Nathalie Hubert

Use of Pitzer's model to calculate thermodynamic properties of aqueous electrolyte solutions of Na2SO4 + NaCl between 273.15 and 373.15 K

Abstract Pitzers model has been used to calculate various thermodynamic properties of NaCl–Na2SO4–H2O system. A methodology taking into account enthalpy and solubility data has been proposed. This leads to the proposal of a unique set of parameters allowing simultaneous representation of dissolution enthalpies, activity coefficients and solubility data in binary and ternary systems.

Influence of the distribution general shape of n-alkane molar concentrations on the structural state of multi-alkane mixtures

Abstract X-ray diffraction analyses were carried out on four commercial multi-alkane samples and their fifty-fifty weight mixtures which present molar concentration distributions of the “normal logarithmic” type: all these systems form a single solid phase which is isostructural to the β′ ordered intermediate solid solution of n-alkane binary molecular alloys: they are the n-alkanes with carbon atom numbers, n, close to the mean composition in carbon atoms of the mixtures which are in the majority and which impose a single molecule layer thickness. Structural and differential thermal analyses highlighted in the course of cooling from liquid state the successive appearance of three solid solutions in a synthetic mixture whose the molar concentration distribution (from C18 to C36) has a shape of the “decreasing exponential” type as observed in petroleum cuts: the smaller chains, which here are in the majority, do not succeed in making the longer chains bend, too numerous, to form a single solid solution.

Dissolution enthalpy of NaCl in water at 25°C, 45°C and 60°C. Determination of the Pitzer's parameters of the {H2ONaCl} system and the molar dissolution enthalpy at infinite dilution of NaCl in water between 25°C and 100°C

The dissolution enthalpy of sodium chloride in water was measured until saturation at 24.4°C, 44.3°C and 59.2°C using a C80D differential calorimeter (SETARAM). The experimental data were fitted using Pitzers ion interaction model and the value of the dissolution enthalpy per mole of salt at infinite dilution was found at each temperature by treating ΔsolH∞ as adjustable parameter. Using these calorimetric measurements, the analytical expression of the variation of Pitzers parameters were established with respect to temperature. The validation of this expression is made by the measurement of vapour equilibrium of binary solution {H2ONaCl} closed to saturation. Excellent agreement between experimental and calculated values of water activity has been found using our Pitzers model parameters.

Influence of addition of pure components on the structural state of multi-n-alkane mixtures

Abstract Commercial multi-alkane product whose normal alkane distribution is of the ‘normal logarithmic type’, has been modified by adding pure components (n-C 20 H 42 , n-C 32 H 66 , n-C 50 H 102 ) in different proportions. The molecular alloys resulting have been studied by means of X-ray diffraction, at room temperature. It appears that addition of pure components to the initial product has no influence on the structural state of the wax when the proportion is lower than that of the component of higher concentration contained in the initial multi-alkane mixture. On the contrary, when the added proportion is higher than the concentration of the average alkane representative of the distribution, a new series of 00 l reflections characteristic of the pure component structure added, is observed. Moreover, this work has shown the possible existence of a solubility of wax in pure n-alkane.

Aqueous solution of sodium sulfate : determination of the dissolution enthalpy at 25, 27.5 and 45°C

Abstract The dissolution enthalpy of sodium sulfate in water was measured until saturation at 24.4, 27.5 and 44.3°C using a C80D differential calorimeter (Setaram). The experimental data were fitted using Pitzers ion interaction model and the value of the dissolution enthalpy per mole of salt at infinite dilution was found at each temperature, by treating ( Δ diss H ) ∞ as an adjustable parameter.

Binary phase diagram (C14 + C25) and isopleth (C14 + wax): comparison of solubility of n-pentacosane and multiparaffinic wax in n-tetradecane

Abstract The crystallization of n-alkanes is known to be responsible for the solid deposit in flowlines of middle distillate fuels or petroleum cuts. To solve the problems of solid deposits in industrial equipment, it is essential to determine the thermodynamic and structural behaviour of the components of crude oils. The aim of the study was to determine the phase diagram of (n-tetradecane+n-pentacosane) was determined by simple and differential thermal analyses and X-ray diffraction; then it was compared with the pseudo-binary isopleth of the (n-tetradecane+multiparaffinic wax) multicomponent system. In both cases, the solid phases of compounds ( γ o (P 1 )- C 14 H 30 ; β o ( Pbcm )-C 25 H 52 ; β′-wax multiparaffinic solid solution) were observed separately, but there was co-solubility in the liquid state. In the C 25 -rich side of the diagram, the binary mixtures (C 14 +C 25 ) undergo structural changes corresponding to those of pure C 25 when the temperature increases and those of β′ orthorhombic multiparaffinic solid solution of the wax are observed for the multicomponent system (C 14 +wax) in the wax-rich side. The binary diagram (C 14 +C 25 ) displays an eutectic solidification ( liquid ↔ γ o (P 1 )- C 14 H 30 + β o (Pbcm)- C 25 H 52 ) and the isopleth (C 14 +wax), a solidification that resembles an eutectic binary equilibrium ( liquid ↔ γ o (P 1 )- C 14 H 30 + β ′- wax ). The comparison of the two diagrams, particularly the curves of the crystallisation in the heavy n-alkane-rich side, shows that the solubility in C 14 of the multiparaffinic solid solution, whose medium number of carbon atoms per molecule is equal to 25.6, is greater than that of C 25 .

PHASE DIAGRAM OF NAPHTHALENE AND N-PENTACOSANE: EXPERIMENTAL DETERMINATION

The phase diagram of (naphthalene + n-pentacosane) has been determined by thermometrical and differential thermal analyses and X-ray diffraction carried out on the two pure compounds and twelve binary mixtures. The solids phases of the pure compounds are immiscible, but there is cosolubility in the liquid state. With increasing temperature, the binary mixtures undergo structural changes corresponding to those of pure n-pentacosane, while naphthalene seems to retain its initial crystalline structure until fusion occurs. The binary diagram, determined by experiments, displays an eutectic solidification with immiscibility in the solid state: