Hierarchically Structured Reactors

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Sanchez, C., and Yang, X.-Y. (eds) (2012) Hierarchically Structured Materials, Wiley-VCH Verlag GmbH, Weinheim. 

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Figure 1.13 Hierarchic structure of entities that are related to using micro reactors - fundaments, impacts, and applications.

The hierarchical approach is a simple but powerful methodology for the development of process flowsheets. It consists of a top-down analysis organized as clearly defined sequence of tasks aggregated in levels. Each level handles a fundamental conceptual problem input/output structure, reactor design, structure of separa-... 

Aligned multiwall CNT arrays were synthesized as a basis for a microstructured catalyst, which was then tested in the Fischer-Tropsch reaction in a microchannel reactor [269]. Fabrication of such a structured catalyst first involved MOCVD of a thin but dense A1203 film on a FeCrAlY foam to enhance the adhesion between the catalyst and the metal substrate. Then, multiwall CNTs were deposited uniformly on the substrate by controlled catalytic decomposition of ethene. Coating the outer surfaces of the nanotube bundles with an active catalyst layer results in a unique hierarchical structure with small interstitial spaces between the carbon bundles. The microstructured catalyst was characterized by the excellent thermal conductivity inherent to CNTs, and heat could be efficiently removed from the catalytically active sites during the exothermic Fischer-Tropsch synthesis. 

Figure 1 Hierarchical structure of photobioprocesses. The outer layer is the reactor itself being the link between the environment and the biomass. The cells can be understood from the quasi-stationary metabolic network. Acclimation to different environmental situations is mainly done on the level of functional macromolecules controlling the fluxes in the framework of stoichiometric constraints.

Also worth noting is that for chemical processes involving chemical reactions, the chemical reactor is the heart of the process and its operation affects the operation of other units (separation units, utility requirements, etc.) [3]. Therefore, the design of the chemical reactor should be a distinct step in the hierarchical design structure of chemical processes with chemical reactions. Recently, Mann and Hoo [2] have proposed a modified hierarchical structure, shown in column 3 of Table 1. 

The subgroups were determined based on a hierarchical decomposition strategy for a FCCU (Ramesh et al, 1992). The FCCU was decomposed into four separate units feed.system, catalyst.system, reactor/regenera-tor.system, and separation.system as shown in Fig. 32. Each of these units was further divided into more detailed functional, structural, or behavioral... 

The reactor/separation/recycle level allows plantwide control issues to be included in the hierarchical approach at an early level of design. In most cases, the Separation is considered as a black-box for which targets are set, for example as species recovery or product purities. A stoichiometric or a kinetic reactor can be used. In the first case, plantwide control structures can only be proposed, while in the second case these can be also evaluated. 

Fig. 11. Three possible general separation structures when reactor exit is (a) liquid (b) vapor, and liquid (c) vapor only [Reproduced from Hierarchical Decision Procedure for Process Synthesis, J. M. Douglas, AIChE J., 31, 353 (1985), by permission].

The chemical reactor has a determinant role on both the material balance and the structure of the whole flowsheet. It is important to stress that the downstream levels in the Hierarchical Approach, as the separation system and heat integration, depend entirely on the composition of the reactor exit stream. However, a comprehensive kinetic model of the reaction network is hardly available at an early conceptual stage. To overcome this shortcoming, in a first attempt we may neglect the interaction between the reactor and the rest of the process, and use an analysis based on stoichiometry. A reliable quantitative relationship between the input and the output molar flow rates of components would be sufficient. This information is usually available from laboratory studies on chemistry. Kinetics requires much more effort, which may be justified only after proving that the process is feasible. Note that the detailed description of stoichiometry, taking into account the formation of sub-products and impurities is not a trivial task. The effort is necessary, because otherwise the separation system will be largely underestimated. 

The key result of the Hierarchical Approach is the development of the basic flowsheet structure, formed by Reactor-Separations-Recycles. This structure defines the material balance envelope. In this respect of highest importance is the behaviour of the reaction system, which should deliver a realistic image of the reaction mixture. Other constraints regarding the reactor operation, as molar ratio of reactants, or safety requirements, are determinant for the structure of recycles. Optimal conversion represents a complex optimisation problem between the valorisation of raw materials and the cost of reactor, separators and recycles. 

This paper describes an integrated MINLP synthesis of overall process schemes using a combined synthesis / analysis approach. The synthesis is carried out by a multilevel-hierarchical MINLP optimization of the flexible superstructure, whilst the analysis is performed in an economic attainable region (EAR). The role of the MINLP synthesis step is to obtain a feasible and optimal process structure, and the role of the subsequent EAR analysis step is to verify the MINLP solution and to propose in the feedback loop, any profitable superstructure modifications for the next MINLP. The main objective of the integrated synthesis is to exploit interactions between the reactor network, separator network and the remaining part of the heat/energy integrated process scheme. 

This paper presents a Fuzzy Clustering of Fuzzy Rules Algorithm (FCFRA) that allows the automatic organisation of the sets of fuzzy IF. .. THEN rules of one fuzzy system in a Hierarchical Prioritised Structure. The proposed FCFRA algorithm has been successfully applied to the modelling of a nonlinear small scale Pilot Plant Reactor. 

A control structure obtained using the hierarchical procedures in the previous section normally can be expected to work reasonably well. However, the only valid test of that conjecture is actually to perform simulations or plant tests after individual controllers have been tuned. In that way, one can determine just how well the controlled system deals with disturbances, production rate changes, and so on. For our purposes, we have focused initially on the core process units in the plant (reactor, flash unit, and recycle tank) to determine how well a design likely would work if it were developed using heuristics, strongly guided by simplified structural analysis. Other credible alternatives are possible. Which of the many alternatives are acceptable... 

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