53 research outputs found
An Integrated approach for the early detection of runaway reactions by using UV-visible and temperature sensors
No full textWe report here on a novel approach for the early runaway detection in chemical reactors, based on the
integration of two kinds of sensors: i) a set of three Pt thermo-resistances for measuring the temperatures
both within the reactor and in the cooling jacket and ii) an UV-visible probe for the indirect evaluation of the
conversion through measurements of light absorbance. The measured variables (temperature and
conversion) were used as input to our model based on the divergence calculation. The early warning
detection system (EWDS) was tested for the sulphuric acid catalyzed esterification of acetic anhydride and
methanol, a very simple reaction but releasing ~ 70 kJ per mole of anhydride consumed. The responses
given by EWDS were examined during the simulation of runaway reactions in a lab-scale reactor working
under batch isoperibolic conditions. Different chemical heat flows were generated by varying the
concentration of the sulphuric acid and adding at once the acetic anhydride into the reactor. The behaviour
of the detection criterion was evaluated comparing the EWDS signals using both temperature and
conversion as input variables, with the responses obtained from only temperature measurements. A
detailed kinetic model was also developed to solve the differential energy and mass balance equations
and define the runaway boundaries. Results showed the importance of an input variable indirectly related
to conversion in such kinds of processes where other enthalpy variations (i.e. due to an endothermic
mixing of the reagents) may hide a runaway reaction occurring
AN INTEGRATED APPROACH FOR THE EARLY DETECTION OF RUNAWAY REACTIONS BY USING UV-VISIBLE AND TEMPERATURE SENSORS
No full textWe report here on a novel approach for the early runaway detection in chemical reactors, based on the integration of two kinds of sensors: i) a set of three Pt thermo-resistances for measuring the temperatures both within the reactor and in the cooling jacket and ii) an UV-visible probe for the indirect evaluation of the conversion through measurements of light absorbance. The measured variables (temperature and conversion) were used as input to our model based on the divergence calculation. The early warning detection system (EWDS) was tested for the sulphuric acid catalyzed esterification of acetic anhydride and methanol, a very simple reaction but releasing ~70 kJ per mole of anhydride consumed. The responses given by EWDS were examined during the simulation of runaway reactions in a lab-scale reactor working under batch isoperibolic conditions. Different chemical heat flows were generated by varying the concentration of the sulphuric acid and adding at once the acetic anhydride into the reactor. The behaviour of the detection criterion was evaluated comparing the EWDS signals using both temperature and conversion as input variables, with the responses obtained from only temperature measurements. A detailed kinetic model was also developed to solve the differential energy and mass balance equations and define the runaway boundaries. Results showed the importance of an input variable indirectly related to conversion in such kinds of processes where other enthalpy variations (i.e. due to an endothermic mixing of the reagents) may hide a runaway reaction occurring
An Isotropic Analytical Vector Preisach Model Based on the Lorentzian Function
No full textThe modeling of vector behavior of magnetic materials is an open research topic, it is used in a large number
of different applications. Several models have been proposed, these must satisfy two physical properties:
saturation and loss. This paper introduces a new vector Preisach hysteresis model that, differently by
the Mayergoyz approach, is based on the super-position of 2-3 Classical Scalar Preisach Models in orthogonal
directions along the principal axes of the system, assuming that the Preisach functions are independent
for each axis. The Lorentzian function used as analytical Preisach approximation allowing to
write the magnetization and its derivative with respect to the magnetic field in an analytical closed form.
Lastly, the paper shows as the model satisfies saturation and loss properties
Reaction inhibition as a method for preventing thermal runaway in industrial processes
No full textIn this work inhibitors were used to prevent runaway reactions during
methylmethacrylate suspension polymerization processes. The main problem that
may more frequently occur in chemical reactors, carrying out free radical polymerization
reactions, is the loss of temperature control. The addition of an inhibitor
during polymerization processes can be considered as a good method to stop or at
the least slow down the reaction. In this work two inhibitors were used: hydroquinone
and 1,4-benzoquinone in a series of polymerization experiments. In order to identify
situations that can lead to a runaway reaction, an early warning detection system
based on the divergence criterion was used. When this system signalled an alarm,
small amounts of inhibitor were added to the reaction mixture. The results showed
that hydroquinone and 1,4-benzoquinone behave slightly differently and the reactor
temperature can be kept within safe limits
Applicazione della Calorimetria a Temperatura Oscillante allo studio di processi di polimerizzazione
No full textTemperature Oscillation Calorimetry (TOC) is a technique for the on line determination
of heat transfer coefficient UA, and is therefore suitable for studying processes where there
is a large increase in the viscosity and a consequent decrease in UA as the reaction
proceeds.
The free radical polymerization of methyl methacrylate in ethyl acetate has been studied
in a stirred, jacketed calorimeter with a volume of two litres. Sinusoidal temperature oscillation
of amplitude 1-2°C and period 5-6 minutes are imposed on the fluid circulating in
the jacket. The heat transfer coefficient is obtained from an analysis of the oscillating
parts of the jacket and reactor temperatures, while the chemical heat flow is obtained from
the non-oscillating parts of these temperatures.
The results obtained show the utility of TOC in studying polymerization processes. A limitation
has been encountered with concentrated solution where both the temperature
oscillation and the value of UA may become irregular. Under these conditions a correlation
between UA and the stirred torque may be used to correct the values of UA
Fitting isoperibolic calorimeter data for reactions with pseudo-first order chemical kinetics
No full textThe non-linear least squares fitting of the chemical heat flow and the reactor temperature are compared for reactions with
pseudo-first order chemical kinetics carried out in an isoperibolic calorimeter operating quasi-isothermally. Both methods give very
similar results for the reaction rate constant and enthalpy of reaction but fitting the reactor temperature appears to have some advantages
especially when there is an enthalpy of mixing of the reagents
Control of Polymerization Process using an Inhibitor
No full textIn a previous paper (Ampelli et al., 2006, J. Loss Prevent. Proc., 19, 419-424) inhibitors were used to prevent runaway reactions during methylmethacrylate (MMA) emulsion polymerization processes. In this work the reaction inhibition was extended to suspension polymerizations.
Radical polymerization processes are often characterized by high exothermicity and self-accelerating kinetics, which influence strongly the heat and mass transfer phenomena. The main problem that more frequently may occur in chemical reactors carrying out polymerization reactions is the loss of temperature control. In this situation, if the rate of heat generation by chemical reaction exceeds the rate of heat removal by the cooling system, the temperature of the reaction mass will rise, increasing in turn the heat generation rate and a runaway reaction may occur.
The use of aqueous suspensions allows to carry out the polymerization process in presence of water as dissipation means, but leads to several difficulties, such as the homogeneity of particle size, the stability of the suspension and the possible separation of the mixture in two phases with the formation of hot spots within the reactor. All these phenomena are strongly dependent on the operative conditions (reaction temperature and stirring of the reactor). For this reason control of suspension polymerization reactions is of paramount importance and safety considerations are necessary to prevent runaway reactions.
Reaction inhibition involves injecting small quantities of a particular substance into the reactor at an early stage of the runaway. The substance used can either stop the reaction completely (inhibitor) or lower the reaction rate (retarder). The suitability of a system for inhibition is dependent on the reaction mechanism occurring in the reactor. Inhibition is ideally suited to processes involving free radicals in the initiation phase, such as suspension polymerization of MMA.
In this work two inhibition substances were used: hydroquinone and 1,4-benzoquinone. Preliminary tests were carried out using a Differential Scanning Calorimeter (DSC) to understand the influence of these two substances in bulk polymerization reactions of MMA; afterward other experiments were carried out using a small stainless steel jacketed reactor (200 ml) under isothermal batch conditions.
An early warning detection system (EWDS) was used to identify situations that can lead to runaway reactions. EWDS is based on the calculation of the reactor divergence; the detection criterion was introduced by Strozzi et al. (1999, AIChE Journal, 45, 2429-2443). When EWDS warned of alarm, small quantities of an inhibition substance were injected into the reactor in order to control the process.
The results have shown the reaction inhibition as being a good method to stop runaway phenomena and control the polymerization process. The use of a retarder, such as 1,4-benzoquinone, should be preferred with respect to an inhibitor, because it does not completely stop the reaction and allows to obtain a high final conversion value
Studio di processi di polimerizzazione in emulsione mediante l'uso di tecniche calorimetriche integrate
No full textThe Integration of Calorimetric Techniques with an Early Warning Detection System
No full textThe reliability of a chemical reactor installed in a industrial plant is very important for the safety conditions of a process. It depends on the capability of the control and supervision system to estimate its state and to identify in time malfunctions or failure modes that can take place during the operating phases. The major problem that more frequently may occur in chemical reactors carrying out exothermic reactions is the loss of temperature control. In this situation, if the rate of heat generation by chemical reaction exceeds the rate of heat removal by the cooling system, the temperature of the reaction mass will rise , increasing in turn the heat generation rate. In this case, if no countermeasures are taken, a runaway reaction may occur.
This work has been carried out within the European Research Project AWARD, (Advanced Warning And Runaway Disposal). This project intends to improve the quality, safety and reliability and reduce accidents in chemical plants by developing and testing a new device for early warning detection of runaway events (EWDS), based on the results of a previous research carried out at JRC of Ispra on the application of chaos theory techniques to discontinuous reactors. A prototype of the EWDS has been realized by Segibo and coupled to a reaction calorimeter at the University of Messina. The experiments carried out at Messina have allowed the validation of the EWDS in small scale reactors with reactions of industrial interest.
The reaction studied is the free radical emulsion polymerisation of acrylic monomers, an important class of reactions of great industrial relevance. These reactions are characterized by a high exothermicity and self-accelerating reaction kinetics. Because of these characteristics polymerisation reactions have given rise to incidents due to thermal runaway. Batch and semi-batch experiments have been carried out in a jacketed, stirred reactor (capacity of 2 litres). For the batch tests, made in the temperature range 60-90°C, the emulsion is initially prepared in the reactor and brought to the reaction temperature: Before continuing the experiment the stability of the emulsion is checked; then the reaction is started by introducing an aqueous solution of the initiator into the reactor. The results obtained show that, overall for high temperatures, the reaction starts immediately after the addition of the initiator and there is such a rapid and large release of heat that the cooling system is not able to prevent a large increase in the reactor temperature. In some case rapid temperature increases of 40°C have been observed. It is not difficult to understand that, if a similar temperature increase should occur in an industrial reactor, a dangerous event could take place. The anomalous behaviours of the reactor temperature occurred during the batch experiments have been early detected by the EWDS, without any false alarms.
The semi-batch experiments were made by initially charging the reactor with a partial amount of an aqueous solution of the emulsifier and bringing the system to the reaction temperature. When the system had stabilized a small quantity (~5%) of the monomer emulsion was added to the reactor. When the system had reached a stable temperature a small quantity (~5%) of the initiator solution was added and the reaction was considered to have started. After 15 minutes the semi-batch process was started by starting the constant flows of the monomer emulsion and initiator solution into the reactor. For both the batch and semi-batch experiments a constant stirrer speed in the range of 200-250 rpm has been used
La calorimetria di processo: uno strumento per il monitoraggio e controllo di reattori industriali
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