Walnut Shell Filter Explanation

This page depicts the micro-level mechanism by which black walnut shell filters capture and coalesce oil droplets as part of the de-oiling process in oil production facilities.

There are three parts to this discussion.

Surface 1 is hypothetical: a completely flat oleophilic (oil-loving) surface.

Surface 2 is also hypothetical: a completely flat oleophobic (oil-hating) surface.

Surface 3 is the actual mechanism for how walnut shell filters work. It features a rough surface with both oleophilic and oleophobic regions, and incorporates aspects of both Surface 1 & Surface 2.


Update

Mar 23, 2021: An animated version of this page was originally created in Adobe Flash in November of 2013. This page covers most of the main points of the animation (using static images published from Flash).

Since Flash was phased out at the end of 2020, the original SWF file can no longer be accessed. The following link is kept for archival purposes only.

Click for the Walnut Shell Filter Flash Animation

  • Walnut Shell Filters. Original Flash animation by Joseph Pecsenyicki. Special thanks to Ian Mackay and Kari McDonald

Model #1:
Flat Oleophilic Surface

I

The influent flows through the filter from top to bottom. Best capture rates occur when oil droplet diameter is equal to or greater than 2 microns.

  • Flat oleophilic surface (brown) with arrow (blue) showing influent flow moves from top to bottom. Oil droplets should be at least 2 micrometers in diameter.

II

Flow through the filter has begun.

The green area depicts the area where forces of oleophilic attraction occur.

  • Oleophilic forces (fuzzy radiating green area) on the surface of the oleophilic media will attract oil droplets.

III

An oil droplet falls & is captured by oleophilic forces on the surface.

  • Oil droplet falls towards oleophilic surface.
  • Oil droplet sticks to the oleophilic surface.

IV

A second oil droplet falls & is also captured on the surface.

  • Another oil droplet falls towards the oleophilic surface.
  • Another oil droplet sticks to the oleophilic surface.

V

A third oil droplet falls to the surface. But this time, it coalesces with one of the previously captured droplets.

  • Third oil droplet falls towards the oleophilic surface.
  • The third oil droplet begins to coalesce with the first captured droplet.
  • The third oil droplet completely coalesced with the first droplet on the oleophilic surface.

VI

The oleophilic forces help break the surface tension of the large, coalesced oil droplet. The oil begins to cover the filter media surface.

  • Coalesced oil droplet breaks and begins to cover the oleophilic surface.

VII

The spreading oil makes contact with another captured droplet, and breaks the surface tension on that droplet as well.

The oil proceeds to wet the entire surface of the oleophilic media.

  • Spreading oil makes contact with a droplet stuck on the oleophilic surface.
  • Coalesced oil droplet breaks on contact with spreading oil. All this oil starts to cover the oleophilic surface.

VIII

The entire surface is completely wetted with oil.

  • The entire oleophilic surface is completely covered with a layer of oil.

IX

Polyvinyl chloride (PVC) is a good example of this kind of media. It captures oil very well, but gets wetted with oil very easily.

This fouls the media, requiring ever-higher pressures to run influent through the filter.

(And because of its strong affinity for oil, this media cannot be cleaned without using costly chemicals).


Model #2:
Flat Oleophobic Surface

I

In this model, the flat oleophilic media has been wetted with water for a half-hour.

It is therefore covered with a film of oil-repelling water.

  • The entire oleophilic surface is completely coated with a film of oleophobic water.

II

Although the media itself is oleophilic, the film of water atop it is oleophobic. Therefore, the latter creates a region of oleophobic repulsion.

  • Region of oleophobic repulsion (fuzzy radiating red area) will repel oil on the media surface.

III

This type of media never attracts oil droplets, and is of no use whatsoever as an oil filter.

  • Oil droplet is repelled from flat oleophobic surface.
  • Second oil droplet is repelled from flat oleophobic surface.

Model #3:
Rough Surface
(Oleophilic & Oleophobic)

I

Black walnut shells contain a small amount of natural oils, and are slightly oleophilic.

  • Oleophilic surface of a black walnut shell.

II

Black walnut shells are also rough: they have sharp points (asperities) as well as crevices & pores.

  • Rough surface of a black walnut shell.

III

The filter is wetted with water for a half hour. This covers the flat surfaces with a layer of oil-repelling water (as in Model #2).

However, the asperities and crevices can still attract oil, since the water layer doesn't completely cover them (as per Model #1).

  • Water coating the flat surfaces of a black walnut shell. The water does not cover the asperities or pores.

Apart from any intermolecular forces, oil droplets can also get physically trapped in the crevices (just as dust gets trapped in nooks and crannies in one's home).

IV

Oil droplets that impact the oleophobic water film are not captured (as in Model #2, Part III).

  • Oil droplet is repelled from the flat water-covered surface of the black walnut shell surface.
  • Second oil droplet is repelled from the flat oleophobic surface of the black walnut shell granule.

V

However, the asperities and pores of the walnut shell DO capture oil droplets (as per Model #1, Part III).

  • Oil droplet stuck in the pore of the black walnut shell filter.
  • Another oil droplet stuck on the asperity of the black walnut shell filter.

VI

Other droplets coalesce with the captured droplets.

  • Another oil droplet falls near one of the captured droplets.
  • The oil droplet coalesces with the droplet which was previously captured in the pore of the black walnut shell filter.

VII

When the coalesced droplets within the pores reach a certain size, the surface tension breaks.

This wets the pores of the black walnut shell filter completely with oil (in a manner somewhat similar to Model #1, Part VIII).

  • Oil droplets completely coalesced in the pore of the black walnut shell filter.
  • Pore of the black walnut shell filter completely covered in oil.

VIII

The captured oil on the asperities and pores is strongly oleophilic, and serves as sites of capture for other droplets in the influent.

  • Oil droplet coalesces with oil captured in the pore of the black walnut shell filter.
  • Another oil droplet coalesces with oil captured in the pore of the black walnut shell filter.
  • Oil droplet coalesces with oil captured on the asperity of the black walnut shell filter.
  • Oil droplet coalesces with oil captured on the asperity of the black walnut shell filter.

IX

The black walnut shell filter is now completely saturated with oil.

  • Black walnut shell filter with two very large oil droplets captured. The filter is completely saturated with oil.

X

At this stage, the forces in the system are in equilibrium:

On the one hand, the oleophilic forces (green arrows) draw the captured oil droplets together.

On the other hand, the oleophobic forces from the water film (red arrows) serve to repel the oil just enough to keep it in droplet form. (Similar to rain drops beading on the surface of a newly-waxed automobile).

  • Oleophilic and oleophobic forces in equilibrium on oil-saturated black walnut shell filter.

XI

However, if the system continues to operate, the filter will become fouled with oil.

This occurs because the captured oil droplets become too large and coalesce with each other.

The oil would then coat the surface irreversibly, as in Model #1, Part VIII.

  • Strong oleophilic forces between captured oil droplets on a black walnut shell filter over-saturated with oil.
  • Black walnut shell filter fouled with oil (oil completely covers surface). The 3 layers from bottom to top are: 1) black walnut shell surface, 2) water layer, and 3) oil layer.

XII

To prevent the filter from becoming fouled, it is periodically BACKWASHED.

This entails reversing the direction of the flow at high pressure, using relatively "clean" water as the influent.

  • Influent direction reversed as the backwash procedure is initiated in the saturated black walnut shell filter. The large captured oil droplets on the surface are NOT in contact with each other.

XIII

During the backwash, the high-pressure influent loosens & removes captured oil droplets from the black walnut shell filter.

  • Large coalesced droplet captured in the pore of the black walnut shell granule stretches and loosens under high-pressure reverse flow.
  • Large coalesced droplet captured in the pore of the black walnut shell granule breaks off and is removed under high-pressure reverse flow.
  • Mid-sized coalesced droplet captured on the asperity of the black walnut shell granule stretches and loosens under high-pressure reverse flow.
  • Mid-sized coalesced droplet captured on the asperity of the black walnut shell granule breaks off and is removed under high-pressure reverse flow.

The large droplets are routed to a holding pond or slop tank where they coalesce further with each other.

XIV

The backwash has served its purpose, with the black walnut shells having been cleansed for the most part of captured oil droplets.

  • Black walnut shell filter, with a single droplet attached to an asperity and a small amount of oil completely covering the pore. Flow is still from bottom to top.

XV

Normal operation resumes, with the influent flowing once again from the top of the filter to the bottom.

The filter can now be re-used to capture more small oil droplets (Part VIII).

  • Black walnut shell filter, with a single droplet attached to an asperity and a small amount of oil completely covering the pore. Flow is now from bottom to top.

Credits

Static images from: Walnut Shell Filter Flash Animation

This animation was originally part of a more macro-level explanation given in a Chem 260 PowerPoint presentation.

The PowerPoint presentation itself was based in part upon a walnut shell filter presentation by Mike Howdeshell, Chris Catalanotto and Eric Lorgeused.

Header font: Veteran Typewriter by Magique Fonts

Paragraph font: My Underwood by Tension Type

Special thanks again to Ian Mackay and Kari McDonald.


Walnut Shell Filter Explanation (static non-animated version of this page)
Background black walnut shell granules image from: AliBaba.com