David A. Storm

FT-IR and solid-state NMR investigation of phosphorus promoted hydrotreating catalyst precursors

The effect of phosphorus on the structure of NiMo/Al2O3 hydrotreating catalyst precursors has been investigated. Calcined and reduced P/Al2O3, PNi/Al2O3, P-Mo/Al2O3, and PNiMo/Al2O3 (where the wt% P = 0.0 to 10.0, wt% Mo = 8.0 to 12.0, and wt% Ni = 4.0) have been studied using FT-IR, XRD, and 31P and 27Al MAS NMR techniques. Phosphoric acid reacts with alumina hydroxyls forming monomeric and polymeric phosphates. At the higher phosphorus loadings, aluminum phosphate is also formed. On calcined PNi/Al2O3, nickel phosphate is formed. This leads to a decrease in density of NO sites in the reduced state as measured by CO adsorption. The addition of up to 1.5 wt% P to Mo(8)/Al2O3 promotes the formation of octahedral molybdena on the alumina surface. However, the addition of > 2.0 wt% P results in the formation of bulk M003 and Al2(MoO4)3 in both PMo(8)/Al2O3 and PMo(12)/Al2O3. CO adsorption on reduced PNi(4)Mo(8)/Al2O3 samples shows that the presence of 0.5 wt% P causes a significant increase in the number of sites adsorbing CO. Increasing the P loading further causes a decrease in the number of adsorbing sites; this decrease can be attributed to the formation of either nickel phosphate or nickel molybdate.

Macrostructure of asphaltenes in vacuum residue by small-angle X-ray scattering

Abstract Small-angle X-ray scattering was used to investigate the macrostructure of the asphaltenes (heptane-insolubles) in Duri, Ratawi, Oriente and Merey vacuum residues at 93 °C. Synthetic vacuum residues prepared from these materials with different amounts of asphaltenes were also studied. To determine the shape and average size of the colloidal particles, the measured scattering intensities were fitted by applying the constraint that the contrast must remain independent of shape and size distribution parameters as the asphaltene concentration is varied. According to this analysis the asphaltenic colloidal particles appear spherical. The average radii are in the range 30–60 A, depending on the crude oil; the radii obey the Schultz distribution. The degree of polydispersity is ~15%. The average particle size does not appear to depend on heteroatom content. Larger asphaltenic particles appear to dissociate when vacuum residue is diluted with the non-asphaltenic fraction.

Upper bound on number average molecular weight of asphaltenes

Abstract An upper bound on the number average molecular weight of a molecular mixture is derived in terms of quantities measured in mass spectrometry. The upper bound is useful for samples that do not completely sublime, such as the less soluble subfractions of vacuum residue. Using the upper bound it is shown that the number average molecular weights of Ratawi and Alaska North Slope asphaltenes must be less than 814 and 1270, respectively. Values measured by VPO at 50 °C using toluene are too high.

Characterization of colloidal asphaltenic particles in heavy oil

Abstract Studies with the vacuum residue from Ratawi crude oil and the asphaltenes obtained from this residue support certain aspects of both the Pfeiffer-Saal and Yen models of the nature of asphaltenes in heavy oil. However, the asphaltenic molecules are not found to be macromolecular. Instead, ∼ 100 asphaltenic molecules, with molecular weights of ∼ 1000 u, spontaneously self-associate to form micelle-like particles. The average asphaltenic micelle-like particle is 66 A in diameter and has an apparent molecular weight of ∼ 100 000 u in toluene-pyridine mixtures or in vacuum residue. These asphaltenic micelles are solvated in the vacuum residue by non-asphaltenic molecules, as suggested by Pfeiffer and Saal, making their effective volumes and apparent molecular weights about three times larger, at 93°C. The volume of the average solvated particle in Ratawi vacuum residue is ∼450 000 A 3 and its apparent molecular weight is approximately 300 000 u, at 93°C.

Fractal structure of asphaltenes in toluene

Abstract Small-angle neutron scattering (SANS) was used to study the fractal structure of Ratawi petroleum asphaltenes in toluene. For analysis of the SANS data, the asphaltene aggregates are modelled as polydisperse fractal objects containing monodisperse asphaltene micelles as unit particles. This model is applicable to asphaltene concentrations of 10–80 wt%. The results show that the asphaltenes are well dispersed with a low agglomeration number when the concentration is ≤ 40%, and the fractal dimension is ∼ 3.0. Above 40 wt%, large clusters start to form, and the fractal dimension decreases to a minimum of ∼ 1.8 at 60 wt%. Above 60 wt% the average cluster size gradually decreases, while the fractal dimension increases, regaining ∼ 3.0 at > 75 wt%. This is attributed to a phase inversion phenomenon. In addition, the clustering of asphaltene micelles is found to follow a reaction-limited aggregation process.

FLOCCULATION OF ASPHALTENES IN HEAVY OIL AT ELEVATED TEMPERATURES

ABSTRACT It has been observed that there is a dispersed phase of particles in vacuum residue that is associated with the heptane insoluble asphaltenes. The particles are in the size range of 100 Aˇ. In this paper we present rheological and small angle X-ray scattering(SAXS) data for the vacuum residue of Arabian Medium/Heavy crude oil that suggests the dispersed particles flocculate at elevated temperatures. We find the asphaltenic particles are stabilized below 200 ° C against the attractive dispersion forces between asphaltenic particles by an adsorbed layer of non-asphaltenic molecules. However the strength of the interaction holding the adsorbed layer to the particles is only about 1.8 kcal/mole(3kT), and so this layer is dissipated as the temperature is increased. We estimate the particles are unprotected at 200 °C. The strength of the attraction due to the dispersion force is about 7kT in this state, and so the dispersion becomes thermodynamically unstable with respect to flocculation. Additionally,...

COLLOIDAL NATURE OF VACUUM RESIDUE

Abstract Vacuum residues from Duri, Alaska North Slope, Ratawi, Oriente and Merey crude oils are observed to be colloidal dispersions of the asphaltenes (heptane insolubles) in the other molecules that make up the vacuum residue. It is suggested that molecules in the heptane-insoluble portion self-associate, and form solid-like particles that can be distinguished from the other molecules in vacuum residue by rheological measurements. Intrinsic viscosities are measured for these colloidal particles (7–9 at 93 °C). The differences in intrinsic viscosities for the various asphaltenes are small in spite of significant differences in their heteroatom content. This suggests that the heteroatoms are buried inside the colloidal particles, and therefore have only a small influence on the hydrodynamic disturbance caused by the particles in the suspending fluid.

Rheological studies of Ratawi vacuum residue at 366 K

Abstract Ratawi vacuum residue is a typical bottom-of-the-barrel fraction from an asphaltenic sour crude oil, and previous work indicates that Ratawi asphaltenes are typical petroleum asphaltenes. The rheology of Ratawi vacuum residue is analysed in this work by employing theories given by Pal and Rhodes, Eilers, Campbell and Forgacs, and Grimson and Barker to describe the rheology of concentrated suspensions of solid particles. The excellent and self-consistent description provided by these theories shows that Ratawi asphaltenes are a distinct species in vacuum residue. Information about the macrostructure of asphaltenes in their natural environment, the crude oil or residue, is also obtained. Ratawi asphaltenes are solvated spheres of different sizes in the vacuum residue. Their solvated volume is two to three times the dry volume at 366 K. Direct interparticle interactions between solvated asphaltenic particles are weak at 366 K. These forces are repulsive, and account for the stability of Ratawi vacuum residue with respect to asphaltene precipitation. A method of employing the Grimson-Barker theory suggests that the Ratawi asphaltenes are charged. The solvation effect arises because polar molecules in the non-asphaltenic fraction surround the particles and screen the charge.

Rheological properties of vacuum residue fractions in organic solvents

Abstract We report our rheological study on Ratawi vacuum residue fractions in various organic solvents. Both light and heavy ended fractions form colloidal particles in these solvents, and the mixtures exhibit Newtonian behaviour. The viscosity data was analysed using various available theories, to determine their colloidal structure and the degree of solvation. We also studied the viscosity response to temperature for the heavy ended fractions in toluene, where a glass-like phase boundary was observed at low temperature.

Adsorption kinetics of asphaltenes at toluene/acid solution interface

Abstract The adsorption kinetics of asphaltenes at the asphaltene-in-toluene/acid solution interface was studied using dynamic interfacial tension measurements. The pH of the aqueous phase was varied from 0.5 to 7 to study the effect of pH. Three asphaltene concentrations were used, one below the critical micelle concentration (CMC), one near the CMC and one above it. The results showed that for asphaltene concentrations below or near the CMC, at pH between 3 and 7, the dynamic interfacial tension (DIFT) decays monotonically. The decay rate followed a diffusion-controlled process closely. At a concentration above the CMC, the DIFT behaved differently. It exhibited similar behaviour initially, but reached a minimum at ∼ 40–70 min, depending on the pH, and slowly increased thereafter. A physical and a chemical process were used to rationalize this phenomenon.