Evolutionary modification is the conventional approach for process synthesis based on the experience of engineers and researchers. This approach condenses this experience into a programmed set of heuristic rules intended to making deci­sions on the process structure. Considering that this method corresponds to a trial — and-error approximation, its main limitation consists in the difficulty of obtaining relevant information in a way suitable for computational calculations.

2.2.1 Hierarchical Decomposition

The hierarchical heuristic method is an extension of the purely heuristic approach that entails the evolutionary modification and combines the heuristic rules with an evolutionary strategy for process design (Li and Kraslawski, 2004). Douglas (1988) proposed a method by which any process can be decomposed into five lev­els of analysis for its design. This strategy has a hierarchical sequential character considering that in each level of analysis different decisions are made based on heuristic rules. This allows generating different alternatives, which are evaluated from an economic point of view using short-cut models. As the method is applied in each one of the five levels, more information becomes available and the tech­nological scheme of the process evolves until its completion. According to this author, hierarchical decomposition comprises the analysis of the process in the following levels:

• Batch versus continuous

• Input-output structure of the flowsheet

• Recycle structure of the flowsheet

• Separation system synthesis

• Heat recovery network

A slightly different hierarchical decomposition scheme corresponds to the onion diagram (Smith, 2005), which starts from the reaction step toward effluent treatment:

• Reactor

• Separation and recycle system

• Heat recovery system

• Heating and cooling utilities

• Wastewater and effluent treatment

The design begins with the reactor selection to move toward the outer surface of the diagram by adding other layers, such as the separation and recycle system. These heuristic methods emphasize the decomposition strategy and screening of alternatives, which allow the fast identification of technological configurations often located near optimal solutions (Li and Kraslawski, 2004). However, the main drawback of these methods is the impossibility of handling the interactions among the different levels or layers due to their sequential character. In spite of this, the nature of this approach allows rapidly discarding many alternative configurations not leading to “good” designs. In addition, the analysis by design levels permits the utilization of process simulators with which the process flow­sheets are completed in an evolutionary way. This methodology has been mostly applied to chemical and petro-chemical processes. Nevertheless, the utilization of these procedures and design schemes is less frequent in the processes of the microbiologic industry.