Although the thermodynamics imposes constraints to different chemical pro­cesses, it also sets itself up as a powerful source of design guidance for generating alternative technological configurations with better performance. For instance, the consideration of the second law of thermodynamics for analyzing the energetic efficiency of technological schemes through the exergy balance allows evaluating and choosing the best alternative configuration for a given process. This balance takes into account that the energy always will be degraded, which implies a lost of work by the mass and energy flows entering or leaving the system, according to the second principle of thermodynamics (Bastianoni and Marchettini, 1997). Sorin et al. (2000) proposed the application of some indexes based on the energy balance, such as the utilizable exergy coefficient and local contribution of exergy for each operation. They used them as criteria for reducing a superstructure of technological schemes in the case of benzene production from cyclohexane. The process synthesis methodology developed allowed obtaining a technological con­figuration with better performance with respect to the feedstock consumption, amount of emissions, and utilizable exergy coefficient compared with process synthesis procedures based on hierarchical decomposition or mathematical pro­gramming for the same benzene production process. In fact, the analysis of the second law of thermodynamics is the base for exploration and generation of tech­nological alternatives in the framework of the algorithmic methods for process design (Seider et al., 1999).

For synthesizing sustainable and environmentally friendly processes, the employment of the emergy concept has been proposed. This approach exploits the fact that industrial systems also obey the laws of nature as ecological systems. Since all the materials and services are transformed and stored forms of solar energy, the amount of solar energy directly or indirectly used to make any prod­uct can be employed as a measure of the ecological input or investment in that product or service. In this way, the incorporated solar energy (emergy) is used as a comparison standard for process synthesis (Bakshi, 2000).