Search

We show how the modification by Henry et al. of the earlier van der Waals−Plateeuw thermodynamic model can be used to interpret data for hydrates in small pores even when there is a broad distribution of pore sizes. Pore size distribution effects explain the previously reported discrepancy between calculated and measured equilibrium pressures for nominal 7.5 nm radius pores. On the basis of the model, pore volume distributions are reconstructed for the silica gel samples in which the hydrates were formed and compared with distributions constructed from desorption isotherms. The excellent agreement between the calculations and experimental data supports the interpretation of how the pore size distribution present in the silica gel samples affects the observed equilibrium pressures.

Equilibrium pressures for the dissociation of propane hydrate confined in silica gel pores of nominal radii 7.5, 5.0, 3.0, and, 2.0 nm were measured across a wide temperature range, and were higher than those for bulk propane hydrate. The upward shift from the bulk pressures, which depended on the pore size, was 200−300% for the smallest pore size. In each of the pressure−temperature profiles, the temperature at which the four phases (hydrate, ice, liquid water, and gas) apparently coexisted was identified, and this temperature was a linear function of the reciprocal pore size. The enthalpy of dissociation for the equilibrium involving hydrate, liquid water, and gas was estimated from the data. The change with pore size in this apparent enthalpy appeared to be analogous to the variation in the heat of melting of pore ice as sometimes reported.

Self-assembled monolayer films of a metallotetraazaphthalocyanine were prepared using two different reaction conditions. The films were characterized by ellipsometry, AFM, electronic absorption spectroscopy, semiempirical MO calculations, core-level XPS, and projection of a calculated structure of the chromophore onto a surface to obtain its silhouette area for comparison with surface concentration from absorption spectroscopy. A denser packing of the chromophores is found in the SAM film prepared with bromonaphthalene at 250 °C as compared with the film from the more polar solvent DMF at 120 °C. Data is consistent with an almost vertical orientation of the chromophores in the former film. A different number of bonds connecting the chromophore with the coupling molecule is suggested as an explanation to differences in the absorption bands for the two films.

Using electrochemical and chemical probes, we demonstrate that thin films of crystalline polymethylene (PM), formed via solution decomposition of diazomethane on gold surfaces, exist in the form of a heterogeneous distribution with microscopically small pores at low average PM coverages. For evaporated gold substrates, as the PM loading increases above ~0.8 μg·cm^-2, a mass density equivalent to an ~7 nm thick, fully dense planar film, a transition from a micropore to a blocked electrode occurs. This microstructural picture is confirmed by the ability to backfill open areas with alkanethiolates and electropolymerized aniline. Parallel experiments on sputter-deposited gold films show that the blocking threshold occurs at much lower average PM coverages and alkanethiolate chemisorption data suggest that the open pores are approaching the sizes of individual molecules. These results can be rationalized by a more uniform nucleation of PM nanocrystals across sputter-deposited relative to evaporated surfaces. This ability to regulate the conformal deposition of an inert, low dielectric material at the nanometer scale on gold surfaces offers a new way to engineer electrodes with controlled, micropore dielectric barrier structures and “quasi-two-dimensional” nanocomposite films.

A distillation fraction of a coal-derived liquid (tar) was cracked over a char−dolomite mixture, calcined dolomite, and silicon carbide in a fixed-bed reactor. The char−dolomite mixture (FWC) was produced from Pittsburgh No. 8 coal and dolomite in a Foster Wheeler carbonizer. The experiments were conducted under nitrogen and simulated coal gas (SCG), which was a mixture of CO, CO2, H2S, CH4, N2, and steam, at 1 and 17 atm. The conversion over these materials under nitrogen was much higher at 17 atm than at 1 atm. At higher pressures, tar molecules were trapped in the pores of the bed material and underwent secondary reactions, resulting in the formation of excess char. However, when nitrogen was replaced by SCG, the reactions that induce char formation were suppressed, thus increasing the yield of gaseous products. The analysis of the gaseous products and the spent bed materials for organic and inorganic carbons suggested that the product distribution can be altered by changing the carrier gas, temperature, and pressure.

This paper describes our recent research on vapor and particle deposition rate processes in combustion systems. Inorganic compounds which deposit on heat exchanger surfaces and gas turbine blades are formed when the fuel and/or ingested air contain inorganic impurities. A quantitative understanding of the deposition rate of inorganic oxides, salts or soot can suggest efficient test/control strategies. Accordingly, we have developed optical methods for precise, real-time measurements of the growth rate of deposits under laboratory conditions. To demonstrate these techniques, and provide data for theory development, a deliberately simple chemical system and target geometry are described; viz. BCl3(g) is added to a premixed C3H8-air flat flame and the B2O3(l) deposition rate on an electrically heated Pt ribbon is measured interferometrically or ellipsometrically over a range of fuel/air ratios and seed levels. The stability of B2O3(l) at present seed levels and surface temperatures (ca. 1200–1300 K) reveals kinetic restrictions on B2O3(l) gasification reactions. Measured deposition rates agree with predictions of our multicomponent mass-transfer boundary layer (BL) theory, with a constrained equilibrium ((HBO2)3precluded) boundary condition. These encouraging results suggest a more general applicability for the optical methods and BL theory outlined herein, and demonstrate the role of heterogeneous kinetic barriers in determining dew points and deposition rates in combustion systems.

Results of experimental and numerical studies of counterflow, partially premixed, flames burning methane in air are reported. Experimental data is obtained for the temperature profile, the axial velocity profile, the concentration profiles of various stable species and the critical conditions of the flame at extinction. The results are compared with numerical calculations performed at conditions identical to those used in the experiment. Experimental measurements and numerical calculations show that partial premixing of the reactant streams of a counterflow diffusion flame makes the flame less resistant to stretch. In addition, numerical calculations show that the reaction zone of a partially premixed flame exhibits a diffusion flame and a premixed flame-like structure. Both results are consistent with previous asymptotic analyses of the structure of partially premixed flames.

Molecular constraints for the localization of active site directed ligands (competitive inhibitors and substrates) in the active site of phospholipase A2 (PLA2) are characterized. Structure activity relationships with known inhibitors suggest that the head group interactions dominate the selectivity as well as a substantial part of the affinity. Theab initiofitting of the amide ligands in the active site was carried out to characterize the head group interactions. Based on a systematic coordinate space search, formamide is docked with known experimental constraints such as coordination of the carbonyl group to Ca2+and hydrogen bond between amide nitrogen and ND1 of His48. An optimal position for a bound water molecule is identified and its significance for the catalytic mechanism is postulated. Unlike the traditional “pseudo-triad” mechanism, the “Ca-coordinated-oxyanion” mechanism proposed here invokes activation of the catalytic water to form the oxyanion in the coordination sphere of calcium. As it attacks the carbonyl carbon of the ester, a near-tetrahedral intermediate is formed. As the second proton of the catalytic water is abstracted by the ester oxygen, its reorientation and simultaneous cleavage form hydrogen bond with ND1 of His48. In this mechanism of esterolysis, a catalytic role for the water co-ordinated to Ca2+is recognised.

Reciprocal salt-pair system 2KC1 + MgSO4⇌ K2SO4+ MgCl2has been studied at 35° C to eliminate the discrepancies reported by different workers and for correlating the experimental data with natural evaporation of brine (without NaCl) so as to recover potash salts.

This is a report on the use of 13C NMR spectroscopy to study the structure of polyethylenes, polyethylene-olefin copolymers, and polyethylene model systems, through the observation of chemical shifts and spin-lattice relaxation times of individual carbons. Information is obtained on the type and numbers of side chains of polyethylenes from solution spectra at higher temperatures. The level of detectability of side chains is established at 2 side chain carbons per 1000 skeletal carbons. The observed chemical shifts are accounted for well by standard additivity schemes. The low-density polyethylenes show spectra which are distinctly differentfrom high-density polyethylenes. The 13C-determined branch frequencies show the same general trend as the IR results, but it is also argued that the former are more accurate. Relaxation times of the various carbons of the model system C44paraffin vary from 2.94 s for the backbone methylenes to 8.33 s for the terminal methyls to 11.11 s for α-methyls. Indicative of the behavior of olefin/polyethylene copolymers is the finding that all carbons of the copolymer have spin-lattice relaxation times of about 1.5 s, except for branch methyls which are 6 s. A series of polyethylenes at this high temperature shows relaxation times for the backbone methylene varying from 1.4 to 1.7 s, indicating that at this temperature, branching does not substantially affect the dynamic behavior of polyethylene.

A conceptual model for hydrate equilibrium in media with broad pore-size distributions is used to interpret methane hydrate equilibrium experiments in silica gel pores with nominal radii of 2, 5, and 7.5 nm. While the hydrate equilibrium pressure is an intensive property, it is demonstrated that the presence of a broad pore-size distribution in a porous medium causes the experimental pressures to appear extensive. It is observed that the sample size, the headspace volume, the temperature at which the experiments are started, and the amount of hydrate allowed to dissociate to establish equilibrium at this initial temperature all affect the observed pressure at any given temperature. The pore-volume distribution of the porous medium (not the nominal pore radius) and the percent conversion of pore-water to hydrate also affect the measured equilibrium pressures. As a result, measured equilibrium pressures at a specific temperature for identical media can be significantly different, while those for different media may be superposed. These seeming inconsistencies, their causes, and interpretations are examined and explained. Apparently fundamental problems in experimental data are removed when the number of degrees of freedom is properly included in the interpretation.