Pilot Testing of a New Generation of Induced Gas Flotation Equipment

Abstract

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.

The Effect of Temperature and Impeller Speed on Mechanically Induced Gas Flotation (IGF) Performance in Separation of Oil from Oilfield-Produced Water

“…The oil appears lighter because the density difference between the oil agglomerate and water is increased; consequently, the oil rises faster, enabling a more rapid and effective separation from the aqueous phase. The oil droplets and oil-coated solids rise to the surface where they are trapped in the resulting foam/scum and removed from the flotation chamber when the foam/scum is skimmed off (Strickland, 1980;Kitchener, 1985;Arnold and Stewart, 1998;Moosai and Dawe, 2002;Movafaghian et al, 2004). The internals of flotation cells such as nozzles, rotors, baffles, and foam/scum skimmers for these units are patented designs and still are not well understood (Chabot et al, 1998;Arnold and Stewart, 2008).…”

“…Therefore, it is believed that the performance of Mastour flotation cell in different conditions is not limited to its type. Hydraulic retention time (HRT) for the IGF pilot was set at 1 min as suggested by Movafaghian et al (2004) and Arnold and Stewart (2008) and it was controlled and calibrated by the emulsion preparation tank outlet valve.…”

Introduction of a Dual-Cell Depurator for FPSO Application-Theory to Practice

“…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 3 . This design benefited from mechanical floatation mechanism and in its conventional form utilized four consecutive gasification chambers or cells.…”

Improved Mechanical IGF Technology for Floating Production Storage and Offloading Application-First Installation Theory and Practice

“…Among the ongoing attempts to improve the flotation technology and adapt it to the new demands of the petroleum industry was introduction of the first generation of the cylindrical Depurator* (Movafaghian et al 2004). This design benefited from a mechanical flotation mechanism and, in its conventional form, used four consecutive gasification chambers or cells.…”

“…The kinetic approach to the complexflotation phenomenon considerably reduces the number of variables to a system's kinetic-rate constant. For a given set of design parameters, the kinetic-rate constant will be a unique variable, capturing the macroeffect for the sum total of the system's microscale physical and electrochemical characteristics-proof and application of which have been thoroughly discussed in other publications (Movafaghian et al 2004). …”