D. Petitjean

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.

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.

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.

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.

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.

Isothermal Sections of Ternary Mixtures: n-docosane + n-tricosane + n-tetracosane

Abstract The crystallographic long c parameter of the binary and ternary β′1, β′2, β″1, intermediate solid solutions (n-C22H46 + n-C23H48 + n-C24H50) is approximately equal to the average c parameter of mixture equivalent orthorhombic pure n-alkanes: this finding shows that the longer n-alkane chain is not straight in unit cell of the mixture, but flexible near the chain end, as earlier observed from spectroscopy analyses by other authors. Four ternary isothermal sections were determined at 305 K, 308 K, 311 K and 313 K from binary diagram data and differential scanning calorimetry and X-ray diffraction analyses of thirty ternary mixtures: the ternary intermediate solid solutions undergo the same first-order solid-solid equilibrium transitions as those of binary alloys with the observation of β(Fmmm)-RI and α(R3m)-RII Rotator states below the solidus point.

Ternary Phase Diagram n-Docosane: n-Tricosane: n-Tetracosane Molecular Alloys at 293 K

Abstract The ternary phase diagram of the mixtures n-C22H46: n-C23H48: n-C24H50 has been established at 293 K by X-ray diffraction experiments on forty one samples. This study indicates the existence of limited terminal solid solutions in the neighbourhood of three pure n-alkanes and three domains of intermediate phases, identical to those observed in the binary systems and denoted β‘1, β“ 1, β’ 2 : on the basis of powder X-ray diffraction photographs, the patterns of the phases β‘ 1 and β’2 are indistinguishable and these two intermediate solid solutions on both sides of the middle intermediate phase β” 1 are isostructural as in the binary systems.

Binary phase diagram of the system: n-docosane-n-tricosane

Abstract The binary phase diagram of the system: n -docosane- n -tricosane has been established by means of calorimetric and structural analyses. These technical methods have made it possible to show the existence of seven solid single-phase domains of equilibrium: three terminal solid solutions denoted γ 0 (C 22 ), β 0 (C 23 ), β 0 ′(C 23 ); and two intermediate solid-solutions noted β 1 ′, β 1 ″ of orthorhombic structures. All these phases are stable at 298K. On increasing temperature, a limited miscibility field has been shown in which the orthorhombic phase β(Fmmm), with appearance of the rotator state called RI, is stable. At the highest temperature, this latter phase changes into the rotator phase α-RII (R 3m ), which has been detected on the whole concentration field. Moreover, several eutectoid and/or peritectoid decompositions were proposed in order to explain the experimental results and to respect thermodynamic rules.