Ind. Eng. Chem. Res. 1991,30,367-375 erties observed. These features include having the phenolic OH and the aminomethyl N ortho to each other and having a 2-hydroxyethyl group attached to the amine nitrogen. The structural features that appear to be essential for corrosion-protection performance also appear to be necessary for the hydroxybenzylamine to have the ability to form a reactive film with a steel surface and to form a complex with ferric ions in solution. Infrared reflection-absorption studies of the film formed on polished steel and of the iron complex (I1 + Fe3+)isolated from solution suggested that the two species are similar if not identical. It seems likely that corrosion protection is associated with the ability of the ohydroxybenzylamine to react with Fe(II1) ions at or near the metal surface to form water-insoluble complexes that deposit on the surface.
Acknowledgment We gratefully acknowledge helpful discussions with A. Lindert and J. L. Pierce of Parker Chemical Company and M. Chattha of Ford Motor Company, and the technical assistance of D. Muschott. Registry No. 11, 118328-17-9; steel, 12597-69-2.
Literature Cited Bellamy, L. J. Alcohols and Phenols. In The Infra-red Spectra of Complex Molecules, 3rd. ed.; Chapman and Hall: London, 1975; Vol. 1, p 108. Bertrand, J. A,; Kelly, J. A.; Breece, J. L. Sub-Normal Magnetic Moments in Copper(I1) Complexes: Five Coordinate Copper in an Oxygen-Bridged Dimer. Inorg. Chim. Acta 1970,4, 247-250. Carter, R. 0.;Gierczak, C. A.; Dickie, R. A., The Chemical Interaction of Organic Materials with Metal Substrates. Part 11: FT-IR
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Studies of Organic Phosphate Films on Steel. Appl. Spectrosc. 1986,40, 649-655. Casellato, U.; Vigato, P. A. Transition Metal Complexes with Binucleating Ligands. Coord. Chem. Reu. 1977,23, 31-117. Kato, M.; Muto, Y.; Jonassen, H. B.; Imai, K.; Harano, H. Magnetic Moments and d-d Bands of N-n-Propanolsalicylaldiminato-copper(I1). Bull. Chem. SOC.Jpn. 1968, 41, 1864-1870. Lindert, A. US.Patent 4 376000, 1983. Lindert, A.; Maurer, J. I. Post-Treatment of Phosphate Coatings. Chromate-Free Options. Polym. Mater. Sci. Eng. 1985, 53, 709-7 13. McCrackin, F. L. A Fortran Program for Analysis of Ellipsometer Measurements. NBS Tech. Note 1969, 479. Miners, J. 0.; Sinn, E. Alkoxy and Phenoxy Bridged Dimeric Copper(I1) Complexes with Salicylaldimine Ligands. Bull. Chem. SOC. Jpn. 1973, 46, 1457-1461. Pasto, D. J.; Johnson, C. R. Separation and Purification. In Organic Structure Determination; Prentice-Hall: Englewood Cliffs, NJ, 1969; pp 7-56. Ryzhkina, I. S.; Boos, G. A.; Kudryavtseva, L. A.; Bel’skii, V. E.; Ivanov, B. E. Spectroscopic Study of the Complexing of Copper(11) with 2-Diethylaminomethylphenol. Russ. J . Inorg. Chem. 1983, 28, 1151-1154. Shriner, R. L.; Fuson, R. C.; Curtin, D. Y.; Morrel, T. C. The Detection and Confirmation of Functional Groups: Complete Structure Determination. In The Systematic Identification of Organic Compounds, 6th ed.; Wiley: New York, NY, 1980; pp 134-355. Sinn, E. Schiff Base Ligands from 3-Aminopropanol. Synthesis, Magnetism, Structure, and Mass Spectroscopy of the Binuclear Copper(I1) Complexes C U ~ C ~ ~ O ~ C~zO&~CzoHzo, N ~ C ~ ~ H ~ and ~ , Cu2O4N2CzsHz6Inorg. Chem. 1976, 15, 358-365. Yamada, S.; Kuge, Y.; Yamanouchi, K. Copper(I1) Complexes with a Subnormal Magnetic Moment. Inorg. Chim. Acta 1967, 1, 139-1 40.
Received for review May 17, 1990 Accepted September 10, 1990
Chemical Demulsification of Petroleum Emulsions Using Oil-Soluble Demulsi f iers Mark A. Krawczyk, Darsh T. Wasan,* and Chandrashekar S . Shetty Department of Chemical Engineering, Illinois Institute of Technology, Chicago, Illinois 60616
The factors affecting the coalescence and interfacial behavior of water-in-crude-oil emulsions in the presence of oil-soluble demulsifiers were investigated. The emulsion-breaking characteristics and interfacial properties of East Texas Crude and a model system were compared. T h e variation of interfacial tension with demulsifier concentration for the model system was ascertained by measuring the interfacial tensions between the oil and water phase. Interfacial activity, adsorption kinetics, and partitioning were shown to be the most important parameters governing demulsifier performance. A conceptual model of drop-drop coalescence process in demulsification was presented which indicates that the interfacial activity of the demulsifier must be high enough to suppress the interfacial tension gradient. This accelerates the rate of film drainage, thus promoting coalescence.
Introduction According to Bancroft (1913), the stability of any emulsion is largely due to the nature of the interfacial film that is formed. The stability of this film is strongly dependent upon the surfactant adsorption-desorption kinetics, solubility, and interfacial rheological properties such as elasticity, interfacial tension gradient, and interfacial viscosity. Zapryanov et al. (1983) have conducted a parametric study describing the rate of drainage of axisymmetric plane-parallel emulsion films associated with droplet-droplet coalescence phenomena. In their analysis, they * To whom correspondence should be addressed. 0888-5885/91/2630-0367$02.50/0
determined that the stability of a surfactant-stabilized emulsion film is dependent not only on the interfacial viscosity of the film but also on the interfacial activity and solubility of the surfactant. The velocity of thinning is dependent upon the balance of forces acting at the interface of the approaching fluid particles. As these fluid particles come close to each other (Figure l),liquid flows out of the film toward its thicker parts and thinning occurs. However, as the liquid flows out of the film, it conveys surfactant due to the convective flux, thus perturbing its equilibrium distribution. This generates reverse fluxes tending to restore the equilibrium distribution: interfacial diffusive flux and bulk fluxes from the film and the droplet. 0 1991 American Chemical Society
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Water (Droplet) phase
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Diffusion
Figure 1. Surfactant mass balance at the film interface.
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Figure 2. Tangential stress balance at the film interface.
Interfacial flow resulting from nonuniform surfactant distribution gives rise to interfacial stresses (Figure 2). The difference in concentration along the interface produces a variation in the local values of interfacial tension that generate a force (interfacial tension gradient) opposite to the liquid flow. In addition, the surfactant monolayer may experience shearing and dilating deformations that also generate interfacial stresses. Therefore, the sum of the interfacial stresses and the tangential bulk stresses from the liquid in the droplet must counterbalance the tangential bulk stress from the film liquid which causes interfacial flow. Demulsification can be conceived as a physical process that disrupts the interfacial stress resulting from film flow. Since the interfacial stress (Figure 2) opposes the tangential bulk stress produced by the convective flux, the principal effect of chemical demulsification is to enhance film drainage by suppressing the tension gradient. It will be shown that not only must the interfacial viscosity be low for demulsification to proceed, but the interfacial activity of the demulsifier must be sufficiently high in order to suppress the tension gradient.
Experimental Section Materials. Emulsion stability in the presence of chemical additives was studied. The categories of demulsifier studied were alkoxylated polyol, alkoxylated resin, ethoxylated polyol, and a blend of phenol resins. The blend of phenol resins was provided by Exxon Research and Engineering Co. (ER&E Co.), while the other demulsifiers were provided by Nalco Chemical Co. These demulsifiers were chosen for this investigation because of the differences in their effectiveness and chemical structure. The crude oil provided was centrifuged to remove all the finely suspended particles naturally present in the oil. Batch tests were conducted to classify the demulsifiers according to their emulsion-breaking ability and to de-
termine the concentration of demulsifier required. On the basis of the time required for a known quantity of water to settle, the separation rate constants for the crude oils were determined. The separation rate constant K is simply the least-squares slope of the variation of the fractional water separation with time. The model crude oil system consists of 1.9 g of asphaltenes derived from East Texas Crude per liter of an alkane-aromatic solution of 70% heptane and 30% toluene. The solution in which the asphaltenes are dissolved shall henceforth be called "heptol". The procedure for the preparation of asphaltenes involved the dissolution of crude oil in pentane using 40 parts of pentane to each part of crude. This mixture was then filtered to withdraw the precipitated asphaltenes. The asphaltenes were placed in a desiccator; when completely dried the oil phase was prepared. The oil phase was finally passed through a 0.5-pm filter to remove residual solids. Emulsion Stability and Demulsifier Effectiveness. Batch emulsion stability tests for a 30% water-in-oil (W/O) emulsion were conducted on the crude oil and model systems. A known quantity of demulsifier was added to the oil phase by use of a microsyringe. The concentration of the active component of the demulsifier was varied from 100 to 1000 ppm in the crude oil and from 0.1 to 50 ppm in the model system. The emulsion was generated by vigorous agitation of the oil and water phase in a graduated cylinder. The volume of water settled was noted against time. These experiments were performed to establish the demulsifier concentration required to effectively resolve the water-in-oil emulsions. The method of preparation for all emulsions used in this investigation was identical to ensure that the mechanical energy input to the systems remained constant. Interfacial Tension. The interfacial tension as a function of the concentration of the active component of the demulsifier in the model system was measured at room temperature (26.0 " C ) using the Wilhelmy plate method with a Kruss K8 tensiometer (Lankveld and Lyklema, 1972). Before each measurement the scratched platinum was well washed and fired over a Bunsen burner flame to ensure proper wettability. Whether the plate had been properly wetted was judged from its reflectivity and the contact angle the water phase formed with the plate. All steady-state values are an average of at least two individual observations. The interfacial tension so measured was found to be reproducible within 1.0 dynlcm. The demulsifiers were initially dissolved in the oil phase, Con.centrations of the demulsifier are stated with respect to this phase or the total emulsion. Interfacial Shear Viscosity, The interfacial shear viscosity of the oil-water interface containing only asphaltenes and demulsifier (Le., for the model system) was measured by use of a viscous traction deep channel viscometer (Wasan et al., 1971). Precise values of the interfacial shear viscosity of the crude oil-water interface in the presence of demulsifier could not be determined due to the high bulk viscosity of the oil (26 CPa t 25 O C ) . In order to distinguish the extent of interfacial viscosity lowering due to demulsifier adsorption at the crude oilwater interface, the ratio of the center-line velocity uo of the deep channel viscometer dish to the particle velocity up is reported. 4u0/up = n cosh sY'sinh nY'
where Y = y / d , Y'= y ' / d (1)
Ind. Eng. Chem. Res., Vol. 30, No. 2, 1991 369
Tensiomcm m D k d m of dispascd phase flow
Glass capillary arrangrment
Figure 3. Diagram of dynamic interfacial tension apparatus.
This parameter, x, is a direct measure of the interfacial shear viscosity with a correction factor for changes in depth of the lower and upper phases, y and y'. The channel width of the viscometer is given by d. Partition Coefficient, The partition coefficient is defined as the equilibrium ratio of the demulsifier concentration in the water phase to the demulsifier concentration in the oil phase (Berger et al., 1987). Thus
K = c,/c, The partition coefficient for asphaltenes which are virtually insoluble in deionized water is zero. Partition coefficients greater than unity are indicative of preferentially watersoluble compounds. Preferentially oil-soluble compounds are characterized by partition coefficients less than unity. The determination of the partition coefficient requires ascertaining a calibration plot of the surface tension of aqueous solutions containing known concentrations of the demulsifier. Since alkoxylated resin and ethoxylated polyol are only slightly soluble in water, it was reasoned from the surface tension isotherms of the other demulsifiers (viz., alkoxylated poly01 and phenol resin) that alkoxylated resin and ethoxylated poly01 possess partition coefficients less than 0.01. Equilibrated systems were prepared by contacting the water and oil phase in a graduated cylinder under vigorous agitation. The initial concentration of the demulsifier in the oil phase was 0.001 g/cm3. After a period of equilibration lasting 3-4 days the contents were transferred to a centrifuge tube. The oil and water phases were separated by use of a Beckman ultracentrifuge. The demulsifiers were separated using a rotor speed of 5000 rpm for 2 h. The remaining aqueous phase was centrifuged again to ensure complete oil removal. The surface tension of the equilibrated and separated water phase was measured to determine the demulsifier concentration from the calibration plot. The resulting mass balance allows the calculation of the equilibrium demulsifier concentration in the oil phase and the partition coefficient. Dynamic Interfacial Tension. Chemical demulsification is a dynamic process since it is a phenomenon that occurs under nonequilibrium conditions. Coalescence of the dispersed phase often happens before the interface is at equilibrium. Therefore, consideration of dynamic properties, such as dynamic interfacial tension, in the
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Time, t (min) Figure 4. Effect of asphaltene concentration on model emulsion stability.
analysis of the demulsification mechanism is paramount. The newly designed experimental apparatus used to determine the variation of dynamic interfacial tension with demulsifier concentration includes a manometer and pipet assembly, a glass capillary arrangement, an oscilloscope, and a frequency counter (Figure 3). The dispersed lower density oil phase of the model system consisting of 2 g of asphaltenes/L of heptol was forced through the capillary by the pressure difference between the liquid in the pipet and that a t the tip of the capillary. As liquid was removed from the pipet the level dropped; therefore, the pipet was raised to ensure that constant pressure conditions were maintained. Data ascertained on the variation of droplet volume with the dispersed-phase flow rate using this experimental arrangement was used to determine the dynamic interfacial tension. The dynamic interfacial tension was calculated from the force balance about the expanding droplet. Details of the dynamic interfacial tension calculations are given elsewhere (Krawczyk, 1990).
Results and Discussion A destabilization study on a typical water-in-oil emulsion using asphaltenes derived from East Texas Crude was conducted. The partition coefficient, interfacial activity, surface activity, and adsorption kinetics were determined for demulsifiers of varying separation efficiency using a model system. The effect of asphaltene concentration on the stability of the model emulsion in the absence of demulsifier was investigated to determine a suitable concentration at which the demulsifier effectiveness tests were to be conducted. Figure 4 indicates that the fractional water separation x, for the emulsion in the absence of demulsifiers does not become significantly low unless the asphaltene concentration is increased to approximately 2 g/L. In other words, the emulsion must be prepared so that it is stable
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Table I. Physical Properties of Demulsifiers in the Model System ro, dyn cm-’ demulsifier av MW c min-I K 1.00 1.3 0.98 alkoxylated poly01 2.5 X lo4 phenol resin 2.5 X lo4 0.95 0.6 0.10 0.85 3.3
