Improvements in the Modeling of the Self-ignition of Tetrafluoroethylene

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1 Exerpt from the Proeedings of the OMSOL onferene 010 Paris Improvements in the Modeling of the Self-ignition of Tetrafluoroethylene M. Bekmann-Kluge 1 *,. errero 1, V. Shröder 1, A. Aikalin and J. Steinbah 1 ederal Institute for Materials Researh and Testing, Tehnial University Berlin *orresponding author: Unter den Eihen 87, 105 Berlin, martin.bekmann-kluge@bam.de Abstrat: In order to prevent inidents in the industrial prodution of Teflon from Tetrafluoroethylene TE, the safety related aspets of the self-heating proess of TE must be taken into aount. Simulations of this self heating proess were performed with OMSOL Reation Engineering Lab and OMSOL OMSOL Multiphysis. A omplex reation net was employed, resulting in a model allows for a good agreement with experimentally ahieved results. Keywords: heating, free onvetion, reative flow, gas reation, self ignition 1. Introdution Tetrafluoroethylene TE is the monomer of the widely used polytetrafluoroethylene PTE, also known as Teflon. In addition to its high flammability, TE belongs to the small group of deomposable gases. These gases are apable of exothermi reations without the need of an oxidant. Under speifi onditions, whih an even our in the prodution proess, an exothermi dimerization of TE an start, leading to a self heating of the gas phase [Eq. 1] H R = 83kJ kj mol -1 Dimerization In some ases this an initiate an explosive exothermi deomposition reation of the TE, forming tetrafluoromethane and arbon blak. By reahing loal temperatures of more than 1000 K a thermal ignition is likely to be happening. In the past years a number of inidents in PTEproduing plants all over the world were aused by this self-ignition of TE. The deomposition reation produes tetrafluoromethane and arbon blak, aording to the following reation. + [Eq. ] H R = 57 kj mol -1. Theory Deomposition In a previous paper the self heating of TE in a losed vessel using a simple reversible reation was desribed [3]. The reation rate of a hemial reation is mainly depending on the temperature. If an exothermi reating gas or mixture is enlosed in a vessel and surrounded by walls with a onstant temperature then it is possible to define a speifi wall temperature at whih the heat loss through the walls is equal to the heat prodution by the reations. By inreasing the wall temperature, the heat release by the reation would inevitably lead to a runaway resulting in a so alled thermal explosion. or the oxidation proess of flammable gases a speial test proedure aording to EN 15 is used to determine the minimum temperature at atmospheri pressure when an ignition by hot walls takes plae. The regarding wall temperature is alled the Minimum Ignition Temperature MIT. Beside this MIT deomposable gases an also reat without an oxidant when heated. To determine an analogue safety related property for deomposable gases another method is required to avoid the ontat with other reatants, suh as the oxygen in the air. Several deomposable gases an deompose only above a speifi pressure, the so alled limiting stability pressure. Therefore a losed pressure resistant vessel is used. With an adequate setup, tests at even elevated onditions an be arried out. The minimum temperature leading to an explosive deomposition in a losed vessel is then alled the Minimum Ignition Temperature of Deomposition MITD. 3. Motivation This study originally started by the initiation of a researh projet subsidized by an industrial assoiation, PlastisEurope, to improve the safety in PTE produing fatories.

2 The aim of the study was to determine the MITD for TE at elevated ondition and to reate a numerial model whih is apable of simulating the self heating proess of TE. It is expeted that the researh will also reveal the ritial onditions that are responsible for the self ignition of TE gas. If this an be ahieved further aidents may be prevented. Parallel to the numerial work the model is ontinually validated by experiments. additional reations are onsidered, as shown in igure [Eq. 3] H R = 0 kj mol Deay of Dimer e. Numerial model e In order to transfer the results to real sale geometries used in industry, OMSOL Reation Engineering Lab and OMSOL Multiphysis were employed to model the problem. or the simulations with OMSOL Reation Engineering Lab, a heat and mass balane in a bath reator was onsidered. Up to 11 hemial reations were oupled to desribe the self heating proess. urthermore, a heat transfer oeffiient gas to wall was alulated for eah time step, based on the Grashoff, Pelet and Nusselt numbers. Regarding the OMSOL Multiphysis software, three appliation modes of the hemial Engineering Module are linked. The non-isothermal flow mode hns is used to desribe the free onvetion aused by the volume fore generated by different densities due to the exothermi dimerization reation. As there is no phase hange during the dimerization reation and the analyzed system is losed, the total gas density stays onstant and the nonisothermal appliation mode ould be used wherein the gas density only depends on the temperature. However there is a prediament regarding the pressure that is generated in this appliation mode. This is beause the pressure alulated with this module depends on the temperature only and not on the number of moleules, but during the dimerization the number of moleules dereases. However, the total density remains unhanged, beause there is no phase hange during the reations, as mentioned. The onvetion and ondution mode h is used to model the heat transfer resulting from the extended reation net. Beside the exothermi dimerization reation [Eq. 1] and the endothermi bak reation [Eq. 3], four igure 1: extended reation net []. 3 3 e To lose the equation system, the onvetion and diffusion mode hd is used to model the mass balane by linking the reation kinetis. When onsidering, the omplex reation net, there are nine unknown field variables depending on eah other: The veloity field omponents, u and v; The pressure, p; The temperature, T; The onentration of Tetrafluoroethylene, 1; The onentration of the dimer 8, ; The onentration of 3 ; 3; The onentration of ;. The geometry and the orresponding mesh used in the alulations are shown in igure. Due to the symmetry of the problem a D geometry using the axial symmetry modus was used. Table 1 shows the mesh statistis for the used geometries in the Multiphysis model. The used mesh had always a quality higher than 0.. Table 1: Mesh statistis for different vessel sizes Mesh parameter 0.-dm³-vessel 3-dm³-vessel Number of elements Degrees of freedom Element aria ratio

vessels via the pressure drop meassuring inside the autolave [7] using a method desribed by Matula []. 1 3 igure : Geometry and mesh used in the alulations, left: 3-dm³-vessel, right: 0.

Experimental work The dimerization reation was studied experimentally in several test series in losed vessels with volumes ranging from 0. dm³ to 3 dm³.

Before eah experiment the vessel was evauated and then heated to the desired initial temperature. Then, TE was filled to the defined initial pressure.

3 vessels via the pressure drop meassuring inside the autolave [7] using a method desribed by Matula []. 1 3 igure : Geometry and mesh used in the alulations, left: 3-dm³-vessel, right: 0.-dm³-vessel This model mainly onsists of a gas domain that is surrounded by fixed walls vessel at a onstant temperature. There is no inlet or outlet, beause a prefilled losed vessel is modeled. 5. Experimental work The dimerization reation was studied experimentally in several test series in losed vessels with volumes ranging from 0. dm³ to 3 dm³. igure 3 shows a shemati view of the 3 dm³ autolave, where the employed measuring devies are detailed. The internal pressure and internal temperature of the vessel were measured during the tests. Before eah experiment the vessel was evauated and then heated to the desired initial temperature. Then, TE was filled to the defined initial pressure. The inlet was losed and pressure and temperature inside the vessel were reorded. If a peak in the pressure and temperature urves was observed, this was defined as an ignition. Tests were repeated for eah initial pressure, where the temperature was varied in steps of ± 10 K, until the lowest temperature leading to an ignition was deteted. This temperature is the MITD for the speifi initial pressure and vessel volume. urthermore, in a previous researh projet the kinetis of the dimerization reation of TE also for elevated onditions was studied in losed igure 3: Setional view of a 3 dm³ vessel used for the experiments to determine the MITD for TE; 1 thermoouple, pressure transduer, 3 internal vessel volume ylindrial reation hamber The pressure in a losed vessel drops during the dimerization reation beause the number of moleules is redued. This pressure derease an be desribed by linking the ideal gas law, the reation equation and the rate equation, aording to the following equation. = 1 1 p t + p t 0 k 1 T t + RT p t0 [Eq. ] In this equation the rate onstant k also depends on the temperature. By hoosing relatively low temperatures for the walls the temperature inrease by the exothermi reation ould be nearly neutralized in the experiments. Then the temperature in [Eq. ] stays onstant and for eah wall temperature a rate onstant ould be determined.

Aording to the performed experiments, an Arrhenius seond order reation rate forward reation [Eq. 1] was observed, in agreement with previous results [Matula, 198; Buravtsev et al.

4 Aording to the performed experiments, an Arrhenius seond order reation rate forward reation [Eq. 1] was observed, in agreement with previous results [Matula, 198; Buravtsev et al., 1985; Babenko et al., 1998]. The following expression represents the reation rate: RT [ J/mol] m³ k f = 8800 e [Eq. 5] mol s The previous equation is valid for low temperatures. In fat, all experiments for the determination of the dimerization kinetis were done at temperatures below 550 K, due to the neessity of a small reation rate. The neessary bakward reation [Eq. 3] whih is not negligible above temperatures of 550 K was taken from several authors found in the NIST kinetis data base, and is expressed as follows: [ J/mol] 1 m³ RT,1 10 [Eq. ] k b = e mol s where: R 1 rf,, 1 + r b = k f + kb = [Eq. 9] R 1 is the omplete reation rate for TE inluding forward and bakward reation. H Rf is the reation enthalpy for the forward reation forming the dimer. As mentioned previously, a omplex net of reation was finally used in the simulations presented in this paper igure 1. Aordingly, to the heat soure presented by [Eq. 8] further endothermi of exothermi terms are onsidered, whih take into aount for the side reations.. Disussion The temperature distribution in the gas phase alulated some seonds after the filling of TE is presented in igure and igure 5, for a 0. dm³ and a 3 dm³ vessel, respetively, alulated by OMSOL Multiphysis. or the numerial modeling of hazardous ondition often temperatures above 50 K are reahed while the alulation. Therefore a more detailed literature study revealed a dereasing reation rate above temperatures of 50 K. In fat, in preliminary alulations performed, the model did not onverge for temperatures above 700 K. Therefore, different kinetis were introdued for the higher temperature range, whih led to a further improvement of the model. Here a -stage-kinetis for the forward reation was introdued. Generally, the soure term for the heat prodution results from the reation enthalpy of the dimerization proess. Sine this is a seond order reation the equation is the following. Dim R TE p Ea RT q& = V H A e [Eq. 7] The orresponding heat soure term is: igure : Temperature field in K in a 0.-dm³-vessel after s at an initial pressure of 0.5 MPa absolute Daxial symmetry and a onstant wall temperature of 30 [alulation with OMSOL Multiphysis ] Q = R 1 H Rf [Eq. 8]

50000 90000 p in Pa 0000 390000 p omsol, 330 p omsol, 30 p omsol, 350 p omsol, 30 Test_TE_011, 30 30000 0 8 10 1 1 1 18 0 igure : omparison of simulated pressure and experimental data for a 30 wall

5 p in Pa p omsol, 330 p omsol, 30 p omsol, 350 p omsol, 30 Test_TE_011, igure : omparison of simulated pressure and experimental data for a 30 wall temperature and an initial pressure of 0.5 MPa absolute in a 3 dm³ vessel [alulation with OMSOL Multiphysis ] t in s igure 5: Temperature field in K in a 3-dm³-vessel after 1 s at an initial pressure of 0.5 MPa absolute D-axial symmetry and a onstant wall temperature of 300 [alulation with OMSOL Multiphysis ] Both figures show a hot zone in the upper part of the vessels. This is due to the buoyany effet whih is generated by the loal temperature differenes due to the heat generation by the dimerization reation. There are several small downward streams whih are formed at the top and move to the walls where they disappear. This is due to the ooling effet of the walls and produes a lot of turbulenes in the gas leading to ontinuous supply of non reated TE from the lower part of the vessel into the hot upper reation area. The effet of temperature layering ould be verified by experiments with several small thermoouples in a 0.-dm³-vessel. In igure the validation of the developed model is done by omparing the experimentally measured pressure with the pressure generated by OMSOL Multiphysis ]. The experimental initial pressure in the test was 0.5 MPa absolute and the initial wall temperature amounted to 30. If these onditions are set in the model, a good agreement with the experimental data is observed. In igure it is also shown, that if the wall temperature in the model is inreased to 350, a steep pressure inrease would be observed. This means, that the heat loss through the wall an not ompensate the heat generated by the dimerization reation. Therefore, the reation rate inreases aording to [Eq. 5] and the pressure primary rises aording to the ideal gas law before dropping down by the redution of mole numbers resulting in a thermal explosion: RT p = with V = onst. p ~ T [Eq. 10] V or the speifi ase presented in igure, the MITD predited by the model would be 350, sine this is the first temperature value in a range ±10 K, where an ignition is observed. As mentioned, alulations with improved kinetis for the predition of the MITD were performed both with OMSOL Reation Engineering Lab and OMSOL Multiphysis Good agreements with the experimental data have been obtained for both alulation methods, as shown in igure 7 and igure 8. Simulations performed with OMSOL Multiphysis show the best agreement and allow for the observation of loal hot spots. urthermore, with this method the pressure evolution up to the ignition an be modeled with good auray. By using the OMSOL Reation Engineering Lab to determine the MITD no signifiant influene of the side reations ould be found, unless the primary reversible dimerization reation was inluded. The reason might be the missing loal distribution of heat, as it was observed within the experiments. In fat, the

6 influene of the side reations will not affet the alulation, beause these reations need higher temperatures to generate non negligible reation rates. Nevertheless, nearly all results simulated with the OMSOL Reation Engineering Lab delivered lower values for the MITD ompared to the experimental ones. Thus the results are on the safe side. MITD Simulated in Bisetor + 0% - 0% 0.7 L Partially heated pipe 0. L ully heated vessel 3 L ully heated vessel MITD Experiment in igure 7: Auray of the MITD preditions with OMSOL Reation Engineering Lab MITD Simulated in Bisetor + 0% - 0% 3 L ully heated vessel MITD Experiment in igure 8: Auray of the MITD preditions with OMSOL Multiphysis By inluding the side reations into the OMSOL Multiphysis model a muh better agreement of the simulated and the experimental values ould be ahieved. Here the side reations influene the heat generation, due to high loal temperatures where their reation rates overome the rate of the dimerization reation. 7. Solver Problemati OMSOL Multiphysis or solving the model the UMPAK solver was used first. This worked well only after a manual arrangement of the net point distribution at the walls. Due to the ooling effet of the walls while the exothermi dimerization reation ours, the highest veloities are found in its proximity. Therefore, the element size had to be redued. Otherwise even the UMPAK solver ran into an error. or the autolave interior the predefined mesh size was set to extra fine and for some boundaries a speifi number of elements where set. Sine OMSOL Multiphysis is apable of using more than one PU, the PARDISO solver was used instead. Here all eight ores of the available workstation were used. Nevertheless the solving time ould not be signifiantly redued. In fat, the PARDISO solver did not work for the same initial mesh and solver parameters whih were used for the UMPAK solver. The mesh size had to be redued and the relative and absolute tolerane had to be set to a tenth of the value used for the UMPAK solver. These effets only ourred when linking the non-isothermal flow mode hns appliation mode to the ondution and diffusion hd mode. Without the non-isothermal flow mode hns the PARDISO solver solved the model very fast and without presenting any error. Moreover the non-isothermal flow alulation worked only fine by ativating the stabilization method isotropi diffusion using a parameter of onlusions The safety related onsiderations of the self heating proess of TE by the exothermi dimerization reation to otafluoroylobutane is neessary to prevent inidents in PTE prodution plants. The released temperature due to the dimerization reation an lead to the self heating of the system and finally to an explosive deomposition that may results in a massive destrution of plant equipment or even worse might lead to fatalities. As to gain information on the self heating proess of TE, simulations were performed with OMSOL Reation Engineering Lab and OMSOL Multiphysis. A omplex reation sheme was used to model the self-heating proess, whih inludes not only the dimerization reation but also the most important side reations. Good agreements with the experimentally determined MITDs have been obtained for both alulation methods. Simulations performed with OMSOL Multiphysis show the best agreement and allow for the observation of loal

7 hot spots. urthermore, with this method the pressure evolution up to the runaway an be predited with good auray. 9. Outlook On the basis of the suessful validation of the model presented, OMSOL Multiphysis might be used to simulate other geometries like pipes with different diameters, different flow regimes and orientations as well as vessels with larger volumes and internals will be modeled. Depending on the flow veloity of the TE gas inside a supply pipe, the wall temperature ould be higher as higher the gas veloity is before a signifiant self heating takes plae. In the simulations performed with a 3-dm³-vessel, the veloity field showed downward streams in the range of 0.13 m/s, but this might be strongly affeted by the studied geometry. Moreover, it might be possible to determine the maximum temperature of a loal hot spot inside a vessel filled with TE, whih would indue the self heating proess and finally initialize the explosive deomposition. Espeially for situations where a high buoyany effet or fored onvetion ours the Multiphysis model might give better preditions than the Reation Engineering Lab model. In a next step the presented models will be used to predit the MITD of TE in a 100-dm3- vessel. In a final validation state tests in a large vessel with a volume of ~ 100 dm³ will be arried out. Journal of hemial Kinetis, Volume 30, Pages Bekmann-Kluge, M., Krause, H., Shröder, V., Aikalin, H.A., Steinbah, J.; Study of a Self Heating Proess of Tetrafluoroethylene by the Exothermi Dimerization Reation to Otafluoroylobutane, Proeedings of the OMSOL onferene, November -, 008, Hannover, Germany.. Buravtsev, I.I., Grigor ev, A.S., Kolbanovskii, Yu.A.: Kinetis of tetrafluoroethylene ylodimerization reations and thermal otaylobutane deay., Kinet. Katal., No.1, EN 15:005, Determination of the Auto Ignition Temperature of Gases and Vapours, European ommittee for Standardization, Matula, R.A., ombustion Kinetis of Tetrafluoroethylene, A Offie of Sientifi Researh, Virginia, Shröder, V., Bekmann-Kluge, M.:, Explosion harateristis of TE and TE mixtures, R&D Projet VH 15, ederal Institute for Materials Researh and Testing BAM, 005. Aknowledgement The authors would like to thank the PlastisEurope assoiation for the finanial support whih made this projet possible. Moreover the harring of information with the TE safety task fore gave a grate input to drive this study to its urrent state. Referenes 1. Babenko, Yu.I., Lisohkin, Ya.A. Poznyak, V.I.: Explosion of Tetrafluoroethylene during Non-isothermal Polymerization, UD 51.1, 199 Plenum Publishing orporation.. Bauer, S.H., Javanovi, S.; The Pyrolysis of Otafluoroylobutane - Revisited, International

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