Bubblewashing Biodiesel & Dealing With Wash Problems
by Maria "Mark" Alovert from Local B100
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There are a number of water-soluble impurities left in biodiesel after the reaction and initial settling is complete. They are found in everyone’s homebrew biodiesel, regardless of the quality of your reaction or of your oil. The contaminants include (primarily) soaps, a small amount of leftover methanol, a small amount of leftover lye, and some free glycerine. Washing the fuel with water removes these nasties. Washing also has two additional advantages- it stops the very slow remaining reaction that sometimes occurs (in unwashed fuel you can sometimes see glycerine settle out over a period of weeks or months, and washing and removing methanol or lye will stop this) and it provides you with some quality control feedback.
Washing is commonly done using the U of Idaho bubblewash method or the ‘mist wash’ system developed by members of the infopop biodiesel discussion group. Less common is bulk washing. All washes can be done after the initial settling time (18 hours) and after draining the glycerine layer, but you can get better results by waiting at least 36-48 hours or longer before the first wash, so that soaps and free glycerine have more of a chance to settle out.
Bubble washing involves gently adding 1/3 water to 2/3 biodiesel, adding a cheap aquarium air stone and air pump setup to the water portion (water sinks to the bottom and biodiesel will float on top) and bubbling air through the water. The air bubbles allow a sort of indirect agitation of the two fluids- they pick up a tiny amount of water and gently carry it through the biodiesel, picking up soaps and other contaminants. When the bubble bursts at the surface it drops the water which picks up more of the soaps and contaminants on it’s way back down. After about 6 hours of this (low-wattage) ‘washing’, the air is stopped, then the water is drained, more fresh water is added, and the process repeats. These ‘wash water changes’ are repeated about 3 times on average- until the water measures the same pH of your tap water, and is perfectly clear. The wash water can be reused a number of times to wash succeeding batches (explained below as ‘Counter Current’).
Advantages are that bubblewashing uses less water than others, and that it uses very cheap equipment (aquarium air pump is under $10 new, very easy to find ‘used’ at flea markets and thrift stores, and uses 3 watts of electricity). It is easy to leave the wash unattended (especially with a cheap timer to make the process even more userfriendly).
The disadvantage: if you have made poor quality biodiesel, or are washing a very small batch, bubblewashing can agitate the water and the biodiesel too vigorously- causing emulsification of the two liquids. Emulsification is the quintessential ‘wash problem’- but it is also a form of quality testing and feedback on your process. Once you know what causes it, it is also easily avoided.
Mist washing was developed as a way to address emulsification issues. It uses more water and more complicated equipment. It also masks quality problems- you can get a ‘good wash’ but will have much less of an idea of what you’ve made. This system uses a very fine mist head from a garden supply store, suspended over a container of biodiesel, with a way to drain the water after it falls through the fuel. The mist stirs up the fuel less than in bubblewashing, and removes soaps gradually. The gentler agitation gives less of an opportunity for the soaps and any mono and diglycerides to form emulsion. Many mist-users also do bubblewashing as a final step- after the soaps are gone the mg and dg alone won’t usually emulsify the fuel and water as much, so bubbling after misting usually ‘works’.
The disadvantages of misting are higher water consumption, more complicated equipment, and the masking of potential problems- Unfortunately, the mg and dg will still be there regardless of washing method- they are not watersoluble and won’t wash out- and in misting, you won’t know if your biodiesel was the result of an incomplete reaction you’re doing some other form of routine testing.
The last method, ‘bulk washing’ is not usually used by homebrewers now. It involves mixing equal amounts of water and biodiesel, agitating gently, letting separate, draining water, and repeating again a number of times. It is more work, uses more water, and isn’t an automated unattended process like bubblewashing.
I am going to focus on bubblewashing for the rest of the article:
Valve (optional) or siphon to drain water from under the biodiesel
Aquarium air pump of at least 3 watts
--(if washing a large- 25 gallon or bigger- batch)
Aquarium air stone- the bigger 6” ones sink to the bottom easily, the small ones will need to be weighted down with some washers, and some airline tubing.
Cheap timer to turn off air bubbler automatically after 6 hours.
You can do washing in your processor or in a separate container. I use a 55gallon closed head drum which has been turned upside down (bungs are now on the bottom) and the end without bungs is cut open (sawzall works great for this). I then attach a ball valve to the 3/4” bung and mount the whole thing on a stand (or some cinderblocks, or just even set it on top of a two fullsize American milk crates stacked together to a bucket-clearing height).
Using a separate washing container rather than your processor frees up the processor for making and settling the next batch, keeps any residual water out of your next batch, and keeps any glycerine that didn’t drain well from contaminating the wash water (which can lead to emulsification)
The rest of the equipment is fairly straightforward.
USING THE CONTAINER
Add 1/3 water to a container with 2/3 biodiesel.
Dribble the water into the tank gently- don’t spray it hard from a hose under high pressure.
Drop a bubblestone into the water, turn on the air pump, and let bubble for a few hours. The air pump should be mounted (I use thick wire and sort of tie it to the outside edge of the drum) above the level of the water so that if the power goes off there’s no chance of liquid siphoning back into the pump.
The exact amount of time you bubble is not crucial.
We use 6 hours for the first wash, and more for the others. The water will become somewhat saturated in relation to the soaps content of the biodiesel, and will stop taking in any more soaps/contaminants. At this point (6 hours or overnight), turn off the bubbler. It is useful to let everything sit for an hour or several so that the water and biodiesel separate well (optional). This is where the timer is useful- to turn bubbler off and let settle before you get there to drain it. You will then drain the water as much as possible, add more water, turn on the bubbler. This wash water change and overnight bubbling is repeated three or more times, until the wash water is clear and measures the same pH as your tap water.
If you have made less-than-perfect biodiesel, the dreaded “emulsification” is most likely to rear it’s ugly head in the first wash:
An explanation of emulsion:
If you mix water with some clean washed 100% biodiesel, the two liquids will separate quickly into two layers- oily biodiesel on top and water on the bottom, with nothing in between (try this with a commercial biodiesel sample- shake it up in a jar with an equal amount of water). Well-made and well-washed biodiesel will act just like salad oil and water if they were mixed this way. Unfortunately, with the soaps (or free glycerine, or moono and diglycerides) present in unwashed fuel, the emulsification factor comes in, and if there is a lot of these soaps or other emulsifiers, the liquids don’t separate easily. This is more like what would happen if you mixed salad oil, water, and lots of dish soap, and shook it up in a jar- there would be more of a homogenous mixture instead of two layers.
Emulsification will also happen if these soaps/mg/dg are present, and too much/too vigorous agitation of the mixture occurs. This is the reason for the mist washing method- to take vigorous agitation out of the picture. To me it seems that this masks the actual problem- which isn’t agitation, it is fuel quality. I would rather produce fuel that contains less soaps and less emulsifying mg and dg instead of decreasing the agitation- because the amount of agitation produced by standard bubbledrying isn’t so strong that an average homebrew fuel shouldn’t be able to handle it without emulsifying. Two-stage acid-base process, while a little complicated for beginners, produces much much less soap than the usual single-stage, and gives a consistently easier wash. Try it when you get experience handling the singlestage.
Mono and diglycerides explanation:
The additional factor in ‘wash problems’ is the presence of mono- and diglycerides, which result from an incomplete reaction in a less-than-perfect batch of homebrew biodiesel. Mg and Dg also emulsify oils and water, are soluble in biodiesel, and do not wash out. Mg and dg are common ingredients in junk food- as emulsifiers in cheap ice cream and sauces and the like. They are unwanted guests in homebrew biodiesel, however. Mono and diglycerides can cause injector clogging and corrode some metals when burned as part of poor quality fuel. When making fuel, it’s usually not hard to correct for the appearance of mg and dg- use the right amount of lye, higher temperature (I use 130 F during reaction), enough agitation (go longer if in doubt), more methanol. Different oils need differing amounts of the above to produce good biodiesel. Skimping on any of the above factors, or inaccuracy in measurement, is the first thing to look for if you suspect poor conversion in your fuel.
Home testing for mg and dg:
One test for mono and diglycerides is to re-process a liter of your fuel, and to see if more glycerine drops out.
1. Take a liter of biodiesel (prior to washing, or after washing, but not in the middle of the washing process somewhere as there will be water present that can disrupt the process and yield soap).
2. Re-process the liter of biodiesel as though it were new oil instead of biodiesel- 3.5 g lye, 200 ml of methanol, and
3. See if more glycerine drops out, indicating that some mg and dg was present and finished converting to glycerine in the reprocess test. Be aware that some of the ‘glycerine’ dropping out will be watered down by the methanol- so the total glycerine quantity isn’t as large as it may seem. Good fuel won’t drop any glycerine at all- again, try this test on some commercial fuel.
WHAT EMULSIFICATION LOOKS LIKE:
Normal, no emulsion:
Biodiesel made from WVO should stay relatively dark or golden-brown even during the first step of washing. A sample will be hazy/cloudy. It could turn slightly orange and lighter, and this is still normal. The orange color is from some small amount of emulsification occurring- there is some water emulsified into the biodiesel layer (probably held there by soaps), but it will probably go away in the next wash water change or two.
Emulsification can happen two ways: either all the way through an entire container of washing fuel and water (really bad emulsification) or in a less stubborn form- an emulsified middle layer in between the biodiesel and the water. It’s been described as looking like cream or mayonnaise or ‘chicken soup’ (funny how it’s all food references, must be the fryer oil exhaust smell that makes biodieselers hungry)
1. All The Way Through (bad):
If you look into the top of your wash tank and instead of brown biodiesel see the contents turn a creamy whitish-yellow, opaque color, with a thicker viscosity (like a thin milkshake or very runny pancake batter instead of the biodiesel viscosity)- then you’ve got emulsion all the way through your container.
2. Emulsified Middle Layer (more common, less of a problem):
If on the other hand the wash has formed an emulsion layer between the water layer and the biodiesel layer, you will come across it when you drain your first wash water. Normal ‘first wash’ water should be an opaque white color, and of ordinary water-like viscosity, looking just like milk sometimes. If you hit some emulsion the water will look more creamy (slightly yellow) color and can sometimes also change viscosity- the emulsion can be more like that runny milkshake rather than like milk or water.
Dealing with it:
If you‘ve gotten yourself the middle layer variety of emulsification- don’t despair- its somewhat normal and chances are that you can just let it sit and break on it’s own.
If you’re in a hurry, drain the wash water until you hit emulsion, then divert the emulsion into another bucket or container. Then wash the rest of the biodiesel. Separating emulsion from the rest of the biodiesel lets you keep washing the majority of the biodiesel in the wash tank while the emulsion sits and separates. If it does not separate by itself in time (half a day to a couple of weeks) you can then experiment with breaking emulsion by using salt, heat, or acid, and you will not compromise the quality of the rest of your washed batch with the salt or acid. Not removing the emulsion and continuing to just try and wash it out doesn’t seem to work as well as removing the emulsion and it’s associated soaps/emulsifiers- you’d have to wash the whole batch longer if you just recirculated the emulsified matter.
Since I have the space and make a lot of fuel, I now use an ‘emulsion aggregate tank’ to which I add any of the middle-layer emulsion I’ve drained out of the wash- it’s a small drum (a 15gallon would work well for someone washing in a 55gallon drum) with a drain at the bottom. Most emulsion will break or at least release a lot of water very rapidly. I drain out the released water periodically. Eventually the aggregate tank builds up enough ‘released’ biodiesel that I pump that fuel into a wash tank and do a regular wash on it. Before I had the aggregate tank I just stuck the emulsion into buckets or restaurant oil jugs, let it sit, and then pumped or siphoned off the biodiesel when it released (usually a day or so). I had a lot of emulsion to deal with because I have for the past 6 months worked with some feedstock oil that produced exceptionally bad biodiesel. I got poor conversion no matter what factors I and other local biodieselers changed- but now that I am back to using regular restaurant fryer grease I rarely have much emulsion to deal with, so the tank isn’t very necessary and may not be necessary for you either.
Severe Emulsion Formation:
If your whole batch of washing fuel turned light yellow and emulsified, you have a number of options:
In order of preference, try: time, gentle heat, salt, acid.
Turn off the air bubbler and let it sit. It might ‘break’ in a day or several.
Drawback: time, and your wash tank being ‘tied up’, which may or may not be a problem.
Heating an emulsion can break it, but it is somewhat dangerous if the heat approaches boiling point of water- someone told me that he once did this, accidentally overheated the water, the weight of the unbroken thick emulsion layer above the water layer held down all the steam that was forming, and the guy got a nasty steam explosion and blew the lid off the tank. Obviously this can be done safely, now that you know how to avoid the unsafe method.
1. The obvious danger above if you overheat it, and
2. The additional energy required, unless you’re using solar hydronic with a heat exchanger
Adding salt will break an emulsion. The water and salt molecules have more affinity for each other than water and soaps do, and the water portion of the emulsion to drop the soaps and take on the salt instead. With no soaps bonding to the water, the oil (biodiesel) drops out of emulsion with that water.
1. Salt is a toxin depending on how you dispose of your wash water- ie don’t just run this stuff over your garden!
2. I have found that if you salt, you will then need to use more wash water changes to get to the finished fuel point- ie where your wash water is clear and of the pH of your tap water.
My theory is that in between wash water changes, there is still quite a bit of water present in the biodiesel, and that this dissolved water holds the salt as well as the soaps.
While salt shouldn’t dissolve in perfect biodiesel ordinarily, remember that during the intermediate steps in a wash the biodiesel layer contains a lot of water and other impurities.
The salt then prevents subsequent washes from being as effective as normal. White color of wash water is caused by soaps, and if you salt out an emulsion you will find a water with very little white to it- and the next few washes will also have less white color to them than normal.
3. Salt of course accelerates corrosion of metals. It seems very important to make sure that all the salt is washed out.
The University of Idaho and many published instructions about washing used to recommend using acid to help make washing easier. People used to use it routinely in their first wash water- which acid to use doesn’t matter very much, and household vinegar or citric acid was routinely used.
You can also add acid to an emulsion and might see the emulsion break. However it isn’t the best method to use as it can compromise the quality of your finished fuel:
Acidifying a wash or an emulsion works by breaking up the soaps into their constituent parts: forming a salt and a free fatty acid. The ffa will end up in your biodiesel and is indistinguishable from it.
FFA content is a concern of the ASTM and other specifications for biodiesel. FFA promotes some corrosion of metals. Raising the FFA level doesn’t seem to be a good thing to do.
I also believe I’ve had a harder time washing fuel that I have acidified- same situation as salt- a harder time getting clear water/neutral pH for some reason- but I could be wrong on this one as I don’t routinely do the acid treatment.
Some people recommend breaking really stubborn emulsion with acid, then drying the biodiesel (big energy input there), then re-reacting the biodiesel portion to remove the ffa (and to get a better reaction if mg and dg were a problem). The re-reaction will again form more soaps- using two-stage acid-base process in this case seems like a good idea. But really, really stubborn emulsion isn’t all that common, and one of the other methods should fix it without resorting to acid.
Counter-Current Wash Water Reuse:
If you have the space and do a fair amount of biodiesel production, it’s good to know that you can reuse the wash water for subsequent batches, up to a point. This is called a counter current system.
The water needs to be changed because there is reached something like an equilibrium between the amount of soaps in the water and the amount of soaps in the biodiesel, where the water won’t take in any more from that batch of fuel. But if you then saved this water and reused it on ANOTHER batch, one step BACK in the process (ie for fuel that’s soapier than the first batch you used it for), it will again absorb more soaps. The example is, to reuse the water from Wash #2 in the Wash#1 step of the next batch you bubblewash. So you only need to dispose of wash 1’s water, and only after it’s already been reused for three other washes in previous batches.
Keeping this much water sitting around isn’t practical for everyone, given space limitations.
At our co-op, we don’t have any room for any more storage of water. However we try and have two wash tanks ‘going’ at once. The wash on one batch is started one day before the other. We change the wash water on both tanks at the same time- draining the water out of the older batch into the newer one. So even though we dispose of water before it’s been reused as often as it could ideally be, we DO reuse it at least twice. We also leave the last wash water in the tank, remove the biodiesel by pumping it off the top, and save that last wash water for the first wash of the next batch.
It is theoretically possible to use biological filtration- a constructed wetlands graywater system- to break down the soaps in wash water, cleaning it enough to reuse it many times. I don’t know of anyone actually doing this yet, though many biodieselers are talking about it. See the book “Building an Oasis With Graywater” available through RealGoods , for a discussion of a few homebuilt graywater systems- in brief, a graywater system uses water plants- cattails and water hyacinth or the like- along with gravel, sand, baffles, and microorganisms, to break down impurities in water. This can be done in a trio of drums or old bathtubs set on a staggered stand (so water flows slowly from one to the others by gravity) and planted with the above plants. Remember, you’re only trying to get the water clean enough to absorb more impurities, not to necessarily clean it enough to water a food garden with it.
Barring graywater filtration, you can do several things: add acid to the water, checking the pH, until it is neutral, then sewer it. Most of us currently sewer the wash water, figuring that it is no worse than the soaps, detergents, shampoos, laundry soaps, bleach, fabric softeners, drain cleaner, shower scrubbers, tile polish, floor cleaner, car cleaners, tire cleaners, windshield bug deflector cleaner, ammonia, and host of other nasties flushed by the average American household. The soaps in the biodiesel wash water are fairly watered down- you won’t get ‘suds’ from your first wash- and the amount of lye and methanol left to be washed out has been shown to be fairly low (most of your excess lye and methanol ends up in the glycerine layer, not the biodiesel layer we wash). One improvement could be to do methanol recovery on the biodiesel, but it’s usually not worth the energy to do it to the biodiesel (as opposed to glycerine) layer. I’m also waiting for someone to try reverse osmosis to clean up their wash water, but the units are VERY expensive.
Drying Washed Biodiesel:
The current conventional wisdom on dry fuel is that if it is ‘clear’- referring to lack of haze, not color- it does not contain much water (in reality biodiesel will absorb some small and harmless amount of moisture from the atmosphere).
Washed fuel will often look slightly cloudy or hazy. If you let it sit it will eventually drop or evaporate it’s water content. You can also heat it (I don’t do this as the MSDS implies that biodiesel vapors are harmful) . I have been experimenting with bubbling air through it, and it yields clear fuel, but I am not certain that actually indicates dry fuel.
More importantly than these external drying aids, is the fact that the better-washed the fuel, the faster it will clear. If there aren’t any soaps left to hold on to water, the water will settle out rapidly. I have made fuel that cleared almost immediately, and I think the factor that helped this was the number of times I washed it. So like with everything in biodiesel, this ‘drying’ seems to be a form of ‘feedback’ on the process- if it refuses to clear in a few days I usually give it another wash (using a little less water than usual).
A test for water content is to weigh a sample, heat it until it passes the boiling point of water, then re-weigh to see how much water it has lost. Biodiesel contains 1200-1500 ppm water normally, this is the water absorbed from the atmosphere naturally.