Phase separation: meeting the technical challenges

Paul Boughton

Dr John Turner looks at how separation technology can ensure operational efficiency for every stage in the hydrocarbon E&P process chain.

The 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.

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 towards refinement (and, possibly, optimisation) of the separation systems, concentrating on reducing size and weight, while attempting to enhance performance and reliability.

A mix of physical laws and empirical data, together with a good deal of prior experience, forms the background to the process and mechanical design procedures. Unfortunately, the close confines in which the internals operate means that some interaction between the internals should be expected. The operator may be able to observe the overall influence of these effects but it will usually be impossible to measure the performance of the individual separators, directly.

In operation, the rates of separation of the fluid components are typically governed by the rates at which oil droplets rise through water to form the oil layer, or settle out of the gas phase, under gravity or in a cyclonic device. These fundamental physical processes therefore determine both the maximum flow rate through the pressure vessel, and the dimensions of each separation device. Furthermore, it is evident that the flow conditions will show large spatial variations across the diameter and length of the vessel, because of the evolving separation processes and the changes in fluid composition. This complexity implies that the internals should (ideally) be considered in sequence, from the flow diverters and distribution devices at inlet to the liquid/liquid separation and gas cleaning systems downstream. Such detailed understanding of the flow behaviour is not easily achieved but both the client and the supplier, despite their different standpoints, must ultimately recognise the need for this understanding if satisfactory reliability and performance is to be achieved.

The reducing output from established centres of production, coupled with sharply rising demand from developing nations, has dramatically increased the value of all petrochemical reserves. Consequently, there is emphasis on increasing the recovery from maturing fields and on re-assessing fields that were once considered to be marginal. As one consequence of the growing economic and geo-political pressures on energy supplies, the plans to exploit deep water resources by means of sub-sea operations must, surely, present one of the most exciting challenges ever to face the industry. The intention is to transfer process control and separation devices to the sea bed, thereby dispensing with the need for a rigid or floating platform to provide support and access to the well-head. Among the many benefits claimed, sub-sea processing offers the opportunity to re-inject bulk water back into the well, thereby increasing the yield and delivery rate, delivers a significant reduction in the capital and operating costs, and provides much greater operational flexibility over the life of the field. However, taking the decision to locate the pressure vessel and separation systems, together with pumps and valves, and the associated instrumentation and control equipment, on the sea bed, places extra demands on the designer in terms of size, weight, reliability and performance. The assembly of this equipment at depths up to 3000m, coupled with greater demands for efficient and automatic operation, brings fresh technical challenges that will require a great deal of ingenuity for solution. However, the enormous potential of hitherto untapped sub-sea gas and oil reserves off the coast of West Australia and West Africa, together with the chance to enhance maturing or marginal fields in the North Sea and off the East Coast of South America, makes it certain that all related suppliers, including those specialising in separation equipment, will move towards even greater design sophistication over the next few years. This is the route chosen by Zeta-pdm.

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Dr John Turner is Technical Director, Zeta-pdm, Newport, Isle of Wight.

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