Dynamic Measurement Method of the Emulsified Water Content Based on Interfacial Polarization

Dynamic Measurement Method of the Emulsified Water Content

Based on Interfacial Polarization

Oil-water two-phase system in the flow shear conditions tends to form a certain stable emulsion, affecting the flow characteristics of oil-water two-phase in gathering lines. Compared to the quiescent state, the emulsion in the flowing state has a higher emulsified water content. Conventional emulsified water content studies are usually based on static conditions. The objective of this study is to propose an effective method for measuring the emulsified water content of oil-water two-phase system under flowing conditions.

There are interfaces films between oil and water phase in crude emulsions because of the existence of some natural surfactants including asphaltenes, resins, and naphthenic acids et al. In the alternating electric field, the charged particles will accumulate on interface film, which caused the interfacial polarization. According to the difference between the conductivity of oil phase and water phase, and the polarization of the phase boundary film, the emulsion system in the alternating electric field can be equivalent to a circuit model composed of resistors and capacitors in series or in parallel. Based on the circuit model, the complex impedance formula capable of describing of emulsion system is thus deduced. There exists a maximum value of the response curve of imaginary part of complex impedance due to the occurrence of interfacial polarization of emulsion. The corresponding frequency of imaginary maximum is defined as the interfacial polarization frequency f₀(w), which is strongly correlated to the water content of the emulsion.

A dynamic testing apparatus (Figure 1) of complex impedance of emulsion is developed in our work based on the principles stated above. To realize the dynamic testing and improve the sensitivity of the measurements, the storage tank is made of non-metal material with two half-cylindrical surface electrodes. The measuring circuit is based on a Frequency Response Analyzer PSM1700, which is used to measure the impedance of emulsions in the frequency range of 50Hz-1MHz.

Using this apparatus, different stirring rates and test temperatures were controlled for the oil-water mixtures with different water contents in experiments to prepare emulsions with varied amount of emulsified water content. The experimental results suggest linear relation for the interfacial polarization frequency and emulsified water content of the oil-water emulsions in the observed range. The relationship between the emulsified water content C_e and the interfacial polarization frequency f₀(w) can be well described by C_e=k₁f₀(w)+k_2, The coefficients k₁(<0) and k₂ are obtained by linear fitting with the temperature and the shear rate, respectively.

Furthermore, it is found that the value of interfacial frequency will significant step increase when the phenomenon of full emulsification occurs. In this way, the step increase in interfacial frequency can be used to determine the complete emulsification of the oil-water mixture.

Fig. 1 Dynamic testing apparatus of emulsion