Novel Coalescer Technology in First-Stage Separator Enables Single-Stage Separation and Heavy-Oil Separation
- Tor A. Fjeldly (Vetco Aibel AS) | Erik B. Hansen (ABB Corporate Research Center) | Paal J. Nilsen (ABB Corporate Research Center)
- Document ID
- Society of Petroleum Engineers
- SPE Projects, Facilities & Construction
- Publication Date
- June 2008
- Document Type
- Journal Paper
- 1 - 5
- 2008. Society of Petroleum Engineers
- 5.3.4 Integration of geomechanics in models, 6.3.6 Chemical Storage and Use, 5.2.1 Phase Behavior and PVT Measurements, 4.1.5 Processing Equipment, 4.3.1 Hydrates, 4.5 Offshore Facilities and Subsea Systems, 4.1.2 Separation and Treating, 6.1.5 Human Resources, Competence and Training, 2.2.2 Perforating
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Aibel (former ABB Offshore Systems) has for a long while carried out fundamental research on compact separation. A significant part of the development has been to incorporate robust electrostatic coalescers into three phase separators. The current technology enables single-stage separation and accelerates separation of oil and water. The most recent product developed to enable single-stage separation is the Low Water Content Coalescer (LOWACC). The LOWACC is an electrostatic coalescer element, enhancing and accelerating the separation in an oil/water/gas separator. It is designed to operate as a polishing element for removal of the remaining droplets of water and is designed to be located downstream the Vessel Internal Electrostatic Coalescer (VIEC), which is a bulk separation device. A prototype polishing element, together with the bulk separation device, has been tested on several different real crude oils ranging from API 17 to API 29. For the API 29 crude, the water in oil (WiO) content was successfully reduced below 0.5% without the use of chemicals. For the challenging API 17 oil, the oil in water (OiW) was reduced to below 2-5%. All the test results verified good separation even with tough separation conditions that created stable emulsions and small droplets. The produced water leaving the separator contained 2 to 200 ppm oil in water.
Separation of water from crude oil is of great importance for the petroleum industry. The traditional oil, water, and gas separation train consists of a first-stage gravity separator, a second-stage gravity separator, and an electrostatic coalescer unit with bare electrodes. The free gas and most of the water is removed in first and second stages so that a low-water-content oil phase is passed on to the coalescer. A careful tuning of the water content, retention time, and temperature, together with de-emulsifying chemicals, is necessary to obtain a satisfying separation process. For heavy-oil emulsions, the process becomes very difficult and often unstable.
Electrocoalescence is a mechanism for enhancing oil/water separation that has been known since the beginning of of the 20th century, when it was patented by Cottrell (Eow 2001). Many of the commercially available electrostatic coalescers are made with bare metal electrodes. When the WiO level is too high, it becomes difficult to establish the electrical field in the fluid because of short-circuiting or electrical arcing. This is why the traditional electrostatic coalescer is located after the first- and second-stage gravity separator. A coalescer with insulated electrodes will tolerate higher water contents and can therefore be placed earlier in the process. A more thorough discussion of the different configurations has been presented by Noïk et al. (2006).
By introducing an electrostatic coalescer with insulated electrodes already in the first-stage separator, a much more efficient separation process can be achieved (Wolff and Knutsen 2004; Kvilesjø et al. 2005; Mosland et al. 2005). This is further demonstrated through the combined use of the VIEC and LOWACC. The LOWACC is an electrostatic coalescer element, enhancing and accelerating the separation in an oil/water/gas separator. It is designed to operate as a polishing element for removal of the remaining droplets of water and is located downstream the VIEC, which is a bulk separation device. VIEC and LOWACC both have insulated electrodes that allow the coalescers to operate in all kinds of fluids without short-circuiting. One additional feature with the coalescer elements is that they have built-in transformers that will produce a high-voltage electric field across the electrodes, but only require low-voltage penetrators on the power supply side.
A single-stage separator is a separator that separates the water from the crude oil so efficiently that there is litte need for a separate electrostatic coalescer, or even a second-stage separator. This paper describes how insulated electrode coalescers can enable single-stage separation and related experimental results. The experimental data demonstrates that this technology greatly enhances oil/water separation and improves the produced water quality.
Possible applications of this technology are for oil producers on fields struggling with separation problems, and for solving problems related to hydrate formation in subsea separation processes. By implementing the single-stage separation, the footprint and weight of separation equipment topside will be significantly reduced. In addition, the operational expences could be lowered as the requirement for emulsion breaker chemical is also significantly reduced. The technology can be very useful for subsea applications as it enables long tiebacks because of reduced water content in oil.
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