Aspen HYSYS

The process simulation code Aspen-HYSYS 3.2 has been used for residential PEM fuel cell system calculations. Natural gas has been simulated as three different sources for hydrogen production. The chemical compositions of the natural gas fuel are summarized in Table 1. The average molecular weight of natural gas is around 16.6 kg/kmol. All simulation studies are performed based on this composition. 

The major units of the Aspen-HYSYS simulation for natural gas steam reforming based fuel cell system are presented in Figure 3. 

Nowadays, several process simulators such as Aspen Plus and Aspen HYSYS are commercially available for simulating complete chemical processes. Common process units and a property database for numerous chemicals are available in such simulators. However, models for less common and/or new process units (for example, membrane separation) are not readily available in the simulators, but they may be available in the literature or can be developed from first principles. Mathematical model for a new process unit can be implemented in Aspen Custom Modeler (ACM), and then it can be exported to (included in) Aspen Plus or Aspen HYSYS for simulating processes having a new process unit besides common process units such as heat exchangers, compressors, reactors and columns. Process simulators for simulation and ACM for implementing models of new process units are... 

As mentioned earlier, some process models are not available in the simulators such as Aspen Plus. In such situations, ACM can be used to implement models available in the literature or newly developed models. Subsequently, these ACM models can be included in Aspen Plus and/or Aspen HYSYS for use like built-in models in any process. The above model for gas separation using membrane (Section 4.2.3) can be implemented and solved in ACM see Appendix 4A for more details on the ACM model for no permeate mixing membrane module. In order to implement the membrane model in ACM, all chemicals are defined from the component list in the Aspen Properties User Interface program, and then... 

Global SOO and MOO optimization methods, particularly recent ones, have been implemented and are readily available in Excel, MATLAB, C-i-i-, FORTRAN, and so on (Table 4.1). But, they may not be available in process simulators. So, interfacing between a process simulator (for example. Aspen HYSYS, Aspen Plus or ACM) and a global optimization program (for example, I-MODE in Excel) is necessary for improving the design of chemical processes. In particular, students and practitioners are familiar with Excel, and can easily use worksheets in Excel for computations, data analysis, plotting, and so on. 

For using the model in Aspen Plus, the membrane model in ACM has to be saved as a. msi file using the export wizard (right click on the NGSep in All Items pane). For this, Microsoft C++ compiler is required (ACM V7.3 requires Professional/Premium/ Ultimate edition of Microsoft Visual Studio 2008, whereas ACM V8.0-8.6 requires Professional/Premium/Ultimate edition of Microsoft Visual Studio 2010). After this, the. msi file can be installed in Aspen Plus and/or Aspen Hysys library for use like built-in models in simulating chemical processes... 

Appendix 4C Interfacing of Aspen HYSYS v8.4 with Excel 2013... 

To illustrate the interfacing between Aspen HYSYS and Excel, an ammonia synthesis process (Figure 4.C.1) is taken from simulation examples available in Aspen HYSYS. This process consists of several reactors, separators and heat exchangers/coolers. The main steps of this interfacing are shown below. Some familiarity with Aspen HYSYS, Excel and VBA is required for adopting/using them. Note that sample Aspen HYSYS and Excel files for interfacing can be downloaded from this book s website. 

Open a new Excel file, and create a new macro (name Ammonia Synthesis). Add Aspen HYSYS library (that is, HYSYS 8.4 Type Library for Aspen HYSYS v8.4) to VBA (by going through Excel - VBA - Tools - References). Note that the user needs to install Aspen HYSYS on his/her computer before this addition. Object Browser in VBA is used to access the Aspen HYSYS library, which gives VBA syntax for all variables in process streams or units... 

Figure 4.C. 1 Ammonia synthesis process example in Aspen HYSYS V8.4.

As shown in Figure 4.C.2, Aspen HYSYS simulation file is opened first. In VBA, Dim statement is used for declaration of variables, whereas Set statement is used for creating new objects. Then, values of feed flow rate, length and diameter of each reactor, and temperatures of inlet streams of both separators are transferred from cells C3 to Cll in Excel worksheet named DV to Aspen HYSYS , to Aspen HYSYS. To reduce computational time, remember to deactivate HYSYS solver before this transfer, and then activate it after transferring values of all decision variables to proceed with the calculations... 

After the convergence of the simulation, values of product flow rate and duties of both coolers are transferred from the simulator to cells C2 to C4 in Excel worksheet named Data from Aspen HYSYS . VBA code for these variables and/or data transfer is given in Eigure 4.C.2 include this code in the new macro... 

Feed Flow Rate.MolarFlow = Sheets("DV to Aspen Hysys").Ran e("C3") Reaetorl.TubeLength = sheets("DV to Aspen Hysys").Ran9e("C4")... 

Reaetor2.TubeLength = sheets("DV to Aspen Hysys").Range("C ")... 

Figure 4.C.2 VBA code for interfacing Aspen HYSYS file (for simulating ammonia synthesis...

DECISION VARIABLES TRANSFER FROM EXCEL TO ASPEN BSYYS 4 DEACTIVATE THE ASPEN HYSYS ACTIVATE THE ASPEN HYSYS... 

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

Simulate the ammonia synthesis process (Figure 4.C. 1 in Appendix 4C) in Aspen HYSYS. Identify important objectives, decision variables and constraints for this ammonia process. Interface the simulation file with the optimization program (in Excel or MATLAB), and then optimize the ammonia synthesis process. 

Hamid, M.K.A. (2013) Aspen HYSYS An Introduction to Chemical Engineering Simulation, Lambert Academic Publishing. 

The membrane module used for the simulation of the HMD process in Aspen HYSYS is developed using Aspen Custom Modeler (ACM) v8.4, and then compiled using Microsoft Visual Studio 2010. 

Here, F, Zf and h are, respectively, the molar flow rate, mole fraction of component of i and total enthalpy, all in cell k their subscripts, ret and perm, refer to retentate and permeate streams. Equations (10.4) and (10.5) are mass balances and mass-transfer equations for each of the components present in the membrane feed. The cross-flow model [Equations (10.3)-(10.7)] was implemented in ACM v8.4 and validated against the experimental data in Pan (1986) and the predicted values of Davis (2002). The Joule-Thompson effect was validated by simulating adiabatic throttling of permeate gas through a valve in Aspen Hysys. Both these validations are described in detail in Appendix lOA. 

The developed membrane model/ACM module was also validated against the results of Davis (2002). In his study, a mathematical model of a hollow-fiber membrane was developed in Aspen HYSYS for air separation. Parameters used in the simulation of this separation are given in Table 10.A.2. As can be seen in Table 10.A.3, the ACM model predictions are very close to the simulation results of Davis (2002), with a maximum difference of 0.41% in permeate O2 concentration. 

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