-Hydrolysis of all fatty acyl ester bonds by saponification.
-Removal of water via standard industrial methods used to prepare an acid oil, a current product used in the animal feed industry.
-Acid-catalyzed esterification of the resulting free fatty acids.

While the above description seems complicated, the process is simple and efficient, and can operate at ambient pressure and relatively low temperature in simple equipment. This process appears to have promise, particularly if the market spread between soybean oil and products such as recycled grease continues to increase.


Processing considerationsAll biodiesel feedstocks, including soybean oil, require some amount of pre-conditioning, often chemical refining, before being processed into biodiesel. For example, crude soybean oil must be degummed to remove sources of phosphorous, which degrades the catalysts employed in biodiesel production.

Free fatty acids should also be removed from the feedstock before transesterification, especially if recycled cooking oils, which are high in free fatty acids due to their exposure to high temperature during frying, are used. Free fatty acids are corrosive, and therefore hard on fuel storage facilities, fuel systems and engines. ASTM standards set forth tight tolerances on the allowable levels of free fatty acids in biodiesel.

A common approach for removing free fatty acids from the feedstock is via acid catalyzed esterification in the presence of methanol. This converts the free fatty acids to methyl esters (e.g., biodiesel). Acid esterification equipment requires the use of stainless steel equipment due to the corrosive nature of the process.

Crude oil from a crushing operation also may be pre-treated by caustic refining to remove free fatty acid. The soapstocks generated in this step are then removed by washing the oil with hot water. The oil must be dried to remove the water before transesterification, since water inhibits the biodiesel-producing reaction. In at least one large biodiesel plant, the feedstock is RBD soybean oil (refined, bleached and deodorized).

As mentioned previously, soybean oil or other virgin vegetable oils are reasonably consistent. The feedstock used today will be approximately the same as the feedstock tomorrow. This usually is not the case when using recycled oil. Recycled oil requires accurate chemical analysis to determine what pretreatment may be required.

Used cooking oil may also contain sulphur compounds, often the result of frying sulphur-bearing foods such as onions. There are several proven methods to strip out any unwanted sulphur compounds during pretreatment.

Obtaining used cooking oil for use as a biodiesel feedstock involves more than just driving up to your local McDonalds and filling a tank truck. The recycling process includes not only the pickup of the cooking oil, but also the proper removal of food particles and other physical contaminants. In addition, used cooking oil contains water, and the oil must be dried. An experienced rendering company is in the best position to do these pre-process steps and then deliver a cleaned and dried recycled product to the biodiesel plant.

The use of feedstocks other than soybean oil may involve extra processing steps and more equipment. But to be financially successful, a biodiesel plant needs to be prepared to process multiple feedstocks based on regional availability, market prices and processing costs.

When processing multiple feedstocks, consideration should be given to using a blended feedstock. Instead of processing feedstock "A" for a few days and then switching to feedstock "B" for another period of time, various feedstocks can be pretreated and temporarily stored in separate agitated storage for final blending prior to transesterification. In this way, feedstocks can be analyzed and monitored, yielding a final blend that will be more consistent and result in a more efficient and controlled process.

In conclusion, remember that the goal is to produce biodiesel that will help reduce America's dependence on imported oil, and at the same help the environment. To accomplish this, the primary cost of biodiesel production—the feedstock—must be carefully considered. Over the long term, economics will dictate that the use of multiple feedstocks will be a significant factor in the development of a successful alternate fuel business.

Bob Stroup, president of Ohio-based R.L. Stroup Co. LTD, is an oilseed and biofuels consultant. He can be reached by e-mail at rlstroupco@earthlink.net

<-- Previous Page	1   2   3
View Entire Article