UniSim Design RTO Reference Guide

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In the standard UniSim Design modeling environment, the sub-flowsheet lets you provide a logical grouping of operations to facilitate understanding of the process behaviour. In addition, it provides the mechanism to encapsulate a solver (i.e., the Column sub-flowsheet) or to use different fluid packages (thermodynamics, component slates, etc.) within a simulation. For optimization, the sub-flowsheet provides the same benefits plus a number of additional capabilities for the simultaneous modular approach. Foremost, it provides a location where the standard propagation of information can be broken. Once a model is torn for optimization, information does not propagate from one sub-flowsheet to another.

This limits calculations to only those needed at a point in time. Similarly, by selecting the structure of the Sub-flowsheets appropriately, unnecessary equations are never posed to the optimizer. In addition, if derivatives are being generated numerically, the potential for noise in the generated derivatives is minimized by constructing suitably sized Sub-flowsheets. For operations that deliver analytical derivatives, these must be encapsulated within a single sub-flowsheet. For example, UniSim Design columns being solved by the Newton solver are able to deliver the Jacobian matrix to the optimizer directly. Extension unit operations that deliver analytical derivatives are handled in the same manner.