Elementary Process Functions Methodology (EPF)
State-of-the-art methods for chemical process design are (1) experience-based and model-supported heuristics, which are easy to apply and lead often to good results at least for not too complex systems; (2) attainable region concepts, which typically make use of predefined ideal reactors to find an optimal reactor network, but are hard to apply to processes with integrated recycles; (3) rigorous optimization methods, in which either superstructures of units are optimized, where MINLP problems are to be solved, or dynamic programming approaches from which optimal temperature and concentration profiles along the reaction coordinate can be found. Main drawbacks of approaches (1) and (2) are the use of predefined units, and the difficulties in designing process systems with recycle loops. They are mostly used to design single-phase systems and are choosing devices from a predefined set of solutions. In recent years rigorous optimization methods have been intensively used. The superstructure optimization approach was applied to both, homogeneous and multiphase systems. Dynamic optimization approaches developed so far have not been used to derive optimal designs for devices or curtail the optimal design in advance.
In 2008, the PSE group has proposed a new methodological approach for the optimal design of chemical production processes, illustrated in the Figure below. In its original form, this approach is based on the idea that in each chemical process Lagrangian matter elements are manipulated along their travel route through the process by mass, energy, and momentum fluxes. In terms of the process hierarchy introduced, the consideration of matter elements is linked to the Phase Level. At this level, independent on specific devices or unit operations, we can determine an ideal process route via dynamic optimization at unlimited fluxes but with predefined system-inherent constraints, provided that we have access to adequate knowledge about the reaction kinetics. Then, on the Process Unit Level the ideal process route is approximated by application of a combination of existing devices or new tailor-made equipment. This is achieved in a three-step procedure under consideration of many alternatives, operated either in semi-batch or continuous mode.
In the recent years, we have further extended the EPF process design concept to four main directions . On the Molecular Level, we have started to include the computer-aided molecular selection and design of solvents used as reaction media or separating agents [52, 59, 61, 62, 63]. At the Phase Level, the methodology was extended to multi-phase systems [16, 44, 45, 46] and to complex reaction networks . On the Process Unit Level, we robustified the design by accounting for the uncertainties of the fluid residence times, uncertain kinetic parameters and uncertain control actions . On the Plant Level, we have investigated how the optimal process unit, e.g. the best chemical reactor, can be identified under constraints imposed by the loops necessary for recycling non-converted reactants  and auxiliary agents (homogeneous catalysts and/or solvents). Furthermore, the Elementary Process Functions Methodology (EPF) was adapted to new challenging process examples including chemical energy conversion processes [47, 71, 72, 90], biochemicals production processes [77, 89], and production processes where solid particles are involved [54, 91].
Figure: The Elementary Process Functions (EPF) methodology provides a conceptual framework for taking rational process design decisions at the four levels of the process hierarchy.[less]
Figure: The Elementary Process Functions (EPF) methodology provides a conceptual framework for taking rational process design decisions at the four levels of the process hierarchy.