Flowsheet embedding

Consider now ways in which the best arrangement of a distillation sequence can be determined more systematically. Given the possibilities for changing the sequence of simple columns or the introduction of prefractionators, side-strippers, side-rectifiers and fully thermally coupled arrangements, the problem is complex with many structural options. The problem can be addressed using the optimization of a superstructure. As discussed in Chapter 1, this approach starts by setting up a grand flowsheet in which all structural features for an optimal solution are embedded. 

Steady-state process simulation or process flowsheeting has become a routine activity for process analysis and design. Such systems allow the development of comprehensive, detailed, and complex process models with relatively little effort. Embedded within these simulators are rigorous unit operations models often derived from first principles, extensive physical property models for the accurate description of a wide variety of chemical systems, and powerful algorithms for the solution of large, nonlinear systems of equations. 

The older modular simulation mode, on the other hand, is more common in commerical applications. Here process equations are organized within their particular unit operation. Solution methods that apply to a particular unit operation solve the unit model and pass the resulting stream information to the next unit. Thus, the unit operation represents a procedure or module in the overall flowsheet calculation. These calculations continue from unit to unit, with recycle streams in the process updated and converged with new unit information. Consequently, the flow of information in the simulation systems is often analogous to the flow of material in the actual process. Unlike equation-oriented simulators, modular simulators solve smaller sets of equations, and the solution procedure can be tailored for the particular unit operation. However, because the equations are embedded within procedures, it becomes difficult to provide problem specifications where the information flow does not parallel that of the flowsheet. The earliest modular simulators (the sequential modular type) accommodated these specifications, as well as complex recycle loops, through inefficient iterative procedures. The more recent simultaneous modular simulators now have efficient convergence capabilities for handling multiple recycles and nonconventional problem specifications in a coordinated manner. 

Step 1 Representation of Alternatives A superstructure is postulated in which all process alternative structures of interest are embedded and hence are candidates for feasible or optimal process flowsheet(s). The superstructure features a number of different process units and their interconnections. 

At the process level, efficient flowsheet optimization strategies based on lumped parameter models are now widely used in practice (Biegler et al., 1997). At this scale, the PEFC is embedded within a power plant flowsheet model, as shown in Figure 3. The process comprises... 

Another class of total flowsheet synthesis algorithms is that of "embedding," where (as stated earlier) a superstructure is invented which contains all the desired flowsheet arrangements as substructures within it. Optimization is used to remove those portions of the superstructure which are not worthwhile, leaving the optimal flowsheet. 

There appears to be three fundamental approaches to the synthesis of chemical process flowsheets. The first, systematic generation, builds the flowsheet from smaller, more basic components strung together in such a way that raw materials eventually become transformed into the desired product. The second, evolutionary modification, starts with an existing flowsheet for the same or a similar product and then makes modifications as necessary to adopt the design to meet the objectives of the specific case at hand. The third, superstructure optimization, views synthesis as a mathematical optimization over structure this approach starts with a larger superflowsheet that contains embedded within it many redundant alternatives and interconnections and then systematically strips the less desirable parts of the superstructure away. 

Figure 10.5 Water use and reuse in a plating line (A) the original process flowsheet and (B) the new process flowsheet with an embedded optimal water use and reuse network design technology.

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