Simulation thermodynamic packages

A number of design calculations require a knowledge of thermodynamic properties and phase equilibrium. In practice, the designer most often uses a commercial physical property or a simulation software package to access such data. However, the designer must understand the basis of the methods for thermodynamic properties and phase equilibrium, so that the most appropriate methods can be chosen and their limitations fully understood. 

The mathematical description considered in Section 2.3 and Appendix A was used as a modeling basis for the specially developed completely rate-based simulator DESIGNER (155). This tool consists of several blocks, including model libraries for physical properties, mass and heat transfer, reaction kinetics, and equilibrium, as well as a specific hybrid solver and thermodynamic package. 

Physical Properties (Thermodynamics) Packages An important functionality of a process simulator is its ability to calculate thermodynamic and physical properties of materials (e.g. density or boiling point). 

In using a flowsheet simulator, one of the most important decisions is the choice of thermodynamics package. The choice of thermodynamics options affects the accuracy of the material and energy balances. Appropriate choices depend on the compounds in the system, temperature, pressure, and the availability of parameters. Equations of state and activity models are used to calculate stream properties number of phases, phase composition, PVT relationships, enthalpy, and entropy. 

The mathematical description considered in Section 10.3.3 was used as a modeling basis for the specially developed completely rate-based simulator [80]. This tool consists of several blocks including model libraries for physical properties, mass and heat transfer, reaction kinetics and equilibrium as well as specific hybrid solver and thermodynamic package. It also contains different hydrodynamic models (e.g., completely mixed liquid - completely mixed vapor, completely mixed liquid - vapor plug flow, mixed pool model, eddy diffusion model [80]) and a model library of hydrodynamic correlations for the mass-transfer coefficients, interfacial area, pressure drop, holdup, weeping and entrainment that cover a number of different column internals and flow conditions. 

In the first part of this chapter, we opened it with how to start HYSYS and get familiar with its desktop environment. We also discussed how to select components that will be used in simulation. Selecting the right fluid/thermodynamic package is veiy important and therefore we provided a flowchart that will assist users to select the right thermodynamics models. 

The structure of a typical process simulator and the basic process information required to simulate a process are discussed. The various types of equipment that can be simulated, and the differences between alternative modules used to simulate similar process equipment, are reviewed. The inportance of choosing the correct thermodynamic package for physical property estimation is enphasized, and strategies to eliminate errors and solve simulation problems are presented. 

Other Models. Different simulators have a variety of additional models beyond those mentioned above. For example, some have non-ideal electrol5 e thermodynamic models that calculate species equilibria, some have polymer thermodynamic packages, and some allow petroleum cuts to be represented automatically by pseudoconponents. Presently, these packages are less consistent across simulators and are not discussed here. However, the user should always investigate the range of models available for the simulator being used. 

The following thermodynamics packages are strongly recommended for simulation of this process. 

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. 

Build a new system consisting of two streams, two tanks, and a mixer using the Wilson thermodynamic package. Pick any two components that are liquid phase at ambient temperatures. The first stream should be pure component A at 25°C and 100 kPa. The second stream should be pure component B at the same temperature and pressure. Set the flow of the first stream to 400 kg h and the second stream to 100 kg h These flows are consistent with the desired ratio of 4 1 between components A and B. The tanks are used to simulate dead time in the system, so choose relatively small volumes for the tanks and locate them in series with the first stream. Both tanks should be on level control rather than liquid flow control. Simulate process noise with a sine-wave input to the first stream using an amplitude of 50 kg h and a period of 10 min. The system should resemble the one shown in Figure W6.4. 

The above flowsheet can be simulated by means of an appropriate simulation package. In the absence of a comprehensive kinetic model and of fundamental thermodynamic data the results will be only approximate, namely with respect to satisfying the quality specifications. However, the simulation allows the designer to obtain an overall view of streams, utilities and equipment, needed for an economic assessment. 

Unsteady-state or dynamic simulation accounts for process transients, from an initial state to a final state. Dynamic models for complex chemical processes typically consist of large systems of ordinary differential equations and algebraic equations. Therefore, dynamic process simulation is computationally intensive. Dynamic simulators typically contain three units (i) thermodynamic and physical properties packages, (ii) unit operation models, (hi) numerical solvers. Dynamic simulation is used for batch process design and development, control strategy development, control system check-out, the optimization of plant operations, process reliability/availability/safety studies, process improvement, process start-up and shutdown. There are countless dynamic process simulators available on the market. One of them has the commercial name Hysis [2.3]. 

Professor Wakeham is interested in the relationship between the bulk thermophysical properties of fluids and the intermolecular forces between the molecules that comprise them. Thus, at one extreme, he is involved in the determination of intermolecular forces from measurements of macroscopic properties and the development and application of the statistical mechanics and kinetic theory that interrelate them. He is also actively involved in the measurement of the thermophysical properties of fluids under a very wide variety of thermodynamic states. The same thermophysical properties find application in the process industries within the design of a plant. A part of Professor Wakeham s activities are therefore concerned with the representation and extension of a body of accurate information on thermophysical properties in a fashion that allows their use with software packages for process simulation. 

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. 

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