Aspen HYSYS convergence loops

Figure 4.C3 Convergence check for recycle loop (RCY-1) and distillation column (COL-I) present in an Aspen HYSYS simulation file.

As a process simulator, we used Aspen HYSYS (see Figure 8.15). In all the scenarios, as initial values, we use the stream values obtained when the simulation is done in open loop. Moreover, instead of using the simnlator tools for converging the system (the recycle unit operation in HYSYS), we connect the simulator with external modules developed in MATLAB . In that way, we have a complete control over the numerical methods used for converging the system. In all the cases, a termination tolerance is eqnal to 10 , using a norm 1. 

Figure 4.12a shows a simulation flowsheet with two recycle loops for ASPEN PLUS. Flowsheets for CHEMCAD and PRO/II are identical except for the subroutine (or model) names for the units. Note that no recycle convergence units are shown. This is typical of the simulation flowsheets displayed by most process simulators. The flowsheet for HYSYS.PIant is an exception because the recycle convergence unit(s) are positioned by the user and appear explicitly in the flowsheet. For ASPEN PLUS, CHEMCAD, and PRO/Il, to complete the simulation flowsheet, either one or two convergence units are inserted, as described below. Note that a single convergence unit suffices because stream S6 is common to both loops, as illustrated in Figure 4.12b. Stream S6 is tom into two streams, S6 and S6, with guesses provided for the variables in S6. Since no units are outside of the loops, all units are involved in the iterative loop calculations. The calculation sequence is...

Emphasis is placed in this chapter on the usage of process simulators to carry out the optimization simultaneously with converging the recycle loops and/or decision variables. To do the optimization efficiently, simulators use one of three methods (1) successive linear programming (SLP), (2) successive quadratic programming (SQP), and (3) generalized reduced gradient (GRG). Emphasis in this chapter is placed on SQP, used by ASPEN PLUS and HYSYS.Plant. GRG, which is used by CHEMCAD, is not discussed here, but is covered by Edgar et al. (2001). 

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