Direct Radiometric Measurement of Vapor-Liquid Equilibria: Hydrogen Sulfide-Diethanolamine

Direct radiometric measurements of the vapor concentration of H₂S in equilibrium with aqueous diethanolamine absorbing solution were made using ³⁵S-labeled H₂S. A thin-window Geiger-Müller tube viewed the gas phase only, while the absorbing solution was held in another part of the reaction flask. The equilibrium data were measured at H₂S partial pressures below 0.01 mm Hg; the agreement with published results obtained by chemical analysis was good. The radiometric results were obtained very rapidly; a test run with seven data points took about two hours to complete. The results were applied to processes for reducing H₂S concentrations in petroleum refinery fuel gases needed to meet tightening air pollution requirements.     

On the Instability Intervals of the Mathieu–Hill Operator

Consider the Mathieu–Hill operator

Turbulent Flame Speeds of Premixed Supersonic Flame Kernels

It is unclear whether turbulent flame speed scalings established in low speed regimes are applicable to supersonic flames. To investigate this question, the canonical flame kernel is investigated in a scramjet-like channel having a one degree wall divergence. The growth, shape and internal kernel dynamics are investigated. Results are presented for three Mach numbers, four equivalence ratios, and three turbulence generators. Schlieren photography provides flame images for growth rate statistics and particle image velocimetry (PIV) provides turbulence statistics and investigation of internal kernel dynamics. Supersonic flame kernels are self-propagating and respond to the equivalence ratio in a fashion that is similar to low speed flames. However, supersonic flame kernels have features that are not present in subsonic flame kernels. Baroclinicity, resulting from pressure-density misalignment, creates a reacting vortex ring structure. Further, the mean kernel shape has a Mach number dependence and the vortex ring enhances the turbulent flame speed through entrainment of reactants and augmented flame surface growth. Hence, the previously established (low speed) flame speed scalings are inappropriate for supersonic flame kernels. Drawing motivation from vortex ring literature, the ring propagation velocity is used as the characteristic velocity and a new flame speed scaling is proposed.     

Practical effects of pressure-transmitting media on neutron diffraction experiments using Paris–Edinburgh presses

ABSTRACT

To understand the practical effects of pressure-transmitting media (PTM) on neutron diffraction using Paris–Edinburgh presses, diffraction patterns of MgO were collected to approximately 20?GPa using PTMs of Pb, AgCl, 4:1 methanol–ethanol (ME) mixture with and without heating, N₂, and Ar. Hydrostaticity in the sample chamber estimated from the MgO 220 peak width improves in the order of Pb, AgCl, Ar, ME mixture, N₂, and the heated ME mixture. Unlike previous results using diamond anvil cells, the unheated ME mixture is superior to Ar even after freezing, probably due to the cup on the anvil face. Considering these results and the sizable coherent scattering of Ne, which would show good hydrostaticity, we conclude that the ME mixture (preferably the heated one) is the best PTM in neutron experiments up to 20?GPa, while Ar can be substituted when a sample is reactive to alcohols.     

An analytic solution of capillary rise restrained by gravity

We derive an analytic solution for the capillary rise of liquids in a cylindrical tube or a porous medium in terms of height h as a function of time t. The implicit t(h) solution by Washburn is the basis for these calculations and the Lambert W function is used for its mathematical rearrangement. The original equation is derived out of the 1D momentum conservation equation and features viscous and gravity terms. Thus our h(t) solution, as it includes the gravity term (hydrostatic pressure), enables the calculation of the liquid rise behavior for longer times than the classical Lucas-Washburn equation. Based on the new equation several parameters like the steady state time and the validity of the Lucas-Washburn equation are examined. The results are also discussed in dimensionless form.     

Myxochelin biosynthesis: direct evidence for two- and four-electron reduction of a carrier protein-bound thioester

Microorganisms produce small molecules known as siderophores to scavenge iron from the environment. Insight into iron acquisition in myxobacteria has been provided recently by the sequencing of the gene cluster for the catecholate myxochelins A and B, from the myxobacterium Stigmatella aurantiaca Sg a15. The gene cluster contains enzymes (MxcCDEF) for assembly of 2,3-dihydroxybenzoic acid (DHBA), an amino transferase, MxcL, and a nonribosomal peptide synthetase (NRPS) subunit, MxcG. In the proposed pathway to the myxochelins, two molecules of DHBA are condensed with the two amino groups of lysine, which is itself tethered to the peptidyl carrier protein domain (PCP) of MxcG. The resulting thioester is then reduced by the NADPH-dependent reductase (Red) domain of MxcG to generate an aldehyde intermediate; subsequent Red-catalyzed reduction yields myxochelin A, while transamination by MxcL produces myxochelin B. Although myxochelin A has been obtained successfully in vitro, it has not been possible to date to reconstitute the transamination reaction to give myxochelin B nor to unequivocally establish the intermediacy of the aldehyde. We report here the successful biosynthesis of myxochelin B in vitro. Furthermore, we demonstrate for the first time the existence of an aldehyde intermediate in the four-electron reduction of a PCP-bound thioester. Finally, we show that the relative levels of myxochelin A and B are likely to be controlled by the direct competition of MxcL and the MxcG Red domain for a free aldehyde intermediate.