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Calorimetric study of the inibition of runaway reactions during methylmethacrylate polymerization processes
Get PDFReaction inhibition was adopted as a method to halt runaway phenomena during polymerization experiments. Use of reaction calorimetry coupled with a particular system for early detection of the onset of runaway (early warning detection system) has allowed to investigate the behaviour of two substances that can influence the reaction rate: hydroquinone and 1,4-benzoquinone. The process studied was the free-radical polymerization of methylmethacrylate carried out under batch conditions in bulk or in emulsion. The results show that hydroquinone and 1,4-benzoquinone behave differently: the first is an inhibitor because it completely stops the process; the second behaves as a retarder and could be used industrially to control the process and keep the reactor temperature within safe limits
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
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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
Control of polymerization processes using an inhibitor
No full textIn this work the behaviour of two substances (inhibitors or retarders) has been studied in a series of methylmethacrylate polymerization experiments by using calorimetric techniques.
The substances used were: hydroquinone and 1,4-benzoquinone.
In order to identify situations that can lead to a runaway reaction, an Early Warning Detection System (EWDS) has been used.
The detection criterion for runaway is based on the calculation of the divergence of the reactor.
When the EWDS signals an alarm, small amounts of inhibitor or retarder have been added to the reaction mixture.
The use of substances that can destroy the free radicals during polymerization processes is a good method to stop the reaction if a runaway phenomenon occurs.
In this work the behaviour of hydroquinone and 1,4-benzoquinone was investigated in methylmethacrylate polymerization processes.
The first behaved as an inhibitor, the second as a retarder.
In order to control an emulsion polymerization process benzoquinone is preferred because, if added during the reaction, it does not completely stop the process but diminishes the polymerization rate giving a higher final conversion.
An Early Warning Detection System (EWDS) has been used to identify anomalous process conditions that can lead to runaway reactions.
The EWDS performance has been satisfactory: in fact anomalous temperature behaviours have been early detected without producing false alarms
Application of an Early Warning Detection System to batch and semi-batch polymerization processes
No full textThe EWDS is an electronic device designed to recognize, on-line and well in advance, anomalous process conditions in chemical reactors that can lead to runaway reactions.
The EWDS consists of hardware and software parts:
In the hardware part we have:
PT resistance thermometers to measure reactor and jacket temperatures; a data acquisition unit; a supply unit that complies with industrial safety standards and a PC equipped with a special data acquisition card.
The software part is used to save and elaborate data, to show the results on line and to provide warning and alarm signals.
For the bench scale reactor experiments the EWDS performance has been satisfactory. The algorithm, in fact, has detected the runaway events in advance without producing false alarms.
For the industrial experiments two different algorithms have been used.
Algorithm n.1 was able to distinguish between runaway and non-runaway situations, because it is mainly focused on the dynamics inside the reactor.
Algorithm n.2, instead, produced false alarms during the manual opening of the jacket valve by the operator.
The results that I have presented refer to the first versions of the algorithms.
In the future a new version of the EWDS algorithm will be tested. This version uses only the reactor temperature and it does not use derivatives. So the calculation will be simpler compared to the first versions
Progetto AWARD: Applicazione del sistema EWDS ai processi di polimerizzazione batch e semi-batch
No full textVariations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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