Systems with Multiple Operational Objectives

Synthesizing of process systems with multiple objectives Given the importance and potential impact of early design decisions in synthesis, it is highly desirable to develop computational frameworks that allow the evaluation and determination of trade-offs for a number of different attributes. Besides traditional economic measures, these include operability, safety, and environmental aspects. 

Some of the problems that concern the proper methods for consideration of several different objectives in reservoir planning are discussed. Classical systems analysis approach to decision making for multiple objective problems is outlined and the inherent difficulties associated with multiple objectives and subjective estimates are identified. Techniques used in reservoir design and operation are reviewed. An alternate technique for considering noncommensurate, objectives, which relates the objectives in terms of real trade-off costs and eliminates the need for a priori estimates of objective worth is presented. The method is illustrated with three examples, including a reservoir operation problem and a cooling tower design problem. 31 refs, cited. 

Mujtaba and Macchietto (1996) presented a more general formulation for optimal design and operation, dealing in particular with multiple separation duties, multicomponent mixtures, more complex operations (involving off-cuts) and more general objective functions. The method utilises a dynamic model (Type IV, Chapter 4) of the column in the form of a generic system of DAEs. Models of various rigor (type III and V, etc. of Chapter 4) can therefore be used. 

We noted earlier in this chapter that many reactions in the chemical industries are exothermic and require heat removal. A simple way of meeting this objective is to design an adiabatic reactor. The reaction heat is then automatically exported with the hot exit stream. No control system is required, making this a preferred way of designing the process. However, adiabatic operation may not always be feasible. In plug-flow systems the exit temperature may be too hot due to a minimum inlet temperature and the adiabatic temperature rise. Systems with baekmixing suffer from other problems in that they face the awkward possibilities of multiplicity and open-loop instability. The net result is that we need external cooling on many industrial reactors. This also carries with it a control system to ensure that the correct amount of heat is removed at all times. 

The above mentioned models can be used to support operational distribution decisions under consideration of the available capacities. On the tactical and strategical level such capacities are subjects of interest. To be able to find improved SC configurations, the effects of alterations of the system s capacities have to be measured. These effects are hard to anticipate since stochastic processes often complicate theoretical deductions which also affect the planning at the operational level. Therefore, this work advocates simulation models as an appropriate approach to model complex chemical SCs with multiple stochastic processes and multiple objectives. This modelling approach integrates operational planning models in a stochastic environment to accurately reflect the dynamic effects of alternative SC configurations. 

Fault-tree analysis This level of analysis concerns critical infrastructures, where multiple conditions are necessary for the systems to ensure its task. This type of approach aims to evaluate the remaining operating capacity (residual operation capacity) of objects such as health-care facilities. The system is broken down into structural, nonstructural, or human components, each one of them being connected with logic operators. 

The designation of 1-1-1 or B-R-G where B is the number of Brayton units, R is the number of recuperators sized to support the operation of one Brayton unit at 100% of its rated power and G is the number of gas cooiers sized to support the operation of one Brayton unit at 100% of its rated power. It does not indicate the number of individual recuperators or gas cooler assemblies as more than one unit may be housed within a common pressure boundary to achieve design objectives e.g. minimize mass, improve reliability, facilitate system arrangement. For example a 2-2-2 for a 200 kWe system with both Brajrtons running (no spares) would incorporate two 100 kWe Brayton units with two recuperators and two gas coolers, each sized to support a 100 kWe Brayton unit. The gas cooler and/or recuperator may be physically one unit with multiple flow paths or independent flow paths or multiple units. See Section 3.11 for additional discussion. 

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