Method for preparing high purity biodiesel

A method for preparing biodiesel from crude oils, comprising:

• 1) providing crude oil raw material, and pre-treating the crude oil raw material to remove impurities;
• 2) adding the pre-treated raw material into a reaction kettle;
• 3) pre-esterification step:
• adding methanol and 0.1-2 wt % concentrated sulfuric acid by the weight of the pre-treated raw material into the reaction kettle, and maintaining at 70-100° C., preferably 70-80° C., for 30 min-2 h, wherein the weight ratio of methanol to the pre-treated raw material is (1:1)-(1:20);
• 4) esterification step of vapor phase:
• heating the above reaction system to 110-130° C., supplementing vapor phase methanol from the bottom of the reaction kettle under atmospheric pressure, reacting under the temperature until the acid value of liquid phase system is ≦2, and stopping supplementing methanol;
• 5) ester exchange step:
• cooling the reaction material obtained in the step 4) to below 70° C., adding mixture of potassium hydroxide and sodium hydroxide withequal weight ratio in the same reaction kettle to maintain pH of the reaction material in the reaction kettle at 8-9, and then reacting at 70-90° C., preferably 70-80° C., for 2-3 h;
• 6) recycling methanol, and separating glycerol via gravity settling, so as to obtain a crude fatty acid methyl ester;
• and
• 7) subjecting the crude fatty acid methyl ester obtained in the step 6) to purification and distillation to obtain biodiesel.