A single flow helical coil reactor (SFHR) and reverse flow helical coil reactor (RFHR) were designed for continuous biodiesel synthesis from a mixture of oils using KOH. BBD (Box-Behnken design) coupled with RSM (Response Surface Methodology) was carried out to optimize reaction variables for the transesterification reaction in the continuous reactor. The R-2 value for SFHR and RFHR were 0.97 and 0.98 respectively which indicate the accuracy of the model. The results indicated that RFHR gives highest biodiesel yield than SFHR. Residence time due to reverse flow mixing shows a positive effect on biodiesel yield. The optimum findings include residence time of 8.3 min, the process temperature of 57.6 degrees C, the catalyst concentration of 2.2 g and residence time of 5 min, the process temperature of 63 degrees C, the catalyst concentration of 2.1 g for SFHR and RFHR respectively. At the predicted optimized process parameters, the observed and predicted biodiesel yield was 99.0% and 99.2% for SFHR and 99.9% and 99.8% for RFHR, respectively. These values experimentally verified the model accuracy. The batch and helical coil reactor processes were also compared for biodiesel production and found that helical coil reactors are 8-9 times more time efficient as compared to the batch reactor. (C) 2018 Elsevier Ltd. All rights reserved.