The Case for Compact Separation

Abstract

Technology Today Series articles are general, descriptive representations that summarize the state of the art in an area of technology by describing recent developments for readers who are not specialists in the topics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and present specific details only to illustrate the technology. Purpose: to inform the general readership of recent advances in various areas of petroleum engineering. Introduction The mission of a production facility is to separate produced well fluids into individual phase components (oil, gas, water, and sand) and process these phases into marketable products or dispose of them in an environmentally acceptable manner.1 Gravity-based vessels (two- or three-phase, horizontal or vertical) perform most of the separation duty; however, the demands of deepwater production and remote environments command a new portfolio of technology. The term compact separation is commonly applied to separation-unit processes that do not rely on (large) gravity-settling vessels. In the evolving world of facilities technology, compact separation promises to revolutionize topside design by saving space and weight while improving process performance. Compact separation is both necessary and unavoidable because it provides benefits to separation design beyond minimal facilities. However, evaluation of any compact-separation technology should fully review all aspects of performance, not just size. Separating Forces Compact separation applies to partitioning of distinct phases - specifically oil, water, gas, and sand. All four phases are separable by gravity settling; however, "enhanced" physical forces reduce equipment size. Specie separation within a phase (e.g., hydrocarbon fractionation, water deoxygenation, or desalination) is not covered in this review, because it requires more than mechanical forces to be accomplished. Gravity. Horizontal or vertical vessels relying on gravitational force provide most of the separation duty in oil and gas production. Efficient separation can be easily achieved with minimal complexity. However, the large size and weight of these vessels, especially at high pressure, is the main driver for development of compact-separation technology. Additionally, floating production performance deteriorates because of wave motion. Normally, these vessels are the largest and heaviest pieces of production equipment in the process facility. When relying only on gravitational forces for separation, residence time and low velocity are the keys to efficient separation. Impact/Interception. One of the first developments in compact separation was the addition of impact or interceptor internal devices to reduce gravity vessel size. Including devices such as momentum breakers to aid in gas/liquid separation, perforated plates for motion suppression, matrix packing to aid oil/water separation, and vane packs and mist pads to aid in droplet removal from gas greatly increase the separation efficiency compared with bare gravity vessels and enable significant size/weight reduction in many cases. Momentum breakers provide a partial defense against motion sensitivity, but they are susceptible to plugging or channeling. Cyclonic. The current trend in compact separation is cyclonic technology. Harnessing vortex flow provides an effective separation force of 10 to 5,000 times gravitational acceleration (g force), significantly decreasing separation equipment size. Static cyclones use pressure drop to produce spinning flow patterns, which in turn applies the high g forces to separate phases of different densities. Common equipment in this category includes cyclonic inlet devices for separators, axial flow and multicyclones for gas scrubbing, and deoilers and desanders for produced-water treatment. Cyclones provide the highest throughput-to-size ratio of any separation equipment, coupled with the least complexity. They are a net consumer of pressure drop, and the high velocities produced may be a source of wear. Rotordynamic. Rotordynamic forces are developing into the next generation of compact-separation technology. Applied energy to a rotating element promises to increase separation efficiency over that of cyclones because the static wall (boundary layer) effects are eliminated. Moving to rotordynamic equipment is a major step because the complexity of equipment increases significantly; however, in many cases, the benefits are worthwhile. Equipment in this category includes produced-water centrifuges, solid-bowl centrifuges, dynamic cyclones, self- and external-powered separator turbines, and the in-line rotary scrubber.