The reuse of the liquid enzymes Eversa Transform and Resinase HT for castor oil transesterification was investigated. For each enzyme, the reactions were carried out at 35 °C with 6:1 methanol-to-oil molar ratio, 5 wt% enzyme and 5 wt% water by weight of oil, for 8 hours. After completion, the reaction mixture was centrifuged at 4000 rpm for 30 min and the enzyme phase was collected. The reuse of the enzyme solutions were tested using three different separation techniques. In the first, enzymes were reused after the centrifugation step without further separation. In the second, enzymes were reused after centrifugation followed by water diafiltration using an ultrafiltration membrane in order to remove glycerol and methanol from the recycle solution. In the third, the enzyme solution obtained after diafiltration was concentrated in the membrane set-up prior to its reuse. Tubular ceramic ultrafiltration membranes with MWCO of 15 and 25 kDa were used.
The experiments showed that ultrafiltration removed glycerol and methanol from the enzyme solution, increasing the enzyme activity during the transesterification. Furthermore, concentration of the enzyme solutions after the diafiltration had a positive effect on the FAME production when recycled enzyme solutions are used. For Eversa Transform, the biodiesel content increased from 67 % with reuse after centrifugation to 83 % when a concentrated and diafiltrated enzyme solution was used as catalyst. For Resinase HT, the biodiesel concentration improved from 69 to up to 79 %.