Study On Abnormal Situation Self-Recovery Theory And Engineering Application For Chemical Process

In order to obtain more economic benefits, chemical industries always adopt complex modern chemical plants. Furthermore, the plants are often operated at extremes of pressure and temperature to achieve optimal performance, which makes equipment more vulnerable to failures. Chemical process system is a complex system which includes human, process, equipment, environment, instrument and control system and so on. The system state will become abnormal because of every deviation from design intent. It will bring catastrophic consequences to the society if an accident occurs. Although emergency shutdown can eliminate this abnormal situation, it may cause economic losses. The current DCS and other control system have some disadvantages, such as too many alarm points, too simple warning logic and too many difficulties in judging the importance of alarm instantly.Engineering practice shows that accident which has a gradual process from normal situation to abnormal situation can be avoided if the development trend of the accident can be understood beforehand and prevented instantly. The abnormal situation self-recovering system studied in this thesis, taking interactional relationship models of variables and other mechanism models which can correctly simulate the system as a basic and monitoring the fluctuation of the key variables which were determined beforehand in this system, can integrate the hazard and operability (HAZOP) analysis report and other information into the practical chemical industry producing process. Applying this system into real plant can decrease the times of non-plan shutdown to ensure the inherently safe of the industry and obtain more economic benefits.The main contributions of this dissertation are summarized as follows:(1) Abnormal situation self-recovery regulation for chemical process is presented in this thesis. Chemical process system is not only a dissipative structure, but also a complex system made up of a lot of factors which are interrelated and real-time changing. The objective of the system is preventing accidents before emergency shutdown system (ESD) or shutdown by adopting diagnosis and prediction, intelligent decision and active control method used to automatically find the initial cause, and eliminate the accidents at their very early stage after the real-time monitoring of conditions which will cause accident and analyzing symptoms of accident.(2) According to most of accidents caused by misoperations, this thesis summarizes presentation forms of common misoperations and the reasons which will cause misoperations, and then it introduces the misoperation hazard and operability (MO-HAZOP) software and an analysis method developed for misoperation, which makes up for the deficiency that HAZOP analysis can not systematically analyze the operation step neglect and inversion. The misoperations are arranged in terms of the probability, with high probability misoperations assigned as the critical situations. Adopting preventive measure , may prevent misoperation or control the influence of the accident consequence to the least extent after the risk rank.(3) According to the problem, the report of the computer aided HAZOP analysis has a huge number of HAZOP results and the important issues can not be separated from the general ones, this thesis presents a SDG (signed directed graph)- HAZOP analysis method based analytic hierarchy process (AHP), taken as an example to introduce the computer aided HAZOP analysis method. Establishing hierarchical structure of nodes and variables in the process, dangerous paths are arranged in terms of the relative importance degree, with high relative importance degree paths assigned as the critical situations which will decrease the effect of human factor to the analysis result. These advantages will make up for the deficiencies that computer aided HAZOP analysis has a huge number of HAZOP results and the important issues can not be separated from the general ones, and this method will provide technological basis for the combination of qualitative and quantitative method. This method not only can help experts ignore small probability events, but also can help experts focus on the urgent and serious problems and provide correspondingly preventive measures for potential hazards which are important in the chemical process.(4) According to the explosion repeated emergence problem in chemical process, 5 categories and 46 kinds of explosion accidents and 65 kinds of influence factors are summarized in this thesis. Taking them as one of the basics to establish the knowledge database of abnormal situation self-recovery system for chemical process will correspondingly prevent the repeated emergence of major explosions.(5) The separation process of nitrobenzene primary tower was taken as an example to explain real application of misoperations prevention and the explosion accidents analysis technology in chemical process. Based on the results got from the above analysis, the measures in terms of full automatic mechanism, programmed control system and interlock mechanism, can be adopted to reduce and prevent misoperations and to maintain the normal production.(6) Abnormal situation operation guide system for propylene polymerization process based multifunction process and control experiment system (MPCE) can ensure the starting of process successfully, and it may resolve problems occurring in the production process instantly. Before ESD or shutdown, the system utilized in the propylene polymerization process, on the one hand, may automatically find the initial cause and then eliminate the accidents at their very early stage as much as possible. On the other hand, it may extend the range of normal situation of chemical process and gain more benefits for the industry to a certain extent.(7) Inherently safe system means that the risk of a system is 0 or close to 0. At the design stage, some measures, such as intensification, minimization, substitution, attenuation or moderation, can be utilized to eliminate or , decrease risk. At the operation stage of the system, some control measures can be used to control risk. All of those technological methods can enhance the level of inherently safe of chemical industry. Taking the production process of light magnesium carbonate as a research objective, this thesis studies the inherently safe process of chemical process and provides some suggestions for improving of the inherent safe level for chemical industry.