Phase Equilibrium Using Aspen Plus

The phase equihbrium of this system is complex because of the existence of azeotropes. The inert components in the C5 feedstream include isopentane, n-pentane, 1-pentene, and 2-pentene. Essentially all of these inerts go out the top of the reactive distillation column. To illustrate the vapor-hquid equilibrium issues involved in the separation, we consider the ternary system iCs, methanol, and TAME. 

Note that the UNIFAC physical property package predicts that the iCs/methanol azeotrope is heterogeneous (two liquid phases). 

The boiling point of the first azeotrope (339 K) is lower than the boUing point of the lightest component iCs (348 K). This means that the overhead from the column will have a composition close to the azeotropic composition. 

Ternary Diagrams. Ternary diagrams are very useful in analyzing three-component systems and in helping to design columns, particularly for systems with complex vapor— liquid equilibrium. We illustrate some of their features in this section using the ternary iCs/ methanol/TAME system at 4 bar. 

We can locate various streams on this ternary diagram. For example, plotting a point with coordinates ZMeOH = 0.3 and Zjcs = 0.4 will indicate a feedstream to the column with these compositions. In the same way, the distillate and bottoms points can be located on the diagram. Because of the ternary mixing mle, the feed coordinates must lie on a straight line connecting the distillate and bottoms coordinate points. 

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Next, we specify the x, between 0 and 1, and estimate the total pressure P and yx from Eq. (1.193) to prepare the total pressure and equilibrium compositions shown in Table 1.10. In Figure 1.9, we can compare both the Tyx and Pyx diagrams obtained from Raoult s law and the NRTL model using the Aspen Plus simulator. As we see, ideal behavior does not represent the actual behavior of the acetone-water mixture, and hence we should take into account the nonideal behavior of the liquid phase by using an activity coefficient model. 

In Aspen Plus, solid components are identified as different types. Pure materials with measurable properties such as molecular weight, vapor pressure, and critical temperature and pressure are known as conventional solids and are present in the MIXED substream with other pure components. They can participate in any of the phase or reaction equilibria specified in any unit operation. If the solid phase participates only in reaction equilibrium but not in phase equilibrium (for example, when the solubility in the fluid phase is known to be very low), then it is called a conventional inert solid and is listed in a substream CISOLID. If a solid is not involved in either phase or reaction equilibrium, then it is a nonconventional solid and is assigned to substream NC. Nonconventional solids are defined by attributes rather than molecular properties and can be used for coal, cells, catalysts, bacteria, wood pulp, and other multicomponent solid materials. 

To solve equations of state, you must solve algebraic equations as described in this chapter. Later chapters cover other topics governed by algebraic equations, such as phase equilibrium, chemical reaction equilibrium, and processes with recycle streams. This chapter introduces the ideal gas equation of state, then describes how computer programs such as Excel , MATLAB , and Aspen Plus use modified equations of state to easily and accurately solve problems involving gaseous mixtures. 

In this chapter, you have derived the equations governing phase equilibrium and seen how the key parameters can be estimated using thermodynamics. You have solved the resulting problems using Excel, MATLAB, and Aspen Plus. You also learned to prepare a T-xy diagram as a way of testing the thermodynamic model chosen to represent the phenomenon. 

Vapor-Liquid Equilibrium Data Collection (Gmehling et al., 1980). In this DECHEMA data bank, which is available both in more than 20 volumes and electronically, the data from a large fraction of the articles can be found easily. In addition, each set of data has been regressed to determine interaction coefficients for the binary pairs to be used to estimate liquid-phase activity coefficients for the NRTL, UNIQUAC, Wilson, etc., equations. This database is also accessible by process simulators. For example, with an appropriate license agreement, data for use in ASPEN PLUS can be retrieved from the DECHEMA database over the Internet. For nonideal mixtures, the extensive compilation of Gmehling (1994) of azeotropic data is very useful. 

One of the most important issues involved in distillation calculations is the selection of an appropriate physical property method that will accurately describe the phase equilibrium of the chemical component system. The Aspen Plus library has a large number of alternative methods. Some of the most commonly used methods are Chao-Seader, van Laar, Wilson, Unifac, and NRTL. 

Figure 9.13 gives a ternary diagram for the isopentane-methanol-TAME system at 4 bar. The phase equilibrium of this system is complex because of the existence of azeotropes. The UNIFAC physical property package in Aspen Plus is used to model the VLB in all units except the methanol/water column where the van Laar equations are used because of their ability to accurately match the experimental data. 

Note that Aspen Plus gives a huge amount of results. Spend some time exploring these. Write down the values for vapor and liquid mole flow rates and drum temperature. Also look at the phase equilibrium and record the x and y values or print the xy graph. Of course, all these numbers are wrong, since we used the wrong VLE model. 

FIGURE 1.10 Phase equilibrium diagrams for acetone(l)-water(2) mixture (a) Tyxat 101.3 kPa, (b) Pyx at 50 °C both estimated from the Raoult s law (c) and (d) from the NRTL model using the Aspen Plus simulator. 

To account for nonideal vapor-hquid equilibrium and possible vqxjr-liquid-liquid equilibrium (VLLE) for these quaternary systems, the NRTL model or UNIQUAC model is used for activity coefficients. Table 7.2 provides the model parameters for these five quaternary systems where the EtAc, IPAc, and AmAc systems are described by the NRTL model and MeAc and BuAc systems are represented using the UNIQUAC model. Because of the near atmosphaic pressure, the only vapor phase nonideality considered is the dimerization of acetic acid as described by the Hayden-O CoimeU second virial coefficient model. The Aspen Plus built-in association parameters are used to compute fiigacity coefficients. 

To account for nonideal vapor-liquid equilibrium and possible VLLE for this quaternary system, the NRTL model is used to calculate the activity coefficients. Model parameters are taken from Chapter 7. Vapor-phase nonideality caused by the dimerization of acetic acid is also taken into consideration using the Hayden-O Connell second virial coefficient model. Aspen Plus built-in parameters values are used. 

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