KINETIC SYSTEM-IDENTIFICATION BY USING SHORT-CUT TECHNIQUES IN EARLY SAFETY ASSESSMENT OF CHEMICAL PROCESSES

The chemical industry uses complicated reacting systems under a wide r ange of physical conditions. Besides production maximisation, stable a nd safe operation are important goals. This objective has to be consid ered in all the engineering calculations from the process design phase until the plant operation. Modelling and simulation of the processes accelerates and improves design. It may include qualitative informatio n and/or heuristic rules. Process modelling, optimisation and control allow safe plant design and operation if enough information about the process is available. An experimental and a numerical technique will b e combined in this paper. The 'Differential Scanning Calorimetry' (DSC ; Hohne, Hemminger & FIammersheim, 1996, Differential scanning calorim etry) is an experimental technique that can be used to investigate the mechanism and kinetics of a chemical process by measuring the thermal effect of the reaction following a very elaborated strategy. At early stages in process design, DSC is used as a screening tool to assess t he thermal safety. Under the assumption of zero order Arrhenius kineti cs, the activation energy and therefore the time to maximum rate under adiabatic conditions (TMRad) may be estimated (ANSI/ASTM, E 698-79; K eller, Stark, Fierz, Heinzle & Hungerbuhler, 1997, J. Loss Prev. Proce ss Ind., 10, 31-41). A similar way can be applied to determine the kin etic constants and TMRad for an nth order reaction. Attempts to fit mo re complex kinetic models to DSC thermograms encounter many difficulti es; one of them is model discrimination, a question not yet answered. The 'Modified Integral Transformation Procedure' (MIP; Maria & Rippin, 1997, Comp. & Chem. Eng., 21, 1169-1190) was proposed for quick proce ss identification by considering previous information stored in data-b anks and incomplete information about the process. The estimation tech nique is effective even if few but distributed process data are availa ble and it can be easily coupled with other statistical data analysis and estimation techniques. The MIP is integrated in an expert system f or process identification (Maria & Rippin, 1996, Comp. & Chem. Eng., S 20, S587-S592) which facilitates computer-based plant analysis. It is the scope of this paper to investigate the effectiveness of using thes e coupled experimental and numerical short-cut techniques and an inter active data-bank in quick identification of complex chemical kinetics. If model discrimination is not possible, an experimental procedure to close existing data gaps most efficiently can be developed. The ident ified model can be further used in predicting optimum operating condit ions for a chemical process by considering the desired product maximis ation, waste and by-product minimisation, safety and operability as go als. The developed quick experimental and PC-coupled numerical identif ication and process analysis are exemplified in some simple complex ki netic cases. The effectiveness of the elaborated calculation methodolo gy is discussed together with possibilities of further improvements. ( C) 1998 Elsevier Science Ltd. All rights reserved.