ASPEN convergence calculations

For convergence calculations ASPEN employs some advanced features with the well-proven sequential modular architecture. 

In addition, convergence calculations may be combined simultaneously with design specifications. The usual methods would be to embed the design in a convergence loop and meet the design specification in each recycle calculation. A quasi-Newton method convergence calculation in ASPEN will allow a simultaneous, more efficient solution for the more difficult problems. 

Under Properties, Specifications, select the base property method. Since these components are liquids, NRTL thermodynamic package is the most convenient fluid package. Install CSTR reactor under Reactors in the model library, and connect inlet and exit streams. Specify the feed stream conditions and composition. Input the reactor specifications double click on the reactor block. The reactor Data Browser opens. Specify an adiabatic reactor and the reactor volume to 4433 liters the value obtained from hand calculations (Figure 5.11). Add the reactions to complete the specifications of the CSTR. Choose the Reactions block in the browser window and then click on Reactions. Click New on the window that appears. A new dialog box opens enter a reaction ID and specify the reaction as Power Law. Then click on Ok. The kinetic data are very important to make Aspen converge. Mainly specifying accurate units for pre-exponential factor A, is very important (see the k value in Figure 5.12). The value MUST be in SI units. 

This chapter listed many of the possible units in the Model Library of Aspen Plus. The ammonia process illustrated the procedures (and computer windows) you used to set the process conditions and examine the results. The thermodynamics choices can be verified by comparison with data reported in the literature. Sometimes the calculations do not converge, and then the Wegstein method, or Broyden s method, are useful for accelerating convergence. 

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...

Note that this is the calculation sequence prepared by ASPEN PLUS. Alternatively, when the user prefers to provide guesses for the two recycle streams, S5 and SIO, the simulation flowsheet in Figure 4.12c is utilized. To accomplish this in ASPEN PLUS, select Convergence from the Data pulldown menu. Then, select Tear which produces the Tear Streams... 

Complete the simulation flowsheets using sequences acceptable to ASPEN PLUS. If any of the streams are tom, your flowsheets should include the recycle convergence units. In you should indicate the calculation sequences. 

Finally, specify the makeup feed stream input to the second mixer to have a temperature of 35°C and pressure of 2 bar. To be an effective makeup stream, the flow rates of MDEA and H2O must equal the flow rates of those two components lost via the CO2 product and clean syngas streams. Although trivial to compute at the moment, once the recycle loop is closed, this number must be computed for each flow sheet convergence iteration in order to be able to physically achieve a steady state and prevent eventual dry-up of the solvent. Unlike ProMax, which has a Make-up/Blow-down block to handle this specific scenario. Aspen Plus has more generic tools that can be used instead. For example, a calculator block can be added that computes and sets the inlet composition and flow rate of the makeup feed stream for every flowsheet iteration. To do this, first configure the makeup stream to have a mole flow of H2O and MDEA of 1 kmol/h each. These are strictly dummy variables. However, it is critical that the Total Flow Rate is left blank. By specifying it in this manner, we are creating a specification in which the calculator block will be able to directly overwrite the dummy numbers of 1 kmol/h with its own calculations just before the second Mixer block executes. 

The next step is not required by Aspen HYSYS, but is good practice to ensure that columns converge regardless of the method used. We estimate the top and bottom temperatures from measured plant data in Figure 2.34. For the initial run, the calculated values may differ from the given temperature estimates. 

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