Characteristics of a motionless mixer for dispersion of immiscible fluids—I. A modified electroresistivity probe technique

Abstract

A modification of the electroresistivity probe technique of Neal and Bankoff was made allowing its use in liquid-liquid dispersions with drops in the range of 0.2–2 mm diameter. Modifications of hardware and on-line microcomputer signal processing permit the sensor length to be considered, as negligible with respect to the drop size. The principle of operation, the hardware and the validation of the method by independent photographic measurements are presented in this paper. The method has the advantage that hold-up, average drop size and interfacial area can be obtained on-line from a single sensor. The experiments were carried in a 3 4 in. diameter Sulzer motionless mixer with 15 elements. Both stainless steel 316 (SS316) and Teflon-coated mixing elements were used. The measurements and the data processing were carried out using an Apple microcomputer which was connected on-line to the system through an A/D interface. A fast assembler program was written to achieve high resolution in the signal processing. Steady-state data were obtained by photography and were used to validate the method. The photographic study also allowed the distribution of size in the drop population to be observed. In general, the distribution is Gaussian and the variance of the distribution decreases as the velocity increases. It was found that at low velocities there is a large difference in the average drop diameter in the two kinds of mixer. This suggests that the surface of the mixing elements has an important role in the processes of drop break-up and coalescence.