An electrical driven pump unit having two hydraulic pumps and two electric motors that are fitted in such a way as to make it possible to obtain the unit’s power by adding together the power of the two motors. Additional keys are stored in relatively slow system memory that has high storage capacity. The method for preparing fatty acid alkyl ester for bio-diesel fuel comprises the step of carrying out a counter current type esterification reaction of fatty acid and alcohol in each tray of a counter current column reactor at a temperature of 200 to 350\xb0 C.