By Rick Da Tech
The primary advantages of using enzymes are that it works on low-quality oil as well as high-quality oil, it does not make soap, and the glycerol is 97% pure or better, making it worth something. It has several disadvantages. It takes a day of mixing rather than an hour, and methanol is added over time rather than up front, calling for a more complex processor.
I feel it is still out of reach for most home brewers, primarily because we do not have an open source processor design that uses it. So far, all the engineering using enzyme catalyst has been for profit. Engineers are people too; they deserve to get paid for their work, including their processor designs. However, the homebrew community also has the skills to design a processor using enzymes, and most of the research is available at your local public library. We also have several sources for the enzymes that are used to make biodiesel.
I spoke with the Novozymes Rep, and they have a one-time use process designed for their Eversa Transform 2.0 catalyst. The process calls for filtering the WVO to 10 microns and caustic stripping the oil as a pretreatment to rid the oil of any mineral acids. Even trace amounts of mineral acids can inactivate the enzymes. Then the process calls for 2% w/w of water. It does not matter if that water is in the methanol or the oil or both. It is needed to activate the enzyme. Heat to 40°C (104°F) then add 25% of your methanol, adding a little bit each hour for 24 hours. The method calls for a 1.2-1.5 molar equivalent of methanol and 0.3-0.4% enzyme. Using more enzyme makes the reaction go faster. Using less enzyme makes the reaction go slower. The result is a product with around 3% FFA. If you are to meet ASTM standards, additional polishing is required.
We process with enzymes at a lower temperature as heat above 120˚F destroys the enzyme. The Novozymes Eversa® Transform 2.0 is a liquid enzyme made from genetically modified Aspergillus oryzae. It is delivered with at least a three-month shelf life if kept between 0-25°C. They sell It in both 1000L totes and jerry cans of about 5 gallons. The jerry can is enough to make about 1,500 gallons, give or take a few hundred. It costs from 8 to 15 cents per gallon of oil when used at the recommended 0.3-0.4% concentrations. With 15 cents being the cost of the one-time use prescription, and 8 cents being the cost when recovering the enzyme for reuse.
One of the fundamental truths behind processing with enzymes is that water in the mix pushes the reaction in the direction of FFA, away from esters. In fact, chemistry defines lipase enzymes as enzymes that hydrolyze triglycerides into fatty acids. One way to use them is to make medical and food grade glycerin. One research paper reported that they used four parts water to one part oil with the enzyme to make 100% FFA in the light phase and nearly a mix of pure glycerin and water in the heavy phase. The paper went on to talk about esterifying the FFA into biodiesel using enzymes and ethanol. However, esterification generates water as a byproduct, and a process is required to remove the water to prevent the esterification from stalling out.
Enzymes require some water for transesterification, and water is what limits how far we can push the reaction towards completion. For ASTM biodiesel, we need an additional step to purify the biodiesel. One such purification step is a caustic wash to remove the FFA. Another method mentioned is using resins with immobilized acid catalysts to esterify the remaining FFA. Distillation is another purification process used for the removal of gums and phospholipids, but it does not remove FFA.
One of the major drawbacks is that too much methanol poisons the enzyme catalyst. The solution is to add methanol in steps throughout the processing. The result is that we end up using less methanol per batch. One research paper reported using 25% of their methanol initially then add 25% of the remaining methanol in steps during processing. They used a 1.5 molar equivalent of methanol or about half of the standard single stage NaOH recipe.
There are several dozen specific lipase enzymes. Candida antarctica lipase B (CALB) is most frequently used to make biodiesel and is commercially available in both liquid and immobilized forms. The immobilized form has been attached to beads to make it easier to filter. Both forms are expensive requiring reuse to be economical, with the immobilized form being much more expensive than the liquid. Thermomyces lanuginosus Lipase (TTL) is another commercially available lipase enzyme. The research shows that TTL is better for hydrolysis and CALB is better for esterification and transesterification.
Enzyme-catalyzed biodiesel made from low-quality oils - Biodiesel Magazine
Commercializing Enzymatic Biodiesel Production - Biodiesel Magazine
Enzymatic Biodiesel Production - 2014 Collective Biodiesel Conference - video