A Study of Pure and Surfactant-Stabilized Oil-Water Emulsions in Batch Separators

Abstract

Understanding the influence of mixing speed and volumetric water concentration (WC) on emulsion stability in batch separators, particularly pure (emulsifier-free) oil-in-water (O/W) and water-in-oil (W/O) emulsions, remains a significant challenge. Also, previously published models do not consider the effect of mixing speed on emulsion type. Therefore, this work proposes a unique strategy for incorporating the role of initial mixing speed and WC into several variables of the separation dynamic in batch separators. The technique was based on the framework developed by Jeelani et al. in 1999. According to comparisons made against literature data, the improved model was shown to capture the initial and later stages of the separation fairly well. Furthermore, it requires assumption of only one variable, ranging from 0.80 to 2.70, as opposed to the four variables required by Jeelani et al.'s (1999) original model. The study also carried out many experiments to understand the significance of mixing speed (600-2500 rpm), water salinity (0-60 g/L), water acidity (1.88-4.80 pH), and temperature (25-80°C) on mineral oil (ExxsolTM D110) and distilled water dispersion stability. Subsequently, the work considered dissolving 0.050% wt. of Tergitol 15-S-7 (T15S7) in the water phase and 0.050% wt. of Span® 80 (SP80) in the oil phase. Each surfactant was introduced separately. Data of surfactant-stabilized emulsions were gathered at mixing speeds of 800 rpm and 2500 rpm, iii temperatures of 25°C-80°C, and in the presence of monovalent salt (NaCl), divalent salt (CaCl2 and MgCl2), and their mixture. Unlike pure W/O emulsion, pure O/W emulsion was substantially impacted by mixing speed, water salinity, and water acidity. Indeed, the stability under these three conditions changed from a few minutes to over four hours. Although the increase in temperature accelerated the separation kinetic of the oil phase, it was shown to delay the initiating time of water separation and was overall ineffective as compared to water salinity and acidity. At WCs ≤ 50%, the increase in mixing speed from 800 rpm to 2500 rpm showed no visible effects on the volume of T15S7-stabilized emulsions (especially after a few hours of the separation process). At WCs ≥ 75%, however, the emulsion volume increased enormously with mixing speed. In the absence of salts, the increase in temperature showed no effects on 75% WC T15S7-stabilized emulsions for at least 4 hours. On the contrary, in the presence of salt, 70°C and 80°C temperatures resulted in immediate separation of T15S7-stabilized emulsions. Furthermore, in the presence of salts, 75% WC emulsions stabilized with SP80 showed full separation within a few seconds at 800 rpm. However, at 25% WC and 800 rpm, the SP80-stabilized emulsion was extremely stable, with or without salt. However, after increasing the mixing speed to 2500 rpm, both monovalent and divalent salts improved the stability of 25% WC SP80-stabilized emulsions. Furthermore, data of T15S7-stabilized emulsion (at 25% WC) showed less stability with salts. In summary, at 25°C, salt concentrations ranging from 1 to 60 g/L impacted the stability of pure O/W emulsions significantly, but not emulsions stabilized by SP80 or T15S7.