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Application of Predictive Control (PFC) in Vinyl Chloride Distillation Process
In response to the control challenges in the VCM rectification process, our company implemented Zhejiang University Central Control's APC-PFC advanced control software to develop a sophisticated control system for the distillation process. This system was successfully launched in March 2005, marking the introduction of advanced control in VCM rectification. The system significantly reduced the workload on operators by utilizing predictive models to forecast changes in controlled variables. It also performed rolling optimization and deviation correction on control loop setpoints, thereby enhancing the precision and stability of key process variables. As a result, product quality was ensured, operational conditions were maintained smoothly, and satisfactory control outcomes were achieved.
The VCM rectification process typically involves a low-boiling column system and a high-boiling column system, forming a typical two-stage cascade distillation process. The crude vinyl chloride from the compressor first enters a full condenser, where most of the gas is condensed. After water separation, it moves into the low-boiling tower. Uncondensed gas goes to the exhaust condenser, and condensate is returned to the water separator. The low-boiling tower reboiler is heated by hot water, and the distillate from the top is sent to the tail gas condenser. Uncondensed gases are recovered by an adsorber before being discharged. The bottom liquid flows into the high-boiling tower through an intermediate tank, where it is heated again, evaporated, and separated into refined VCM that is condensed and stored.
Due to the continuous nature of production, the VCM rectification process is closely linked with both the preceding synthesis and the following polymerization stages. Fluctuations in feed flow rate and composition from the synthesis section greatly impact the VCM rectification process, affecting both material and energy balances. The long lag time of the crude VCM in the column makes traditional feedback control ineffective in responding to these disturbances. Additionally, the complex and unmeasurable feed composition introduces significant challenges in maintaining product quality.
For the high-boiling tower, frequent and large variations in feed also make liquid level control difficult. These fluctuations affect column temperature, which in turn influences the top temperature and pressure, creating complex coupling relationships among operating parameters. Therefore, controlling the high-boiling tower remains a major challenge in the VCM rectification plant.
The APC-PFC control software, based on Predictive Functional Control (PFC), uses a predictive model, rolling optimization, and feedback correction. PFC, developed in 1986, focuses on the structural form of control inputs and incorporates error feedback for online optimization. Its main features include simplicity, low computational load, and ease of integration with existing DCS systems. It also allows for efficient adjustment of parameters and effective suppression of measurable disturbances.
To address the complex and coupled nature of the VCM rectification process, APC-PFC was applied to both the low- and high-boiling towers. A block diagram of the advanced control system shows a supervisory layer using PFC and a control layer using conventional PID. This hybrid structure combines the reliability of PID with the dynamic performance of PFC. Feedforward control was added to the PFC controller to enhance disturbance rejection.
After implementation, the system significantly improved process stability and product quality. Before the predictive control was introduced, the top temperature of the low-boiling tower and the liquid level of the high-boiling tower showed large fluctuations. Post-implementation, these variations were drastically reduced, and other process parameters also improved. The system demonstrated strong anti-disturbance capability, as seen in the minor fluctuations caused by high-boil discharge. Monomer purity reached 99.999%, meeting PVC production requirements.
In conclusion, the APC-PFC software was seamlessly integrated without disrupting the production process. It allowed for bumpless switching between PID and predictive control with a single switch. Minimal operator training was required, and system maintenance was straightforward. Overall, the APC-PFC solution proved highly effective in improving control performance and operational efficiency.
Author: Xue Jian, Qinqiao Liang, Chen Dongmeng, Fan Minghua, Nantong Jiangshan Agrochemical & Chemical Co., Ltd.
Source: Control Engineering China