In this work we have optimized the operation of a geothermal facility. Benzene, toluene and cyclohexane are selected as candidate fluids based on previous studies. We formulated a superstructure optimization problem for the optimal use of geothermal energy using a binary cycle with either one or two turbine expansions. Surrogate correlations for the thermodynamic properties of the fluids have been developed (i.e. enthalpy, entropy) to model the operation of the turbine and the heat exchanger network. A single economic objective and a normalized multiobjective function accounting for economic, environmental and safety issues are used. The economic optimization selects a two expansion cycle using toluene as organic fluid, producing 10.4 MW at 0.075 (sic) kWh with an investment of 102 M(sic) Safety considerations slightly change the operating conditions, reducing the pressures and temperatures, but not the selection of the working fluid. Sustainable and economic terms overcome safety issues. The results are competitive with other renewable-based technologies for thermal power production such as CSP or biomass.