Theses of the doctoral (PhD) dissertation. Pannon University PhD School of Chemical and Material Engineering Science. Supervisor: dr.

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1 Theses of the doctoral (PhD) dissertation PROCESS MODELS AND DATA MINING TECHNIQUES IN DETERMINATION AND CHARACTERIZATION OF SAFE OPERATING REGIMES TAMÁS VARGA Pannon University PhD School of Chemical and Material Engineering Science Supervisor: dr. János Abonyi Pannon University Department of Process Engineering Veszprém 2009

2 1. INTRODUCTION AND THE AIM OF THE WORK Nowadays in industrial practice the possible capacities of the process are more and more exploited with the development of advanced process control systems. Hence, the operating conditions are getting closer and closer to boundaries determined by laws of nature. For many applications, it is necessary that process variables should be maintained within strict limits. Therefore stricter safety regulations must be determined in the characterization of optimal operating regimes. Due to complexity of production systems the fault prevention, diagnoses and the control of abnormal events became more and more complicated for the process operators. Many thousands of process variables are observed and stored every second in a large process plant. To support the operators in improving the quality of products, reducing the energy and materials waste, and increasing the flexibility of production it is necessary to increase their insight in the behavior of the process. Due to this demand the number of claims against the development of diagnostic systems is continuously increasing. To support the work of process operators such a tool should be developed in which the simulator of safety elements are integrated with the simulator of the process. In addition to development of the proposed tool, algorithms should be developed too, to determine the safe operating regimes of complex processes and to calculate process safety time by using the simulators. The aim of this work was to analyze the reactor runaway phenomenon. The root cause event of reactor runaway can be described as the most critical safety boundary in industrial reactors in which exothermic reactions take place. Runaway phenomenon means a sudden and considerable change in the state-variables of the process, e.g. in temperature which is usually called thermal runaway. It has contributed to the most serious industrial chemical accidents 1

3 the most notably, the 1984 explosion of a Union Carbide plant in Bhopal, India that produced methyl isocyanate and the disaster in Seveso, Italy in 1976 which elevated and stiffened industrial safety regulations. Thermal runaway is also a concern in hydrocracking, an oil refinery processes. Detection of runaway has two main important aspects. In one hand, the runaway forecast has a safety aspect, since it is important for avoiding the damage of constructional material or in the worst case the explosion of the reactor. In the other hand, it has a technology aspect, since the forecast of the runaway can be used for avoiding the development of hot spots in catalytic bed, which speed up the ageing of catalyst, or for preventing the production of byproducts in huge mass like at the synthesis of 2- octanone from 2-octanol Since both the optimal and safe operation are really important, it is necessary to develop intelligent expert systems to support the operators making proper decisions. The expert system in addition to detect the undesired events should determine possible strategies to avoid the development of these events. The most important step to develop such an expert system is to collect all the necessary information about the investigated process. The extraction of information can be done by applying data mining tools, elaborating a priori models, or combining these techniques. The thesis presents some novel tools which can be the base or just a part of an intelligent expert system to support operators. The applied techniques related to the following science areas: mathematical modeling and identification, simulation, data-mining, quality trend analysis, process alarm management, stability analysis. 2

4 2. NEW SCIENTIFIC RESULTS 1. Hierarchical modeling concept can be efficiently applied in model based analysis of safe operating regimes of heterocatalytic reactors. (Related publications: 1, 5, 8, 10, 11, 14, 17, 18, 19, 24, 26, 27) Due to the evolution of process control systems the importance of a priori models increases in the chemical industrial practice. I developed the models with different complexity of an industrial used packed bed tube reactor to introduce hierarchical modeling concept. In model development the main aspect was the cover of optimal and safe operation of the investigated reactor. All the developed models have been assigned to possible engineering applications. The reactor is a two-phase system where a standing solid and a moving gas phase are connected. The two-phase system is considered as a quasi-single phase system in the developed steady-state model. A modified Langmuir-Hinshelwood kinetic expression with the consideration of reaction equilibrium has been applied to calculate the rate of the overall process. To support the worked with the developed steadystate simulator of the industrial colleges I develop graphical user interfaces. For the analysis of the evaluation of reactor runaway and boundaries of unstable operating regimes in time a quasi-single phase dynamical model has been developed. The next step was the investigation of the transport processes between phases and to allow model based design of the startup strategies of the reactor. Hence, I describe the two-phase dynamical model of the studied reactor. To study the runaway in micro scales the model of a single catalyst pellet and its close environment has been developed. I apply this model to calculate the evaluation of state-variables in specific points 3

5 of the reactor. The developed model set is applicable for the analysis of the reactor stability, for the design and checking of start up and operating strategies, as well as for the investigation of phenomena related to the stability of the system. 2. For the evaluation of the stability analysis a decision tree based approach has been proposed. The developed tool can be applied to determine the safe operating regimes in the reactor. (Related publications: 1, 2, 3, 4, 6, 8, 11, 12, 14, 16, 17, 18, 19, 20, 22, 23) (a) As the state of the reactor is getting to the unstable regime the reactor runaway develops since in the unstable regime the produced heat exceeds the cooling capacity of the reactor. Hence, the development of runaway can detected based on the stability analysis of the process. I have analyzed the runaway in an industrial tube reactor and in a fed batch reactor. I have described the phase map of the tube reactor. The phase map has been defined with the conversion and the temperature of the reactor. I have analytically determined the equilibrium and optimal temperature as the function of the conversion based on the applied reaction kinetic expression in the model. The equilibrium temperature represents the theoretical maximum temperature at given state since at this temperature the rates of exothermic production and endothermic decomposition are equal. The optimal temperature has been determined by calculating the real roots of the first derivative of reaction rate expression respect to the temperature. Applying the optimal temperature profile as setpoint of the temperature control the maximum reaction rate can be achieved in the reactor during the operation. In these calculations abilities of the reactor have not been considered. Hence, I have analyzed the stability of the developed steady-state reactor model using Lyapunov s indirect stability analysis method. Based on the applied stability analysis I have analytically determined the 4

6 boundaries of the unstable operating regime of the investigated system. I have plotted the results of these calculations to construct the phase map of the reactor. The resulted phase map can be applied to design operating strategies or it can be a base of reactor design. (b) A great number of inlet conditions have been randomly generated to define different operating regimes. The profiles of statevariables at each inlet condition have been calculated by solving the developed steady-state model of the reactor. I have developed an algorithm to select those inlet conditions at which reactor runaway occurs, i.e. the applied stability analysis method detects at least one unstable working point. I have used the results of stability analysis as learning samples to induct a decision tree. The generated tool can be applied to forecast the development of reactor runaway based on the measured inlet condition of the reactor. In addition to forecast of runaway the tool can be applied to characterize the stable operating regimes of the process with a set of easily comprehensible linguistic rules. From a different point of view, I have demonstrated in this thesis that data mining techniques are not only useful to extract applicable and potentially useful information from large historical databases, but there can be used for the analysis of first-principle models. 3. With the application of dynamical process simulators the process safety elements, i.e. the variables which can applied to drive back the system to the safe operating regime, can be efficiently ranked through the analysis of the process safety time. (Related publications: 7, 8, 9, 13, 15, 21, 25) In real world every modification needs time to have an effect on something, it is called time delay. Hence, in industrial practice the process safety elements need time to prevent the process from the development of unsafe situations. Therefore if the appropriate 5

7 process safety element is not applied in proper time the undesired event cannot be avoided. I have proposed a framework based on the consideration of the dynamic behavior of safety elements next beside the process model. Applying the algorithm the time domain or the safety time, in which the process is controllable by applying the possible safety elements, can be determined. The base of this algorithm is beside the detection of process hazards, is the calculation of process safety times in case of each safety elements. By the knowledge of process safety time the safety elements can be ranked and the proper one can be chosen in different scenarios. I have showed the applicability of the proposed framework in two case studies, for a fed-batch reactor and a packed bed tube reactor. 3. UTILIZATION OF RESULTS A part of the results presented in the thesis has been already used, since the developed steady-state reactor and reactor system simulators have been applied by the industrial partner to solve some practical problems. The most important problem is the decreasing of the maximum temperature in the catalyst bed to increase the lifetime of the applied catalyst. Simulators have been applied to simulate the effect of the modifications in operating conditions and in the structural parameters of the reactor. The developed tool for the forecast of reactor runaway can be applied to detect the unsafe events or to characterize the safe operating regimes in any other chemical industrial unit, e.g. in absorbers where the absorption heat is highly exothermic. The proposed framework for the determination of process safety time can also be applied as a part of an operator support system to help the operator making decisions during the operation. 6

8 4. FUTURE WORK The aim of this research was to develop some novel techniques to determine and characterize the safe operating regimes of complex processes. These new scientific results raise some other interesting questions. The applied engineering softwares, such as MATLAB and COMSOL Multiphysics satisfy the CAPE-OPEN standard, which ensures the interactions of engineering softwares. Hence, any application made in one of these softwares can be integrated into another software. Due to the CAPE-OPEN standard the pellet model implemented in COMSOL can be applied as a building block of the model a whole catalyst bed. The construction of this bed can be made in MATLAB from the pellet model, which represents a single catalyst pellet. By the development of this framework more sophisticated analyzes of catalyst beds can be performed with much less necessity of computer memory (Random-access Memory - RAM) for the calculations. It is really important, since the memory requirement of the calculation in case of a sophisticate model can easily exceeds the available RAM. The developed tool can be applied to design the shape and size distribution of catalyst pellets which build up the catalyst bed. Another research topic can be the development of a novel searching algorithm which applies the information extracted from process models with DTs to determine the constraint of search variables. That kind of algorithm can be dependably applied to determine the optimum of operating variables. The main goal is to solve the online application of the algorithm to keep the state variables of the process at the optimum to achieve maximal production efficiency with safe operation. The results of the introduced method for the analysis of process controllability can be evaluated with DTs and a novel tool can be developed, which can be applied to alarm the process operators when state variables of the process leave the controllable region. Therefore further simulation 7

9 experiments should be constructed and evaluated to generate the necessary learning samples for DT induction. 5. PUBLICATIONS OF THE AUTHOR RELATED TO THE THESIS Articles in international journals: [1] T. Varga, F. Szeifert, J. Réti, J. Abonyi, Analysis of the runaway in an industrial heterocatalytic reactor, Computer Aided Chemical Engineering, Vol. 24, pp , 2007 [2] T. Varga, F. Szeifert, J. Abonyi, Evolutionary Strategy for Feeding Trajectory Optimization of Fed-batch Reactors, Acta Polytechnica Hungarica, Vol. 4, pp , 2007 [3] T. Varga, F. Szeifert, J. Réti, J. Abonyi, Decision tree based qualitative analysis of operating regimes in industrial production processes, Computer Aided Chemical Engineering, Vol. 25, pp , 2008 [4] T. Varga, G. Horváth, J. Abonyi, Determination of safety operating regimes based on the analysis of characteristic equation of state-space model, Hungarian Journal of Industrial Chemistry, Vol. 36, pp , 2008 [5] T. Varga, G. Rádi, T. Chován, Improving the mathematical model of heterocatalytic tube reactor, Hungarian Journal of Industrial Chemistry, Vol. 36, pp , 2008 [6] T. Varga, F. Szeifert, J. Abonyi, Decision tree and first-principles model based approach for reactor runaway analysis and forecasting, Engineering Applications of Artificial Intelligence, Vol. 22, pp , 2009 [7] T. Varga, F. Szeifert, J. Abonyi, Detection of Safe Operating Regions a Novel Dynamic Process Simulator Based Predictive Alarm Management Approach, Industrial Engineering Chemistry Research, accepted with corrections, 2009 [8] T. Varga, F. Szeifert, J. Abonyi, Application of mathematical models with different complexity in reactor runaway detection, Mathematical and Computer Modelling of Dynamical Systems, accepted with corrections,

10 [9] T. Varga, J. Abonyi Novel method for the determination of process safety time, Chemical and Biochemical Engineering Quarterly, accepted with corrections, 2009 [10] T. Varga, F. Szeifert, J. Abonyi, Detailed description of a packed-bed tube reactor based on hierarchical modeling concept, International Review on Modelling and Simulations, in progress, 2009 Articles in Hungarian journals: [11] Varga T., Abonyi J., Szeifert F., Heterokatalitikus reaktorok vizsgálata, Acta Agraria Kaposváriensis, Vol. 10, pp. 1-9, 2006 [12] Varga T., Abonyi J., Szeifert F., Applying decision trees to investigate the operating regimes of a production process, Acta Agraria Kaposváriensis, Vol. 11, pp , 2007 [13] Varga T., Baradits G., Abonyi J., The role of dynamic process models for the detection of safe operating regions of process systems, submitted, 2008 Refereed presentations: [14] T. Varga, F. Szeifert, J. Réti, J. Abonyi, Analysis of the runaway in an industrial heterocatalytic reactor, 17th European Symposium on Computer Aided Process Engineering, Bukarest, 2007 [15] T. Varga, F. Szeifert, J. Abonyi, Application of process models for determining the region of controllability in fed-batch reactor, Modeling, Identification, and Control, Innsbruck, 2008 [16] T. Varga, F. Szeifert, J. Réti, J. Abonyi, Decision tree based qualitative analysis of operating regimes in industrial production processes, 18th European Symposium on Computer Aided Process Engineering, Lyon, 2008 Non-refereed presentations: [17] Varga T., Abonyi J., Chován T., Nagy L., Szeifert F., Réti J., Heterokatalitikus reaktorok vizsgálata, 34. Mőszaki Kémiai Napok, Veszprém,

11 [18] Varga T., Abonyi J., Szeifert F., Heterokatalitikus reaktorok vizsgálata, V. Alkalmazott Informatika Konferencia, Kaposvár, 2006 [19] Varga T., Chován T., Szeifert F., Réti J., Reaktorelfutás idıbeli viselkedésének vizsgálata heterokatalitikus csıreaktorban, 35. Mőszaki Kémiai Napok, Veszprém, 2007 [20] Varga T., Abonyi J., Szeifert F., Applying decision trees to investigate the operating regimes of a production process, VI. Alkalmazott Informatika Konferencia, Kaposvár, 2007 [21] T. Varga, J. Abonyi, F. Szeifert, Characterizing operating regimes of complex process systems - model based approaches, 27 th International Workshop on Chemical Engineering Mathematics, Veszprém, 2007 [22] T. Varga, F. Szeifert, J. Abonyi, Evolutionary Strategy based Feeding Profile Optimization of Fed-batch Reactors, International Conference of Hungarian Researchers on Computational Intelligence, Budapest, 2007 [23] Horváth Gy., Varga T., Abonyi J., Komplex technológiai rendszer biztonságos üzemeltetési tartományainak feltárása állapottér modell karakterisztikus egyenletének analízise alapján, 36. Mőszaki Kémiai Napok, Veszprém, 2008 [24] Rádi Gy., Varga T., Chován T., Heterokatalitikus csıreaktor matematikai modelljének fejlesztése, 36. Mőszaki Kémiai Napok, Veszprém, 2008 [25] T. Varga, G. Baradits, J. Abonyi, The role of dynamic process models for the detection of safe operating regions of process systems, VII. Alkalmazott Informatika Konferencia, Kaposvár, 2008 [26] T. Varga, G. Rádi, T. Chován, Improving the mathematical model of a heterocatalytic tube reactor, COMSOL conference 2008, Budapest, 2008 [27] Rádi Gy., Varga T., Chován T., Hierarchikus modellezés a vegyipari gyakorlatban, 37. Mőszaki Kémiai Napok, Veszprém,