Analysis of Gravity Separation in Freewater Knockouts
- Maston L. Powers (Conoco Inc.)
- Document ID
- Society of Petroleum Engineers
- SPE Production Engineering
- Publication Date
- February 1990
- Document Type
- Journal Paper
- 52 - 58
- 1990. Society of Petroleum Engineers
- 4.1.3 Dehydration, 4.6 Natural Gas, 5.3.2 Multiphase Flow, 4.1.2 Separation and Treating, 5.6.5 Tracers, 4.1.5 Processing Equipment
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The selection of a freewater knockout depends on the desired performance. The design criterion developed here is based on the perception performance. The design criterion developed here is based on the perception that most FWKO's are installed to remove the bulk of the water from -a high-watercut flow stream so that the oil can be dehydrated economically to salable specifications and to discharge water requiring minimal treatment before disposal. Only the gravity separation process is considered; the contribution of coalescing devices is excluded. Vertical FWKO's are discussed briefly, but the bulk of this paper is devoted to horizontal FWKO'S, which constitute a majority of vessels. Hydraulic similarity of horizontal FWKO's and API separators is demonstrated, and a design criterion for FWKO's is developed with the basis used in the design development for API separators. This analysis discloses that commonly used criteria overrate the ideal flow capacity of FWKO's by a factor of 4/ . It shows that maximum FWKO capacity occurs when the oil/water interface is maintained at a level of 0.769 diameters in the case of ideal vessels and somewhat higher for nonideal vessels, rather than 0.50 as commonly believed. Published FWKO residence-time distributions (RTD's) indicate nonideal flow. Consequently, a design allowance for short-circuiting and turbulence effects was incorporated into the FWKO design. The effect of vessel proportions on cost per unit capacity is analyzed and an equation is developed for determining optimum proportions.
FWKO's (also known as pressurized skimmers) are commonly used on high-watercut flow streams for bulk separation of hydrocarbon phases from water. Liberated gas may be removed either with the oil or separately. An FWKO should incorporate means of dissipating the momentum of the incoming flow and confining the gas phase to the top of the vessel. These requirements are essential for providing a tranquil environment for gravity separation of oil and water. Many FWKO's incorporate coalescing media, which enhances separation; however, only gravity separation is ad- dressed in this paper. FWKO designs include both horizontal and vertical orientation, the latter being a small percentage of units in service.
Stokes' law best describes the gravity-separation velocity for droplet sizes of concern in oil/water mixtures. This law, expressed in oilfield terms, is
vt = 178.74( w - o)do2/uw..................(1)
Values of , ranging from 0.015 to 0.020 cm are commonly used in sizing FWKO'S.
Vertical FWKO'S. Because it is demonstrated here that vertical FWKO's are not a competitive alternative, these units are given minimal coverage. A vertical FWKO should be constructed so that the flow stream enters near the top and passes through a gas/liquid separating chamber. Inside the open vessel, separation of the degassed liquid mixture begins, with the oil- continuous phase rising to the top and the water-continuous phase settling to the bottom . Removal of water from the vessel is regulated by a dump valve that normally is controlled by an interface float. The oil/gas outlet is usually restricted only by superimposed backpressure. As with all vessels, outlets should be equipped with vortex breakers. Entrained oil droplets must rise countercurrent to the water-displacement velocity, which, assuming plug flow, would equal the flow rate divided by the vessel cross-sectional area. Equating this velocity to the rising velocity of an oil droplet as expressed in Eq. 1 yields the commonly used design-capacity equation:
qVC = 2.1603 X 10-6( w - o)do2di2/uw,..........(2)
which demonstrates that diameter is the only vessel parameter contributing to capacity. This equation, as modified later to compensate for nonideal flow, is appropriate for sizing.
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