Optimisation of Three-Phase Separator Design through Computational Fluid Dynamics Simulation and Experimental Investigation

  • Tariq Galadanchi Ahmed

    Student thesis: Doctoral Thesis

    Abstract

    A literature review has been performed to evaluate the design process for multiphase separators. This survey identified that no formal hierarchical rules exist for the selection of a separator type for a given duty. To remedy this, selection criteria have been formulated.
    Existing design models use slenderness (length to diameter) ratio as the key sizing parameter. The slenderness ratio sets the oil and water pad thicknesses and ultimately the separation achieved. There are inconsistencies between extant models for the optimum slenderness ratio. An experimental test rig (300mm Diameter) was designed to investigate separation performance for slenderness ratios from 3:1 to 8:1. The rig operates at gas flowrates of 30 to 100LPM, oil and water flow up to 10GPM and compositions of 1:1, 2:1 and 1:2 oil/water (v/v). Oil phase residence time is the rate-limiting process, this was fixed at a constant value by varying the water-oil interface level to remove it as a factor in the achieved separation. The results indicated that slenderness ratios 3:1 and 4:1 are strongly influenced by mixing, preventing stratified flow that is conducive to droplet settling. For slenderness ratios of 4:1 to 6:1, separation increase. For ratios above of 6:1, the separation increases but at a slower rate implying a reducing return in terms of separation beyond this point. Further, these results were used to develop a new separation performance indicator (SPI) to predict the separation of a three-phase separator for known droplet size.
    Extant design models do not provide the product streams oil-water ratios. To address this, new CFD models of three pilot-scale separators with different configurations and operating ranges were developed. A further model was developed for the new variable-length apparatus, shown to represent experimental data well and used to investigate the separation performance beyond the experimental operating envelope. Extant models do not consider the importance of droplet size on design often assuming a constant droplet diameter. Fine-tuning of the CFD model proved that this parameter is very significant, it must be considered in the design process and that further work is required to incorporate it into the design procedure.
    A new Capital Cost Optimisation model was developed to address the time dependence of extant models. This model was combined with a case study to identify the significance of operating cost for seven separator configurations for 30 years. The optimum slenderness ratio for this study was 6:1 which agrees with that predicted from the CFD results.
    In summary, the new contributions to knowledge are: a set of heuristic rules for separator selection has been developed; Experimental work investigating the effect of slenderness ratio on separator outlet quality; A new separator performance indicator (SPI) dimensionless group has been derived to predict separation for a known droplet size; CFD models have been developed to predict separator performance and, a finally new flexible separator sizing model has been developed.
    Date of Award2021
    Original languageEnglish
    Awarding Institution
    • Teesside University
    SupervisorPaul Russell (Supervisor), Faik Hamad (Supervisor) & Samantha Gooneratne (Supervisor)

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