Browsing by Subject "Thermal degradation"
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Item Modeling and simulation of linear thermoplastic thermal degradation(2012-05) Bruns, Morgan Chase; Ezekoye, Ofodike A.; Ganesan, Venkat; Howell, John R.; Koo, Joseph H.; Nyden, Marc R.; Ruoff, Rodney S.Thermal degradation of linear thermoplastics is modeled at several scales. High-density polyethylene (HDPE) is chosen as an example material. The relevant experimental data is surveyed. At the molecular scale, pyrolysis chemistry is studied with reactive molecular dynamics. Optimization is used to calibrate several pyrolysis mechanisms with thermogravimetric analysis (TGA) data. It is shown that molecular scale physics may be coupled to continuum scale transport equations through a population balance equation (PBE). A PBE solution method is presented and tested. This method has the advantage of preserving detailed information for the small species in the molecular weight distribution with minimal computational expense. The mass transport of these small species is modeled at the continuum scale with a bubble loss mechanism. This mechanism includes bubble nucleation, growth, and migration to the surface of the condensed phase. The bubble loss mechanism is combined with a random scission model of pyrolysis to predict TGA data for HDPE. The modeling techniques developed at these three scales are used to model two applications of engineering interest with a combined pyrolysis and devolatilization PBE. The model assumes a chemically consistent form of the random scission pyrolysis mechanism and an average, parameterized form of the bubble loss mechanism. This model is used to predict the piloted ignition of HDPE. Predictions of the ignition times are reasonable but the model over predicts the ignition temperature. This discrepancy between model and data is attributed to surface oxidation reactions. The second application is the prediction of differential scanning calorimetry (DSC) data for HDPE. The model provides detailed information on the energy absorption of the thermally degrading sample, but the literature data is too variable to validate the model.Item Thermal degradation and oxidation of aqueous piperazine for carbon dioxide capture(2011-05) Freeman, Stephanie Anne; Rochelle, Gary T.; Maynard, Jennifer A.; Reible, Danny D.; Katz, Lynn E.; Critchfield, JamesAbsorption-stripping with aqueous, concentrated piperazine (PZ) is a viable retrofit technology for post-combustion CO2 capture from coal-fired power plants. The rate of thermal degradation and oxidation of PZ was investigated over a range of temperature, CO2 loading, and PZ concentration. At 135 to 175 °C, degradation is first order in PZ with an activation energy of 183.5 kJ/mole. At 150 °C, the first order rate constant, k1, for thermal degradation of 8 m PZ with 0.3 mol CO2/mol alkalinity is 6.12 × 10-9 s-1. After 20 weeks of degradation at 165 °C, 74% and 63%, respectively, of the nitrogen and carbon lost in the form of PZ and CO2 was recovered in quantifiable degradation products. N-formylpiperazine, ammonium, and N-(2-aminoethyl) piperazine account for 57% and 45% of nitrogen and carbon lost, respectively. Thermal degradation of PZ likely proceeds through SN2 substitution reactions. In the suspected first step of the mechanism, 1-[2-[(2-aminoethyl) amino]ethyl] PZ is formed from a ring opening SN2 reaction of PZ with H+PZ. Formate was found to be generated during thermal degradation from CO2 or CO2-containing molecules. An analysis of k1 values was applied to a variety of amines screened for thermal stability in order to predict a maximum recommended stripper temperature. Morpholine, piperidine, PZ, and PZ derivatives were found to be the most stable with an allowable stripper temperature above 160 °C. Long-chain alkyl amines or alkanolamines such as N-(2-hydroxyethyl)ethylenediamine and diethanolamine were found to be the most unstable with an allowable stripper temperature below 120 °C. Iron (Fe2+) and stainless steel metals (Fe2+, Ni2+, and Cr3+) were found to be only weak catalysts for oxidation of PZ, while oxidation was rapidly catalyzed by copper (Cu2+). In a system with Fe2+ or SSM, 5 kPa O2 in the inlet flue gas, a 55 °C absorber, and one-third residence time with O2, the maximum loss rate of PZ is expected to 0.23 mol PZ/kg solvent in one year of operation. Under the same conditions but with Cu2+ present, the loss rate of PZ is predicted to be 1.23 mole PZ/kg solvent in one year of operation. Inhibitor A was found to be effective at decreasing PZ loss catalyzed by Cu2+. Ethylenediamine, carboxylate ions, and amides were the only identified oxidation products. Total organic carbon analysis and overall mass balances indicate a large concentration of unidentified oxidation products.Item Thermal degradation of aqueous amines used for carbon dioxide capture(2009-08) Davis, Jason Daniel; Rochelle, Gary T.Aqueous amine solutions loaded with CO2 were degraded in stainless steel sealed containers in forced convection ovens. Amine loss and degradation products were measured as a function of time by cation chromatography (IC), HPLC, and IC/mass spectrometry. A full kinetic model was developed for 15-40 wt% MEA (monoethanolamine) with 0.2 – 0.5 mol CO2/mol MEA at 100°C to 150°C. Experiments using amines blended with MEA demonstrate that oxazolidone formation is the rate-limiting step in the carbamate polymerization pathway. With 30 wt% MEA at 0.4 mol CO2/mol MEA and 120°C for 16 weeks there is a 29% loss of MEA with 13% as hydroxyethylimidazolidone (HEIA), 9% as hydroxyethylethylenediamine (HEEDA), 4% as the cyclic urea of the MEA trimer, 1-[2-[(2-hydroxyethyl)amino]ethyl]-2-imidazolidone, 3% as the MEA trimer, 1-(2-hydroxyethyl)diethylenetriamine, and less than 1% as larger polymeric products. In the isothermal experiments, thermal degradation was slightly more than first order with amine concentration and first order with CO2 concentration with an activation energy of 33 kcal/mol. In a modeled isobaric system, the amount of thermal degradation increased with stripper pressure, but decreased with an increase in amine concentration and CO2 concentration due to a reduction in reboiler temperature from the changing partial pressure of CO2. Three-fourths of thermal degradation in the stripper occurred in the reboiler due to the elevated temperature and long residence time which offset the decrease in CO2 concentration compared to the packing. The amount of degradation for other amines tested starting with the least degraded include; cyclic amines with no side chains < long chain alkanolamines < alkanolamines with steric hindrance < tertiary amines < MEA < straight chain di- and triamines. Piperazine and morpholine had no measurable thermal degradation under the conditions of this experiment and were the most resistant to thermal degradation. Diethyelenetriamine and HEEDA had the largest amount of degradation with over 90% loss at 135°C for 8 weeks.