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- Gas sorption (4)
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- Molecular modeling (3)
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Organisationseinheit der BAM
Um Oxidationsprozesse in der chemischen Industrie zu optimieren, wird verstärkt versucht, Luftsauerstoff durch reinen Sauerstoff zu ersetzen und den Prozessdruck zu erhöhen. Dadurch werden weitergehende sicherheitstechnische Betrachtungen notwendig. Bisherige Untersuchungen an Systemen bestehend aus einer glatten, ruhenden brennbaren Flüssigkeit unter einer Gasphase aus reinem Sauerstoff bei erhöhtem Anfangsdruck zeigten, dass in solchen Systemen nicht nur deflagrativ verlaufende Reaktionen, sondern auch detonative Reaktionen, sogenannte Oberflächendetonationen, auftreten. Bis dato wurde bei diesen Versuchsreihen hauptsächlich untersucht, unter welchen Anfangsbedingungen Oberflächendetonationen auftreten.
Im Rahmen der vorliegenden Arbeit wurden die Vorgänge in einem heterogenen System, bestehend aus Toluol und Sauerstoff, systematisch untersucht. Dabei wurde eine Einteilung der gesamten Verbrennungsreaktion in unterschiedliche Stadien vorgenommen. Diese Stadien sind die Zündung, der Abbrand, die Oberflächendeflagration und die Oberflächendetonation. Mit Experimenten in einem 50 l Kugelautoklav konnte nachgewiesen werden, dass sich die Mechanismen bei einer solchen Oberflächenexplosion grundlegend von den Abläufen bei Explosionen in einem homogenen System unterscheiden. Darüber hinaus konnte nachgewiesen werden, dass Oberflächendetonationen im Gegensatz zu bisherigen Vermutungen über den gesamten untersuchten Druckbereich auftreten können.
Ein weiteres Ziel der vorliegenden Arbeit war eine Untersuchung, ob mit einfachen, mechanischen Hindernissen ein sicherheitstechnisches Konzept zur Vermeidung von Oberflächendetonationen möglich ist. Dies muss nach den Ergebnissen der vorliegenden Arbeit verneint werden. Die Versuchsreihen mit unterschiedlichen Hindernissen erbrachten jedoch weitere Informationen über die Mechanismen von Oberflächenexplosionen. Diese Informationen führten zur Formulierung einer neuen Hypothese. Diese Hypothese diskutiert den Einfluss des Energieeintrags von der Flamme in die Flüssigkeit und einen sich daraus ergebenden selbstbeschleunigenden Prozessverlauf.
Heterogene Detonationen
(1997)
Detailed atomistic simulations were carried out for swelling polymer/gas systems related to experimental sorption and dilation data for CO2 and CH4 in three glassy polymers (polysulfone PSU, the polyimide 6FDA-TrMPD, and a polymer of intrinsic microporosity PIM-1) at 308 K (35 °C) and pressures up to 50 bar. Corresponding experiments were performed with a gravimetric sorption balance and a dilatometer based on a capacitance distance sensor. For each polymer/gas system molecular packing models were prepared and equilibrated for two reference states: the pure polymer is taken as reference for the respective 'unswollen' state and similarly the state of the highest penetrant pressure reached in the corresponding experiment is taken to represent the 'swollen' state. Models for the latter were constructed in agreement with experimental data (pressure, temperature, gas concentration and volume dilation). Concentration–pressure isotherms of each polymer/gas system were obtained using Grand Canonical Monte Carlo (GCMC) simulations for both reference states (depleted of gas molecules), which are in good agreement with the experimental data in the respective pressure range. As expected these isotherms – due to the simulation technique used, merely based on hole-filling in a static host matrix – do not represent the sorption behavior over a broader range of gas pressures which may involve significant structural rearrangements as well as swelling and relaxational phenomena. Nevertheless, a linear combination of the two GCMC-isotherms allows the interpolation in order to describe the nonlinear gas sorption in the glassy polymers under investigation covering the penetrant pressure range between the reference states in good agreement with the experimental results.
A molecular modeling investigation of dilation effects induced by sorbed gas molecules in two glassy polymers is presented. As experimental reference, integral sorption of CO2 and CH4 was measured for polysulfone (PSU) and a polyimide (6FDA-TrMPD, PI4) at 308 K and a pressure of 10 bar. Simultaneously, the gas induced swelling effect was measured with a dilatometer based on a capacitive distance sensor recorded. The experimental evidence of the (on the observed time scale and concentration levels) elastic nature of the gas induced dilation is supported by the dilation and contraction behavior observed in molecular dynamics (MD) simulations of respective detailed atomistic packing models. These models were constructed in accordance with gas concentration levels obtained from the experimental sorption results. Quantitative deviations between simulated and measured dilations are discussed as a consequence of an anelastic response of the polymer matrix which is too fast to be resolved in the experiments whose kinetics is dominated by diffusional processes. In the simulation, the initial insertion of penetrant molecules into equilibrated packing models circumvents the slow diffusional process of the experiment and allows a reasonable representation of the dilation process as well as a closer investigation. Our simulation approach reveals a different behavior for PSU and PI4 on the corresponding time scale. Most likely, the different chain mobility of the two polymers is responsible for the respective response to the inserted amount of gas molecules which is discussed in terms of the different chain mobilities of the two polymers.
Atomistic packing models have been created, which help to better understand the experimentally observed swelling behavior of glassy polysulfone and poly (ether sulfone), under CO2 gas pressures up to 50 bar at 308 K. The experimental characterization includes the measurement of the time-dependent volume dilation of the polymer samples after a pressure step and the determination of the corresponding gas concentrations by gravimetric gas-sorption measurements. The models obtained by force-field-based molecular mechanics and molecular dynamics methods allow a detailed atomistic analysis of representative swelling states of polymer/gas systems, with respect to the dilation of the matrix. Also, changes of free volume distribution and backbone mobility are accessible. The behavior of gas molecules in unswollen and swollen polymer matrices is characterized in terms of sorption, diffusion, and plasticization.