Reaction Calorimetry and UV-Vis Spectrophotometry Integration aimed at Runaway Reaction Early Detection

The use of different calorimetric techniques for process design and scale up is well consolidated, allowing the definition of the kinetic and thermodynamic of the process and the evaluation of several parameters useful to optimization and process safety. During a calorimetric analysis temperature (or a temperature difference) is measured; from experimental data it is for example possible to calculate the heat flow and the heat evolved by the reaction and consequently the conversion and the reaction rate constant just if kinetics of the reaction is known. To overcome these limitations and to obtain additional information about the reaction, it is possible to couple to a calorimeter different type of sensors such as densitometers, refractometers, electrochemical, chromatographic and spectrophotometric probes (Schwedt, 1997). Some of these sensors are able to log data on line during the process and so to monitor the reaction at the same time of the calorimetric analysis. For safety aims, it is important to choose a sensor whose time for the analysis is comparable to the reaction time in order to have an accurate profile of the monitored parameter during the reaction (Moritz, 1989). In a previous study (Parisi, 2002; Ampelli et al. 2003) an Ultra Violet - Visible spectrophotometer was integrated to a reaction calorimeter to analyse the kinetic of a specific reaction. In this work a similar UV - Vis probe has been coupled to an isoperibolic reaction calorimeter in order to study the feasibility of the application of an Early Warning Detection System (EDWS) to a spectrophotometric signal. The innovation of this integration of techniques is the analysis of the spectrophotometric signals with a reactor stability criterion based on divergence theory in order to check if this method of monitoring allows detecting runaway reactions at an early stage of the process, when it is still possible to take protective measures. The innovation of the integration between the calorimetric and the spectrophotometric techniques is the application of an Early Warning Detection System based on divergence criterion to spectrophotometric signals in order to check if this method of monitoring high reactive processes could lead to a greater advance in the detection of the thermal explosion with respect to temperature monitoring. The results are that EWDS does not give a good result in the detection of the onset when applied to Absorbance signal: it is not possible to define a ΔVLIM, and the profile of ΔV does not show a peak when adding catalyst. This makes the analysis not reliable and subject to false alarm. This monitoring cannot be used for process safety purpose. The application of the method to the Intensity signal gives different results. There is the detection of the onset 1 second before the profile of Intensity starts to decrease and the noise is more smoothed than in the previous case. The algorithm is able to detect the runaway start 59 seconds before the maximum of temperature is developed (6 seconds before the application to temperature signal). This monitoring could be used at the same time than temperature monitoring, in order to improve the efficiency of the system in detecting runaway reaction decreasing the possibilities of false alarms. The method could be very useful if applied to those processes in which the accumulation of a reaction intermediate, that develops no temperature increase in the system, could lead to thermal explosion by starting a secondary reaction: in this case the EWDS working on Intensity signal (that can be monitored on line, during the process) could detect the runaway at a very early stage, when the temperature in the system could still be kept under control by some protective measure activated after the alarm. The integration of calorimetry and spectrophotometry gave good results: spectrophotometry is able to supply useful data for process modelling and also for safety purpose. This methodology could be easily implemented in a plant-scale reactor providing a valid process safety monitoring against runaway reactions. The divergence criterion account reasonably well for the thermal runaway in the considered reaction. An advantage of the Strozzi and Zaldívar criterion over existing criteria is that it is possible to reconstruct, using non-linear time series analysis techniques, the divergence of the system from a monitored signal without the necessity to have a model for the process. Hence, all the results and conclusions obtained from an off-line analysis may be extended and applied on-line to develop a general early warning detection device, based on a robust criterion.