responsible for the popularity of the transesterification method for reducing the viscosity related problems of plant oils. The popularity of methyl esters has contributed to the term “biodiesel” now usually referring to plant oil esters and not neat plant Oils.
It was shown that in homogeneous catalysis, alkali catalysis is a much more rapid process than acid catalysis in the transesterification reaction. At 320C, transesterification was 99% complete in 4 h when using an alkaline catalyst (NaOH or NaOMe). At 600C and a molar ratio alcohol:oil of at least 6:1 and with fully refined oils, the reaction was complete in 1 h to give methyl, ethyl, or butyl esters. The reaction parameters investigated were molar ratio of alcohol to vegetable oil, type of catalyst (alkaline vs. acidic), temperature, reaction time, degree of refinement of the vegetable oil, and effect of the presence of moisture and free fatty acid. Although the crude oils could be transesterified, ester yields were reduced because of gums and extraneous material present in the crude oils.
Besides sodium hydroxide and sodium methoxide, potassium hydroxide is another common transesterification catalyst. Both NaOH and KOH were used in early work on the transesterification of rapeseed oil8. Recent work on producing biodiesel (suitable for waste frying oils) employed KOH. With the reaction conducted at ambient pressure and temperature, conversion rates of 80 to 90% were achieved within 5 minutes, even when stoichiometric amounts of methanol were employed9. In two steps, the ester yields are 99%. It was concluded that even a free fatty acid content of up to 3% in the feedstock did not affect the process negatively and phosphatides up to 300 ppm phosphorus were acceptable. In a study similar to previous work on the transesterification of soybean oil, it was concluded that KOH is preferable to NaOH in the transesterification of safflower oil. The optimal conditions were given as 1 wt-% KOH at 69±10C with a 7:1 alcohol:vegetable oil molar ratio to give 97.7% methyl ester yield in 18 minutes.
Most patents dealing with transesterification emphasize the engineering improvement of the process. Using patented procedures, a transesterification process permitting the recovery of all byproducts, such as glycerol and fatty acids, has been described. The use of alkaline catalysts is also preferred on the technical scale, as is documented by patents using sodium hydroxide, sodium methoxide, and potassium hydroxide. Different esters of C9-24 fatty acids were prepared with Al2O3 or Fe2O3 containing catalysts. A sulfonated ion exchange catalyst was preferred as catalyst in the esterification of free fatty acids.
Methyl and ethyl esters of palm and coconut oils were produced by alcoholysis of raw or refined oils using boiler ashes, H2SO4 and KOH as catalysts. Fuel yields > 90% were obtained using alcohols with low moisture content andEtOH-H2O azeotrope. Instead of using the extracted oil as starting material for transesterification, sunflower seed oils were transesterified in situ using macerated seeds with methanol in the presence of H2SO4. Higher yields were obtained than from transesterification of the extracted oils. Moisture in the seeds reduced the yield of methyl esters. The cloud points of the in situ prepared esters appear slightly lower than those prepared by conventional methods.
Another study reported the synthesis of methyl or ethyl esters with 90% yield by reacting palm and coconut oil from the press cake and oil mill and refinery waste with MeOH or EtOH in the presence of easily available catalysts such as ashes of the waste of these two oilseeds (fibers, shell, husk), lime, zeolites, etc. Similarly, it was reported that the methanolysis of vegetable oils is catalyzed by ashes from the combustion of plant wastes such as coconut shells or fibers of a palm tree that contain K2CO3 or Na2CO3 as catalyst. Thus the methanolysis of palm oil by refluxing 2 h with MeOH in the presence of coconut shell ash gave 96-98% methyl esters containing only 0.8-1.0% soap. The ethanolysis of vegetable oils over the readily accessible ash catalysts gave lower yields and less pure esters than the methanolysis.
Several catalysts (CaO, K2CO3, Na2CO3, Fe2O3, MeONa, NaAlO2, Zn, Cu, Sn, Pb, ZnO, and Dowex 2X8 (anion exchange resin)) were tested (mainly at 60-630C) for catalytic activity in the transesterification of low-erucic rapeseed oil with MeOH. The best catalyst was CaO or MgO. At 2000C and 68 atm, the anion exchange resin produced substantial amounts of fatty methyl esters and straightchain hydrocarbons. An enzymatic transesterification method utilizing lipases and methanol, ethanol, 2-propanol, and 2-methyl-1-propanol as alcohols gave alkyl esters of fatty acids. This method eliminates product isolation and waste disposal problems.
Among the recent developments in the transesterification of plant oils is a new process called “advanced dry biodiesel production process”. This process has the following advantages over the conventional alkaline transesterification process: (a) oil with high free fatty acid and water content can be used as feedstock; (b) high conversion efficiency of over 99.5% can be achieved in a singlestage reaction (compared to 90-95% conversion efficiency in the conventional process) using nearly theoretical amount of methanol (15% compared to 30% in conventional process) and catalyst (1.5% KOH compared to 3% NaOH in the conventional process); (c) no waste water is produced; and (d) high quality biodiesel can be produced at relatively low temperature (water content <200 ppm). Commercial plants using the advanced dry biodiesel production process of up to 5,000 liters per day of product are now in successful operation in Japan.