Heat Explosion In A Two-Phase Medium

Introduction In this work we study heat explosion in a heterogeneous medium consisting of reacting particles surrounded by a gas. Chemical reaction can occur inside the particles or between the particles and the gas, but it does not occur inside the gaseous phase. The particles are sufficiently small and there is no heat conduction in this phase. The gas is heat conducting and there is heat exchange between the two phases. Under the gravity conditions the temperature distribution can be influenced by convection. Convective motion of particles is determined by the gas motion in the sense that both phases have the same velocity field. We consider the two-temperature model in terms of dimensionless variables oe ` @` 1 @t + (ur)` 1 ' = ke Z` 1 \Gamma ff(` 1 \Gamma ` 2 );&lt

Uranyl sorption species at low coverage on Al-hydroxide: TRLFS and XAFS studies

International audienceDetailed understanding of the respective roles of solution and surface parameters on the reactions at uranyl solution/Al-(hydr)oxide interfaces is crucial to model accurately the behaviour of U in nature. We report studies on the effects of the initial aqueous uranyl speciation in moderately acidic solutions, e.g. of mononuclear, polynuclear uranyl species and/or (potential) U(VI) colloids, on the sorption of U by large surface areas of amorphous Al-hydroxide. Investigations by Extended X-ray Absorption Fine Structure (EXAFS) and Time-Resolved Laser-induced Fluorescence Spectroscopy (TRLFS) reveal similar U coordination environments on Al-hydroxide for low to moderate U loadings of sorption samples obtained at pH 4–5, independently of the presence of mononuclear or polynuclear aqueous species, or of the potential instability of initial solutions favoring true U-colloids formation. EXAFS data can be interpreted in terms of a dimeric, bidentate, inner-sphere uranyl surface complex as an average of the U surface structures. TRLFS data, however, provide valuable insights into the complex U surface speciation. They indicate multiple uranyl surface species under moderately acidic conditions, as predominant mononuclear and/or dinuclear, inner-sphere surface complexes and as additional minor species having U atoms in a uranyl (hydr)oxide-like coordination environment. The additional species probably occur as surface polymers and/or as adsorbed true U colloids, depending on the aqueous U concentration level (1–100 μM). These results are of importance because they suggest that Al-hydroxide surface characteristics strongly control uranyl surface species in moderately acidic systems