Yusoff, Nazrul Hizam
(2012)
Stratifying of liquid-liquid two phase flows through sudden expansion.
PhD thesis, University of Nottingham.
Abstract
The transport and separation of oil and water is an essential process to the oil and chemical industries. Although transporting the mixtures is often necessary due to few reasons, it is generally beneficial to separate out the phases in order to reduce installation and maintenance costs, at the same time, avoiding safety problems. Thus, separation of liquid-liquid flows is a necessary part of many industrial processes. Hence, knowledge of two-phase flow dynamics is important for the design optimisation of separators. Therefore, the aim of this research is to investigate the feasibility of a sudden pipe expansion to be used as phase separator because it compact in design and capable for converting dispersed flow to stratified flow.
In the test section, spatial distribution of the liquid-liquid phases in a dynamics flow system was visualised for the first time for by means of capacitance Wire Mesh Sensor (CapWMS), providing instantaneous information about the interface shapes, waves and phase layer evolution of oil-water flow. Visual assessment and analysis of the WMS data showed three distinct layers: an oil layer at the pipe top; a water layer at the pipe bottom and a mixed layer between them. The interfaces that form between the separated phases (oil or water) and the mixed layer were classified as oil interface or water interface. Results showed interface shapes were initially concave or convex near to the inlet of the test section and became flat further downstream the expansion, especially for water interfaces. There were no waves observed for horizontal and downward pipe orientations at all flow conditions and axial position downstream of the expansion. As for the upward inclined pipe orientation, waves were found, and they formed at position close to the inlet at all input oil volume fraction except at 0.2 OVF. The amplitude of the waves was: ~ 0.29D for 0.8 OVF; ~ 0.22D for 0.6 OVF and ~ 0.26D for 0.4 OVF. The higher the input oil volume fraction, the larger the waves become. In conclusion, the WMS results demonstrated that spatial distributions are strongly dependent on the mixture velocity, input oil fraction and inclination angles for the far position.
In this present work, droplets were found to be larger near the interface. Drops were large nearer to the interface at the near position (10D) for all pipe orientations and throughout the test section for horizontal flow. The drops size decreased when the distance from the interface increased for these pipe configurations. As for the furthest position from the expansion for upward and downward inclined pipe orientation, larger droplets could also be seen at distance away from the interface and vice versa.
The gravity or buoyant force is one of the contributing factors to the settling of the droplets. These forces are acting simultaneously on the droplets i.e. if the buoyant force which tends to spread the droplets throughout the pipe cross-section, is not large enough to overcome the settling tendency of gravity settling of the droplets occurs. Hence, the droplets that are non-uniformly scattered within the continuous phase begin to coalesce as they flow further downstream the pipe, producing larger drops. In addition, as the distance from expansion increased, the mixed layer becomes narrow and more drops begin to coalescence to form large drop due to increased droplet-droplet collision. Owing to these factors, results indicate that the mechanisms of coalescence occurred faster at the bottom, for water droplets and at the top, for oil droplets than the other locations in a pipe cross-section. For a better separation design, the coalescence process should occur at the aforementioned (bottom for water and top for oil) locations within the expansion pipe. However, at higher mixture velocities the mixed layer would be responsible for the smaller droplet size for horizontal and both inclinations of pipe orientation. The mixed layer dominated almost entirely in the pipe cross-section.
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