Catalysts for Biodiesel
By Rick Da Tech
NaOH is Sodium Hydroxide, also called Caustic Soda and sometimes lye. KOH is Potassium Hydroxide, also called Caustic Potash. Both are commonly used to make biodiesel. A major side reaction in making biodiesel is converting the FFA or Free Fatty Acid to soap. Soap made from NaOH is solid. Soap made from KOH is liquid.
NaOH and KOH are dangerous chemicals. Please read the MSDS sheets before handling these hazardous chemicals. They can severely burn your skin on contact. If you get it in your eye, it can blind you. If you get any caustic on you, the prescribed treatment is to flush with LOTS of water for up to 30 minutes. Always wear safety goggles when using NaOH and KOH.
KOH vs NaOH
KOH dissolves a bit easier in methanol than NaOH. Although not everyone notices the difference, most do. Under certain conditions, NaOH makes a glycerin byproduct that turns into a gel or even solid. It is not a problem after you drain the glycerin, but a big problem if it is still in your processor or plumbing and goes solid.
KOH is more forgiving on high titration oils than NaOH. High titration oils make more soap than low titration oils. If we use NaOH and there is enough soap you get a tank full of gelatinous goo that contains mostly soap, WVO, and some biodiesel. KOH, on the other hand, being liquid does not seem to interfere with the reaction. The result is that we can process much higher titration oils with KOH than we can with NaOH.
KOH is a heavier molecule than NaOH, so, we use more KOH by weight than NaOH. Mathematically we use 1.4 times as much KOH as NaOH. When making biodiesel, we use a base of 5 grams per liter of NaOH and 7 grams per liter for KOH. To this, we add our titration values. While NaOH is relatively pure, KOH is usually only available with about 90% purity. The lack of purity requires us to divide the 7 grams by .9 to adjust our base up to 7.7 grams per liter.
If we titrate our oil using NaOH, to find it titrates to 3, we end up with a recipe of 8 grams per liter. The same oil titrates to 4.6 grams per liter using KOH, giving us a recipe of 12.6 grams per liter. If NaOH is 50 cents per pound and KOH is 80 cents per pound, then NaOH would cost 3 cents per gallon of biodiesel and KOH would cost 8 cents per gallon. Of course, since the amount of catalyst varies with the titration value of the oil, high titration oils use more and cost more, while low titration oils require less catalyst and cost less to process.
NaOH may be available in the hardware stores in 2lb containers for about $8 each as drain cleaner. However, since it can be used to make illicit drugs, most manufacturers no longer offer pure sodium hydroxide as a drain cleaner. Manufacturers have added aluminum flakes and other materials that poison the chemistry. So, make sure it says "100% Sodium Hydroxide" on the bottle somewhere.
Both NaOH and KOH are available over the Internet from various sources, like companies that supply soap makers. Typically the sizes available range from 50lb bags to 2lb jars. Due to the unique handling requirements of strong caustics and the extra charges for shipping HAZMAT chemicals, they usually cost about $3 to $4 per pound with shipping.
If you look in the Yellow pages for your area under chemical suppliers, you can often find a local source for either NaOH or KOH. Sizes range from 50kg bags to 400lb drums and cost about 50cents per pound for NaOH and 80 cents per pound for KOH. Chemical suppliers usually require a business license before they sell bags or drums of caustics.
If you go pick up your catalyst, make sure it does not ride in the passenger compartment. Use the trunk only if it is well sealed and completely separate from the passenger compartment. In the back of a pickup is the best way to transport catalyst. Even in a bag, it gives off fumes and dust that can cause major breathing problems for those confined in the same compartment with the catalyst. Those that have done it once, never do it again.
Both NaOH and KOH are commonly available as an aqueous solution (mixed with water). Both are less expensive when purchased as a liquid. There is even an old technique for mixing methoxide by first dissolving the catalyst in water then mixing it with the methanol. This method came about because of the difficulty people were having dissolving NaOH directly in methanol. The biodiesel forums often refer to it as the concentrated aqueous catalyst method. The advantages are that it is safer to deal with liquids than dusty caustics and it is a lot easier to make your methoxide this way. The disadvantages are that the water lowers the quality of your biodiesel, and it sometimes unexpectedly makes glop. ASTM spec biodiesel is very difficult to make using this technique. If your oil is not perfectly dry or you are working with high titration oils, this method often leads to a processor full of soap, in the form of gelatinous glop.
NaOH and KOH catalysts are most economically purchased in 50 lb bags from a chemical supply house. Both NaOH and KOH are hygroscopic, meaning they draw moisture out of the air. Both will react with CO2 from the air to become a useless carbonate. A 50 lb bag of catalyst will last you a long time giving you lots of opportunities to contaminate your catalyst with water and CO2. Once you expose your catalyst to water, it clumps up and becomes difficult to dissolve in methanol. The answer is to keep your catalyst dry and minimize it's exposure to air!
There are two basic methods used to keep catalyst dry.
- Put it in a 5-gallon bucket, bag and all, with a good sealing lid. This method is great for areas with low relative humidity.
- Divide up the 50lb bag as soon as you get it, into smaller containers like half gallon milk jugs or zip-lock plastic bags. The containers do not all need to be the same size or hold the same amount of catalyst. The best ones would be largemouth HDPE bottles with good sealing lids.
If you have super high humidity, you might want to divide up your bag in the winter on a cold day when there is not much moisture in the air.
Having proper labels on your chemicals may seem petty, but if someone from the fire department sees your processing area, not having the proper labels can cause a conflict with the authorities. There are two types of labels that are found on chemicals, "Hazmat Placards" and "Health and Safety Labels." These labels should be on the container showing what was originally in the container until it has been washed out and filled with another chemical.
Hazmat Placards are required by the D.O.T. when transporting hazardous materials. Section 14 of the MSDS sheet provides you with the "Hazard Class" for that material. On the left is the placard for Sodium Hydroxide and Potassium Hydroxide.
The NFPA is the organization defining Health and Safety Labels. It consists of a diamond divided into four boxes, each box is a different color and represents a different hazard. The number found in the box indicates the relative degree of each hazard. 0 being no hazard and 4 being the maximum. Section 5 of the MSDS sheet sometimes lists the proper number to write in each box. You do not need the fancy labels. You can draw the boxes on the drum with chalk. Then fill in the numbers and write the chemical name beneath the boxes.
High Humidity Biodiesel Tip
When I lived in Atlanta, the humidity could get so high on a summer day that my NaOH would absorb its weight in water while weighing it. So, I divided up my NaOH on a dry winter day, carefully marking each container with its weight. Then when I made biodiesel on humid days, I would adjust the oil and methanol quantities to match my catalyst.
KOH and NaOH are not the only catalysts that can be used in transesterification to make biodiesel. NaOH and KOH are the only catalysts used by homebrewers. There are two types of catalyst, those that dissolve in methanol and biodiesel and those that do not. The ones that do not dissolve are called heterogeneous catalysts. The ones that do not dissolve in biodiesel or its byproducts have the advantage of being filtered out and reused. Soap poisons heterogeneous catalysts, so using them to make biodiesel from WVO is not efficient. You end up only getting a couple of uses out of the catalyst before it becomes useless. Calcium Oxide CaO and Barium Hydroxide Ba(OH)2 are the most often mentioned.
Many people use the term methoxide to describe the methanol/lye solution we use to make biodiesel. It is a chemistry term for another chemical. Industry makes Methoxide, or Sodium Methylate in very controlled environments by dissolving the metallic Sodium into methanol. They then boil off the methanol. The remaining powder is Sodium Methylate. It is a hazardous material that if left exposed to humid air can spontaneously catch fire on a cold day. It is a very nasty and HIGHLY regulated chemical. You can’t buy it, so don't try.
When Sodium Methylate comes up on the forums, they are talking about a "premix" solution of 30% sodium methylate in methanol. Commercial producers frequently use it. When we dissolve NaOH in methanol, we create water. Sodium Methalyate in methanol contains none, and even reacts with the water in WVO, removing it from the reaction resulting in a reduction in the amount of soap produced. It makes great biodiesel, but it is expensive and extremely hard to get.
Lipase or enzymes are a new development in heterogeneous catalysts. They can be used to esterify or transesterify without using strong acids or bases. So far they are not available to the homebrewer and require different processing equipment to use. They produce great biodiesel and in a commercial environment can reduce costs.