Summary Induced gas flotation (IGF) technology is well-known in the oil industry for effectively treating produced water to meet environmental oil-in-water discharge requirements. This paper presents technical considerations for development of a new generation of IGF units, a discussion of the process kinetics, actual operating data, and performance analysis of a full-sized, four-cell WEMCO* cylindrical depurator. A pilot test program was developed and executed in collaboration with ChevronTexaco Overseas Petroleum on platform-gathering- station Mike, located offshore in the Cabinda Province, Angola. Pilot data indicate that the flotation kinetic rate for oil removal from a four-cell cylindrical depurator is comparable with a conventional depurator. Cylindrical depurator design captures the main technical advantage of conventional, mechanical IGF units for effective oil-particle flotation and offers the promise of fewer maintenance issues as well as viable alternatives for higher-pressure applications. Field data validate theoretical predictions for the performance of the cylindrical depurator. Introduction Industrial application of the flotation process can be traced to the 1930s. Flotation machines were originally developed as devices for removing suspended solids in the mining and mineral industries. By the 1960s, economic considerations triggered modifications to flotation methods that resulted in the development of large, multicell units with capacities of up to 3,000 ft3 per cell.1 Success of flotation machines in the mining and mineral industries paved the way for using this equipment in oilfield water-treatment applications. Very little has changed in the equipment's design since its first application in the oil industry. IGF technology uses a motor-driven, vaned rotor or impeller for inducing and mechanically mixing gas with produced water. Gas is drawn from the vapor space in the vessel and mixes with the produced water, resulting in small bubbles that rise to the surface, as shown in Fig. 1. Negatively charged oil droplets will adhere to the bubbles, which are positively charged and quickly rise to the surface, forming an oil froth layer that is mechanically or hydraulically removable through the oil trough. Cationic polymer is typically added upstream of the IGF unit to enhance oil-removal efficiency. Produced water flows through each of the four cells in series, resulting in overall oil-in-water (OIW) removal efficiency of approximately 95%. The WEMCO cylindrical depurator is specially designed to incorporate many new features that enhance equipment performance and result in lower capital and operating and maintenance costs. Additional features broaden IGF applications where high operating pressure and off-gas containment are specified. Mechanical IGF Design Methodology Leading mechanical IGF manufacturers employ a systematic scaleup methodology for the development of larger units2 from smaller ones. As a general rule, scaleup can be performed through one or a combination of geometric, metallurgic, and hydrodynamic scaleup criteria. In a geometrical scaleup, as the name implies, a new model is designed to scale from a smaller test unit on the basis of physical dimensions only. Metallurgic scaleup is designed around flotation kinetics and contaminant recovery, whereas hydrodynamic scaleup seeks to identify and maintain certain relationships between the gas and liquid interaction within the cell. Hydrodynamic parameters of a flotation cell have been studied in detail and published by independent sources.3 The design of a cylindrical depurator takes into account the effects of major parameters on the process hydrodynamics, including rotor diameter, engagement, speed, and submergence as well as cell residence time, skimming rate, bubble rise velocity, gas ingestion rate, etc.
This paper is intended to be part of the continuous development and adaptation of mechanical-cylindrical induced-gas-flotation (IGF) technology that meets the demands of today's offshore production practices as the best-available technology (BAT). The focus of this paper relates theory to practice for this new, yet proven, technology, and presents the real-life example of one of its applications. This paper presents the problem-solving and selection process adopted by Chevron Corp.'s North America Operations Team at the Typhoon production facility, located in the deepwater Gulf of Mexico. The paper compares alternative solutions, and the economic advantages and disadvantages of each. It also provides an in-depth analysis of the performance achieved after installation of the dual cell on the basis of the theoretical model developed for this equipment.
This paper is intended as part of the continuous development of WEMCO® Dual-Cell Cylindrical Depurator® and adaptation of Induced Gas Floatation technology to meet the demands of today's offshore production practices as the best-available-technology (BAT). The focus of this paper is relating theory to practice for this new and proven technology and the real life example of one of its applications. This paper is presenting the problem solving and selection process adopted by ChevronTexaco North America's Operations Team at Typhoon Production Facility, located in deep water Gulf of Mexico. The paper compares alternative solutions and economical advantages and disadvantages of each alternative. It also provides an in-depth analysis of the performance achieved after installation of the Dual-Cell, based on the theoretical model developed for this equipment. Introduction Produced water production is by far the most challenging problem for the mature oil fields of our time. The actual production numbers are unknown, but various literatures have reported an estimated 210 million bwpd on a global scale. American Petroleum Institude reports 18 billion barrels of water were produced in 1995 from onshore operations in addition to large amounts from offshore in the United States alone. A study for Department of Energy estimates from 1985 to 2002 in the United Sates, there has been a steady increase in water production from 7.4 to 9.5 barrels of water for each barrel of oil produced. Regardless of the locality of these estimates, similar trends have been reported elsewhere on the global scale. These facts are inline with increased de-bottlenecking activities for major operators at oil production facilities worldwide. In addition, increased environmental awareness, discharge limits set by governmental regulatory organizations or self-imposed regulations are other factors in the produced water treatment problem. The shear production volume and the ever tightening regulations are imposing an economical burden on all producers at large. It is this demand in recent years, which has revived a new interest in water treatment technologies, and acted as the catalyst to the efforts for improving these technologies. Review of the Technology Induced gas floatation (IGF) continues to be identified as the best-available-technolgy by both the operators and environmental organizations. In the United States, Environmental Protection Agency (EPA) continues to recognize the "improved gas floatation" as the BAT for removal of oil and grease (O&G) from the produced water. The law sets the mandatory O&G discharge limits at 29 mg/L monthly average and 42 mg/L daily maximum for the Territorial Seas and Outer Continental Shelf. Among the ongoing attempts to improve and adapt the floatation technology to the new demands of the petroleum industry, first generation of Cylindrical Depurator was introduced. This design benefited from mechanical floatation mechanism and in its conventional form utilized four consecutive gasification chambers or cells. A modified compact version of the cylindrical design specialized for offshore and motion sensitive applications, is offered as Dual-Cell Cylindrical Depurator. This model is designed based on similar principles to its predecessor but comprised of two gasification chambers, each with dedicated flotation mechanism. This design benefits form smaller rotor assembly and employs draft tubes to enhance internal dynamic characteristics of each cell (figure 1).
TX 75083-3836 U.S.A., fax 01-972-952-9435. AbstractInduced Gas Flotation (IGF) technology is well known in the oil industry for its effective treatment of produced water to meet environmental oil-in-water discharge requirements. This paper presents technical considerations for development of a new generation of IGF units, a discussion of the process kinetics, and actual operating data and performance analysis of a full size four-cell WEMCO® cylindrical depurator.
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