Process Synthesis by Hierarchical Approach

The flowsheet synthesis of continuous chemical-like process can be performed following a systematic strategy known as the hierarchical approach. The procedure, initially proposed by Jim Douglas and coworkers in the decade 1980-90 [1, 2], describes the conceptual design process as a logical sequence of analysis and synthesis steps grouped in levels. Each level involves a flowsheet development mechanism based on design decisions. The result is not a unique solution but a collection of alternative flowsheets from which an evaluation procedure eliminates the less attractive ones. 

Another option is merging the alternatives into a large superstructure that can be submitted to global optimization by appropriate MINLP techniques. However, this approach is not workable at present by using commercial packages. 

We should mention that the modern process-synthesis methods can ensure intrinsically the optimization of subsystems, such as reactions, separations and heat exchange. Therefore, by searching in the first stage the structural optimization of the flowsheet and by performing later the optimization of units should lead quite soon to the true overall optimum in a large number of situations. 

We may define the objectives of conceptual process design as follows  

Chemical Process Design Computer-Aided Case Studies. Alexandre C. Dimian and Costin Sorin Bildea Copyright 2008 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 978-3-527-31403-4 

---

Process Synthesis by Hierarchical Approach (c) Formation of byproducts and impurities... 

Process Synthesis by Hierarchical Approach Table 2.1 Heuristics at input/output level. 

The process flowsheet of the hydro-dealkylation (HDA) of toluene to benzene is the ubiquitous example used in the pedagogy of the process design. Moreover, it has been studied extensively in the open literature [1,7]. Luyben et al. [7] have developed a plantwide control structure for the HDA process using their nine step approach to control structure synthesis Fisher et al. [19] provide a control structure development for the HDA process using a hierarchical approach. Stephanopoulos [47] also presents a control structure for the HDA process. These flowsheets provide a basis by which the outcome of the proposed methodology can be compared. 

As mentioned, the process synthesis section should retrace the rationale of flowsheet development by applying the Hierarchical Approach, as explained in Chapter 7. Deviations from the standard flowsheet are encouraged. This is necessary because often the flowsheets described in the open literature are either oversimplified or being based design decisions that could be not longer valid in the context of the actual design. 

The materials which have been mentioned here so far are predominantly shaped in planar films of hierarchical order. However, the synthesis of hierarchically structured particles is also highly desirable, as they might be further processed and used for the preparation of composite porous materials. Wu et al. showed the synthesis of raspberry-like hollow silica spheres with a hierarchically structured, porous shell, using individual PS particles as sacrificial template [134]. In another intriguing approach by Li et al. [135], mesoporous cubes and near-spherical particles (Fig. 10) were formed by controlled disassembly of a hierarchically structured colloidal crystal, which itself was fabricated via PMMA latex and nonionic surfactant templating. The two different particle types concurrently generated by this method derive from the shape of the octahedral and tetrahedral voids, which are present in the template crystal with fee lattice symmetry. 

---