Hierarchical recycle structure

The two-time-scale behavior of the material balance of integrated processes with large recycle suggests the use of a hierarchical control structure with two tiers of control action ... 

Hierarchical Approach is a simple but powerful methodology for the synthesis of process flowsheets. It consists of a top-down analysis organised as a clearly defined sequence of tasks grouped in levels. Each level solves a fundamental problem as, number of plants, input/output structure, reactor design and recycle structure, separation system, energy integration, environmental analysis, safety and hazard analysis, and plantwide control. At each level, systematic methods can be applied for the synthesis of subsystems, as chemical reaction, separations, or heat exchangers network. 

This factor is used to categorize the option within the waste reduction hierarchal structure. The preferred hierarchy is source reduction, resource recovery and recycling, and alternative treatment (The fact sheet on waste reduction for metal finishers can help identify an option s place within the hierarchy.) Each category or level has a point value assigned to it. The six levels are ... 

The purpose of this chapter is to create a general framework that captures the dynamic effect of the simultaneous presence of both significant material recycle streams and purge streams. We will use this result presented in (Baldea and Daoutidis 2007) to rationalize at the theoretical level the development and use of a hierarchical process control structure, consisting of several interconnected control and optimization layers. 

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. 

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. 

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... 

Lin YX, Cai WP, He H, Wang XB, Wang GZ (2012) Three-dimensional hierarchically structured PAN gamma-AlOOH fiber films based on a fiber templated hydrothermal route and their recyclable strong Cr(VI)-removal performance. RSC Adv 2(5) 1769-1773. doi 10.1039/c2ra00945e... 

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