M. Dirand

Multicomponent paraffin waxes and petroleum solid deposits: structural and thermodynamic state

The X-ray diffraction analyses, carried out on eight commercial and industrial waxes and a heavy crude oil, show the following remarkable results: (i) each multicomponent paraffin wax (from 20 to 33 n-alkanes), which has a continuous distribution of consecutive n-alkanes (19 < n < 53), forms a single orthorhombic solid solution; (ii) the molecule packing identity period along the long c-axis of this solid solution corresponds to a chain length of a hypothetical orthorhombic n-alkane whose carbon atom number is equal to the average carbon atom number of n-alkanes contained in each multicomponent paraffin wax. This multicomponent phase, whose orthorhombic structure is analogous to one of the two intermediate solid solutions, β′n or β″n, of binary and ternary molecular alloys of consecutive n-alkanes, is also observed in the deposit of the heavy crude oil with the presence of an amorphous solid.

Mixtures of numerous different n-alkanes: 2. Studies by X-ray diffraction and differential thermal analyses with increasing temperature

Abstract The structural behaviour of the orthorhombic multi- n -alkane crystalline β ′ phase, observed in mixtures consisting of 23 (19 n n n -alkanes, respectively, is studied by X-ray diffraction with increasing temperature. A Rotator single-phase domain of rhombohedral (or hexagonal) α -RII type is observed below the solidus point. The product, consisting of 23 n -alkanes, undergoes this structural transition ( β ′→ α -RII) through two two-phase domains: first, a region with the β ′ and β (Fmmm) phases; then, a domain with the two β (Fmmm)-RI and α (R 3 m)-RII Rotator phases. For two β ′ solid solutions consisting of 33 n -alkanes, a single two-phase domain ( β ′+ α -RII) is observed in the course of this structural crystal–Rotator transition.

Modifications to the binary phase diagram of the alkane mixtures n-C23–n-C24

Abstract X-ray diffraction patterns of binary mixtures of normal tricosane and tetracosane at 20°C (powder method) show the presence of two new phases, here denoted β′ and β ′ 1 . The crystal structure of these phases is orthorhombic; the structure of the β′ phase is isomorphous with the tricosane β′ phase observed after the δ transition. With increasing temperature and concentration, the experimental observations, both by X-ray diffraction and differential thermal analysis, made it possible to modify the binary phase diagram of n -C 23 – n -C 24 .

Structural behaviour of n-tetracosane and n-hexacosane mixtures

Abstract The study of the n-alkane binary system n-tetracosane (n-C24H50) : n-hexacosane (n-C26H54) was carried out using calorimetry and X-ray analyses. In spite of what is generally found in the literature for the two consecutive even-numbered n-alkanes mixtures, the X-ray diffraction results at 293 K show the presence of five solid solutions when the molar concentration in n-hexacosane (n-C26H54) increases : two terminal solid solutions γ1 and γ2, isostructural with n-tetracosane (n-C24H50) and n-hexacosane (n-C26H54) respectively and three orthorhombic intermediate phases, isomorphic with the odd-numbered pure alkanes (n-pentacosane n-C25H52, for example), noted β′1, β″ and β′2 respectively. On the basis of powder X-ray patterns, the phases β′1 and β′2 are indistinguishable and they are isostructural with the intermediate solid solutions β′1 and β′2 of the n-docosane (n-C22H46) : n-tetracosane (n-C24H50) system and probably with the phases β1 and β2 of the n-eicosane (n-C20H42) : n-docosane (n-C22H46) diagram. The intermediate solid solution β″ is also isostructural with the phase β″ of the n-docosane (n-C22H46) : n-tetracosane (n-C24H50) system and certainly with the phases β of the n-eicosane (n-C20H42) : n-docosane (n-C22H46) mixtures : these two phases have been compared and discussed. With increasing temperature, X-ray diffraction and differential calorimetry led to the determination of the phase transitions at their equilibrium temperatures. The terminal solid solution γ1 and γ2 transformations are similar to those observed in n-tetracosane (n-C24H50) and in n-hexacosane (n-C26H54). The transitions of the intermediate phases β′1, β″ and β′2 are equivalent to those of the n-docosane : n-tetracosane system and the odd-numbered n-alkanes, above the transition δ. The n-pentacosane (n-C25H52) transformations versus temperature have been described with the appearance of the rotator phases β-RI and α-RII just below the melting point. They are compared with those that are observed in the phase β″. The phase denoted β0 in the n-C20H42 : n-C22H46 diagram probably corresponds to the rotator phase β-RI.

Crystallization of a multiparaffinic wax in normal tetradecane

Abstract The thermodynamic and structural studies of the formation of solid deposits in solutions, that consist of a commercial multiparaffinic wax in the normal tetradecane, used as solvent, are carried out by X-ray diffraction as the temperature decreases from the liquid state, chromatography analyses of (liquid and solid) phases separated at equilibrium, and simple and differential thermal analyses. The experimental results highlight that the first deposits, observed just below the liquidus point, form a single orthorhombic multi-Cn solid solution that consists of all the n-alkanes of the commercial wax from C20 to C42; the thermodynamic behaviour of (wax+C14) mixtures in the course of the crystallization resembles binary eutectic solidification of the C14 solvent on the one hand, and the wax as a single pseudo-component on the other hand.

On the ideality of liquid mixtures of long-chain n-alkanes

Abstract Calorimetric measurements show that the heats of mixing two n-alkanes (of similar chainlength) in the liquid phase, are very small, and may often be neglected. Also, calculation of the entropy of n-alkane binary mixtures in the liquid and in the high temperature solid α-RII Rotator phase, by integration of experimentally obtained enthalpies, show that the excess entropies are negligible for all practical purposes. n-Alkane liquid mixtures between neighbouring homologs may therefore be considered as ideal solutions.

Diagram of the n-tricosane-n-tetracosane mixtures: Corrections

Abstract Following the rules of solid phase sequence in binary mixtures of n -alkanes, a new orthorhombic intermediate phase is observed in the rich n -tetracosane concentration range, situated near the triclinic terminal solid solution γ 0 (C 24 ). The joint calorimetric (D.T.A) and structural studies, realized on thirty-nine samples, complement the n -tricosane : n -tetracosane diagram.

Dissolution of some normal alkanes in ethylbenzene: deduction of the enthalpy of mixing two n-alkanes in the solid state

Abstract Drop calorimetry has been used to measure the heat changes occurring upon the dissolution of some single n -alkanes (octadecane to tetracosane) or their binary mixtures (C 22 H 46 C 24 H 50 and C 23 H 48 C 24 H 50 ), in ethylbenzene, at 311.45 K. The enthalpy changes measured show the even-odd alternation effect known for n -alkanes. The binary mixtures show pronounced non-ideality. Their enthalpies of formation at 293.15 K were deduced from the measurement of their heats of dissolution in ethylbenzene and found to depend on the crystal type. It is shown that these mixtures may consist of several phases of solid solutions.

Thermodynamic Properties of the n-Alkanes C19H40 to C26H54 and Their Binary Phase Diagrams

The shapes of the C22H46-C24H50 and C23H48-C24H50 binary phase diagrams were analyzed. In the C22H46-C24H50 binary system the increased stability of the binary compounds with increasing temperature can be explained by the much larger heat capacity and entropy of the binary compounds compared to that of the components C22H46 and C24H50. In the C23H48-C24H50 system this effect is much less pronounced. The measured enthalpy data of n-alkanes C19H40 to C24H50 and of the binary system C22H46-C24H50 were analyzed to obtain the ‘excess’ heat capacity per atom of carbon {[Cp/(Rm)]-3} (Rm being the number of carbon atoms). The ‘excess’ heat capacity per carbon atom is the value of the heat capacity above the Debye high temperature value of 3R. At low temperatures (below 280 K) one is in the Debye temperature θD region. At higher temperatures the large ‘excess’ heat capacity of the solids explains the movements in the carbon chains. In the liquid the excess heat capacity is small and corresponds numerically to the anharmonic vibrations in low melting metals. In contrast to metals, where the difference in heat capacity between liquid and solid below the melting point is positive Cp(L-s)>0, in the alkanes studied it is strongly negative Cp(L-s)≪0. This explains the shape of the binary phase diagrams C22H46-C24H50, C24H50-C26H54, C22H46-C23H48 and C23H48-C24H50.

Phase diagram of n-heneicosane and n-tricosane molecular alloys

Abstract The phase diagram of the mixtures n-C21H44:n-C23H48 has been established by joint calorimetric and structural analyses. This study indicates the existence of nine solid single-phase domains of equilibrium: four terminal solid solutions, denoted βo(C21), β′o(C21), β′o(C23) and β′o(C23) with the n-heneicosane and n-tricosane structures, three orthorhombic intermediate solid phases, called β″1, β′1 and β″2 (β″1 and β″2 on both sides of β″1 are isostructural), with increasing temperature, a total miscibility field: β, with the orthorhombic structure Fmmm, above the intermediate solid solution regions (a second order transition which is characterized by the Rotator RI state is observed in this phase) and below the solidus line α-RII with the rhombohedral structure R3m. These results complete the literature diagrams which only presented two solid single-phase domains.