NOVELTY - Pretreatment reaction kettle comprises a reaction kettle body. The outer upper wall of the reaction kettle body is covered with a heating jacket. The upper axial of the reaction kettle body is provided with a stirring shaft and a frame-type stirrer. The stirring shaft and the frame-type stirrer set on the reaction kettle body are connected with a stirring speed reducer and a stirring motor. The upper part of the reaction kettle body is provided with a material inlet and a safety valve joint. The lower part side wall of the reaction kettle body is provided with a product outlet.    USE - Used as a pretreatment reaction kettle.    ADVANTAGE - The method produces biodiesel oil which meets national standard GB/S500-2007. The pretreatment reaction kettle has scour prevention facility. The esterification-transesterification reaction kettle ensures one-step process, and is provided with methanol refluxing and dealcoholization functions.    DETAILED DESCRIPTION - Pretreatment reaction kettle comprises a reaction kettle body. The outer upper wall of the reaction kettle body is covered with a heating jacket. The upper axial of the reaction kettle body is provided with a stirring shaft and a frame-type stirrer. The stirring shaft and the frame-type stirrer set on the reaction kettle body are connected with a stirring speed reducer and a stirring motor. The upper part of the reaction kettle body is provided with a material inlet and a safety valve joint. The lower part side wall of the reaction kettle body is provided with a product outlet. The bottom of the reaction kettle body is provided with a slag discharge port. The top part of the reaction kettle body is connected with an anti-impact tank by a rising pipe and a return pipe. The top part of the anti-impact tank is connected with a breathing valve. The lower part of the reaction kettle body is provided with a thermometer sleeve. INDEPENDENT CLAIMS are also included for:    (1) an esterification-transesterification reaction kettle, comprising a reaction kettle body, the outer upper wall of the reaction kettle body is covered with a heating jacket, the upper axial of the reaction kettle body is provided with a stirring shaft and a frame-type stirrer, the stirring shaft and the frame-type stirrer set on the reaction kettle body are connected with a stirring speed reducer and a stirring motor, the upper part of the reaction kettle body is provided with a material inlet and a safety valve joint, the lower part side wall of the reaction kettle body is provided with a product outlet, the bottom of the reaction kettle body is provided with a slag discharge port, the top part of the reaction kettle body is connected with an anti-impact tank by an anti-impact tank rising pipe and a return pipe, the anti-impact tank is connected with a reflux tank by the anti-impact tank rising pipe and the return pipe, the top part of the reflux tank is connected with a breathing valve, the bottom part of the reflux tank is connected with a methanol recycling tank, and the lower part of the reaction kettle body is provided with a thermometer sleeve;    (2) preparing biodiesel, comprising either (i) introducing waste oil and a strong electrolyte into the pretreatment reaction kettle, performing ionization and flocculation, then adding water, refluxing under stirring condition, allowing for sedimentation and separating to obtain upper layer degummed waste oil and lower layer sediment solution, (ii) adding degummed waste oil, methanol and a catalyst into the esterification-transesterification reaction kettle, performing esterification-transesterification reaction under reflux condition, and recycling methanol using the methanol recycling tank, where catalyst is an acidic catalyst or a basic catalyst, (iii) adding an acid or an alkali to the esterification-transesterification reaction kettle, neutralizing to pH of 6-8, allowing for sedimentation and separating to obtain upper layer biodiesel oil mixture and lower layer aqueous glycerol, and (iv) vacuum distilling the biodiesel oil mixture in a vacuum distillation tower to obtain heavy biodiesel oil and collecting heavy biodiesel oil from top of the vacuum distillation tower, or (i) adding edible oil soapstock and a strong acid catalyst into the pretreatment reaction kettle, performing acidification reaction, allowing for sedimentation and separating to obtain upper layer acidified edible oil fatty acid soap and lower layer sediment solution, (ii) adding acidified edible oil fatty acid soap, methanol and a catalyst into the esterification-transesterification reaction kettle, performing esterification-transesterification reaction under reflux condition, and recycling methanol using the methanol recycling tank, where catalyst is an acidic catalyst or a basic catalyst, (iii) adding an acid or an alkali to the esterification-transesterification reaction kettle, neutralizing to pH of 6-8, allowing for sedimentation and separating to obtain upper layer biodiesel oil mixture and lower layer aqueous glycerol, and (iv) vacuum distilling the biodiesel oil mixture in a vacuum distillation tower to obtain heavy biodiesel oil and collecting heavy biodiesel oil from top of the vacuum distillation tower; and    (3) biodiesel oil prepared by the above method.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic representation of the pretreatment reaction kettle.