Flowsheeting thermodynamic options

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 appropriate selection of thermodynamic models is probably the most important aspect of a simulation work. Sometimes preliminary work is necessary to estimate physical properties for non-library components, or to identify the parameters of thermodynamic models from experimental data. A thermodynamic model may be valid for the whole flowsheet, or only for some units. Specific thermodynamic options at unit level will increase the reliability of the results. For the HDA process we have the following possibilities ... 

The Hashimoto process (b) was formd to be infeasible for 99.9% purity. In fact, the process is thermodynamically infeasible for the production of pure A. The scheme achieves only low performance for 90% purity. This is in agreement with literature [2]. The hypothetical option of a fully integrated process with distributed reactivity (c) allows for a significant improvement of process performance. This holds in particular for component A, where SR and EC are strongly reduced. The main reason is that here the required 3 / -value is 16% lower than in case (a) m [ is even lower than the Henry constant of B. The explanation is that any B in the reactive zone I also reacts to produce A, which is desorbed more easily and transported towards the non-reactive zones. A similar benefical effect (which is somewhat less pronounced) is found for m [v, which is higher for the fully integrated schemes than for the flowsheet-integrated processes. 

Step 1 The flowsheet is Figure 6.1, and you enter the components and feed conditions in the usual manner. (See other examples in Chapters 2-5 and Appendix C.) The thermodynamics model chosen is the Refinery, Chao-Seader option. The parts specific to the distillation column are the block parameters. Shown in Figure 6.2 is the screen where you select the block parameters. You choose the light and heavy keys (propane and Z-butane), the splits desired (99 and 1 percent), the pressure of the column (138 psia), a total condenser, and the desired number of stages (10). 

As mentioned, cubic EOS models are often default options in flowsheeting software. The reason is that these make possible to execute most of the basic engineering calculations involving thermodynamic properties and phase equilibria with the minimum amount of data. The intensive research in this domain enlarged considerably the area of applications. Among specialised works in this field we recommend the review of Sandler (1993), as well as the monograph on equations of state edited by lUPAC Commission on Thermodynamics (2000). 

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