Studies of Water-Continuous Emulsions of Heavy Crude Oils Prepared by Alkali Treatment

Abstract

Summary Pumping of heavy crudes as concentrated oil-in-water (O/W) emulsions may be a feasible pipeline transport scheme for viscous crudes. Many crude oils can be emulsified by treatment with alkali without the addition of expensive surfactants. In experimental studies of seven crudes, stable emulsions could be formed with four by alkali treatment; two could not be emulsified; and one formed unstable emulsions. High shear viscosity, particle size, and stability measurements on 60% emulsions formed with various amounts of alkali are reported. High shear viscosities below 100 mPa.s [100 cp] at 25 degrees C [77 degrees F] indicate that concentrated emulsions of some crudes can be transported in pipelines at concentrations of 60% and higher. pipelines at concentrations of 60% and higher. Introduction Significant reserves of heavy crudes exist in the U.S., Canada, and Venezuela. Production of these heavy crudes is expected to increase significantly in the near future as low-viscosity crudes are depleted. Conventional pipelining is not suitable for transporting these heavy crudes from the reservoir to the refinery because of the high viscosities involved (1000 to >100 000 mPa.s at 25 degrees C [1,000 to is greater than 100,000 cp at 77 degrees F]). Several other transport methods have been proposed, including dilution with lighter crudes or petroleum distillates, injection of a water sheath around the crude, preheating the crude and heating the pipeline, and encapsulation preheating the crude and heating the pipeline, and encapsulation of the crude. Transport of viscous crudes as concentrated O/W emulsions is an alternative pipeline technique. This method has been demonstrated on a large scale in an Indonesian pipelines and in a 21- km [13-mile] -long, 20-cm [7.9-in.] -diameter pipeline in California. While emulsion transport has the disadvantage of requiring dewatering of the crude after transport, it may have unique advantages in some cases. For instance, at some locations, makeup water suitable for emulsification may be more readily available at low cost than materials suitable for dilution of heavy crudes. In other cases, environmental concerns or energy costs may make heated pipelining unattractive, particularly for extremely viscous crudes. Emulsions may be transported at ambient temperatures. Water-continuous emulsions containing high concentrations of crude can have viscosities of 100 mPa.s [100 cp] or less. Commercial, nonionic surfactants have been used to form O/W emulsions containing as much as 85 % crude. The presence of natural saponifiable acids in some crudes, however, may eliminate the need for expensive commercial surfactants. With the addition of alkali, these acids will react and form compounds that may lower interfacial tension. Emulsions suitable for pipeline transport, containing as much as 75 % crude, have been formed for some crudes by mixing them with alkaline water. Surface-active agents in crudes have been shown to be carboxylic acids of various molecular weights and chemical structures. Here, data are presented to show the characteristics of 60 vol% crude emulsions formed by NaOH or KOH treatment of seven heavy crudes of widely varying origin to illustrate the effects of varying the alkali/oil ratio. For the four crudes that formed stable emulsions over reasonably wide alkali ranges, the effects of NaOH or KOH addition on emulsion properties were investigated in some detail. The emulsion properties reported are apparent viscosity, mean particle diameter, and water separation after 3 days. particle diameter, and water separation after 3 days. Experimental Methods and Materials Table 1 summarizes the properties of crude oils studied. Values for specific gravity and viscosity are reported at 25 degrees C [77 degrees F]. Total acid numbers (TAN) were measured by titration with an alcoholic KOH solution in accordance with ASTM Method D 644–81. Demineralized double-distilled water was used for all emulsions. Reagent-grade, anhydrous NaOH or KOH in pellet form was used to prepare alkali solutions. The hydroxide was dissolved in the water phase before mixing. Emulsions were made in 45-mL batches with phase before mixing. Emulsions were made in 45-mL batches with 60 vol% crude with a Sorvall Omni-mixer TM at a rotational speed of 9,000 rev/min for 180 seconds. Crude oils were preheated to between 65 and 95 degrees C [149 and 203 degrees F], depending on the crude viscosity. During homogenization, the mixing vessel was immersed in a 70 degrees C [159 degrees F] water bath. Relative emulsion stability was roughly measured under stagnant conditions by periodic monitoring of the volume of the water settling to the bottom of a 60-mL test tube. The height of the 45 mL of emulsion in the test tube was 100 mm [3.9 in.]. Apparent viscosities of emulsions were measured at 25 degrees C [77 degrees F] with a Haake Rotovisco TM viscometer with a concentric-cylinder measuring system. Inner cylinder diameter was 40.08 mm [1.6 in.] with a gap width of 0.96 mm [0.038 in.]. The maximum apparent-shear rate range that could be obtained with this instrument for the emulsions studied here was 152 to 1,370 seconds. For the low-viscosity emulsions (which were Newtonian), this range was limited to 457 to 1,370 seconds. Viscosities were measured by initially increasing shear rate. Measurements were repeated after the highest shear rate was reached to check for hysteresis. Significant hysteresis was not observed for any of these samples. Apparent viscosities reported were measured at 685 seconds This shear rate provided the most reliable data for all emulsions, particularly for the low-viscosity emulsions, and therefore provided particularly for the low-viscosity emulsions, and therefore provided the best comparison of different emulsions. Particle sizes were analyzed by the electrozone technique with an Elzone/ADC-8O TM manufactured by Particle Dynamics Inc. The measurement principle of this instrument is based on changes in conductivity caused by particles immersed in an electrolyte solution as they pass through a small orifice. Results and Discussion Of the seven heavy crudes of varied origin, stable emulsions could be made with four of them over a range of alkali additions less than and greater than their TAN values (see Table 1). The water-separation data in Table I show the hydroxide ranges in which fairly stable emulsions (less than 3-mL separation in 3 days) could be formed. With St. Lina crude, a moderately stable emulsion could be formed with only one concentration of KOH. This concentration was greater than the equivalence amount. No stable emulsions at any alkali concentration could be formed with the Vic Bilh 6 or Rospo Mare crudes. Thus, with the alkali treatment, only the Boscan, Cold Lake, Guadalupe, and Martinez crudes are potential candidates for emulsion pipeline transport. The Martinez and Cold Lake emulsions had lower viscosities than the Boscan and Guadalupe crudes.