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- 2012 (8) (entfernen)
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- Tetrafluoroethylene (3)
- Decomposition (2)
- Large-scale reactor (2)
- Self-ignition (2)
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Eingeladener Vortrag
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Preventing the explosion of acetylene cylinders involved in fire with help of numerical modeling
(2012)
The current paper describes a mathematical model, which was developed to simulate the heat transfer in acetylene cylinders during exposure to a fire. The cases of a direct engulfment of the cylinder in the flames and of exposure to a distant fire were considered. Furthermore, the model was also applied to the prediction of the heat transfer during the cooling with water of heated acetylene cylinders, in order to assess the effectiveness of this procedure as a measure to prevent the burst of the cylinder. To provide data for the definition and validation of the model a total of 13 bonfire tests with 8.9-, 10- and 50-dm³-cylinders were performed, where pressure and temperature measurements in the samples were performed. During 5 experiments the fire was extinguished before the expected cylinder burst and a cooling with water was applied. In the paper a short description of the experimental set-up and of the test results is given. Finally, a comparison with the model predictions is provided, showing reasonable agreement.
The Minimum Ignition Temperature of Decomposition (MITD) of tetrafluoroethylene in a partially heated pipe was analyzed for different initial pressures (5, 10 and 15 bara). The pipe used had an internal length of 1 m, an internal diameter of 30 mm with a volume of about 0.7 dm³ and was vertically oriented. Pressure at the pipe top and temperature at four different locations along the pipe axis were measured. Tetrafluoroethylene was found to decompose at lower temperatures for increasing initial pressures, in agreement with previous tests with reactors with fully heated walls. A complete passive quenching in the non-heated part of the pipe was observed only for an initial pressure of 5 bara, while for higher initial pressures, the decomposition propagated completely along the test pipe. Moreover, the test results on the MITD were compared with data from previous experiments in fully heated 0.2 and 3 dm³ cylindrical reactors and showed a decrease of the MITD with the heated volume through heated surface ratio of the vessel. Furthermore, the prediction of the MITD of tetrafluoroethylene by simplified calculation methods was attempted, showing a good agreement with the experimental results.
Acetylenflaschen im Feuer
(2012)
Acetylen ist eines der wichtigsten technischen Gase und findet insbesondere Anwendung beim Schweißen und Schneiden verschiedener Werkstoffe. Damit Acetylen in entsprechenden Mengen vor Ort zur Verfügung gestellt werden kann, wird es in Druckgasbehälter abgefüllt. Im Falle eines Brandes stehen Acetylenflaschen im Verdacht, jederzeit – auch nach Löschen des Feuers – aufgrund einer möglicherweise initiierten Zerfallsreaktion zu bersten. Um insbesondere Personenschäden zu vermeiden, evakuieren und sperren die Rettungskräfte einen Sicherheitsbereich mit einem Radius von mindestens 200 m um die verdächtige Flasche ab. Gleichzeitig wird umgehend mit der Kühlung der Flasche begonnen. Wie lange muss aber eine Acetylenflasche gekühlt werden, bis sie sicher abtransportiert werden kann, und wie lange muss der Sicherheitsbereich mit dem großen Radius aufrecht erhalten werden? Diesen Fragen ist die BAM in den vergangenen Jahren im Rahmen verschiedener Untersuchungen und Projekte nachgegangen.
Adiabatic compression of gases can work as an ignition source and is still one of the main causes of accidents in chemical plants processing tetrafluoroethylene (Reza and Christiansen, 2007). The ignition of tetrafluoroethylene induced by adiabatic compression has been studied experimentally with a setup which allowed for the rapid opening of a high speed valve connecting two portions of a pipeline at different initial pressures. Due to the fast opening time and to the high pressure difference, a shock wave in the pipeline was generated. The propagation of the shock wave and its reflection at the end of the pipeline caused pressure and temperature increase. This led to some ignitions in the experiments performed. Nonetheless, in some test an ignition was not achieved, even if this was expected according to the theoretical temperatures predicted by the Rankine-Hugoniot equations. In order to understand the discrepancy between the experimental results and the theoretical predictions, shock wave simulations have been carried out with COMSOL Multiphysics. The 'High Mach Flow' interface was used, since it solves the heat and impulse equations for fast flows. Figure 1 and Figure 2 show, respectively, the velocity and temperature distribution over time for a simulation in a 0.2 m pipeline of 20 mm in diameter with the following settings:
- high pressure section: nitrogen initially at 20 bar;
- low pressure section: nitrogen initially at 1 bar;
- initial temperature of the system: 20 °C;
- adiabatic walls with slip condition for the flow;
- laminar flow.
As from Figure 1 and Figure 2 the shock wave generation and propagation has been properly computed and the physical properties of the shock wave reflected the prediction of the Rankine-Hugoniot equation. Nonetheless, divergence problem occurred when trying to add turbulence to the system and strange temperature and profiles after the shock wave reflection were achieved if the no slip condition at the walls was chosen. Despite these limitations, it was possible to perform a parametric study and to analyze the effect of the pipe diameter and length on the shock wave evolution. Here simulations with constant wall temperature were carried out, in order to account for the heat losses to the pipe surroundings. Figure 3 shows that the temperature of the reflected wave is maintained for a longer time, if the pipe diameter is larger, due to minor heat losses. On the other hand, Figure 4 shows that higher average temperatures are achieved and maintained for a longer time if the pipe length increases. These results suggest that in the experiments performed by Meyer (2009) the pipe geometry was probably not optimal for the achievement/conservation of high temperatures and might explain the difficulty in inducing ignitions by adiabatic compression.
There is a lack of data on the self-ignition behaviour of tetrafluoroethylene in industrial sized equipment. Therefore, a facility was designed and constructed for the determination of the Minimum Ignition Temperature of Decomposition of tetrafluoroethylene in a cylindrical reactor with a volume of 100 dm3. Tests with initial pressures of 5 and 10 bar(a) were performed. The Minimum Ignition Temperature of Decomposition of tetrafluoroethylene was observed to decrease with the initial pressure, in agreement with previous experiments with small scale cylindrical vessels. This paper describes the test set-up und gives an overview of the achieved experimental results. In particular the effect of the reactor orientation (vertical or horizontal) is discussed. Furthermore, simplified equations from the Semenov thermal explosion theory are used to attempt extrapolations of previous and current data on the Minimum Ignition Temperature of Decomposition of tetrafluoroethylene to other vessel volumes or initial pressures. Moreover, the experimental data are plotted together against the heated volume to heated surface ratio, which should provide a better extrapolation to other vessel dimensions by taking into account that the efficiency of the dispersion of the heat generated by the reaction is different for two reactors with the same volume but different diameter. Finally, simplified methods for predicting the Minimum Ignition Temperature of Decomposition of tetrafluoroethylene presented previously by the authors are validated for large scale reactors with the experimental data collected within the current work.
In the industry there is a lack of data on the Minimum Ignition Temperature of decomposition of tetrafluoroethylene in industrial sized equipment. In order to determine the Minimum Ignition Temperature of Decomposition of tetrafluoroethylene in large scale vessels, a facility was designed and constructed. Tests were carried out in a cylindrical reactor with a volume of 100 dm³ with initial pressures of 5 and 10 bar(a). In agreement with previous experiments with small scale cylindrical vessels, the Minimum Ignition Temperature of Decomposition of tetrafluoroethylene was observed to decrease with the initial pressure. The current paper describes the test setup and summarizes the experimental results achieved. The effect of the reactor adjustment (vertical or horizontal) is discussed. Moreover, simplified equations from the Semenov thermal explosion theory are used to attempt extrapolations of previous and current data on the Minimum Ignition Temperature of Decomposition of tetrafluoroethylene to other initial pressures and vessel volumes.