The phenomena-driven approach for process synthesis considers as a starting point for the design, not the unit operations, but the phenomena occurring in them at a lower level of aggregation (Gavrila and Iedema, 1996). Reactions, phase changes, heat and mass transfer, and mixing are considered among the phenomena included in this method. The design problem is divided (decomposed) into three tasks: role assignment, phenomena grouping, and operating condition analysis. The goals of these tasks can be formulated through the following questions:

• What should occur in the process in order to achieve the global design target?

• Where should it occur?

• When and how should it occur?

In the second task, the alternative designs are proposed by grouping the phe­nomena in units and the continuous variables are described as Boolean variables (for instance, is the rate of a phenomenon equal to zero or greater than zero?). In the third task, the favorable conditions in the units are defined employing ordinal relations among quantities (dR/dt > dS/dt; Gavrila and Iedema, 1996). This method is oriented to explore innovative units and processes in order to sup­port the creativity during the design process. However, the method is based on an opportunistic identification and integration of the tasks as pointed out by Li and Kraslawski (2004). This approach has been applied to very particular cases as in the production of methyl tert-butyl ether (MTBE) by reactive distillation (Tanskanen et al., 1995).