Process hierarchy generation

As noted in a previous article in this Encyclopedia, self-assembly processes often generate compoimds and materials displaying hierarchies of stmcture. also known as higher stmctures. Self-assembled catenanes display similar stmctural hierarchies, although these are dififerent to the hierarchies common in biology and chemistry. 

It would obviously be desirable to eliminate the generation of waste. Practical alternatives are to minimize it by reduction at source or to recycle it. Table 16.7 illustrates the hierarchy of waste minimization practices. Source reduction includes increased process efficiency economies in the use of energy are also relevant, e.g. it may result in a decrease in the consumption of fossil fuels ... 

Because of the low success rate for the commercialization of new processes, we will continue to develop new processes by proceeding through a hierarchy of designs, and therefore we will still need shortcut models. To decide whether simple models are applicable in a particular situation, we can develop a perturbation solution around a complex model, so that the simple model is the generating solution. With this approach, we can establish an error criterion that will indicate the validity of the simple model. 

The Task Level is where the formulation problemsolving activity takes place. The formulation process is driven via the generation of a hierarchy of tasks. A task represents some well-defined activity. The hierarchy has an indefinite number of task levels. Domain knowledge about the formulation application is distributed throughout the hierarchy, with more abstract knowledge represented towards the top of the hierarchy and more specific knowledge toward the bottom. 

To handle the complexity of the plant and to still achieve the overall goal, a control hierarchy has been developed and used for many years. This architecture gets the company policy (several weeks time resolution) and refines it to the current action to be applied on any actuator of the plant (ms-sec resolution time). The procedure is to observe the state of the plant through thousands of sensors and evaluate the next action for any resolution time. Implicit, explicit, heuristic and first principles models are used in order to generate the adequate action. The common process control architecture has four control levels. The lower level of the architecture is the basic regulatory control, this control is achieved by single decentralized loops. Most of these loops are controlled by standard PID controllers. The actuating horizon at this level is just one. 

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