Pre-Treatment is the process of eliminating impurities from Oils and Fats so that the FAME produced from such Oils and Fats is also free of such impurities. All feedstock (i.e., Animal Fats and Vegetable Oils) have impurities such as water, solids, gums, free-fatty acids (FFA), waxes, and compounds of Sodium, Potassium, Magnesium, Calcium, and other metals.
Pre-Treatment process can be custom designed to be feedstock-dependent or feedstock-independent. Feedstock-dependent means the plant is custom-designed for a specific type of feedstock. For example, it could be designed for processing Extracted Corn Oil (XCO). Feedstock independence comes at a higher capital cost but allows the plant to be multi-feedstock ready. Such a plant can process different types of feedstock even though some feedstock may not require each processing step in the plant.
The goal of Pre-Treatment is to reduce the impurities such as water, solids, gums, FFA, and metallic compounds. The Trans-Esterification-ready feedstock should typically meet the following specifications:
FFA Preferably less than 0.1% (but no more than 0.25%)
Gums Preferably less than 10 ppm (measured as Phosphorus).
The following four Pre-Treatment processes (or combination thereof) of biodiesel process are possible depending upon the nature of the impurities in the feedstock:
Refining is suitable for virgin vegetable oils such as Canola, Camelina, Cottonseed, Rapeseed, Soybean Oil or Animal Fats, which have low FFA (less than 3%).
b. FFA Stripping (Physical Refining)
FFA stripping is suitable for virgin oils such as Coconut, Jatropha, Palm Oil, Used Cooking Oil, or Animal Fats, which have high FFA (above 3%). If in place, this process is preferred even for lower FFA oils.
Esterification is suitable for oils which have moderately high or very high FFA (usually above 2%).
Glycerolyis is suitable for oils which have very high FFA (usually above 20%)
Degumming and dewaxing may be required in addition to the above-mentioned steps if the Oils have higher levels of gums and waxes.
There are advantages and disadvantages of each type of process. The biggest disadvantage of Esterification is that it requires the use of “Sulfuric Acid” as a catalyst. This poses challenges with respect to operator hazard and equipment failure due to corrosion. Although such challenges can be overcome with the use of heterogenous (solid) catalyst; however, solid catalysts pose other challenges that may require additional investment in Pre-Treatment equipment. Another disadvantage of Esterification is that the amount of Methanol required during Esterification is very high; thus requiring additional investment in methanol recovery and rectification equipment.
The main disadvantage of FFA stripping is that it removes the FFA from the Oil resulting in yield loss. Although such loss is not a complete loss of yield because FFA can be sold to partially recover the value of the feedstock. A challenge of FFA stripping is that it works under very high temperature and vacuum; thus requiring additional investment in heating and vacuum equipment.