In light of the energy crises and skyrocketing cost of gasoline and heating oils over the last year, wouldn't it be great if there were a simple process that could convert a toxic waste material into an inexpensive and nonpolluting fuel?It is estimated that American restaurants discard about 13 billion gallons of used grease and oil from deep fryers each year. Most of them have to pay somebody to haul it away, and although some of it is eventually made into animal feed or other products, most of it winds up in some toxic waste dump. Doesn't that give you confidence in your burger & fries?
When Dr. Rudolph Diesel introduced the internal combustion engine that bears his name at the World Exhibition in Paris in 1900, his prototypes ran on peanut oil. The fledgeling petroleum industry, seeking a market for the portion of their product now marketed as "diesel fuel" jumped on the band wagon, seeing in Dr. Diesel's invention the promise of greater sales than the kerosene lantern was likely to produce. Today, of course, most of the world's trade is moved by diesel engines fueled by petroleum-derived diesel fuel. But the fact remains that the diesel engine can run on vegetable oil, or even lard, just as well.
The main reason that those 13 billion gallons of waste vegetable oil don't become fuel for the truck industry is that modern engines (and for that matter modern home furnaces, kerosene heaters, and even those kerosene lanterns sometimes used for decorative lighting) have all been designed to run on the petroleum product, which has different enough physical properties to make significant engine modifications necessary.
There is a way, however, to modify the waste fryer oil so that the physical properties are close enough to petroleum diesel fuel or kerosene so that it becomes interchangeable with petroleum diesel. The product is called biodiesel, and in fact, it can be mixed in any proportion with petroleum diesel, and except for an exhaust stream that smells like french fries, the casual bystander or truck driver would never notice a difference. Actually, fueling trucks with such modified waste oil is environmentally better than using petroleum diesel: emissions are lower for all the pollutants of concern, and unlike petro diesel, which adds fossil carbon to the greenhouse gases in the atmosphere, biodiesel actually removes carbon from the atmosphere, since the plants grown to make it remove about 3 times as much carbon as is present in the biodiesel itself.
For this experiment, imagine that you have been hired to help a waste recycler "do something useful" with the tons of fryer grease collected at restaurants all over the city. Supplying fuel for the city buses and the school system has been suggested, and you are about to make it into production.
The process you are going to use is called "transesterification", and it uses methanol and a caustic such as lye (sodium hydroxide) or potash (potassium hydroxide). It is similar to the making of soap, and in fact, one of your by-products is likely to be soap.
Fats, whether vegetable or animal in origin, chemically are esters made from three organic acid molecules and the tri-functional alcohol called glycerin. The organic acid treats the alcohol as a base, reacts with it to form the fat (the tri-ester) and water. Thus these substances are called tri-glycerides.
Our ancestors discovered that adding a strong base (made by leaching water through wood ashes, which dissolved the sodium and potassium hydroxide in the ashes) could liberate the glycerin and form the sodium or potassium salt of the organic acid. We call that salt "soap".
If instead of water we use an alcohol such as wood alcohol (methanol) or grain alcohol (ethanol) and just enough caustic lye or potash to catalyze the reaction, we will get glycerin and the methyl (or ethyl) ester of the organic acids. These esters effectively cut the molecular weight of the vegetable oil in thirds, bringing the physical properties of density, viscosity, boiling point, etc. very close to the properties of petroleum diesel fuel or kerosene, and making them attractive as fuels.
The problems are these:
Many experimenters have found that a level of 3.5 g of NaOH per liter of vegetable oil (or 9.0 g of KOH) produces the most rapid and complete conversion. To this must be added enough alkali to remove the free fatty acids, so your first step will be to perform a titration to characterize the oil at hand. Adding the amount of alkali needed for catalysis to the amount determined necessary for neutralization will tell you how much alkali you need.
- the reason that fryer oils are discarded is that after some time at temperature, oxidation liberates enough fatty acid to affect the taste of the food; free organic acids will consume some of whatever caustic you ad and must be taken into account. How badly oxidized the oil becomes depends upon the chef, so you have no control over the free acid content of your starting material. You have to add enough caustic to catalyze the reaction as well as enough to neutralize the free acid, but no more, as caustic costs money and will steal some of the fatty acids by making soap.
- you also have no control over the length of the organic acid molecule. Fryer oils in common use have acids between 14 and 24 carbons long depending upon the species from which the oil is derived, as well as growing conditions, and this will require adjustment in your process. To get the maximum conversion of dense, viscous tri-glyceride into the thinner and less dense methyl esters, you will need to add enough methanol. At the same time, you want to use only the methanol needed and no more, since unlike the waste vegetable oil, methanol will cost you money. It also has deleterious effects on rubber fuel lines, so to keep your customers happy, you will want to see that the product has as little free alcohol as possible.
- storage tanks also cost money, so you want to get the oil in, treat it, and ship product out the door as quickly as possible freeing up tank space for the next batch. So you need to get the reaction to proceed as rapidly as possible.
It has also been found that using between 10% and 20% by volume of methanol to veggie oil works best. But the ideal amount depends upon the oil at hand.
Finally, we are going to separate our products, characterize them for quality control purposes, and demonstrate their usefulness in an actual diesel engine, oil lamp, or kerosene heater.
We are going to make a small (200 mL) batch test reaction in preparation for making a much larger batch "in the factory", a common technique in industry.
For the titration, we wish to make the NaOH or KOH solution and use isopropyl alcohol because we want the alkali to mix thoroughly with the waste oil. A water solution would not work, but would form two distinct layers, which would complicate the process. Using an alcohol to dissolve the alkali works well, but we also want to minimize the transesterification reaction. Isopropyl alcohol will react much more slowly than either methanol or ethanol, so we are going to use it.
SAFETY: Be sure to look up the MSDSs for the chemicals you are going to use. Safety glasses or goggles are required at all times in the lab. Be aware that both sodium hydroxide and potassium hydroxide can cause chemical burns, either from the solid form or the alcohol solutions. What other hazards should we watch for in using these chemicals?
Examine the container of waste fryer oil and note its appearance. Straight from the fryer, we might expect to find pieces of french fry, bits of fish stick, chunks of chicken wing, and god alone knows what else. Depending upon the oil, it may also be more or less solidified, because frying oils vary widely from "lard" which is an animal fat, to lighter oils such as corn or soy oil.
Bits of french fry, etc., are not desirable in the finished product, so we might as well remove them early from the process. Cut some cheesecloth into pieces large enough to serve as a filter over the top of a 400 mL beaker. You will want to use 3 or 4 thicknesses, and push them down into the beaker while leaving the edges hanging over the top. Pour enough oil through the cheesecloth to get a little over 200 mL of filtered oil. You can discard the used cheesecloth in the trash. Be sure to examine the filtered oil and note its appearance.
Carefully pour 1 mL of the oil into a graduated cylinder. Add enough isopropanol to it to make 10 mL, cover with a piece of parafilm and invert several times to mix. Pour the resulting solution into a 25 mL Erlenmeyer flask and cover it with parafilm, too.
Use a small piece of pH paper to measure the pH of the solution, and note the pH in your notebook as well.
A solution containing 1 gram of alkali per liter of water has been made and will be put in a buret. Use this buret to add 1 mL of this alkali solution to the contents of your 25 mL Erlenmeyer, cover and mix carefully by swirling. Measure the pH with pH paper. Repeat as often as necessary to cause the pH to change to around 8 or 9. Note the total volume of alkali needed.
The concentration of the titrant was chosen so that the number of ml of titrant equals the number of extra grams of alkali needed to neutralize the free fatty acids. To this must be added the amount of alkali needed to catalyze the reaction. If the alkali used is to be sodium hydroxide, this will be 3.5 g of NaOH per liter of oil. If potassium hydroxide is to be used, we will need 9.0 g of KOH per liter of oil. Add up the amount of alkali you will need. You should show the calculations in your notebook.
We are going to make a small batch of biodiesel "in the lab" to prove that it will work properly before taking the recipe to the plant. A 200 mL batch will be sufficient for our purposes. Thus you will need to divide your calculated amount of alkali by 5 for this smaller batch. Weigh out enough alkali for your small batch in a weighing boat.
Measure the amount of methanol you will need in a graduated cylinder. You'll be assigned an amount from 10% to 20% of the oil by volume. Add that amount to a 250 mL Erlenmeyer flask and immediately cover it with a piece of parafilm so it doesn't evaporate. Carefully slide a stirring bar down the side of the flask, add the alkali from your weighing boat, and cover with the parafilm. Put the flask on the stirrer and start it mixing to dissolve the alkali. It will take a few minutes to dissolve.
Using a graduated cylinder, measure 200 mL of filtered oil and add it to the 250 mL flask while stirring. Re-cover with parafilm, and let it stir for 1 hour.
Remove from the stirrer and pour your mix into a separatory funnel and cap. Be careful not to let the stirring bar drop into the separatory funnel. Make sure your name is on the funnel, and let the mixture settle at least overnight.
Use a little soap and water to clean up the glassware and stirring bar.
Use the separatory funnel to drain as much of the glycerin as you can into a graduated cylinder. It tends to coat the sides of the funnel, so it may take several minutes to get it out. Note how much you have. If there is a soap layer, drain it into a separate graduated cylinder and note its volume as well. Pour the biodiesel from the top of the funnel into another graduated cylinder and note its volume. Did you get the same total volume of components as you put in? If not, why do you think that might be?
Use pH paper to check the pH of both the top biodiesel layer and the bottom glycerin layer. If there was a soap layer, check its pH as well. Knowing that the alkali was present as a catalyst, and that catalysts are not consumed in the reaction, where did your alkali end up?
If the ratio of alcohol to oil was not exactly correct for the given oil, there will remain either some triglyceride or some alcohol in our reaction products. If there is excess oil, it will be part of the biodiesel layer, but if there was excess alcohol, it will be in all the layers. We can tell what has happened by measuring the specific gravity of our biodiesel. Fryer oils will have a specific gravity generally around 0.94-0.96, while biodiesel will have a specific gravity in the 0.86-0.89 range. We generally consider biodiesel specific gravities above 0.9 to be incompletely transesterified. If a hydrometer measuring in the appropriate range is not available, you can weigh a small amount of the biodiesel using a volumetric flask to calculate the density, and from that the specific gravity.
Viscosity is also an important property, since most applications such as diesel engines and oil furnaces use a pump to spray the fuel into the combustion chamber. Just as a thin liquid like water sprays nicely from a small plant mister, but a viscous liquid like honey would not spray at all unless very much higher pressures were used, biodiesel must have a viscosity similar to petroleum diesel to be useful in the same equipment. Devise a way to measure the relative viscosities of the waste oil, vs petroleum diesel or kerosene, vs the biodiesel.
We will attempt to fuel a device with the product biodiesel.
CLEAN UP: When finished, place your esters and the glycerine in the containers provided. Any excess or left over vegetable oil can be put back into the Waste Fryer Oil container. Any alcohol or lye can go down the drain. Clean out the glassware with soap and water before rinsing with distilled water.
Report the amount of biodiesel produced, the specific gravity and viscosity of your sample, and include a picture of it burning in a lamp.