Aspen HYSYS physical properties

Each of the property information systems has an extensive set of subroutines to determine the parameters for vapor pressure equations (e.g., the extended Antoine equation), heat capacity equations, etc., by regression and to estimate the thehnophysical and transport properties. The latter subroutines are called to determine the state of a chemical mixture (phases at equilibrium) and its properties (density, enthalpy, entropy, etc.) When calculating phase equilibria, the fugacities of the species are needed for each of the phases. A review of the phase equilibrium equations, as well as the facilities provided by the process simulators for the calculation of phase equilibria, is provided on the CD-ROM that accompanies this book (see ASPEN- Physical Property Estimation and HYSYS Physical Property Estimation). 

Throughout this book, we have seen that when more than one species is involved in a process or when energy balances are required, several balance equations must be derived and solved simultaneously. For steady-state systems the equations are algebraic, but when the systems are transient, simultaneous differential equations must be solved. For the simplest systems, analytical solutions may be obtained by hand, but more commonly numerical solutions are required. Software packages that solve general systems of ordinary differential equations— such as Mathematica , Maple , Matlab , TK-Solver , Polymath , and EZ-Solve —are readily obtained for most computers. Other software packages have been designed specifically to simulate transient chemical processes. Some of these dynamic process simulators run in conjunction with the steady-state flowsheet simulators mentioned in Chapter 10 (e.g.. SPEEDUP, which runs with Aspen Plus, and a dynamic component of HYSYS ) and so have access to physical property databases and thermodynamic correlations. 

As mentioned, the ability of a cubic equation of state to describe accurately the physical properties and behaviour of a fluid depends both on the alpha function and the mixing rules (see Chapter 6). A discussion about the last development in commercial simulators, namely in Aspen Plus and Hysys, can be found in a recent article written by Twu, Sim and Tassone (2002). 

Aspen HYSYS used the concept of the fluid package to contain all necessary information for performing flash and physical property calculations. This approach allows you to define all information (property package, components, hypothetical components, interaction parameters, reactions, tabular data, etc.) inside a single entity. 

Process simulators, steady state, dynamic, and batch, are used throughout the textbook (ASPEN PLUS, HYSYS.Plant, CHEMCAD, PRO/II, BATCH PLUS, and SUPERPRO DESIGNER). This permits access to large physical property, equipment, and cost databases... 

The ability to model Selexol-based unit operations in Aspen Plus or Aspen HYSYS was recently made possible by the inclusion of the Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT) physical property model. As in Aspen HYSYS (see Section 6.1.1), a single chemical DEPG be used as a proxy for the mixture. Simple example files for using PC-SAFT with Selexol for one- or two-unit operations are included with the Aspen Plus distributiOTi, and an example for using PC-SAFT in Aspen HYSYS is available for download to subscribers of the Aspen Technology support website. 

Use Hysys, Aspen, PRO/II, and SuperPro softwares to estimate physical properties. 

Hysys, SuperPro Designer, and Aspen Plus results are exactly the same. There is discrepancy between hand calculations and the result obtained with SuperPro mainly due to physical properties such as specific heat. 

For any process simulation that involves only vapor-liquid phases, certain key physical and thermodynamic properties must be available for each phase. Table 1.3 lists these properties for all phases. We can typically obtain these properties for pure components (i.e. n-hexane, n-heptane, etc.) from widely available databases such as DIPPR [2]. Commercial process simulation software (including Aspen HYSYS) also provides a large set of physical and thermodynamic properties for a large number of pure components. However, using these databases requires us to identify a component by name and molecular structure first, and use experimentally measured or estimated values from the same databases. Given the complexity of crude feed, it is not possible to completely analyze the crude feed in terms of pure components. Therefore, we must be able to estimate these properties for each pseudocomponent based on certain measured descriptors. 

The first step in creating the model is the selection of a standard set of components and a thermodynamic basis to model the physical properties of these components. When we create a new simulation, we must choose the components and thermodynamics appropriate for the process using the Simulation Basis Manager. The Simrdation Basis Manager allows us to define components and associated thermodynamics in Aspen HYSYS. Components maybe added manually through the Add button shown in Figure 4.41. However, we have a predetermined set of components for the FCC model. 

It is important to note that even when we choose an equation-of-state approach. Aspen HYSYS does not calculate all physical properties from the equation of state. For hydrocarbons, equations of state do not generally predict the equilibrium properties of very light components such as hydrogen. In addition, density predictions (especially in the heavy hydrocarbon range) can be quite poor. We almost always modify the equation of state to account for these deficiencies. For the FCC process, we choose the COSTALD method to predict the liquid density (Figure 4.46). 

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