P.F. Luckham

Dewatering of crude oil emulsions 2. Interfacial properties of the asphaltic constituents of crude oil

The importance of the interfacial rheology in determining the stability of water-in-Buchan crude oil emulsions has been demonstrated in part 1 of this series of papers (R.A. Mohammed, A.I. Bailey, P.F. Luckham and S.E. Taylor, Colloids Surfaces A: Physicochem. Eng. Aspects, 80(1993)223). In part 2, interfacial tensions of crude oil, and solutions of asphaltenes and resins in a model oil have been investigated. Surface pressure vs. area (Π—A) curves of monolayers of asphaltenes, resins and their mixtures have been established. In its dependence on the ratio of resins to asphaltenes, the pseudostatic dilatational modulus has high values for low resin-to-asphaltene ratios and low values for high resin-to-asphaltene ratios. This is expected to throw light on the cause of the enhanced stability of water-in-crude oil emulsions.

Dewatering of crude oil emulsions 3. Emulsion resolution by chemical means

Abstract In Parts 1 and 2 of this series of papers, the importance of interfacial properties in determining the stability of water-in-Buchan crude oil emulsions has been demonstrated. In this third part of the series, we report on the chemical demulsification of water-in-crude oil emulsions. Two classes of experiment were carried out; bottle tests in which emulsion resolution was followed by the appearance of a separated aqueous layer in the bottom of the tube, and microscopic examinations, where resolution was directly followed by observing coalescence of the droplets. Demulsification was achieved by adding one of the commercial demulsifiers Unidem120 and BJI8. Unidem120 is a blend of non-ionic and anionic surfactants, whereas BJ18 was specially developed for the demulsification of Buchan crude. Some of the pure constituents of these blended surfactants were also used for resolving water-in-Buchan crude oil emulsions. Two types of behaviour were observed: type I behaviour, showing constant water separation with increased demulsifier concentration, and type II, showing reduced water separation with increased demulsifier concentration. To a small extent, there exists a degree of functional specificity in the demulsifier molecules towards interaction with the constituents of the interfacial film, and to a larger extent good emulsion resolution is governed by the interfacial properties of the demulsifier blend.

The effect of demulsifiers on the interfacial rheology and emulsion stability of water-in-crude oil emulsions

Abstract The interfacial rheological properties of the crude oil-water interface have been measured. It is found that the rheological properties are time dependent and the film takes at least 8 h to attain equilibrium. During this period the surface elasticity and viscosity increase markedly with time, with the surface viscosity increasing from 3 × 10 −2 mN s m −1 after 2 h of ageing to 3 × 10 3 mN s m −1 after 8 h of ageing, and the surface elasticity increasing from zero (i.e. the film was simply viscous) to 2 mN m −1 . The effect of the addition of the demulsifier Unidem 120 to an 8 h aged film was to decrease the surface viscosity by an order of magnitude. In the demulsification experiments, this corresponded to a high resolution of the emulsion. The addition of the demulsifier BJ18, however, produced markedly different results. If the demulsifier was added to a pre-aged film, there was little change in the interfacial rheology of the film; indeed the interfacial rheological properties seemed to increase. However, if the demulsifier was added to the crude oil before the interface between the crude oil and water had formed, the interfacial properties of that film remained constant with time, i.e. no thick viscoelastic film typical of the crude oil-water interface was formed. This result indicates that this demulsifier is poor at displacing the naturally occurring asphaltene surfactants from the crude oil-water interface, but if they adsorb at that interface first they prevent the formation of the stable, rigid asphaltene films. Despite this, however, BJ18 was effective in demulsifying freshly formed water in crude oil emulsions.

Correlation of the rheology of concentrated dispersions with interparticle interactions

The viscoelastic properties of concentrated latex dispersions were investigated as a function of the volume fraction of particles. Both electrostatically and sterically stabilized dispersions were studied. The dispersions showed a change from a predominantly viscous to a predominantly elastic response when the volume fraction of the dispersion exceeded a critical value φcr. With electrostatically stabilized dispersions, φcr was determined by the electrolyte concentration. At low electrolyte concentrations (10−5 mol dm−1 NACl) elastic response occurred at relatively low volume fraction φ of a latex dispersion of radius 700 nm. This is due to the high effective volume fraction in this case (φeff = 1.50), i.e. when φ ≈ 0.4, one approaches the maximum packing fraction at which double layer overlap occurs. At higher electrolyte concentrations (10−3 mol dm−3 NACl), φeff is slightly higher than φ (φeff = 1.05φ) and elastic response occurs at φ values approaching 0.6. In other words, the dispersion approaches a hard-sphere system. The high frequency modulus G′ was calculated from knowledge of the double layer parameters. Plots of theoretically obtained G′th vs φ were compared with the experimentally determined values obtained from rheology. With sterically stabilized dispersions, the change from predominantly viscous to predominantly elastic response is governed by the ratio of the adsorbed layer thickness Δ to the particle radius R. Using latex particles with three different radii (core radii 78, 303 and 502 nm) with the same grafted poly(ethylene oxide) (PEO) chains of molecular weight M 2000 (Δ ≈ 8–21 nm depending on R) it was shown that the dispersions behaved like hard spheres when Δ/R (defined as the compressibility of the chains) was relatively small, and as soft spheres when Δ/R was significant. The elastic modulus G′ vs φ could be fitted to a scaling law of the form G′ = kφm and the exponent m was correlated with the compressibility of the chains. Force distance curves (F—h) were obtained using mica containing an adsorbed graft copolymer of methyl methacrylate and PEO chains with M = 2000. The F—h curves were converted to G′—φ curves, and the results were compared with those obtained from rheological measurements.

Adsorption of poly(ethylene oxide)—poly(propylene oxide) ABA block copolymers on carbon black and the rheology of the resulting dispersions

Abstract The adsorption of ABA block copolymers, where A is poly(ethylene oxide) (PEO) and B is poly(propylene oxide) (PPO), on carbon black was investigated at room temperature (20 ± 2°C). Two series of block copolymers were studied: series A containing 56 PO units and 16–148 EO units per chain; series B containing 30 PO units and 30 or 77 EO units per chain. The results showed a decrease in the amount of adsorption Γ (μmol m−2) with increase in the ethylene oxide (EO) chain length. Adsorption increased as the number of propylene oxide (PO) units in the chain decreased. The area per molecule σ showed a linear increase with n 1 2 , where n is the number of EO units, indicating that adsorption is governed by the size of the PEO chain. Concentrated dispersions of carbon black that were stabilised using the block copolymers were studied using steady state (shear stress—shear rate) and oscillatory measurements. The relative viscosity—volume fraction φ curves were compared with the theoretical curves for hard-sphere dispersions (calculated according to the Dougherty—Krieger equation) to obtain the adsorbed layer thickness as a function of φ. Storage modulus G′-φ curves showed a rapid increase above a critical volume fraction φcr for the carbon dispersions. With the ABA block copolymer containing the shortest EO chain (16 EO units per chain) φcr was significantly lower than the values obtained with the other block copolymers, indicating weak flocculation of the suspension. All other block copolymers stabilised the carbon dispersions, and φcr decreased with increase in the EO chain length, as expected. The critical flocculation temperature (CFT) of carbon dispersions, stabilised using a block copolymer containing 56 PO units and 37 and 148 EO units per chain, was measured as a function of K2SO4 concentration using rheological measurements. The results showed that for both polymers the CFT is lower than the θ temperature of the PEO chain at the same electrolyte concentration. This indicated that the block copolymers were not as effective for stabilisation of carbon black dispersions as the previously studied ABC surfactants containing the nonylphenyl group in addition to the PPO chain, indicating that the nonylphenyl group plays a major role in anchoring the chain to the carbon surface.

The measurement of interparticle forces

Abstract Over the last decade an experimental technique has been developed which essentially measured the forces between two surfaces as they approach. The technique is capable of measuring small forces (nN) and small distances (

Adsorption of nonyl phenol propylene oxide—ethylene oxide surfactants on carbon black and the rheology of the resulting dispersions

Abstract The stability of concentrated aqueous dispersions of carbon black has been investigated using rheological measurements. The dispersions are stabilised by a series of Synperonic NPE non-ionic surfactants. These materials contain nonyl phenol, 13 units of propylene oxide and between 27 and 175 ethylene oxide units. The adsorption of the surfactants onto carbon black was studied. The adsorption isotherms revealed that the weight of surfactant adsorbing per unit area was approximately constant (or the number of molecules of surfactant adsorbing decreases with increase of the molecular weight of the polyethylene oxide chain). All the surfactants studied stabilised the carbon dispersions. However, those stabilised by the higher molecular weight surfactants showed higher viscosities and elastic moduli than those stabilised by the lower molecular weight materials. With these surfactants it was possible to prepare dispersions with a phase volume of carbon up to 55%. From the elastic and viscous properties of these dispersions it was possible to estimate the thickness of the adsorbed surfactant layer. This agreed closely with that obtained from light scattering. Critical flocculation temperature of the surfactant-stabilised dispersions corresponded to the 0 temperature of the polyethylene oxide, indicating that the mechanism for stabilisation of carbon black is steric in nature.

The measurement of volume fraction profiles for adsorbed polymers under compression using neutron reflectometry

Abstract A new surface force apparatus is described which enables simultaneous measurement of the interaction forces between adsorbed polymer layers as well as their surface structure by neutron reflectometry. The apparatus utilises two optically flat quartz plates that are brought together by applying a known external force. Surface force and structural profiles have been obtained for polystyrene adsorbed on quartz from cyclohexane, i.e. a poor solvent.

Recent developments in the measurement of interparticle forces

Abstract In the last twenty years there has been an explosion of experimental work devoted to determining the forces between colloidal particles. There have been considerable experimental and theoretical advances. In this review we shall concentrate on the experimental aspects. Currently there is no technique which directly measures the interaction between two individual particles as a function of their separation. Particle - particle forces can only be inferred. In this paper we review the experimental procedures which have been used to determine the nature and range of interparticle forces in this indirect way. The many different experimental techniques which have been developed are outlined and typical data presented. The relative merits and demerits of each technique is discussed and the way forward to measuring particle - particle interactions directly is proposed.

Forces between graft copolymers adsorbed to mica surfaces

Etude de la stabilisation dune suspension de particules de mica par adsorption a la surface des particules dun copolymere doxyethylene et de methacrylate de methyle