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In order to prevent incidents in the
industrial production of Teflon® from
Tetrafluoroethylene (TFE), the safety related
aspects of the self-heating process of TFE must
be taken into account. Simulations of this self
heating process were performed with COMSOL
Reaction Engineering Lab® and COMSOL®
COMSOL Multiphysics®. A complex reaction
net was employed, resulting in a model allows
for a good agreement with experimentally
achieved results.
COMSOL Multiphysics® was used as a tool for the better understanding of the evolution of flow patterns during the induced ignition of gases. A simplified model was developed for the scope, by coupling the weakly compressible Navier Stokes module and the convection and conduction module. The current paper presents the results of the performed simulations.
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.
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.
This work investigates the ignition of tetrafluoroethylene induced by the adiabatic compression that can arise by activating a high speed valve separating two portions of a pipeline with a high pressure difference. In the tests performed the high pressure zone contained tetrafluoroethylene at pressures between 15 and 30 bar. For the low pressure zone, experiments with nitrogen, air and tetrafluoroethylene were carried out. The pressure range in the low pressure zone was comprised between 0.05 and 1 bar. The pipe diameters analyzed were 15 and 20 mm. For the analyzed geometries, special conditions were required in order to reach reproducible ignitions, namely air at temperatures of at least 105 °C had to be present in the compression pipe. Furthermore, a minimum length of the compression pipe had to be used. The current work describes the experimental setup employed for the tests and discusses the achieved results. Numerical simulations were performed in order to clarify unexpected findings.
The Ignition Temperature (IT) of stoichiometric tetrafluoroethylene–air mixtures on hot walls was determined in a 3-dm³-reactor. Tests at elevated pressure conditions were performed, namely at 5, 15 and 25 bar(a), showing a decrease of the IT with the initial pressure. Furthermore, the measured ignition temperatures of stoichiometric tetrafluoroethylene–air mixtures were lower than the ignition temperatures required for the decomposition pure tetrafluoroethylene (Minimum Ignition Temperature of Decomposition, MITD) reported in previous works.
Equations from the Semenov thermal explosion theory on spontaneous ignition were used to identify approximate combustion kinetics of tetrafluoroethylene from the experimental results. The determined kinetics was used for the prediction of the IT of stoichiometric tetrafluoroethylene-air by simplified calculation methods. A very good agreement with the experimental results was observed.
There is a lack of data on the self-ignition behaviour of tetrafluoroethylene (TFE) in industrial sized
equipment. In order to assess the tendency of TFE for
decomposition in large scale vessels, , a facility was
designed and constructed. Tests were
carried out in a cylindrical reactor with a volume of 100 L with initial
pressures of 5 and 10 bar(a). The effect of the reacto
r adjustment (vertical or horizontal) was taken into
account. The current work describes the test set
up and summarizes the experimental results achieved.
Furthermore, this paper reports on a numerical model for
the prediction of the self
-heating of TFE in closed
vessels, which was previously developed for small scale reactors and has been here validated for larger
dimensions with the experiments performed in the new facility.