Biodiesel at Crescent Moon
Restaurants that buy Crescent Moon's produce send their used vegetable oil out to the farm. At the farm it is converted into biodiesel. The biodiesel is used to run all of the farm's tractors and other equipment.
The Biodiesel Process
Startup
Jack fills the reactor -- previously an old honey vat a neighbor gave him -- with 150 gallons of used restaurant oil strained through a window screen to remove large food particles. He starts the wood stove, which has pipes running through it from the vat. A small electric pump circulates the oil through the stove and back to the vat.
Titration test
This test indicates the amount of free fatty acids in the oil, which determines how much potassuym hydroxide (KOH) must be added to the methyl alcohol and mixed with the vegetable oil.
Mini-batch
This scaled-down test is run to veify that the mixture of KOH, methyl alcohol and cooking oil is correct. It takes about 45 minutes and if successful, the biodiesel will rise to the top of the jar and the thicker glycerin will collect at the bottom. If not, Jack will have a jar of soft soap.
Heat the Oil
The reactor is heated until the oil is 160 degrees F. At that point, Jack boosts the heat to 220 degrees. He then turns on a shower head suspended above the vat and sprays the hot oil back into the vat, which removes any water present and cools the oil to a level below 125 degrees.
Reaction
Next, the KOH is weighed and the methyl alcohol is measured according to the mini-batch ratios. Jack mixes the two together and adds that to the vat of oil, stirring with a wood paddle. The electric pump is restarted and keeps the mix circulating for 4 hours. He shuts it off after that and lets it stand overnight.
Drawdown
After settling, the glycerin has formed a layer on the bottom of the vat. It is drained out of the vat from a valve at the bottom. Jack uses a clear hose so he can see when the glycerin is gone and the clearer biodiesel is emerging.
Wash
The biodiesel is then washed using a water stream applied with a series of misters above the wat. The glycerin and methyl alcohol in the biodiesel cling to the water droplets, which collect at the bottom.
Wash water transer
A transfer pump is turned on to send the wash water to the treatment tank. Eventually, the last remaining gallons are transferred by hang using buckets to avid introducing any biodiesel into the treatment tank.
Drying
The wood stove is refired and Jack heats the biodiesel to 180 degrees, then runs it back through the shower head. That removes any residual water or methyl alcohol.
Testing
The finished biodiesel is tested for dissolved glycerin and the pH is checked with a hydrometer. This is done to verify that the batch meets the fuel specification of the ASTM, the American Society for Testing and Materials.