Solubilities and diffusivities of N2, O2, CO2, and He in a variety of polyethylenes were measured in the temperature range 0–50°C. Polyethylene films studied covered a range of crystallinities (43–82%) and branching indices, and were prepared under a variety of known thermal histories. Diffusivities were determined by the time‐lag method; solubilities, by time‐lag, and also by a newly developed and more accurate static method. Solubilities were found to obey Henry's Law; the solubility constants determined by both methods were found to agree within the limits of accuracy, confirming the applicability of the unsteady‐state diffusion equation to essentially isotropic crystalline polymers. For a given gas at constant temperature, the solubility constant is directly proportional to the volume fraction of amorphous material in a polyethylene sample (as determined by density), irrespective of its origin or thermal history; the concept of the crystallites as an impermeable, dispersed phase thus appears justified. Diffusivities were found to vary widely (nearly fivefold) depending on polymer crystallinity and thermal history and were as much as tenfold lower than the values estimated for completely amorphous polyethylene. Application of principles of flow through porous media to this system leads to a conclusion that abnormally low diffusivities arise predominantly from the impedance to gas flow offered by the dispersed crystalline phase. Variations in diffusivity with branching index and thermal history correlate qualitatively with expected corresponding variations in crystallite growth kinetics and shape; the existence of highly anisometric, laminar crystallites in annealed linear polyethylene is indicated from these studies. The combined influence of crystallinity on solubility and diffusivity permits as much as a tenfold variation in gas permeability of polyethylene depending on polymerization method or fabrication process.
Accurate, predictive reaction models are critical for the design and optimization of chemical looping combustion (CLC) reactors. The formulation and estimation of kinetic parameters for these reaction models using a first‐principles equation‐oriented (EO) approach is particularly beneficial as large amounts of experimental data spanning process‐relevant conditions can be used to estimate parameters in a computationally tractable way. This work demonstrates the application of a novel EO framework to develop reduction reaction kinetic models of an iron‐based CLC oxygen carrier (OC). An optimization problem is formulated to estimate kinetic parameters that provide the best fit to the experimental data. The model predicts the state of the OC with mean square error values of 2.5%–4.4% across the full range of validation data, including multiple reduction cycles.
Where rapid heating is encountered, as in a prompt-burst nuclear reactor, a thermocouple with a rapid time-response is necessary to monitor the temperature of the specimen in question. Because all thermocouples have some mass, they cannot have an infinitely fast response. However, an intrinsic-type thermocouple has less thermal inertia than the usual welded-bead type. Thus it is a natural choice for the measurement of transient surface temperatures of conducting solids. In this report an analytical expression is obtained for the transient response of an intrinsic thermocouple. In support of the analysis, experimental data are presented which were obtained by means of a capacitor bank pulse-heating technique. It is concluded that thermocouple wires of small diameter and low thermal conductivity respond the fastest. As an example, a 1-mil constantan wire on a copper substrate produces 95 percent of the steady-state emf in 3 μsec.
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