Ioannou, K; (2006) Phase inversion phenomenon in horizontal dispersed oil/water pipeline flows. Doctoral thesis , UCL (University College London).

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Abstract

This thesis reports on experimental and theoretical investigations relevant to the understanding of the phenomenon of phase inversion and its effect on pressure drop during dispersed flow of two immiscible liquids in horizontal pipelines. Experimental studies of phase inversion and associated phenomena were carried out in the liquid flow facility in the Department of Chemical Engineering at University College London (UCL), and at the Norwegian University of Science and Technology, Trondheim, Norway (NTNU). Detailed local conductivity measurements have been obtained at UCL (using conductivity ring probes, a local needle conductivity probe and a flush probe mounted on the pipe wall), which revealed phase continuity at different locations in the pipe cross section as the system approaches phase inversion and after it. In both systems, pressure gradient was measured and phase inversion identification measurements along the pipe were enabled with the use of the conductivity ring probes. A new probe that enables phase and drop size distribution measurements was designed and developed for use at UCL. At the UCL facility, velocity ratio of the two phases, the dispersed phase droplet velocity profiles, and phase distribution at the pipe cross section and droplet chord length were also measured. This revealed a significant increase in the dispersed drop size at inversion point. The results also enabled the equal surface energy criterion validation, based on droplet size considerations. The velocity ratio of the two phases was found to have a higher value than unity at all conditions studied, while inversion from water to oil continuous mixtures results in a decrease in its value. The drop velocity was also becoming lower with increasing dispersed phase fraction and it was found to be affected by the presence of high dispersed phase concentrations. Various parameters and their effect on inversion were studied. Three types of oil (with viscosities of 1.7, 5.5 and 11 mPa s) were used while different pipe diameters and materials were tested (namely, acrylic with 32 and 60 mm ID, stainless steel with 38 and 60 mm ID and an epoxy coated stainless steel pipe with 60 mm ID). Mixture velocities from 2.5 m/s to 6.2 m/s (depending on the test section) were used, selected so that the mixture away from the inversion was dispersed. Also, two experimental routes were followed, starting from oil continuous and water continuous dispersions to investigate the existence of a possible hysteresis at the occurrence of inversion. It was found that phase inversion is accompanied with significant changes in pressure gradient it was preceded by a sharp peak when the less viscous oils were used, while no peak was recorded with the use of the more viscous oil. An ambivalent range was seen for the less viscous oil, possibly related to the creation of secondary dispersions. A mechanistic model that describes the layered structure of the flow during inversion (detected experimentally) was proposed for the prediction of flow characteristics and pressure gradient at the region of inversion. It is suggested that inversion starts when a thin layer of the dispersed phase (that is to become continuous) forms at the top or the bottom of the pipe. A clear layer of the continuous phase may also exist at the bottom or the top of the pipe respectively. Two or three layer models were used for these configurations. Results showed that the two layer model predicts pressure gradient and layer thickness well. The homogeneous model was found to agree well with the experimental results, especially in the water continuous region when considerations for the mixture velocity. The friction factor was modified to compensate for the appearance of the drag reduction in the conducted measurements. In addition, a commercial feasibility study has been carried out which confirmed the considerable and immediate potential for the commercialisation of the impedance probe developed within this research for phase and drop size distribution measurements.

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