Energy integration superstructure

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. 

Within each of the three general approaches toward process synthesis, key decisions are made about the flowsheet design that have a bearing on the operability characteristics of the plant. For example, in a hierarchical procedure (Ref. 6) we will make decisions about whether the plant is batch or continuous, what types of reactors are used, how material is recycled, what methods and sequences of separation are employed, how much energy integration is involved, etc. In a thermodynamic pinch analysis, we typically start with some flowsheet information, but we must then decide what streams or units to include in the analysis, what level of utilities are involved, what thermodynamic targets are used, etc. In an optimization approach, we must decide the scope of the superstructure to use, what physical data to include, what constraints to apply, what disturbances or uncertainties to consider, what objective function to employ, etc (Ref. 7). 

Fig. 15. (a) Integrated LEED intensity of an oxygen-induced second-order p(2x2) superstructure spot vs. temperature at an electron energy — 65 eV. Dots are data from a... 

The material and energy balance equations associated with every unit in the superstructure are included as the equation constraints of the optimisation problem. Other than the balance equations associated with all units, models of gas emissions and environmental impacts are also integrated into the optimisation model. Binary variables are used to signify the existence or non-existence of units in the superstructure. The resulting multi-objective optimisation problem is formulated as an MINLP model. The decisions to be made by the multi-objective optimisation model include the configuration of the utility system, the values of the operating pressures and temperatures of different steam headers, the types of fuels used by the units, and all stream flowrates. 

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