PRODUCTION/PROCESSING/HANDLING 51
Phase separation: meeting the technical challenges
Dr John Turner looks at how separation technology can ensure operational efficiency for every stage in the hydrocarbon E&P process chain.
T
he separation methods used to treat the complex mixtures of fluid and solid components arriving at the surface from a typical well do not differ significantly,
whatever the location or type of petrochemical extraction system, so it is convenient to consider rigid and floating marine production platforms, and even land-based operations, in the same general terms. In each case, the fluid is delivered to a pressure vessel then separated into components (oil, water, and gas) and any solid particulate material is removed to minimise the risk of blockage and erosion. Dependent on the duty, the axis of the pressure vessel may be horizontal or vertical, with the ‘process internals’ being designed to accept complex fluid mixtures at high pressures, temperatures, and flow rates. In practice, the actual conditions will vary on a daily basis and both the pressure and flow rate will decrease, over the longer term, as depletion of the well takes place.
Many devices fall under this general description
of process internals, including flow diverters and distribution devices, liquid coalescing and gas/oil/
water separation systems. Operating in this same restricted space within the pressure vessel may also be devices such as slug and sand catchers, flow smoothing and de-foaming systems. The intention, here, is to provide a broad overview of the design and operational constraints placed on the devices routinely employed, without introducing too much technical detail.
Fig. 1. A typical 3-phase configuration.
Rather than considering any particular type of separator, it is instructive to treat all devices in the same way when defining the efficiency. The separation efficiency is then the amount removed (from the inlet mixture) per unit time, divided by the total amount per unit time at the inlet for that same component. Consequently, in a perfect separator, all of the component would be removed, irrespective of the operating conditions whereas, in practice, the efficiency will vary with the actual flow conditions, worsening with increasing flow rate, a higher liquid fraction, or a greater concentration of small droplets. In contrast, the separation efficiency will be improved if the fluid is allowed to remain in the pressure vessel for a longer (residence) time and the blockage, hence pressure loss, of the internals is increased. These factors are likely to be in conflict with the operator’s specification, where a minimum vessel size and maximum flow rate would be welcomed. An understanding of the physical basis for design and an appreciation of how the operating conditions influence the performance of each type of separator are essential to meet these requirements. The problem is to satisfy the process demands, together with the usual mechanical engineering and commercial constraints, within the dimensions imposed by the pressure vessel. Given that all components must be inserted through the manway, final assembly of the internals within the vessel is another factor for consideration. Consequently, the fluid flow behaviour (eg, efficiency of separation, pressure loss, off-design performance), must be balanced against methods of fabrication, cost, and mechanical reliability. Software design tools such as AutoCAD and Inventor have proved valuable in carrying out this mechanical design work and there is a growing awareness of the advantages offered by computational modelling using finite element analysis (FEA) and computational fluid dynamics (CFD). In the first place, FEA and CFD allow examination of those regions where the stress levels and fluid loading might be limiting. Subsequently, these same methods provide a route
www.engineerlive.com
➠
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76