Improvements in the Modeling of the Self-ignition of Tetrafluoroethylene

Transcription

1 Presented at the COMSOL Conference 2010 Paris Improvements in the Modeling of the Self-ignition of Tetrafluoroethylene M. Beckmann-Kluge 1, F. Ferrero 1, V. Schröder 1, A. Acikalin 2 and J. Steinbach 2, 1 BAM Federal Institute for Materials Research and Testing, 2 Technical University Berlin COMSOL Conference, Paris,

2 Content I. Introduction to topic and motivation II. Numerical Model with REL and Comsol Multiphysics III. Results of different numerical approches IV. Conclusion and outlook COMSOL Conference, Paris,

3 Introduction to TFE Tetrafluoroethylene (TFE, C 2 F 4 ) is monomer of Polytetrafluoroethylene (PTFE) and other copolymers ( t/year) PTFE is resistant to most reactive and corrosive chemicals and has non-sticky properties COMSOL Conference, Paris,

4 Motivation Several incidents in PTFE-production-plants in the last decades TFE is a decomposable gas possibility of explosive decomposition Sources for ignition: - Spark ignition, electrostatic + - Hot surfaces content of this work Research project subsidized by PlasticsEurope to determine hazardous conditions, started 2007 Exothermic Dimerization reaction of TFE to Octacyclofluorobutane can cause ignition 2C F c C F H = R 103 kj mol TFE COMSOL Conference, Paris,

5 Reaction Engineering Lab FEM simulation without fluid dynamic Easy integration of complex reaction kinetics Thermodynamic properties are calculated via the NASA polynomial coefficients Energy and mass balance are solved COMSOL Conference, Paris,

6 Validation data base by experiments 3-dm³ autoclave thermocouple for gas temperature pressure transducer 0.2-dm³ autoclave internal volume (cylindrical reaction chamber) Steel vessel Aluminium (with heating jacket) COMSOL Conference, Paris,

7 1 st Model: Dimerization Reaction forward reaction 2. order reaction f 2C F c C F k f 2 4 = m³ mol 4 8 exp s RT [ J/mol] r f = ( ) 2 cc F k f 2 4 New 2-stage kinetics was determined backward reaction c C b 4F8 2C2F4 1. order reaction [ J/mol] k f r b = 2, = cc C4F8 m³ exp mol s k b RT COMSOL Conference, Paris,

8 Enhanced reaction net in REL and COMSOL CFD Model Reaction RO A0 in [m³ mol-1 s-1] bzw. [s-1] Ea in [J/mol] C2F4+C2F4 C4F E C4F8(c) 2*C2F E acc. NIST [kj/mol] -166 ( ) 166 ( ) acc. REL (NASA Polynoms) -161,5 C4F8(c) C3F6(e)+CF E ,3 164 C3F6(e) C2F4 + CF E ,7 288 C3F6(c) C2F4 + CF E ,7 288 C3F6(e) C3F6 (c) E H r 0 H r 161,5 C2F4 2*CF E ,6 C4F8(c) + CF2 C3F6(e)+ C2F E ,7-126,5 C4F8(i) 2* C2F E C4F8(i) C3F6(e)+ CF E ,3 - COMSOL Conference, Paris,

9 Reaction Engineering Lab Model dt V r ( ci cpi ) = Qreaction + Qloss + V dt r dp dt Pressure work is considered dynamic calculation of alpha, depending on T(t) is implemented Input window of REL COMSOL Conference, Paris,

10 Reaction Engineering Lab Model Prediction of the MITD with the REL method experimental MITD = 310 C, 5 bar, 0.2-dm³ COMSOL Conference, Paris,

11 II. Reaction Engineering Lab Comparison of REL results and experimental values in C MITD REL Bisektor + 20% - 20% MITD 3-dm³ REL in C MITD 0,2-dm³ REL in C MITD Rohr REL in C 200 Partially heated pipe MITD Experiment in C COMSOL Conference, Paris,

12 Conclusions from REL method Pro Contra Easy to calculate Volume of vessel is considered Height of vessel can be considered (via alpha) Complex reaction net possible All predicted MITD on the safe side No fluid dynamic considered (buoyancy) Geometry specification can only be considered in calculation of alpha Prediction of MITD might be to conservative COMSOL Conference, Paris,

13 Application modes in Multiphysics Convection and diffusion mode: mass balance based on chemical reactions R TFE, R Dimer, R T(x) Convection and conduction for heat transfer u, v u, v T(x), ρ ρ, η, p Weakly compressible Navier-Stokes for impulse balance COMSOL Conference, Paris,

14 FEM Model: Enhanced reaction net in COMSOL Multiphysics k in s -1 bzw. m³mol -1 s -1 0,1 0,09 0,08 0,07 0,06 0,05 0,04 0,03 0,02 0, T in K C2F4+C2F4 --> C4F8 C4F8(c) -->2*C2F4 C4F8(c) -->C3F6(e)+CF2 C3F6(e) --> C2F4 + CF2 C3F6(c) --> C2F4 + CF2 C3F6(e) --> C3F6(c) C2F4 -->2*CF2 C4F8 (c) + CF2 -->C3F6(e)+C2F4 C4F8(i) -->2*C2F4 C4F8(i) -->C3F6(e)+CF2 additional reactions are important in the relevant temperature range (green) COMSOL Conference, Paris,

15 Enhanced reaction net in COMSOL CFD Model 4 additional reactions and two additional species were integrated New reaction net shows very good numerical results Additional reactions prevent a too early runaway at lower temperatures At higher temperatures the primary dimerization reaction generates a runaway ignition of decomposition reaciton C + ( 2F4 C2F4 C4F 8 c ) C ( + + C F 4 F 8 c ) CF 2 C 3 F 6 ( e ) 2 4 C 2 F 4 C ( + CF 4 F 8 c ) C 3 F 6 ( e ) 2 C + 2 F 4 CF 2 CF 2 C ( C F + CF 3 F 6 c ) C 3 F 6 ( e ) C 3 F 6 ( c ) COMSOL Conference, Paris,

16 Enhanced reaction net in COMSOL CFD Model New model shows for the volumes of 0.2-dm³ and 3-dm³ a maximal deviation of 10 K in the MITD For both volumes a pressure peak in the simulation could be observed and was taken as the ignition criterion Exponential pressure increase p in Pa p (Comsol, 330 C) p (Comsol, 340 C) p (Comsol, 350 C) p (Comsol, 360 C) Test_TFE_011, 340 C t in s COMSOL Conference, Paris,

17 Enhanced reaction net in COMSOL CFD Model Comparison of CFD Simulation and experimental values, 0.2 dm³ OL in C MITD COMSO Bisektor + 20% - 20% MITD 0,2-dm³ COMSOL in C MITD Experiment in C COMSOL Conference, Paris,

18 Enhanced reaction net in COMSOL CFD Model - results Comparison of CFD Simulation and experimental values, 3 dm³ SOL in C MITD COMS Bisektor + 20% - 20% MITD 3-dm³ COMSOL in C MITD Experiment in C COMSOL Conference, Paris,

19 Conclusions from CFD method Pro Contra Most accurate prediction of MITD Complex geometries can be considered Complex fluid flow possible Complex reaction kinetics possible Partially heating possible Intense knowledge of software is necessary Long computational times Not applicable on standard PC COMSOL Conference, Paris,

20 Outlook Validation in larger volumes Tests in 100-dm³-vessel Vessel is fixed in rotational rack Tests for MITD in horizontal and vertical orientation will be carried out Prediction with COMSOL REL and Multiphysics was done MITD dependence on variation in geometry for similar volumes could be found when using COMSOL Multiphysics COMSOL Conference, Paris,

21 100-dm³ vessel: construction Permanent sealing with graphite gasket Easy removable lid From existing 6,5-dm³ autoclave Trough put for 4 thermocouples, 90 offset COMSOL Conference, Paris,

22 5,2 100-dm³ vessel: Simulation of MITD, 5 bar TFE Ignition criterion based on exponential pressure increase p in bar (a absolute) 5,15 5,1 5,05 5 4,95 4,9 Vertical orientation p in bar (260 C) p in bar (270 C) time in s COMSOL Conference, Paris,

23 100-dm³ vessel simulation: MITD dependence on geometry Simplified geometry real geometry 58 s Temperature field at t(t max ) in C 30 s COMSOL Conference, Paris,

24 100-dm³ vessel simulation: MITD dependence on geometry Comparision of p-t curves for different geometries (similar volume) 5,2 5,15 p in bar (a absolute) 5,1 5,05 5 4,95 4,9 p in bar (real geometry) p in bar (270 C) time in s COMSOL Conference, Paris,

25 Prediction of MITD for 100-dm³-vessel (simple geometry) MITD dependence on pressure for 100-dm³-vessel MITD in C MITD COMCOL in C MITD ( C)= *ln(p) Logarithmisch (MITD COMCOL in C) p in bar COMSOL Conference, Paris,