Experimental study and machine learning modeling of water removal efficiency

Experimental determination of the water removal efficiency from crude oil

Influences of time and demulsifier concentration

Figure 3 represents the variations of WRE with time at various levels of demulsifier concentration under a constant temperature of 25 °C. As can be seen, a low concentration of demulsifier (10 ppm) is not enough to break the W/O emulsion, so that the phases remain unseparated. This can be explained by the presence of elastic and viscous film around the water droplets under this condition²⁷. By increasing the demulsifier concentration up to 40 ppm, the WRE from crude oil at the initial moments is gradually increased. After that, it is dramatically enhanced over time at any concentration. When the demulsifier concentration is 40 ppm, a WRE of 23% is obtained after 5 min. The foregoing value reaches more than 99% after 30 min. A high concentration of demulsifier may increase the rate of coalescence of the water droplets due to the thinning of the interfacial film²⁷. The size of water droplets is increased by sticking the droplets (coalescence), which results in the reduction of the water-in-oil emulsion stability²⁸. Since the demulsifier used in this study (RP968Q) is immiscible with water, it migrates to the two-phase interface during mixing with crude oil. In fact, demulsifiers have interfacial tension gradient that arises from the difference between interfacial tension inside and outside of the film. Due to the high surface activity of demulsifiers, they can invert the interfacial tension gradient after mixing with crude oil. Hence, the coalescence of water droplets can be occurred²⁹.

The effect of demulsifier concentration on WRE at temperature of 25 \(\mathrm{^\circ{\rm C} }\).

The demulsifier concentrations exceeding 40 ppm not only make no positive impacts on WRE, but also cause it to decrease. As shown in Fig. 3, the WRE values obtained at 60 ppm concentration are even less than those obtained at 20 ppm. Accordingly, the overdose of the demulsifier only leads to a higher economic cost. The adsorption rate at the interface of water drops is directly dependent to the demulsifier concentration. In fact, the adsorption of demulsifier at the interfacial layer can prevent the emulsification phenomenon. Afterward, the interfacial tension between oil and water is decreased, which contributes to the enhancement of the coalescence process^4,12,28. In other words, the demulsifier injection affect the dynamic properties of the oil–water interface²⁹. During the overdose of demulsifier, the orientation of the hydrophobic part of the demulsifier molecules are changed, and this gives the demulsifier opportunity to bond with the rest of the water droplets in the opposite direction²⁹. In fact, at very high concentration (overdose), the demulsifier act as an emulsifier agent, which enhances the stability of crude oil emulsion¹².

While increasing time contributes to the gravitational settling, and enhances the demulsifier diffusion through the interface⁶, it can also increase operational costs. Additionally, a high settling time may be associated with the re-emulsification process. This is why determining the optimal settling time is crucial. The settling time can be minimized by adjusting temperature and demulsifier concentration.

The variations of WRE with time at various levels of demulsifier concentration under a constant temperature of 35 °C have been depicted in Fig. 4. As it is evident, by increasing temperature, higher WRE values can be obtained even at very low demulsifier concentrations. This can be explained by the reduction of viscosity and interfacial surface tension at higher temperatures¹². When the demulsifier concentration is 10 ppm, the WRE values of 8% and 79% are observed after 15 and 30 min, respectively. As the concentration of the demulsifier is increased up to 40 ppm, a noticeable improvement is made in WRE. At the concentrations of 10, 20, 30 and 40 ppm, the WRE from crude oil reaches 8.33%, 45.83%, 62.92% and 99.17%, respectively. This observations highlight the fact that at the concentration of 40 ppm, a higher WRE value can be obtained in a short settling time. Similar to those observed under the temperature of 25 °C, a further increase in the concentration of demulsifier (overdose) is associated with the reduction of WRE, and the WRE curves pertinent to the concentrations of 50 and 60 ppm lie below that of 40 ppm. Hence, determining the optimal concentration of demulsifier is of special importance in the water removal process from crude oil.

The effect of demulsifier concentration on WRE at temperature of 35 \(\mathrm{^\circ{\rm C} }\).

Figure 5 illustrates the variations of WRE with time at various levels of demulsifier concentration under a constant…