aiming at the defects of fuzzy control quantization error, a linear interpolation method was used to refine the fuzzy rules, the corresponding formulas were derived and this method was implemented in PLC program, which can eliminate the quantization error and improve the system performance significantly.
(2) Flow field and temperature field simulation
The mathematical model of gas flow and heat transfer of large pre-oxidation equipment was established based on the basic computational fluid dynamics and heat transfer equations. Using the finite volume method, the governing equations of flow and heat transfer were discretized and discrete equations of transient flow and heat transfer were derived. According to the SIMPLE algorithm, speed and pressure correction formulas were derived.
Analyzing the structure characteristics of large-scale pre-oxidation equipment, a two-dimensional model with reasonable simplification was adopted to study the fluid field and temperature field. Models with different conditions were established and meshed in GAMBIT. FLUENT software was used for the simulation.
Based on the simulation results, a series of optimization suggestions was proposed for structure improvement of large-scale pre-oxidation equipment, optimized rotation angles of wind scoopers and PAN fibers'distribution in chamber.
The temperature filed simulation was conducted with improved structure and optimized rotation angles of wind scoopers, and the result shows that improved and optimized large-scale equipment without load has good temperature uniformity.
(3)Control system design and software development
Based on the research results of this paper, a set of temperature control system with high performance, high reliability, high cost performance and scalability was designed and it has been successfully applied to the development of a carbon fiber production line with annual output of800tons.
The PLC software achieved high measurement precision of0.02℃using polynomial fitting and external compensation. Fuzzy adaptive PID algorithm and segmental temperature control strategy ware used to ensure the heating-up rate and control accuracy. USS communication based on RS485bus was utilized to control72inverters divided into3subnets and the real-time performance can meet the production requirements by reasonable parameter settings and software optimization. This paper also proposed an intelligent pre-oxidation scheduling algorithm which has been programmed. This intelligent scheduling algorithm can schedule all the available power by step heating and time-sharing control, it can reduce about35%of transformer capacity without temperature control precision loss.
According to the research results, the air supply system of large-scale pre-oxidation equipment was optimized. The precision of two different large-scale pre-oxidation equipments with optimized parameters was tested and the temperature control precision is plus or minus1degrees C. Distributed multiple-thermocouple measurement and oxygen content tests of pre-oxidized fiber both proved that optimized large-scale pre-oxidation equipment has good temperature uniformity. The maximum temperature deviation in the chamber is2.6degrees C.
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