**4. Conclusions**

The main findings in this research work are as follows:

Two-step transesterification was an e ffective and economic method to produce biodiesel from castor oil. An analysis of the main variables of the process—catalyst concentration and methanol/oil molar ratio—in both steps showed that this method was quite robust, since a lot of experimental conditions produced a methyl ester yield in excess of 96.5%. An economic assessment of the main variable costs of biodiesel production showed an important reduction of annual expenses when the optimum conditions were used. This decrease was caused by the use of a two-step process instead of one-step, and the optimization of the conditions allowed for the use of response surface methodology. In the complete economic analysis, raw material costs accounted for a major portion of the total manufacturing costs. For this reason, the total manufacturing costs of biodiesel in the smallest industrial plant were close to the value in the biggest plant. The decrease in castor oil price could considerably improve the profitability of the process.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4344/9/10/864/s1, Figure S1: Experimental methyl ester content versus predicted values, Table S1: Experimental conditions which lead to an ester content greater than 96.5 %, according to the SRM.

**Author Contributions:** Conceptualization, N.S. and J.M.E.; methodology, N.S.; software, N.S.; validation, N.S., J.M.E. and J.F.G.; formal analysis, N.S.; investigation, N.S.; resources, J.M.E and J.F.G.; data curation, N.S. and S.N.; writing—original draft preparation, N.S.; writing—review and editing, S.N.; visualization, S.N. and J.M.E.; supervision, J.M.E. and J.F.G.; project administration, J.M.E. and J.F.G.; funding acquisition, J.M.E. and J.F.G.

**Funding:** This research was funded by the "Ministerio de Ciencia, Innovación y Universidades", the "Junta de Extremadura" and "FEDER" (ENE2009–13881, PRI09B102, IB18028, GR10159 and GR18150).

**Acknowledgments:** The authors would like to thank to the "Ministerio de Ciencia, Innovación y Universidades" and the "Gobierno de Extremadura" for the financial support received to perform this study. Nuria Sánchez thanks the Spanish Ministry of Education for the gran<sup>t</sup> received.

**Conflicts of Interest:** The authors declare no conflict of interest.
