Plantoil/diesel conversion basics
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danalinscott

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Prefiltering

There are three main types of equipment used for filtering the unwanted particulates from wvo.

1. The "bag filter"  or "sock filter".
This is descriptive of its' appearance. Essentially they are a tube of fabric filter media sewn closed on one end and open on the other. Often a  ring of poly tube  is sewn into the open end to act as a seal when they are installed in a filter support housing.

                  
Bag filter element           Bag filter support/housing    

Filtering is accomplished by simply warming/thinning the wvo sufficiently to allow it to pass through the filter without exceeding the elements design limits  and pumping it through the filter. Since bag filter housings are fairly expesnsive some of earliest DIY designs for prefilters simply used bag filters suspended in a 55 gallon drum by the sealing ring sewn into the elements top opening.   

Nore: String wound and blown poly filters
Several individuals have reported some success with using modified whole house water filters for prefiltering wvo. Care must be taken to prevent overpressure and provide secure end mountings for the filter elements or unfiltered WVO tends to leak around the ends resulting in contamination.
Also because of the limited surface area the amount of wvo that can be filtered before the element clogs and must be replaced is usually 1/10th to 1/15th compared to a bag filter.

2. Motor driven centrifuges                                   
These devices use centrifugal force generated by a rapidly spinning  bowl to drive particulates onto the inner surface of the bowl where they are held while the liquid they were suspended in is allowed to pass out of the bowl. These are very effective since they allow removeal of particulates AND suspended water in a single pass but expensive when compared to most other methods of prefiltering.

 
Cutaway of a typical motor driven centrifuge.

3. Fluid driven centrifugal filters.
Originally designed for continuous cleaning of crankcase oil in large engines these have been adapted to WVO particulate filtration. WVO is first heated to thin it and then pressurized to 90+psi and then fed to the filter at the design volume and pressure. The pressurized wvo is forced through two small ports in the bottom of the rotor assembly causing it to spin at high speed and forcing some of the particulates in the wvo to stick to the inner surface of the rotor. Because flowthrough cannot be regulated as it can in a motor driven centrifuge wvo must be passed though it many times until it has reached the desired filtration level.


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danalinscott

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Dewatering

Consistently dewatering wvo is much more difficult than pre-filtering.
Generally speaking wvo that has passed through a 10 micron filter can be considered to be filtered to 10 microns. However...wvo must be tested to determine if it has been dewatered to 300ppm (total water content TWC) and can be considered adequately dewatered to safely use as fuel in a diesel engine.
Although several tests exist only one (the "Sandy Brea") is consistently accurate. Both the "Hot Pan Test" and its derivatives are ONLY accurate for wvo dewatered by Heat and Settle and Motor Driven Centrifuge equipment since they rely on the microdroplets of water in the wvo sample converting to bubbles of steam. If these are used to test for water content in wvo dewatered via evaporative methods the results will not reflect the actual TWC of the wvo being tested.

This is because all Evaporative Dewatering Methods(EDM) tend to concentrate the water soluble acids, salts, sugars, etc present in the microdoplets of water suspended in wvo. As they become more concentrated they alter the point at which the remaining microdroplets will turn to bubbles of steam until the remainig microdroplets of water will not turn transform to steam at temperatures below where the wvo itself begins to form microbubbles of smoke.

1. Heat And Settle
One of the most common method of dewatering wvo is referred to as Heat and Settle (HS). This method consists of prefiltering wvo to roughly 10 microns and then heating it and allowing it to sit for 24-48 hours while suspended water falls out of suspension and concentrates in the lower levels of the wvo in teh settling vessel. Any movement or convective currents must be avoided since these will tend to remix microdroplets of water in the lower levels of the wvo column which have not yet coalesced into large harder to move droplets. HS is NOT an EDM.

2. Motor driven centrifuges   
These devices use centrifugal force generated by a rapidly spinning  bowl to drive microdroplets of suspended water onto the inner surface of the bowl where they coalesce and are held while the liquid they were suspended in is allowed to pass out of the bowl. These are very effective since they allow removal of particulates AND suspended water in a single pass but eare xpensive when compared to most other methods of dewatering. Motor Driven Dewatering with a motor driven centrifuge is NOT an EDM.



Cutaway of a typical motor driven centrifuge.

3. Vacuum Dewatering
The lower the atmopheric pressure the lower the temperature at which water turns to steam. By applying strong vacuum to warmed prefiltered oil in a "pressure  vessel" the microdroplets of water suspended in it boil at very low temperatures requiring far less energy input in the form of heat. As more vacuum and heat is applied more water tends to "boil out". This allows much more water to be "boiled out" of wvo than is possible at atmospheric pressure. Vacuum dewatering is an EDM

4. Pressure dewatering
When hot prefiltered WVO is put under high pressure and sprayed into open air the microdroplets of water present tend to turn to steam as the pressure suddenly drops. I strong air circulation is present much of that steam will escape to the atmosphere.  Pressure dewatering is an EDM.

5. Absorbant gel/crystals/beads
WVO is prefiltered and then passed through a fluidized bed of highly hygroscopic material. This material absorbs the microdroplets of water upon contact. This method is not an EDM.

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