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A nut shell filter is a device to remove oil from water. In the oil & gas industry, the term walnut shell filter is common since black walnuts are most often used. Typically nut shell filters are designed for loadings under 100 mg/L oil and 100 mg/L suspended solids and operate with 90–95% removal efficiency.Howdeshell, Mike, Chris Catalanotto, and Eric Lorge. "Walnut Shell Fitler Reuse Potential." 2011 Saudi Arabian Water Environment Association (SAWEA). http://www.sawea.org/pdf/Mike_Howdeshell.pdf High oil and solids loadings reduce run times between backwashes and results in reduced effluent quality.
Periodical backwashes are initiated to regenerate the media. Typically, backwash is triggered by one of the following:
* Differential pressure
* Timer (often 24 hours)
* Operator initiated (often due to exceeding limit for effluent quality)
Backwash occurs through mechanical agitation such as
* Backwash through draft tube
* Backwash through external media scrubber
* Mechanical mixer
If backwash is not sufficient, oil can cause media to agglomerate, known as mudballing.
Typical flux of nut shell filters is 7 to 27 gpm/ft2.Catalanotto, Chris. "Walnut Shell Filter Raises Flux for On-shore and Off-shore Produced Water Treatment." Industrial Water Treatment. January/February 2012. Commercial vessels are sized to accommodate the flow rate of water and range up to 14 feet in diameter. For continuous operation, multiple vessels are frequently used so flow can continue to be treated while backwash occurs in one vessel. For large flows, several vessels may be used. Unlike some oil / water separators, no chemicals are required for oil removal in nut shell filters.
Design
A bed of nut shell media is contained in a vessel. Vessels are typically vertical, but may also be horizontal. Particles are captured as flow penetrates through the media bed. Although it is possible to use other medias for this purpose,walnut
A walnut is the edible seed of a drupe of any tree of the genus ''Juglans'' (family Juglandaceae), particularly the Persian or English walnut, '' Juglans regia''.
Although culinarily considered a "nut" and used as such, it is not a true ...
and pecan
The pecan (''Carya illinoinensis'') is a species of hickory native to the southern United States and northern Mexico in the region of the Mississippi River. The tree is cultivated for its seed in the southern United States, primarily in Georgia, ...
shells are most commonly used since they have several desirable properties making them well suited for oil
An oil is any nonpolar chemical substance that is composed primarily of hydrocarbons and is hydrophobic (does not mix with water) & lipophilic (mixes with other oils). Oils are usually flammable and surface active. Most oils are unsaturated ...
removal. First, nut shells are hard with a high modulus of elasticity
An elastic modulus (also known as modulus of elasticity) is the unit of measurement of an object's or substance's resistance to being deformed elastically (i.e., non-permanently) when a stress is applied to it. The elastic modulus of an object is ...
, resulting in a low attrition rate and minimal media replacement, typically <5% per year. Nut shells also have an equal affinity for water and oil, allowing oil to be adsorbed during normal operation, but also enable oil removed from the bed during agitation allowing for media reuse.Felch, Chad, et al. "Water treatment apparatus incorporating walnut shell filter media and a draft tube system." US Patent 9,580,335. During normal operation, water typically flows down through the media bed where oil is coalesced and attracted to the nut shells and accumulates in the interstitial spaces between the media.Kashaev, Slava and Douglas Lee. "Deep Bed Nutshell Filter Evolution." 20th Annual Produced Water Seminar. 2010. Typical nut shell media is 12/20 (0.8 to 1.7 mm) and 12/16 mesh
A mesh is a barrier made of connected strands of metal, fiber, or other flexible or ductile materials. A mesh is similar to a web or a net in that it has many attached or woven strands.
Types
* A plastic mesh may be extruded, oriented, ex ...
(1.2 to 1.7 mm). Although not designed for solids removal, an added benefit is that solids accumulate in the bed. As solids are collected, the differential pressure across the bed increases.
Periodical backwashes are initiated to regenerate the media. Typically, backwash is triggered by one of the following:
* Differential pressure
* Timer (often 24 hours)
* Operator initiated (often due to exceeding limit for effluent quality)
Backwash occurs through mechanical agitation such as
* Backwash through draft tube
* Backwash through external media scrubber
* Mechanical mixer
If backwash is not sufficient, oil can cause media to agglomerate, known as mudballing.
Typical flux of nut shell filters is 7 to 27 gpm/ft2.Catalanotto, Chris. "Walnut Shell Filter Raises Flux for On-shore and Off-shore Produced Water Treatment." Industrial Water Treatment. January/February 2012. Commercial vessels are sized to accommodate the flow rate of water and range up to 14 feet in diameter. For continuous operation, multiple vessels are frequently used so flow can continue to be treated while backwash occurs in one vessel. For large flows, several vessels may be used. Unlike some oil / water separators, no chemicals are required for oil removal in nut shell filters.
Uses
Nut shell filters were designed to separatecrude oil
Petroleum, also known as crude oil, or simply oil, is a naturally occurring yellowish-black liquid mixture of mainly hydrocarbons, and is found in geological formations. The name ''petroleum'' covers both naturally occurring unprocessed crude ...
from oilfield produced water
Produced water is a term used in the oil industry or geothermal industry to describe water that is produced as a byproduct during the extraction of Petroleum, oil and natural gas, or used as a medium for heat extraction. Produced water is the kind ...
in the 1970s, which remains the principal use. Nut shell filters can be used onshore and offshore, but are more common onshore where the treatment requirements are typically more stringent and footprint is not limited. Nut shell filters are used for tertiary treatment following primary and secondary treatment which removes the bulk of the oil and suspended solids. Typically, effluent is reinjected for reuse or disposal or discharged to a surface body of water.
Categories
*Media filter
A media filter is a type of filter that uses a bed of sand, peat, shredded tires, foam, crushed glass, geo-textile fabric, anthracite, crushed granite or other material to filter water for drinking, swimming pools, aquaculture, irrigation, storm ...
References
{{reflist Filters Water filters