CHEMCAD simulation package

The best path to an in-depth understanding of a reactor simulation is a solid understanding and familiarity of how to arrange equations for a reaction system, but when the reactor system becomes too complicated to be simulated with a mathematical package (MATLAB, POLYMATH), then it is the turn of the process simulator package (ASPEN plus, ANSYS, ChemCAD, HYSYS, etc.) to simulate it. 

Solutions of formaldehyde and water are very non-ideal. Individually, the volatilities are, from most volatile to least volatile, formaldehyde, methanol, and water. However, formaldehyde associates with water so that when this three-component mixture is distilled, methanol is the light key and water is the heavy key. The formaldehyde will follow the water. The ESDK K-value package in CHEMCAD simulates this appropriately and was used for the simulation presented here. Latent heat should be used for enthalpy calculations. The expert system will recommend these choices. Alternatively, the data provided in Table B.7.4 can be used direcdy or to fit an appropriate non-ideal VLE model. 

Isopropyl alcohol and water form a minimum boiling point azeotrope at 88 wt% isopropyl alcohol and 12 wt% water. Vapor-liquid equilibrium (VLE) data are available from several sources and can be used to back-calculate binary interaction parameters or liquid-phase activity coefficients. The process presented in Figure B.3 and Table B.6 was simulated using the UNIQUAC VLE thermodynamics package and the latent heat enthalpy option in the CHEMCAD simulator. This package correctly predicts the formation of the azeotrope at 88 wt% alcohol. 

All the hydrocarbon corrponents used in the simulation can be considered to be well behaved, i.e., no azeotrope formation. The simulations were carried out using the SRK VLE and enthalpy packages using the CHEMCAD simulator. 

The following hints were developed for students using the CHEMCAD simulator. These should also provide help to people using other simulator packages. 

Figure 21.5 Comparison of (a) one-stage, (b) two-stage, and (c) two-step membrane processes, aU producing the same volume of inert gas purge (54.4 scfm). These calculations are performed using a computer process simulation package (ChemCAD 5.0, Chemstations, Inc., Houston, TX) modified with code written at MTR for the membrane separation step.

Note that separate packages as Batchfrac of Aspen and CC-ReACS of ChemCad are more convenient for simulating discontinuous processes involving both reaction and separation, as encountered in speciality chemicals. 

In recent years, property information systems have become widely available in computer packages. Some are available on a stand-alone basis, such as PPDS2 (1997), while others are available within the chemical process simulators, such as ASPEN PLUS, HYSYS.Plant, PRO/n, CHEMCAD, BATCH PLUS, and SUPERPRO DESIGNER. Commonly, constants and parameters are stored for a few thousand chemical species, with programs provided to estimate the property values of mixtures, and determine the constants and parameters for species that are not in the data bank using estimation methods or the regression of experimental data. Virtually all of the property systems estimate the properties of mixtures of organic chemicals in the vapor and liquid phases. Methods are also provided for electrolytes and some solids, but these are less predictive and less accurate. 

Many packages are available for steady-state simulation, as discussed in Chapter 4. To manipulate the linearized models in the Laplace, frequency, and time domains, MATLAB and SIMULINK are used commonly, and example scripts are introduced in Section 21.6. The most recent commercial packages permit steady-state and dynamic simulations. These include HYSYS.Plant, CHEMCAD, and ASPEN DYNAMICS, with the former used in this section and in Section 21.5. 

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