Physical Separation

Fact Sheet - Physical Separation

   
 
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This fact sheet describes several physical separation technologies that remove oil and grease and other organics from produced water. These technologies, which include advanced separators, hydrocyclones, filters, and centrifuges, are primarily deployed at offshore facilities where produced water is treated prior to ocean discharge

Oil and grease occurs in at least three forms:

  • Free oil: large droplets - readily removable by gravity separation methods;
  • Dispersed oil: small droplets - somewhat difficult to remove; and
  • Dissolved oil: hydrocarbons and other similar materials dissolved in the water stream - very challenging to eliminate.
   
  Schematic of skim pile
Schematic of skim pile; Source: U.S. Environmental Protection Agency.

The basics of oil and water separation are discussed in another fact sheet. At onshore sites, this generally involves some form of oil/water separator or free water knockout vessel (for separation of the free oil). In offshore settings, oil/water separators and skim piles are deployed to remove oil droplets greater than 100 microns in diameter. More physical separation steps are added to remove any remaining free oil and some dispersed oil. Additional treatment iterations may be required to achieve compliance with all applicable discharge limits. These treatments are discussed in other fact sheets.

 

Frankiewicz (2001) provides a helpful table for selecting treatment equipment based on the size of the particles that need to be removed. The information is presented below.

 

 

Particle Size Removal Capabilities

Technology Removal Capacity by Particle Size
(Units in Microns)
API gravity separator 150
Corrugated plate separator 40
Induced gas flotation without chemical addition 25
Induced gas flotation with chemical addition 3-5
Hydrocyclone 10-15
Mesh coalescer 5
Media filter 5
Centrifuge 2
Membrane filter 0.01
Source: Frankiewicz (2001)

Advanced Separators

Separators rely on the difference in specific gravity between oil droplets and produced water. The lighter oil rises at a rate dependent on the droplet diameter and the fluid viscosity (Stokes Law). Smaller diameter droplets rise more slowly. If insufficient retention time is provided, the water exits the separator before the small droplets have risen through the water to collect as a separate oil layer. The table shows that corrugated plate separators can remove more oil than a standard API gravity separator.

         
Schematic of corrugated plate separator
Schematic of corrugated plate separator; Source: Natco.
  Corrugated plate separators
Corrugated plate separators; Source: Natco.
  Inclined plate separator
Inclined plate separator; Source: J. Veil, Argonne National Laboratory

Likewise, inclined plate separators show better performance. Advanced separators contain additional internal structures that shorten the path followed by the oil droplets before they are collected. This gives smaller oil droplets the opportunity to reach a surface before the produced water overflows and exits the separator. The figures show a cross-section of a corrugated plate separator and a unit on an offshore platform.

   
  Schematic of hydrocyclone
Schematic of hydrocyclone; Source: Natco.
Multiple hydrocyclones inside a vessel
Multiple hydrocyclones inside a vessel; Source: Natco.

Hydrocyclones
Hydrocyclones have been used for surface treatment of produced water for several decades. By the mid-1990s, over 300 hydrocyclones were deployed at offshore platforms (Hashmi et al. 1994). Hydrocyclones, which do not contain any moving parts, apply centrifugal force to separate substances of different densities. Hydrocyclones can separate liquids from solids or liquids from other liquids. The liquid/liquid type of hydrocyclone is used for produced water treatment. Produced water is pumped tangentially into the conical portion of the hydrocyclone. Water, the heavier fluid, spins to the outside of the hydrocyclone and moves toward the lower outlet. The lighter oil remains in the center of the hydrocyclone before being carried toward the upper outlet.  

Filtration 
Tyrie (1998) reviews several types of media filtration devices used for offshore produced water treatment. He describes the designs, advantages, and disadvantages of upflow sand filters, walnut shell filters, downflow sand filters, and multimedia filters containing anthracite and garnet. Media filters operate until they reach a pre-determined level of solids loading, then they are taken offline and backwashed to remove the collected material.

   
Media filter
Media filter; 
Source: Natco.
 
Schematic of multi-media filter
Schematic of multi-media filter; Source: U.S. Environmental Protection Agency.

Membrane filters have also been used offshore. They are typically deployed as cartridges, which can be replaced when filled. Membrane filtration is discussed in a separate fact sheet.

Centrifuges 
Faucher and Sellman (1998) describe centrifuges that remove oil and solids. Like hydrocyclones, centrifuges separate oil from water by centrifugal force. However, centrifuges use a rapidly spinning bowl and generate much stronger forces than hydrocyclones. Hence, centrifuges are capable of removing oil droplets with smaller diameters. In produced water centrifuges, the spinning axis is vertically positioned. Centrifuges are often used to help achieve compliance with strict oil and grease discharge standards.

Another technology that employs a variation of the centrifuge process is the Voraxial separator. The Voraxial Separator is a continuous flow turbo machine that spins on a horizontal axis, produces a high centrifugal force, and generates a vortex to separate a mixture of fluids or a combination of fluids and solids by their different densities (DiBella and Samela, 2010).

References
DiBella, J.A., and D. Samela, 2010, “The Voraxial® Separator – A New Technology for Separation of Oil and Solids from Produced Water,” presented at the 20th Produced Water Seminar, Houston, TX, January 20-22, 2010.

Faucher, M., and E. Sellman, 1998, "Produced Water Deoiling Using Disc Stack Centrifuges," presented at the API Produced Water Management Technical Forum and Exhibition, Lafayette, LA, Nov. 17-18.

Frankiewicz, T., 2001, "Understanding the Fundamentals of Water Treatment, the Dirty Dozen - 12 Common Causes of Poor Water Quality," presented at the 11th Produced Water Seminar, Houston, TX, Jan. 17-19.

Hashmi, K.A., H.A. Hamza, and M.T. Thew, 1994, "Liquid-Liquid Hydrocyclone for Deoiling Produced Waters in Heavy Oil Recovery," in Proceedings of the International Petroleum Environmental Conference, Houston, TX, March 2-4.

Tyrie, C.C., 1998, "The Technology and Economics of the Various Filters That Are Used in Oil Field Produced Water Clean Up," presented at the API Produced Water Management Technical Forum and Exhibition, Lafayette, LA, Nov. 17-18.

Veil, J.A., M.G. Puder, D. Elcock, and R.J. Redweik, Jr., 2004, "A White Paper Describing Produced Water from Production of Crude Oil, Natural Gas, and Coal Bed Methane," prepared by Argonne National Laboratory for the U.S. Department of Energy, National Energy Technology Laboratory, January. Available at http://www.evs.anl.gov/pub/dsp_detail.cfm?PubID=1715 .

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