Drywashing Biodiesel with Sawdust
By Rick Da Tech
Purifying Biodiesel with wood shavings and sawdust has become the most common form of drywashing biodiesel for homebrewers. It's apeal to home brewers is that it's available locally and either cheap or free. It started with Eco2pureTM and has continued development with wood chips and sawdust. Wood, it seems, soaks up the crud from unwashed biodiesel. Specifically, it reduces the soap and glycerin.
DIY biodiesel hobbyists are some of the cheapest people on the face of the earth. Unwilling to pay the 5-8 cents per gallon for Eco2pure, they started investigating using sawdust and wood chips as a drywash media. Hardwoods seem to do better than softwoods while kiln dried does better than green wood. Red Oak seems to be the best of all. It is speculated that Red Oak performs best because of it's high tanic acid content. The wood chip mulch you get at the nursery is usually too wet to be useful. Some hobbyists in dry climates have been able to dry it out enough to use, but most just avoid the stuff.
The consensus seems to be that sawdust is better than wood shavings and mulch because of the increased surface area of the sawdust. When sourcing your sawdust, make sure your sawdust comes from solid lumber, not from glued up or fabricated wood products like plywood. These products contain glue that could contaminate your biodiesel.
In a Thesis by Jacob Wall dated Aug 2009 for the University of Idaho with Dr Jon Van Gerpen as the major professor, sawdust from a lumber mill was tested for it's soap reduction properties. They tested the sawdust with raw biodiesel containing 4% methanol and 2000 ppm of soap. This is similar to what we would expect for raw unwashed biodiesel made from low titration oils. They found through several tests that if less than 14.9g of biodiesel was purified per gram of sawdust, it would effectively remove the soap and glycerin.
There are multiple equipment designs discussed on the forums, I'm a proponent of KISS or simple is best. Following the KISS theology, the simplest equipment seems to be gravity fed drums inspired by Jehu of infopop. The are filled about one half to two thirds full of sawdust. Start with an open head drum with a removable lid. I prefer steel drums mainly because there have been reports of spontaneous combustion of oil soaked sawdust. Make sure the lid has the standard bungs located in it for the fill port and the vent. A ring clamp will be your best bet for securing the lid to the drum. You will drill a hole in the bottom and install a bulkhead fitting for a drain. to the drain you will attach a valve to control the flow of the biodiesel flowing out of the drum. On top of the bulkhead mount, you will attach a fine screen to prevent the sawdust from flowing out with the biodiesel. Legal Eagle documented how RolfQuo made their filters.
Pump the biodiesel into the top of the drum, through one of the bungs, into the sawdust drum. slowly drain the biodiesel out the bottom into another drum. There is a cadre that will let the biodiesel sit in the sawdust for a time before starting the drain. The absolute maximum flow rate would be about a gallon per minute. Most people set it up for a few gallons per hour. When set up as the lead with ion exchange as the lag, the flow rate through the sawdust drum will match the flow rate through your resin towers.
Over time, the soap will form a 'cake' that will prevent the biodiesel from flowing through the sawdust. This is resolved by breaking up the cake with a stirring rod before adding a batch to the drum. Then once the biodiesel has been added, wait for the bubbling to finish before opening the drain. It helps to leave a few gallons in the sawdust to help the next batch go in more smoothly.
There have been multiple reports of spontaneous combustion of wood chip systems when they are drained. Typically several days after draining the wood chips, they start oxidizing from the center of the drum out. It is usually spotted after smelling either smoke or some other off odor. Touching the wood chip tank reveals it to be very hot.
The systems that do not drain the biodiesel from the sawdust do not have an issue with spontaneous combustion, except when it is time to change the sawdust. At that time, special care can be taken to prevent a fire by spreading out the sawdust to prevent heat buildup or by soaking the sawdust in water.
The easiest way to make sure the sawdust is always covered is to use an up flow system. In this design, the biodiesel is introduced in by the bottom of the tank and drains off the top, trapping enough biodiesel inside to keep the sawdust covered. This deprives the sawdust of oxygen preventing oxidation from taking place.
While the Eco2pureTM does not seem to have any wood resins to leach out on initial use, By most reports, sawdust and wood chips do. Several methods of washing the resins out of the sawdust have been discussed. They include using either biodiesel or methanol or both to wash out the sawdust. If you are distilling the glycerin to recover the methanol, then using a methanol wash would probably be best for you. Just add the methanol to the next batch of glycerin being distilled. The people washing with methanol are suggesting one bed volume as the amount of methanol to use in the wash. Pump in the methanol, let is sit for a day or so, then drain it out.
Several people have said they use biodiesel, noting that the bodiesel will pick up some resin from the sawdust. A water wash of the biodiesel will cause the resins to precipitate out and form a layer between the biodiesel and water. Then a non-critical use is found for this batch.
A few people using sawdust and wood shavings from kiln dried wood indicate they do nothing special at all. They don't see an issue with resins in their sawdust.
Dealing with the Methanol
How the methanol is removed has not been standardized. Some will leave it in. Some will use the GL Method to remove most of it before the sawdust. Some let the bio sit for up to a week after GL before sawdust. Some don't. Some remove the methanol after the sawdust. Some use bubbling, some use water washing, and some feel the sawdust gets enough of the methanol out that they don't worry about it. More testing is needed to determine the best process for dealing with the methanol, Although the most popular is some variant of distillation before the biodiesel is sent to the sawdust.
The academic research shows that you do not need to remove the methanol before sending the biodiesel to the sawdust. They achieved complete absorption of the soap with 4% methanol. Our raw unwashed biodiesel will usually contain about 3% methanol. The sawdust will soak up some methanol. We do not know how much, but we do know the sawdust reeks of methanol after the biodiesel goes through it.
Disposal of Spent Sawdust
When the sawdust becomes ineffective, replace it, and dispose of the spent sawdust ASAP. It will burn fast and hot if you have a wood burning fireplace.
You can make fireplace logs out of the sawdust by either stuffing the sawdust into paper milk cartons or by rolling it up inside newspapers. Either way works good, just be sure to let the sawdust drip dry for several days to extract all the usable fuel before disposal.
One person reported reusing the spent sawdust after washing with methanol and air drying.
Whatever you do, do not keep the spent sawdust laying around the shop, It is a fire hazard and likely to burst into flames spontaneously.